Note: Descriptions are shown in the official language in which they were submitted.
Z009641
1 B 35136
Methanol
Thls lnventlon relates to methanol and ln partlcular to
the productlon of methanol synthesls gas, le a gas contalning
hydrogen and carbon oxides, by steam reforming a hydrocarbon
feedstock such as natural gas.
Background _ the Invention
The steam reforming process i8 well known and involves
passage of a mixture of the feedstock and steam over a reforming
catalyst, usually disposed in externally heated tubes. However,
partlcularly where the feedstock is predominantly methane, eg
where the feedstock is natural gas, the synthesis gas contains an
excess of hydrogen over that required for methanol synthesis. In
synthesis gas having stoichiometric proportions of hydrogen and
carbon oxides for methanol synthesis, the ratio (R) of the molar
amount of hydrogen (less the molar amount of carbon dioxide) to
the total molar amount of carbon oxides equaLs 2. Typically
synthesis gas made by the conventional steam reformLng process has
a ratio R of the order of 2.5 or more, eg about 3.
It has been proposed in US-A-4337170 to provide a fired
reformer and an auxiliary reformer. Part of the feedstock
bypasses the primary reformer and is fed to the auxiliary reformer
and reformed in cataLyst-containing tubes disposed thereLn. The
reformed gas from the auxiliary reformer tubes is mixed with the
reformed gas from the fired reformer and the mixture of reformed
gases is then fed, in counter-current to the gas undergolng
reforming in the auxiliary reformer tubes, past the tubes of the
auxiliary reformer so as to supply the heat required for the
reforming in the auxillary reformer tubes. As a result of the use
of an auxiliary reformer, the throughput of a steam reformLng
stage can be increased and so this development is of use for ~ -
uprating an existing plant.
For metallurgLcal and efficiency reasons, the pressure
at which the reforming stage is conducted is generaLly in the
range 10 to 40 bar abs. However, methanol synthesis is normally
conducted at higher pressures, eg 50 to 120 bar abs. or even
B 35, ~009641
higher in old processes, and so, after removing unreacted steam
but prior to use for methanol synthesis, the synthesis gaY ha~
generally to be compressed. Because the reforming process gives a
gas containing more hydrogen than is required in the methanol
synthesis gas, energy is unnecessarily consumed in the compression
of this excess of hydrogen. Furthermore, the provision of an
auxiliary reformer giving an increase in the throughput of the
reforming stage increase~ the amount of gas that has to be
compressed. Not only does thls mean that more power is requlred
to effect the compression, but if an exlstlng plant 18 belng
modlfled, the exlsting synthesis gas compressor may be inadequate
to handle the increased amount of ~ynthesis gas.
It has been propo~ed in GB-A-2140801 to produce methanol
synthesis gas by a process involving partial oxidation of a
hydrocarbon feedstock with air, followed by sub~ecting the
resultant gas stream to the shift reaction and then membrane
separatlon to remove most of the nltrogen introduced by the use of
alr ln the partlal oxldatlon step. However such a process, unless
employlng prellmlnary preheating, uslng a fired heater, and
adiabatlc steam reforming stages, gives a hydrogen-deficlent
synthesls gas. Furthermore such a process is not amenable to the
uprating of a plant having a conventlonal primary reforming stage.
Brlef Description of the Invention.
We have realised that by the use of a membrane
separation technlque it is possible to remove most, or all, of the
excess of hydrogen from the synthesls gas produced by steam
reformlng prlor to compression. This renders it possible to
operate a process using a combination of a fired reformer and an
auxiliary reformer such that the volume of synthesls gas fed to
the compressor ls slmilar to, or even less than, the amount that
would be produced if the auxlllary reformer had not been used.
