WO2023126294A1

WO2023126294A1 - Apparatus and process for hydrocarbon steam cracking

Info

Publication number: WO2023126294A1
Application number: PCT/EP2022/087412
Authority: WO
Inventors: Walter Vermeiren; Hayato Hagi; Alexandre JEGOUX
Original assignee: Totalenergies Onetech Belgium
Priority date: 2021-12-30
Filing date: 2022-12-22
Publication date: 2023-07-06

Abstract

The present invention relates to an apparatus for hydrocarbon steam cracking, more specifically to a cracking furnace for steam cracking a hydrocarbon feedstock, wherein the furnace comprises one or more reactor tubes for transporting the hydrocarbon feedstock and dilution steam; wherein each reactor tube comprises one or more tube passes, and wherein the furnace comprises an electrically heated infrared emitter comprising one or more electrically powered heating elements for transferring heat to the reactor tubes, and wherein the furnace comprises an enclosure surrounding the one or more reactor tubes and the electrically heated infrared emitter. The present invention also relates to a process for hydrocarbon steam cracking using said apparatus.

Description

APPARATUS AND PROCESS FOR HYDROCARBON STEAM CRACKING

FIELD OF THE INVENTION

The present invention relates to an apparatus for hydrocarbon steam cracking. The present invention also relates to a process for hydrocarbon steam cracking using said apparatus.

BACKGROUND

Steam cracking is a well-known petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing lighter alkenes, including ethylene and propylene.

In steam cracking, a gaseous or liquid hydrocarbon feed - like naphtha, LPG (liquified petroleum gas) or ethane - is diluted with steam and then heated (pyrolyzed) in a furnace, without the presence of oxygen. Typically, the reaction temperature is very hot (around 800 - 850 °C) but the reaction is only allowed to take place for a very short time. In modern cracking furnaces, the residence time is even reduced to milliseconds (resulting in gas velocities reaching speeds beyond the speed of sound) in order to improve the yield of desired products.

After the maximum cracking temperature has been reached (related to the Coil Outlet Temperature), the gas is quickly quenched to stop the reaction in a transfer line exchanger. The products produced in the reaction depend on the composition of the feed, on the hydrocarbon to steam ratio, and on the cracking temperature and furnace residence time.

Light hydrocarbon feeds, such as ethane, LPG, or light naphthas, give product streams rich in the lighter alkenes, including ethylene, propylene, and butadiene. Heavier hydrocarbon feeds (full range and heavy naphthas as well as other refinery products) give some of these, but also give products rich in aromatic hydrocarbons and hydrocarbons suitable for inclusion in gasoline or fuel oil.

Steam cracking is typically carried out in a steam cracking furnace. Steam cracking furnaces that are heated by burning fuels generally include a convection section and a radiant section. The radiant section includes a plurality of tubular coils which are typically referred to as “radiant tubes”. Conventionally, the radiant tubes are located proximate to one or more fired heaters, e.g., burners, in the radiant section which heat the outer surface of the radiant tubes. Flue gas from combustion carried out with the fired heaters travels upward from the radiant section, through the convection section, and then away from the steam cracker furnace's flue gas outlet. The hydrocarbon cracking feed and the dilution steam is typically preheated by indirect exposure to the flue gases (heat exchange bundles) in the convection section. The pre-heated hydrocarbon cracking feed is then combined with steam to produce the steam cracker feed. The steam cracker feed is typically subjected to additional pre-heating in the convection section. The pre-heated steam cracker feed is then transferred to the radiant section, where the steam cracker feed is indirectly exposed to the combustion carried out by the burners.

One of the disadvantages of a steam cracking process is that such process is one of the most energy-intensive processes in the chemical industry, due to the highly endothermic chemical reactions it involves. Since hydrocarbon pyrolysis is highly endothermic, energy is needed to “crack” large hydrocarbon molecules, to produce an effluent stream comprising molecular hydrogen and desirable hydrocarbon products such as light olefins. The effluent stream is typically compressed and cooled to facilitate separation and recovery from the effluent stream of the desired products and co-products, which requires still more energy, e.g., for turbomachinery and refrigeration equipment. When the required reaction heat is produced by combustion of fuel, still a lot of the produced combustion heat is entrained with the fuel gases as fatal sensible heat to the convection section. Generally, 40-50% of the produced combustion heat is absorbed in the radiation section for cracking while the other 60-50% is entrained with the flue gases. In the convection section, this sensible heat is used as much as possible for preheating the feed (hydrocarbons and dilution steam) and raising high pressure steam. The latter is used essentially for the compression (including refrigeration equipment) and fractionation section. In particular, turbomachines for compression that are driven on steam have not a high energy efficiency. Hence if the production of this fatal steam could be minimized, turbomachines could be driven by electricity at a much higher energy efficiency.

Moreover, the convection section is a very complex and expensive piece of equipment consisting of several heat exchange bundles. In particular those bundles where heat exchange occurs between two gaseous phases have to develop a lot of heat exchange surface.

Another important disadvantage of a steam cracking process is that it produces considerable emissions of air pollutants, particularly NOx, and greenhouse gas, such as CO2; emissions that can be related to the high energy consumption of the endothermic conversion in the cracking furnaces and the use of fossil fuels. The flue gas from steam cracking furnaces may comprise for instance about 71% N2, about 8% CO2, 18% H2O, about 3% O2, and ppm level contaminates like CO, SO_X, and NO_X depending on the combustion conditions and fuel composition used. During combustion of fuels, the flames can reach temperatures above 1600°C where the formation of NO_X from the nitrogen in the air becomes significant. When levels of NO_X are too high, selective catalytic reduction units (using ammonia or urea as reductant) need to be installed on top of the exhaust of the convection section. Given the increasingly pressing climate issues, there is therefore a particular need for optimizing steam cracking processes in order to reduce the emissions of greenhouse gases (CO2) and other exhaust emissions. There is also an ongoing need in the art to increase energy efficiency and/or to reduce energy losses in steam cracking processes, for instance, by improving heat transfer in the radiation section of steam cracking furnaces and/or by reducing or eliminating the amount of fuels, especially fossil fuels, burned to provide energy.

Overall, while steam cracking hydrocarbons is and will continue to be the main industrial process to produce light olefins in the coming decades, the need for improvements in this field persists in light of at least the aforementioned drawbacks.

It is therefore an object of the present invention to provide an apparatus and process for hydrocarbon steam cracking which addresses at least some of the above indicated needs and disadvantages.

Therefore, there is a need for an improved apparatus and process for hydrocarbon steam cracking.

SUMMARY OF THE INVENTION

It has now been found that the above objectives can be attained either individually or in any combination by using the specific and well-defined apparatus and process as disclosed herein.

In a first aspect, the present invention provides a cracking furnace for steam cracking a hydrocarbon feedstock, wherein the furnace comprises one or more reactor tubes for transporting the hydrocarbon feedstock and dilution steam; and an electrically heated infrared emitter comprising one or more electrically powered heating elements for transferring heat to the reactor tubes, and an enclosure surrounding the one or more reactor tubes and the electrically heated infrared emitter, wherein the enclosure comprises an enclosing inner wall surface.

More in particular, the present invention relates to a cracking furnace for steam cracking a hydrocarbon feedstock, wherein the furnace comprises: one or more reactor tubes, wherein each reactor tube comprises one or more tube passes for transporting the hydrocarbon feedstock and dilution steam, and wherein the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane; and an electrically heated infrared emitter comprising one or more electrically powered heating elements for transferring heat to the reactor tubes, wherein the one or more electrically powered heating elements are arranged in contact with a 2^nd plane, and an enclosure surrounding the one or more reactor tubes and the electrically heated infrared emitter, wherein the enclosure comprises an enclosing inner wall surface, and wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

In some embodiments of the present invention a cracking furnace for steam cracking a hydrocarbon feedstock is provided, wherein the furnace comprises: one or more reactor tubes, wherein each reactor tube comprises one or more tube passes for transporting the hydrocarbon feedstock and dilution steam, and wherein the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane; and an electrically heated infrared emitter comprising one or more electrically powered heating elements for transferring heat to the reactor tubes, wherein the one or more electrically powered heating elements are arranged in contact with a 2^nd plane, and an enclosure surrounding the one or more reactor tubes and the electrically heated infrared emitter, wherein the enclosure comprises an enclosing inner wall surface, and

- wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and

- wherein the electrically heated infrared emitter is configured to produce a radiation heat flux of at least 20 000 W/m², preferably at least 40 000 W/m², and more preferably at least 60 000 W/m², for example about 80 000 W/m².

In some preferred embodiments of a cracking furnace of the invention the 1^st and 2^nd planes are parallel to one another, and separated by a distance equal to at most 15 times an outer diameter of the reactor tube, such as at most 10 times, or at most 8 times, the outer diameter of the reactor tube.

In some preferred embodiments of a cracking furnace of the invention the 1^st and 2^nd planes are parallel to one another, and separated by a distance of between 1 and 15 times an outer diameter of the reactor tube, such as between 2 and 10 times, or between 3 and 8 times, the outer diameter of the reactor tube. In some preferred embodiments of the invention, the 1^st and 2^nd planes are facing each other.

In some preferred embodiments, the one or more tube passes are disposed parallel to each other.

In some preferred embodiments, the one or more electrically powered heating elements are disposed parallel to each other. In some preferred embodiments, the one or more electrically powered heating elements are disposed parallel to the one or more tube passes.

In some preferred embodiments, the tube passes are arranged in a lane, wherein said lane is configured as a single-lane, or a dual-lane, or a triple-lane, or a x-lane arrangement of tube passes, wherein x is 4 or more.

In some preferred embodiments, the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) of at least 0.10 m to at most 1 .20 m, preferably at least 0.15 m to at most 1 .00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

In some preferred embodiments, the one or more electrically powered heating elements are arranged in contact with a 2^nd plane, and the enclosing inner wall surface of the enclosure and the 2^nd plane are parallel to one another, and separated by a distance (d_e) of at most 1.0 m.

In some preferred embodiments, the invention provides a cracking furnace as defined herein, wherein the tube passes and electrically powered heating elements are arranged in a reaction unit, wherein the reaction unit comprises one lane of tube passes, wherein one side of the lane contacts the 1^st plane, and the other side of the lane contacts a 3^rd plane parallel to the 1^st plane, wherein the 3^rd plane is separated from the 1^st plane by the lane of tube passes, wherein some of the electrically powered heating elements are arranged contacting the 2^nd plane and some of the electrically powered heating elements are arranged contacting a further 4^th plane, wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and wherein the 3^rd and 4^th planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

In some preferred embodiments, a furnace is provided, comprising more than one reaction units (as defined herein) and preferably wherein the reaction units are arranged in parallel, and preferably in a direction perpendicular to a longitudinal axis of the lane.

In some preferred embodiments, the enclosing inner wall surface of the enclosure comprises a material with a thermal conductivity A, measured at 1000°C, of at most 0.50 W/m.K, preferably at most 0.40 W/m.K, preferably at most 0.30 W/m.K, preferably at most 0.25 W/m.K, preferably at most 0.20 W/m.K. In some preferred embodiments, the enclosing inner wall surface of the enclosure comprises a material with a reflectance for wavelengths in the infrared range of at least 0.1 , such as at least 0.2; or at least 0.3, or at least 0.4, or at least 0.5.

In some preferred embodiments, the enclosing inner wall surface of the enclosure comprises a material selected from the group comprising alumino-silicate refractory bricks, silicate bricks, and corundum (alumina) bricks.

In some preferred embodiments, the enclosing inner wall surface of the enclosure comprises one or more layers concentrically arranged around the inner wall surface. Preferably, each layer comprises a material with a thermal conductivity A that is lower than the nearest enclosed wall within, for example at least 0.05 W/m.K lower. In some preferred embodiments, one or more reactor tubes or reactor tube passes are coated with an infrared receiving material and/or wherein the one or more reactor tubes or reactor tube passes are made of infrared receiving material, preferably wherein said infrared receiving material is a material having an emissivity of at least 0.80, preferably at least 0.90, and most preferably at least 0.93.

In some preferred embodiments, the electrically heated infrared emitter is configured to produce a radiation heat flux of at least 40 000 W/m², and preferably at least 60 000 W/m², for example about 80 000 W/m².

In some preferred embodiments, said one or more electrically heating elements are selected from the group comprising: ceramic heating elements, carbon-based heating elements, carbide-based heating elements, silicide-based heating elements; heating elements in a quartz tube, and metal or metal alloy heating elements, and any combinations thereof.

In some preferred embodiments, the electrically heating elements are planar emitters or scallop emitters configured to emit radiation at one side (i.e. single side emitter).

In some preferred embodiments, the electrically heating elements are planar emitters or scallop emitters configured to emit radiation at both sides (i.e. dual side emitter).

In some preferred embodiments, the electrically heating elements are cylindrical emitters configured to emit radiation in all directions (i.e. cylindrical emitter).

In some preferred embodiments, wherein the electrically powered heating elements emit at a temperature of at least 1200 K to at most 1800 K, for example at least 1400 K to at most 1600 K, for example at least 1450 K to at most 1550 K, for example about 1500 K.

In some preferred embodiments, the one or more electrically powered heating elements are configured to emit electromagnetic radiation with a wavelength of between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15 pm. In some preferred embodiments of a cracking furnace of the invention, the one or more electrically powered heating elements are configured to emit their electromagnetic radiation with a wavelength of between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15 pm, and preferably to emit at least 50%, preferably at least 60%, more preferably at least 70% or even 100%, of their electromagnetic radiation with a wavelength of between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15 pm.

