CA1246874A

CA1246874A - Steam reforming hydrocarbons

Info

Publication number: CA1246874A
Application number: CA000503339A
Authority: CA
Inventors: Sydney P.S. Andrew; Antony P.J. Limbach; Ralph J. Doy
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Johnson Matthey PLC
Priority date: 1985-03-05
Filing date: 1986-03-05
Publication date: 1988-12-20
Also published as: NO168574B; NO860813L; EP0194067B2; DE3663652D1; US4690690A; EP0194067A2; NO168574C; IN170072B; EP0194067B1; JPS61222903A; US4810472A; AU5419086A; JPH07481B2; NZ215292A; AU577926B2; EP0194067A3

Abstract

ABSTRACT

"Steam Reforming Hydrocarbons"
Hydrocarbon steam reforming process and apparatus wherein blind ended, ie double pass, externally heated reformer tubes are employed. The inner, ie return, tubes are insulated so that there is only a small temperature drop between the reacted gas leaving the catalyst zone and entering the return tubes and the gas leaving the return tubes. The outer, blind-end, tubes preferably have fins and are surrounded by sheaths through which the heating medium passes. The heating medium is preferably hot gas obtained by subjecting the primary reformed gas to secondary reforming.

Description

1 ~ 3339 Stea~ reformln~ hydrocarbon~
Thls lnventlon relate~ to a process of ~team refor~lng hydrocarbon~ to peoduce a gas containing carbon oxlde~ and hydrDg~n and to a reactor a~e~bly or carrylng out tha proc~
In the ~a~orlty of steDm sefor~ing peDc~ prcpo~cd or in lndustrial use, the endother~lc ~team hydro~arbon rea~tlon typifled by CH4 + H20 ~ C0 + 3H2 ~ ~1 ~ 49-3 kcal/~mol 18 carried out over a cataly~t dl~posed 8~ a bed in tube~ e~ternally heated ln a combuatlon furn~c~ and heat i8 recovered by stea~
ralslng. Nowever, in a long known and occa~ionally lndustrlslly used proce~ r the hot refor~ed ga8 leaYin2 ~he cat~ly~t bed 1B
~ithdrawn through an inne~ tube wlthin the cataly~t bed, 80 th~t heat i8 exchanged between that hot ga~ and reactants reactlng over the cataly~t. Examples nf thl~ proce~s ar~ de~cri~ed ln CB-A-3308~2~ US-A-4271086 and our eP-A-124226. Thl~ arran8emen~ of the reactLon zone, reEerred to co~only ~ "bayonet-t~be" or fleld tube" or "double-paas" prl~sry reformer 1~ e~ldently ~dvantageou~ ln provldlng a u~eful hest eecovery and in avolding con~truction proble~ due to dlfferentlal thermal e~pan~ion.
We have now reali~ed that the heat ~ransfer propertie~ o the double pas~ type of reformer can be l~proved in a way that i~
e~pecially sd~antageou~ when the source of heat 1~ not independent of the refor~lng ~tage.
Accordlng to the in~entlon in lt~ first aspect a process for produclng a product ga8 strea~ contalnln~ hydrogen and c~rbon oxlde~ by lncomplete cstalytlc ~eaction o A hydrocarbon ~eed~toc~
with ste~m and/or carbon dloxlde by passlng a reactnnt~ streAm contalnlng s~ld feed~tock and ~aid ~tea~ and/or cnrbon dloxld~ over a ca~alyst dlspo~ed ln an elon~ated, c~teenally heneed, zone and wlth~rawing the product ga~ ~tr~am through ~ tube wlthln that zone and countercurrently to thc flow of ~sld re~ctants ~treA~ over the c~tsly~t:
18 ch~racterlsed by llmltlng heat exchange between the product gAs ~tream and the re~ctant~ ~trea~ acro~ the wall Oe the tube ~o

2 B 33397 that the te~perature of the product 8aa ~tream leaving the tube 1~
le9~ than that of ~uch stream leavlng the elongated zone by at ~ost lOO~C.
The effect of ll~ieing the hest e~chan8e bet~een the 5 product g98 ~r~a~ and the raactant~ 1~ that, ~t r~glo~ of eh~
elon~ated zone inter~edlate the ends thereof, ~he te~pQrature of the react~nts 1~ lower, and coneequently the tempera~ure dlfferenca be~ween the reactane~ and ehe beatln8 medlum exeeenal to thQ
elongat2d zone 18 greater, th~n lf ~uch heat e~change hsd taken place. Therefore he~t tran~fer beewee~ the heatlng ~edlu~ and eh~
re~ctant~ 1~ more eff~cient and the nu~ber snd/or lengtb of the resctlon ~ones required to proces~ a g~ven quantlty Oe eedstock 18 less; partlcul~r expedlent~ can b~ used to enhance thl~ heat eransfer further, a~ will be descrlbed below.
The process oE the lnvention 1~ e~pecially valuable when the elongated zone 1~ heated by secondary refor~er gas; tha~ 1~, the hot ga~ re~ultlng from ~eactl~g ehe product gas ~trea~ wlth ~ 8a~
contalnlng free oxygen in an adiabatlc c~talyst zone whereby to effect reaction of further hydroc~rbon to carbon oxldea and hydeogen.
In the process of the inventlon, unllke the u~e of the prlor double-pa~Y refor~er, lnternal heat recovery betwaen the product 8a8 streAm ~nd the reactant~ 18 not ~vallable. Howeve~
heat recovery equlv~lent to sAld lnternal he~t recovery 1Y obtained Z5 where th~ medium for external heatlng of th~ elongated zon~ 13 ~econdsry reformer gas ~5 aforeflald. ~ven though ehere 1~ no lnternal heat recovery between the product gas ~tream nnd the reactants, th~ process mny stlll be vnluabl~ when usln~ other aource~ of hent, ~uch nH fuel combustlon producta or hot hellu~
nuclenr reactor coolant.
' PreEerably the medlum Eor e~ternal heatlnR oE the elongated ~one 18 ga~ at a prea~ure ln the range 5 to ~0 bar abs, slnce thls allow~ lmproved l~eat tran~fer in comparlson with a conventlon~l reformsr furnace to be nchleved snd, ln the event that an lndependent fuel ia combu~ted, permlt~ a u~eful " ~ ~
, 7~

