JPS6338888A - Heat exchanger, method of forming heat exchanger and hydrocarbon decomposing furnace including said heat exchanger - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION In a first aspect, the present invention relates to a heat exchanger and in particular a heat exchanger for use in a pyrolysis furnace. In a second aspect, the invention relates to a particular pyrolysis furnace and method of making a pyrolysis furnace.

Pyrolysis of hydrocarbons is a widespread process in the hydrocarbon industry, and many types of pyrolysis furnaces have been proposed.

For example, European Patent Specification No. 0 074 435 discloses a pyrolysis furnace of the type in which a mixture of hydrocarbons and superheated steam flows through a reactor channel and at the same time a stream of heated gas surrounds this channel.

Certain features are particularly desirable for the pyrolysis of hydrocarbons, as described in European Patent Specification No. 0074435. In particular, high reaction temperatures that facilitate short residence times generally increase the yield of the target product of pyrolysis while minimizing the production of by-products.

Therefore, it is desirable for the pyrolysis reactor to be as short as possible, yet still provide maximum heat transfer over its length.

U.S. Pat. No. 4,412,975 describes a type of pyrolysis furnace in which the tubes containing the hydrocarbons to be cracked pass through a radiant enclosure and are heated within the radiant enclosure by radiation from the furnace walls. This system is similar to the system disclosed in European patent specification no. H has the disadvantage that it changes. Therefore, it is not possible to maintain the heat transfer surface at an optimum temperature to provide maximum heat transfer along the entire length of the heat transfer surface.

The factors that limit the maintenance of heat transfer in tubular cracking furnaces of the type disclosed in European Patent No. 00.74435 and US Pat. No. 4.4], 2,945 are generally: is the thermal breakdown temperature of Thus, in the furnace of Figure 2 of U.S. Pat. No. 4,412,975, even though the furnace tube approaches its thermal breakdown temperature in the area adjacent to the burner,
The temperature is believed to be substantially lower than the temperature downstream of the burner.

U.S. Pat. No. 4,412,975 discloses various attempts to overcome this problem using pack mixing of flue gases and (in view of the prior art) multiple burner arrangements. Such attempts have met with only limited success, as substantial temperature gradients along the length of the reactor still occur, resulting in significantly higher flue gas temperatures and poor fuel efficiency. It is from.

In a first aspect of the invention, the radiant heat exchanger, for example for use in a pyrolysis furnace, is preferably coaxial with the surroundings to enable substantial radiant heat transfer between the outer wall of the outer duct and the inner duct. an inner duct defining means having an outer duct outer wall defining means disposed in the outer duct, the cross-section of the outer duct being arranged over at least a portion of the length of the outer duct so as to form a radiant heat flux inwardly from said outer duct in use; The radiant heat flux varies along said portion of the length of the external duct to compensate for the temperature reduction of the fluid entering the external duct.

The change in the cross section of the external duct is the change in the cross section of the duct (
(usually a decrease) and/or an increase in the surface area per unit length of the outer duct wall, preferably in the form of a continuous increase.

When the flue gas from the burner at or adjacent to the inner duct end is removed through the outer duct,
Preferably, the structure is such that the temperature of the surface of the inner duct outer wall over a corresponding length of the outer duct is substantially constant during operation due to the heat transfer properties of the outer duct outer wall.

Said portion of the duct length where the cross-section changes is preferably at least 1/4, more preferably 1/4 of the duct length.
2, even more preferably at least 3/4, most preferably the full length.

The variation in the cross-section of the external duct tends to compensate for the temperature drop in the flue gas as it passes through the external duct. In a typical embodiment, the temperature of the flue gas decreases along the duct from about 2000°C at the burner end to about 1500°C at the discharge end. The above-described variation in the outer duct cross-section allows the outer wall temperature to be maintained sufficiently high to provide a desirable level of radiant heating to the inner duct. According to the present invention, the temperature of the outer duct wall of the cracking furnace is typically 1600°C at the end of the burner.
°C to 1200 °C at the discharge end, preferably from 1500 °C at the burner end to 1450 °C at the discharge end.