In some preferred embodiments, the one or more electrically powered heating elements (132) are provided in the form of a lamp, a panel, a tube, a scallop, a cylindrical rod, or a cylindrical spiral.

In a second aspect, the present invention provides a process for steam cracking a hydrocarbon feedstock to produce olefins, comprising the steps of: feeding a hydrocarbon feedstock and dilution steam to one or more reactor tubes in a cracking furnace as defined herein, and embodiments thereof; and, exposing the hydrocarbon feedstock and dilution steam in the one or more reactor tubes to infrared radiation from the electrically heated infrared emitter to crack at least a portion of the hydrocarbon feedstock.

(Preferred) embodiments of the first aspect are also (preferred) embodiments of the second aspect, and vice versa.

The independent and dependent claims set out particular and preferred features of the invention. Features from the dependent claims may be combined with features of the independent or other dependent claims as appropriate.

The present invention will now be further described. In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic illustration of an embodiment of a cracking furnace (100) in accordance with the present invention.

FIG. 2A to F schematically illustrate a top view of various embodiments of a furnace of the invention, wherein the arrangement of reactor tubes/reactor tube passes (110) and the infrared emitter/heating elements (132) are schematically illustrated. FIG. 3 illustrates a simulation of the heat distribution for a furnace (100) for various distances between the heating elements (132) and the reactor tubes (110). FIG. 3A illustrates a simulation for a furnace (100) without an enclosure (150). FIG. 3B illustrates a simulation for a furnace (100) with an enclosure (150).

FIG. 4 illustrates a simulation of the heat distribution for a furnace (100) as a function of the emissivity of the reactor tubes (110).

DETAILED DESCRIPTION OF THE INVENTION

When describing the invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art.

The terms "comprising", "comprises" and "comprised of" as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements, or method steps. It will be appreciated that the terms "comprising", "comprises" and "comprised of" as used herein comprise the terms "consisting of", "consists" and "consists of". As used in the specification and the appended claims, the singular forms "a", "an," and "the" include plural referents unless the context clearly dictates otherwise. By way of example, "a step" means one step or more than one step.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art.

The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1 , 2, 3, 4 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of endpoints also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

The term "about" as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of +/-10% or less, preferably +/-5% or less, more preferably +/-1% or less, of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier "about" refers is itself also specifically, and preferably, disclosed.

The terms “wt%”, “vol%”, or “mol%” refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, which includes the component.

When describing the present invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

Preferred statements (features) and embodiments and uses of this invention are set herein below. Each statement and embodiment of the invention so defined may be combined with any other statement and/or embodiment unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features or statements indicated as being preferred or advantageous. Hereto, the present invention is in particular captured by any one or any combination of one or more of the below numbered statements and embodiments, with any other aspect and/or embodiment.

1 . A cracking furnace (100) for steam cracking a hydrocarbon feedstock, wherein the furnace (100) comprises one or more reactor tubes, wherein each reactor tube comprises one or more tube passes (110) for transporting the hydrocarbon feedstock and dilution steam, and wherein the one or more tube passes are arranged in a lane (115), wherein one side of the lane is in contact with a 1^st plane (111); and an electrically heated infrared emitter (130) comprising one or more electrically powered heating elements (132) for transferring heat to the reactor tubes (110), wherein the one or more electrically powered heating elements (132) are arranged in contact with a 2^nd plane (112), and an enclosure (150) surrounding the one or more reactor tubes (110) and the electrically heated infrared emitter (130), wherein the enclosure (150) comprises an enclosing inner wall surface (152).

2. A cracking furnace (100) according to statement 1 , wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

3. A cracking furnace (100) for steam cracking a hydrocarbon feedstock, wherein the furnace (100) comprises one or more reactor tubes, wherein each reactor tube comprises one or more tube passes (110) for transporting the hydrocarbon feedstock and dilution steam, and wherein the one or more tube passes are arranged in a lane (115), wherein one side of the lane is in contact with a 1^st plane (111); and an electrically heated infrared emitter (130) comprising one or more electrically powered heating elements (132) for transferring heat to the reactor tubes (110), wherein the one or more electrically powered heating elements (132) are arranged in contact with a 2^nd plane (112), and an enclosure (150) surrounding the one or more reactor tubes (110) and the electrically heated infrared emitter (130), wherein the enclosure (150) comprises an enclosing inner wall surface (152), and wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

4. A cracking furnace (100) for steam cracking a hydrocarbon feedstock, wherein the furnace (100) comprises one or more reactor tubes, wherein each reactor tube comprises one or more tube passes (110) for transporting the hydrocarbon feedstock and dilution steam, and wherein the one or more tube passes are arranged in a lane (115), wherein one side of the lane is in contact with a 1^st plane (111); and an electrically heated infrared emitter (130) comprising one or more electrically powered heating elements (132) for transferring heat to the reactor tubes (110), wherein the one or more electrically powered heating elements (132) are arranged in contact with a 2^nd plane (112), and an enclosure (150) surrounding the one or more reactor tubes (110) and the electrically heated infrared emitter (130), wherein the enclosure (150) comprises an enclosing inner wall surface (152), and wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and wherein the electrically heated infrared emitter (130) is configured to produce a radiation heat flux of at least 20 000 W/m².

5. A cracking furnace (100) according to any one of the previous statements, wherein the 1^st and 2^nd planes are facing each other.

6. A cracking furnace (100) according to any one of the previous statements, wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at most 15 times an outer diameter of the reactor tube, such as at most 10 times, or at most 8 times, the outer diameter of the reactor tube.

7. A cracking furnace (100) according to any one of the previous statements, wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) of between 1 and 15 times an outer diameter of the reactor tube, such as between 2 and 10 times, or between 3 and 8 times, the outer diameter of the reactor tube.

8. A cracking furnace (100) according to any one of the previous statements, wherein the one or more tube passes (110) are disposed parallel to each other.

9. A cracking furnace (100) according to any one of the previous statements, wherein the one or more electrically powered heating elements (132) are disposed parallel to each other.

10. A cracking furnace (100) according to any one of the previous statements, wherein the one or more electrically powered heating elements (132) are disposed parallel to the one or more tube passes (110). 11. A cracking furnace (100) according to any one of the previous statements, wherein the one or more electrically powered heating elements (132) are placed parallel to the one or more tube passes (110), and are placed in the same direction or perpendicular to the tube passes.

12. A cracking furnace (100) according to any one of the previous statements, wherein the tube passes are arranged in a lane (115), wherein said lane is configured as a single-lane, or a dual-lane, or a triple-lane, or a x-lane arrangement of tube passes, wherein x is 4 or more.

13. A cracking furnace (100) according to any one of the previous statements, wherein the one or more electrically powered heating elements (132) are arranged in contact with at least one plane facing the tube passes (110), preferably in 2 opposing planes facing the tube passes (110).

14. A cracking furnace (100) according to any one of the previous statements, wherein the one or more electrically powered heating elements (132) are placed at a shortest distance to the tube passes (110), which shortest distance corresponds to a distance of at least one times the outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

15. A cracking furnace (100) according to any one of the previous statements, wherein the one or more electrically powered heating elements (132) are placed at a shortest distance of at least 0.10 m to at most 1.20 m from the tube passes (110), preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

16. A cracking furnace (100) according to any one of the previous statements, wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) of at least 0.10 m to at most 1.20 m, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

17. A cracking furnace (100) according to any one of the previous statements, wherein the lane of tube passes is centrally disposed in said furnace.

18. A cracking furnace (100) according to any one of the previous statements, wherein one side of the lane (115) contacts the 1^st plane (111), and the other side of the lane (115) contacts a 3^rd plane (113) parallel to the 1^st plane, wherein the 3^rd plane is separated from the 1^st plane by the lane of tube passes (115), and wherein some of the electrically powered heating elements (132) are arranged contacting the 2^nd plane (112) and some of the electrically powered heating elements (132) are arranged contacting a further 4^th plane (114), and wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and wherein the 3^rd and 4^th planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

19. A cracking furnace (100) according to any one of the previous statements, wherein the tube passes (110) and electrically powered heating elements (132) are arranged in a reaction unit (120), wherein the reaction unit comprises one lane (115) of tube passes, wherein one side of the lane contacts the 1^st plane (111), and the other side of the lane contacts a 3^rd plane (113) parallel to the 1^st plane, wherein the 3^rd plane is separated from the 1^st plane by the lane (115) of tube passes (110), wherein some of the electrically powered heating elements (132) are arranged contacting the 2^nd plane (112) and some of the electrically powered heating elements (132) are arranged contacting a further 4^th plane (114), wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and wherein the 3^rd and 4^th planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

20. A cracking furnace (100) according to statement 18 or 19 wherein said reaction unit (120) comprises one lane of tube passes, and wherein said lane is configured as a single-lane, or a dual-lane, or a triple-lane, or a x-lane configuration of tube passes, wherein x is 4 or more.

21. A cracking furnace (100) according to any one of statements 18 to 20, wherein said 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) of at least 0.10 m to at most 1.20 m, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

22. A cracking furnace (100) according to any one of statements 18 to 21 , wherein the 3^rd and 4^th planes are parallel to one another, and separated by a distance (d) of at least 0.10 m to at most 1.20 m, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m. 23. A cracking furnace (100) according to any one of previous statements, comprising more than one more reaction units (120, 120’), preferably wherein the reaction units are arranged in parallel.

24. A cracking furnace (100) according to any one of previous statements, comprising more than one reaction unit (120, 120’), wherein the reaction units are arranged in parallel, and preferably in a direction perpendicular to a longitudinal axis of the lane.

25. A cracking furnace (100) according to any one of previous statements, wherein there are at least two lanes of tube passes (115, 115’), wherein adjacent lanes of tube passes are separated by one array of electrically powered heating elements (132), and wherein one side of the array contacts a plane (e.g. 2^nd plane), and the other side of the array contacts other plane (e.g. 4^th plane).

26. A cracking furnace (100) according to any one of previous statements, wherein there are at least two lanes of tube passes (115, 115’), wherein a lane of tube passes adjacent to an enclosure inner wall is separated from the adjacent wall by one array of electrically powered heating elements, and wherein one side of the array contacts a plane (e.g. 2^nd plane), and the other side of the array faces the enclosure inner wall.

27. The furnace (100) according to any one of the previous statements, wherein the one or more electrically powered heating elements (132) are placed parallel to the enclosing inner wall surface (152) of the enclosure (150).

28. The furnace (100) according to any one of the previous statements, wherein the electrically powered heating elements (132) are placed at a shortest distance of at most 1.0 m from the enclosing inner wall surface (152).

29. The furnace (100) according to any one of the previous statements, wherein the electrically powered heating elements (132) are placed at a shortest distance of at least 0.1 m from the enclosing inner wall surface (152), such as at a distance of 0.1 m, 0.2 m, 0.3 m, 0.4 m, 0.5 m, 0.6 m, 0.7 m, 0.8 m, 0.9 m or 1.0 m from the enclosing inner wall surface (152).

30. The furnace (100) according to any one of the previous statements, wherein the electrically powered heating elements (132) are placed at a shortest distance of at least 0.1 m from the enclosing inner wall surface (152) and at a shortest distance of at most 1.0 m from the enclosing inner wall surface (152).

31. The furnace (100) according to any one of the previous statements, wherein the one or more electrically powered heating elements (132) are arranged in contact with a 2^nd plane (112), and wherein the enclosing inner wall surface (152) of the enclosure and the 2^nd plane are parallel to one another, and separated by a distance (d_e) of at most 1.0 m. 32. The furnace (100) according to any one of the previous statements, wherein the one or more electrically powered heating elements (132) are arranged in contact with a 2^nd plane (112), wherein the enclosing inner wall surface of the enclosure and the 2^nd plane are parallel to one another, and separated by a distance of at least 0.1 m from the enclosing inner wall surface (152), such as at a distance of 0.1 m, 0.2 m, 0.3 m, 0.4 m, 0.5 m, 0.6 m, 0.7 m, 0.8 m, 0.9 m or 1.0 m from.

33. The furnace (100) according to any one of the previous statements, wherein the enclosing inner wall surface (152) of the enclosure (150) comprises a material with a thermal conductivity A, measured at 1000°C, of at most 0.50 W/m.K, preferably at most 0.40 W/m.K, preferably at most 0.30 W/m.K, preferably at most 0.25 W/m.K, preferably at most 0.20 W/m.K.

34. The furnace (100) according to any one of the previous statements, wherein the enclosing inner wall surface (152) of the enclosure (150) comprises a material with an emissivity of at most 0.90, such as at most 0.80, or at most 0.70, or at most 0.60, or at most 0.50.

35. The furnace (100) according to any one of the previous statements, wherein the enclosing inner wall surface (152) of the enclosure (150) comprises a material with a reflectance (values between 0 and 1) for wavelengths in the infrared range of at least 0.1 , such as at least 0.2, or at least 0.3, or at least 0.4, or at least 0.5.

36. The furnace (100) according to any one of the previous statements, wherein the enclosing inner wall surface (152) of the enclosure (150) comprises a material selected from the group comprising alumino-silicate refractory bricks, silicate bricks, and corundum (alumina) bricks.

