WO2003031050A1

WO2003031050A1 - Heat exchange reactor

Info

Publication number: WO2003031050A1
Application number: PCT/GB2002/004189
Authority: WO
Inventors: Bernard John Crewdson; Stuart Ballentyne Brady; Peter William Farnell
Original assignee: Jonhson Matthey Plc
Priority date: 2001-10-09
Filing date: 2002-09-13
Publication date: 2003-04-17
Also published as: GB0124172D0

Abstract

A heat-exchange reactor suitable for e.g. reforming hydrocarbons, having polygonal, e.g. hexagonal, heat-exchange tubes is described. The conformation of the tubes permits high shell-side heat transfer whilst minimising complex internal construction of the reactor.

Description

HEAT EXCHANGE REACTOR

This invention relates to a heat exchange reactor.

In a heat exchange reactor a process fluid is passed from a process fluid feed zone, through heat exchange tubes optionally containing a particulate catalyst, disposed within a heat exchange zone defined by a casing through which a heat exchange medium passes, and then into a process fluid off-take zone. Means, such as tube-sheets, are provided to separate the zones. Thus a tube-sheet may separate the heat exchange zone through which the heat exchange medium passes from a zone, such as a plenum chamber, communicating with the interior of the heat exchange tubes to permit feed of process fluid to the tubes or off-take of process fluid from the tubes. An alternative arrangement involves the use of header pipes disposed within the heat exchange zone to define the process fluid feed zone: the process fluid is fed to the header pipes from whence it flows into and through the heat exchange tubes. Similarly header pipes may be provided for the off-take of process fluid from the tubes. Alternatively there may be a combination of tube-sheets and header pipes, for example the process fluid may be fed to the heat exchange tubes from a plenum chamber separated from the heat exchange zone by a tube-sheet while header pipes are provided disposed within the heat exchange zone for off-take of process fluid from the tubes. Such tube-sheets or headers are herein termed boundary means as they define boundaries between the heat exchange zone and the process fluid feed and off-take zones. In conventional heat exchange apparatus having tubes of circular cross-section, heat exchange medium flows through the irregular spaces between adjacent tubes. Such designs do not generally provide very efficient heat transfer between the medium and the tubes. Thus in one form of design, the heat-exchange tubes are surrounded for at least a part of their length by a sheath tube. The sheath tube provides an even, annular passage through which the heat-exchange medium flows that enhances the heat transfer between the heat-exchange tube and heat-exchange medium by forcing at least some of the medium to flow in a controlled manner over the external surface of the heat-exchange tube. Optionally, the heat-exchange tubes may additionally have fins or other means to increase their surface area in order to further enhance the heat transfer. Although provision of sheath tubes can improve heat transfer, the irregular spaces between the sheath tubes do not contribute to efficient heat transfer and the sheath tubes require a separate tube-sheet in order to support their weight. The sheath tube-sheet requires separate construction and generally is disposed below the boundary means supporting the heat exchange tubes, i.e. in the heat exchange zone. Hereforeto the heat-exchange tubes have a circular cross-section and typically have a length of several metres, e.g. 5 to 15 m and a diameter in the range 7 to

20 cm. Likewise, the sheath tubes have a generally circular cross-section with an inner diameter providing an annulus between the sheath tube and the heat-exchange tube of between 1 and 10 mm. The circular cross-section of the heat-exchange tubes is dictated by the need for them to withstand the potentially substantial pressure differential between the pressure of the process fluid within the tubes and the pressure of the heating medium. For example in many applications, the heating medium is at a pressure in the range 1 to 4 bar abs. while the process fluid may be at a pressure in the range 20 to 80 bar abs. In other applications, both the process fluid and the heating medium may be at a pressure in the range 10 to 80 bar abs. with a differential pressure between them in the range 0.5 to 10 bar abs.

In a steam reforming process the process fluid, i.e. a mixture of a hydrocarbon feedstock and steam, and in some cases also carbon dioxide or other components, is passed at an elevated pressure through catalyst-filled heat exchange tubes which are externally heated by means of a suitable heating medium, generally a hot gas mixture. The tubes are generally disposed vertically within the heat exchange reactor. The catalyst is normally in the form of shaped units, e.g. cylinders, rings, saddles, and cylinders having a plurality of through holes, and are typically formed from a refractory support material e.g. alumina, calcium aluminate cement, magnesia or zirconia impregnated with a suitable catalytically active material which is often nickel and/or ruthenium.

