Note: Descriptions are shown in the official language in which they were submitted.
~754~
This invention relates to the treatment of synthesis
gases, particularly to methane-containing synthesis gases
derived, for example, by the gasification of coal. More
particularly, the invention relates to removal of sulfur-
containing compounds from such synthesis gases.
Processes have been developed and are being developed
for the production of fuel gases, e.g., synthetic natural
gas (SNG) by the pressure gasification of coal in the presence
of steam and oxygen. The coal derived synthesis gas principally
contains hydrogen and carbon oxides with usually not more than
20 volume percent methane, and in order to manufacture SNG,
the synthesis gases have to be upgraded. Upgrading is con-
ventionally a catalytic methanation process wherein the
carbon oxides present in the gas are reacted with the co-present
- hydrogen. Coal derived synthesis gases contain appreciable
amounts of sulfur compounds, and it is known that such sulfur
compounds have a deleterious effect upon the performance of
methanation catalysts. One such methanation catalyst comprises
nickel oxide plus promoters supported on alumina, alumina
~0 silica mixtures, ~ieselguhr, or the like. Such catalysts
are highly sensitive ~o accumulative sulfur poisoning. It
has therefore been common practice to subject the coal derived
synthesis gas to a desulfuri~ation step, usually accompanied
by adjustment of the hydrogen and carbon oxide ratios prior
to methanation.
Such adjuRtments are commonly accomplished prior to
desulfuri~ation by a CO shift conversion processO Such a
process is shown by the reaction
CO + H2O - > CO2 2
--2--
1~754~L
Carhon dioxide is removed from the reaction product to provide
a stream rich in hydrogen for mixture with the synthesis gas
feed to the methanation reaction zone. The methanation
reaction comprises the foll~wing reactions:
CO ~ 3H2 3 C 4 2
C2 ~ 41~2 ~ CH4 + 2H20
These reactions are normally conducted in the presence of
the catalysts described above, and it is clear that the
desulfurization step must precede the methanation reaction.
The dry, shifted gas-synthesis gas mixture fed to the
methanation reaction i3 desulfurized hy routes such as the
Recticol acid ga~ absorption process or in the case where
organic sulur level~ are low by the Benfield acid gas
ab~orption process. In some instances, the gas mixture is
purified of sulfur by a hydrogenation process in which the
sulfur compounds are catalytically hydrogenated to hydrogen
sulfide which is subsequently removed in a catch vessel which
often contains zinc oxide. ~i~h each of these processes, a
zinc oxide vessel normally comprises the last zone priox to
the methanation reactor since the zinc oxide functions to
remove hydrogen sulfide and the like from the synthesis gas.
The zinc oxide functions by reacting with hydrogen
sulfide to produce zinc sulfide according to the eqUAtiOn
H2S + ZnO~ > ZnS + ~12O
The reaction is reversible, and therefore it is desirable
that the water level be kept low in the synthesis gas
: mixture pa~sing to the zinc oxide reactor.
It has been discovered that the use of such zinc
oxide reactors, while effective in removing the small amount~
Of hydrogen sulfide which remain after the first de~ulfurization
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44~
treatment, have a major disadvantage in that when the synthesis
yas is 8ub tantially dry, as is normally the case, carbon
disulfide is produced in minor amounts in the zinc oxide
reactor. The carbon disulfide functions as an accumulative
poison to the methanation catalyst and as a result shortens
the effective life of the catalyst, thus reducing the SNG
yield and resulting in the necessity for the replacement of
the catalyst at short intervals.
We have surprisingly found that carbon disulfide
is not formed to any substantial extent as the synthesis
gas is passed through the zinc oxide reactor when a small
amount of water is deliberately included in the synthesis gas
mixture contacting the zinc oxide. Further, we have found
that the performance of the zinc oxide in the removal of
hydrogen sulfide and similar compounds is not adversely affected
by the presence of such small amounts of water n Accordingly,
the present invention provides a method for the purification
of synthesis gases to render them suitable for methanation
reactions wherein a sulfur sensitive methanation catalyst is
~o used wherein the process comprises contacting a ~ynthesis
ga~ con~aining hydrogen, carbon oxides, hydrogen sulfide,
and from 0.5 to 5.0 volume percent of steam with the zlnc
oxide absorbent.
The synthesis yas preferably contains methane in
addition to hydrogen, carbon oxides, and hydrogen sulfide.
Furthermore, the synthesis gas may also contain small amounts
of carbonyl sulfide. The hydrogen content may range from 1 to
95 volume percent, preferahly from about 10 to 80 volume
percent. Carbon oxides may be present in the following
proportions:
~4--
~7544~
Carbon monoxide (volume percent) 0.5-50, preferably ]-40
Carbon dioxide (volume percent) 0.6-75, preferably 1-40
Hydrogen sulfide may be present in amounts ranging
from 0.1 to 500 ppmv (paxts per mil]ion by volume) but is
typically pre4ent in amounts from 0.2 to 100 ppmv. When methane
is present, it together with nitrogen and other inert gases may
comprise up to 95 volume percent of the synthesis gas. Pre-
ferably, however, the total of methane, nitrogen, and other
inert gases will range from 5 to 50 volume percent.