37. The furnace (100) according to any one of the previous statements, wherein the enclosure (150) has a thickness of at least 20 cm, preferably at least 30 cm.

38. The furnace (100) according to any one of the previous statements, wherein the enclosure (150) comprises one or more layers (154) concentrically arranged around the inner wall surface (152).

39. The furnace (100) according to statement 38, wherein each layer (154) comprises a material with a thermal conductivity A that is lower than the nearest enclosed wall within, for example at least 0.05 W/m.K lower.

40. The furnace (100) according to any one of the preceding statements, wherein said reactor tube (110) is configured as a single pass reaction tube which is preferably essentially straight and vertically arranged in said furnace; or as a coiled tube having more than one reactor tube passes, such as U-shape tubes or M-shape tubes or W-shape tubes; or as a coiled tube with more than one tube passes wherein certain tube passes split into two or three or more tube passes; or as a coiled tube with more than one tube passes wherein two or three tube passes merge into one pass; or as any combinations thereof. The furnace (100) according to any one of the preceding statements, wherein said reactor tube(s) (110) has (have) a circular cross section. The furnace (100) according to any one of the preceding statements, wherein the reactor tubes and the reactor tube passes have a same outer diameter, and preferably wherein all reactor tube passes have a same outer diameter. The furnace (100) according to any of the preceding statements, wherein the one or more reactor tubes or reactor tube passes are coated with an infrared receiving material and/or wherein the one or more reactor tubes or reactor tube passes are made of infrared receiving material. The furnace (100) according to statement 43, wherein said infrared receiving material is a material having an emissivity of at least 0.80, preferably at least 0.90, and most preferably at least 0.93. The furnace (100) according to any one of the preceding statements, wherein said one or more reactor tubes (110) have a tube inner diameter comprised between 4.0 and 15.0 cm, such as between 6.0 and 8.0 cm. The furnace (100) according to any one of the preceding statements, wherein said one or more reactor tubes (110) have a tube outer diameter comprised between 4.5 and 17.0 cm, such as between 6.0 and 10.0 cm. The furnace (100) according to any one of the preceding statements, wherein said one or more reactor tubes have a wall thickness of at least 0.5 cm, such as at least 0.6 cm, or at least 1.0 cm. The furnace (100) according to any one of the preceding statements, wherein the electrically heated infrared emitter (130) is configured to heat the hydrocarbon feedstock and dilution steam in the reactor tubes (110) to a temperature of at least 650°C. The furnace (100) according to any one of the preceding statements, wherein the electrically heated infrared emitter (130) is configured to produce a radiation heat flux of at least 40 000 W/m², and more preferably at least 60 000 W/m², for example about 80 000 W/m². The furnace (100) according to any one of the preceding statements, wherein the one or more electrically powered heating elements (132) are configured to emit electromagnetic radiation with a wavelength of between 0.7|jm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15pm.

51. The furnace (100) according to any one of the preceding statements, wherein the one or more electrically powered heating elements (132) are configured to emit at least 50%, preferably at least 60%, or preferably at least 70%, or preferably at least 80%, or preferably at least 90%, or even 100%, of their electromagnetic radiation with a wavelength of between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15pm.

52. The furnace (100) according to any one of the preceding statements, wherein said one or more electrically heating elements (132) are selected from the group comprising: ceramic heating elements, carbon-based heating elements, carbide-based heating elements (such as silicon carbide, SiC), silicide-based heating elements (such as molybdenum silicide, MoSi2) heating elements in a quartz tube, and metal or metal alloy heating elements (such as iron-chromium-aluminum alloy, FeCrAI), and any combinations thereof.

53. The furnace (100) according to any of the preceding statements, wherein the electrically heating elements (132) are provided in the form of a lamp; a panel (such as a flat panel); a tube; a scallop (such as a concave or convex scallop); a cylindrical rod, a cylindrical spiral.

54. The furnace (100) according to any of the preceding statements, wherein the electrically heating elements (132) are planar emitters or scallop emitters configured to emit radiation at one side (i.e. single side emitter).

55. The furnace (100) according to any of the preceding statements, wherein the electrically heating elements (132) are planar emitters or scallop emitters configured to emit radiation at both sides (i.e. dual side emitter).

56. The furnace (100) according to any of the preceding statements, wherein the electrically heating elements (132) are cylindrical emitters configured to emit radiation in all directions (i.e. cylindrical emitter).

57. The furnace (100) according to any of the preceding statements, wherein the electrically heating elements (132) are not electrical wires.

58. The furnace (100) according to any one of the preceding statements, wherein the electrically powered heating elements (132) emit at a temperature of at least 1200 K to at most 1800 K, for example at least 1400 K to at most 1600 K, for example at least 1450 K to at most 1550 K, for example about 1500 K. 59. The furnace (100) according to any one of the preceding statements, wherein the electrically powered heating elements (132) have an emissivity of at least 0.75, preferably at least 0.80, preferably at least 0.85, preferably at least 0.90, preferably at least 0.95, on the side facing the reactor tubes (110).

60. The furnace (100) according to any one of the preceding statements, wherein the one or more electrically powered heating elements (132) are attached to a support element, such as, e.g., a supporting frame.

61. The furnace (100) according to any one of the preceding statements, and wherein said support element is placed parallel to the reactor tubes passes.

62. The furnace (100) according to any one of the preceding statements, wherein said support element is placed at a shortest distance of at least 0.10 m to at most 1.20 m from the reactor tube passes (110), preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

63. The furnace (100) according to any one of the preceding statements, wherein said support element is placed parallel to the enclosing inner wall surface (152) of the enclosure (150).

64. The furnace (100) according to any one of the preceding statements, wherein said support element is placed at a shortest distance of at least 0.10 m to at most 1.0 m from the enclosing inner wall surface (152) of the enclosure (150), preferably at least 0.15 m to at most 1 .00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

65. The furnace (100) according to any one of the preceding statements, wherein the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane, and wherein said support element and said 1^st plane are placed parallel to one another.

66. The furnace (100) according to any one of the preceding statements, wherein the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane, and wherein said support element and said 1^st plane are placed parallel to one another, and are separated by a distance equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

67. The furnace (100) according to any one of the preceding statements, wherein the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane, and wherein said support element and said 1^st plane are placed parallel to one another, and are separated by a distance of at least 0.10 m to at most 1 .20 m, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

68. The furnace (100) according to any one of the preceding statements, wherein said support element is placed parallel to the enclosing inner wall surface (152) of the enclosure (150).

69. The furnace (100) according to any one of the preceding statements, wherein said support element is placed at a shortest distance of at least 0.10 m to at most 1.00 m from the enclosing inner wall surface (152) of the enclosure (150), preferably at least 0.15 m to at most 1 .00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

70. The furnace (100) according to any one of the preceding statements, wherein the furnace comprises a radiation chamber comprising one or more reaction units as defined herein.

71. The furnace (100) according to any one of the preceding statements wherein the furnace does not comprise a convection chamber.

72. The furnace (100) according to any one of the preceding statements, wherein the furnace comprises at least one radiation chamber that comprises one or more reaction units, and wherein the furnace does not comprise a convection chamber.

73. A process for steam cracking a hydrocarbon feedstock to produce olefins, comprising the steps of: feeding a hydrocarbon feedstock and dilution steam to one or more reactor tubes (110) in a cracking furnace (100), preferably according to any one of the preceding statements; and, exposing the hydrocarbon feedstock and dilution steam in the one or more reactor tubes (110) to infrared radiation from the electrically heated infrared emitter (130) to crack at least a portion of the hydrocarbon feedstock.

74. The process according to statement 73, wherein the hydrocarbon feedstock comprises one of more hydrocarbons having at least two carbon atoms, in particular selected from the group comprising: ethane, propane, butane, liquefied petroleum gas, naphtha, gasoils, and crude oils.

75. The process according to any one of statement 73 or 74, wherein the transfer of heat to the hydrocarbon feedstock and of the dilution steam is carried out separately in at least two pipes before mixing the two streams; or wherein the hydrocarbon feedstock and the dilution steam are mixed, followed by transfer of heat to the mixture; or wherein the transfer of heat to the hydrocarbon feedstock and the dilution steam is carried out separately in at least two pipes to a temperature less than 500°C, followed by mixing the two streams and further transferring heat to the mixture.

76. The process according to any one of statements 73 to 75, wherein sensible or latent heat of effluent exiting the furnace (100) is extracted at least partially by means of one or more heat exchangers and used to at least partially preheat the hydrocarbon feedstock, dilution steam or mixture thereof.

In a first aspect, the present invention relates to a cracking furnace. More in particular, the present invention relates to a cracking furnace for steam cracking a hydrocarbon feedstock. The furnace comprises one or more reactor tubes for transporting the hydrocarbon feedstock and dilution steam. The furnace comprises an electrically heated infrared emitter comprising one or more electrically powered heating elements for transferring heat to the reactor tubes.

In addition the furnace comprises an enclosure surrounding the one or more reactor tubes and the electrically heated infrared emitter, wherein the enclosure comprises an enclosing inner wall surface. As used herein, the term “enclosure” refers to a surrounding structure around the reactor tubes and the electrically heated infrared emitter. The enclosure need not be closed in all dimensions, but is preferably as closed as possible, only allowing for the reactor tubes to enter and leave and for wiring of the electrically heated infrared emitter to enter and leave. The enclosure typically comprises an enclosing inner wall surface, /.e., the surface inside the enclosure directed towards the reactor tubes and the electrically heated infrared emitter. Preferably, the inner wall surface is a homogeneous surface. The enclosure advantageously directs any lost infrared energy back towards the reactor tubes by reflectance or by absorption/re-emission. In some embodiments, the enclosure comprises multiple layers including the inner wall surface and one or more layers concentrically arranged around the inner wall surface.

In accordance with the present invention, a hydrocarbon feedstock is fed to one or more reactor tubes in a cracking furnace.

The “reactor tube” is herein sometimes also referred to as reactor coils, or coils or radiant coils. Suitable reactor tubes for use in cracking furnaces are generally known. The reactor tubes may be formed of one or more cylindrical tubular conduits, preferably with a circular cross-section. The conduits or pipes may be connected by connecting devices such as but not limited to connecting bends to provide a number of tune passes.

As used herein the term “reactor tube” refers to a tube that comprises at least one “tube pass”. The term “tube pass” (or “reactor tube pass”) as used herein intends to refer to a section/portion of a tube, such as an essentially straight section of a tube. Preferably a straight section of a tube. In certain embodiments of the invention the terms “reactor tube” and “reactor tube pass” or “tube pass” or “coil pass” are used as synonyms and may be used interchangeably.

In certain embodiments of the invention, a cracking furnace for steam cracking a hydrocarbon feedstock is provided, wherein the furnace comprises one or more reactor tubes, wherein each reactor tube comprises one or more tube passes for transporting the hydrocarbon feedstock and dilution steam, and wherein the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane; and an electrically heated infrared emitter comprising one or more electrically powered heating elements for transferring heat to the reactor tubes, wherein the one or more electrically powered heating elements are arranged in contact with a 2^nd plane , and an enclosure surrounding the one or more reactor tubes and the electrically heated infrared emitter, wherein the enclosure comprises an enclosing inner wall surface.

In certain preferred embodiments of a cracking furnace of the invention the 1^st and 2^nd planes are parallel to one another, and separated by a distance equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

The term “an outer diameter” of the reactor tube as used herein refers to the distance defined by the outer diameter of the reactor tube.

A reactor tube for use in a furnace as described herein may have any suitable cross-section (cross-sectional shape). The cross-section of a reactor tube as described herein can for instance be circular, oval, elliptical, or have any other regular geometry. Preferably, the reactor tube(s) has (have) a circular cross section. When the cross-section is not circular, “outer diameter of a reactor tube” refers to the maximal distance between the two most distantly located points on the cross section.

In certain embodiments of the invention, the reactor tubes and the reactor tube passes have a same outer diameter. In preferred embodiments, all reactor tubes as applied in the present furnace have a same outer diameter. In preferred embodiments, all reactor tube passes as applied in the present furnace have a same outer diameter.

In certain preferred embodiments, said reactor tubes have a tube inner diameter comprised between 4.0 and 15.0 cm, such as between 6.0 and 8.0 cm. In certain preferred embodiments, said reactor tubes have a tube outer diameter comprised between 4.5 and 17.0 cm, such as between 6.0 and 10.0 cm. In certain preferred embodiments, said reactor tubes have a wall thickness of at least 0.5 cm, such as at least 0.6 cm, or at least 1.0 cm.

In certain preferred embodiments, a cracking furnace for steam cracking a hydrocarbon feedstock is provided, wherein the furnace comprises one or more reactor tubes, wherein each reactor tube comprises one or more tube passes for transporting the hydrocarbon feedstock and dilution steam, and wherein the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane; and an electrically heated infrared emitter comprising one or more electrically powered heating elements for transferring heat to the reactor tubes, wherein the one or more electrically powered heating elements are arranged in contact with a 2^nd plane, and an enclosure surrounding the one or more reactor tubes and the electrically heated infrared emitter, wherein the enclosure comprises an enclosing inner wall surface, and

- wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

In certain preferred embodiments of a cracking furnace of the invention the 1^st and 2^nd planes are parallel to one another, and separated by a distance equal to at most 15 times an outer diameter of the reactor tube, such as at most 10 times, or at most 8 times, the outer diameter of the reactor tube.