In some types of heat exchange apparatus, the heating exchange medium is the process fluid that has passed through the tubes but which has then been subjected to further processing before being used as the heat exchange medium. An example of this type of process and heat exchange apparatus for effecting the primary reforming of hydrocarbons is described in GB 1 578 270, which describes a process where a primary reformed gas is subjected to partial oxidation (where it is partially combusted with oxygen or air) and optionally, the process known as secondary reforming. The resultant partially combusted gas, by which term we include secondary reformed gas, is then used as the heat exchange medium heating the tubes. We have realised that in processes where the pressure differential is low, for example where the heating medium is the process fluid that has passed through the tubes but which has then been subjected to further processing, the heat-exchange tubes need not have a circular cross-section and that polygonal-shaped heat exchange-tubes may be used. Polygonal heat-exchange tubes can provide a tube bundle having defined passages between adjacent tubes through which heat-exchange medium flows, that maximise the heat transfer between the heat-exchange medium and the tubes. In addition, sheath tubes and the sheath tube-sheet may be eliminated and the heat exchange reactor design considerably simplified. Elimination of the sheath tubes also presents the opportunity to decrease the reactor size or increase the number of heat-exchange tubes in a particular vessel. Accordingly the present invention provides a heat exchange reactor including a process fluid feed zone, a heat exchange zone and a process fluid off-take zone, first and second boundary means separating said zones from one another, a plurality of heat exchange tubes moveably attached to at least one of said boundary means and extending through the heat exchange zone, whereby process fluid can flow from the process fluid feed zone, through the heat exchange tubes and into the process fluid off-take zone, characterised in that the heat exchange tubes have a polygonal cross-section for a major portion of their length.

The invention further provides a process wherein a process fluid is subjected to heat exchange in a heat exchange reactor and then to a further processing step, said heat exchange reactor having a process fluid feed zone, a heat exchange zone, a process fluid off-take zone and first and second boundary means separating said zones from one another, said process comprising feeding the process fluid to said process fluid feed zone, passing said process fluid from said process fluid feed zone through a plurality of heat exchange tubes moveably attached to at least one of said boundary means and extending through said heat exchange zone, subjecting said fluid to heat exchange with a heat exchange medium in said heat exchange zone, passing the process fluid from said heat exchange tubes to a process fluid off-take zone, subjecting the process fluid from said process fluid off-take zone to the further processing step and passing the further processed process fluid through the heat exchange zone as the heat exchange medium, characterised in that the heat exchange tubes have a polygonal cross-section for a major portion of their length. By the term "major portion", we mean greater than 50%.

The process and apparatus are of particular utility for steam reforming hydrocarbons wherein a mixture of a hydrocarbon feedstock and steam is passed through the heat exchange tubes which contain a steam reforming catalyst so as to form a primary reformed gas. The primary reformed gas may be then subjected to partial combustion with an oxygen- containing gas, e.g. air, and the resultant partially combusted gas used as the heating fluid in the heat exchange zone. Preferably the partially combusted primary reformed gas is passed through a bed of a secondary reforming catalyst, so as to effect further reforming, before being used as the heat exchange fluid.