3 8 333g7 e~ergy recovery by expan~lon of th~ combu~t~on products ln an ~nglne.
The hydrocarbon feed~tock csn in principl~ be any that can be vaporised and can b~ drsulphurl~ed thDroughly eno~gh to permit the catalytic reaction with ~team. V~UQ11Y it h~ ~ boiling polnt under 220C and preferably lt 1~ nor~ally ga~eou~, natural gas being very ~itable. If deslred, the reactant ml~ture u~ed can be or lnclude the product of reactlng hydrocarbon~ higher tha~ ~ethane with s~ea~ in adlabstic conditloRs ~o give a methane-elch ga~.
The molar ratlo of (steam + C02) to hydrocarbon csrbon a~om~ ln th~ feedstock 13 typlcally ln the ran8e 2 to 5.
Tb~ presaure of the product ga~ ~tream at the outl~t of the elongated zone, l.e. where the product g8~ enter~ the tube, ie typlcally ln the ran8e 25 to 80 bar ab~. and preferably dlffe~ from that eYternal to the elongated zonY by no more than 5 bar.
The elongated zone reactane~ lnlet temperature I B
typlc~lly in the rnnge 200 to 600~C, e~pecially 350 to 450C, nnd can be convenle~tly brought to thi~ temperaturc by heat e~change ~ith the product gaB ~tres~ leavlng the tube, or in the event tb~t ~0 the product g88 ~trea~ 1~ sub~ected to ~econdary refonmlng 8a aforesald and the secondsry refor~sr 8a~ i~ ueed a~ the e~tQrn~l heating-medlum for the elongated zone, wlth cooled ~econdsry reformer gas, that 1~, such g8B after use a~ the e~ternal heatlng medlum.
The te~perature at which the product gss ~tream leRves the elongated zone and enters th~ tube 18 typically 1~ the ran8e 650 to 850C~ ospecially 700 to 800G.
The combi-lation o~ ( st.eam ~ C02) rati.o, p.ress~re and temperature 1~ prcfQrably such that the product 8~0 cont~lns 15 -30X V/v of C1l4 o~ a dry bas1~.
The limltation of haat e~ch~nga acro~a the ~uba wall 1B
preferably ~uch that after le~vlng the elon~ated zone, the product gaB streQm 18 cooled by les than 30C during lt~ pa~sge along the tube; thl~ 1~ effected by ln~ul~tion in~lde and/or outslde the tube, snd thus expedlent~ such a~ ~lnnlng or ~ turbulstor lnslde the . ,

4 B 33397 ~ube or extended ~ube length are inappropriate to the invention, in contrast with prlsr proce~ses.
Where the elongated zone i5 heated by secondary reformer gas, preferably the product gas stream is fed from ~he outlet of the elongated zone, via the tube, to the secondary reformer with as little hea~ loss as pract$cable, preferably with no more than a 50C
fall in temperature. In the secondary reformer the product gas stre~m, and optionally a further amount of feedstock is reacted wieh a gas containing free oxygen, preferably air, moderately enriched air (up to 35~ V/v 2)~ or possibly oxygen depleted air ~down to 15% V/v 2) As usually carried out, this reaction 1nitially produces a flame whereafter the temperature decreases as methane in the product gas stream reacts endothermally with steam and C02~ but a flameless catalytic reaction could be used. The outlet temperature of the secondary reformer gas is typically in the range 950 - 1050GC.
The rate of feed of oxidant gas mixture to the secondary reformer is preferably such as to give a secondary reformer gas having a methane content of under 5, for example 0.01 to 1, % V/v on a dry basis.
The reforming pro~ess of the invention is of particular - use in the production of a~monia synthesis gas, with secondary reforming effected using air as the gas containing free oxygen. For ammonia synthesis the secondary reformer gas is normally subjected to the shift reaction, carbon tioxide removal, and methanation to remove residual carbon oxides. In conventional processes the amount of air employed in secondary reforming is often controlled to give a secondary reformer gas wherein the molar ratio of hydrogen plu9 carbon monoxide to nitrogen (which is ineroduced from the air) is about 3 (or ~lightly above to allow for the hydrogen consumed during methanation). However in the process of the lnventioc where the elongated zone is heated by secondary reformer gas and the secondary reforming ls effected using air as the gas containing free oxygen, at the preferred product gas, and secondary reformer gas, methane contents, the quantity of air required is such that the amount of ~4~

nitrogen added ls in a substantlal excess of what would be suitable for ammonia synthesis gas: generally the amount of air required i9 such tha~ the aforesald H2 + C0 ratio of the se~ondary refor~er gas is less than 3.Q, for example in the range 1.0 to 2.7. Taking into account the argon introduced in the air and any unreacted methane, the amount of air employed, and the reformlng conditions, are preferably such that the Molar ratio of hydrogen plus carbon monoxide to methane plus argon plus nitrogen is in the ran8e 1.25 to 2.5, especially 1.4 to 2.1 and at least 90~ V/v of the methane plus argon plus nitrogen i5 nitrogen. Such a secondary reformer gas compo~ition is especially suitable when it is to be processed ~o ammonia synthesis gas by the process of our EP-A-157480. As ~n alternative the e~cess nitrogen ran be removed cryogenically before ammonia synthesis, e.g. as described in GB-A-11$6002, or from reacted ammonia synthesis ga~, after separation of am~onia therefrom and before recycle thereof to ammonia synthesis, e.g. a~ described in EP-A~00993.
In alternative applications of the process the oxidant in the secondary reformer can be substantially pure oxygen, whereby to produce a nitrogen-free gas stream suitable for synthesis of organic compou~ds such as methanol. In another form of the process the heating medium is the product of combu6ting a waste g~s from downstream stage, for example a pressure swing adsorption waste gas or synthesis purge gas.
Whichever heating medium is brought into heat exchange with the elongated zone, such heat exchange is highly intense and is preferably carried out by passing a strenm of the heating medium in a direction countercurrent to the flow of reactant~ through the elongated zone through a sheath surrounding the elongated zone and extending for at least the ma~or part of the length thereof. The dimensions of the sheath should be such that the cross-sectional area of the space between the sheath and the elongated zone is between 0.1 to 1.0, especially 0.2 and 0.5, times the cross-sectional area of the space between the wall of the elongated zone and the tube located therein. Alternatively, or additionally, th~
outer ~all of ehe elongated zone pref~rably has, for at least the major part of the length thereof, an extended 3urface such as is given by longitudinal or helical fins, or studs.
The extent and dlmensions of the sheath, and the extent and nature of th extended surface, are preferably selected such that there is a substantially uniform heat flux from the heating medium into the elongated zone along the length thereof.
In some cases, if the sheath and/or extended surface extended right to the outlet end of the elongated zone, there ~ay be a risk of overheating of the outlet end region of the elongated zone and/or the sheath, which i5 preferably of llght gauge metal, may be subject to unacceptably high temperature~. Thus in preferred arrangements, the outlet end region of the elongated zone extends beyond th~ sheath and likewise the extended surface of the elongated zone terminates short of the outlet end of the elongated zone. In a particularly preferred arrangement the extended surface of the elongated zone terminates, in the outlet end region of the latter, at a position closer to the outlet end of the elongated zone than does the shesth~ In ~his way the heating of the elongated zone is ~ainly by radiation at the outlet end region where the elongated zone is un~heaehed and hai no extended surface, by a mixture of radiation and convection over the intermediate part of the outlet end end region where it is unsheathed but has an extended surface, and mainly by convection in the region where it is sheathed.
Preferably the unsheathed outlet end region of the elongated zone constitutes 10 to 30Z of the length of the elongated zone while the part of the outlet ~nd region of the elongated zone that does not have an extended surface constitutes 4 to 20% of the length of the elongated zone.
To enhance the heat flux over the inlet region of the elongated zone, the extended surface may be of greater surface area over this region, for exa~ple by increasing the number of fins for the first part, for example the first 30 to 60% of the length of the elongated zone.