In certain preferred embodiments of a cracking furnace of the invention the 1^st and 2^nd planes are parallel to one another, and separated by a distance of between 1 and 15 times an outer diameter of the reactor tube, such as between 2 and 10 times, or between 3 and 8 times, the outer diameter of the reactor tube.

The inventors have found that the herein described furnace configurations provide homogeneous heating of the reactor tubes thanks to the properties such as reflecting or reemitting properties, of the enclosure inner wall surface and improved cracking properties. Furnace configurations according to the invention also allow to provide more compact furnaces by using adapted heating elements (emitters) such as multi-side emitters and by requiring a limited distance between emitters and reactor tubes/tube passes.

In certain preferred embodiments of a cracking furnace of the invention the 1^st and 2^nd planes are facing each other. In certain preferred embodiments of a cracking furnace of the invention the one or more tubes are disposed parallel to each other. In certain preferred embodiments of a cracking furnace of the invention the one or more tube passes are disposed parallel to each other.

In certain preferred embodiments of a cracking furnace of the invention the one or more electrically powered heating elements are disposed parallel to each other.

In certain preferred embodiments of a cracking furnace of the invention, the one or more electrically powered heating elements are disposed parallel to the one or more tube passes.

In certain preferred embodiments of a cracking furnace of the invention the one or more electrically powered heating elements are placed parallel to the one or more tube passes, and are placed in the same direction as the tube passes.

In certain preferred embodiments of a cracking furnace of the invention the one or more electrically powered heating elements are placed parallel to the one or more tube passes, and are placed perpendicular to the tube passes.

In accordance with the invention, a furnace of the invention comprises one or more tube passes that are arranged in a lane. The term “lane” in this respect may include different layouts of reactor tube and/or reactor tube passes. For instance, in some embodiments of the invention, the tube passes are arranged in a lane, wherein said lane is configured as a singlelane arrangement (layout) of tube passes. In another example, the tube passes are arranged in a lane, wherein said lane is configured as a dual-lane or a triple-lane arrangement of tube passes. In another example the tube passes are arranged in a lane, wherein said lane is configured as a x-lane arrangement, wherein x is 4 or more. In certain preferred embodiments, the lane of tube passes is centrally disposed in said furnace.

In certain embodiments of a cracking furnace of the invention, the one or more electrically powered heating elements are placed parallel to the one or more reactor tube passes.

In certain embodiments of a cracking furnace of the invention, the one or more electrically powered heating elements are arranged in contact with at least one plane (e.g. the 2^nd plane as defined herein) facing the tube passes, preferably in 2 opposing planes facing the tube passes (e.g. the 2^nd lane and a 4^th lane as defined herein).

According to preferred embodiments of the invention, wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) of at least 0.10 m to at most 1.20 m, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

In some embodiments of a cracking furnace of the invention the one or more electrically powered heating elements are placed at a shortest distance to the tube passes, which shortest distance corresponds to a distance of at least one times the outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

In certain embodiments of a cracking furnace of the invention the one or more electrically powered heating elements are placed at a shortest distance of at least 0.10 m to at most 1 .20 m from the tube passes, preferably at least 0.15 m to at most 1 .00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

In certain embodiments, the invention relates to a cracking furnace for steam cracking a hydrocarbon feedstock, wherein the furnace comprises one or more reactor tubes for transporting the hydrocarbon feedstock and dilution steam; and wherein the furnace comprises an electrically heated infrared emitter comprising one or more electrically powered infrared heating elements for transferring heat to the reactor tubes; and wherein the furnace further comprises an enclosure surrounding the one or more reactor tubes and the electrically heated infrared emitter, wherein the enclosure comprises an enclosing inner wall surface; wherein said furnace is characterized in that: i) the one or more electrically powered heating elements are placed parallel to the one or more reactor tubes; ii) the one or more electrically powered heating elements are placed in at least one plane facing the reactor tubes, preferably in 2 opposing planes facing the reactor tubes; and, iii) the one or more electrically powered heating elements are placed at a shortest distance of at least 0.10 m to at most 1 .20 m from the reactor tubes, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m, and/or the one or more electrically powered heating elements are placed at a shortest distance from the reactor tubes wherein said distance is equal to at least one times to at most 15 times an outer diameter of the reactor tube, preferably by a distance of between 1 and 15 times an outer diameter of the reactor tube, preferably between 2 and 10 times, preferably between 3 and 8 times, and preferably wherein the one or more electrically powered heating elements are placed parallel to the enclosing inner wall surface of the enclosure; and are preferably placed at a shortest distance of at least 0.1 m to at most 1.0 m from the enclosing inner wall surface.

The present invention is thus at least in part based on the use of electric power to heat reactor tubes provide in a cracking furnace.

Steam cracking furnaces are in general known that use radiant heat delivered by burners such as conventional gas or oil burners as its source of heat. The burners are often placed on the floor and/or the walls of the furnace and they define a high temperature zone in the furnace, also referred to as the “radiation” zone of the furnace. Immediately above said zone, there is a convection zone through which the hot combustion gases escape from the radiation zone, which convection zone is generally used for preheating the mixture of hydrocarbons to be cracked, also known as the “feedstock” to be cracked.

The present invention now provides a cracking furnace in which the heat conventionally supplied in known cracking furnaces as thermal energy via burners by the combustion of a fuel (e.g., natural gas/fossil fuels) is replaced by electrical heating, and more in particular, by heating (heat transfer) through infrared (IR) radiation.

To that end, embodiments of a cracking furnace according to the invention comprises an electrically heated infrared emitter (or radiator), which comprises one or more electrically powered heating elements for transferring heat to the reactor tubes in the cracking furnace.

Benefits of infrared heaters compared with other heating technologies are:

• Because the energy is directly transferred from the heater to the target object, infrared heating systems can be more efficient and require less energy for similar results.

• Infrared heaters are electrically powered, allowing for controls that respond rapidly and precisely.

• These heaters can be compactly designed and fitted in small spaces.

• Maintenance is very minimal and have generally long lifetimes and often the heating elements can be replaced easily and without having to stop operation.

• There is less heat wasted as the air inside the furnace does not have to be heated for the process to be effective.

In accordance with the present invention, heat transfer through radiation in the cracking furnace of the invention takes place in the form of electromagnetic waves mainly in the infrared region. Material surfaces absorb and emit radiation at all frequencies. Whereas a black body will radiate and absorb energy over a much wider wavelength range, gases absorb and emit radiation at certain discrete frequencies, depending on the gas composition.

In the present context, the terms “infrared radiation” or “infrared wavelength” or “wavelengths in the infrared range” are used interchangeably and refer to electromagnetic radiation with wavelength between 0.7 micrometer and 1 millimeter, and preferably between 0.7 micrometer and 50 micrometer, such as between 0.7 and 40 micrometer, or between 0.7 and 20 micrometer, or between 0.7 and 15 micrometer. Infrared energy travels at the speed of light without heating the air (essentially composed of nitrogen and oxygen, exhibiting no absorption bands in the infrared region of interest) it passes through, and gets absorbed or reflected by objects it strikes. When classical burners are used, the infrared radiation may be absorbed by carbon dioxide and water vapor. However, since the present invention avoids the use of such burners, the amount of infrared radiation absorbed by the air is typically negligible.

A black body is a hypothetical body that completely absorbs all wavelengths of thermal radiation incident on it and does not reflect light (perfect emitter and absorber of radiation). When heating black bodies to a given temperature, they emit thermal radiation. The radiation energy per unit time from a black body is proportional to the fourth power of the absolute temperature and can be expressed with Stefan-Boltzmann Law. Perfect black bodies do not exist, but real bodies approach them and incident radiation (also called irradiation) is partly reflected, absorbed or transmitted. The Stefan-Boltzmann Law for non-ideal black or grey bodies is: The total power density of a radiating surface is proportional to the fourth power of the surface temperature

P = E o T⁴ where

P = heat transfer per unit time and per surface area (W/m²) o = 5.6703 10'⁸ (W/m²K⁴) - The Stefan-Boltzmann Constant

T = absolute temperature in kelvin (K)

A = area of the emitting body (m²)

£ = emissivity coefficient of the object (one - 1 - for a black body)

The heat flux (W/m²) can be derived from this formula.

In general, the temperature of an IR emitter determines its peak wavelength: the higher the emitter temperature, the shorter the peak wavelength and higher the intensity. When radiant energy impinges on an object, it can either be absorbed (A) or reflected (R), or transmitted (T) by that object, where A + R + T = 1 when expressed relatively to the total amount of incident radiation intensity. The amount of absorption, reflection, or transmission is affected by the wavelength of the radiant energy and the physical and surface properties of the object and only absorbed energy will contribute to the heating of the product. The amount of energy an object absorbs can be controlled by selecting the proper emission wavelength. The materials absorption coefficient is directly related to its emissivity. According to Kirchhoff law, on thermal equilibrium the material emissivity should be equal to its absorption. Hence, the emissivity of the emitters should be as high as possible while also the emissivity of the reactor tubes should be as high as possible in order to maximize radiation absorption. For a gray surface, the total radiative flux leaving a surface is defined as the radiosity and is made up of two contributions: the radiation emitted by the surface on the one hand and the reflected radiation on the other where the reflectance equals “1 - emissivity”, as opaque bodies to not transmit radiation and hence A + R = 1.

The reflection phenomenon (R) occurs when radiation reaches an interface between two media with distinct refractive index. In specular or “mirror-like” reflection, a beam coming from a single direction is redirected to the same plane and with the same angle. The diffuse reflection occurs when the reflected beam intensity is dependent on the angle of observation and hence depends on particle size and microstructure. If the surface exhibits a high reflectance, or a very low emissivity, the energy striking the surface would be reflected back from the surface still in its untransformed state (same wavelength), therefore more readily absorbed by the furnace atmosphere molecules. The effect is to “super-heat” the furnace atmosphere, resulting in wasted energy. On the other hand, a surface with a high emissivity, hence tendency to absorb radiation and heat up the surface, becomes a back body emitter emitting radiation in a broad band form (many wavelengths). It may be of advantage to bypass more absorbing gases in the furnace or to align more the re-emitting wavelengths with the reactor tube absorbing wavelengths.

In accordance with the present invention, a furnace is provided in which an electrically heated infrared emitter comprising one or more electrically powered heating elements is applied for transferring heat to the reactor tubes. As indicated the infrared emitter is heated by electricity. In certain embodiments, the electrically heated infrared emitter (130) is configured to produce a radiation heat flux of at least 20 000 W/m², preferably at least 40 000 W/m², and more preferably at least 60 000 W/m², for example about 80 000 W/m².

Embodiments of the present application therefore replace a hydrocarbon, CO2 emitting energy source by electrical power. The electrical power applied herein, required for heating the emitter, is preferably from a renewable energy or low-carbon source. Renewable energy refers to energy from natural sources or processes that are constantly replenished on a human timescale, such as sunlight, wind, rain, tides, waves, hydropower, and geothermal heat. Low- carbon energy source refers to energy from processes or technologies that produce power with substantially lower amounts of carbon dioxide emissions than from conventional fossil fuel power generation. It includes low carbon power generation sources such as wind power, solar power, hydropower and nuclear power. The term largely excludes conventional fossil fuel sources. The apparatus and process of the present application are particularly advantageous in that the electrically powered heat sources can use electricity originating from renewable or low-carbon energy. In this case, the furnace of the invention, and corresponding cracking process in which this furnace is applied, lead to less emission of CO2 than a conventional steam cracking furnace/process.

As a proposed definition of renewable or low-carbon electricity, it is considered that electricity produced with a standard emission factor of less than 0.2 ton CO2 per MWh electricity is renewable or low-carbon electricity, preferably less than 0.1 ton CO2/MWh or most preferably less than 0.05 ton CO2/MWh. The standard emission factors are the emissions taking place due to consumption of energy carriers using the standard approach, /.e., by applying IPCC “standard” emission factors in line with IPCC principles for stationary combustion of the energy carriers. For clarity, other greenhouse gases like methane and nitrogen oxides are not accounted for in these standard emission factors. Also emissions related to the supply of the energy carriers are not accounted for in these emission factors as they can vary a lot according to regions and over time.

By way of example, power generation from fossil resources emits the following amounts of CO2: 320-330 kg CO2/MWh emissions from the highest efficiency combined-cycle gas turbine (CCGT) plants, 500-520 kg CO2/MWh for modern open-cycle gas turbine (OCGT) plants, and 750-800 kg CO2/MWh for a modern supercritical coal-fired power plant whereas nuclear energy plants, geothermal, photovoltaic, hydropower and wind turbines emit 0 kg CO2/MWh (excluding the emissions related to the manufacturing of such power generating equipment).

Hence, the replacement of energy obtained from a hydrocarbon fuel by electrically powered heat transfer through infrared (IR) in accordance with the present invention provides several important advantages.

Using electrically powered infrared radiation sources for the purpose of steam cracking by heating the outside of the tubes containing the hydrocarbon feedstock, avoids producing hot combustion gases that need to be processed to recover its contained sensible heat. In other words, in some embodiments, recirculation of gas does not take place in the furnace of the invention. More specific, in some embodiments, the furnace of the invention is configured not to allow the recirculation of gas.

Another advantage is that using electrically powered infrared radiation sources as provided herein allows to increase the overall energy efficiency of the process, while decreasing carbon dioxide emissions. In some cases, is also allows to improve reliability and operability of the process decrease emissions of, for example, NOx, SOx, CO, and/or volatile organic compounds.