In operation the heat-exchange tubes are heated to a high temperature, typically to a maximum temperature in the range 700°C to 900°C. This heating necessarily means that the tubes are subject to considerable thermal expansion, both longitudinally and radially, as the tubes are heated from ambient temperature to the operating temperature and likewise to thermal contraction as the tube is cooled upon shut down of the process. Because the heat- exchange tubes are of considerable length, typically several metres, the tubes expand longitudinally by a considerable amount, often 10 cm or more, relative to the casing to which the boundary means is fastened. Thus there is a requirement for the heat-exchange tubes to be moveably attached to at least one of the boundary means. By the term "moveably attached" we mean that the tube is attached to the boundary means by means that allow for the thermal expansion and contraction of the heat-exchange tubes. Normal practice is to provide flexible elements known as "pigtails" at one or both ends of the tubes to permit such differential expansion so that the pigtails, rather than the tubes themselves are fastened to boundary means. Alternatively, bellows arrangements may be employed to permit such expansion. More recently, venturi-seal designs as described in our EP-B-0843590 may be employed where leakage of heat exchange medium into process fluid is permissible and the heat exchange medium is at a pressure above, or not more than a few bar below, that of the process fluid. Preferably the heat-exchange tubes are moveably attached to one boundary means and non-moveably attached to the other. Thus the heat-exchange tubes preferably extend from a first boundary means to which they are non-moveably attached, through the heat exchange zone, and are moveably attached by means of e.g. pigtails, bellows or venturi seal tubes, to a second boundary means. The heat-exchange tubes are of polygonal cross-section for the major portion of their length. The cross-section is particularly a polygon having 3 to 8 sides, preferably 3 to 6 sides. The polygons may be regular, for example an equilateral triangle, square or hexagon or irregular, such as a quadrilateral or rectangle. The tube bundle may comprise tubes of the same or different widths, and may comprise tubes of the same or mixed polygonal cross- section. The polygonal tubes are arranged to provide spacing between adjacent tubes that provides a suitably high heat transfer between the heat exchange medium and the tubes, for example by providing parallel-walled passages through which the heat-exchange medium is able to flow. While the sides of the polygon may be straight, so that the cross-sections are those of e.g. an equilateral triangle, square, regular pentagon, hexagon, heptagon or octagon, the sides of the polygon may also be inwardly curved. The curve may be an arc of a circle or in particular is in the form of a catenary (hyperbolic cosine curve). However, preferably the cross-section of the tubes is a regular hexagon.

The spacing of the tubes along their length is preferably maintained by means of, e.g. lugs, pins or longitudinal spacers. Single or multiple spacers may be used on each face of the polygon. Additionally, it may be desirable to increase the surface area of the tubes, and therefore the heat transfer between the tubes and the heat exchange medium, by providing longitudinal fins for at least a part of the length of the tubes. Such longitudinal fins may also act conveniently as spacers, separating the heat-exchange tubes in the heat exchange zone. The spacing means, i.e. the lugs, pins, longitudinal spacers or fins or the like should be attached to the tubes so as to permit the lateral and radial movement anticipated by thermal expansion and contraction. For example fins may be attached only at one end and extend for only a portion of the distance between opposing faces. The spacing between the heat- exchange tubes should be small enough to provide the necessary velocity of heat transfer material and yet provide sufficient volume for the heat transfer medium to effectively flow through the reactor with e.g. an acceptable pressure drop. Spacing between the major portion of the heat-exchange tubes along their length should be in the range 1-10 mm and preferably 2-6 mm at operating temperature.

The heat-exchange tubes of the present invention may be formed by extrusion or reshaping of circular tubes. Alternatively the tubes may be formed by seam-welding rolled, folded or otherwise formed plate metal. In a preferred arrangement, the heat-exchange tube is formed by welding, e.g. by electrical resistance or laser seam welding, together the edges of metal strips of substantially uniform thickness. The strips are preferably relatively thin; thus the maximum thickness of the walls of the inner tube, i.e. at the seam-welded edges in the aforesaid form of construction, may be 3-6 mm.

The heat-exchange tubes may be attached to boundary means such as a tube-sheet in the configuration of the major portion of the tube, i.e. a polygon, or may be attached via a short length of tube whose cross-section is different in order to simplify the attachment process. For example a short length of tube having a circular cross-section may be attached to the end of the heat-exchange tube and the circular tube then attached to the boundary means, e.g. by welding. The circular tube may be of the same width, but is preferably of narrower cross section than the major part of the tube. The length of the attachment tube will depend amongst other things on the heat-exchange tube and reactor dimensions but may be between 1 and 20% of the length of the heat-exchange tube. Alternatively, the heat- exchange tube may be 'necked-in' by a suitable forming process to provide a shape suitable for attachment to the tube-sheet. The tube-sheet thickness is determined in part by the width of the tubes attached to it. Generally, the wider the tubes, the thicker has to be the tube-sheet. Thick tube-sheets, e.g. >30 cm are disadvantageous because of their weight and difficulty of construction. In order to reduce the thickness of a single tube-sheet acting as boundary means, more than one tube-sheet may be used, each tube-sheet being thinner than if a single one were to be used. For example the heat-exchange tubes may be attached to two thin tube-sheets, spaced apart by a short distance, e.g. between 20 - 30 cm, preferably 30 cm.

