Integrated steam methane reforming process for producing...

Details Integrated steam methane reforming process for producing... Integrated steam methane reforming process for producing...

1996-04-08

2001-12-11

Hendrickson, Stuart L. (Department: 1754)

Chemistry of inorganic compounds

Carbon or compound thereof

Oxygen containing

C423S652000, C423S654000

Reexamination Certificate

active

06328945

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention is a process for producing carbon monoxide (CO) by reforming methane and steam in the presence of a reforming catalyst to produce a reformate product enriched in CO, carbon dioxide and hydrogen. Carbon dioxide (CO
2
) in the enriched reformate is shifted to CO in an integrated sorption enhanced reaction (SER) cycle which employs a series of cyclic steps to perform the reverse water gas shift reaction to convert CO
2
to CO, to partially separate the shift reaction product mixture to recover an enriched CO stream and to prepare the SER reactors for a subsequent SER process cycle.
BACKGROUND OF THE INVENTION
Carbon monoxide is typically produced by catalytically reforming a hydrocarbon feed with steam, and optionally, carbon dioxide, at high temperatures. The reaction occurs in a steam methane reformer (SMR) which contains catalyst-filled tubes housed in a furnace. The synthesis gas exiting the reformer contains carbon monoxide (CO) along with hydrogen, carbon dioxide (CO
2
), steam and unconverted methane according to the equilibria established in the following reactions:
CH
4
+H
2
O&rlarr2;3H
2
+CO Steam Reforming
H
2
+CO&rlarr2;H
2
+CO
2
Water Gas Shift
CH
4
+CO
2
&rlarr2;2H
2
+2CO CO
2
Reforming
The above-mentioned reactions are generally carried out at high temperatures (800°-1000° C.) and at high pressures (5-30 atmospheres) wherein the reactants are contacted with a nickel based catalyst. These reactions are thermodynamically controlled. Therefore, the reformate effluent composition shall depend on many variables including pressure, temperature, molar ratio of steam/methane in the reactor feed and carbon dioxide concentration in the reactor feed. A typical SMR effluent composition (mole fractions) possesses 73% H
2
, 13% CO, 8.5% CO
2
and 5.5% CH
4
when the SMR reaction is conducted at 850° C. and 25 atmospheres using a CO
2
-free feed mixture containing a 3:1 water/methane molar ratio. The SMR effluent is subjected to a series of reaction and separation operations in order to recover a high purity H
2
product (99.9+mole %) or a high purity CO product (99.5+mole %).
Carbon monoxide provided in commercial SMR Plants is typically used to manufacture isocyanates and polycarbonates through phosgene chemistry. Alternatively, certain processes for producing oxoalcohols require a synthesis gas having a 1:1 ratio of hydrogen to carbon monoxide. By-product hydrogen and export steam formed during such SMR processes may have fuel value, but may not be required as products.
As is well known in the industry, synthesis gas having a high CO content is produced by injecting CO
2
into the reformer feedstock and by reducing the ratio of steam to hydrocarbon in the SMR feedstock. The SMR feedstock can be further enriched in CO
2
by recycling CO
2
produced and separated from the synthesis gas or recovered from the furnace flue gas or by importing additional CO
2
into the feedstock from an outside source. SMR feedstocks having a high CO
2
to methane ratio and reduced amounts of steam inhibit the water gas shift reaction from producing additional H
2
from CO and will reverse this reaction to produce additional CO from H
2
under extreme reaction conditions. Some CO
2
also reacts with methane in the SMR feedstock to yield syngas having a low H
2
/CO ratio.
The amount of CO produced in conventional SMR processes is limited by reaction thermodynamics wherein a relatively low conversion to CO (~10-15%) necessitates a significant separation effort to recover the desired CO product. Numerous prior art SMR processes for producing synthesis gas are known which utilize a variety of separation cycles to recover the desired CO product from the SMR reformate effluent which typically contains a mixture hydrogen, CO, CO
