Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Catalytic combustion
This inventlon relates to catalytic combustion, and in
particular to a process, and apparatus suitable therefor, for
producing a hot pressurised gas stream. In one form of the
invention, a hydrocarbon feedstock is converted to a hot
pressurised hydrogen-containing gas stream by the reaction of said
feedstock with an oxidant gas containing free oxygen, in the
presence of a catalyst.
Such processes are well known and include the so-called
catalytic partial oxidation and secondary reforming processes.
In these processes, which are operated continuously and
generally effected at elevated pressure, the feedstock stream is
partially combusted and then the combustion products are passed
over a conversion catalyst, to bring the combustion products
towards equilibrium. Where employed, steam and/or carbon dioxide,
is included in one or both of the reactant streams or may be fed
as a separate stream.
While there have been proposals, for example in
"Chemical Engineering" January 3 1966 pages 24-26, G~-A-1137930,
and US-A-4522894, of autothermal reforming wherein the combustion
is effected catalytically, for example by the provision of a bed
of a combustion catalyst upstream of the conversion catalyst, such
processes suffer from the disadvantage that there is a risk that
the combustion catalyst will become deactivat~d by cont~nued
exposure to high temperatures and/or by the deposition of carbon.
Also there is a risk that the feedstock may autoignite and the
resulting flame will damage the combustion catalyst and/or the
vessel.
It is therefore more usual to employ non-catalytic
combustion by feeding the reactants to a burner whereat a flame is
formed.
EP-A-254395 (which was not published until after the
claimed priority date of the present application and which
corresponds to US Serial No. 52004) discloses the use of catalytic
combustion of a gaseous stream containing a hydrocarbon feedstock
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to provide a hot gas stream for use in the start-up of a process
employing a burner to effect non-catalytic combustion, wherein a
feedstock stream is reacted with an oxidant gas stream containing
free oxygen. In preferred forms of that process the catalytic
combustion was operated under conditions giving a hot pressurised
gas stream containing a proportion of hydrogen.
The presen~ invention relates to a process, and
apparatus, of particular utility in such processes where the hot
pressurised gas stream is desired to contain hydrogen, and/or is
desired to have a low concentration of free oxygen, e.g. for use
in downstream operations where the presence of free oxygen can not
be tolerated.
However as will be de~cribed hereinafter, the invention
is also of utility in other processes.
Accordingly the present invention provides a process for
the production of a hot pressurised gas stream by catalytic
combustion comprising:
a) feeding to a mixing zone at superatmospheric pressure a
first first gas stream containing combustible gas;
b) separately feeding, at superatmospheric pre~sure, to the
inlet of a combustion zone containing a combustion
catalyst:
the gas stream from the mixing zone, and
a second gas stream containing free oxygen;
c) effecting at least partial combustion of said
combustible gas in the combustion zone to produce a hot
product gas stream; and
d) recycling part of the hot product gas stream to said
mixing zone and therein mixing that recycled hot product
gas stream with the first gas stream;
whereby the gas stream from said mixing zone that
is fed from the mixing zone to the inlet of said
combu&tion zone is a mixture of the recycled hot product
stream and the first gas stream.
Provision of recycle of part of the hot product gas
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stream thus has the effect of preheating the gas fed to the
combustion catalyst, thereby rendering the catalytic combustion
easier.
In a preferred form of the invention the first gaæ
stream comprises a feedstock containing at least one hydrocarbon
and and at least one of said fir!~t and second streams contains
steam, and the amount of free oxygen as supplied by said second
gas stream is insufficient to effect complete combuation of said
first gas stream.
The first and second gas streams are preferably fed at a
pressure in the range 2 to 70 bar abs, particularly 5 to 60 bar
abs.
Combustion catalysts may also promote other reactions
such as shift conversion, and steam reforming. The provision of
steam in at least one of said first and second streams, together
with a combustion catalyst which shows some steam reforming
activity, allows a proportion of hydrogen to be formed in the hot
product gas stream. In this case the recycle of part of the hot
product gas stream thus also has the effect of introducing
hydrogen into the gas mixture that is subject to the partial
combustion: since hydrogen is more readily combusted
catalytically than hydrocarbon components such as methane in the
feedstock, this also renders the catalytic combustion more
effective. Also, since hydrogen has a lower autoignition
temperature than such hydrocarbon components, the autoignition
temperature may be attained more readily (wherej as described
below, autoignition is desired).