In an alternative embodiment, the boundary means are not provided by a tube-sheet or header pipes but rather the heat exchange tubes are welded together with a weld that provides a spacing between opposing faces of the heat-exchange tubes in the range 1- 10 mm and preferably 2-6 mm at the operating temperature. The weld may comprise at least one weld around the periphery of the heat-exchange tube that extends longitudinally between 2 and 30 cm in depth such said at least one weld is continuous and thereby effectively provides a seal between the zones within the reactor. Consequently the weld or welds act as the boundary means and the reactor design is further simplified. Where more than one weld is present, it is preferable to provide orifices, or to have discontinuities present such that thermal expansion and contraction of any gas trapped between the welds is accommodated. Alternatively, the tubes may have at the boundary end, a short length of polygonal tube of larger cross section (e.g. by 1-10 mm) and the boundary formed by placing all tubes touching, e.g. for hexagonal tubes in a honeycomb pattern and welding, e.g. by laser beam welding, the adjacent tubes together. Alternatively, a piece of spacer material of thickness, e.g. 1-10 mm, may be welded between opposing tube walls.

Where welds are used to provide the boundary means, it is preferable that the tube bundle is not welded directly to the internal walls of the vessel and that, for example, a support ring is provided around the periphery of the tube bundle which is then attached to the vessel wall.

In order to improve cross-flow of heat exchange medium within the heat exchange reactor it is preferable that where a weld is used as a boundary means, at least one portion of the heat-exchange tubes is provided that has a narrower cross-section than the major part of the tube. The narrower portion may be of the same or different polygonal cross-section or may be circular. The narrower portion may be formed by attaching a suitable length of narrower tube, for example by welding, or by 'necking-in' the heat-exchange tube using a suitable forming process. The length of this narrower cross-section portion will depend amongst other things upon the desired flowrate of heat-exchange medium but may be between 1 and 20% of the length of the heat-exchange tube.

The flow of heat exchange medium over the heat-exchange tubes may be further improved if a shroud or casing is present that surrounds the tube bundle and prevents by- pass of heat exchange medium in the space between the heat-exchange wall and the outside of the tube bundle. Such a shroud may be constructed from similar material to the heat-exchange tubes and, for example has a thickness of 3-25 mm. The shroud preferably closely surrounds the tube bundle and may be separated from it by means of e.g. lugs, pins, longitudinal spacers and/or fins. Where a shroud is present, the spacing between the shroud and the tube bundle is preferably in the range 1 to 25 mm at operating temperature.

Preferably, the shroud is not attached to the tube bundle directly and is supported by support means, e.g. a tube sheet or a support ring around the periphery of the vessel. Alternatively, the walls of the heat exchange reactor may be shaped so as to prevent by-pass of the process fluid around the tube bundle. The number of tubes in the reactor, and hence the reactor size, is dictated by its intended use. For example, in a steam reforming process the number of tubes is usually greater than about 10 and typically between 50 and 500 although smaller and larger reactors may be used. The width of the tubes will depend upon the application of the heat exchange reactor but may be in the range 5 to 25 cm. In a further alternative embodiment, the tubes may be of a double-tube configuration wherein the tubes are closed at one end and contain additionally a second inner tube disposed longitudinally therein through which the process fluid may pass. Such a double- tube configuration is described in US 4690690. Furthermore, the inner tube may be circular or non-circular. Non-circular inner-tube configurations are described in WO 01/37982. Several embodiments of the invention are illustrated by reference to the accompanying drawings in which Figure 1 is a diagrammatic cross section of a heat exchange apparatus according to a first embodiment of the invention wherein upper and lower boundary means are tube-sheets, Figure 2 depicts one configuration of the heat-exchange tubes in a hexagonal arrangement, Figure 3 depicts a second embodiment wherein one of the boundary means is a header bearing venturi-seals, Figure 4 depicts a third embodiment wherein upper the boundary means is provided by welds between the adjacent heat- exchange tubes, and Figure 5 is a flow-sheet depicting a process according one embodiment of the invention. In Figure 1 there is shown heat exchange apparatus, such as a heat exchange reformer, having an outer pressure shell 10 enclosing three zones 11 , 12, and 13, defined by the shell walls and upper tube-sheets 15 and lower tube-sheet 14.