The particular operating temperature will vary from device to device and will depend not only on the geometry of the system, but also on the decomposition reactions that are desired to occur.

The specific temperature profile used on the external duct wall is
The temperature of the internal duct wall in contact with the material to be decomposed is selected to be maintained at at least 950° C. over a substantial portion of its length.

Preferably, the temperature of the inner wall of the external duct is as high as possible over a substantial portion of the external duct, typically above 1070°C, as high as material limitations permit.

Some heat is clearly transferred from the flue gases to the internal ducts by radiant and convective heat transfer. However, in typical embodiments, at least 60% of the heat transfer to the internal ducts;
Preferably at least 75% is due to radiant heat transfer from the external duct wall.

The surface area per unit length is given by the internal surface of the external duct, which has a cross-sectional area that increases continuously along said part of the external duct.
This can be increased by providing multiple ribs.

Preferably, the increase in surface area over said portion is continuous, for example by providing ribs of continuously increasing cross-sectional area as described above.

However, for example, by constructing an external duct by combining multiple blocks, each with a slightly increased internal area per unit length, along the length of the duct, the cross-sectional shape of the external duct can be changed little by little. It is less desirable to make many changes to increase the surface area of the external duct.

Preferably, the cross-sectional area of the external duct decreases over its length or at least over said portion of its length. This has the effect of increasing the velocity of the gas flow in the air gap between the inner and outer ducts and enhancing the convective heat transfer between the gas and the outer duct walls. Said reduction in cross-sectional area is
This is achieved by providing said ribs increasing in size in two series. Alternatively or additionally, the cross-sectional area is reduced by stepwise or continuous reduction in the diameter of the external duct.

The inner duct consists of a pair of coaxial sleeve tubes, the inner tube of the two coaxial tubes supplies the fluid to be heated, such as a mixture of steam and hydrocarbons to be cracked, and the air gap between the inner tube and the outer tube is It constitutes the area where the fluid is substantially heated and, in the case of a cracking furnace, where the cracking takes place.

This arrangement is particularly advantageous since radiant heat transfer from the outer tube wall (ie the outer surface of the inner duct) to the inner tube is very easily carried out via the fluid to be heated (eg steam/hydrocarbon mixture). The inner tube surface is therefore heated to a higher temperature than the temperature of the surrounding fluid.

This not only serves to preheat the liquid entering the inner tube, but also provides additional heating of the fluid in the gap between the inner and outer tubes by radiant heat transfer from the inner tube.

According to this first aspect of the invention, by varying the heat exchange characteristics of the external duct continuously over its length as described above, the temperature of the fluid within the internal duct is increased;
Even though the temperature of the surrounding flue gas is reduced, the outer surface of the inner duct (i.e. the inner surface of the outer tube of the two coaxial tubes in the embodiment) that exchanges heat with the liquid in the inner duct over its entire length. can be maintained at a substantially constant temperature along the line. Additionally, the outer wall of the internal duct, which is generally a metal tube, can be operated along substantially its entire length at a temperature very close to its thermal breakdown temperature to achieve maximum heat transfer.

Since the operating temperature of the outer duct is generally substantially higher than the temperature of the inner duct, the outer surface limiting means of the outer duct is preferably formed from a ceramic material (as used herein, the term ceramic material includes within its scope (including various types of refractory materials capable of withstanding the associated high temperatures) along part or all of the length of the exterior duct exterior wall to provide the desired variation in surface area and cross-section of the exterior duct;
Raised ribs can be provided. Both the size and number of such ribs can be increased along the length of the external duct.

The outer surface of the duct is preferably formed with the desired number of ribs on the outer surface, for example by molding the ceramic material, such as by molding the ceramic material around a disposable miniature.

Disposable miniatures can be manufactured from foamed plastic materials such as polystyrene. After the ceramic has hardened, the disposable miniature can be removed by dissolving it with a solvent or, more preferably, by simply heating the ceramic to a temperature at which the plastic material pyrolyzes.