In the practice of the present invention, the water
content of the feed gas should range from 0.5 to 5.0 volume
percent. It has been found that a minimum of 1.0 volume percent
is preferred, and desirably, the water content will be between
2 to 3.5 volume percent. When the hydrogen sulfide content in
the synthe~is gas is significant, higher concentrations of
water are desirable to suppress the carbon disulfide formation.
At levels significantly above 5 volume percent, the water
begins to affect the equilibrium of the reaction between
hydrogen sulfide and zinc oxide, with the result that the
tendency for hydrvgen sulfide to slip through the zinc oxide
reactor increase~.
The water content of the feed gas may be adjusted by
mean~ known to those skilled in the art, such as by the
injection of steam into the synthe~is gas charged to the zinc
oxide reactor. Typically, the synthesis gas after removal of
the carbon dioxide and desulfurization prior to charging to the
zinc oxide reactor will contain substantially no water. For
instance, in the Rectisol process, cold methanol is used to
dissolve the carbon dioxide from the reaction stream from the
30 C0 shift reactor. The water content of the stream leavin~
--5--
:.
1~'75~
the Recti~ol unit is less than 0.2 percent water and typically
is less than 1 part per million water. Clearly, it is nece~sary
that water be added to such streams prior to charging such
streams to the zinc oxide reactor in the practice of the
present invention. In those instances wherein processes have
occurred prior to charging the stream to the zinc oxide reactor
which result in the presence of substantial amounts of water
in the synthesis gas charged to the zinc oxide reactor, it may
be necessary to remove substantial amounts of water. It is
particularly desirable that the amount be reduced below 5
volume percent.
The sulfur compounds in the synthe~is gas stream
charged to the methanztion reaction must be less than about
0.2 ppmv to result in an economically viable process. The
presence of higher amounts of sulfur compounds results in
, fouling of the catalyst very quickly, and of course, such
- renders the operation of the process impractical until the
ca~alyst has been replaced. Such is obviously an expensive
and time-con~uming operation and results in an impractical
proce~s.
The process of the present invention may be operated
at a pre~sure of up to 3,000 psig and preferably is operated
at pressure3 from 50 to 1,500 psig.
The reaction between the sulfur components and
zinc oxides may be effected at any temperature from 100 to
800F but preferably is effected at temperatures from 400 to
750DF. The invention will now be illustrated by reference
to the following examples.
5~
EXAMPLE I
A ~ynthesis gas consisting of
Component V l _3
Methane 10.5
Ethane 0.8
I~ydrogen 59.9
Carbon Monoxide10.0
Carbon Dioxide14.0
Nitrogen 0.2
Water 4.6
~Iydro~en Sulfide200 ppmv
Carbonyl Sulfide0.13 ppmv
Carbon DisulfideNil
Thiphene Nil
was contacted with 12,560 lbs of zinc oxide in an abosrber
at a flow rate of 242 X 106 SCF/hr. The operating temperature
and pressure were 679F and 325 psig, respectively. The
gas exiting from the absorber was then analyzed and found
to contain
Hydrogen Sulfide0.06 ppmv
Carbonyl Sulfide0.12 ppmv
Carbon DisulfideNil
Thiophene Nil
At the termination of the experiment, a compara~ive experiment
was carried out with a dry synthesis gas in which the same
zinc oxide was con~acted wi~h the following synthe~is gas at
a flow rate of 262 X 106 SCF/hr:
1~75q~
Composition Feed Gas, Volume Percent
Methane 9.0
Ethane 0-7
Hydrogen 71.1
Carbon Monoxide 8.4
Carbon Dioxide 10.0
Nitrogen 0.6
Water 0.2
Sulfur Compounds in Feed Gas, ppmv
Hydrogen Sulficle 90
Carbonyl Sulfide 0.09
Carbon Disulfi.de Nil
Thiophene Nil
The experiment was carried out at a temperature of
690F and at a pressure of 325 psig.
The exit gas from the absorber had the following
: sulfur component analysis (ppmv)
Hydrogen SulfideNil
Carbonyl Sulfide0.11
20 Carbon Di~ulfide5.55
Thiophene Nil
;~ It will be apparent from a comparison of the results
of the two experiments that no carbon disulfide was formed
or detected in the outlet gas when the process of the present
:: invention was carried out, whereas with the essentially dry
feed gas, the carbon disulfide content of the product gas
was ~igh.
` EXAMPLE II
A zinc oxide absorber containing 10,875 lbs of
30 absorbent was first contacted with a dry eed gas IComparative II)
and then a wet feed gas (Invention II). The experiment results
are given below: .