Accordlngly the present inventlon provldes a process for
the production of methanol synthesis gas comprising:
a) forming ma~or and minor streams, each containing a
hydrocarbon feedstock and steam;
, ! .',: ' ' . ,. ' ~
B 351~964
b) passing, at a pressure in the range 10-40 bar abs.,
the ma~or stream over a steam reforming catalyst
di~posed in tubes heated by means of a fired furnace,
thereby producing a reformed ma~or stream;
c) passing, :~180 at said pressure in the range 10-40
bar abs., the minor stream over a steam reforming
catalyst disposed in tubes of an auxlliary reformer,
thereby forming a reformed minor stream;
d) mixing the reformed mlnor stream with the reformed
ma~or stream, thereby forming a comblnet reformed gas
stream;
e) passlng the reformed ma~or steam, before or after
the mixing thereof with the reformed minor stream, past
the exterior of the auxillary reformer tubes,
thereby supplying heat thereto;
f) cooling the combined reformed gas stream to
condense unreacted steam therein as water, and
separatlng the condensed water to give a water-depleted
gas stream;
g) sub~ecting at least part of the water-depleted gas
stream to membrane separation effectlve to separate a ~ -
permeate gas stream contalning some of the hydrogen from
an impermeate gas stream contalning hydrogen and carbon
`~ oxides, said impermeate gas stream, together with the
reoainder, if any, of said water-depleted gas stream,
forming a synthesls gas stream; and
h) compressing the synthesis gas stream to a pressure
above 50 bar abs., ~ -
the amount of hydrogen separated as sald permeate stream
30 béing such that said synthesis gas stream has a ratio R, as
hereinbefore deflned, in the range l.8 to 2.5.
Description of Drawing
The drawing shows in diagrammatlc form a preferred
flowsheet of the process of the invention.
.
'
.
2009641
4 B 35136
General Description of the Invention
In the process of the invention the feedstock is
preferably methane or natural gas containing a substantial
proportlon, eg over 90% v/v, methane. If the feedstock contains
sulphur compounds, prior to feeding to the reformer tubes, the
feedstock ls sub~ected to desulphurisation, eg by passage over a
hydrodesulphurisation catalyst followed by absorption of hydrogen
sulphide using a suitable absorbent, eg a zinc oxide bed. Usually
it is desirable to incorporate a hydrogen-contalning gas into the
feedstock prior to hydrodesulphurisation: this may be achieved by
recycling a small amount of the reformed gas, or a hydrogen-
containing gas produced therefrom, eg part of the permeate gas
stream or purge gas from a methanol loop to which the synthesls
gas ls fed, to the feedstock prlor to passage over the hydro-
desulphurisatlon catalyst.
Prior to reforming, steam is mixed with the feedstock:this steam introduction may be effected by dlrect ln~ection of
steam and/or by saturation of th~e feedstock by contact of the
latter with a stream of heated water. The a unt of steam
introduced is preferably such as to give 2 to 4 moles of steam per
gram atom of hydrocarbon csrbon ln the feedstock. Some of the
steam may be replaced by carbon dioxide, where a supply thereof is
available, as this decreases the amount of hydrogen that has to be
separated as the permeate stream in order to obtaln a synthesls
gas of approxlmately stolchlometric compositlon.
The feedstock/steam mixture 18 preferably preheated by
heat exchange with, for example, the comblned reformed gas stream
and/or the flue gases from the flred reformer and then part
thereof is fed as the major stream to the tubes of the flred
reformer. The ma~or and mlnor streams may be preheated
separately, eg to different temperatures and/or may contain
differing proportions of steam and/or carbon dioxide. For example
steam may be introduced separately into the feedstock streams of
the ma~or and minor streams. The major stream preferably contains
75-90% of the total amount of feedstock in the major and mlnor
Z00964~
5 B 35136 -
streams. The fired reformer is preferably operated 80 that the
temperature of the reformed ma~or stream leaving the catalyst of
the fired reformer i9 in the range 750 to 950C, especially 850 to
900C.
In a preferred form of the invention, the auxiliary
reformer tubes are of the "double tube" configuration, ie where
each tube comprises an outer tube having a closed end and an inner
tube disposed concentrically within the outer tube and
communlcating wlth the annular space between the lnner and outer
tubes at the closed ent of the outer tube, with the steam
reforming catalyst disposed in said annular space. The minor
stream is fed to the open end of the annular catalyst-containing
space between the inner and outer tubes whlle the reformed ma~or
stream is fed past the external surface of the outer tube. The
reformed minor stream leaves the annular space at the end thereof
ad~acent the closed end of the outer tube and flows back through -
the inner tube. One form of double-tube reformer is described ln
EP-A-194067: in this reformer lnsulation is provided to minimi6e
the amount of heat transferred through the walls of the inner tube
from the reformed minor stream flowing through the inner tube.