Accordingly, in a second aspect of the present invention, the steps of: injection molding a ceramic material around at least one elongate compact; curing the ceramic material; and breaking the core mold to form at least one elongate through-hole. Ceramic with channels
leaving a block; and inserting a heat exchange tube into said at least one through passage so as to confine a gas duct in the flow path around the zero heat exchange tube. Preferably, several miniatures are used to form multiple elongated channels in a single injection molding operation.

In one embodiment of this aspect of the invention, the length of the ceramic mold is between 7.5 and 10m. This method of formation has low material and manufacturing costs and can produce structures that are substantially stronger with respect to thermal stress than prior art structures.

In an alternative preferred method, the ceramic material may be formed by a compression molding method, for example using a metal mold. When using this method, it is generally necessary to form the external duct-limiting ceramic material as a number of vertical sections nailing a maximum length of, for example, 1 m or less.

Preferably, a plurality of elongated longitudinal grooves are provided extending along the radiating surface of the ceramic material. Such elongated grooves have been found to be of substantial advantage in order to maximize the resistance of the ceramic material to thermal shock. This elongated groove is, for example, approximately 1 cm deep and 0.5 to 1 mm wide. Such grooves are conveniently formed by passing a toothed comb-like element through the ceramic material before firing the ceramic material.

The inner duct defining means of the heat exchanger according to the invention consists of a pair of coaxial tubes, the inner tube supplying a heat exchange fluid into the gap defining the inner duct in which heat exchange between the inner tube and the outer tube takes place. used for The two coaxial tubes are generally in communication at one of their ends, which end is in the heat exchanger section occupied by the hottest flue gases. With this arrangement, there is no need for the heat exchange fluid feeding the internal duct to pass through the burner chamber where spatial temperature fluctuations can occur. Such temperature fluctuations also render the heat exchanger inoperable at the temperature limit of the heat exchange surface of the internal duct.

Accordingly, in another aspect of the invention, a first tube connected to the feedstock inlet and preferably coaxially disposed within a second tube communicating with the decomposition product outlet is -G; and a second tube in communication at a first end of the reactor, and a flow of hot flue gas around the second tube in a cocurrent direction with the feedstock flow in the second tube. a burner disposed proximate said first end of the reactor to flow a flue gas, said first end of the reactor being disposed within said flue gas flow from said burner; A hydrocarbon cracking furnace is provided that extends downstream with respect to flue gas flow from one end.

The particular arrangement of the hydrocarbon cracking furnace according to the invention is advantageous in providing an optimal temperature distribution in the reactor tubes without local "hot spots". The cracking furnace according to said other embodiment of the invention preferably, but not essentially, comprises a heat exchanger according to the first aspect of the invention. Similarly, the heat exchanger according to the invention is particularly suitable for use in pyrolysis furnaces, but can be used in situations where maximum heat transfer with minimum residence time of at least one heat exchange fluid is desired. One thing will be understood.

Similarly, the method of forming a heat exchanger according to the second aspect of the invention is particularly suitable for forming a heat exchanger according to the first aspect of the invention for use in a pyrolysis furnace. However, the method is also advantageously applied to form other types of heat exchangers.

The pyrolysis furnace according to the invention preferably comprises a number of heat exchange elements arranged in parallel. Each such heat exchange tube is mounted within a generally hexagonal ceramic block having a continuously varying radiant area along a portion of its length. Such polygonal blocks advantageously engage each other to provide maximum structural integrity with minimum weight. The heat exchanger is formed in a vertically offset array of tubes to facilitate the supply of feedstock to the various heat exchange sections. A particular advantage of the heat exchanger according to the invention with multiple flue gas ducts molded within a ceramic block is that it is possible to produce a pyrolysis furnace that is also easily applicable to the cracking of alternative feedstocks. It is a characteristic. Different feedstocks generally require different pyrolysis temperatures and heats of reaction. In heat exchangers of the type with raised ribs of different cross-section, the heat exchange temperature of the internal duct through which the feedstock flows can be adjusted by appropriate selection of the size and number of ribs formed on the ceramic material. . Therefore, different ceramic blocks with different pyrolysis properties can be replaced in the same pyrolysis furnace to accommodate different feedstocks.

Furthermore, the design of the pyrolysis furnace according to the present invention is such that an existing furnace can be easily converted to the furnace of the present invention by simply equipping the existing furnace with a ceramic block and tube structure.

