Purification of Synthesis Gas Produced from Pulverized Coal

INDUSTRIAL AND ENGINEERING CHEMISTRY

April 1948

a - b - ~ +1 2 LIMITINGCASEI I A SOLUTION. r4 [K4

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The equations given for the partial oxidation of methane are merely special cases of the general equations. This may be readily demonstrated by substituting s = 4 in the general equations, whereupon they are transformed into the more special equations. Literature Cited

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(1) Am. Petroleum Inst., Research Project 44, Tables of Selected

Values of the Properties of Hydrocarbons.

(17b)

(2) Scarborough, J. B., “Numerical Mathematical Analysis,” pp. 178, 198, Baltimore, Johns Hopkins Press, 1930. (3) Wagman, D. H., Kilpatrick, J. E., Taylor, W. J., Pitaer, K. S., and Rossini, F. D., J. Research Natl. Bur. Standards, 34, 14361 (1945); ’Reamrch Paper 1634.

(lSb)

RBCEIVED November 4, 1947.

S

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KAr2a - a

607

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Purification of synthesis gas produced from pulverized coal Specifications, Purification Processes, and Analytical Methods A. E. SANDS, H. W. WAINWRIGHT, AND L. D. SCHMIDT U. S. Bureau of Mines, Synthesis Gas Production Division, Morgantown, W. Va. THE production of synthesis gas directly from coal, instead of the usual practice of using coke as generator fuel, presents new and greater difficulties in connection with purification of the gas to render it suitable for synthesis purposes. With special reference to gas made directly from pulverized coal, this paper outlines the purification specifications for Fischer-Tropsch synthesis gas, some of the established purification processes which may be used to meet those specifications, and finally, analytical methods which may be employed to deal with the extremely low concentrations of impurities that may be tolerated in the purified gas.

A

S PART of the program authorized by Congress (65),the

Office of Synthetic Liquid Fuels of the Bureau of Mines, United States Department of the Interior, is preparing to operate a Fischer-Tropsch demonstration plant t o demonstrate the commercial feasibility of producing liquid fuels from coal by this method. To further this program, the Synthesis Gas Production Division has established laboratory and pilot plant facilities a t We3t Virginia University to perform the necessary development work in connection with the gasification of pulverized coal and the treatment of gas so produced to render i t suitable for use in the Fischer-Tropsch synthesis of liquid fuels. For a number of years ammonia, alcohols, and other chemicals have been synthesized from coal as a basic raw material. I n such work, however, the synthesis gas usually has been made from coke by the water-gas process. Although a few German plants used coal for making synthesis gas during the war, these were exceptions t o the almost universal practice of using coke or char. F. Martin of the Ruhrchemie A. G. (86)stated that synthesis gas made from brown coal or reformed coke-oven gas always gave operating difficulties owing to tar, carbon, resins, and other disturbing substances. Attempts to use

coke-oven gas for synthesis were unsuccessful because of the difficulty of removing thiophene a n d , other organic sulfur compounds. In view of these greater difficulties presented i n the purification of synthesis gas made directly from coal, i t appears desirable to consider the purity requirements of synthesis gas in terms of impurities which may be expected t o be concomitant with pulverized-coal gaqification, to review the existing processes which, by commercial plant performance, have demonstrated thzir ability to effect the required purification, and finally to investigate the available analytical methods for dctermining impurities in the extremely low concentrations with which we are concerned.

Purity Specifications An ideal synthesis gas for the Fischer-Tropsch process would be a mixture of hydrogen and carbon monoxide in proportions that may vary according to the catalyst and pressure used and the products desjred. Any other constituents of the gas may be considered impurities. Starting from this definition, impurities may be classified as either inert or injurious. The inert substances are constituents that have no deleterious effect on the catalyst or process equipment but dilute the reaction mixture and result in lessened space-time yields. The injurious impurities are those that e x x t a n injurious effect on either the process equipment or the catalyst. The inerts are chiefly carbon dioxide, methane, and nitrogen. The proportions of all these may be controlled, within limits, by the selection of the gasification process and operating conditions. With a conqinuous gasification process using oxygen, steam, and pulverized coal, i t is anticipated that the methane a n d nitrogen together will not exceed 7%. I n the same process, however, carbon dioxide may be as high as 20 or 250j0. This high carbon dioxide concentration is in contrast to the conventional wates

