CA1075441A

CA1075441A - Zinc oxide desulfurization process

Info

Publication number: CA1075441A
Application number: CA262,887A
Authority: CA
Inventors: William B. Watson; Leslie J. Dekanski
Original assignee: Conoco Methanation Co
Current assignee: Conoco Methanation Co
Priority date: 1976-03-29
Filing date: 1976-09-30
Publication date: 1980-04-15
Also published as: JPS52117880A; BR7701877A; DE2650711A1; AU1980676A; AU503929B2; GB1568703A; ZA77923B

Abstract

IMPROVED ZINC OXIDE DESULFURIZATION
PROCESS

Abstract of the Disclosure An improvement in a method for desulfurizing gaseous streams by contacting the gaseous streams with zinc oxide, the improvement comprising adjusting the water content of the gaseous streams to from about 0.5 to about 5.0 volume percent prior to contacting the gaseous streams with the zinc oxide.

Description

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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 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 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~

:.

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: .

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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.

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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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method for removing sulfur compounds and carbon dioxide from a gaseous stream containing carbon oxides and hydrogen to produce a gaseous stream having a sulfur content of less than 0.2 ppmv and a reduced carbon dioxide content, said sulfur compounds including at least one compound selected from the group consisting of hydrogen sulfide, carbonyl sulfide and carbon disulfide, the method consisting essentially of passing said gaseous stream to a first desulfurization zone to remove a major portion of said carbon dioxide and said sulfur compounds and thereafter passing said gaseous stream into contact with zinc oxide in a second desulfurization zone at temperatures from about 100 to about 800°F and a pressure up to about 3000 psig, the improvement comprising adjusting the water content of said gaseous stream recovered from said first desulfurization zone to from 0.5 to 5.0 volume percent prior to contacting said gaseous stream with said zinc oxide in said second desulfurization zone thereby minimizing the production of carbon disulfide in said zinc oxide.

2. The improvement of claim 1 wherein said gaseous stream comprises a synthesis gas feedstream to a methanation reactor.

3. The improvement of claim 2 wherein said gaseous stream charged to said zinc oxide contains from 0.1 to 500 ppmv hydrogen sulfide.

4. The improvement of claim 1 wherein said first desul-furization zone reduces the water content of said gaseous stream to less than 0.2 volume percent water.

5. The improvement of claim 4 wherein said first desul-furization zone utilizes cold methanol to remove at least a major portion of said carbon dioxide and said sulfur compounds from said gaseous stream.

6. The improvement of claim 1 wherein said gaseous stream passing from said first desulfurization zone contains more than 5.0 volume percent water.