Preferably, at least one additional heat exchanger is provided at its outlet from the furnace area for rapid cooling of the pyrolysis products. Such rapid cooling minimizes the production of undesirable by-products. In a preferred embodiment, this additional heat exchanger is of the coaxial type as described above and generates high pressure steam in the outer tube by supplying high pressure water to the inner tube.

A number of implementations of various aspects of the invention will now be described in conjunction with the accompanying drawings.

Referring first of all to FIGS. 1 and 2, a hydrocarbon pyrolysis furnace includes a casing 1 having an inner layer 2 of thermal insulation, for example a ceramic material. A burner 3 present at one end of the casing 1 is used to burn a suitable hydrocarbon feed H feed supplied from line 4. Combustion air for the burner 3 is supplied through the flow path 6a by the pump 7 and is preheated in the heat exchanger 18a and J-8b,
It is supplied near the burner 3 via the flow path 6b. If additional heating of this air is desired, it is provided by flowing it between the casing 1 and the insulation 2 on its way to the burner 3. The hydrocarbon fuel supplied by line 4 can be heated before being delivered to burner 3, if desired.

The furnace includes a block of eight heat exchangers, two of which are shown generally at 8 in FIG. 1 and described in more detail in FIGS. 2 and 3. The heat exchanger 8 consists of a ceramic block 9, within which are shown diagrammatically at 10 in FIG. 1 and in more detail in FIG.
A limited duct is placed between the This will be explained in more detail below. A large number of ducts are typically used in practice, for example six or eight.

FIG. 3 is a schematic view taken in the direction of arrows 3--3 in FIG. 1 and shows the structure of the external duct of heat exchanger 8, which generally consists of a plurality of hexagonal ceramic blocks 9a-9h. Blocks 9a-9h of FIG. 3 illustrate hexagonal ceramic blocks of progressively varying cross-section along their lengths, as will be explained in more detail below.

Inside the duct 10, there is an internal duct 1 having three concentric tubes.
1, an outer tube 12 whose outer surface defines the heat exchange surface with the flue gas from the burner 3, an intermediate tube 13 for supplying diluted feedstock and an inner tube 14 for supplying hot steam diluent; is included.

In FIG. 3, the ducts 10 are present in each of the blocks 9a to 9h, but for the sake of brevity, the ducts 10 will be explained only in the block 9a. The various tubes 12, 13 and 14 communicate at the end 15 of tube 10.

The flue gas from the burner 3 flows through the duct 10 and exchanges heat with the ceramic blocks. Heat exchange with the tube 12 is performed mainly by radiation from the inner surface of the ceramic block 9. After passing through the pipes 12, the flue gas from the burner 3 is conducted through a heat exchange column 20. heat exchange tower 2
0 houses various heat exchangers of conventional configuration to preheat the hydrocarbon feed for air preheating and preheat steam for hydrocarbon feed dilution. The heat exchange column 20 can also be used to preheat the fuel in the burner 3, as described above.

Specifically, the heat exchange column 20 includes two heat exchangers 18a and 18b for preheating combustion air. Column 2o also includes heat exchangers 22a, 22b and 22c to preheat the cracked feedstock fed from line 23.
Also includes. Hot steam is supplied from line 25 to heat exchangers 26a and 26b via lines 25a and 25b. The high temperature steam generated in the heat exchangers 28a and 26b is co-fed to the duct 11 together with the heated feedstock.

Feedstock (in conjunction with FIG. 2) is specifically supplied via line 14 and superheated steam is supplied via line 13. When using a furnace for pyrolysis of naphtha,
Typically, naphtha feedstock is fed to tube 14 at a temperature of about 620°C and superheated steam is fed to tube 13 at a temperature of about 1100°C.

The heat exchange column 20 also includes an additional heat exchange element 28 for the 0r heat of the boiler feed water used in the heat exchanger 17.

The structure of the internal duct 11 will be explained in more detail with reference to FIG.