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*

INDUSTRIAL AND ENGINEERING CHEMISTRY

gas made from coke, where the carbon dioxide seldom exceeds 5 or 6%. Similar high concentrations of carbon dioxide may result from the catalytic shift conversion of carbon monoxide and steam to hydrogen. For the Fischer-Tropsch process in its present state of development, however, shift conversion may be necessary to only a limited extent, if a t all. With iron catalysts in the Fischer-Tropsch process, large amounts of carbon dioxide are formed-amounting to as much as 20'30 of the volume of gas converted. When conversion in the first stage is 600j0, the carbon dioxide formed will thus be 12% of the original gas volume (1). If this carbon dioxide is not removed before the second stage, space-time yields and specific yields will suffer. It is difficult to set up a specification for the maximum carbon dioxide content of Fischer-Tropsch synthesis gas, because whether or not this impurity is t o be removed (and a t which points in the system) depends on a balancing of economic factors. These include the cost of the synthesis gas, fixed charges on the converters, catalyst and pressure used, and removal-process costs. For estimating purposes, i t has been calculated that, in terms of value t o the conversion, reducing the carbon dioxide content from 20 t o 5% is worth approximately 0.2 cent per gallon of liquid hydrocarbon (1) product. The impurities that may be classified as injurious include tar, dust, gum formers, iron carbonyl, sulfur, hydrogen cyanide, ammonia, and naphthalene. Tar. Unlike water gas produced from coke, gas produced from coal may contain tar. Tar must, of course, be completely removed from the synthesis gas before it reaches the converters, and it is nearly as essential to effect this removal prior t o the gaspurification system. If it is present in the gas entering the sulfurremoval processes, a detrimental effect on the equipment or purification materials may be expected, particularly in catalytic units for removing organic sulfur. Liquid purification processes involving solution heaters or heat exchangers would be adversely affected because of formations of film on the heat-transfer surfaces. While active charcoal units end dry boxes, intended for other purposes, would a h o remove tar, such removal would decrease the efficiency of the material for its intended purpose and be more expensive than the use of specific tar-removal equipment. For these reasons, plans for the demonstration plant include a tentative specification of less than 0.001 grain of tar per hundred cubic feet of gas. Du6t. Although dust has been a problem in many plants utilizing water gas made from coke, the problem will be greatly aggravated with gas made from pulverized coal. Some of the dust will be contained in the tar and hence considered and treated as such. If the quantity of tar is small, it is believed that there will be appreciable amounts of tar-free dust in the gas. Dust depositing on the rotors of turbocompressors would cause operating and maintenance troubles because of the resulting unbalance. Because of its chemical composition, the dust might poison the synthesis catalyst if carried through to that point. -4s very fine dust particles sometimes have an amazing ability t o pass through a series of scrubbers] the presence of dust should be anticipated at points remote from the gasification equipment. German experience indicates that, because of the effect on turbocompressors, dust should not exceed 0.05 grain per hundred cubic feet. Dry-box purifiers will remove dust effectively, but even in this case the dust should not exceed 0.25 grain per hundred cubic feet ( 1 ) . Experience a t Oppau, Germany, indicated that 0.2 grain of dust per hundred cubic feet had no serious effect on shift-converter operation (8%). The specification for dust, therefore, is a maximum of 0.05 grain per hundred cubic feet of gas before turbocompressors or synthesis catalyst, with a permissible content of 0.25 grain per hundred cubic feet before dry-box purifiers.