Zone 11 , a process fluid feed zone, is defined by the shell walls and tube-sheet 15, is provided with a feed supply conduit 16 and has a plurality of hexagonal cross section heat exchange tubes, e.g. reforming tubes, 17 fastened to, and extending downwards from, tube- sheet 15 into zone 12. The number of tubes employed will depend on the scale of operation: although only seven tubes are shown in Figure 1 there may be typically be 50 or more such tubes. For steam reforming, the tubes 17 will contain a suitable steam reforming catalyst, for example nickel on a support of a refractory material such as alumina, zirconia or a calcium aluminate cement. The reforming catalyst is normally in the form of shaped units random packed in the tubes. Typically the shaped units have a maximum dimension of less than about one fifth of the reforming tube width and may be in the form of cylinders having a passage, or preferably more than one passage, extending longitudinally through the cylinder. Alternatively, the catalyst may be provided as a coating on the internal surface of the heat- exchange tubes.

The heat-exchange tubes 17 are attached to tube-sheet 15 via a narrowed portion 18 of reduced cross-section, that facilitates easier attachment of the tubes to the tube-sheet. The narrowed portion 18 may be polygonal, e.g. hexagonal, or circular in cross section.

Zone 12 is the heat exchange zone and is bounded by the walls of shell 10 and tube- sheets 14 and 15. A heat exchange medium inlet conduit 19 is provided at the lower end of zone 12 and an outlet conduit 20 is provided suitably at the upper end of the zone 12. Heat- exchange tubes 17 extend through zone 12 and through the tube-sheet 14 at the lower end of zone 12. The tubes 17 each have a portion 21 of reduced cross section at the lower ends and passing through the tube-sheet 14. Surrounding the tube bundle is a shroud 22 to improve the heat transfer around the outside of the tube bundle. Heat exchange medium enters through conduit 19, passes up the spaces between the heat-exchange tubes 17 and between the tubes 17 and the shroud 22 and exits zone 12 via conduit 20. The separation of the tubes 17 in heat exchange zone 12 is maintained by spacer means such as longitudinal fins (not shown). While it is not essential that the heat-exchange tubes 17 have upper portions 18 and lower portions 21 of decreased cross-section, i.e. the tubes 17 could be of full cross section where attached to tube-sheet 15 and on passage through tube-sheet 14, the provision of the upper portion 18 and lower portion 21 of reduced cross-section facilitates design and construction of the boundary means and provides a means to improve cross-flow of the heat exchange medium through the heat exchange zone 12. Zone 13, the process fluid off-take zone, is defined by the walls of shell 10 and the tube-sheet 14, and is provided with a process fluid outlet conduit 23. The lower portions 21 of the tubes 17 pass through tube-sheet 14 and are open at their lower ends thus enabling process fluid from tubes 17 to pass into zone 13 and thence out through conduit 23. The lower portions 21 of tubes 17 are not fastened to tube-sheet 14. So that thermal expansion of the heat-exchange tubes 17 relative to the shell 10 can be accommodated, each tube portion 21 extends into a venturi seal tube 24 of the type described in EP-B-0843590, the disclosure of which is incorporated herein by reference, fastened to tube-sheet 14 and extending into zone 13. The configuration of a group of seven heat-exchange tubes is depicted in Figure 2. Six equivalent regular hexagonal tubes 17 surround a central hexagonal tube with each of the surrounding tubes 17 presenting one face substantially parallel to the central tube. The tubes are separated by means of longitudinal spacer fins 25 which act to maintain the separation of the tubes from each other and provide additional surface area for heat transfer and thereby improve the heat transfer between the heat exchange medium and the tubes.