FIG. 2 illustrates an internal duct of the type shown in FIG. 1 for supplying hydrocarbons to be cracked (e.g. naphtha) from flow path 30 to internal tube 14. FIG. Preferably, a relatively small amount of dilution steam is added to the hydrocarbon in the convection section. For example, the steam dilution ratios used for the cracking of LPG, naphtha and gasura are preferably (by weight) 0.3-0.8, 0.3-0.8, respectively.
4 to 0.8 and 0.6 to 1.0, more preferably about 0.4°0.5 and 0.8, respectively.

Superheated steam is supplied from the flow path 31 to the gap defined between the tubes 14 and 13. Orifices (not shown) are provided along the length of tube 14 to allow naphtha feedstock to mix with the superheated steam supplied from flow path 31. When the mixture of hydrocarbon feed and superheated steam is contained within tube 13, it is at a relatively low temperature, insufficient to effect substantial pyrolysis. The hydrocarbon feed/steam mixture is passed through pipe 13 at end 15 of duct 11.
and tube 12. here,
Heat exchange with the metal surface of the outer tube 12 takes place. Due to the increased surface area of the inner surface of the block 9, the temperature of the outer surface of the tube 12 remains substantially constant over the length where the radiant area per unit length increases.

On the inner surface of the duct 10 in which the pipe 12 is arranged,
Inwardly projecting ribs are provided, as will be described in more detail in connection with FIG.

FIG. 3 is a schematic diagram showing the variation of the internal cross-section along the length of the block 9. Although four general types of blocks are shown in FIG. 3, and all ducts 10 have the same cross-section at any point along their length, the various blocks 9a-9h in FIG. Figure 9 shows cross-sections at various points along the length of 9; Thus, block 9c.

9d, 9c and 9h illustrate the transverse fold planes of the block 9 used at the end of the heat exchanger adjacent to the burner 3. Along this length, the block 9 has a substantially constant cross-section over a length of about 3 m. There are only three large support legs 36, 37 and 38 in the block 9 for supporting and positioning the tube 12. Duct 1
Over the next adjacent portion 3m of 0, there are small teeth 40 between the main support ribs 313.37 and 38, as illustrated in the blocks indicated by reference numbers 9f and 9o in FIG. The size of these support ribs increases continuously over the central part of the duct 10, so that at a distance of 6 m from the burner 3 the ribs 40 have a height of approximately 2.5 cm -6. The total diameter of the duct 10 ranges from approximately 34 cm at the point adjacent to burner 3 to 27 cm at a distance of 6 m from burner 3.
decreases to cm. Block 9a describes a cross-section of block 9 at its end at a distance of approximately 9 m distal from burner 3. The diameter of the duct 10 at this point is approximately 27 cm
The height of the teeth 40 is approximately 3.2 cm. Both the change in height of the teeth 40 and the change in the overall diameter of the duct 10 are substantially continuous over at least a portion of the length of the duct 10.

Additional ceramic blocks 4I are provided to support the profiled blocks 9a-9h.

FIG. 1 also depicts various lines, conduits and heat exchangers, such as a steam drum 45 and a transfer line exchanger 46, of the type commonly used in pyrolysis and whose function does not need to be described in detail. do.

The pyrolysis furnace described above offers many substantial advantages over conventional pyrolysis furnaces. In conventional furnaces, the heat exchange tubes are placed within a large refractory block, and the residence time is shortened by using several small heat exchange tubes instead of a single large heat exchanger, so that the surface area/volume ratio is increased. becomes possible. However, obtaining a uniform heat distribution over the tube surface using such heat exchangers spaced within one refractory block is particularly difficult due to the small size of these tubes. It is very difficult to think about it. This results in different coke ratios in the various heat exchangers, resulting in substantial pressure drops.

In the above-described furnace, each heat exchanger provides its own feedstock supply, so that the flow rate and pressure decrease through each heat exchanger and are independently adjustable.

Various embodiments of the invention are clearly possible within the scope of the claims, in addition to those specifically described above.

In particular, an alternative embodiment internal duct 10a (not shown) provides hydrocarbon feedstock and superheated steam into a single internal lumen.