2
and methane.
U.S. Pat. No. 3,986,849 discloses a SMR process for converting water and a source of methane, such as natural gas, to a hydrogen product as depicted in FIG.
1
. Methane and water are introduced through line
1
into a conventional SMR reactor
2
and reacted under reforming conditions to produce a H
2
-enriched reformate stream
3
. Stream
3
is introduced into condenser
4
to yield steam and cooled reformate stream
6
at an intermediate temperature of 250°-350° C. The cooled reformate is then fed into water-gas shift reactor
7
(high temperature shift reactor, alone or in combination with a low temperature shift reactor) to convert a portion of the CO in reformate stream
6
to hydrogen by reacting CO with H
2
O according to the reaction (CO+H
2
O&rlarr2;CO
2
+H
2
).
The above-mentioned shift reaction plays a key role in the over-all process when hydrogen is the desired product because the shift reaction increases the hydrogen concentration and quantity in the reformate product mixture prior to separating the reformate product mixture to produce essentially pure hydrogen. Shift reactor effluent
8
is further cooled to a near ambient temperature (25°-50° C.) by indirect heat exchange with cooling water in condenser
9
wherein a substantial amount of water is condensed and removed from the reformate via line
10
. Finally, stream
11
exiting the condenser is introduced into a hydrogen pressure swing adsorption unit (H
2
-PSA) to yield essentially pure hydrogen via stream
14
and a waste gas stream
13
which can be used as fuel in the reformer.
U.S. Pat. No. 4,171,206 discloses a SMR process for converting water and a source of methane such as natural gas to simultaneously yield a high purity hydrogen product and a high purity CO
2
product as depicted in FIG.
2
. Methane and water are introduced through line
21
into a conventional SMR reactor
22
and reacted under reforming conditions to produce a reformate stream
23
.
Stream
23
is introduced into condenser
24
to yield cooled reformate stream
26
at an intermediate temperature of 250-350° C. and condensate stream (not numbered). The cooled reformate is then fed into water-gas shift reactor
27
to convert a portion of the CO in reformate stream
26
to hydrogen. Shift reactor effluent
28
is further cooled to a near ambient temperature (25-50° C.) by indirect heat exchange with cooling water in condenser
29
wherein a substantial amount of water is condensed and removed from the reformate via line
30
. Finally, reformate stream
31
exiting condenser
29
is introduced into CO
2
vacuum swing adsorption (VSA) unit
32
wherein the reformate is separated to provide an essentially pure CO
2
product stream
35
. The waste gas from CO
2
VSA unit
32
is introduced into H
2
-PSA unit
38
via line
34
and is separated to yield an essentially pure hydrogen stream
37
and waste gas stream
36
which can be used as fuel in reformer
22
. The CO
2
VSA unit
32
and H
2
PSA unit
36
are integrated to obtain maximum separation efficiency.
A conventional SMR process is depicted in
FIG. 3
wherein water and a source of methane are introduced through line
41
into a conventional SMR reactor
42
and reacted under reforming conditions to produce a reformate stream
43
. Stream
43
is introduced into a CO
2
absorber/stripper
44
which contains a physico-chemical solvent which removes CO
2
from the pre-cooled SMR effluent to provide stream
45
which contains essentially pure CO
2
and a CO
2
-depleted reformate stream
46
which is introduced into thermal swing adsorption unit
47
to remove water and remaining CO
2
which is withdrawn from adsorption unit
47
via line
48
. CO
2
and water depleted stream
49
is introduced into cryogenic cold box
50
to yield essentially pure hydrogen stream
51
, essentially pure CO stream
53
and a waste stream
52
containing CO and unreacted methane which can be used as fuel in reformer
42
.
Another conventional SMR process is depicted in
FIG. 4
wherein water and a source of methane are introduced through line
61
into a conventional SMR reactor
62
and reacted under reforming conditions to produce a reformate stream
63
. Stream
63
is introd

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