However in the present inventlon it 18 preferred that such
lnsulatlon ls omltted so that heat transfer takes place through
the wall of the inner tube from the reformed minor stream passing
through the inner tube to the minor stream undergoing reforming in
the annular ~pace containing the catalyst. This heat transfer has
a dual effect: firstly lt supplies part of the heat required for
the reformlng of the minor stream and secondly lt gives rise to
cooling of the reformed mlnor stream. The latter has the
advantage that the resultant reformed gas stream consistlng of the
mlxture of the réformed major and mlnor streams will be at a lower
temperature and hence will contaln less heat, thereby reducing the -~
heat recovery necessary therefrom for efficient operation.
The use of this type of reformer wherein a process gas
stream, ie the reformed major stream or the mixture thereof with
the reformed minor stream, is used to heat the auxiliary reformer
Z009641
6 B 35136
tubes has the advantage that the auxiliary reformer tubes can be
of thinner gauge material than is customary for reformer tubes
since the pressure differential across the auxiliary reformer
tubes is relatively small, being essentially that resulting from
the pressure drop the major stream experiences as lt passes
through the fired reformer tubes.
In the present invention, the reformed ma~or and minor
gas streams may be combined and the combined gas stream employed
for heating the auxlliary reformer tubes as descrlbed in the
aforesaid US-A-4337170. However it is advantageous,` and hence
preferred, to comblne the reformed minor gas stream with the
reformed ma~or gas stream after the latter has been used to heat
the auxiliary reformer tubes.
The proportion of feedstock that can be reformed in the
auxiliary reformer will depend on the acceptable methane slip in
the water-depleted ga~ and the desired temperature of the combined
reformed gas streams. Thus the methane content of the water-
depleted gas stream will be the sum of the methane contents of the
reformed ma~or and minor streams: for any glven reformer,
feedstock, pressure, and proportion of steam, the methane content
of the reformed ma~or stream will depend on the temperature of the
reformed ma~or stream leaving the catalyst in the fired reformer
whlle the methane content of the reformed minor stream will depend
on the temperature of the reformed minor stream leaving the
catalyst of the auxiliary reformer. The temperature of the
reformed minor stream leaving the catalyst of the auxiliary
reformer will depend on the temperature of the reformed ma~or -
stream used to heat the auxiliary reformer, the heat transferred,
if any, from the reformed minor stream to the minor stream
undergoing refor~ing, and the relative proportions of the major
and minor streams. It is preferred that the reformers are
operated so that the overall methane content of the water-depleted
gas stream is in the range 2 to 10% by volume on a dry basis.
After reforming, the combined reformed gas stream is
cooled to below the dew-point of steam therein to condense
2009641
7 B 35136
unreacted steam as water, which is then separated. This cooling
may be effected in conventional manner, eg by intirect heat
exchange with reactants to be fed to the tubes of the flred
reformer and/or auxiliary reformer, with water, giving hot water
and/or steam (which may be used as process steam), and/or with
steam giving super-heated steam from which power may be recovered
in a turbine. Alternatively, or additionally, at least the final
part of the coollng may be by dlrect heat exchange with water,
giving a warm water stream, containing also the condensed water,
whlch may be used, after further heating, as a hot water stream
that is contacted with the feedstock to effect saturatlon thereof
to introduce process steam.
After separation of the water, at least part of the
water-depleted stream is sub~ected to a membrane separation
process to separate a permeate stream containing hydrogen from an
impermeate stream containing hydrogen and carbon oxides. As is
well known ln the art, a varlety of membrane materlals may be
used: examples of such membrane materlals include polyimldes and
polyethersulphones. It i9 preferred to employ a membrane that has
a relatively low permeabllity to carbon oxides 80 that little
thereof pass lnto the permeate stream: for this reason polylmide
membranes are preferred.
~` It 19 not necessary that all the water-depleted gas 18
sub~ected to the membrane separatlon: thus part thereof may bypass
the membrane separatlon stage. By varylng the proportlon
bypasslng the membrane separation stage, control may be effected
~; on the composition of the synthesls gas. The amount of the
feedstock that is fed to the auxiliary reformer tubes and the
amount of hydrogen removed as the permeate are such that the
synthesis gas formed fro~ the impermeate and that part, if any, of
the water-depleted gas bypassing the membrane separation stage ha6
a R ratio in the range 1.8 to 2.5, and preferably such that the
volume of synthesis gas produced does not exceed the dry gas
volume of the reformed ma~or gas stream by more than lOX. In
particular it is preferred that the amount of hydrogen removed as
.