In another form of the invention the hot gaseous
products from the combustion zone are passed through a further
zone containing a catalyst suitable to effect a separate catalytic
reaction on the hot gaseous products e.g. shift conversion, before
the products from that catalyst zone are in part recycled to the
mixing zone.
The desired recycle of hot product gas can be induced by
creating a suitable pressure differential between the hot product
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4 B 34270
the hot product gas and the first stream~ A suitable pressure
differential can be achieved by the use of a means providing a
constriction, having the effect of an ejector, wherein the flow of
of the first gas stream through the constriction generates a low
pressure region. The difference in pressure between the low
pressure region and the pressure of the hot product stream results
in the desired recycle flow of hot product gas to the low pressure
region, wherein intimate mixing of the two streams is effected.
The above use of the principle of an ejector so as to
effect a recycle of part of the hot product stream may also be
applied in other exothermic reactions to effect preheating of the
gas fed to the catalyst to improve the catalytic performance,
and/or to suppress undesired side reactions. In some such
proces~es there may be no need for a second gas stream.
Accordingly the invention further provldes a process for
the production of a hot pressurised gas stream by an exothermic
catalytic reaction comprising:
a) passing a first gas stream containing a reactant gas, at
superatmospheric pressure, through a constriction
whereby a lower pressure region is created;
b) feeding the gas stream from the lower pressure region to
a zone containing a catalyst for said exothermic
catalytic reaction and therein effecting said exothermic
catalytic reaction to produce a hot product gas stream;
and
c) recycling part of the hot product gas stream to the
lower pressure region;
w~ereby the gas stream that is fed from the lower
pressure region to the inlet of the catalyst zone is a
mixture of the recycled hot product stream and the first
gas stream, and the pasfiage of the first gas stream
through the constriction provides that the pressure in
said lower pressure region is sufficiently below the
pressure of the said hot product gas stream to cause
said recycle.
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As will be described below, for any given apparatus,
when operating under specified design conditions, the proportion
of the hot product gas that is recycled tends to decrease as the
system is started up. It is preferred that, after such start-up
and having achieved steady-state operating conditions, the
proportion of the hot product gas stream that is recycled is from
20 to 95%, particularly at least 40~, and preferably from 50 to
80%, of the hot product gas leaving the combustion zone.
The present invention also provides apparatus for use in
a process wherein a hot pressurised gas stream is produced by
catalytic combustion comprising:
a) an outer cylindrical shell provided with
inlet ports for the first and second streams,
and
an outlet port for the hot product gas stream;
b) a hollow member, having inlet and outlet ends, dispo6ed
within, and extending for a major proportion of the
length of, the shell and defining an annular space
between said shell and said hollow member, the interior
of said hollow member communicating with said annular
space at each end thereof, and at its outlet ènd,
communicating with the outlet port;
c) first supply means, connected to the inlet port for the
first gas stream and terminating adjacent the inlet end
of said hollow member, for supplying the first gas
stream from the inlet port therefor to the interior of
the hollow member,
d) second supply means for supplying the second gas stream
from the inlet port therefor to the interior of the
hollow member coMprising means connected to the inlet
port for the second gas stream and extending into the
interior of the hollow.member, said second supply means
terminating within said hollow member at a location
downstream of the termination of said first supply
means; and
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e) a combustion catalyst bed located ~ith said hollow
member downstream of the termination of the second
supply means.
The combustion catalyst is preferably a precious metal,
e.gO platinum, supported on a suitable 6upport such as a
refractory material, e.g. alpha alumina. The support preferably
has a monolithic honeycomb structure, as such structures give rise
to a low pressure drop hence allowing a significant recycle of the
product gas.
As previously stated the production of a hot pressurised
gas stream by an exothermic reaction may under some conditions not
require the supply and use of a second gas stream. The provision
of a recycle of hot product gases could however be of benefit. A
constriction means can be employed within the apparatus in which
such an exothermic reaction takes place to produce the effect of
an ejector, thereby inducing the recycle of hot product gases.