According to the embodiments described herein, the thermal decomposition can be carried out over a long period of time without thermal breakdown of the reactor at high reactor inner wall temperatures, such as 950° C. or higher, due to the favorable temperature profile of the reactor. It turned out that it is possible. This is important because at such high temperatures, the rate of the chemical reaction that decomposes the carbon deposits in the reactor (the so-called "shift reaction") is greater than the rate of the chemical reaction that causes the carbon deposit. Because the reactor is operated above this critical temperature such that the shift reaction is more rapid than the carbon forming reaction, the reactor can be used for extended periods of time substantially without coke formation.

Furthermore, the particular design of the cracking furnace according to the present invention allows for a substantial reduction in the total furnace volume for a given throughput. For example, throughputs that by conventional design require a furnace with a volume of 300 m3 are typically handled by the use of a furnace with a volume of about 25 m3 according to the present invention.

[Brief explanation of the drawing]

1 is a schematic diagram of a pyrolysis furnace including a heat exchanger according to the invention; FIG. 2 is an enlarged view of one embodiment of the internal duct; and FIG. 3 is a diagram of a portion of the furnace of FIG. FIG. 3 is a schematic end cross-sectional view taken in the direction of arrow 3-3. 1...Casing 2...Inner layer 3...Burner 4...Feed supply lines 6a, b...
・Flow path 7...Pump 8...Heat exchanger 9・
・Ceramic block 10 ・Dat 18a,
b...Heat exchanger 20...Heat exchanger procedure Amendment Book October 7, 1988 1, Incident indication 1988 patent! IT No. 176922 Hydrocarbon Cracking Furnace Including a Thermal Converter 3, Relationship with the Amendment Case Patent Applicant Address Name A (723) The Gra Chemical Company 4
, Agent 5, [Claims] column of the specification to be amended (attached sheet) 1. The claims are corrected as follows. [(1) means for defining an interior duct having means for defining an exterior duct exterior wall disposed around the exterior duct for substantial radiant heat transfer between the exterior duct exterior wall and the interior duct; surface is at least 18'l of its goodness
During operation along S, the temperature of the fluid flowing into the external duct varies so that radiant heat from the external wall inwardly occurs, and the radiant heat decreases the temperature of the fluid flowing into the external duct along that portion of the external duct length. The heat exchanger changes to compensate for the (2) The heat exchanger of claim 1, wherein the external duct external wall defining means comprises a plurality of inwardly convex ribs of continuously increasing cross-sectional area along the portion of the external duct length. vessel. (3) the internal duct limiting means consists of a pair of coaxial pipes;
The heat exchanger according to claim 1, wherein the inner tube of the coaxial tubes is used for supplying the primary heat exchange fluid to the gap defined by the inner tube and the outer tube. . (4) The heat exchanger according to claim 1, wherein the outer wall of the external duct is made of a ceramic block. (5) The heat exchanger according to claim 1, wherein the internal duct limiting means comprises a metal tube disposed within the external duct. 6. The heat exchanger of claim 1, wherein the cross-sectional area of the external duct decreases over said portion of the external duct length. (7) A first pipe connected to the 91'1 supply port and disposed within a second pipe communicating with the decomposition product and personal material discharge port, and the first pipe and the second pipe are connected to the reactor. A flow of hot flue gas is directed around the second pipe in a parallel flow direction with the feedstock flow of the duct defined by the gates of the first and second ducts and the at least one decomposition reactor communicating with the first part. a burner disposed near the first end of the reactor for flow, the first end of the reactor extending downstream with respect to the flue gas flow from the burner. ) radiant heat 7 in which the cross section of the flue gas duct is directed inwardly from the outer wall along at least part of the length of the flue gas duct;
The radiant heat 7 changes as occurs during operation.
The flue has an outer wall formed in such a way that the lux varies along said part of the outer wall so as to compensate for the temperature drop of the fluid entering the flue. Claim Ff5$ in which the pipe is arranged
(9) The hydrocarbon cracking furnace of claim 8, wherein the cross-sectional area of the external duct varies over said portion of the length of the external duct. (10) The outer wall of the flue gas duct has a plurality of inwardly projecting ribs, the cross-sectional area of the ribs increasing continuously along the portion of the outer duct length. Hydrocarbon cracking furnace. (11) A hydrocarbon feed to be decomposed is supplied to the feedstock supply port of the furnace according to claim 7, and a flue gas flow is generated from the burner within the flue gas flow to generate the hydrocarbon feed. A method of hydrocarbon decomposition consisting of heating and decomposition. 12. The method of claim 11, wherein the furnace is operated such that the temperature of the second g is maintained substantially constant over at least a portion of the second pipe temperature corresponding to the portion of the flue gas duct. . (13) Inject ceramic material around at least 1 (1i5 elongated disposable 1 bun), harden the ceramic material, and destroy the front lining) to form at least one m
leaving a ceramic block with elongated passages and inserting a heat exchange tube within the at least one elongated passage;
"A method of forming a radiant heat exchanger comprising confining a gas duct in a flow path around a heat exchange tube."