.~
zoas64~ ,
8 B 35136
the permeate stream is such that the volume of synthesis gas is no
greater than the dry gas volume of the reformed ma~or gas stream,
so that no additional load i8 placed upon the synthesis gas
compressor.
The permeate stream may be used as fuel for the flred
reformer or exported to a user of hydrogen. Part of the permeate
stream may be used, as aforesald, as a hydrogen-containing gas
stream added to the feedstock prior to hydrodesulphurisatlon of
the latter,
It has been the practlce to take a purge stream from the
methanol synthesls loop and to use this purge as fuel for the
flret reformer. Such a purge has been necessary to avold a
build-up in the loop of inerts, eg methane, and possibly nitrogen
(which may be present in small amounts in natural gas), and the
excess of hydrogen resulting from the use of a hydrogen-rich
synthesis gas. By means of the present inventlon it is posslble
to recycle some or all of the methane-containing purge to the
reforming stage as feedstock as the membrane unit acts to remove
some or all of the excess of hydrogen. If the membrane employed
ls such as to separate nltrogen into the permeate stream, then in ;-
some cases it may be posslble to recycle all of the purge. That
portlon, lf any, of the purge that is not recycled may be used as --
fuel for the flred reformer, together with the hydrogen-rich
permeate stream from the membrane unit as aforesald. The recycled
purge forms part of the feedstock to the reformer, thus decreasing
the amount of fresh feedstock required. Furthermore, since the
recycled purge contains hydrogen, it may be used as the hydrogen-
contalnlng gas added to the fresh feedstock prior to hydro-
desulphurisation of the latter.
Part or all of the recycled purge may be subjected to a
further membrane separation step to separate hydrogen as a
hydrogen-containing permeate stream. The impermeate stream is
then recycled to form part of the feedstock. This has the
advantage of reducing the amount of hydrogen that is recycled and
so decreases the load on the membrane separation unit treating the
i..
Z009641
9 B 35136
water-depleted gas. The hydrogen-containing permeate stream may
be used as part of the fuel for the fired reformer. ~here there
is a need to recycle part of the purge as a hydrogen-containing
gas, eg for hydrodesulphurisation, the part to be used for hydro-
desulphurisation is desirably not sub~ected to such a membrane
separation step.
In some cases it may be desirable that the ma~or and
minor feed streams fed to the reformers contain different
proportions of the recycled purge. For example, the ma~or feed
stream may contain only a small proportion of the recycled purge,
eg merely that required to supply the amount of hydrogen required
to ensure satisfactory hydrodesulphurisation, while the remainder
is used as feedstock in the minor feed stream. Indeed, in ~ome
cases the feedstock of the minor feed stream may consist entirely
of recycled purge, preferably after subjecting that purge to a
membrane separation step.
Detailed Description of the Invention.
One form of the invention is illustrated by reference to
the drawlng which is a diagrammatlc flowsheet wherein for
simplicity the reformers are shown with only single catalyst tube
ln each reformer: in practice there will of course generally be a
multiplicity of tubes in each reformer.
In the drawing i8 shown a fired furnace 1 containing a
main reformer tube 2 in which a steam reforming catalyst 3 is ~;
disposed. A heat exchanger 4 is disposed in the flue gas duct 5
of the fired furnace 1. An auxiliary reformer 6 is provided
wherein each catalyst tube is of the "double tube" construction
having the catalyst 7 disposed in the annulus 8 between outer tube - -
g and inner tube 10. Outer tube 9 is closed at its lower end,
while the upper end of the outer tube 9 opens into a plenum
chamber 11. At the lower end of reformer 6, a hot gas inlet 12 is
disposed, connected to the outlet of the tube 2 of the fired
reformer. The reformer 6 is al~o provided with an outlet 13 for
the gas from the space outside the outer tube 9 and an outlet 14
with which the inner tube 10 communicates. Outlets 13 and 14 lead ~-
.', ~:
10 B 351~6
to a reformed gas line 15. A feedstock/steam feed 16 leads to the
heat exchanger 4 and a preheated reactants line 17 from heat
exchanger 4 to the inlet of tube 2. An auxiliary reformer feed 18
is taken from heat exchanger 4 to the plenum chamber 11 of the
auxiliary reformer 6. The reformed gas line 15 leads, via one or
more heat exchangers 19, to a catchpot 20 having a drain 21. A
water-depleted gas line 22 leads from catchpot 20 to a membrane
separation unit 23 provided with a bypass 24 having a flow control
valve 25. Membrane separation unlt 23 has a permeate line 26 and
an impermeate line 27 to which bypass 24 connects forming a
synthesis gas delivery line 28 feeding to a synthesis gas
compressor 29.