By means of the above features the average heat flu~ in~o the elongated zone can be very high, over 100 kw per m2 o the interior surface of the external wall of ~he elo~gated zone. This average h~at flux may be as high as 200 kW m 2 but is typically in the range 120 - 150 kW m 2 Where the elongated zone is heated by second~ry reformer gas, the latter cools as it passes along the exterior surface of the elongated zone. During such cooling, especially at the preferred temperatures and pressures, there is a strong driving force for the unwanted side reactions 2C0 ~ C + C02 "Boudouard reaction' C0 + ~2 ~' C + H20 C0 + 3~3 -~ Ca4 + H20 and hese will proceed especially if the metal contacted by the gas catalyses ehese reactions and/or forms carbides and/or removes elemental carbon by catalysing or nucleating the formation of solid carbon. In the high intensity process mentioned above, the sheath and fins provide a particularly large area on which such reactions can take place.
We have found that such unwanted side reactions may be prevented or limited by contacting the hot gas undergoing cooling only with surfaceQ made of one or more metals having substantially no catalytic actlvity, whether in metallic or oxide form, for reactions of carbon monoxide.
The metal presented to the hot secondary reformer gas undergoing cooling while in heat exchange with the elongated zone is typically one or more of those whose oxide is at least as difficult to reduce to metal as chromium II oxide, yet whose oxide forms a coherent "pas~ivating" layer substantially preventing further o~idation of the metal. Preferably such metal does not resdlly form csrbides or nitrides. As examples of suitable metals there may be mentioned aluminium, titanium, zirconium, niobium and tantalum and alloys thereof containing not more than 10% W/w of metals outside that list. Very suitably the metal is aluminium containing no other metals or at most 5% W/w of chromium. Because of its low melting ~ 37~

point (659C, aluminium is unsuitable a~ a constructiona1 metal in the plant in which the process is carried out; however it can be u~ed as a diffusion-bonded layer on a ferrous alloy, such as mild steel, low-chromium steel or ~hromium nickel steel such as AISI type 304 or 316 and Incoloy (RTM) depending on the eemperature to be encountered. The diffusion bonding is carried out so as preferably to form a distinct phase of iron alloy, such as an ir~n-aluminium alloy contalning at least 20% W/w of aluminium. Ferrous alloys 50 treated are availabl~ co~mercially under the trade-name "ALONIZED
in standard units and procedures for such treatment after fabrication are well established.
The reforming catalysts in the elongated zone and secondary reformer (if used) can be respectively conventional primary and secondary reforming catalysts, such as refractory-suppor~ed nickel or cobalt. The elongated zone catalyst should preferably be one having relatively high activity and stability at "low' temperatures, that is,~in the range 550 to 650C. It can be random-packed but can, if desired, be structured.
After leaving the tube (if there is no secondary reforming stage) or the space out~ide the elongated zone (if there is a secondary reforming stage and the hot gas therefrom is used to heat the elongatecl 20ne), the gas is preferably subjected to catalytic shift to convert carbon monoxide to hydrogen plus carbon dioxide.
The! catalytic shift reaction can be carried out in conventional ways, for example 'high temperature"p with an inlet temperature of 330 to 400C, outlet temperature 400 to 500C, usually over on lron oxide/chromia catalyst, and affording an outlet carbon monoxide content in the range 2 to 4% V/v on a dry basls;
"low temperature", ~ith an inlet temperature of 190 to 230~C, outlet temperature 250 to 300C, usually over a catalyst comprising metallic copper, zinc oxide and one or more other difficultly reduclble oxides such as alumina or ~z~

chromia, and affording an outlet carbon mono~ide content in the range 0.1 to 1.0% Vlv on a dry basis;
"combination", using the sequence of high temperature shift, cooling by indlrect heat e~change and lo~
temperature shift; if desired, either shift step can be subdivided with interbed cooling.
Alternatively a "medlum temperature" shift can be used, in which the inlet temperature is in the range 250 to 325C and the outlet temperature up to 400C. A suitably formulated supported copper catalyst can be used. The outlet carbon monoxide content is up to 2~ V/v on a dry basis.
Whichever shift reaction methcd is used, it is preferably operated in indirect heat exchange with a coolant, especially water under pressure. Thus the catalyst can be disposed in tubes surrounded by the water, or vice versa.
Utilisation of the heat taken up by the water may be by generating steam at for example 15 to 50 bar abs. pressure and use of such s~ea~ as feed to the shift step or the steam/hydrocarbon reaction.
The resulting shifted gas is cooled to condense out unreacted steam. If it is to be treated further by pressure swing adsorption (PSA) it can be subjected to selective o~idation or ~ethanation to remove carbon monoxide but need not be contacted ~ith a carbon dio~ide absorption liquid. If it is to be sub~ected to cryogenic nitrogen removal it should be contacted with such a llquid, then methanated and dried. Such further treatments can remove part or all of any nitrogen present depending on the desired composition of the final product.
The present invention al90 provides apparatus for conducting an endothermic catalytic re~ction comprising:
(a~ a tubular reactor having (i) a first tube, blind at one end, provided with an inlet at the other end, 1~ ~ 333g7 ~li) a ~econd tube dlspo~ed wlthln, and extending along, the flrst tube, thereby prov~dlng a 3pace betweYn the flr~t nnd aecond eube6 for receip~ of cat~lyst, the interior of ~ald ~econd tube com~unicstlng wl~h the ~pace between sald flr~t and second tube~ at tha blind end of ~aid flrst tube, and ~nld ~cond tube havlng an outl~t at the lnlet end of said first tube, and (b) ~esn~ for aupplying a heatin8 fluid to the e~eernsl Yurface of the fir~t tube;
ch~racterl~ed in that the 3econd tube csrrle~, for at least part of lt~ length, internally or externally, or both, a layer of ther~sl in~ulation.
More partlcularly ~uch app&ratus i8 part of sn a~se~bly (referred to h~rein a~ th~ reactor asoembly) ncludlng a plurality of ~uch tubular reactor~ wlthl~ an outer ~hell havlng sn lnlet and vutlet for fluld to be brought lnto heat exchange wlth the flr3t tube~ of the tubular reactor~. ~uch nn outer shell iH con~tructed to withstand a pre~sure ln the rsnge preferably 5 to 80 bar ab~, so that ~he heat excbange fluld can be under ~uperat~o~pherlc pres~ure and can thu~, If aQproprlate, be utlllaed i~ a power recover~ englne, and ao that the flrst tube~ of tha tubular r~actor~ of the ~D~embly can be ~ade of relatlv~ly llght gsuga ~etAl.
The reactor anse~bly ~ay be co~bined ~lth the ~ourc~
of hent exchange Eluld. For exa~ple the source can be an e~ternal furnsce providing hot co~buatlon ga~es or a nuclenr reactor proYldin~ hot pre~surlsed heliu~ or, In the event that the tubular r~actor provlde~ thq elong~ted ~one Oe th~ ~tea~-hydr~carbon reectlon proces~ Oe the flr~t a~pect of th~
Inventlon whereln the heatlng medlum 1~ secondAry refor~er g~, an o~ldative secondary reformer.
Accordingly the inveutlon further provlde~ a resctor a~embly aA afore~ald ln comblnstlon wlth A ~econdery reformer 7~