Accordingly the present invention further proYides an
apparatus for use in such a process wherein a hot pressurised gas
stream i8 produced by an exothermic catalytic reaction
comprising:
a) an outer cylindrical shell provided with
an inlet port ~or an inlet gas stream, and
an outlet port for the product gas stream;
b) a hollow member, having inlet and outlet ends, disposed
within, and extending for a major proportion of the
length of the shell, and defining an annular space
between said shell and said hollow member, the interior
of said hollow member communicating with said annular
- space at each end thereof, and at its outlet end,
communicating with th outlet port;
c) supply means connected to the inlet port and term~nating
in a constriction means adjacent the inlet end of said
hollow member, for supplying the inlet gas stream from
the inlet port to the interior of the hollow member;
d) a catalyst bed located within said hollow member
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7 B 34270
downstream of the termination of the said supply means.
A particular embodiment of the apparatus will now be
described, wherein a hot pressurised gas stream is produced by
catalytic combustion.
For convenience of description the apparatus will be
described as if the shell is mounted vertically wlth the inlet
end of the hollow member uppermost. However it will be
appreciated that such spatial disposition of the apparatus is not
essential.
The hollow member is preferably of circular cross
section having a cylindrical inlet region of smaller diameter than
the region thereof containing the combustion catalyst, with a
conical transltion section between these two regions. The
cylindrical inlet region and the conical transition section thus
can act as a diffuser. The first supply means may extend into the
inlet reglon of the hollow member or, where the hollow member does
not extend at its upper, inlet 9 end to the end of the shell, the
first supply means may terminate above the hollow member. The
second supply means for the second stream extends into the hollow
member to a location below the end of the first supply means and
preferably extends for at least a major portion of the length of
the inlet region. The first supply means is preferably provided
at its outlet end with a constriction to act as an ejector. The
second supply means may be similarly provided. As described
below, where the upper end of the hollow member is open so that it
does not extend to the end of the shell, the open end of the inlet
region may be flared in the vicinity of the ejector on the first
supply means.
The interior of the hollow membPr communicates at each
end with the annular space between the hollow member and the
shell. Where one or both ends of the hollow member extends to the
end of the shell, the appropriate communication may be provided by
holes in the wall of the hollow member adjacent the relevant end
thereof. Where the combustion catalyst is provided in the form of
a plurality of longitudinally spaced catalyst sections, in some
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cases lt may be desirable to provide openings in the wall of the
hollow member communicating with the region, or regions, between
adjacent catalyst sections to provide, as described below, for
some recycle of the gas stream before passage thereof through the
whole of the combustion catalyst~
The invention is illustrated by reference to the
accompanying drawings wherein
Figure 1 is a diagrammatic longitudinal section through one
embodiment of the apparatus, and
Figure 2 is an enlargement of that part of Figure 1 within
the dotted line.
In the embodiment of Figures 1 and 2, the apparatus
consists of an outer cylindrical shell 10 designed to ~ithstand
the process pressure, which is typically in the range 5 to 60 bar
abs. At one end of the shell 10 is an inlet port 12 for a first
gas stream consisting of a steam/natural gas mixture, and an
outlet port 14 for the product gas stream. At the other end 16 of
the shell 10 is an inlet port 18 for the second gas stream, air.
Located within the shell 10 and sealed thereto at the end adjacent
inlet port 12 is a liner 20. Liner 20 extends almost to the other
end 16 of the shell 10 and thus de~ines an annular conduit 22
between the interior surface of the shell 10 and the exterior
surface of the liner 20. Inlet port 12 connects with this annular
conduit 22. At the end 16 of the shell 10, liner 20 extends
across the end of the shell 10, and terminates in a cylindrical
portion 24 surrounding, but spaced from, an air supply pipe 26
forming the air supply means from air inlet port 18. The end of
the cylindrical portion 24 that is remote from the end 16 of the
shell 10 is provided ~ith an in~ard enlargement 28, see Figure 2,
thus providing a constriction between the end of cylindrical
portion 24 and the air supply pipe 26 to act as an ejector.
The conduit defined by liner 20, the wall of shell 10,
the cylindrical portion 24, and the external surface of the air
supply pipe 26, thus forms the supply means for deltvering the
first gas stream from the inlet port 12. Since the structure is
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9 B 34270
thus of the hot-wall type so that the gas flowing through conduit
22 acts as a coolant, the amount of refractory insulation, if any,
required on the shell 10 can be kept relatively small.