In a typical operation a feedstock/steam mixture at a
pressure of about 24 bar abs. is preheated in heat exchanger 4 and
a ma~or part stream is then fed to the reformer tube 2, while a ~ -
minor part stream is fed to plenum chamber 11 via line 18. The . . .
ma~or part stream passes over the catalyst 3 and is reformed by
heat supplied by the fired furnace 1 giving a reformed ma~or
stream which is then fed, via inlet 12 to the space outside the
outer tube 9 of the double tube reformer 6, and then via outlet 13
to the reformed gas line 15. The minor part stream is fed, from
plenum chamber 11, over the catalyst 7 in the annulus 8 between
tubes 9 and 10 wherein it is reformed. The reformed minor stream
leaves the lower end of the annulus and then passes up through the
inner tube 10 to outlet 14 and thence to reformed gas line 15.
The heat required for the reforming of the minor part stream is
: supplied from the reformed ma~or stream passing past the outside
of outer tube 9 and from the reformed minor stream passing up
through the inner tube 10.
From reformed gas line 15, the combined reformed gas
stream is cooled in heat exchanger 19 to below the dew point of
the steam therein to condense the unreacted steam as water. The
condensed water is separated in catchpot 20 from which it is
removed via drain 21. The resultant water-depleted gas is fed,
via line 22, to the membrane separation unit 23 and therein
Z00964~
11 B 35136
separated into a permeate stream 26 and an impermeate stream 27.
Part of the water-depleted gas bypasses membrane separation unit
via bypass 24. The amount bypassing the membrane unit is
controlled by valve 25.
Example
In a calculated example using the flowsheet described
above in relation to the drawing and using a feedstock of
desulphurised natural gas and a reforming pressure of 24 bar abs.,
the gas composition, flow rates, and temperatures at various
stages of the reforming operation are as shown in Table 1.
Table 1
________________________________________________________________
I I I Gas flow rate (kmol.h 1)
I Position I Temp l----------------------------------------------l
1 ( C) I CH4 I H2 I C0 I C02 I H20
l__________l______l________l_________l________l________l_________l
1 17 1 520 1 83.4a 1 2.0 10.0 10.3 1 250.1
1 18 1 520 1 25.1b 1 0.6 l0.0 10.1 1 75.2
I 12 1 870 1 14.8 1228.6 144.4 124.5 1 157.3
1 13 1 664 1 14.8 1228.6 144.4 124.5 1 157.3
1 10* 1 770 1 10.4 151.6 16.8 18.0 1 52.6
1 14 1 658 1 10.4 IS1.6 16.8 18.0 1 52.6
1 15 1 662 1 25.2 1280.2 151.2 132.5 1 209.9
________________________________________________________________
Inlet, ie bottom.
a~ b includes 6.5 and 2.0 kmol.h 1, respectively, of higher
hydrocarbons expressed as CH2 96 -
In order to achieve the heat transfer across the outer -~
walls of the outer tubes 9 necessary to effect the degree of
reforming of the minor feed stream shown in Tablè 1, it is ~ -
calculated that, for a given number of tubes of a given diameter,
the tubes 9 need to have a length of 7.2 m exposed to the major
reformed gas stream 12.
If the inner tubes 10 were omitted and there were used
the same number of outer tubes 9 of the same diameter but with
L
200964~
12 B 35136
open lower ends so that the reformed minor stream leaving the
tubes 9 mixed with the reformed maJor stream 12 and the resultant
mlxture used to heat the tubes 9, the temperatures and gas flow
rates are calculated to be as set out in Table 2.