provided with means for supplylng a gas containing free-oxygen thereto, sald assemb1y including flrst condult mea~s connecting the outlet of said secondary reformer with the inlet of said outer shell, second condult means communicatlng with the inle~
of each tubular reactor for supplying a gas stream thereto from outside sald outer shell, and third conduit means communicating with the outlet of each tubular reactor for delivering said ga~ stream, after passage through said tubular reactor, to said secondary reformer.
As an alternative, the reactor assembly can include ln the same outer shell an additional section ia which such combustion or oxidative reforming takes place.
To complete the heat e~change arrangements of the combination, a further heat exchanger is preferably providedO
the hot side of this further heat exchanger being in communication with the outlet for the heat exchange fluid fro~
the shell, and the cold side being in communication with the inlets of the tubular reactors.
The reactor assembly preferably includes fluld flow guide means effective to enhance heat transfer at the surface of the first tubes of the tubular reactors. Such means can include baffles but preferably includes a sheath surroundlng each tubular reactor and extending for at least the major part of the len~th thereof as aforesaid and means to obstruct or prevent flow of such fluid other than through such sheaths.
The cros~-sectiosal area between each sheath and its associated tubular reactor is preferably between 0.1 and 1.0, especially 0.2 and 0.5, tlmes the cross-sectlonal area of the space between the flrst and second tubes of that reactor. As mentioned herelnbefore the sheath preferably termlnates short of the blind end of ies associated first tube, leavlng 10 to 30% of the length of the flrst tube unsheathed at the blind end.
As a further enhancement of such heat exchange, the first tube preferably has, for at least a major part of its ~æ~

length as aforesald, an extended outer ~urface. If, in the nbe~nce of an e~tended outer ~urface, the outer aur~nce were con~ldered to be a cyllnder of radlua equ~l to th~ ~lnl~u~
cro0~uectional radiu~ of the e~tended o~ter 3urfsc~, it 18 preferred that~ over the part having the e~tended 3ur~Ace, the exterasl surfaoe area of the irst tuba is 1~5 to 10 tl~e~ the are~ of the c~rved aurfar~ of that cyllnder.
For the reason~ msntioned herelnbefore, the e~tended ~urface preferably ten~lnates ~hort of the blind end of the tube. The ~lrst tuhe ~hus preferably haa ~ cyllndrical portlon between the portlon havlng the e~tended wrface snd the bllnd e~d: lt 18 preferred that thl~ cyllndrlc~l port10n con~tltute~
4 to 20~ of the length of the first tube.
The extented surfacQ ~y be provlded by~ for ax~npl~
flns, ~tuds, or by the ug~ of ~ corrugated profile fir6t tube.
The thermal ln~ulatlon of the second tube preferably i8 ~ssociated therewlth for at lea~t that portlo~ o~ the second tube that occuples the reglon e~tesdlng fro~ lOX eo 30~ and partlcularly nt leu~t the region e~tendln8 fro~ 5% to 50Z, of tbe le~gth of the flr~t tube, as ~es~ured fro~ the lnlet end o ehs flr~t t~be~
Preferably the lnsulatlon e~tends fcr æub~t~ntlally all the leagl:h of the second tube that 1~ ~lthln the fir~t tube. The lnaulatlon csn be provlded, for exa~ple by refrsctory oxldlc coatlng or a ~leeve affordlng nn e~pty gn~
space or a sleeve encloslng ~ layer of ~olld or flbrous refractory oxide and ~ny lnclude a rntlatlon-reflectlng 13yer.
Whlle a ~etal sleeve enclonlng an eNpty 8ao Hpace or n flbrouc, purticul~rly cera~lc flbre, ln~ul~nt 1~ attrnctlve, difflcultleH ari~e a~ a re~ult of the elon~ated nature of the reactors ~nd the sub~tantlal temperature difference that wlll occur acro~ the ln~ulatlon 2 thu~ the ther~al exp~n~lon dlfference ~etween the second tube and lts sleeve presents problema.
A preferred for~ of con~tructlon 19 to hsve one end ~ .

6~7~

of the ~leeve fa~tened to the ~econd tube with thz other end of the ~leeve not a~tened to ehe tube but beln~ free to e~p~nd or contr~ct.
In ~oma cas~a lt ~ay ~e d~slrabl~ to fabr~c~t~ th~
tube, and ~leeve, ln tub~ Qectton~ ~hlch Are then welded together. In ~uch case~ lt 1~ preferred th~t at le~t two adJacent ~ec~lon~ esch have a ~leeve~ wlth one end of each ~leeve f~tened to itR a~soclated ~ection and the other end free to expund or con~rnct~ the freQ end Oe the ~le~v~ of one oE ~sld ad~acent ~ectiona belng adjace~t the fD~tened end of the sleeve on the ~ectlon ad~acent that one sectlon. In such c88e8 it i8 preferred that ~h~ sleeY~ of ad~ncent sleeved sectloD~ sre dl~ensloned ~o that, when fully expanded at the worklng te~perature, the ~leeve on one ~leeved aeetlon e$tends over ~ub~tsntlally all the gep between that ~ectlon and the ad~ace~t ~ection, thu8 providing insulation ~t lea3t by ~tatlonary gas In such gap. If practlcable, the flbrous ln~ulatlon may be exeeodabl~ wlth the e~pandlng sleeve.
The, or each, sleeve 18 preferably f~stened to it~
tube, or sectlon therof, at the up0trea~ ~nd of th~ tuba or ~ction. Slnce, ln that caae, the lnsulatlon 1~ at thn pre~sure of the ~a~ at the free edge, ~hlch pre~ure 1~ lower thAn that at the ~ecured edge, fallure of the sleeve wlll cause lt to yleld on to the cersmic inuulatlon, ratber than to detach ltselE. A~ a result the ~10eve 1~ ln effect ~tructur~lly supported by the ~ube and need be made only of thln gauge ~etal. Slnce there 1~ to be ~ cat~ly~t between the flrot and second tube~ and that catalyst ~ay be ln a random packed form, the thermslly lnsulatlng ~tructure lu preferablg ln~lde the 8econd, ie lnner, tube ~o that the cat~lyut unlts bear agaln0t ~tru~tur~l metal rathee than the relatlvely thln leeve.
The lnventlon 18 lllustrated by re~erence to the accompanying drawinR0 whereln5 Flgure 1 1~ a longltudlnsl ~ectlon of ~ re~ctor a~sembly uhown ln dlagra~matlc form ,.~