To permit movement caused by thermal expansion and contraction, the fins are preferably not attached at both ends. Rather they may be fixed to one face of a polygon and extend for a portion of the distance such that at operating temperature, the tubes are suitably spaced. The spacing between the opposing faces of adjacent tubes 17 is about 3 mm at operating temperature. A shroud 22 is depicted surrounding a portion of the tube bundle. The shroud

22 closely follows the outer surface of the tube bundle and is separated from the outer tubes by means of, e.g. spacer fins 25. The spacing between the shroud 22 and the faces of the outer tubes 17 of the tube bundle is about 3 mm.

In the embodiment of Figure 3, the lower tube-sheet 14, the process fluid off-take zone 13 and the process fluid outlet conduit 23 of Figure 1 are replaced by a series of header pipes 30 connected to a process fluid outlet conduit 31 extending through the vessel wall 10. Process fluid passes into zone 11 through conduit 16, down through the heat exchange tubes 17, into header pipes 30 and out of the vessel via conduit 31. The heat exchange tubes are moveably attached to the header pipes by venturi seal tubes 24, fastened to the header pipes 30 and extending upward into the heat exchange zone 12. The heat exchange medium enters the vessel at the lower end through conduit 19 and passes through the spaces between adjacent header pipes 30, past the lower ends of the tubes 17 and then up through the spaces between the tubes 17 and between the tubes 17 and the shroud 22 and exits the vessel through outlet conduit 20. Figure 4 depicts a further embodiment in which the reformer tubes 17 do not have an upper narrow portion 18 and tube-sheet 15 is omitted. The boundary means separating process fluid entry zone 11 from heat exchange zone 12 is provided by welds 26 that separate the adjacent heat-exchange tubes 17. A first weld is provided near the top of each of the heat exchange tubes and the second about 30 cm below it. The welds are about 3 mm in width and extend about 10 cm in length. One of the welds is provided with orifices (not shown) to permit the expansion and contraction of gasses contained in the spaces between said welds. A shroud 22 surrounds the tube bundle and is attached to the vessel wall 10 via a support 27 that extends around the internal walls of the vessel. To improve cross flow of heat exchange medium in heat exchange zone 12, the tubes 17 have a narrow portion 28 formed at a suitable point, e.g. near the top of the tubes about in the plane of the heat exchange medium outlet 20. The narrow portion 28 is wide enough to permit loading of catalyst shaped units and their recovery, e.g. by means of vacuum recovery.

The reactor of the present invention is particularly suitable for processes involving primary and secondary reforming of hydrocarbon feed-stocks with an oxygen-containing gas, e.g. those designed to produce reformed gas for use in the production of methanol or ammonia or in gas-to-liquid processes. For example, Figure 5 depicts a process for the steam reforming of a hydrocarbon feedstock. Process fluid comprising a mixture of a hydrocarbon feedstock and steam is fed via line 40 to a heat exchange reactor having a process fluid feed zone 1 , a heat exchange zone 12, a process fluid off-take zone 13 and first 15 and second 14 boundary means separating said zones from one another. The process fluid is subjected to steam reforming in a plurality of heat exchange tubes 17 having a polygonal cross-section for a major portion of their length and containing a steam reforming catalyst to give a primary reformed gas stream. The primary reformed gas stream is then passed from said heat exchange tubes 17 to the process fluid off-take zone 13, and thence via line 41 to further processing. The further processing comprises partial combustion in a vessel 42 with an oxygen-containing gas, supplied via line 43, over a bed of secondary reforming catalyst 44, for example nickel supported on calcium aluminate or alumina. The resultant partially combusted gas is passed via line 45 to heat exchange zone 12 as the heat exchange medium. The heat exchange medium passes up through the spaces between the heat-exchange tubes and exits the reactor via line 46.

