WO2001023169A1

WO2001023169A1 - Coated furnace component and catalyst

Info

Publication number: WO2001023169A1
Application number: PCT/US2000/026825
Authority: WO
Inventors: Donald M. Trotter, Jr.
Original assignee: Corning Incorporated
Priority date: 1999-09-29
Filing date: 2000-09-29
Publication date: 2001-04-05
Also published as: AU7839300A

Abstract

A laminated component (10) for a thermal cracking furnace comprising a metal substrate (12) and a layer of glass-ceramic (14) adherent to a surface on the metal substrate, at least the exposed surface portion of the glass-ceramic layer containing a catalyst that promotes a carbon reaction, a coating material (14) for coating the substrate surface, and a method of preventing carbon deposition on a metal surface.

Description

COATED FURNACE COMPONENT AND CATALYST

FIELD OF THE INVENTION

A metal component for a thermal cracking furnace coated with a glass- ceramic containing a catalyst for promoting a carbon reaction.

BACKGROUND OF THE INVENTION

The invention relates to components for a furnace used in thermal cracking a stream of hydrocarbons to form olefins. It is particularly concerned with a component having a lessened tendency for carbon deposits, known as "coke," to form on the component surface during a thermal cracking process. At the heart of a thermal cracking process is the pyrolysis furnace. This furnace comprises a fire box through which runs an array of tubing that is composed of tubing lengths and fittings. The array may be manifold-fed, straight tubes, or a serpentine array. The array of tubing is heated while a stream of feedstock is forced through the heated tubing under pressure and at a high velocity. The product is quenched as it exits. The tubing array is commonly operated at a temperature greater than 750° C. During passage of the hydrocarbon stream, a carboniferous residue forms and deposits on the tube walls and fittings in a process known as "coking."

Initially, carbon residue appears in a fibrous form on the walls. The carbon fibers on the tube wall appear to form a matte by trapping pyrolytic coke particles formed in the gas stream. This leads to build-up of a dense coke deposit on the walls of the tubing and fittings.

The problem of carbon deposits forming during the thermal cracking of hydrocarbons is one of long standing. It results in restricted flow of the gaseous stream of reaction material. It also reduces heat transfer through the tube wall to the gaseous stream. The temperature to which the tube is heated must then be raised to maintain a constant temperature in the stream flowing through the tube. This not only reduces process efficiency, but ultimately requires a temperature too high for equipment viability and safety. A shut down then becomes necessary to remove the carbon formation, a process known as decoking.

Numerous solutions to the problem of coking have been proposed. One such solution involves producing metal alloys having special compositions. Another proposed solution involves coating the interior wall of the tubing with a silicon-containing coating such as silica, silicon carbide, or silicon nitride. In still another proposal, the interior wall of the tubing is treated with a chromium and/or an aluminum compound.

My United States Patent No. 5,807,616 proposes a method of control which comprises constructing a furnace with components having their exposed surfaces coated with a glass-ceramic material. The principles involved are isolation of the metal from the hydrocarbon stream, and presentation of an exposed surface that is not prone to carbon attachment.

Bench testing indicated that the glass-ceramic material tended not to have coke form on its surface. However, a scale-up to partial furnace testing was not encouraging. The tube interior could not be observed, of course, but thermal testing of the exterior of the coated tube showed little or no difference from an uncoated tube. This indicated that coke was forming in the normal manner on the coated tube as well as on the uncoated tube.

Further, when a test in a pilot furnace was terminated, a coke deposit was found on the coated tubes. Surprisingly, however, unlike deposits on uncoated tubes, the coke deposit on the coated tubes was loosely adhered. This meant that large chunks of coke could be easily lifted off the tube wall. ~> j

This unexpected finding suggested that a minor disruption of the mechanisms for forming and bonding of coke on a metal surface could facilitate removal, or even eliminate the forming process. It is a basic purpose of the present invention to provide such a mechanism. A further purpose is to provide an improved material to disrupt coke deposition on a metal surface.

Another purpose is to provide a coated component for a cracking furnace that promotes a carbon reaction during a thermal cracking process.

A still further purpose is to provide a method of facilitating removal of carbon on a furnace component during a thermal cracking process.

Still another purpose is to provide a coating on the exposed surface of a furnace component to resolve the problem of coke deposition on the component during a thermal cracking process.

SUMMARY OF THE INVENTION

The invention resides in part in a coated, furnace component comprising a metal substrate and a layer of glass-ceramic that is adherent to a surface on the metal substrate and that is exposed to carbon, at least a portion of the exposed glass-ceramic layer containing a catalyst that promotes a carbon reaction.

The invention further resides in a component for a furnace for cracking or reforming a hydrocarbon stream, the component comprising a metal member having a surface facing the hydrocarbon stream, that metal surface having an adherent layer of a glass-ceramic, the glass-ceramic layer containing on its surface, or in a surface layer, a catalyst that promotes a carbon reaction.

A further aspect of the invention is a coating material comprising a mixture of a crystallizable glass frit in a vehicle, the mixture being of such viscosity that it will form, on a metal surface, an adherent, continuous coating that does not flow when applied to the undersurface of the metal, at least the surface of the coating material containing a catalyst that promotes a carbon reaction. The invention also resides, in part, in a method of removing carbon that deposits on a metal surface when that surface is exposed, while heated, to a gaseous stream containing a source of carbon particles, the method comprising isolating the metal surface from the gaseous stream containing the source of carbon particles with a thin, adherent coating of a glass-ceramic material prior to contacting the metal surface with the hot gaseous stream, and doping at least the exposed portion of the glass-ceramic with a catalyst that promotes a carbon reaction.

PRIOR ART

References that may have relevance are listed in a separate document.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a front elevation view, partly broken away, showing a segment of a reactor tube coated in accordance with the invention.

FIGURE 2 is a front elevation view, partly broaken away, showing a segment of a reactor tube having a dual layer coating in accordance with the invention.

FIGURE 3 is a graphical representation comparing burn-off temperatures for different catalytic materials in accordance with the invention.

DESCRIPTION OF THE INVENTION

The invention is described with reference to a thermal cracking process for olefin production, and to a reactor tube and fittings for a cracking furnace used in practicing that process. However, the coking problem also occurs in other thermal cracking and reforming processes wherein a feed material is fed through a pyrolysis furnace to crack or reform the material into desired components. Accordingly, the invention is equally applicable to such other processes as well. The present invention represents an improvement on the invention described in my earlier patent. It has been found that a glass-ceramic coating does not completely prevent deposition of carbon on a metal surface, at least not in a thermal cracking furnace. However, the coating does provide a bond with carbon that is greatly reduced from that encountered when carbon is deposited directly on the metal substrate. The bond that does exist is of such a small degree that a coke layer is easily dislodged from a metal surface. This suggests that some means of further disrupting the tenuous bond could facilitate the desired removal of coke build-up. I have now found that such further disruption can be accomplished by including a minor amount of a selected catalyst in a glass-ceramic layer adhered to the surface of a metal furnace component. The selected catalyst is one that promotes an oxidation reaction with carbon to produce a carbon oxide. As indicated later, the catalyst selected will be iridium (Ir), rhodium (Rh), or ruthenium (Ru). The catalyst may be incorporated as the chloride or oxide, the rhodium being effective only as the oxide.

The minimum amount of catalyst employed is an effective amount to promote carbon burn-off. The maximum is limited by the material cost and enhanced effectiveness. Preferably, at least about 10 ppm is employed, but amounts greater than 100 ppm, while effective, do not warrant the cost. This is equivalent to 0.001-0.010 wt. % of the glass.

The production of either the monoxide, or the dioxide, of carbon disrupts the already tenuous bond sufficiently to prevent adhesion of the carbon to the metal. The atmosphere in a thermal cracking furnace is generally considered to be reducing in nature. However, it has been observed that there is sufficient oxidation potential to convert chromium carbide to C0₂ and Cr₂0₃. It is believed that this oxidation potential is, likewise, sufficient to form the indicated carbon oxides.

FIGURE 1 is a front elevational view, partly broken away, of a segment 10 of a commercial reactor tube. Such a commercial tube may be up to 12 meters (40 ft.) in length and have a diameter of 2.5-20 cm (1-8 inches). Segment 10 comprises a cast alloy tube 12 having a glass-ceramic coating 14 on its inner surface. It will be appreciated that a cracking furnace will comprise tubes and fittings, such as elbows, connecting adjacent lengths of tubing. It is contemplated that a complete cracking furnace, including tubes and fittings, will be coated in accordance with the invention. However, short length of tubing may be coated and joined, as by welding.

The catalyst may be combined in the glass-ceramic coating as a separate constituent. However, for various reasons, including homogeneous distribution, it is preferable to include the catalyst in the batch for the glass precursor of the glass-ceramic. In order to conserve catalyst, two otherwise identical batches for the precursor, crystallizable glass are mixed and melted. The exception is that the selected catalyst is included in only one of the two batches. The two glasses are prepared in frit form and incorporated in a suitable vehicle for application to a substrate surface. FIGURE 2, like FIGURE 1 , is a front elevational view, parity broaken away, of a segment 20 of a commercial reactor tube. Segment 20 comprises a cast alloy tube 22 which may be identical with tube 12. However, coating 24 on the interior surface of tube 22 is a dual layer coating composed of a first layer 26 on the surface of tube 22 and a second layer 28 applied over layer 26. A first layer of material 26 containing the catalyst-free glass frit, is applied to the appropriate surface of metal tube 22. This forms the major portion of the ultimate protective coating 24. Then, a thin, continuous layer of the material 28, containing the glass frit with a catalyst, is applied over the first layer 26. The dual layers are then converted to a unitary, adherent glass- ceramic layer by heating. This dual layer expedient is designed for situations where there is either little erosion of the coating during use, or the use is of a short-term nature that does not remove the catalyst-containing outer layer.

The thickness of the outer layer, or the need for catalyst throughout a single layer, is dictated by the degree of erosion anticipated. In the event that surface erosion is not a problem, the catalyst could be applied as a surface coating on the glass-ceramic. The basic need is to have an exposed surface containing an operative catalyst throughout the life of the glass-ceramic coating on the metal

Except as modified to include the featured catalyst, it is contemplated that the glass-ceramic coating materials described in my prior application, as well as their methods of preparation and application, will be used in practicing the present invention Accordingly, that application, and its disclosures, are incorporated in toto by reference

The composition, as well as the physical properties, of the glass-ceramic employed will depend on the particular application involved For example, any element known to poison, or otherwise be detrimental to, a particular process, or to the present catalyst, should be avoided Also, the glass-ceramic must not soften, recrystallize, or otherwise undergo detrimental change, at the maximum temperature of the process in which it is used

As initially applied to the metal, the coating is a flowable material composed essentially of a precursor glass for the glass-ceramic The glass is in particulate form, and preferably embodies the catalyst The coated metal is then heated to a temperature at which the glass flows and wets the metal surface During this heating, and prior to complete ceramming, the glass must become sufficiently fluid to form a continuous, essentially non-porous coating The coated metal is then held at this temperature, or at a somewhat lower temperature depending on the glass, for a time sufficient to permit ceramming, that is, uniform crystallization of the glass The maximum temperature reached in this procedure must be well below that at which the metal undergoes structural modification or other changes Another consideration is a reasonable match in coefficient of thermal expansion (CTE) between the glass-ceramic and the metal which it coats It is this consideration, as well as a high operating temperature, that generally precludes use of a glass as such The CTE is particularly important where austenitic-type metals are employed, since these metals tend to have high CTEs on the order of 180x10^"7/ C In such case, a relatively high silica content is desirable This provides a cnstobalite crystal phase, the inversion of which creates an effective CTE that provides an adequate expansion match The presence of alumina in the composition is beneficial to increase glass flow and surface wetting prior to crystallization of the frit However, it may inhibit cπstobalite formation as the frit crystallizes

Where the feedstock is diluted with another material, the coating must be unaffected by the diluent For example, hydrocarbon cracking is usually carried out in the presence of steam In that case, the coating must not interact with the steam, either physically or chemically

In summary, a glass-ceramic suited to present purposes should exhibit these characteristic features 1 Have a composition free from elements detrimental to a thermal cracking process

2 Capable of withstanding an operating temperature of at least 850° C without undergoing detrimental physical or chemical change

3 Thermal expansion characteristics compatible with austenitic-type metals

4 Have processing temperatures below a temperature at which the coated metal undergoes change

5 Form an adherent, continuous, essentially non-porous coating Any glass-ceramic material that meets these several conditions may be employed The alkaline earth metal borates and borosi cates and alkaline earth metal silicates are particularly suitable In general, based on properties, alkali metal silicates and aluminosilicates are less suitable due to physical and/or chemical incompatibility, including low coefficients of thermal expansion For use in a hydrocarbon thermal cracking process our preferred coating is a barium aluminosilicate or strontium-nickel aluminosilicate glass-ceramic

The barium aluminosilicate will have primary crystal phases of sanbornite and cnstobalite, a minor phase of BaAI₂Sι₂O₈, and will contain 20-65% BaO, 25- 65% SιO₂ and up to 15% Al₂0₃ The strontium-nickel aluminosilicate will contain primary crystal phases of SrSιO₃ and Nι₂SιO₄, a minor phase of cnstobalite and will contain 20-60% SrO, 30-70% Sι0₂, up to 15% Al₂0₃ and up to 25% NiO Glass-ceramics having compositions 14 and 12, respectively, in TABLE I are presently preferred TABLE I sets forth, in weight percent on an oxide basis as calculated from the precursor glass batch, the compositions for several different glass- ceramics having properties that adapt them to use for present purposes. Examples 1-6 illustrate alkaline earth metal alumino borates or borosilicates. Examples 7-14 illustrate alkaline earth metal silicates which may contain minor amounts of alumina or zirconia.

TABLE I Ex. Si0₂ B₂0₃ Al₂0₃ BaO MgO CaO ZnO Zr0₂ MnO SrO NiO F

1 - 19.1 27.9 42.0 1 1.0 - - - - -

2 - 25.4 18.6 56.0 - - - - - -

3 17.5 20.2 29.7 - - 32.6 - - - -

4 9.6 22.2 32.5 - - 35.8 - - - -

5 30.6 12.7 3.8 15.9 23.5 - 13.5 - - -

6 - 27.0 19.8 29.7 7.8 - 15.8 - - -

7 32.0 - - 40.9 - - - 8.2 18.9 -

8 33.9 - 2.9 43.3 - - - - 20.0 -

9 33.2 4.8 - 42.4 - - - - 19.6 -

10 65.0 - 6.9 - - - - - - 28.1

11 47.2 - - - - - - 12.1 - 40.7

12 54.1 - 5.7 - - - - - - 23.3 16.8

13 38.3 - - - - - - 5.9 22.7 33.1

14 62.7 - 5.3 31.4 - - - - - 0.6

The invention is further described with reference to the base glass composition of Example 14 in TABLE I. Six glass batches, each containing a different catalyst, were mixed and melted. In each batch, the catalyst was added as the chloride (Cl) or oxide (ox) in an amount equal to 0.01 % by weight of the total batch. The glass batches were melted 16 hours at 1600° C. in platinum crucibles, and the melts rolled in the form of thin ribbons. The ribbons were fragmented, and the fragments ball-milled to a powder having a mean particle size of 6-8 microns. This provided self nucleation when the powders were cerammed. The glass powders were placed in silica crucibles for ceramming. The ceramming cycle was: heat to 1150° C; hold 10 minutes to soften the glass; cool to 1050° C; hold for four (4) hours to ceram the glass; cool to ambient. The resulting solid masses were chipped from the crucibles, and the fragments reduced by milling to granules passing through a 40-mesh sieve but retained by a 60-mesh sieve Granules were used to increase the surface/volume ratio to produce more carbon for a given quantity of glass- ceramic Carbon was deposited on the granules using a coke-deposition furnace consisting of a quartz tube furnace through which a mixture of steam, argon, and ethane were passed Samples comprising about 1 cm³ of granules were maintained in the furnace at 850° C for four (4) hours to form a uniform deposit of carbon on them The effectiveness of each catalyst was determined by a carbon burn-off test Each carbon-coated, glass granule sample was slowly heated in air at 5° C /minute to a temperature of 1000° C The test was designed to determine the temperature at which oxidation of carbon was initiated This was considered the necessary temperature for burn-off of carbon deposits in a processing apparatus

The carbon burn-off was quantified by thermogravimetπc analysis (TGA) A known mass of sample was slowly heated in air and its mass was continuously determined Data were normalized to the initial mass of the sample A decrease in mass shows that carbon has burned off of the specimen The temperature at which the mass starts to decrease is the temperature at which burn-off of the deposited coke has started The results are shown in TABLE II, below TABLE II shows the catalyst added, the form in which it was added (Cl or ox), the temperature at which carbon burn-off was initially observed TABLE II

CATALYST FORM TGA (° C.)

Au Cl 525

Pt Cl 525

Pd Cl 525

Ir Cl 400

Rh ox, Cl 450, 525

Ru ox 475

FIGURE 3 is a graphical representation showing carbon burn-off curves for various catalysts The curves represent measurements of temperature (T ° C.) versus normalized mass of the test sample by TGA. Mass is plotted in weight percent on the vertical axis. Temperature is plotted on the horizontal axis.

The burn-off curves for each catalyst are identified as follows: Curve A for iridium; Curve B for ruthenium; Curve C for rhodium as the oxide; Curve D for rhodium as the chloride; Curve E for gold, platinum and palladium. Inspection of the curves shows that the addition of Ir has been particularly effective at catalyzing the burn-off of the carbon. The carbon on the Ir-doped sample begins to burn off around 400° C, while that on the Au, Pt and Pd samples does not begin to burn off until around 525 ° C. Also effective, although less so than Ir, is Ru doping, which shifts burn-off initiation to around 475° C. The relatively small amount of carbon on the two Rh-doped samples may result from burn-off during deposition.

The effectiveness of Rh doping depends on the manner in which the Rh is added. Rh added as the chloride has little effect on burn-off initiation temperature, while Rh added as the oxide is effective in reducing the temperature at which burn-off starts to around 450° C.

In summary, iridium is very effective at low levels in reducing the temperature at which carbon burns off of the glass-ceramic coating. Ruthenium and rhodium are also effective, although less so than Ir at the levels employed here. Au, Pt, and Pd seem ineffective at catalyzing carbon burn-off.

Claims

I CLAIM

1 A coated furnace component comprising a metal substrate and a layer of glass-ceramic that is adherent to a surface on the metal substrate and that is exposed to carbon, at least a portion of the exposed glass-ceramic layer containing a catalyst that promotes a carbon reaction

2 A coated component in accordance with claim 1 wherein the catalyst is an oxidation catalyst

3 A coated component in accordance with claim 2 wherein the oxidation catalyst contains an element selected from the group composed of indium, rhodium and ruthenium

4 A coated component in accordance with claim 3 wherein the selected element is indium

5 A coated component in accordance with claim 1 wherein the catalyst is dispersed throughout the glass-ceramic coating

6 A coated component in accordance with claim 1 wherein the layer of glass-ceramic material consists of a first portion that adheres to the surface of the metal substrate and that contains no added catalyst, and a second portion that overlays the first portion and that does contain added catalyst

7 A component for a furnace for cracking or reforming a stream of hydrocarbons, the component comprising a metal member having a surface facing the hydrocarbon stream, that metal surface having an adherent coating of a glass-ceramic, the glass-ceramic coating containing, on its surface or in a surface layer, a catalyst that promotes a carbon reaction

8. A component in accordance with claim 7 having a tubular form with the glass-ceramic coating being on the interior surface of the tube.

9. A component in accordance with claim 7 wherein the catalyst promotes a reaction that oxidizes carbon to a gas.

10. A component in accordance with claim 7 wherein the catalyst contains an element selected from the group composed of iridium, rhodium and ruthenium.

11. A component in accordance with claim 7 wherein the catalyst is dispersed throughout the glass-ceramic coating.

12. A component in accordance with claim 7 wherein the glass-ceramic coating consists of a first portion that adheres to the component surface and that contains no added catalyst, and a second portion that overlays the first portion and that does contain added catalyst.

13. A coating material comprising a mixture of a crystallizable glass frit in a vehicle of such viscosity that it will form, on a metal surface, an adherent, continuous, coating that does not flow when applied to the under surface of the metal, the coating material containing a catalyst that promotes a carbon reaction.

14. A coating material in accordance with claim 13 wherein the catalyst promotes a reaction that oxidizes carbon to a gas.

15. A coating material in accordance with claim 13 wherein the catalyst contains an element selected from the group composed of iridium, rhodium and ruthenium.

16. A coating material in accordance with claim 13 wherein the catalyst is a part of the glass frit composition.

17. A method of removing carbon that deposits on a metal surface when that surface is exposed, while heated, to a gaseous stream containing a source of carbon particles, the method comprising isolating the metal surface from the gaseous stream containing a source of carbon particles with a thin, adherent coating of a glass-ceramic material on the metal surface prior to contacting that surface with a hot gaseous stream, and doping at least the exposed portion of the glass-ceramic with a catalyst that promotes a carbon reaction.

18. A method in accordance with claim 17 which comprises isolating the metal surface with a glass-ceramic coating that is doped with an oxidation catalyst.

19. A method in accordance with claim 18 which comprises isolating the metal surface with a glass-ceramic coating containing on or within its surface a catalyst containing an element selected from the group consisting of iridium, rhodium and ruthenium.

20. A method in accordance with claim 19 which comprises isolating the metal surface with a glass-ceramic coating containing rhodium.

21. A method in accordance with claim 17 which comprises isolating the metal surface with a glass-ceramic coating having the catalyst dispersed throughout the coating.

22. A method in accordance with claim 17 which comprises isolating the metal surface with a coating of glass-ceramic material that is formed by applying a glass frit to the metal surface, heating the metal to cause the glass to adhere to the metal surface and to crystallize to a glass-ceramic state in situ. 23 A method in accordance with claim 22 which comprises applying the glass frit to the metal surface by mixing the frit with a vehicle to form a slurry and applying that slurry to the interior surface of an element in a thermal cracking furnace

24 A method in accordance with claim 23 which comprises incorporating the catalyst in the batch for the glass frit and melting the batch

25 A method in accordance with claim 17 which comprises isolating the metal surface with a first glass-ceramic coating which does not contain added catalyst, and applying over that first coating a second glass-ceramic coating that does contain a catalyst