The IR technology also offers the following additional advantages over conventional heating technology, including but not limited to: Infrared heating can be started, switched off and adjusted within seconds, much easier than open-fire-heated furnaces.

Electrically powered infrared heating allows variation of temperature between tubes and across the reactor tube length to achieve any desirable heating profile.

Electrically-powered infrared heating completely avoids the loss of heat from the system in the form of hot flue gases, and the complicated process steps to recover that heat for re-use (and inevitable loss).

Infrared heating may also allow for better furnace design because an infrared heating system decouples the heat generation and the containment functions of reactor tubes, enabling separate optimization.

- An infrared heating system also stays close to current furnace design, potentially even allowing modification of existing assets as opposed to building completely new ones.

Infrared heating devices may be placed around the reactor tubes in a more optimized and concise manner and hence reducing the size of steam cracking furnaces.

In this context, it will be understood that the setup of a cracking furnace as provided in accordance with the present invention can be the same as a conventional combustion/cracking furnace. The present application however provides that part or all of the conventional burners, using fuel gas, can be replaced by an electrically heated infrared emitter. However, other configurations may be provided herein as well, which configurations will have specific advantages, as detailed below.

Since in accordance with the invention no combustion gases are produced, no convection occurs and no heat recovery is required. In addition, it is preferred to select infrared transparent gases inside the electrical furnace in order to minimize selective absorption of radiation resulting in super-heated gases that might become more corrosive in relation to the materials used in such furnaces.

In some embodiments, the furnace of the present invention comprises a radiation section, wherein this radiation section may comprise one or more reaction units, as defined herein. Preferably a furnace of the invention does not comprise a convection section.

The term “radiation section” or “radiation chamber” are used herein as synonyms and intend to refer to that part of a cracking furnace, as provided herein, wherein the tubes receive their heat by radiation from the infrared emitters.

In traditional combustion furnaces, also a convention section (also known as convection chamber) is typically provided. Such convection section is typically located above the radiation section and is cooler than the radiation section, in order to recover additional heat. Heat transfer takes place by convection here. In accordance with the present invention, the presence of such convection section becomes superfluous, and the convection section in a cracking furnace as defined herein can be omitted.

Moreover, in accordance with certain embodiments of the present invention, it is preferred that at least 50%, such as at least 60% or at least 70% or at least 80% or at least 90% or at least 95% of the heat produced by the IR emitters is absorbed in the radiation section for cracking.

It is further preferred in accordance with certain embodiments of the present invention, that at least 50%, preferably at least 60%, preferably at least 70%, more preferably at least 80%, and even at least 90% of the heat applied in the furnace is radiation heat.

The radiative heat flux in an environment can be measured using a meter that is designed to measure the radiation absorbed by the environment, such as a conductive radiation heat flux meter, a capacitive radiation heat flux meter, a calorimetric radiation heat flux meter or a heat pipe heat flux meter, as is well-known by a skilled person.

In accordance with the present invention, it has been established that the configuration of the infrared emitter comprising said one or more electrically powered heating elements in the furnace is particularly advantageous.

The term “configuration” as used herein, refers to the positioning of the electrically heated infrared emitter in the cracking furnace, in particular vis-a-vis the reactor tubes/tube passes, and optionally vis-a-vis other elements that are provided in the furnace, such as, e.g., an enclosure as defined herein. It was found that the way the infrared emitter is placed in the furnace, especially vis-a-vis the reactor tubes (tube passes), allows to provide accurate control of the heating in the furnace, and permits to enhance and optimize the resulting temperature profile. The positioning of the infrared emitter in the furnace in accordance with the invention allows to obtain a more homogenous distribution of the heat flux emitted by said emitter at the surface of the reactor tubes.

In some preferred embodiments, the one or more electrically powered heating elements are placed parallel to the enclosing inner wall surface of the enclosure. This allows for any lost infrared radiation to be directed back towards the reactor tubes and provides a more homogeneously heated reactor tubes, resulting in improved cracking properties.

In some preferred embodiments, the electrically powered heating elements are placed at a shortest distance of at most 1.0 m from the enclosing inner wall surface, for example at most 0.7 m, for example at most 0.3 m. In some preferred embodiments, the electrically powered heating elements are placed at a shortest distance of at least 0.1 m from the enclosing inner wall surface, such as at a distance of 0.1 m, 0.2 m, 0.3 m, 0.4 m, 0.5 m, 0.6 m, 0.7 m, 0.8 m, 0.9 m or 1.0 m from the enclosing inner wall surface.

In some preferred embodiments, the electrically powered heating elements are placed at a shortest distance of at least 0.1 m from the enclosing inner wall surface and at a shortest distance of at most 1.0 m from the enclosing inner wall surface.

In certain preferred embodiments, the one or more electrically powered heating elements are placed in at least a 2^nd plane, and the enclosing inner wall surface of the enclosure and the 2^nd plane are parallel to one another, and separated by a distance (d_e) of at most 1.0 m, such as at most 0.8 m, for example at most 0.3 m.

In certain preferred embodiments, the one or more electrically powered heating elements are placed in at least a 2^nd plane, and the enclosing inner wall surface of the enclosure and the 2^nd plane are parallel to one another, and separated by a distance of at least 0.1 m from the enclosing inner wall surface, such as at a distance of 0.1 m, 0.2 m, 0.3 m, 0.4 m, 0.5 m, 0.6 m, 0.7 m, 0.8 m, 0.9 m or 1.0 m from.

In some preferred embodiments, the one or more electrically powered heating elements are placed in at least a 2^nd plane, and the enclosing inner wall surface of the enclosure and the 2^nd plane are parallel to one another, and separated by a distance of at least 0.1 m to most 1.0 m from the enclosing inner wall surface.

Such distances provide a more homogeneous radiation towards the heated reactor tubes, resulting in improved cracking properties.

The inner wall of the enclosure allows for any lost IR radiation to be reflected (or alternatively absorbed and re-emitted) towards the reactor tubes. This does not only improve the efficiency of the cracking furnace, but also allows for improved homogeneous heating of the reactor tubes. Therefore, the properties of the inner wall of the enclosure will preferably comprise specific characteristics that have been found to improve the cracking process.

In some preferred embodiments, the enclosing inner wall surface of the enclosure comprises a material with a thermal conductivity A, measured at 1000°C, of at most 0.50 W/m.K, preferably at most 0.40 W/m.K, preferably at most 0.30 W/m.K, preferably at most 0.25 W/m.K, preferably at most 0.20 W/m.K. This allows for a minimum of heat to be lost outside the furnace.

In some preferred embodiments, the enclosing inner wall surface of the enclosure comprises a material with an emissivity of at most 0.90, preferably at most 0.8, preferably at most 0.7, preferably at most 0.60, preferably at most 0.50. This allows for a more homogeneous heating of the reactor tubes.

In some preferred embodiments, the enclosing inner wall surface of the enclosure comprises a material with a reflectance for wavelengths in the infrared range of at least 0.1 , such as at least 0.2, or at least 0.3, or at least 0.4, or at least 0.5. This allows for a more homogeneous heating of the reactor tubes.

Such properties of the inner wall of the enclosure have been found to improve the cracking process.

Multiple materials are known and suitable to provide such an enclosure. In some preferred embodiments, the enclosing inner wall surface of the enclosure comprises a material selected from the group comprising alumino-silicate refractory bricks, silicate bricks, and corundum (alumina) bricks.

In some preferred embodiments, the enclosure has a thickness of at least 20 cm, preferably at least 30 cm. This allows for a minimum of heat to be lost outside the furnace.

In some preferred embodiments, the enclosure comprises one or more layers concentrically arranged around the inner wall surface. This allows to use cheaper materials with less ideal characteristics or with other advantages such as structural strength. For example, the layers might have a lower thermal conductivity, or might have inferior emissive/reflective properties.

In some embodiments, the enclosure thus comprises multiple layers or multiple walls. In some embodiments, the enclosure comprises the inner surface wall and one or more additional layers surrounding the inner surface wall.

In some preferred embodiments, each layer of the enclosure comprises a material with a thermal conductivity A that is lower than the nearest enclosed wall within, for example at least 0.05 W/m.K lower. This allows to use other materials, which might be cheaper and/or have other advantageous properties, without necessitating the same IR characteristics of the inner wall surface.

A reactor tube according to the invention may comprise different layouts. For instance, in certain embodiments of the invention, a reactor tube is configured as a single pass reaction tube which is preferably essentially straight and vertically arranged in said furnace.

In certain embodiments of the invention, a reactor tube may also be configured as a coiled tube having more than one reactor tube passes, such as U-shape tubes or M-shape tubes or W-shape tubes; or as a coiled tube with more than one tube passes wherein certain tube passes split into two or three or more tube passes; or as a coiled tube with more than one tube passes wherein two or three tube passes merge into one pass. Also any combination of the herein described layouts may be contemplated in the present invention.

In certain embodiments, the reactor tube(s) or reactor tube passes are coated with an infrared receiving material and/or the reactor tube(s) or reactor tube passes are made of infrared receiving material. Such infrared receiving material is preferably a material having an emissivity of at least 0.80, preferably at least 0.90, and most preferably at least 0.93. Nonlimiting examples of such infrared receiving material suitable for coating include for instance those reported in Journal of the Energy Institute Volume 92, Issue 3, June 2019, Pages 523- 534, or are materials such as commercialized by e.g. Emisshield; CTK-EURO, or Cetek.

The present invention encompasses different arrangements of the one or more reactor tubes, or tube passes thereof in the furnace.

In certain embodiments of a cracking furnace of the invention, the tube passes and electrically powered heating elements are arranged in a reaction unit, wherein the reaction unit comprises one lane of tube passes, wherein one side of the lane contacts the 1^st plane, and the other side of the lane contacts a 3^rd plane parallel to the 1^st plane, wherein the 3^rd plane is separated from the 1^st plane by the lane of tube passes, wherein some of the electrically powered heating elements are arranged contacting the 2^nd plane and some of the electrically powered heating elements are arranged contacting a further 4^th plane, wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and wherein the 3^rd and 4^th planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

In certain preferred embodiments of a cracking furnace of the invention the 1^st and 2^nd planes are parallel to one another, and separated by a distance equal to at most 15 times an outer diameter of the reactor tube, such as at most 10 times, or at most 8 times, the outer diameter of the reactor tube. In certain preferred embodiments of a cracking furnace of the invention the 1^st and 2^nd planes are parallel to one another, and separated by a distance of between 1 and 15 times an outer diameter of the reactor tube, such as between 2 and 10 times, or between 3 and 8 times, the outer diameter of the reactor tube. In certain preferred embodiments of a cracking furnace of the invention the 3^rd and 4^th planes are parallel to one another, and separated by a distance equal to at most 15 times an outer diameter of the reactor tube, such as at most 10 times, or at most 8 times, the outer diameter of the reactor tube. In certain preferred embodiments of a cracking furnace of the invention the 3^rd and 4^th planes are parallel to one another, and separated by a distance of between 1 and 15 times an outer diameter of the reactor tube, such as between 2 and 10 times, or between 3 and 8 times, the outer diameter of the reactor tube.

Examples of such embodiments are for instance illustrated in FIG. 2B, FIG. 2C and FIG. 2F.

In certain embodiments, a reaction unit comprises one lane of tube passes, and wherein said lane is configured as a single-lane configuration of tube passes. Such embodiments are for instance illustrated in FIG. 2B.

In certain embodiments, a reaction unit comprises one lane of tube passes, and wherein said lane is configured as a dual-lane configuration of tube passes. Such embodiments are for instance illustrated in FIG. 2C.

In certain embodiments, a reaction unit comprises one lane of tube passes, and wherein said lane is configured as a triple-lane configuration of tube passes. Such embodiments are for instance illustrated in FIG. 2F.

In certain of the above embodiments, it is preferred that the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) of at least 0.10 m to at most 1.20 m, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

In certain of the above embodiments, it is preferred that the 3^rd and 4^th planes are parallel to one another, and separated by a distance (d) of at least 0.10 m to at most 1.20 m, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

In certain embodiments of a furnace of the invention, said furnace may comprise one reaction unit, as defined herein. Examples of such embodiments are for instance illustrated in FIG. 2B, FIG. 2C and FIG. 2F.

In certain other embodiments of a furnace of the invention, said furnace may comprise more than one reaction units, as defined herein. In such embodiments, it is preferred that the reaction units are arranged in parallel, for instance preferably in a direction perpendicular to a longitudinal axis of the lane. Examples of such embodiments are for instance illustrated in FIG. 2D, 2E and 2G. These figures illustrate a series of two similar reaction units. However, it will be understood from the present invention that a furnace according to the invention may comprise a series of reaction units that are the different from each other. By way of example, the reaction unit illustrated in FIG. 2B may be combined with a reaction unit as illustrated in FIG. 2C and/or FIG. 2F in a same furnace. It will also be understood that FIG. 2D, FIG. 2E and FIG. 2G represent a series of two reaction units but that also more than 2 reaction units may be provided in a same furnace.

In certain embodiments of a cracking furnace wherein there are at least two lanes of tube passes, such adjacent lanes of tube passes are preferably separated by one array of electrically powered heating elements, and wherein one side of the array contacts a plane (e.g. 2^nd plane as defined herein), and the other side of the array contacts other plane (e.g. 4^th plane as defined herein).