~ocated within liner 20 is a elongated hollow member 30
of circular cross section, This hollow member has an inlet region
32 having an open~ flared, end 34 adjacent the ejector terminating
the first gas stream supply means, a combustion region 36 of
greater cross section than the inlet region 32 and containing, at
the end thereof remote from inlet region 32, the combustion
catalyst 38, and a conical transition section 40 connecting the
inlet region 32 with the combustion region 36. Below the
combustion catalyst the lower end 42 of hollow member 30 is
supported on the end of shell 10. Provision is made, e.g. by
providing holes ~4 through the wall of the hollow member 30
adjacent the end 42, for gas exiting the combustion catalyst 38 to
enter the space 46 between the external surface of the hollow
member 30 and the interior surface of liner 20. Part of the gas
leaving the catalyst can thus énter space 46 while the rest leaves
the shell 10 via outlet port 14. The combustion catalyst 38
comprises a number of honeycomb sections 48 on the surface of
which is deposited a suitable metal that has combustion sctivity.
Openings 50 are also provided in the wall of the hollow member 30
between adjacent sections of the honeycomb so that part of the gas
stream can enter space 46 without passage through the whole of the
combustion catalyst 38.
The air s~pply pipe 26 extending from inlet port 18
extends along the length of the inlet region 32 of hollow member
30 and terminates at the commencement of the combustionregion 36
thereof. At the outlet of air supply pipe 26 there is provided a
nozzle 52.
In operation natural gas and steam iB fed under pressure
to inlet port 12 and air is fed under pressure to inlet port 18.
The natural gas/steam mixture flows up the space 22 between shell
10 and liner 20 and emerges through the e~ector formed by inward
enlargement 28, to form a lower pressure region immediately
downstream thereof. The mixture then flows do~n through the inlet
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region 32 and conical transition section 40 of hollow member 30,
where it is mixed with air emerging from nozzle 520 The resultant
mixture then flows through the combustion region 36 and the
combustion catalyst 38 therein. Part of the gas stream leaving
the combustion catalyst 38 flows out through outlet port 14.
~ecause the pressure in the low pressure region is below the
pressure of the product gas, the remainder of the product gas
flows through holes 44 into space 46 between hollow member 30 and
liner 20 and then up towards the end 16 of the shell 10 and is
drawn into the inlet region 32 of the hollow member 30 by the
effect of the natural gas/steam mixture emerging from the ejector
formed by inward enlargement 28. The recycled gas thus mixes with
the natural gas/steam mixture and flows down through the hollow
member 30.
Initially some reaction takes place as the gas stream
passes over the combustion catalyst 38, thereby creating ahot gas
stream. The part of the hot gas stream entering the space 46 via
holes 44 and recycling back to the inlet region 32 of hollow
member 30 heats the natural gas/steam mixture flowing through
annular conduit 22 thereby raising the temperature thereof so that
the gas entering the combustion cataly~.t is preheatedO The
recycled hot gas stream also heats the air as the latter flows
through the air inlet supply pipe 26 extending through the inlet
region 32, and conical transition section 40 of the hollow member
30. With continued operation the temperature of the gas entering
the combustion region 36 increases until the autoignition
temperature is reached whereupon a flame is produced at the nozzle
52. As mentioned above, because of the reforming activity of the
combustion catalyst 38, the hot gas stream leaving the combustion
region 36 of hollow member 30, and hence the hot gas mixture that
is recycled, will contain some hydrogen so tha~ the gas mixture
mixing with the air at nozzle 52 contains hydrogen, thereby
enabling a flame to be established more rapidly at nozzle 52.
It will be appreciated that when a flame is established,
the recycled hot gas flowing up that portion of the space 46
between the combustion region 36 of hollow member 30 and the inner
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11 B 34270
surface of liner 20 will be heated by heat exchange across the
wall of the combustion region 36 and at the same time will heat
the natural gas/steam mixture flowing through the corresponding
portion of annular conduit 22 between the inner surface of shell
10 and the outer surface of liner 20. As the recycled hot gas
flows through that part of the space 46 between the external
surface of the conical transition section 40 and inlet region 32
of the hollow member 30 and the interior surface of liner 20, it
will heat not only the natural gas/steam mixture flowing through
annular conduit 22 between shell 10 and liner 20, but also the gas
flowing through the inlet region 32 and conical transition section
40 of the hollow member 30.
