EP0253633A2

EP0253633A2 - Furnace and process for hydrocarbon cracking

Info

Publication number: EP0253633A2
Application number: EP87306227A
Authority: EP
Inventors: Peter Hubertus Kosters; Nico Jan Vossenberg
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1986-07-15
Filing date: 1987-07-14
Publication date: 1988-01-20
Anticipated expiration: 2007-07-14
Also published as: GB8617214D0; DE3782874T2; JPS6338888A; EP0253633A3; ES2036211T3; EP0253633B1; DE3782874D1

Abstract

A heat exchanger, particularly for use in a thermal cracking furnace for hydrocarbons, comprises a pair of coaxial metal tubes (12, 14) defining an inner duct (11), and a ceramic block (13) defining an outer duct (10) coaxially disposed about the inner duct for substantially radiative heat transfer between the outer duct and the inner duct.

The surface area per unit length of the outer wall of the outer duct increases continuously along at least a portion of the length of the outer duct, and the cross sectional area of the outer duct (10) reduces along its length, to provide an inwardly directed radiative heat flux from the outer wall which varies along the said portion of the length of the outer duct. The heat flux through the heat exchanger can thereby be maximised and short residence times achieved.

The outer wall of the outer duct is formed of a ceramic which may be cast monolithically and has inwardly projecting ribs having a cross sectional area which increases continuously along the said portion of the length of the outer duct.