In certain embodiments of a cracking furnace wherein there are at least two lanes of tube passes, it is further contemplated that a lane of tube passes adjacent to an enclosure inner wall is separated from the adjacent wall by one array of electrically powered heating elements. Preferably, in such embodiments, one side of the array contacts a plane (e.g. 2^nd plane as defined herein), and the other side of the array faces the enclosure inner wall. Preferably the distance between the array facing the enclosure inner wall is as defined herein.

The electrically heated infrared emitter for use in a furnace according to the invention is preferably an electrical infrared heating source that is capable of heat transfer such that the hydrocarbon feedstock and dilution steam in the reactor tubes can be heated to temperatures of 650°C or higher. In certain preferred embodiments of the invention, the electrically heated infrared emitter is configured to heat the hydrocarbon feedstock and dilution steam in the reactor tubes to a temperature of at least 650°C, such as at least 700°C, or at least 750°C, or at least 800°C, or at least 900°C.

The electrically heated infrared emitter itself can have any elevated temperature (1000°C to 3000°C for instance), as long as the heat duty is delivered through a combination of surface area and temperature. For the above temperature range, electrically powered infrared radiation generators are known. German firm Rauschert offers ceramic heaters with element temperatures up to 1000°C. Indian companies Kerone and Ace offers ceramic heaters with power densities up to 77 kW/^m2, 1 kW per unit and temperatures up to 900°C. Other companies include US companies Tempco, Watlow and Protherm. Especially interesting is German firm Bach RC, offering silicon nitride and aluminium nitride heaters for use up to 1000°C and 150 W/cm² (1500 kW/m²).

In certain embodiments of the invention, the electrically heated infrared emitter is configured to produce a radiation heat flux of at least 20 000 W/m². In some embodiments the electrically heated infrared emitter is configured to produce a radiation heat flux of at least 40000 W/m². In some embodiments the electrically heated infrared emitter is configured to produce a radiation heat flux of at least 60 000 W/m², for example of about 80 000 W/m².

The electrically heated infrared emitter for use in the present furnaces comprises one or more electrically powered heating elements.

The term “electrically powered heating elements” and "electrically powered infrared emitting elements” can be used herein as synonyms. According to the invention, the electrically powered heating elements are capable of emitting electromagnetic radiation with a wavelength in the IR spectrum, as defined herein, thereby generating sufficient heat for heating the feedstock and steam in the reactor tubes. Preferably, said electrically powered heating elements are configured to emit electromagnetic radiation with a wavelength of between 0.7pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15pm. In some preferred embodiments, the one or more electrically powered heating elements are configured to emit at least 50% of their electromagnetic radiation with a wavelength of between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15pm. In some embodiments, the one or more electrically powered heating elements are configured to emit at least 60% of their electromagnetic radiation between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15 pm. In some embodiments, the one or more electrically powered heating elements are configured to emit at least 70% of their electromagnetic radiation between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15 pm. In some embodiments, the one or more electrically powered heating elements are configured to emit at least 80% of their electromagnetic radiation between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15 pm. In some embodiments, the one or more electrically powered heating elements are configured to emit at least 90% of their electromagnetic radiation between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15 pm. In some embodiments, the one or more electrically powered heating elements are configured to emit at least 95% of their electromagnetic radiation between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15 pm. In some embodiments, the one or more electrically powered heating elements are configured to emit all, i.e. 100% of their electromagnetic radiation between 0.7 pm and 1 mm, and preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15 pm. Electromagnetic radiation can be measured using a spectrometer wherein the radiation intensity is measured as a function of the wavelength of the radiation (in nm), as is well-known by a skilled person. The percentage of the electromagnetic radiation that is between 0.7 pm and 1 mm, preferably between 0.7 pm and 50 pm, such as between 0.7 and 40 pm, or between 0.7 and 20 pm, or between 0.7 and 15pm, is then calculated by expressing the intensity of radiation in said range versus the total intensity of radiation over the whole range of wavelengths.

The net radiative heat transfer from one surface 1 to another 2 is the radiation leaving the first surface for the other minus that arriving from the second surface and is for a grey body equal to:

QI->₂ = E’ o Ai FI->₂ (TI⁴ - T₂ ⁴) ; with

A surface area of the emitter; and A2: surface area of receiver

FI->2: the view factor from surface 1 to surface 2 with F2->1 = (A1/A2) FI->2 e’ = generalised emissivity coefficient (taking into account the material characteristics of the emitter, £1, A1 as well as those of the receiving object, £2, A2 and the geometric arrangement).

The radiation from the heating elements propagates in straight lines and is generally emitted in all directions. Reflectors can be used to focus the infrared radiation optimally on the receiving tubes, transporting the hydrocarbon feedstock and dilution steam that must be heated. Very high temperatures can be achieved with well-chosen materials. Preferably, the enclosure is used (with an inner wall with preferred characteristics as described above) to obtain homogeneous heating and optimal cracking properties.

Electrically powered heating elements suitable for use in a furnace of the invention exist in a multiplicity of designs and in different materials.

In certain embodiments the electrically powered heating elements are selected from the group comprising ceramic heating elements, carbon-based heating elements, carbide-based heating elements (like silicon carbide, SiC), silicide-based heating elements (like molybdenum silicide, MoSi2), heating elements in a quartz tube, and metal or metal alloy heating elements (like iron- chromium-aluminum alloy, FeCrAI), and any combinations thereof.

Preferred embodiments of the electrically heating elements for use in the present furnace are selected from the group comprising ceramic heating elements, carbon-based heating elements, carbide-based heating elements (like silicon carbide, SiC), silicide-based heating elements (like molybdenum silicide, MoSi2) and any combinations thereof. In certain embodiments the electrically heating elements are provided in the form of a lamp; a panel, such as a flat panel; a tube; a scallop, such as a concave or convex scallop; cylindrical rods or cylindrical spirals. Preferred examples of electrically powered IR heating elements include for instance, but are not limited to ceramic heating elements, silicon carbide, molybdenum silicide or iron-chromium-aluminum alloys (for instance: Kanthal FeCrAI alloys consist of mainly iron, chromium (20-30%) and aluminum (4-7.5 %)).

It will be understood that in accordance with the present invention also combinations of different electrically powered heating elements made of different materials and/or of forms may be applied in the present invention.

In addition, in preferred embodiments of the present invention, the electrically heating elements are not configured as electrical wires. In other words, the electrically heating elements as applied in the present invention do not have the form of wires or cables or strands. The electrically heating elements as applied in the present invention are also not provided in the form of wires (or cables or the like) that are wound in the shape of a coil, helix or spiral.

In certain preferred embodiments of the furnace of the invention, the electrically heating elements are planar emitters or scallop emitters configured to emit radiation at one side. Emitters configured to emit radiation at one side are also denoted herein as “single side emitters”.

In certain preferred embodiments of the furnace of the invention the electrically heating elements are planar emitters or scallop emitters configured to emit radiation at both sides. Emitters configured to emit radiation at both sides are also denoted herein as “dual side emitters”.

In certain preferred embodiments of the furnace of the invention the electrically heating elements are cylindrical emitters configured to emit radiation in all directions. Emitters configured to emit radiation at all directions are also denoted herein as “cylindrical emitters”.

In certain embodiments of the invention the electrically heating elements may also include a combination of any of the above and herein disclosed emitters. It will also be understood that term “radiation” as used in the present context intends to include “IR radiation” as defined herein.

In certain embodiments of the invention, it is preferred that the electrically powered heating elements emit at a temperature of at least 1200 K, for example at least 1400 K, for example at least 1450 K, for example about 1500 K. In certain embodiments of the invention, it is preferred that the electrically powered heating elements emit at a temperature of at most 1800 K, for example at most 1600 K, for example at most 1550 K, for example about 1500 K. In certain embodiments of the invention, it is preferred that the electrically powered heating elements emit at a temperature of at least 1200 Kto at most 1800 K, for example at least 1400 K to at most 1600 K, for example at least 1450 K to at most 1550 K, for example about 1500 K.

Since the enclosure reflects or re-emits IR radiation, and since there are no combustion gases present due to burners resulting in an I R transparent atmosphere, there is less absorption by the gases inside the cracking furnace.

Nevertheless, in certain embodiments of the invention, the electrically powered heating elements have an emissivity of at least 0.75 on the side facing the reactor tubes, such as at least 0.80, such as at least 0.85, such as at least 0.90, such as at least 0.95. The combination of emissivity and surface temperature of the emitter will determine how much heat flux can be generated.

In certain embodiments of the invention, the electrically powered heating elements may be coated with a coating material having suitable emissivity. In preferred embodiments of the invention, the electrically powered heating elements are not coated, for example are solid heating elements. This allows for thermal expansion to occur without the coating cracking or losing attachment to the heating elements.

In certain embodiments, the reactor tubes/tube passes may be arranged in parallel to one another and provided in one lane. In such embodiments, the one or more electrically powered heating elements may advantageously be arranged parallel to said lane of reactor tubes, and preferably are arranged in contact with at least one plane facing the lane of reactor tubes/tube passes, or in 2 opposing planes facing the lane of reactor tubes/tube passes.

In an example, a furnace configuration may comprise a central lane of reactor tube passes arranged in parallel to one another; and wherein the lane (which can for instance be a singlelane, dual-lane, or triple-lane configuration as defined herein) is surrounded at both sides with arrays of emitters (single, dual or cylindrical as defined herein). In this example, the electrically powered heating elements may advantageously be arranged parallel to the lane of reactor tube passes, and preferably arranged in at least one plane facing the lane of reactor tubes and preferably two planes one at each side of the lane of reactor tubes.

In another example, the furnace configuration as described in the above example may be repeated a number of times in the furnace.

In some preferred embodiments, the one or more electrically powered heating elements (132) are attached to a support element, such as e.g. a supporting frame. A frame is less sensitive to thermal expansion of the elements. Preferably, said support element is placed parallel to the reactor tubes or reactor tube passes. In some preferred embodiments, said support element is placed at a shortest distance of at least 0.10 m from the reactor tube passes, preferably at least 0.15 m, preferably at least 0.20 m, for example about 0.30 m. In some preferred embodiments, said support element is placed at a shortest distance of at most 1.20 m from the reactor tube passes, preferably at most 1.00 m, preferably at most 0.70 m, for example about 0.30 m. In some preferred embodiments, said support element is placed at a shortest distance of at least 0.10 m to at most 1 .20 m from the reactor tube passes, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m. Such distances provide an optimal homogeneous distribution of the heat in the reactor tubes.

Preferably, said support element is placed parallel to the enclosing inner wall surface of the enclosure. In some preferred embodiments, said support element is placed at a shortest distance of at least 0.10 m from the enclosing inner wall surface of the enclosure, preferably at least 0.15 m, preferably at least 0.20 m, for example about 0.30 m. In some preferred embodiments, said support element is placed at a shortest distance of at most 1.0 m from the enclosing inner wall surface of the enclosure, preferably at most 0.70 m, for example about 0.30 m. In some preferred embodiments, said support element is placed at a shortest distance of at least 0.10 m to at most 1.00 m from the enclosing inner wall surface of the enclosure, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m. Such distances provide an optimal homogeneous reflection and/or reemission of the heat in from the inner wall surface.

In some preferred embodiments of a furnace of the invention the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane, and wherein said support element and said 1^st plane are placed parallel to one another.

In some preferred embodiments of a furnace of the invention the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane, and wherein said support element and said 1^st plane are placed parallel to one another, and are separated by a distance equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube.

In some preferred embodiments of a furnace of the invention the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane, and wherein said support element and said 1^st plane are placed parallel to one another, and are separated by a distance equal to at most 15 times an outer diameter of the reactor tube, such as at most 10 times, or at most 8 times, the outer diameter of the reactor tube. In some preferred embodiments of a furnace of the invention the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane, and wherein said support element and said 1^st plane are placed parallel to one another, and are separated by a distance of between 1 and 15 times an outer diameter of the reactor tube, such as between 2 and 10 times, or between 3 and 8 times, the outer diameter of the reactor tube.

In some preferred embodiments of a furnace of the invention the one or more tube passes are arranged in a lane, wherein one side of the lane is in contact with a 1^st plane, and wherein said support element and said 1^st plane are placed parallel to one another, and are separated by a distance of at least 0.10 m to at most 1.20 m, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m.

In another aspect, the present invention also provides a process for steam cracking a hydrocarbon feedstock to produce olefins. The process comprises the steps of: feeding a hydrocarbon feedstock and dilution steam to one or more reactor tubes in a cracking furnace, as described herein and (preferred) embodiments thereof; and, exposing the hydrocarbon feedstock and dilution steam in the one or more reactor tubes to infrared radiation from the electrically heated infrared emitter to crack at least a portion of the hydrocarbon feedstock.