Claims (13)

[Claims] (1) means for defining an interior duct having means for defining an exterior duct exterior wall circumferentially disposed for substantial radiative heat transfer between the exterior duct exterior wall and the interior duct;
the cross-section of the external duct is varied along at least a portion of its length in such a way that in operation there is a radiant heat flux inwardly from said external wall; said radiant heat flux is directed into the external duct along said portion of the external duct length; A heat exchanger that changes to compensate for the temperature drop in the incoming fluid. 2. The heat exchanger of claim 1, wherein the external duct external wall defining means comprises a plurality of inwardly projecting ribs of continuously increasing cross-sectional area along the portion of the external duct length. . (3) the internal duct limiting means consists of a pair of coaxial pipes;
The heat exchanger according to claim 1, wherein the inner tube of the coaxial tubes is used to supply the primary heat exchange fluid to the gap defined between the inner tube and the outer tube. . (4) The heat exchanger according to claim 1, wherein the outer wall of the external duct is made of a ceramic block. (5) The heat exchanger according to claim 1, wherein the internal duct limiting means comprises a metal tube disposed within the external duct. 6. The heat exchanger of claim 1, wherein the cross-sectional area of the external duct decreases over said portion of the external duct length. (7) having a first tube connected to the feedstock supply port and disposed within a second tube communicating with the decomposition product discharge port, the first tube and the second tube being connected to the first end of the reactor; at least one cracking reactor in communication with the first end of the reactor for flowing a hot flue gas flow around the second tube in a cocurrent direction with the feedstock flow in the second tube; a burner disposed, the first end of the reactor being disposed in the flue gas flow from the burner, and the reactor extending downstream with respect to the flue gas flow from the first end. Hydrogen cracking furnace. (8) the cross-section of the flue gas duct is varied in such a way that, during operation, a radiant heat flux inward from the outer wall occurs along at least a portion of the length of the flue gas duct, and said radiant heat flux is a fluid flowing into the flue gas duct; 2. The first and second tubes are arranged in a flue gas duct having an outer wall configured to vary along said portion of the outer wall so as to compensate for a temperature drop in the flue gas duct. The hydrocarbon cracking furnace according to item 7. (9) A hydrocarbon cracking furnace according to claim 8, wherein the cross-sectional area of the external duct varies over said portion of the length of the external duct. (10) The outer wall of the flue gas duct has a plurality of inwardly projecting ribs, the cross-sectional area of the ribs increasing continuously along the portion of the outer duct length. Hydrocarbon cracking furnace. (11) Supplying a hydrocarbon feed to be cracked to the feedstock inlet of the furnace according to claim 7, and generating a flue gas flow from the burner in the flue gas duct to heat the hydrocarbon feed. A hydrocarbon decomposition method consisting of decomposition. 12. The method of claim 11, wherein the furnace is operated to maintain the temperature of the second tube substantially constant over at least a portion of the length of the second tube corresponding to the portion of the flue gas duct. . (13) injecting a ceramic material around the at least one elongated disposable core and curing the ceramic material;
breaking the core to leave a ceramic block having at least one elongated passageway, inserting a heat exchange tube into the at least one elongated passageway, and forming a gas duct in the flow passage around the heat exchange tube; A method of forming a radiant heat exchanger comprising: