Steam reforming

The endothermic reaction of a hydrocarbon feedstock with steam and/or carbon dioxide is carried out using a double-tube reactor. In one preferred form of the reactor the catalyst is present as a coating on the outside of the inner tube. In another form inner tubes are mounted in one tube-plate and the other tubes in a second tube plate and the tube plates are disposed across a cylindrical shell so as to define a heat exchange zone (provided with a heat source) a reactants feed zone and a products offtake zone. The heat source is preferably a burner, to be fed with the product of the endothermic reaction, followed by a secondary reforming catalyst. The apparatus makes possible processes for making raw hydrogen-containing gases with advantageous internal heat recovery.

This invention relates to reforming of hydrocarbons with steam and/or 
carbon dioxide and in particular to an apparatus and process for carrying 
it out. 
In our published European application No. 21736 we describe a process for 
producing a gas containing hydrogen which comprises reacting in the 
gaseous phase a hydrocarbon, hydrocarbon derivative or carbon monoxide 
with steam and/or, where appropriate, carbon dioxide, in the presence of a 
catalyst supported on a metal or alloy resistant to the conditions in 
which the reaction is carried out; and more particularly a process for 
producing a gas containing hydrogen which comprises reacting in the 
gaseous phase a hydrocarbon or hydrocarbon derivative with steam and/or 
carbon dioxide, in the presence of a catalyst with a catalyst outlet 
temperature such that the product gas contains at least 30% v/v of 
hydrogen on a dry basis, in which the catalyst units comprise a support 
and at least one active metal selected from the group consisting of 
nickel, cobalt and the platinum group metals, characterised in that the 
catalyst support comprises a primary support made of a metal or alloy 
resistant to the conditions in which the reaction is carried out and a 
secondary support which is a layer of a refractory oxidic material 
adhering to the surface of the metal or alloy. 
We have now realised that the use of the catalyst supported on the metal or 
alloy makes possible a new design of reforming apparatus and a new process 
sequence, which apparatus and sequence conveniently but not necessarily 
include the said catalyst. 
According to the first aspect of the invention an apparatus for the 
endothermic reaction of a hydrocarbon or hydrocarbon derivative with steam 
and/or carbon dioxide comprises at least one outer tube open at one end 
and closed at the other, means to heat the outer tube externally, at least 
one inner tube within the outer tube and having an open end near the 
closed end of the outer tube, means to feed reactants to the space between 
the outer and inner tubes and to withdraw products from the inner tube, 
and a steam reforming catalyst in the space between the inner and outer 
tubes: characterised in that steam reforming catalyst is present as a 
layer adhering to the outer surface of the inner tube. 
To afford adequate geometric surface for the steam reforming catalyst the 
outer tube can be of smaller diameter than has been common in steam 
reforming practice: up to 75 mm I.D., with an inner tube correspondingly 
small, leaving an annular space 5-20 mm wide, would be suitable. Possibly 
a larger outer tube, up to for example 150 mm I.D. can be used with a 
plurality of inner tubes. Preferably the inner tube has an extended 
surface, provided for example by fins or attached spikes or winding wires. 
The surface area is typically 3 to 10 times that of the smooth tube. A 
very suitable extended surface is provided by a multi (e.g. 6-10) start 
helix with in all 80 to 400 turns per m. The fins or helices can be 
continuous or interrupted. 
The inner tube can contain a turbulator, to improve heat exchange between 
product gas and the reactants in the annular space. Neither the inner 
surface of the inner tube nor the turbulator should carry catalyst, of 
course. 
The catalyst layer can itself be a catalyst by virtue of the choice of 
material of construction or of chemical or physical modification of its 
surface for example by cold-rolling of a nickel-containing unit. More 
commonly it is a support for active material, for example one or more 
metals from Group VIII of the Periodic Table. Especially when the support 
has a very low adsorptive surface, for example when it is a metal or 
alloy, it (as "primary support") carries a coating of adsorptive material 
("secondary support") and the active material is associated with that 
coating. Preferably the Group VIII metal content, if non-noble, is in the 
range 30-60% .sup.w /w calculated as equivalent NiO on the total coating. 
Such adsorptive material typically has a pore volume over 0.2 cm.sup.3 
g.sup.-1 and a surface area of at least 1.0, preferably over 15, 
especially in the range 50-200 m.sup.2 g.sup.-1. The secondary support 
preferably has a thickness in the range 0.01 to 0.3, especially 0.02 to 
0.1 mm. 
The secondary support typically comprises alumina, especially gamma or eta 
alumina. Other sesquioxides, for example, chromia and rare earth oxides 
may take up at least part of the secondary support. Other useful secondary 
support oxides are titania, zirconia, hafnia, thoria, vanadia, urania, 
oxides of manganese, molybdenum and tungsten and combined oxides. 
Preferably the secondary support includes a grain growth inhibitor, for 
example at least 0.05, especially 0.1 to 5.0% by weight of yttrium or of 
one or more rare earth oxides, especially of cerium, or praseodymium. 
When nickel and/or cobalt are present in the catalyst, it is expected that 
the secondary support, if it contains a sesquioxide, will include, at 
least after a period of process operation, some nickel and/or cobalt 
spinel. It is within the invention to have the secondary support material 
at least partly in the form of spinel, whether of nickel and/or cobalt or 
of a divalent metal having a difficultly reducible oxide, especially 
magnesium or manganese or, less preferably, zinc. Since nickel and/or 
cobalt present as spinel is in a reduction-resistant oxidic form, it does 
not contribute significantly to the activity of the catalyst: active 
nickel and/or cobalt are additional thereto. 
In a catalyst comprising nickel and/or cobalt there may also be present one 
or more platinum group metals, which are capable of increasing the 
activity of the nickel and/or cobalt and of decreasing the tendency of 
carbon lay-down when reacting hydrocarbons higher than methane. The 
concentration of such platinum group metal is typically in the range 0.005 
to 1% as metal, calculated on the coating. Further, the catalyst, 
especially in preferred forms, can contain a platinum group metal but no 
non-noble catalyst component. Such a catalyst is more suitable than one on 
a conventional support because a greater fraction of the active metal is 
accessible to the reacting gas. A typical content of platinum group metal 
when used alone is in the range 0.005 to 5% .sup.w /w as metal, calculated 
on the coating. 
The specific surface of the catalytic metal is suitably in the range 1 to 
500 m.sup.2 /g of coating. Within these ranges the larger areas are 
preferred for reactions under 600.degree. C. 
When both non-noble and noble metals are present a useful level of 
catalytic activity can be obtained using a notably small concentration of 
such metals, namely under 2%, especially 0.01 to 0.5, % .sup.w /w in all, 
calculated on the total of secondary support and such metals. The 
preferred noble metal is rhodium. 
The catalyst can be made by applying a compound of the active metal and the 
secondary support together to the primary support. In a preferred method 
the secondary support is applied to the primary support, the combination 
is preferably calcined, and then a solution containing a thermally 
decomposable compound of active metal is applied. When it is desired to 
regenerate the catalyst, the inner tubes can be withdrawn and re-treated 
with a compound of the active metal. 
As an alternative to the apparatus just described the invention provides a 
catalyst unit removable from the space between the outer and inner tubes. 
According to the second aspect of the invention a contact material is in 
the form of tubular units having perforated walls and means to distance 
those walls from the walls of a surrounding container in which they are to 
be stacked in axial relationship with one another and with the container. 
The invention provides also such a container charged with such units, which 
are so distanced from the container walls as to afford a space for fluid 
flow in contact with the container walls. 
The means to distance the container walls from the unit walls can be 
integral with the unit walls (in which event the unit is a new article) or 
can be provided by spacers around or stacked between units that need not 
themselves have integral distancing means. 
The purpose of the perforations is to permit fluid flow in at least one 
direction transverse to the general direction of flow through the 
container. This is especially valuable when contact with the units 
involves absorption or evolution of heat, and thus the invention includes 
a heat exchange apparatus including such a charged container, especially a 
tube and means for heat exchange through the container walls. Examples of 
such heat exchange apparatus are a steam reforming furnace, an ammonia 
cracker and a heavy hydrocarbon cracker (all endothermic processes) and a 
methanator, ammonia synthesis reactor, methanol synthesis reactor, 
aromatisation reactor, ammonia/water absorber or ammonia/acid reactor (all 
exothermic processes). 
The perforations in the unit walls typically amount to between 20 and 60% 
of the unit wall area. For many purposes, but especially a hydrocarbon 
steam reforming process at for example 10-60 bar abs pressure, the 
smallest dimension of the perforations is at least 1 mm, and preferably is 
in the range 2-5 mm. 
Especially when, as is preferred, the units are made of metal or alloy, the 
perforations are the result of punching metal foil or sheet, and 
preferably each such punching leaves at least one internal or external 
vane attached to the unit wall; such vanes increase the geometric surface 
of the unit and can, if suitably pitched, ensure better contact with the 
container walls and thus improved heat transfer. 
The distancing means typically define, in cross-section, a circumscribed 
circle having a diameter between 1 and 30%, especially between 5 and 20%, 
greater than that of the unit. If such means is integral with the unit, 
there are at least 3 projections to ensure equidistance from container 
walls, and such projections may occur at least at or near both vertical 
extremities of the unit, to ensure co-axial stacking; alternatively the 
units may interlock, so that one extremity of each is located by the 
neighbouring unit; or only alternating units in a stack need be externally 
projectioned. In an extreme case the outer wall of each unit can be 
corrugated, that is, be formed with a close succession of projections. 
If spacers are used, these can be for example other tubular units of 
greater diameter having internal projections on which the distanced units 
rest. Such other units may fit snugly or loosely in the container. 
Alternatively the spacers can be shorter tubular units or even essentially 
planar, for example, toothed rings or non-slip washers. 
The above-mentioned internal vanes, or possibly other internal projections, 
can also serve to distance the units from an internal heat exchange 
surface or from other units stacked inside them. 
The units can, if desired, be linked together axially, for example in 
stacks of 2 to 100, to facilitate charging to a tubular container. 
The invention in its second aspect provides further a catalyst support in 
which each unit is made of highly calcined ceramic or of metal or alloy 
and carries a layer of catalyst support material as already described, a 
catalyst precursor in which such support carries a compound convertible to 
catalyst by reduction and/or sulphidation, and the so-formed catalyst. 
Such a compound is typically of one or more metals from Groups Ib, V, VII 
or VIII of the Periodic Table especially as described above. If desired, 
any spacers used can also be coated with such catalyst support and/or 
catalyst precursor or active material. 
The invention provides also chemical processes carried out in such 
apparatus or over such catalysts. In particular the reaction of a 
hydrocarbon with steam and/or carbon dioxide to produce a gas containing 
at least 30% .sup.v /v of hydrogen on a dry basis is operated at 
550-1000.degree. C. and at 1-60 bar abs pressure. In a typical process of 
this kind the geometric surface of the catalyst is well below the level of 
about 300 m.sup.-1 common when using conventional ceramic ring catalyst, 
and is, for example in the range 40-200 m.sup.-1. As a result the pressure 
drop can be under 10%, for example 0.1 to 2.0%, that of such conventional 
rings. Catalyst units to be used are for example 40-160 mm in diameter 
excluding external projections or spacers. 
For such processes the hydrocarbon feedstock preferably has a boiling point 
not over 220.degree. C. and is most conveniently normally gaseous, and 
especially has a hydrogen to carbon atomic ratio of at least 3.5. If a 
hydrocarbon derivative is used it is most conveniently methanol or 
ethanol. 
According to a third aspect of the invention an apparatus for the said 
endothermic reaction comprises 
an outer shell; 
mutually parallel first and second tube plates disposed across the shell 
and dividing it into three successive zones, namely a heat exchange zone, 
a reactants feed zone and a products off-take zone; 
at least one relatively wide tube extending into the heat exchange zone 
from the first tube plate bounding that zone and closed at its extremity 
within that zone; 
at least one relatively narrow tube extending from the second tube plate 
into the extremity of each relatively wide tube; and 
a steam reforming catalyst in the annular space between the tubes. 
The outer surface of the relatively narrow tube preferably carries an 
adherent layer of the catalyst, but it can carry catalyst units 
mechanically linked to it, or there can be structured, especially tubular 
units according to the second aspect of the invention, or loose, catalyst 
in the annular space. 
Especially if the adherent catalyst is used, each relatively wide tube 
preferably has its closed extremity upwards, for these reasons: 
(a) piping connections are facilitated; 
(b) removal of the inner tube to permit catalyst replacement is 
facilitated; 
(c) the space between the outer tubes can be packed, thus improving heat 
transfer; 
(d) use of secondary reformer gas as heat source can be facilitated, as 
will be described below. 
The outer shell preferably withstands superatmospheric pressure, especially 
in the range 5-120, for example 25-80, bar abs. 
In the apparatus according to the first or third aspect of the invention 
there is provision to heat the outer (wide) tubes externally. This can be 
for example one or more fuel burners within the external shell: in this 
event, to avoid excessive flame temperatures and production of oxides of 
nitrogen, two such burner systems can be used in series, the first fed 
with a substantial excess of combustion air and the second fed with the 
oxygen-containing effluent of the first. Either system, but more 
especially the second, can include a combustion catalyst. When two such 
burner systems are used in series, the reaction tubes are preferably 
operated in parallel, especially when the product is to be methanol 
synthesis gas or hydrogen. 
In another example the source of heat can be the effluent from a high 
temperature chemical reaction step, especially from a catalytic "secondary 
reformer" in which the product gas from the endothermic reaction is 
reacted with oxygen to decrease its methane contact. Such a secondary 
reforming step can be carried out in a separate vessel; but very suitably 
the heat exchange zone contains, upstream of the tube, a bed for a 
secondary reforming catalyst and, upstream of that bed and spaced from it 
so as to avoid damage to the catalyst a secondary reformer burner, in 
which primary reformer gas and oxygen, possible as air or enriched air, 
are brought together and reacted in a flame. 
In a further example the source of heat is a gas heated in a nuclear 
reactor. 
When the source of heat is combustion of fuel, this is carried out 
preferably at superatmospheric pressure and the combustion gas is expanded 
in an engine to provide useful power. The combustion pressure preferably 
differs from that of the reactants inside the tubes by not more than 20 
bar 
The invention provides also a set of processes for producing raw hydrogen 
streams. 
In particular the invention provides a process for producing raw ammonia 
synthesis gas by the steps of 
(a) primary reforming a hydrocarbon feedstock with steam to give a gas 
containing carbon oxides, hydrogen, methane and unreacted steam; 
(b) reacting the product of step (a) catalytically with an oxygen/nitrogen 
mixture to decrease its content of methane and introduce nitrogen; and 
(c) obtaining part of the endothermic heat required 
for step (a) from the hot effluent of step (b); characterised by carrying 
out step (a) in 2 stages, the first heated by the hot effluent of step 
(b), the second heated from an external heat source. 
Such a process differs from previous proposals, in which the two stages of 
step (a) are to be carried out in the opposite order. The reason for this 
was that previously available catalysts had little activity at below about 
650.degree. C. and therefor required the very intense heating from the 
external heat source at the start of reaction, at which the reactants 
partial pressure is high and the endothermic heat demand correspondingly 
high. In such processes recovery of heat from the effluent of step (b) was 
by heat exchange with gas that had already reacted to a substantial extent 
in step (a); therefore the temperature difference was small and much heat 
had to be recovered by other means. By using the process of the invention 
the effluent of step (b) can be cooled to 450.degree. to 600.degree. C., 
resulting in substantially greater heat recovery within the reforming 
steps than was previously convenient. Less heat need be recovered as high 
pressure steam, the demand for which is in any event less in recently 
developed ammonia production processes including steam reforming at 
relatively low steam ratios and limited synthesis gas compression. 
The second part of step (a) can be carried out in a pressure furnace or an 
ordinary steam reforming furnace fired at atmospheric pressure. 
In the process the oxygen/nitrogen mixture can be air and can be fed at a 
rate producing after shift and CO.sub.2 -removal, a synthesis gas having 
an H.sub.2 :N.sub.2 ratio approximately stoichiometric (usually 2.5-3.0). 
If desired, the ratio can be lower, as for example with nitrogen removal 
from reacted synthesis gas in the process of our European patent No. 993, 
or with nitrogen removal before the gas enters ammonia synthesis. If 
carbon oxides are to be removed by methanol synthesis, the ratio is chosen 
to suit the relative outputs of methanol and ammonia. 
Alternatively raw hydrogen, or a raw synthesis gas for conversion to 
organic compounds, is produced by the process modified by feeding an 
oxygen-rich gas (over 80, especially over 95% .sup.v v) at step (b). 
The invention also provides processes in which the sources of heat are 
entirely external and heat recovery from hot gases is conventional, for 
example by steam superheating and steam raising and preheating of boiler 
feed water. 
Corresponding to the third aspect of the invention the invention provides 
also a process for producing a raw hydrogen stream convertible to ammonia 
synthesis gas or hydrogen-rich gas or organic compounds synthesis gas, by 
the steps of 
(a) feeding a mixture of a hydrocarbon feedstock with steam into a plenum 
zone and therein heat exchanging it with a hot stream to be described in 
tubes to be described surrounded by that zone; 
(b) feeding the resulting heated mixture into a plurality of annular blind 
heated catalyst zones each surrounding one of the said tubes and each 
heated externally in a heat exchange zone by a medium to be described, and 
reacting the mixture therein to produce a gas containing carbon oxides, 
hydrogen, methane and steam; 
(c) feeding the resulting reacted mixture back through the said tubes in 
heat exchange with the reacting mixture and then as the said hot stream 
with the mixture in the plenum zone, whereby partly to cool the reacted 
mixture; 
(d) collecting the partly cooled gas in a products offtake zone, feeding it 
to a burner, partially combusting it with oxygen as such or as air or 
oxygen-enriched air (depending on the intended nitrogen content of the raw 
hydrogen stream to be produced) and passing the combustion product over a 
catalyst whereby to decrease the methane content thereof, the said burner 
and catalyst being disposed in the heat exchange zone specified in step 
(b); 
(e) passing the catalyst effluent as the said medium in heat exchange with 
the exteriors of the said annular catalyst zones. 
In any such processes there follow steps of cooling and removal of excess 
steam as liquid water. When the product is to be ammonia synthesis gas or 
hydrogen-rich gas, the raw gas is subjected also to shift, CO.sub.2 
-removal and fine purification from traces of carbon oxides.

In FIG. 1 the full line paths labelled P refer to the combination and 
dotted paths labelled Q and R to independent operation of A or B. For 
convenience, integers of both apparatuses are indicated by corresponding 
numbers, those in B exceeding those in A by 100. The points of divergence 
of the paths P, Q and R are indicated by numerals 201, 202 and 203. 
The outer shell consists of cylindrical centre portion 10, 110, upper 
portion 12, 112 and lower portion 14, 114. Upper portion 12, 112 is formed 
with a mixing and burner zone 16, 116, to which is connected a fuel inlet 
18 or gas inlet 118 and an air inlet 20, 120, and is joined by flange 22, 
122 bolted to cylindrical centre portion 10, 110. The upper portion 112 of 
apparatus B also contains secondary reforming catalyst bed 113. 
Centre portion 10, 110 is divided horizontally by tube plate 26, 126 the 
space above which constitutes the heat exchange zone 24, 124 in virtue of 
the "outer" heat exchange tubes 28, 128 extending upwards from the tube 
plate with closed upper ends. Zone 24, 124 includes a packed region 30, 
130 to improve heat exchange between hot gases outside and reactants 
inside tubes 28, 128, and an outlet which (32) in A is for flue gas and 
(132) in B is for cooled secondary reformer outlet gas. 
Beneath tube plate 26, 126 is reactants feed zone 34, 134 bounded by tube 
plate 36, 136, which extends outwards to form a flange bolted to flange 
38. Tubes 40, 140 each having an extended, catalyst-coated surface 42, 
142, extended upwards from tube plate 36, 136 almost to the closed end of 
tubes 28, 128. The inner surface of outside tubes 28, 128 may or may not 
have a catalyst coating. 
Beneath tube plate 36, 136 is product offtake zone 48, 148 from which 
outlet 50 in A leads to gas inlet 118 in B, and outlet 150 in B leads to 
gas inlet 46 in, A. 
Outlet 32 in A leads to gas turbine 52 which exhausts at 54 to low grade 
heat recoveries (not shown) and which drives combustion air compressor, 56 
feeding air inlet 20 and alternator 58. 
The process for producing raw ammonia synthesis gas by the first process 
aspect of the invention can follow path P in the apparatus combination. A 
mixture of desulphurised natural gas and steam at for example 
200.degree.-450.degree. C. is fed at 146 of B to reactants feed zone in 
which it is heated by heat exchange with the gas in tube 140. It passes up 
the annular space in contact with catalyst 142 receiving heat both from 
inner tube 140 and from heat exchange zone 124, until at the top its 
temperature is for example 650.degree.-750.degree. C. and its methane 
content is for example 20-'% .sup.v /v on a dry basis. It then passes down 
inner tube 140 as a source of heat for incoming gas and leaves the bottom 
portion 114 of the shell by way of products offtake zone 148 and outlet 
150 en route for reactants inlet 46 of A. The source of heat in zone 124 
of B will be described below. 
In A the path of the gas is as in B but when in contact with the catalyst, 
the reactants are heated more strongly, to a final temperature of for 
example 800.degree.-850.degree. C., by combustion of fuel fed at 18 with 
hot air fed at 20, part of the heat exchange being radiative. Combustion 
is at superatmospheric pressure and the hot flue gas is expanded through 
turbine 52 driving compressor 56. Gas leaving at 50 is fed at 118 to the 
top portion 112 of B and reacted with air fed at 120. A flame is formed 
and the hot gases are brought to equilibrium at for example 
900.degree.-1000.degree. C. over secondary reforming catalyst bed 113, 
whereafter it forms the heat source for the first stage of reforming in B. 
After cooling in heat exchange zone 124 aided by packing 130 the gas 
leaves by 132, whence it is passed to further heat recoveries and to steps 
of shift, CO.sub.2 -removal and fine purification to give ammonia 
synthesis gas. 
Vessels 10 and 110 are preferably provided each with a jacket (not shown) 
through which cool air or water is circulated, to keep down the 
temperature of the pressure-resisting shell and maintain its internal 
refractory lining in compression. When jacket cooling is by air, the 
resulting warm air can be used, preferably after further warming, as 
combustion air for burner 16. 
The process for producing raw hydrogen or a raw synthesis gas for 
conversion to organic compounds according to the second process aspect of 
the invention can follow path Q in the apparatus, using apparatus B only. 
The process is the same as in the combination as far as point 201, except 
that the methane content of the gas leaving at 150 is preferably in the 
range 0.2 to 5% .sup.v /v on a dry basis, and that, if the starting 
hydrocarbon feedstock is hydrogen-rich, the feed at 146 may include carbon 
dioxide. Following path Q from point 201 the gas flows through line 118 
and enters mixer/burner zone 116, in which it reacts with oxygen or 
possibly enriched air fed in by line 120. The resulting heat evolution is 
sufficient to reform the methane present in the gas at 118 and to heat 
tubes 128, without the external heat required (apparatus A) when making 
ammonia synthesis gas. After cooling in heat exchange zone 124 aided by 
packing 130 the gas leaves by 132. Thereafter it is passed to further heat 
recoveries and to steps of shift, CO.sub.2 -removal and fine purification 
to give hydrogen, or to cooling and water removal to give synthesis gas. 
In an alternative process for producing raw hydrogen or a raw synthesis gas 
path R in apparatus A is followed. The starting mixture of hydrocarbon and 
steam, possibly with CO.sub.2, is fed at point 202 and reacts as described 
above, leaving finally at point 203, whence it undergoes heat recoveries 
and process steps as already described. If ammonia synthesis gas is 
required, a conventional air secondary reformer can be used. 
Especially if apparatus A only is to be used, two such apparatuses are used 
side-by-side, with their combustion sections in series and reaction 
sections in parallel. Thus line 20 feeds approximately double the quantity 
of air required in A, line 32 leads to an inlet line corresponding to 20 
in the second apparatus A and the outlet line corresponding to 32 in that 
second apparatus A leads to the inlet of turbine 52. However, line 46 is 
bifurcated and feeds both apparatuses, and the outlet lines 50 are joined 
before passing to the next unit downstream. 
The Table (first part) shows temperatures, pressures and gas compositions 
for the production of raw ammonia synthesis gas using apparatuses A and B 
in combination, with non-enriched air fed at 120. The second part shows 
such data for producing raw methanol synthesis gas using apparatus B only, 
with substantially pure oxygen fed at 120. It is evident that by using 
apparatus B only and feeding non-enriched air at 120 a raw ammonia 
synthesis gas containing nitrogen in excess can be obtained and that to 
use such a gas in ammonia synthesis a subsequent step of nitrogen removal, 
before or after synthesis, will be included. Further, by feeding 
moderately enriched air at 120 the extent of nitrogen removal can be 
decreased, for example to zero when the hydrocarbon feedstock is methane 
and the oxygen percentage is about 35% .sup.v /v. 
TABLE 
______________________________________ 
Press 
Gas composition % v/v 
Temp. bar N.sub.2 + 
Position 
.degree.C. 
abs. CO CO.sub.2 
H.sub.2 
CH.sub.4 
Ar H.sub.2 O 
______________________________________ 
Ammonia synthesis gas 
146 400 40 -- -- 2.0 24.5 -- 73.5 
140 inlet 
733 39.5 2.1 5.3 27.5 13.9 -- 51.2 
150 592 39 2.1 5.3 27.5 13.9 -- 51.2 
40 inlet 
825 38.5 6.1 5.7 41.0 7.3 -- 39.9 
50 700 38.2 6.1 5.7 41.0 7.3 -- 39.9 
113 inlet 
950 38 8.3 5.0 35.8 0.7 17.3 32.9 
132 550 37.5 8.3 5.0 35.8 0.7 17.3 32.9 
52 inlet 
730 7.0 -- -- -- -- -- -- 
Methanol synthesis gas 
46 390 12 -- -- 0.6 19.9 -- 79.5 
40 outlet 
790 11.5 6.2 6.2 43.9 2.5 -- 41.2 
50 580 11.2 6.2 6.2 43.9 2.5 -- 41.2 
52 inlet 
730 7.0 -- -- -- -- -- -- 
______________________________________ 
In FIGS. 2 and 3a, 3b heat exchange tube wall 210 is for example a tube 
externally heated in a steam reforming furnace and the unit 212 is made of 
stainless steel and carries a coating of alumina and metallic nickel. The 
unit includes solid portions 212A, which may at the top, bottom and centre 
form a complete circle or may be split at 213, such that its shape is 
maintained by the resilience of the alloy. From the generally cylindrical 
walls of the unit, vanes 214 project inwardly to provide an internal 
contact surface; and small tongues 216 (FIG. 3a) or ribs 217 (FIG. 3b) 
project outwardly to keep the unit walls at a separation from heat 
exchange tube wall 210. Vanes 214 and 216 are formed by stamping from the 
wall metal, and thus leave perforations in the walls. Ribs 217 are formed 
by pressing between toothed and grooved rollers during the shaping 
operation. Ribs 217 could be formed also on the middle or lower solid 
portions 212A or in the intervening portions. Alternatively or 
additionally one end of each unit can be necked at 218 to fit into the 
full-width end of the next unit above or beneath it, or succeeding units 
can be doubly-necked and of full width. The outer vanes and/or ribs of 
succeeding units need not be mutually aligned. The vanes and/or ribs can 
be pitched to set up a helical flow pattern in contact with the wall. 
If desired, internal vanes 214 can meet or join in axial space 220. More 
usefully, space 220 can have a cross-sectional diameter 30-80% of that of 
unit 212 and can be occupied by a similar unit of such a diameter but of 
the same general shape as unit 212. In a very useful form of the 
invention, also not shown, space 220 can be occupied by a further heat 
exchange tube, and possibly the bottom of tube 210 can be blind, so that 
gas flowing down through tube 210 in contact with units 212 flows upwards 
through the axial tube. Upward flow followed by downward flow is equally 
possible. 
The following further alternative combinations of units are envisaged: 
(a) a unit as shown but without external projection 216 or 217 having above 
and beneath it a unit of greater diameter occupying more of the width of 
tube 210 or possibly fitting snugly as a result of compression of that 
unit to close gap 213. The smaller units in such a combination can at 
their ends lie within the larger units resting on vanes 214. 
(b) units as shown but without external projections 216 or 217, each 
successive pair separated by a ring having at least 3 external projections 
and out-of-plane projections locating the units diametrally--for example a 
highly pitched non-slip washer. 
FIGS. 4a, b show an alternative unit in which internal and external pitched 
vanes have been formed. The outer heat exchange tube having wall 410 is 
blind at is lower end, and inner heat exchange tube suspended within it 
provides the outlet for the reactants. Unit 412, which can be a single 
unit equal in length to the heat exchange tubes or can be one of an 
assemblage as in FIG. 2 has inward vanes 414 (heavy outlines) but at an 
angle of 45.degree. and outward vanes 415 punched from its walls and bent 
also at 45.degree., the root of the bend being indicated by the dotted 
lines. In the unit as shown vanes 414 direct the reactants outwardly, but 
equally the unit could be inverted to provide direction inwardly, or a 
single unit could carry both types of vane, or two types of unit could be 
stacked in alternation. Outward vanes 415 direct the reactants inwardly 
from the outer wall: they could extend outwardly far enough to contact the 
outer wall, but tongues 416 are provided for correct location of the unit. 
When reactants pass through the unit they undergo endothermic reaction 
while in contact with it but at short intervals leave the surface and mix 
with reactants that have entered the unit by way of the perforations, 
having been reheated at wall 210, 410. In this way a high over-all rate of 
reaction is maintained. As the reactants pass downwardly their temperature 
gradually increases as a result of heat fed in from the furnace 
surrounding tube 210, 410 until sufficient conversion has taken place. If 
an axial tube such as 424 is present, the hot converted reactants flowing 
upward through it give up heat to the reactants flowing downwardly over 
units 212, 412 in the annular space between the tubes. 
In experimental trials in standard methane steam reforming conditions as 
described in our European application No. 21736, catalyst units as 
described in FIGS. 2 and 4, made of stainless steel and carrying a coating 
of alumina and metallic nickel showed activity of the same order as that 
of commercial catalyst, but at a much lower pressure drop.