In an alternative embodiment the liner 20 is omitted and
the shell 10 is provided with a refractory insulating layer on lts
interior surface. In this embodiment the firRt gas supply means
comprises a pipe, coaxial with the air supply pipe 26, provided at
its end with an inward enlargement, corresponding to inward
enlargement 28 in ~igure 2, to form the constriction providing the
ejector. In this embodiment there is therefore no preheating of
the first gas stream by the recycled hot gas before the first gas
stream leaves the feed pipe, but a heated mixture of the first gas
stream and the recycled hot gas is formed by the simple mixing of
the two gas streams prior to the mixing with the second gas stream
leaving the air supply pipe 26.
In either embodiment suitable projections may be
provided on the exterior surface of the hollow member 30 to locate
it in the desired spaced relation from liner 20 in the embodiment
of Figure 1 or from the refractory lining in the alternative
embodiment. Likewise suitable spacers may be provided between the
interior surface of the hollow member 30 in the inlet region 32
thereof and the air supply pipe 26 to maintain these components in
the desired spaced relationship.
One advantage of the recycle in a process where thPre is
only partial combustion and one or both of the feed streams
contain steam, is that, aft~r awtoignition has been achieved, the
product gas leaving the combustion catalyst will haYe a
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1~ s 34270
temperature somewhat below the m~ximum temperature in the
combustion zone upstream of the catalyst by virtue o~ the fact
that, 6ince the combustlon catalyst exert6 some steam reforming
activity, such reforming, whlch is endothermic, will take place as
the ga~ passes through the catalyst. The recycled product gfl8,
being cooler than the ga3 lnaide the combustion zone thus serve6
to maintain the hollow member 3Q at sn acceptable temp~rature and
80 ie is not necessary thst the hollow member 30 has to be
constructed of a material that has to withstand very hlgh
temperatures.
The system can conveniently be started up with the first
gas stream being fed at or near the design rate and then the flo~
of the ~econd gafi stream is commenced, at a slow rate, and then
the flow rate of the second stream iB gradually increased. At
slow second gas stream flow rates essentially all the combustlon
takes place in the initial portions of the combustion catalyst 38.
Hence gas that i~ recycled through the holes 50 (lf such holac
are provided) i8 hotter than product gas that passes all the way
through the combustion catalyst 38 (since the latter will cool as
a result of heat transfer with colder combustion catalyst and/or
endothermic reforming taking place) and 80 the recycled gas is
hotter than if there had been no holes 50. By virtue of the
recycled gas mixing with the incoming first stream and, where
there is a liner 20 as in the embodiment of Figure~ 1 and 2, heat
exchange across such a liner, the first stream becomes preheated
before it meets the incoming second ga~ stream. This preheating
enables the catalytic combustion tc occur earlier in the ca~aly6t
containing zone and so enable~ the flow rate of the second stream
to be increased more rapidly. Withln a short time the second ga~
flow rate can be increased to the level at which the product gas
has the desired flow rate and temper~ture. For any given
apparatus and flow rate of first gas of given composition, it will
generally be found that the product gas outlet temperature depends
on the rate of supply of oxygen, e.g. as air, to the combustion
zone. Hence the process may readily be controlled by controlling
the flow rate of the second gas stream.
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As the flow rate of the second stream i8 increased5 the
proportion of recycle will decrease because the addition of the
second gas stream increases the mass of gas that is passing
through the system but the "driving force" effecting the recycle,
i.e. the product of the mass of the first gas stream and the
difference between the product gas outlet pressure and the
pressure in the aforesald region of lower pressure, remains
essentially constant. Furthermore as the recycle gas stream
becomes hotter, the efficiency of the ejector decreases.
It will be appreciated that if the temperature of the
recycled hot gas and the degree of recycle is sufficient that the
mixture of recycled hot gas, first gas stream and second gas
stream attains the autoignition temperature, autoignition will
occur with the production of a flame at the nozzle supplying the
second gas stream. To avoid damage to the combustion catalyst by
such a flame, it is pr~ferred that the second gas stream supply
means terminates well upstream of the catalyst so that the flame
can occur in a catalyst-free space upstream of the ca~alyst.