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Gum Formers. Experience with the formation of gum in the manufactured-gas industry has led to the conclusion that there are two types of gum: liquid- and vapor-phase. The liquidphase type is produced from condensation of indene and styrene, followed by polymerization. Vapor-phase gum is formed by the reaction of nitric oxide, oxygen, and dienes, such as butadiene and particularly cyclopentadiene. All these organic compounds are products of coal carbonization. In coke-oven gas, where the unsaturated hydrocarbons (illuminants) comprise approximately 3 to 4y0by volume, butadiene may be 0.02%, cyclopentadiene 0.006%, styrene 0.00670, and indene 0.013% (61). Coal carbonization or gas-cracking processes at temperatures below 2200" to 2400" F. favor the formation of these compounds ( 1 ) . These gum-forming compounds may be expected in the gasification of pulverized coal. To prevent deterioration of the synthesis catalysts, and particularly of the soda-iron catalyst (if used) for organic sulfur removal, gum formers should be entirely removed. Iron Carbonyl. Iron carbonyl may be formed from the reaction of carbon monoxide and iron. Formation is favored by lower temperatures and elevated pressures. While iron carbonyl has been detected in gas from low-pressure distribution systems, it is expected that in the liquid-fuels plant the time of contact of the gas with the iron of the pipes and vessels will be too low a t the existing pressures to result in any appreciable content in the gas. Iron carbonyl does not appear t o have caused trouble in German synthetic fuel plants. This impurity is mentioned here because, under exceptional conditions-e.g., long storage under pressure-an important concentration might be formed. AS iron carbonyl is decomposed by heat to yield deposits of carbon and iron, the effect on catalysts at elevated temperatures might be serious. No experimental data are available concerning the effect of definite quantities in Fischer-Tropsch synthesis gas, but it would appear that concentrations as low as 0.1 grain per hundred cubic feet might have detrimental effects. Sulfur. Sulfur in all forms results in deterioration of the synthesis catalyst, I n German practice it was considered necessary to reduce the total sulfur t o not more than 0.10 grain per hundred cubic feet of gas. By reducing the total sulfur from the range 0.05 t o 0.10 grain t o 0.01 grain per hundred cubic feet, the life of a cobalt catalyst can be increased 50%. With cobalt catalyst, the value of such increased life has been estimated to have a value t o the synthesis of the order of 0.1 cent per 1000 cubic feet of synthesis gas (1). The specification for maximum sulfur content of the synthesis gas, therefore, is 0.1 grain per hundred cubic feet, with the lower value of 0.01 grain indicated where possible at no greater additional cost than 0.1 cent per 1000 cubic feet of gas. For iron catalysts where the more rapid formation of carbon deposited in the catalyst is the limiting factor in catalyst life, removal below 0.1 grain per hundred cubic feet of gas will have no value. Hydrogen Cyanide. Relatively little, if any, hydrogen cyanide is found in water gas made from coke. In coke-oven gas, this impurity may reach a concentration of 50 to 60 grains per hundred cubic feet. Since gas made from coal will contain distillation products as well as those formed by the water-gas reaction, it is likely that small but appreciable concentrations of hydrogen cyanide will be found in the raw synthesis gas. Since hydrogen cyanide has about the same poisoning effect on the Fischer-Tropsch catalyst as does sulfur, the two impurities should be considered together In specifying maximum tolerances. Sulfur and hydrogen cyanide together should not exceed 0.1 grain per hundred cubic feet. Ammonia. Like hydrogen cyanide, ammonia may be expected in the raw synthesis gas. Although experimental data on the effect of traces of ammonia are lacking, higher concentrations

INDUSTRIAL AND ENGINEERING CHEMISTRY

April 1948

are known to be detrimental. I n one experiment in which 50 grains of ammonia per hundred cubic feet of gas were added to the synthesis gas, immediate decrease in catalyst activity resulted ( 1 ) . When the addition of ammonia was stopped, the original activity of the catalyst returned. Unfortunately, the effect on the life of the catalyst was not determined. As a tentative specification, until more information can be obtained on the effect of small concentrations, it is suggested that ammonia should not exceed 0.1 grain per hundred cubic feet; purification t o this degree can be easily obtained. Naphthalene. Appreciable, concentrations of naphthalene may be found in the raw gas. It is known that, in the manufacture of blue water gas from bituminous coal, naphthalene at the relief holder amounted to 20 grains per hundred cubic feet (21). The effect of pure naphthalene on the synthesis catalyst or reactions is not known, but the term “naphthalene” in gas manufacture is used rather loosely to include a number of other substances, including gum-forming constituents (6). These substances would be removed by the treatment applied for removal of naphthalene. For this reason, if for no other, “naphthalene” should be completely removed from the gas.

Purification Processes It is impossible, i n a paper such as this, even to mention all the individual processes proposed to effect the various steps in gas purification. Seil’s (71)annotated bibliogiaphy gives 1801 abstracts dealing with gas purification and lists 128 patents alone on liquid-puri ficatian processes for removing hydrogen sulfide. In the present work, discussion is limited to processes that have been reduced t o industrial practice. Attention is invited to those used in the German synthetic liquid fuels industry for two reasons: because they have been used commercially to meet the objectives cited in this paper; and because they are perhaps less well known in this country. CARBON DIOXIDE

Carbon dioxide may be removed from gas by some of the liquidpurification processes used for removing hydrogen sulfide; these gases are often removed together in purifying synthesis gas. Hence, removal of carbon dioxide will be discussed in connection with processes for removal of hydrogen sulfide. TAR AND DUST

Tar and dust may be removed from gas by,similar means but not necessarily with the same ease. I n general, both impurities may be removed by water scrubbing, impingement on baffles, filtration, and electrical precipitation. The design of equipment for dust removal may vary somewhat because of differences in the physical nature of the material. In water-gas operation, gross amounts of dust and tar are removed in the equipment forming part of the set. Thus, large quantities of the larger sizes of dust are removed in “dust catchers” or cyclones, while the major portion of the tar is separated in the wash box. At this latter point, further amounts of dust will be separated from the gas, being occluded in the condensed {ar. Incidentally, mixtures of tar, water, and dust frequently lead to serious trouble with emulsion. Following further cooling of the gas, with consequent separation of additional tar and dust, and after passing through the relief holder, water gas may be given a final cleaning by impingement separators, such as P. & A. extractors, or electrical precipitators. If tar extractors are used, these may be followed by shaving scrubbers. A mixture of tar and dust gives a pasty consistency on separation, leading to difficulty in removing the separated material from dry types of mechanical separators. Frequently, water or oil flushing is practiced in electrical precipitators t o remedy this condition.