.~_
~07~4~
Operation Conditions Inventlon II C~
Feed Gas Rate (lo SCH/Hr) 252 260
Temperature, F 520 515
Pressure, psig 325 305
Composition of Gas (Vol. Percent)
Methane 10.8 10.3
Ethane 0-3 0 3
Elydrogen 65.8 70.6
Carbon Monoxide 14.1 10.8
10 Carbon Dioxide 7.7 7.8
Nitrogen 0.3 0.2
Water 1.0 0.0
Sulfur Compounds iIl Feed Gas (ppmv)
Hydrogen Sulfide 0.5 0.8
Carbonyl Sulfide 0.07 0.04
Carbon Disulfide 0.003 0.03
Thiophene Nil Nil
Sulfur Compounds in Product Ga ~
Hydrogen Sulfide Trace 0.002
20 Carbonyl Sulfide Trace 0.004
Carbon Disulfide 0.12 0.41
Thiophene Nil Nil
It will be apparent from a consideration of the above
results that carbon disulfide is produced in marked quantities
even at low hydrogen sulfide concentrations using a d.ry feed
gas, whereas by the practice of the present invention, carbon
disulfide formati.on is significantly suppressed.
75~1
EXAMPLE I I I
Two comparative experiments were run on feed gases
having substantially the same composition except for water
contents. The experimental re~ults for the two runs are
given below. Again the feed gases were contacted with 10,375
lbs of zinc oxide.
Operatin~ Condition~ Invention III Comparative ~I
Feed Ga~ Flow Rate (106 SCH/Hr) 242 252
Temperature (F) 700 700
10 Pressure (psig) 325 325
Composition of Feed Gas (Volume Percent)
Methane 12.9 13.3
Ethane 0 4 0 4
Hydrogen 60~6 62.7
Carbon Monoxide 14.0 14.5
Carbon Dioxide 8.7 g.o
Nitrogen 0.1 0.1
~ater 3.3 0.0
Sulfur Compounds in Feed Gas (ppmv)
20 Hydrogen Sulfide 1.1 1.1
Carbonyl Sulfide 0~12 0.4
Carbon Disulfide Nil 0.17
Sulfur_Com~ound~ in Product &a~ (p~mv)
Hydrogen Sulfide 0.06 Trace
Carbonyl Sulfide Nil 0.03
Carbon Disulfide Nil 0.80
It will be seen from the foregoing results that
carbon di~ulfide formation is completely suppres~ed when
operati~g with the pr0ferred water contents in accordance
w.ith the process of the in~ention.
~10--
~75~
EXAMPLE IV
Two studies of the phenomenon disclosed by this
invention were made on a large pilot plant. In both studies
crude synthesis gas produced by reActing steam and oxygen with
coal was passed through a C0 shift conversion process to
adjust its hydrogen content. It was then processed through
the Recti~ol acid gas absorption process to remove the majority
of its carbon dioxide content and to reduce its total sulfur
content to less than 2 ppmv.
Purified gas from the Rectisol process was passed
through a reactor containing 160 cubic feet of zinc oxide and
then through a second reactor containing 100 cubic feet of
methanation catalyst. Purified gas flow to the zinc oxide
adsorber varied somewhat on a day-to-day basis, but in general
it was about 275,000 standard cubic feet per hour.
Pertinent operations and results of these studies
are given in Tables 1 and 2. A dry feed gas was used initially
for the first s~udy shown on Table 1. Two results are
readily apparent. Carbon disulfide is formed in the zinc
oxide bed with a dry feed gas. As soon as water is added
to the feed gas, the amount of carbon disulfide in the ZnO
adsorber ou~let gas is significantly reduced. Secondly, the
methanation catalyst i8 rapidly poisoned by the sulfur
slippage from the zinc oxide adsorber. The reaction æone depth
increases at a rate of about two inches per day. As soon as
water is added to the feed gas, the reaction zone depth levels
off and remains constant for the balance of the s~udy.
Water was included in the feed gas to the zinc oxide
adsorber throughout the second study as shown on Table 2. The
outlet gas from the zinc oxide adsorber contains essentially no
carbon disulfide. Catalyst poisoning is significantly reduced--
les~ tha~ one-fifth of an inch per day. The reaction zone depth
in the second study reached 35 inches after 100 days. This
depth was reached in about 10 days in the first study.
--11--
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Although the process of the invention has ~een
particularly described with reference to the trea-tment of
synthesis gases derived from coal gasificatlon, it will be
ap~reclated that the process is equal.ly applicable to any
synthesis gas in which fine sulfur purification is required.
The process of the invention may be applied to methane
synthesis routes derived fxom liquid hydrocarbon based
materials such as crude oil or coal-tar extracts where, for
example, the product gases require upgrading by methanation.
Similarly, the process of the invention may be applied to
synthesis rou~es for the production of, for example, oxygen
and nitrogen containing chemicals such as methanol and ammonia,
derived from coal or oil and wherein the primary synthesis
gas is subjected to sulfur sensitive catalyzed reaction,
e.g., reforming, hydrogenation, and isomerization.
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