It will further be appreciated that, since the product
gas temperature c~n be controlled by controlling the rate of
supply of the second gas stream, it is possible to control the
process, if desired, such that the autoignition temperature i8 not
achieved so that the combustion i8 totally catalytic. If it is
intended that the process will be operated without achieving
autoignition, then there is no need for a catalyst-free combustion
zone upstream of the combustion catalyst: however æufficient
space should be provided to ensure good mixing of the first and
second gas streams and even distribution of the mixture before
encountering the combustion catalyst.
In the foregoing description, the start-up has been
described ~ith the assumption that the first gas stream flow rate
is kept essentially constant. It will be appreciated that this is
not necessarily the case. Indeed where a~toignition is
established, the rate of feed of the first and/or second gas
streams can be increased considerably, after autoignition~ since
the rates are no longer limited by the need of obtaining
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14 B 34270
combustion in the catalyst.
The present invention is of particular utility where it
is desired to obtain a hot, fuel-rich, gas stream from relatively
cool reactants. By providing a small heater, for e~ample
electrically operated, to heat the inlet gases to about 150 to
200C during the initial stages of the start-up procedure, it will
be appreciated that it would be possible to operate the process
with a feed of cold reactants, e.g. at ambient temperature.
However, normally sufficient heating can be obtained from the
steam and/or an external source, e.g. as a result of heating
occurring on compression of the reactants to the desired operating
pressure, to enable start-up to be achieved without the need for
such a heater. As mentioned above catalytic combustion is
facilitated by the presence of hydrogen in the first gas stream.
Consequently, where a source of hydrogen is available, e.g. purge
gas from an ammonia synthesis plant, addition of such ~ydrogen-
containing gas to the first gas stream, at least at start-up, is
advantageous.
In addition to the production of a hot pressurised fuel-
rich hydrogen-containing stream as described above, the invention
is also of utility where the desired product is an air-, or
oxygen-, rich hot gas stream from relatively "cold" reactants, for
example for the start-up of a partial oxidation process~ Where an
air-, or oxygen-, rich product is required, the amount of the
second gas stream, i.e,that containing oxygen, should be in an
excess of that required for complete combustion of the combu~tible
gas in the first gas stream.
Another application of the invention is in the treatment
of relatively "cold" gaseous effluents containing combustible
pollutants such as carbon monoxide and/or nitrogen oxides, e.g.
tail gas from nitric acid plants. Such pollutants can be
efficiently combusted by the process of the invention giving a hot
gas stream from which heat may be recovered, e.g. by letting the
stream down through a turbine and thus generating power, before
discharge of the combusted gas to the atmosphere.
99(~8~
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As an example using apparatus of the type depicted in
Pigures 1 and 2 but provided with no openings 48 and dimensioned
so that the proportion of the product gas that i8 recycled, at the
design flowrate after start-up has been accomplished, is about 50%
of the gas leaving the combustion catalyst, the cylindrical shell
is about 3 m length and 40 cm diameter. If a natural gas/steam
mixture of steam to carbon ratio of 2.5 is fed at 162 kg mol/hr at
a temperature of 200C and a pressure of 12 bar abs. as the first
gas stream, and air is fed at 146 kg mol/hr at a temperature of
240C and 12 bar abs. pressure as the second gas stream, it is
calculated that the product gas leaving the shell through outlet
port 14 is at 750C and has the following composition:
Nitrogen and argon31.7%v/v
Garbon dioxide 7.0% v/v
Steam 29.9% v/v
Hydrogen 25.8% v/v
Carbon monoxide 4.8% v/v
Methane 0.8% v/v
Under these conditions it is calculated that the natural
gas/steam mixture is heated to about 330C by the time it leaves
e~ector formed by constriction 28 and the natural gaslsteam/
recycle gas mixture entering the transition region has a
temperature of about 550C. It is calculated that autoignition
and steady state conditions can be attained within 5 to 10 minutes
of co~mencing flow of the reactants.
By way of comparison, in e~periments with a similar
arrangement but in which there was no provision for recycle of
product gas so that reliance was placed upon transfer of heat back
through the catalyst to the combustion zone in order to raise the
temperature of the reactants to the autoignition temperature, the
time taken to achieve autoignition was over one hour.
PA/EJ~/MP
23 March 1988/L197A