Although described above primarily in relation to heat exchange reforming, it will be appreciated that the invention is also of utility in other heat exchange applications where simpler internal designs are desired. Examples include feed/effluent heat exchangers, for example where the feed to a process step such as an exothermic reaction, e.g. methanol or ammonia synthesis, is heated by heat exchange with the effluent from the process step. In such cases the heat exchange tubes may be free of catalyst unless it is desired, as in the aforementioned reforming process, that a catalytic reaction is effected on the process fluid while it is undergoing the heat exchange.

Claims

Claims.

1. A heat exchange reactor including a process fluid feed zone, a heat exchange zone and a process fluid off-take zone, first and second boundary means separating said zones from one another, a plurality of heat exchange tubes moveably attached to at least one of said boundary means and extending through the heat exchange zone, whereby process fluid can flow from the process fluid feed zone, through the heat exchange tubes and into the process fluid off-take zone, characterised in that the heat exchange tubes have a polygonal cross-section for a major portion of their length.

2. A reactor according claim 1 in which the polygonal cross-section is a polygon having 3 to 8 sides.

3. A reactor according to claim 1 or claim 2 wherein spacing for a major portion of the length of the heat-exchange tubes is in the range 1 to 10 mm at operating temperature.

4. A reactor according to any one of claims 1 to 3 wherein the spacing of the heat- exchange tubes along their length is maintained by means of spacer lugs and/or longitudinal fins that extend for at least a part of the length of the tubes.

5. A reactor according to any one of claims 1 to 4 wherein the first boundary means comprises a tube-sheet, header pipes or welds between adjacent heat exchange tubes and the second boundary means is selected from a tube-sheet or header pipes.

6. A reactor according to claim 5 wherein the first boundary means comprise a single or multiple welds around the periphery of the heat-exchange tubes that extend longitudinally between 2 and 30 cm in depth.

7. A reactor according to any one of claims 1 to 6 wherein at least one portion of the heat- exchange tubes in the heat exchange zone is provided with a narrower cross-section than the major part of the length of the tube.

8. A reactor according to any one of claims 1 to 7 wherein a shroud surrounds the heat- exchange tube bundle.

9. A reactor according to any one of claims 1 to 8 wherein the heat exchange tubes contain a steam reforming catalyst.

10. A reactor according to any one of claims 1 to 9 in the form of a heat exchange reformer operatively connected to partial combustion means designed to effect partial combustion of the process fluid after the process fluid has passed through the tubes, and to supply the partially combusted process fluid to the heat exchange reformer as a heat exchange medium.

11. A reactor according to claim 10 wherein the partial combustion means comprises a bed of secondary reforming catalyst through which the partially combusted gas passes before supply thereof to the heat exchange reformer as the heat exchange medium.

12. A process wherein a process fluid is subjected to heat exchange in a heat exchange reactor and then to a further processing step, said heat exchange reactor having a process fluid feed zone, a heat exchange zone, a process fluid off-take zone and first and second boundary means separating said zones from one another, said process comprising feeding the process fluid to said process fluid feed zone, passing said process fluid from said process fluid feed zone through a plurality of heat exchange tubes moveably attached to at least one of said boundary means and extending through said heat exchange zone, subjecting said fluid to heat exchange with a heat exchange medium in said heat exchange zone, passing the process fluid from said heat exchange tubes to a process fluid off-take zone, subjecting the process fluid from said process fluid off-take zone to the further processing step and passing the further processed process fluid through the heat exchange zone as the heat exchange medium, characterised in that the heat exchange tubes have a polygonal cross-section for a major portion of their length.

13. A process according to claim 12 for the steam reforming of a hydrocarbon feedstock wherein the process fluid fed to the process fluid feed zone comprises a mixture of a hydrocarbon feedstock and steam and said heat exchange tubes contain a steam reforming catalyst, whereby said mixture is subjected to steam reforming in said heat exchange tubes to give a primary reformed gas stream, passing said primary reformed gas stream from said heat exchange tubes to the process fluid off-take zone, subjecting the primary reformed gas from said process fluid off-take zone to further processing comprising partial combustion with an oxygen-containing gas, and passing the resultant partially combusted gas through the heat exchange zone as the heat exchange medium.

14. A process according to claim 13 wherein the partial combustion of the primary reformed gas is carried out through a bed of a secondary reforming catalyst.