Description

This invention relates in a first aspect to heat exchangers, and in particular to heat exchangers for use in thermal cracking furnaces. In a second aspect, the invention relates to specific hydrocarbon cracking furnaces, and to methods for making hydrocarbon cracking furnaces.
Thermal cracking of hydrocarbons is a technique which is practised widely in the hydrocarbon industry, and numerous configurations of thermal cracking furnaces have been proposed. For example, European Patent Specification No. 0074435 discloses a form of thermal cracking furnace in which a mixture of hydrocarbon and superheated steam is caused to flow through a reactor conduit, cocurrent with a flow of a heating gas surrounding the conduit.
As is disclosed in European Patent Specification 0074435, certain characteristics are particularly desirable for the thermal cracking of hydrocarbons. In particular, high reaction temperatures, facilitating short residence times will in general increase the yield of the desired products of thermal cracking, whilst minimizing the production of by-products.
It is therefore desirable that a thermal cracking reactor should be as short as possible, but should nevertheless provide for the maximum heat transfer over its length.
U.S. Patent No. 4412975 discloses a form of thermal cracking reactor in which a tube containing the hydrocarbon to be cracked passes through a radiant enclosure, in which the tube is heated by radiation from the furnace walls. This system, like that disclosed in European Patent Specification 0074435, suffers from the disadvantage that because the flue gases cool as they pass through the furnace, the temperature of the heat transfer surface of the heat exchanger varies along the length of the furnace. Thus, it is not possible to maintain the heat transfer surface at the optimum temperature for maximum heat transfer, along the whole of its length.
The factor which limits the heat transfer characteristics of a tubular cracking reactor, of the kind disclosed in EP 0074434 and US 4412975 will in general be the thermal failure temperature of the tubular reactor, which will generally be metallic. Thus, in the furnace of Figure 2 of US 4412975, if the reactor tube is close to its thermal failure temperature in the region adjacent the burners, it will be at a temperature substantially lower than this at a point downstream of the burners.
U.S. 4412975 discloses various attempts to overcome this difficulty, by means of back-mixing of flue gases, and (in the discussion of prior art), the provision of multiple burners. Such attempts have met with only limited success, because a substantial temperature gradient along the length of the reactor still results, and the temperature of the existing flue gases is relatively high, leading to poor fuel economy.
In accordance with a first aspect of the present invention, a radiative heat exchanger, for example for use in a thermal cracking furnace comprises means defining an inner duct having means defining an outer wall of an outer duct disposed thereabout, preferably coaxially therewith, for substantially radiative heat transfer between the said outer wall and the inner duct, wherein the cross section of the outer duct varies along at least a portion of its length in such a manner as to provide in use an inwardly directed radiative heat flux from the said outer wall which varies along the said portion of the length of the outer duct, in such a way as to tend to compensate for fall in temperatures in fluid flowing in the outer duct.
The variation in cross-section of the outer duct may take the form of a variation (normally a decrease) in cross-sectional area of the duct, and/or an increase, preferably a continuous increase, in the surface area per unit length of the outer wall of the outer duct.
The construction is preferably such that, when flue gas from a burner at or adjacent an end of the inner duct is drawn through the outer duct, the heat transfer characteristics of the said outer wall of the outer duct are such as to cause the temperature of the surface of the outer wall of the inner duct over a portion of its length corresponding to the said portion of the outer duct to be substantially constant in use.
The said portion of the length of the duct over which the cross-section varies is preferably at least one quarter, more preferably at least one half, more preferably still at least three quarters, and most preferably substantially all of the length of the duct.
The change in cross section of the outer duct tends to compensate for the fall in temperature of the flue gases as they flow through the outer duct. Thus, the temperature of the flue gases may fall along the duct in a typical embodiment from about 2000°C at the burner end to about 1500°C at the exhaust end. By varying the outer duct cross section as indicated, it is possible to maintain the outer wall temperature sufficiently high to provide the desired level of radiative heating to the inner duct. Typically, the temperature of the outer duct wall in a cracking furnace in accordance with the invention may vary from 1600°C at the burner end to 1200°C at the exhaust end, preferably from 1500°C at the burner end to 1450°C at the exhaust end.
The particular temperature utilised will vary from installation to installation, depending not only on the geometry of the system, but also on the cracking reaction which it is desired to carry out.
The particular temperature profile utilised for the outer wall of the outer duct is preferably chosen so as to maintain the temperature of the wall of the inner duct which is in contact with the material to be cracked at at least 950°C, over a substantial part of its length.
The temperature of the inner wall of the outer duct is preferably as high as possible, typically greater than 1070°C, and as high as material limitations permit, over a substantial part of its length.
Clearly some heat will be transferred to the inner duct by radiative and convective transfer from the flue gases. In a typical embodiment however, at least 60%, preferably at least 75% of the heat transfer to the inner duct will be by radiative transfer from the outer duct wall.
The surface area per unit length may be increased by providing on the inner surface of the outer duct a plurality of ribs, having a cross sectional area which increases continuously along the said portion of the outer duct.
The increase in surface area over the said region may preferably be continuous, for example by providing ribs having a cross sectional area which increases continuously, as described above, However, the increase in surface area may, less preferably, be achieved by means of a number of small stepwise changes in the shape of the cross section of the outer duct, for example by constructing the outer duct from a plurality of blocks, joined along the length of the duct, each having a slightly increased internal area per unit length.
The cross sectional area of the outer duct preferably decreases over its length or preferably at least over the said portion of its length. This has the effect of increasing the velocity of the gas flow in the space between the inner and outer ducts, and thus increasing convective heat transfer between the gas and the wall of the outer duct. The said decrease in cross sectional area may be achieved by providing the said ribs of increasing size as described above. Alternatively, or additionally, the decrease in cross sectional area may result from a stepwise or continuous decrease in diameter of the outer duct.
The inner duct may comprise a pair of coaxial tubes, the inner of the two coaxial tubes supplying the fluid to be heated, for example a mixture of steam and a hydrocarbon to be cracked, and the space between the inner and outer tubes constituting the region in which the substantial heating of the fluid takes place, and in the case of the cracking furnace, in which the cracking takes place.
This arrangement is particularly advantageous, because radiative heat transfer from the outer tube wall (i.e. the outer surface of the inner duct) to the inner tube wall takes place very readily through the fluid to be heated (e.g. the steam/hydrocarbon mixture). Thus, the inner tube surface is heated to a temperature in excess of that of the surrounding fluid. Not only does this serve to pre-heat the incoming fluid in the inner tube, it also provides additional heating to fluid in the space between the inner and outer tubes by radiative transfer from the inner tube.
In accordance with this first aspect of the invention, the outer surface of the inner duct (i.e. in the embodiment just described, the internal surface of the outer of the two coaxial tubes) which is responsible for heat exchange with fluid in the inner duct can be maintained at a substantially constant temperature, along substantially the whole of its lenght, even through the temperature of the fluid within it is increasing, and the temperature of the surrounding flue gas is decreasing, by providing a continuous change in the heat exchange characteristics of the outer duct along its length as referred to above. Furthermore, the outer wall of the inner duct, which is generally a metallic tube, can be operated very close to its thermal failure temperature, along substantially the whole of its length, thus providing the maximum heat transfer.
The means defining the outer surface of the outer duct is preferably formed of a ceramic material (which term as used herein includes within its scope refractory material of various kinds capable of withstanding the high temperature involved), since the temperature of operation of the outer duct will in general be substantially higher than that of the inner duct. Raised ribs may be provided along the whole or part of the length of the outer wall of the outer duct, so as to provide the desired variation in surface area and in cross-section. Both the size and number of such ribs may be increased, over the length of the outer duct.
The outer surface of the duct is preferably formed by moulding a ceramic material, for example by moulding a ceramic material around a disposable former, having the desired number of ribs on the outer surface thereof. The disposable former may be made of a foamed plastics material, for example polystyrene. After the ceramic has been allowed to set, the disposable former may be removed, by dissolution with a solvent, or, more preferably, simply by firing the ceramic to a temperature at which the plastics material is pyrolysed.
Accordingly, in a second aspect of the invention, there is provided a method of forming a radiative heat exchanger, which method comprises casting a ceramic material around at least one elongate disposable former, allowing the ceramic material to set, and destroying the former, to leave a ceramic block having at least one elongate passageway therethrough, and providing in the said at least one passageway a heat exchange tube, in such a manner as to define a gas duct in the passageway around the heat exchange tube. Several formers are preferably utilised in a single casting operation, to provide a plurality of elongate passageways. In one embodiment of this aspect of the invention, the ceramic casting is from 7.5 to 10 m in length. This method of construction results in low materials cost, and low fabrication costs, and can provide a structure which is substantially stronger with regard to thermal stresses, as compared with previous constructions.
In an alternative and preferred method, a ceramic material may be cast by a compression moulding technique, for example using a metallic mould. When this method is employed, it is generally necessary to mould the ceramic material defining the outer duct in a number of longitudinal sections, having a maximum length of, for example 1 m or less.
The ceramic material is preferably provided with a plurality of elongate longitudinal grooves extending along the radiative surface thereof. Such elongate grooves have been found to be of substantial benefit in maximising resistance of the ceramic material to thermal shock. The elongate grooves may be for example about 1 cm in depth and 0.5 to 1 mm in width. They may conveniently be formed by drawing toothed comb-like members through the ceramic material before the material is fired.
The means defining the inner duct of the heat exchanger in accordance with the invention preferably comprises a pair of coaxial tubes, the inner tube being adapted to supply a heat exchange fluid to the space defined between the inner and outer tubes defining the inner duct, where heat exchange takes place. The two coaxial tubes will generally communicate at one end thereof, that end being located in the region of the heat exchanger occupied by the hottest flue gases. With this arrangement, there is no necessity for the supply of heat exchange fluid to the inner duct to pass through a burner chamber, in which spatial temperature fluctuations may occur. Such temperature fluctuations would again result in the inability to operate the heat exchanger at the limit of temperature of the heat exchange surface of the inner duct.
Accordingly, in a further aspect of the invention, there is provided a hydrocarbon cracking furnace, comprising at least one cracking reactor having a first tube, connected to a feedstock inlet, and disposed within, preferably coaxially within, a second tube, connected to an outlet for cracked product, the said first and second tubes communicating at a first end of the reactor, a burner disposed proximate the said first end of the reactor, and adapted to generate a flow of hot flue gas around the second tube, in a direction co-current with the flow of feedstock in the second tube, the said first end of the reactor being disposed in the flow of flue gases from the burner, and the reactor extending downstream with respect to the flow of flue gases from the said first end.
The particular arrangement of the hydrocarbon cracking furnace in accordance with the invention is advantageous in providing optimum temperature distribution of the reactor tubes, without local "hot spots". The cracking furnace in accordance with the said further aspect of the invention may preferably, but not essentially, include a heat exchanger in accordance with the first aspect of the invention. Similarly, although the heat exchanger in accordance with the invention is particularly suitable for use in a thermal cracking furnace, it will be appreciated that it could be used in any situation where maximum heat transfer, coupled with minimum residence times of at least one of the heat exchange fluids is required.
Likewise, the method of constructing a heat exchanger in accordance with the second aspect of the invention is particularly suited to the construction of a heat exchanger in accordance with the first aspect of the invention, for use in a thermal cracking furnace. It may, however, be employed with advantages in the construction of other forms of heat exchanger.
The thermal cracking furnace in accordance with the invention preferably includes a number of heat exchange elements arranged in a parallel fashion. Each such heat exchange tube may be provided within a generally hexagonal ceramic block, having a radiative surface area which varies continuously along a portion of its length. Such hexangonal blocks fit conveniently together, and provide the greatest structural integrity, for minimum weight. The heat exchangers may be provided in an array in which tubes are vertically staggered, to facilitate the supply of feedstock to the various heat exchange sections.
It is a particularly advantageous feature of a heat exchanger in accordance with the invention, having a plurality of flue gas ducts in cast in a ceramic block, that a thermal cracking furnace can be produced which is readily adaptable to the cracking of alternative feedstocks. Different feed stocks will, in general, require different thermal cracking temperatures, and heats of reaction. In a heat exchanger of the kind having raised ribs of varying cross-section, the heat exchange temperatures of the inner duct through which the feed stock is passed can be controlled, by appropriate choice of the size and number of ribs formed on the ceramic material. Accordingly, different ceramic blocks, having different heat exchange characteristics, may be exchanged, within the same thermal cracking furnace, so as to accommodate different feedstocks.
Furthermore the design of the thermal cracking furnace in accordance with the invention is such that an existing furnace may be very easily converted into a furnace in accordance with the invention, simply by providing a ceramic block and tube structure therein.
At least one additional heat exchanger is preferably incorporated to provide rapid cooling of the thermally cracked mateial, on its exit from the furnace area. Rapid cooling in this way minimizes the production of unwanted by-products. In a preferred embodiment, the additional heat exchanger may be of the co-axial type described above, with high pressure water being supplied to the inner tube, whereby high pressure steam is generated in the outer tube.
A number of embodiments of the various aspects of the invention will now be described with reference to the acommpanying drawings in which:-

Figure 1 is a schematic diagram of a thermal cracking furnace incorporating a heat exchanger in accordance with the invention,
Figure 2 is an enlarged view of one embodiment of an inner duct, and
Figure 3 is schematic sectional end elevation in the direction of arrows 3-3 of part of the furnace of Figure 1.

Referring first to Figure 1 and 2, a furnace for the thermal cracking of hydrocarbons comprises a casing 1, containing an inner layer 2 of an insulating material, which may for example be a ceramic material. A burner 3 is located at one end of the casing 1, and is adapted to burn a suitable hydrocarbon feed material, supplied through line 4. Combustion air for the burner 3 is supplied by a pump 7 through a conduit 6a and is pre-heated in heat exchangers 18a and 18b, and delivered to the vicinity of burner 3 via conduit 6b. Additional heating of this air can be achieved, if desired, by causing it to flow between casing 1 and insulating material 2 en route to burner 3. The hydrocarbon fuel supplied by line 4 may if desired also be heated before passage to burner 3.
The furnace includes a block of eight heat exchangers, of which two are indicated generally at 8 in Figure 1 and which are illustrated in more detail in Figures 2 and 3. Heat exchangers 8 comprises a ceramic block 9, having disposed therein a duct, again shown schematically at 10 in Figure 1, and in more detail in Figure 2 defined between block 9 and a tube 12, described in more detail below. In practice, a large number of ducts, for example six or eight will typically be utilised.
Figure 3 is a schematic view in the direction of arrows 3-3 in Figure 1, showing the construction of the outer duct of heat exchangers 8 from a plurality of generally hexagonal ceramic blocks 9a to 9h. Blocks 9a to 9h in Figure 3 illustrate the progressive change in cross section of the hexagonal ceramic blocks along their lenght, as will be described in more detail hereinafter.
Disposed within ducts 10 are inner ducts 11, having three concentric tubes, an outer tube 12, the outer surface of which defines a heat exchange surface with flue gases produced by burner 3, an intermediate tube 13 for supplying the diluted feed stock, and an inner tube 14 for supplying high temperature steam diluent. In Figure 3, ducts 10 are present in each of blocks 9a to 9h, but are illustrated only in block 9a for clarity. The various tubes 12, 13 and 14 meet at end 15 of tube 10.
Flue gases from burner 3 pass through ducts 10 and exchange heat with ceramic block 9. Heat exchange with tubes 12 is primarily by radiation from the inner surface of ceramic blocks 9. After passage over tubes 12, flue gases from burner 3 are directed through heat exchange tower 20. Heat exchange tower 20 includes various heat exchangers, of conventional form, to preheat the hydrocarbon feed, for air preheat, and to preheat steam, for dilution of the hydrocarbon feed. Heat exchange tower 20 may also be used to preheat the fuel for burner 3, as described above.
Specifically, heat exchange tower 20 includes heat exchangers 18a and 18b, for pre-heating the combustion air, as described above. Tower 7 also includes heat exchangers 22a, 22b and 22c for pre-heating the feed stock to be cracked, fed through line 23. High temperature steam is supplied through line 25, via lines 25a and 25b to heat exchanger 26a, and heat exchanger 26b. The high temperature steam produced in heat exchangers 26a and 26b is fed, together with the heated feedstock, to ducts 11. Specifically, (referring to Figure 2) the feed stock is supplied by pipe 14, and super heated steam via pipe 13. When the furnace is used for the thermal cracking of naphtha, the naphtha feed stock is typically supplied to tube 14 at a temperature of approximately 620°C, and super heated steam to pipe 13 at a temperature of approximately 1100°C.
Heat exchange tower 20 also includes an additional heat exchange element 28, for preheating boiler feed water, for use in heat exchangers 17.
The structure of the inner ducts 11 is described in more detail, with reference to Figure 2. Figure 2 illustrates an inner duct of the kind shown in Figure 1 in which the hydrocarbon to be cracked, for example naphtha, is supplied via conduit 30 to inner tube 14. Preferably a relatively low amount of dilution steam is added to the hydrocarbon in the convection section. For example the steam dilution ratios employed for the cracking of LPG, naphtha and gasoil are preferably (by weight) 0.3 to 0.6, 0.4 to 0.8, and 0.6 to 1.0 by weight respectively, more preferably about 0.4, 0.5 and 0.8 respectively. Superheated steam is supplied via conduit 31 to the space defined between tubes 14 and 13. Orifices (not shown) may be provided along the length of tube 14, to enable the naphtha feedstock to mix with superheated steam supplied through conduit 31. Whilst contained within the tube 13, the hydrocarbon feed and superheated steam mixture is at a relatively low temperature, and insufficient for substantial thermal cracking to take place. At the end 15 of the duct 11, the hydrocarbon feed/steam mixture passes into the space defined by tube 13, and outer tube 12. Here, heat exchange takes place with the metallic surface of the outer tube 12. Because of the increasing surface area of the inner surface of blocks 9, the temperature of the outer surface of pipe 12 remains substantially constant over that portion of its length for which the radiative area per unit length increases.
The duct 10 in which the tube 12 is located is provided on its inner surface with inwardly projecting ribs as illustrated in more detail with reference to Figure 3.
Figure 3 is a schematic diagram, showing the change in the internal cross-section of the blocks 9 along their length. Four general types of block are illustrated in Figure 3, and although at any given cross-sectional point along their length, all the ducts 10 will have the same cross-section, the various blocks 9a to 9h of Figure 3 illustrate the cross-section of blocks 9 at different points along their length. Thus, blocks 9c, 9d, 9e, and 9h illustrate the cross-section employed for blocks 9 used at the end of the heat exchanger adjacent the burner 3. Along this section of their length, the blocks 9 have a substantially constant cross-section, over a length of approximately 3 metres. Only 3 large support legs, 36, 37, and 38 are provided in the blocks 9, to support and locate tubes 12. Over the next adjacent section of approximately 3 metres of duct 10, small teeth 40 are provided, between the main support ribs 36, 37, and 38, as shown in the blocks illustrated by references 9f and 9e in Figure 3. The size of these support ribs increases continuously over the central section of duct 10, such that, at a distance of 6 metres from burner 3, ribs 40 have a height of approximately 2.5 cm. The overall diameter of duct 10 decreases from approximately 34 cm adjacent burner 3, to 27 cm, at a distance of 6 metres from burner 3. Block 9a illustrates the cross-section of blocks 9 at their end remote from burner 3, a distance of approximately 9 metres. At this point, the diameter of duct 10 is approximately 27 cm, and the height of the teeth 40, approximately 32 cm. Both the change in height of teeth 40, and the change in overall diameter of duct 10, are substantially continuous over at least a portion of the length of duct 10.
Additional ceramic blocks 41 are provided to support the shaped blocks 9a to 9h.
Also illustrated in Figure 1 are various other lines, vessels, and heat exchangers, for example steam drum 45, and transfer line exchanger 46, of a kind conventionally employed in thermal cracking, the function of which need not be explained in detail.
The thermal cracking furnace disclosed above has a number of substantial advantages as compared with conventional thermal cracking furnaces. In conventional furnaces, heat exchange tubes are placed within a large fireblock, and low residence times are achieved by using several small heat exchange tubes, rather than a single large one, so that the surface area to volume ratio increases. However, the use of such heat exchangers, spaced throughout a fireblock, makes is very difficult to obtain even distribution of heat over the pipe surfaces, particularly in view of their small size. This leads to coking rates which are different for the various heat exchangers, and consequently substantial pressure drops.
In the furnace described above, each heat exchanger has its own feedstock supply, and therefore the flow and pressure drop across each heat exchanger and can be controlled independently.
Clearly, various other embodiments of the invention are possible, other than those specifically described above, within the scope of the appended claims.
Particularly, in an alternative embodiment of inner duct 10a, (not shown) hydrocarbon feedstock and superheated steam are supplied in a single inner lumen.
We have determined that, with the embodiment illustrated, it is possible because of the excellent temperature profile of the reactor, to carry out thermal cracking with reactor inner wall temperature as high as 950°C or more, without thermal failure of the reactor over extended periods. This is significant because at high temperatures such as these, the rate of the chemical reaction which decomposes carbon build-up in the reactor (the so-called "shift reaction") becomes greater than the rate of the chemical reactions which lead to the build-up of carbon. Because the reactor can operate at a temperature above this critical one at which the shift reaction is faster than the carbon formation reaction, it can be operated for long periods without substantial coke formation.
Furthermore the particular design of the cracking furnace in accordance with the invention enables a substantially reduced overall furnace volume to be achieved for a given throughput. For example a throughput which would require a furnace having a volume of 300 m³ with conventional designs can typically be achieved utilising a furnace in accordance with the invention having a volume of about 25 m³.

Claims

1. A heat exchanger, comprising means (12) defining an inner duct (11) having means (13) defining an outer wall of an outer duct (10) disposed thereabout for substantially radiative heat transfer between the said outer wall and the inner duct (11), wherein the cross-section of the outer duct (10) varies along at least a portion of its length in such a manner as to provide in use an inwardly directed radiative heat flux from the said outer wall which varies along the said portion of the length of the outer duct in such a way as to tend to compensate for fall in temperature in fluid flowing in the outer duct (10).

2. A heat exchanger as claimed in claim 1, wherein the means (13) defining the said outer wall of the outer duct is provided with a plurality of inwardly projecting ribs (30, 40) having a cross sectional area which increases continuously along the said portion of the length of the outer duct (10).

3. A heat exchanger as claimed in Claim 1 or Claim 2, wherein the means (12) defining the inner duct (11) comprises a pair of coaxial tubes (12, 13), wherein the innermost (13) of the two coaxial tubes is adapted to supply a first heat exchange fluid to a space (15) defined between the said inner and outer tubes.

4. A heat exchanger as claimed in any one of the preceding claims said means (13) defining the outer wall of the outer duct comprises a ceramic block.

5. A heat exchanger as claimed in any one of the preceding claims wherein the means (12) defining the inner duct comprises a metallic tube disposed within the outer duct (10).

6. A heat exchanger as claimed in any one of the preceding claims wherein the cross-sectional area of the outer duct (10) decreases over the said portion of its length.

7. A hydrocarbon cracking furnace, comprising at least one cracking reactor having a first tube (14), connected to a feedstock inlet, and disposed within a second tube (12), connected to an outlet for cracked product, the said first and second tubes communicating at a first end (15) of the reactor, a burner (3) disposed proximate the said first end of the reactor, and adapted to generate a flow of hot flue gas around the second tube, in a direction co-current with the flow of feedstock in a duct (11) defined between the first and the second tube, the said first end (15) of the reactor being disposed in the flow of flue gases from the burner (3), and the reactor extending downstream with respect to the flow of flue gases from the said first end.

8. A furnace as claimed in claim 7, wherein the said first and second tubes are disposed within a flue gas duct (10) having an outer wall, so shaped that the cross section of the flue gas duct varies along at least a portion of its length in such a manner as to provide from a heat exchanger as claimed in any one of Claims 1 to 6.

9. A hydrocarbon cracking method which comprises supplying a feed of a hydrocarbon to be cracked to the feedstock inlet (30) of a furnace as defined in Claim 7 or Claim 8, and generating from the said burner a flow of flue gas in the flue gas duct (10) to heat and thereby crack the hydrocarbon feed.

10. A method as claimed in Claim 9, wherein the furnace is operated so as to maintain the temperature of the second tube (12) substantially constant over at least the portion of its length corresponding to the said portion of the flue gas duct.

11. A method of forming a radiative heat exchanger, which method comprises casting a ceramic material (13) around at least one elongate disposable former, allowing the ceramic material to set, and destroying the former, to leave a ceramic block having at least one elongate passageway therethrough, and providing in the said at least one passageway a heat exchange tube, in such a manner as to define a gas duct in the passageway around the heat exchange tube.