The hydrocarbon feedstock for the present process preferably comprises one of more hydrocarbons having at least two carbon atoms, in particular selected from the group comprising: ethane, propane, butane, liquefied petroleum gas, naphtha, gasoils and crude oils. For instance, the feedstock for the present process can be ethane, liquefied petroleum gas, naphtha or gasoils. Liquefied petroleum gas (LPG) consists essentially of propane and butanes. Petroleum naphtha or naphtha is defined as the hydrocarbons fraction of petroleum having a boiling point from 15°C up to 200°C. It consists of a complex mixture of linear and branched paraffins (single and multi-branched), cyclic paraffins and aromatics having carbons numbers ranging from 5 to about 11 carbons atoms. Light naphtha has a boiling range from 15 to 90°C, consisting of Cs to Ce hydrocarbons while heavy naphtha has a boiling range from 90 to 200°C, consisting of C? to about On hydrocarbons. Gasoils have a boiling range from about 200 to 350°C, consisting of C10 to C22 hydrocarbons, including essentially linear and branched paraffins, cyclic paraffins and aromatics (including mono-, naphtho- and polyaromatic). Heavier gasoils (like atmospheric gasoil, vacuum gasoil, atmospheric residua and vacuum residua), having boiling ranges above 300°C and C20+ hydrocarbons including essentially linear and branched paraffins, cyclic paraffins and aromatics (including mono-, naphtho- and poly-aromatic) are available from atmospheric or vacuum distillations units. In particular, the cracking products obtained in the present process may include ethylene, propylene and benzene, and optionally hydrogen, toluene, xylenes, and 1 ,3-butadiene. In embodiments, the outlet temperature of the reactor may range from 800 to 1200°C, from 820 to 1100°C, from 830 to 950°C, from 840°C to 920°C. The outlet temperature may influence the content of high value chemicals in the cracking products produced by the present process. In embodiments, the residence time of the feedstock, through the radiation section of the reactor where the temperature may be between 650 and 1200 °C, may range from 0.005 to 0.5 seconds, or from 0.01 to 0.4 seconds. In some embodiments, the temperature may be greater than 750 °C. In some embodiments, the temperature may be greater than 950 °C.

In certain embodiments, steam cracking said hydrocarbon feedstock is done in presence of dilution steam in a ratio of 0.1 to 1.0 kg steam per kg of hydrocarbon feedstock, from 0.25 to 0.7 kg steam per kg of hydrocarbon feedstock, or 0.35 kg steam per kg of feedstock mixture, to obtain cracking products as defined above. In some embodiments, the reactor outlet pressure may range from 500 to 1500 mbar, from 700 to 1000 mbar, or may be approx. 850 mbar. The residence time of the feed in the reactor and the temperature are to be considered together. A lower operating pressure may result in easier light olefins formation and reduced coke formation. The lowest pressure possible may be accomplished by (i) maintaining the output pressure of the reactor as close as possible to atmospheric pressure at the suction of the cracked gas compressor (ii) reducing the partial pressure of the hydrocarbons by dilution with steam (which has a substantial influence on slowing down coke formation). The steam/feedstock ratio may be maintained at a level sufficient to limit coke formation.

In certain embodiments, the present process is characterized in that the transfer of heat to the hydrocarbon feedstock and of the dilution steam is carried out separately in at least two pipes before mixing the two streams; or wherein the hydrocarbon feedstock and the dilution steam are mixed, followed by transfer of heat to the mixture; or wherein the transfer of heat to the hydrocarbon feedstock and the dilution steam is carried out separately in at least two pipes to a temperature less than 500°C, followed by mixing the two streams and further transferring heat to the mixture.

In certain embodiments, the present process is characterized in that sensible or latent heat of effluent exiting the furnace is extracted at least partially by means of one or more heat exchangers and used to at least partially preheat the hydrocarbon feedstock, dilution steam or mixture thereof.

Effluent from the pyrolysis furnaces may contain unreacted feedstock, desired olefins (mainly ethylene and propylene), hydrogen, methane, a mixture of C4 S (primarily isobutylene and butadiene), pyrolysis gasoline (aromatics in the Ce to Cs range), ethane, propane, di-olefins (acetylene, methyl acetylene, propadiene), and heavier hydrocarbons that boil in the temperature range of fuel oil (pyrolysis fuel oil). This cracked gas may be rapidly quenched to 338-510 °C to stop the pyrolysis reactions, minimize consecutive reactions and to recover the sensible heat in the gas by generating high-pressure steam in parallel transfer-line heat exchangers (TLE's). In gaseous feedstock-based plants, the TLE-quenched gas stream may flow forward to a direct water quench tower, where the gas is cooled further with recirculating cold water. In liquid feedstock-based plants, a pre-fractionator may precede the water quench tower to condense and separate the fuel oil fraction from the cracked gas. In both types of plants, the major portions of the dilution steam and heavy gasoline in the cracked gas may be condensed in the water quench tower at 35-40°C. The water-quench gas is subsequently compressed to about 25-35 Bars in 4 or 5 stages. Between compression stages, the condensed water and light gasoline may be removed, and the cracked gas is washed with a caustic solution or with a regenerative amine solution, followed by a caustic solution, to remove acid gases (CO2, H2S and SO2). The compressed cracked gas may be dried with a desiccant and cooled with propylene and ethylene refrigerants to cryogenic temperatures for the subsequent product fractionation: front-end de-methanization, front-end de-propanization or front-end de-ethanization.

EXAMPLES

The following examples serve to merely illustrate the invention and should not be construed as limiting its scope in any way. While the invention has been shown in only some of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the scope of the invention.

Example 1 : embodiment of a furnace according to the invention

FIG. 1 is a schematic illustration of an embodiment of a cracking furnace (100) in accordance with the present invention. As shown in this FIG. 1 , a cracking furnace is schematically represented, comprising a series of five reactor tubes (110) which are arranged in parallel to one another and are disposed in a lane (row) in the furnace (150). The furnace (150) further comprises an electrically heated infrared emitter (130), which comprising a number of electrically powered heating elements (132), schematically represented as a series of flat panels (132) that are arranged in a raster structure. The electrically powered heating elements (132) are placed parallel to the reactor tubes (110) in one plane facing the reactor tubes (110)). Preferably the shortest distance to the reactor tubes (110) is a distance that is at least one time the outer diameter of the reactor tube, such as two time the outer diameter of the tube. This means that if the outer diameter of the reactor tube is 10 cm, the distance of the electrically powered heating elements to the tube (outer surface) is at least 0.1 m, and for instance 0.2 m. The represented furnace (100) further comprises an enclosure (150) surrounding the one or more reactor tubes (110) and the electrically heated infrared emitter (130). The enclosure (150) comprises an enclosing inner wall surface (152). In the represented embodiment of the furnace, the enclosure (150) comprises several layers (154) concentrically arranged around the inner wall surface (152). It is preferred that each layer (154) comprises a material with a thermal conductivity A that is lower than the nearest enclosed wall within, for example at least 0.05 W/m.K lower. In an example, the enclosure may consist of a series of 4 outer walls which may consist (from outside to inside) of the following materials: a layer made of a refractory brick (e.g., JM28 insulation brick (1350-1650°C), with A = 0.37 W/m.K; specific thermal capacity = 1100 J/kg.K; density = 890 kg/m³); a layer made of another refractory brick (e.g., JM28 insulation brick (1100-1315°C), with A = 0.18 W/m.K; specific thermal capacity = 1050 J/kg.K; density = 480 kg/m³); a layer made of an isolating silica-based material (e.g., Superwool Plus 1000 with A = 0.15 W/m.K, specific thermal capacity = 1050 J/kg.K; density = 180 kg/m³); and a layer made of a refractory brick (e.g., Cerablancket “1260°C” with A = 0.08 W/m.K; specific thermal capacity = 1130 J/kg.K; density = 80 kg/m³).

Example 2: Furnace configurations

FIG. 2A to 2F schematically illustrate a top view of various embodiments of a furnace of the invention, wherein the arrangement of reactor tubes/reactor tube passes and the infrared emitter/electrically powered heating elements are schematically illustrated.

FIG. 2A is a schematic representation of an example of a furnace according to the invention, which comprises reactor tube passes (110) (or reactor tubes; for instance if the furnace comprise a series of substantially straight and vertically aligned reactor tubes each having a single tube pass), that are arranged in a lane (115). The reactor tube passes may be arranged in a central portion of the furnace. One side of this lane (115) is in contact with a 1^st plane (111). In this example, the furnace also comprises an electrically heated infrared emitter comprising a plurality electrically powered heating elements (132) for transferring heat to the reactor tubes (110), wherein the electrically powered heating elements (132) are arranged in contact with a 2^nd plane (112). The embodiment in Fig. 2A further schematically illustrates the presence of an enclosure surrounding the reactor tube/tube passes (110) and the electrically powered heating elements wherein the enclosure comprising an enclosing inner wall surface (152). FIG. 2A illustrates that the 1^st (111) and 2^nd (112) planes are parallel to one another, and are separated by a distance (d). This distance is equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube. FIG. 2A also illustrates that the 2^nd (112) plane is parallel to the enclosing inner wall surface (152) and provided at distance (d_e) from this inner wall surface.

FIG. 2B is a schematic representation of another example of a furnace according to the invention, wherein tube passes (110) and electrically powered heating elements (132) are arranged in a reaction unit (120). The illustrated reaction unit comprises one lane (115) of tube passes (or reactor tubes if for instance the furnace comprise a series of substantially straight and vertically aligned reactor tubes each having a single tube pass). This lane of reactor tube passes may be arranged in a central portion of the furnace. One side of the lane (115) of tube passes (110) contacts a 1^st plane (111), and the other side of the lane contacts a 3^rd plane (113) parallel to the 1^st plane, wherein the 3^rd plane is separated from the 1^st plane by the lane (115) of tube passes (110). Some of the electrically powered heating elements (132) are arranged contacting the 2^nd plane (112) and some of the electrically powered heating elements (132) are arranged contacting a further 4^th plane (114). The 1^st (111) and 2^nd (112) planes are parallel to one another, and are separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, such as at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and preferably of at most 15 times, such as at most 10 times the outer diameter. The 3^rd (113) and 4^th (114) planes are also parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and preferably of at most 15 times, such as at most 10 times the outer diameter. In the illustrated embodiments, the electrically powered heating elements (132) are arranged in contact with a 2^nd plane (112) and the enclosing inner wall surface (152) of the enclosure and the 2^nd plane are parallel to one another, and separated by a distance (d_e), which is in accordance with the invention preferably at most 1.0 m.

In FIG. 2B, the reaction unit (120) comprises one lane of tube passes, and wherein said lane is configured as a single-lane layout of tube passes. FIG. 2C and FIG. 2F show examples of a furnace according to the invention which are similar to the example shown in FIG. 2B, with the difference that a reaction unit (120) is illustrated comprising one lane of tube passes, wherein said lane is respectively configured as a dual-lane layout (see FIG. 2C) or a triplelane layout (see FIG. 2F) of tube passes.

In other words, FIG. 2B, FIG. 2C and FIG. 2F thus show examples of a furnace configuration which comprises in a central lane of reactor tube passes (which have a single lane, dual lane and triple lane layout in the respective figures). The tube passes are centrally disposed in a furnace and are arranged in parallel to one another. The lane of tube passes is surrounded/flanked at both sides with electrically powdered heating elements, which can be e.g. single-side emitters, dual-side emitters or cylindrical emitters as defined herein. In this embodiment, the electrically powered heating elements may advantageously be arranged in a plane that is parallel to the lane of reactor tube passes, and are preferably placed in two planes, i.e. one plane at each side of the lane of reactor tubes.

FIG. 2D is a schematic representation of another example of a furnace according to the invention, comprising two reaction units (120, 120’) such as those represented in FIG. 2B. As illustrated the reaction units are arranged in parallel, and in a direction perpendicular to a longitudinal axis of the lane (115, 115’). The embodiment shown in FIG. 2D illustrates that where there are at least two lanes of tube passes (115, 115’), adjacent lanes of tube passes are separated by one array of electrically powered heating elements (132). This array of heating elements is located in between the lanes of tube passes. One side of this array contacts a plane (e.g. 2^nd plane, 112), and the other side of the array contacts other plane (e.g. 4^th plane, 114). The heating elements located in between the lanes of tube passes preferably are dual side or cylindrical emitters as defined herein. In embodiment of FIG. 2D, further illustrates that when there are at least two lanes of tube passes (115, 115’), the lane of tube passes that adjacent to inner wall of the furnace enclosure (152), is separated from this adjacent wall by one array of electrically powered heating elements. One side of the array contacts a plane (e.g. 2^nd plane, 112), and the other side of the array faces the enclosure inner wall (152). The heating elements located in between the enclosure inner wall and the lane of tube passes adjacent to the inner wall preferably are single-side, dual-side or cylindrical emitters as defined herein.

In FIG. 2D, the reaction units (120, 120’) comprise one lane of tube passes, and said lane is configured as a single-lane layout of tube passes. FIG. 2E and FIG. 2G show examples of a furnace according to the invention which are similar to the example shown in FIG. 2D, with the difference that a series of reaction units (120, 120’) is illustrated comprising one lane of tube passes, wherein said lane is configured as a dual-lane layout (see FIG. 2E) or as a triplelane layout (see FIG. 2G) of tube passes.

Example 3: Positioning of electrically heated IR emitter

FIG. 3A and B illustrate a simulation of the heat distribution for a furnace that comprises reactor tubes and an electrically heated infrared emitter, in which the electrically heated IR emitter was applied at different distances from the reactor tubes. The figures illustrate the temperature profile along the reactor tubes (the X axis represents the axial position/length along the reactor tube in m) in a furnace for different distances of the electrically heated infrared emitter.

The represented simulation is based on a finite element method using the Comsol software. In this example for modelling of the heating efficiency, a small number of reactors tubes surrounded by electrically heated infrared emitters has been composed. An embodiment without enclosure and an embodiment with enclosure were considered. The parameters (including number of tubes and dimensions) adopted in the present simulation were selected in function of simulation time. However, it will be understood that also other configurations and dimensions can be applied in modelling concepts.