14 B 333~7 Figure 2 is an enlarged longitudinal sectlon of part of the assembly shown in Fig~re 1 Figure 3 is an enlarged longitudinal section of one of the reactors of Figures l and 2 Figure 4 is a cross-section of one of the rea~tors of Figures 1 and 2 showing in outline the relative location of adjace~t reactors Figure 5 is a view similar to Figure 3 but in which the fins and sheaths have beea omitted Flgure 6 is an enlarged longitudinal section of the in~er tube sf a reactor in the "cold state"
Figure 6a is an enlarged section of part of the inner tube of the reactor of Figure 6 in the "hot" state Figure 7 is a view corresponding to Figure l but showing an alternative reactor assembly in diagrammatic form Figure 8 is a flow diagram of the assembly of Figure l in combination with a secondary refor~er and heat exchanger.
Figure 9 is a graph sho~ing the temperature profiles of the reactants and heating medium in a reactor according to the invention and in a conventional clouble--pass reformer.
Referring first to Figure 1 the assembly lO has upper and lower sections 12, 14, which mate at flanges 16, 18, and an insert 20. The lower section 14 has a metal pressure shell 22, lined with refractory concrete 24 as lnsulation, and is provided with secondary reformer gas inlets and outlets 26, 28 respectively. The lower part of shell 22 is surrounded by a jacket 30 to which water can be supplied via port 32 and from which steam can be removed from port 34. Jacket 30 serves to maintain the shell at a desired te~perature, e.g. 100C.
The upper section 12 of assembly 10 has a metal pressure shell dome 36 lined with refractory concrete 38 as insulation and provided with reactants inlet and outlet ports 40, 4~ respec~ively.
The insert 20 is located in lower section 14 by a flange 44 engaging in a rece~s 46 in the top of the refractory concrete lining 24 of lower section 14. Insert 20 includes a plurality of metal tubular reactors 48 located within a thin gauge metal skirt 50 (shown part cut away) that is spaced from the interior walls of the concrete lining 24 by a distance to allo~ for thermal expansion of i~sert 20. Typically there may be fifty to one hundred or more reactors 48 but, for clarity, only four are sho~ in Figure 1.
Each reactor 48 has a closed, ie blind-end, tube 52 provlded with fins 54 on its surface to increase its surface area and extends f rom a sheath 56. Reactors 48 are located with respect to each other and with respect to skirt 50 by a light gauge horlzontal wire or strip framework (not shown).
Insert 20 has a reactants inlet pipe 58 connected to port 40 of upper section 12 v~a bellows 60 to allow f or thermal expansion. Pipe 58 is provided with insulation 62. Each of reactore 48 haæ a reactants outlet 64 communicating, via the space 66 within the upper section 12, with the reactants outlet port 42. Insert 20 also has a secondary reformer gas outlet pipe 68 leading from insert 20 through the outlet port 28 in lower section 14.
To enable the assembly 10 to be assembled pipe 68 iR
not sealed to insert 20 but is a sliding fit therein so that pipe 68 can be withdrawn through port 28 thereby enabling insert 20 to be lifted out of lower section 14 (after removal of upper section 12 therefrom).
The construction of lnsert 20 is ~hown in more detail in Figures 2 and 3, and include~ upper and lower tube plates 70~ 72 respectively, which, with an annular wall 74 and flange 44 form an enclosure 76 to which reactants are fed via reactants inlet pipe 58.
Each outer reactor tube 52 depends from lower tube plate 72, the underside of which is provided with a layer 78 of ~z~
16 ~ 333~7 ln6ulation, whi~e each lnner reactor tube 80~ which carrles a l~yer of insulation 82 on ltæ i~terlor surface, extend~, Ero~
abo~e upper tube plate 70, through th~ enelosure 76 ~nd ~own ln~lde it~ a~ociated outer tube S2, ter~lnating, a~ sho~n in ~l~ure 3, nenr ~he blind end 84 thereof. Upper tube plate 70 carrle~ a layer of i~ulatioD 86.
Although, for th~ ~ke Df #l~pllclty and cl~ty, ~ubs plate 70 i8 shown a6 beln~ lnee~eal with annulQ~ wall 74 and lower tube plate 72, ln pr~ctlre upper tube plate 70 can be ~epnrated from annular wall 74 ~nd/or low2r t~be pl~te 72, leaYlng l~ner tube~ 8Q withln thelr re~pec~ive o~ter tube~ 52 to enable catalyst to be charged and discharged to the reactor~. The catnlyst particle3 ~re supported by a per~orntQ
grld 88 near the blind end 84 of each outer tube 52 and ~re loaded, ln the space 90 between the inner and outer tubes 80 snd 52 re~pecively, up to about the level lndlcated ~y dotted llne 92.
Extending downwards fro~ the annular wall 74 of enclo~ure 76 i8 a metal shroud 94 which i8 f~tened st it~
lower end to sklrt 50. E~ch ~heath 56 surroundlng a resctor tube 52 depends from a thin plate 96 lntegral ~lth ~kirt 50.
Plste 96 thu~l define~ the lower 31de of nn enclosure 98 bounded by ~hroud 94 and by lower tube plate 72. The enclo~ure 98 co~unicates with the ~paces 100 b~tween the ~heath~ 56 and tbeir assoclated tubes 52: the outlet from enclo~re 98 ls ~leeve ~02 ln the ~all of shroud 94 through which necondary refor~er gB9 outlet plpe 68 1~ a ~lldlng flt.
each outer reactGr tube 1~ typlcally ueveral ~etres long, typically 5 - 15 ~; referrlng to Flgure 3 the length of tube 52 fro~ lower tube plate 72 t~ the cata~yst re~trslnt 88 18 de~l~nated Lo~ 5he f ms 54 on each outer resctoe tubc S2 e~tend for ~ distance Ll, ~tartlng at 9 dlstance L2 below lo~er tube plate 72. Sheaths 56 extend ~or a dl~tance L3 fro~ plste 96, which 1~ located at a dl~tance L4 ~elow lower tube plate 72.
In thla nrrsnge~ent the elongated, catalyst contalnlng, '7~

17 B 33~97 20ne ~d has a length almost equal to L3~ me sheath 56 terminate~ sh~rt o~ ~he bllnd end 84 o~ tube 52~ the unsheathed part at the blind end of the tube 52 thus h~s a length L5 equal to ~ - (L3 + L4).
It 1~ preferred th~t L5 i~ 10 to 30X of Lo~ wi3¢ the fins 54 termlnste ~hort of the bllnd end 84 ~ eube 52: the region of the eatalyst-contalnlng elongat~d zone at the bllnd end of tub~
52 tha~ doe~ not ha~e an extended surfaee thu~ hsa a len~th L6 eqU~1 tO L9 - (LL + L2 ) It 1B pre~erred that L6 18 le~s than ~5 and th~t L~ is 4 ~o 20Z of Lo~
The cro~-secelonal area of sp~ce 90 i~
A~ /4 ~12 _ D~
where Dl ~nd D2 are re~pectively tbe lnterior dia~eter of tube 52 ~nd the exterior dia~eter of tube 80.
Igooring the cross-sectlonal area oecupied by ~lns IS 54, the C~O~h section area of space 100 i~
A2 ~ ~/4 (D3~ - D4 ) where D3 Qnd D4 are re~pectlvely the lnterlor dla~eter of ~he~th 56 and the exterlor dia~eter of the unflnned ~urfac~ of tube 52.
It 19 preferred that Az 1~ 0.1 to 1.0, e~peelally 0.2 t~ 0.5 tl~e~ A1-The fln~ 54 proYlde the 1nned part of tube 52 wlth an e~tended filurface: lf the height of the fln~ i~ h and the length of tbe! fi~ned psrt ~f tube 52 18 1, this e~tRnded ~urf~ce ha~ ~m ~re~ A3 where A3 ~ ~rD41 + 2Nh1 where N 19 the number o flna. There ar~ typlcally 20 to 100 f inH on esch tube 52: nlthough not shown in the drswln~ th2 finned pnrt of tube 52 nenrer lower tube plsto 72 ~y h~v~
8reater numbcr, N1, of ~ln~ thnn thQ ~lnned p~rt remote th~r~fro~. If the flnned pnrt remote fro~ tube plat~ 72 h8~ N2 fln~ then the average e~tended ~urf~ce, per unlt length, of the flnned part of tube S2 ls A4 ~ 1 ~f D4 + 2 (Nlll + N212)h 37~

where l1 and 12 are lengths of the parts having N1 and N2 fins respectively.
It is preferred that A4 = 1.5 1 qr D4 to 10l ~fD4 As shown in Figure 4 the tubular reactors constitutad by the tubes 52 and 80 ~ogether with their sheaths 56 are convenien~ly disposed in an e~uilateral triangullr array: since there ~s no gas flow in the space 104 between the sheaths of the adjacent reactors, the reactors may be spaced a~ close as is co~venient from engineering considerations.
The lnner tube 80 of each reactor ls, as mentioned hereinbefore, provided with in~ulat1On 82. In Figure 3 the insulation is shown e~tending for the whole length of tube 80.
However it i8 not always necessary that it extends the whole length. In Figure 5, which coresponds to Figure 3 but omits the sheath 56, plate 96, and fin 54, the insulation is shown starting and ter~lna~ing at distances L7 and L8 respectively below tube plate 72. It is preferred that L7 is less than 10~, particularly :Less than 5~ of Lo and that L8 is at least 30%, particularly at least 50% of Lo~
The insulation 82 i9 conveniently a ceramic fibrous material located adjacent the interior wall of tube 80. It is conveniently held in place by a thin gsuge ~etal tubular cover 106 (shown in F~gure 6). As a result of the insulation 82, in u~e the gas temperature within cover 106 will e~ceed that outside tube 80 by a conqidarable axtent and 80 there is liable to be a significant difference in the ther~al expansion of tube 80 and cover 106 in the longitudinal direction. To compen~ate for this the construction shown in Figure 6 is preferably adopted. At the lower end of tube ôO the cover 106 ls welded at 108 to the interior surface of tube 80. The upper end 110 of cover 106 is not fastened to the wall of tube 80 but i8 free to expand and contract: the free end 110 of cover 106 is preferably flared.
It is often convenie~t to construct tube 80 in 7~

sections and ~elding together the sections end-to~end ; in Figure 6 a weld 112 is sho~n between two sections 80a and 80b.
~ach section is provided with its own layer of insulation 82a, 82b and cover 106a, 106b. At the lower end of each section S the cover, e.g. 106b is welded, e.g~ at 108b5 to the interior surface of its tube, eg tube 80b. The flared end llOa of the cover 106a of the adjacent section is dimeasioned such that, as shown in Figure 6a, in the hotJ i.e. expanded, state, the flared end llOa e~tends over the welded lower end 108b of the cover 106a of the adjacent section. In this way a space 114 containing staeionary gas is provided as insulation between the end of the insulation 82a of one section and the start of the insulation 82b of the next section. Since the temperature difference, in use, between tube 80 and cover 106 will increaæe along the length of tube 80 from the lower end to the upper end of the insulated part of tube 80, where tube 80 is made in sections, the distance between the end llOa of cover 106a and the welded end 108b of tube 80b preferably increases, in the cold state, for successive joints from the lower end towards ~he upper end of the insulated part of tube 80.
The insulation 82 may be in a plurality of layers (not show~) each layer having, on its surface nearest the wall of tube 80, a metal foil layer (not shown).
Since the lower end of the, or each, cover 106 is fastened and the upper end or ends are free and the gas flow is up the inside of tube 80, the gas pressure at the upper end(s) of cover(~) 106 is less than ae the lower end(s) thereof so that failure of cover 106 leads to yield on to the insulatlon 82 rather than to implosion.
For a reactor to be operated at about 40 bar abs.
pressure with a temperature of 800C at the lower end of tube 80, typical dimension~ are, in mm.:
Tube 80 outer diameter 31 Tube 80 wall thickness 1.65 Cover 106 thickness 0.5 ~aæ~

Cover 106 internal diametes 22 Ceramic fibre insultion layers, thickness, each about The effective in~ernal cross-section available for gas flow within tube 80 may be selected to provide for sufficient pressure drop that, if the 10w of gas is reversed, the catalyst particles In the space 90 between tubes 80 and 52 can be blown out: this enables discharging of the catalyst to be readily accomplished by connecting a source of pressurised gas to the outlet end 64 of tube 80 when discharge of the catalyst is required.
Since the sliding joint between pipe 68 and sleeve 102 will not be gas tight it is preferred that skirt 50 is dimensioned that~ when in the hot state it $s expanded sufficiently to restric~ flow of secondary reformer gas between the outer surface of skirt 50 and concrete lining 24. In this way the amount of secondary reformer gas bypassing the spaces 100 between sheaths S6 and tubes 52 can be minimised.
To minimise carbon lay-down from the secondary reformer gas as it cools as it flows past tubes 52, it is preferred that ae least the outer surface of tubes 52, fins 54 and the inner surface of sheaths 56 are made of stainless steel having a diffllsion bonded surface layer of aluminium Also the upper ~urface of plate 96, the inner walls of shroud 94 and pipe 68 may have a similar construction.
In Figure 7 a slightly modified reactor assembly is shown diagrammatically. The construction is s1milar to that of the reactor assembly 10 of Figure 1 except that the secondary reformer gas outlet 28 is located in the top sectlon 12 of the assembly; dome 116 is provided above and sealed to upper tube plate 70 to enclose space 66 from which the primary reformed gas exits the assembly 10 via a pipe 118 connecting space 66 with port 42; shroud 94 is omitted and provision is made for cooled secondary reformer gas to leave enclosure 98 round the outside of the annular wall 74 of enclosure 76 and thence, via the space 120 between the dome 116 and the concrete lining of upper section 12, to outlet port 28.
In a typical process using the reactor assembly of Figures 1 to 4 (the arrows in Flgures 2 and 3 indicate the flow of gas) a mixture of natural gas (1 vol) and steam (3 vols) at 40 bar abs. pressure is preheated in a heat exchanger 122 (see ~igure 8 wherein, for clarity, only one tubular reactor is shown) to 400C is supplied to enclosure 76 via pipe 58. The reaceants then pass down through the catalyst, a supported nickel steam reforming catalyst, in space 90 between tubes 80 and 52 wherein it $s heated by gas passing up space 100 bet~een sheath 56 and 52. The gas passing up space 100 provides the heat for the endothermic steam reforming reaction over the catalyst. The reaction proceeds further as the reactants ~lxture passes down through the catalyst and as its temperature increases. At the bottom of tube 52, the resulting primary reformed gas, no~ typically containing 15 _ 30~ V/v of methane on a dry basis and is, for example now at a pressure of 37.6 bar abs and a temperature of 720C, returned through tube 80.
As a consequence of the insulation 82 the gas undergoes at most a limited heat loss to the reactant mixture in space 90. The prlmary reformed gas leaves the reactor assembly 10, via space 66 and outlet 42, at, typically 700C, and enters, via line 124, a secondary reformer 126 (whlch can be of the conventional type consisting of a refractory lined, possibly water jacketed vessel) to which hot air is supplied via line 128 to a suitable burner nozzle. The products of the resulting flame reaction are brought towards equllibrium at a methane content of typically 0.01 to 1% Vlv on a dry baGis over a nickel secondary reforming catalyst 130 from ~hich the hot secondary reformed g8S leaves, at typically~ 1000C. This hot gas i9 fed back via line 132 to reactor assemb.1y 10 and enters the lower section 4 thereof via port 26.
The hot secondary reformer gas then passes past the 7~

lower ends of tubes 52 (which do not have fins) ~ then past the unsheathed, f$nned portion~ of tubes 52 and into and up space 100 between sheaths 56 and tubes 52 into enclosure 98. During its passage past tubes 52 the secondary reformer gas c0013 a~
it supplles heat to the reactants within space 90 between tubes 52 and 80. From enclosure 98 the cooled secondary reformer gas leaves the assembly 10 via pipa 68, typically at a pressure of 35.6 bar abs and at a temperature of 500C. The gas is the~
fed, vla line 134, to heat exchanger 122 where it acts as the heat source for pr~heating the natural gas/steam mixture. The reformed gas is then fed, via line 136, to further processing, e.g~ shift and sarbon oxides removal.
In Figure 9 a graph ls shown of calculatet temperature profiles for a tubular reactor in accordance with the invention (lines A, B, C) ie with an insulated inner tube, and, by way of comparison, for an equivalent tubular reactor having no such insulation (lines A1, B1, C1). Lines A and A1 show the temperature profile of the heating medium outside the outer tube, lines B, B1 the temperature profile of the reactants in the catalysC containing zone~ and lines C, C1 the temperature profile of the gas returning through the inner tube. It is evident that at, for example, a reactants temperature of 575C the temperature difference across the tubular reactor outer wall is about 60C according to the invention but only about 20C when heat exchange is permitted, and that the temperature difference is larger when using the invention at all other reactants temperatures. It is calculated that for a given gas output, corresponding for example to the productlon of 1000 metric tons per day of ammonia, the number of tube~ required i~ only 83 to 86% of the number required $f heat exchange across the walls of the inner tube is permitted. Moreover, the tubes can be shorter, with a heated length 66 - 73%, and hence a smaller catalyst volume, 57 -64%, of that required if heat exchange across the walls of the inner tube $s permitted.

Claims

Claims:

1. A process for producing a product gas stream containing hydrogen, carbon oxides, and methane by incomplete catalytic reaction of a hydrocarbon feedstock with steam and/or carbon dioxide comprising:
passing a reactants stream containing a hydrocarbon feedstock, steam and/or carbon dioxide over a catalyst disposed in an elongated zone having an inlet end and an outlet end, said elongated zone being defined by an outer wall and a wall of a tube, disposed within the outer wall and extending from the outlet end of the elongated zone to the inlet thereof;
externally heating said elongated zone with a heating medium withdrawing a product gas stream containing hydrogen, carbon oxides and methane through said tube counter-currently to the flow of said reactants stream over the catalyst; and cooling the product gas stream by at most 100°C by heat exchange with the reactants stream across the wall of said tube as the product gas stream passes through said tube thereby improving heat transfer between the heating medium and reactants stream.

2. A process according to Claim 1 wherein the degree of cooling is such that the product gas stream cools by less than 30°C during its passage through the tube.

3. A process according to Claim 1 comprising reacting the product gas stream with a gas containing free oxygen in an adiabatic catalyst zone whereby to effect reaction of methane in said product gas stream to produce a hot gas stream containing carbon oxides and hydrogen and using the hot gas stream as the heating medium to heat the elongated zone.

4. A process according to Claim 3 wherein the reaction of the product gas stream is effected in a secondary reformer and the product gas stream is cooled during its passage through the tube from the outlet of the elongated zone and to the secondary reformer by no more than 50°C.

5. A process according to Claim 3 wherein, prior to feeding the reactants stream to the elongated zone, heating the reactants stream by heat exchange with the hot gas stream after the latter has been used for heating the elongated zone.

6. A process according to Claim 3 wherein the outer wall of the elongated zone has an outer metal surface and the elongated zone is heated by passing the hot gas stream through a region defined by metal surfaces and through which the elongated zone extends, the outer metal surface of the elongated zone and all the metal surfaces of, and within, said region, with which the hot gas stream contacts while in heat exchange with the elongated zone are surfaces of metal having substantially no catalytic activity, whether in metallic or oxide form, for reactions of carbon monoxide.

7. A process according to Claim 1 wherein said elongated zone is heated by passing a stream of the heating medium, in a direction counter-current to the flow of the reactants stream through the elongated zone, through a sheath surrounding the elongated zone and extending for at least a major part of the length thereof.

8. A process according to Claim 7 wherein, at the outlet end of the elongated zone, the elongated zone extends beyond the sheath.

9. A process according to Claim 1 wherein the outer wall of the elongated zone has, for at least the major part of the length thereof, an extended surface.

10. A process according to Claim 9 wherein the extended surface of the elongated zone terminates short of the outlet end thereof.

11. A process according to Claim 1 wherein the tube has a layer of insulation on its inner or outer surface.

12. A process for the producing a product gas stream containing hydrogen, carbon oxides, and methane by incomplete catalytic reaction of a hydrocarbon feedstock with steam and/or carbon dioxide comprising:
providing a reactants stream containing a hydrocarbon feedstock, steam and/or carbon dioxide and passing this stream over the catalyst in an elongated reaction zone from an inlet end thereof to an outlet end thereof, to form a product gas stream containing hydrogen, carbon oxides and methane;
passing the resulting product gas stream from the outlet end of the said reaction zone back past said reaction zone in counter-current flow to said reactants stream and in indirect heat exchange relationship with said reactants stream so as to indirectly heat the reactants stream by the heat in said product gas stream, applying further heat to the reactant stream as it passes through an elongated reaction zone by an external heating medium, and maintaining insulation between the reactants stream and the product gas stream such that the product gas stream is cooled by at most 100°C by said indirect heat exchange thereby improving the efficiency of heat transfer between the external heating medium and the reactants stream.

13. Apparatus for conducting an endothermic catalytic reaction comprising:
(a) a tubular reactor having (i) a first tube having an internal surface and an external surface, an open end, and a closed end;
(ii) reactant inlet means connected to the open end of said first tube;
(iii) a second tube having an internal surface bounding an interior of the second tube, and an external surface, said second tube being disposed within, and extending along, the first tube, thereby providing a space between the external surface of the second tube and the internal surface of the first tube, the interior of said second tube communicating with said space at the end of the second tube adjacent the closed end of said first tube;
(iv) product outlet means connected to the other end of said second tube; and (v) thermal insulation on at least one of said internal and external surfaces of the second tube for at least part of the length thereof;
(b) a catalyst for the endothermic reaction disposed in said space between the tubes for at least part of the length of said space;
said thermal insulation on said second tube extending for at least part of the length of that part of the space containing the catalyst; and (c) means for supplying a heating fluid to the external surface of the first tube.

14. Apparatus according to Claim 13 wherein the thermal insulation on said second tube extends for at least that portion of the length of said second tube that occupies the region extending from 10% to 30% of the length of the first tube, as measured from the open end of said first tube.

15. Apparatus according to Claim 13 wherein the insulation is provided by a sleeve enclosing a space with one end of the sleeve fastened to the second tube and the other end free to expand or contract.

16. Apparatus according to Claim 15 wherein said space enclosed by said sleeve is filled with fibrous insulation.

17. Apparatus according to Claim 15 wherein the second tube comprises welded together tube sections and at least two adjacent sections each have a sleeve enclosing a space with one end of each sleeve fastened to its associated section and the other end free to expand or contract, the free end of the sleeve of one of said adjacent sections being adjacent the fastened end of the sleeve on the section adjacent that one section.

18. Apparatus according to Claim 17 wherein the fastened end of each sleeve is upstream of the free end.

19. Apparatus according to Claim 17 wherein the sleeves are inside the second tube.

20. Apparatus according to Claim 13 wherein a sheath is provided surrounding the first tube and extending for at least the major part of the length thereof thereby providing a space between the sheath and the first tube having a cross-sectional area between 0.1 and 1.0 times the cross-sectional area of the space between the first and second tube.

21. Apparatus according to Claim 19 wherein the sheath terminates short of the closed end of the first tube, leaving 10% to 30% of the length of the first tube unsheathed at the closed end.

22. Apparatus according to Claim 13 wherein the first tube has, for at least a major part of its length, an extended external surface such that, over that portion having the extended external surface, the external surface area is 1.5 to 10 times the area of the curved surface of a cylinder of radius equal to the minimum cross-sectional radius of the extended external surface.

23. Apparatus according to Claim 22 wherein the portion of the external surface of the first tube having the extended surface terminates short of the closed end of the first tube and the external surface of the first tube is cylindrical over the portion from the end of the extended surface portion to the closed end, said cylindrical portion constituting 4% to 20% of the length of the first tube.

24. An assembly comprising:
(a) a least one apparatus according to Claim 13; and (b) an outer shell within which said at least one apparatus is located, said outer shell having an inlet and an outlet providing the means for supplying heating fluid to the external surface of the first tube of each of said at least one apparatus.

25. A combination comprising:
(i) an assembly according to Claim 24, (ii) a secondary reformer external to said outer shell, said secondary reformer having an inlet and an outlet, (iii) means for supplying a gas containing free oxygen to said secondary reformer, (iv) first conduit means connecting the outlet of said secondary reformer with the inlet of said outer shell, (v) second conduit means communicating with the inlet means of said at least one apparatus for supplying a gas stream thereto from outside said outer shell, and (vi) third conduit means communicating with the outlet means of said at least one apparatus for delivering said gas stream, after passage through said tubular reactor, to the inlet of said secondary reformer.

26. In a double tube reformer having a first tube, closed at one end, and a second tube disposed within said first tube thus defining a zone between the tubes for receipt of a catalyst, said second tube having an open end terminating adjacent the closed end of the first tube, for withdrawing product from the space between the first and second tubes, the improvement whereby improved heat transfer may be achieved between a heating medium heating the external surface of the first tube and reactants passing through said zone, said improvement comprising thermal insulation on at least one of the internal and external surfaces of said second tube for at least part of the length of said zone.