Table 2
_____________________________________________________________ _ ,
I I I Gas flow rate (kmol.h 1)
I Position I Temp 1----------------------------------------------1
I I (C) I CH4 I H2 I CO I C02 I H20
1__________I______I________I _______I________I________I _______I : .
I17 1 520 1 83.4a 1 2.0 10.0 10.3 1 250.1
118 1 520 1 25.1b 1 0.6 10.0 10.1 1 75.2
112 1 ~70 1 14.8 1228.6 144.4 124.5 1157.3
19* 1 762 1 10.7 150.7 16.5 18.0 152.9
15 112 + 9* 1 843 125.5 1 279.3 150.9 132.5 1 210.2
115 1 663 1 25.5 1279.3 150.9 132.5 1210.2
________________________________________________________________
Outlet, ie bottom.
a~ b includes 6.5 and 2.0 kmol.h 1, respectively, of higher
hydrocarbons expressed as CH2 96
In this case in order to achieve the heat transfer
across the outer walls of the outer tubes 9 necessary to effect
essentially the same degree of reforming of the minor feed stream,
with essentially the same inlet gas flows and temperatures and to
give a product stream, ie stream 15, at essentially the same
outlet temperature, it is calculated that, because the gas
effecting the heating has a lower temperature and because there is --
no heating of the minor feed stream undergoing reforming by heat
transfer from the reformed minor stream passing up through tubes
10, the heat exchange surface area of the outer walls of the tubes
9 needs to be increased by about 14Z, eg by increasing the length
of tubes 9 exposed to the combined reformed ma~or and minor
streams, ie stream 12 plus the stream from the outlet of tube 9,
from 7.2 m to 8.2 m.
Table 3 illustrates the format~on of an approximately
Z00964~
13 B 35136
stolchiometrlc synthesis gas from the combined reformed gas stream
15 of Table 1. It is here assumed that there is no bypa~ of the
membrane unit, that the pressure of the gas fed to the membrane
separation unlt ls about 22 bar abs., and that the permeate has a
pressure of about 2 bar abs. It ls also assumed that the membrane
employed is of the polyimide type having a hydrogen to carbon
monoxide permeability ratio of 39, a hydrogen to carbon dioxide
permeability ratio of about 5.4, and that the methane permeability
is similar to that of carbon monoxide.
Table 3
__________________________________________ ~ __________________
I I I Gas flow rate (kmol,h 1)
I Po8ition I Temp 1----------------------------------------------l
I ( C) I CH4 I H2 IC0 IC02 IH20
1 15 1 662 1 25.2 1280.2 1 51.2 1 32.5 1 209.9
1 22 1 35 125.2 1280.2 151.2 132.5 11.0
1 26 1 35 I 0.3 188.7 10.6 1 2.5 10.5
1 27 1 35 124.9 1191.5 150.6 130.0 10.5 1 ~-
The proportion of feedstock that is fed to the auxiliary -
reformer in the embodiment of ~able 1 is about 23X of the total. -
Consequently, if that system is employed to uprate an existing
fired reformer, by the provision of the auxiliary reformer the
throughput can be increased by about 25X at the expense of a lower
I reformed gas temperature and an increa~e in the methane content
(on a dry basis) of the synthesis gas from 4.7% (if no auxiliary
reformer and no membrane separation unit were employed) to 6.5% by ~
volume. ~ -
Since a smalI proportion of carbon oxides, principally
carbon dioxide, are separated into the permeate stream 26, the
full benefit of the increase in the reformer throughput is not
realisable in terms of the amount of methanol that can be
produced: however it is seen from Table 3 that the synthesis gas
stream 27 contains about 17X more carbon oxides than the reformed
Z009~4~.
14 B 35136
maJor stream 12 and so the amount of methanol that can be produced
may be significantly increased.
Further, from Tables 1 and 3 it is seen that the amount
of synthesis gas produced, ie stream 27, is about 95% of the
amount of dry gas in the reformed major stream 12. Thus not only
ls the reformer throughput, and hence amount of methanol that can
be produced, lncreased significantly, but also the amount of gas
fed to the compressor is slightly reduced, resulting in
compression power saving.