In case of absence or presence of an enclosure, it was considered that there were two rectangular arrays of 25 electrical infrared heating plates, each heater being rectangular; the arrays are facing each other, sandwiching 5 tubes, acting as heat sinks, with a length of 1.56 m and an inner diameter of 60 mm.

Characteristics of the heating elements/emitters:

Rectangular parallelepiped: V = 0.23 m x 0.075 m x 0.023 m

One Emitting surface: S = 0.23*0.075 m² ;

Total emissivity: 0.95

Emitter body temperature: 1500 K

Five reflecting surfaces: 51 = 0.23 x 0.023 m² x2;

52 = 0.023 x 0.075 m² x2;

53 = 0.23 x 0.075 m² x1 ;

Total emissivity: 0.02

Distance between emitters: 0.025 m between vertical arrays and 0.05 m for short side in tube direction.

Characteristics of reactor tubes: Internal diameter: 60 mm; Wall thickness: 6 mm; Length: 1.56 m; Total emissivity: 0.91.

In case of presence of enclosure, it was considered that there was a parallelepipedal enclosure (external dimensions: 2 m x 2 m x 1.5 m) in which the electrical infrared heating plates and tubes are inserted.

Insulation of enclosure: The enclosure is composed of 4 successive sheets based on insulating inorganic materials giving an insulating total thickness of 330 mm. The temperature external to the shield is 20°C.

1) Insulating fire brick thickness: 115 mm; Thermal conductivity: 0.37 W/m.K; Density: 890 kg/m^3; Mass heat capacity: 1100 J/kg.K ; Total emissivity: 0.5; 2) Insulating fire brick thickness: 115 mm; Thermal conductivity: 0.18 W/m.K; Density: 480 kg/m^3; Mass heat capacity: 1050 J/kg.K ;

3) Insulating Superwool thickness: 75 mm; Thermal conductivity: 0.15 W/m.K; Density: 180 kg/m^3; Mass heat capacity: 1050 J/kg.K;

4) Cerablanket thickness: 25 mm; Thermal conductivity: 0.08 W/m.K; Density: 0 kg/m^3; Mass heat capacity: 1130 J/kg.K; Total emissivity: 0.6

The infrared heaters approach a black body behaviour (total emissivity = 0.95). The tubes are composed of a refractory alloy with a total emissivity of 0.91 .

For the modelling the following physical conditions have been assumed: the electrical infrared heating plates have a surface temperature of 1500K (1227 °C.) the radiative exchanges between each component are modelled through the radiosity concept assuming that each elementary surface is gray or black and diffuse. the distance between the infrared heaters and the tubes is varied between 0.01 and 1.2 m. the thermal quantities for conduction and radiation are independent from the temperature The atmosphere between the emitters and reactor tube is infrared transparent, corresponding to the presence of air (essentially nitrogen and oxygen)

The hemicube method, based on the Nusselt’s analogy, allows to determine the whole set of view factors between all the elementary facets constituting the mesh depicting the thermal issue. Typical meshes are composed of 60000 elements and a calculation station equipped with two processors (Intel Xeon CPU E5-2637 v4 @ 3.50GHz) and 128 GB RAM is used.

The simulation represented in FIG. 3A, took the following parameters into account: a series of reactor tubes are placed in parallel to each other; an electrically heated IR emitter is applied that is consisting of a series of electrically powered heating elements, e.g., in the form of flat IR radiating panels, that are placed parallel to the reactor tubes; the electrically powered heating elements are placed in one plane facing the reactor tubes; the electrically powered heating elements are placed at various distances from the reactor tubes, i.e. at distances of 0.01 m; 0.1 m; 0.3 m; 0.7 m; and 1.2 m; the furnace does not comprise an enclosure surrounding the reactor tubes and the electrically heated infrared emitter. FIG. 3A (comparative example) illustrates the temperature profile along a tube for different distances between the IR transmitter and the tube. The illustrated profile is a simulation of the pattern in the middle tube (of the series of 5 tubes). The figure shows that the higher the distance of the IR emitter to the tubes, the lower the average temperature will be. In addition, the simulation illustrates that there is a strong temperature variation along the tube for the IR emitter that is positioned closest to the tubes. The results indicate further that when the reflecting or re-emitting enclosure is not present, the mean temperature of the tubes reaches only low values on their outer envelope, that along a tube the thermal gradient is 200K/m and that border effects are significant. Even the small distance of 0.05 m between the emitters (in tube direction) results in a temperature drop of more than 140K. When the emitters are placed at higher distance of the reactor tubes the temperature profile is smoother, but a lot of heat flux is lost.

The simulation represented in FIG. 3B, took the following parameters into account: a series of reactor tubes are placed in parallel to each other; an electrically heated IR emitter is applied that is consisting of a series of electrically powered heating elements, e.g., in the form of flat IR radiating panels, that are placed parallel to the reactor tubes; the electrically powered heating elements are placed in one plane facing the reactor tubes; the electrically powered heating elements are placed at various distances from the reactor tubes, i.e. at distances of 0.01 m; 0.1 m; 0.3 m; 0.7 m; and 1.2 m; the furnace comprises an enclosure surrounding the reactor tubes and the electrically heated infrared emitter.

FIG. 3B illustrates the temperature profile along a tube for different distances between the IR transmitter and the tube. The illustrated profile is a simulation of the pattern in the middle tube (of the series of 5 tubes). FIG.3B illustrates a significantly different pattern as reported in FIG. 3A. FIG. 3B illustrates that the average temperature on the tubes may be even higher for IR transmitters that are located at 0.1 m rather than at 0.01 m. In this embodiment according to the invention, i.e. when an enclosure is integrated to the furnace, the mean temperature of the tubes is about 1285K (1012°C) (at distance of 0,3 m) and the temperature variation falls to 10K. The radiative thermal losses of the enclosure are found to be around 5%. The simulations at different distances, further demonstrate that the border effects (both between the emitters as at the last emitters of the row) starts to be smoothed out at distances higher than 0.1 m.

It is further noted that in this example, a temperature raise is observed at the outer edges. Hence, FIG. 3B illustrates that in accordance with the invention a heat flux may be well distributed by the enclosure, which makes it possible to position the IR emitters at a considerable distance of the tubes. Preferred IR emitter/tube distances are for instance within a range of 0.2 to 1 m. The addition of the enclosure has the advantage of improving thermal efficiency, and reduce heat losses. Example 4: Tube emissivity

FIG. 4 illustrates a simulation of the heat distribution for a furnace as a function of the emissivity of the reactor tubes. This figure illustrates that higher temperatures may be obtained by increasing the emissivity properties of the reactor tubes.

Claims

1. A cracking furnace (100) for steam cracking a hydrocarbon feedstock, wherein the furnace (100) comprises one or more reactor tubes, wherein each reactor tube comprises one or more tube passes (110) for transporting the hydrocarbon feedstock and dilution steam, and wherein the one or more tube passes are arranged in a lane (115), wherein one side of the lane is in contact with a 1^st plane (111); and an electrically heated infrared emitter (130) comprising one or more electrically powered heating elements (132) for transferring heat to the reactor tubes (110), wherein the one or more electrically powered heating elements (132) are arranged in contact with a 2^nd plane (112), and an enclosure (150) surrounding the one or more reactor tubes (110) and the electrically heated infrared emitter (130), wherein the enclosure (150) comprises an enclosing inner wall surface (152), and

- wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and wherein the electrically heated infrared emitter (130) is configured to produce a radiation heat flux of at least 20 000 W/m²

2. The cracking furnace (100) of claim 1 wherein the one or more electrically powered heating elements (132) are configured to emit their electromagnetic radiation with a wavelength of between 0.7 pm and 1 mm, and preferably to emit at least 50%, preferably at least 60%, more preferably at least 70%, or even 100%, of their electromagnetic radiation with a wavelength of between 0.7 pm and 1 mm.

3. The furnace (100) according to claim 1 or 2 wherein the electrically heating elements (132) are provided in the form of a lamp, a panel, a tube, a scallop, a cylindrical rod, or a cylindrical spiral.

4. The cracking furnace (100) of any one of the previous claims, wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at most 15 times an outer diameter of the reactor tube, or at most 10 times, or at most 8 times, the outer diameter of the reactor tube.

5. The furnace (100) according to any one of the previous claims, wherein the one or more tube passes (110) are disposed parallel to each other. The furnace (100) according to any one of the previous claims, wherein the tube passes are arranged in a lane (115), wherein said lane is configured as a single-lane, or a duallane, or a triple-lane, or a x-lane arrangement of tube passes, wherein x is 4 or more. The furnace (100) according to any one of the previous claims, wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) of at least 0.10 m to at most 1.20 m, preferably at least 0.15 m to at most 1.00 m, preferably at least 0.20 m to at most 0.70 m, for example about 0.30 m. The furnace (100) according to any one of the previous claims, wherein the one or more electrically powered heating elements (132) are arranged in contact with a 2^nd plane (112), and wherein the enclosing inner wall surface (152) of the enclosure and the 2^nd plane are parallel to one another, and separated by a distance (d_e) of at most 1.0 m. The furnace (100) according to any one of the previous claims, wherein the tube passes (110) and electrically powered heating elements (132) are arranged in a reaction unit (120), wherein the reaction unit comprises one lane (115) of tube passes, wherein one side of the lane contacts the 1^st plane (111), and the other side of the lane contacts a 3^rd plane (113) parallel to the 1^st plane, wherein the 3^rd plane is separated from the 1^st plane by the lane (115) of tube passes (110), wherein some of the electrically powered heating elements (132) are arranged contacting the 2^nd plane (112) and some of the electrically powered heating elements (132) are arranged contacting a further 4^th plane (114), wherein the 1^st and 2^nd planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube, and wherein the 3^rd and 4^th planes are parallel to one another, and separated by a distance (d) equal to at least one times an outer diameter of the reactor tube, or at least 2 times, or at least 3 times, or at least 4 times the outer diameter of the reactor tube. The furnace (100) according to claim 9, comprising more than one reaction unit (120, 120’), preferably wherein the reaction units are arranged in parallel, and preferably in a direction perpendicular to a longitudinal axis of the lane. The furnace (100) according to any one of the previous claims, wherein the enclosing inner wall surface (152) of the enclosure (150) comprises a material with a thermal conductivity A, measured at 1000°C, of at most 0.50 W/m.K, preferably at most 0.40 W/m.K, preferably at most 0.30 W/m.K, preferably at most 0.25 W/m.K, preferably at most 0.20 W/m.K.

12. The furnace (100) according to any one of the preceding claims, wherein the enclosing inner wall surface (152) of the enclosure (150) comprises a material with a reflectance for wavelengths in the infrared range of at least 0.1 , or at least 0.2, or at least 0.3, or at least 0.4, or at least 0.5.

13. The furnace (100) according to any one of the previous claims, wherein the enclosing inner wall surface (152) of the enclosure (150) comprises a material selected from the group comprising alumino-silicate refractory bricks, silicate bricks, and corundum (alumina) bricks.

14. The furnace (100) according to any one of the previous claims, wherein the enclosure (150) comprises one or more layers (154) concentrically arranged around the inner wall surface (152).

15. The furnace (100) according to claim 14 wherein each layer (154) comprises a material with a thermal conductivity A that is lower than the nearest enclosed wall within, for example at least 0.05 W/m.K lower.

16. The furnace (100) according to any one of the preceding claims, wherein the one or more reactor tubes or reactor tube passes are coated with an infrared receiving material and/or wherein the one or more reactor tubes or reactor tube passes are made of infrared receiving material, preferably wherein said infrared receiving material is a material having an emissivity of at least 0.80, preferably at least 0.90, and most preferably at least 0.93.

17. The furnace (100) according to any one of the preceding claims, wherein the electrically heated infrared emitter (130) is configured to produce a radiation heat flux of at least 40 000 W/m², and more preferably at least 60 000 W/m², for example about 80 000 W/m².

18. The furnace (100) according any one of the preceding claims, wherein said one or more electrically heating elements (132) are selected from the group comprising: ceramic heating elements, carbon-based heating elements, carbide-based heating elements, silicide-based heating elements; heating elements in a quartz tube, and metal or metal alloy heating elements, and any combinations thereof.

19. The furnace (100) according any one of the preceding claims, wherein the one or more electrically powered heating elements are planar emitters or scallop emitters configured to emit radiation at one side; or planar emitters or scallop emitters configured to emit radiation at both sides; or cylindrical emitters configured to emit radiation in all directions; or any combinations thereof.

20. The furnace (100) according to any one of the preceding claims, wherein the electrically powered heating elements (132) emit at a temperature of at least 1200 K to at most 1800 K, for example at least 1400 K to at most 1600 K, for example at least 1450 K to at most 1550 K, for example about 1500 K. 21 . The furnace (100) according to any of the previous claims, wherein the electrically heating elements (132) are not electrical wires.

22. A process for steam cracking a hydrocarbon feedstock to produce olefins, comprising the steps of: feeding a hydrocarbon feedstock and dilution steam to one or more reactor tubes (110) in a cracking furnace (100) according to any one of the preceding claims; and, exposing the hydrocarbon feedstock and dilution steam in the one or more reactor tubes (110) to infrared radiation from the electrically heated infrared emitter (130) to crack at least a portion of the hydrocarbon feedstock.