609

According to one manufacturer of electrical precipitation equipment (88), the maximum precipitator-removal efficiency is generally considered to be 99.9%,*but with equipment designed for the purpose efficiency as high as 99.9995570 could be obtained by successive treatment. This efficiency would be required if an inlet concentration of 100 grains per hundred cubic feet is assumed, with outlet gas having less than 0.001 grain. For dust with the same inlet concentration and a limit of 0.05 grain per hundred cubic feet a t the outlet, an efficiency of 99.9557c would be required for such requirements, four precipitator units having twenty-one 9-foot pipes each would be indicated. The units would be of the film type, provided with nozzles for washing down the pipes with water or oil. Obviously, lower efficiency of the precipitator can be tolerated i f preliminary removal is more effective. Instead of the usual simple types of condensers or primary scrubbers provided in water-gas plants, the condensers may be of a type known as hydroclones, where the effects of centrifugal force and intimate mixing with water remove dispersoids. Two manufacturers of this type equipment state their belief t h a t complete removal can be approached by this method (8, 69). Investment costs for such equipment are relatively low. Captured German documents recently reviewed at the Morgantown, W. Va., Station of the Bureau of Mines show much concern in Germany during the war with the dust problem. T4ese documents contain many fundamental data applicable to the various types of dust separators. One report (81)deals with various devices tried or proposed a t Oppau for removing carbon black from gas made by cracking acetylene and other hydrocarbons by means of incomplete combustion. The gas contained 4 to 20 grains per hundred cubic feet of carbon black and the resulting gas was t o be used for synthesis purposes. Employing two Theisen disintegrators in series and hot water near the dew point of the gas, carbon black content was 2 to 3 grains per hundred cubic feet of gas after the first washer and 0.4 t o 0.8 grain after the second. Addition of a wetting agent (Nekal) gave no improvement. Electric precipitation (Lurgi design) was considered. The design proposed called for operation near the gas dew point, condensing water vapor by injection of cold water. Lurgi was not sure that good removal could be obtained; and this fact, together with anticipated high investment costs ($14,000 to $16,000), led t o abandonment of this method. So-called “dust chambers” or fixed-bed filters in use at Oppau for other dedustina work were considered. but it was believed t h a t they would not b e successful because of’rapid choking with fine Darticles of carbon black. Textile bag fiters were effective, leaving not over 0.04 grain of dust per hundred cubic feet of treated gas, but the high watervapor content in the gas led t o depreciation of the bags in 8 t o 14 days. With filter bags having an area of 23.5 square feet and assuming a n output of 56 cubic feet per square foot per hour, one filter bag will purify 1300 cubic feet per hour. Considering the time required t o change bags, 530 bags were required to be equivalent to one shaft filter.

h shaft filter (81)was finally developed to meet the requirements. One of the present authors inspected a shaft filter installation that successfully removed carbon black from synthesis gas a t Linz, Austria. Shaft filters are moving bed filters (Figure 1) in which the filtering medium is broken pumice or coke, with provision for washing the bed material and returning i t t o the top of the filter without exposure to the air or interrupting operation. Various sizes of filtering media can be used; 0.08- to 0.32-inch size containing a high proportion of 0.08- to 0.20-inch material, has been recommended as best. Coke was found more effective than pumice. I n the shaft filter sketched as Figure 1 gas flows up through the filter bed. Coke is discharged by means of a shaker grate from the bottom of the filter into the washing chamber, where the coke is cleaned by a whirling action of water. Wash water flows out through a pipe provided with a seal. Cleaned coke is discharged into the lower compartment, whence i t is transported by a water stream to the top of t h e filter. T h e hoisting water is separated

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INDUSTRIAL A N D ENGINEERING CHEMISTRY

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I t is probable that, if the active carbon unit had been installed a t first, the wash-oil system would not have been required. Like wash oil, active carbon may be used for recovering light oil (17, 18, 30).

-

Hois*g

Figure 1.

water

Shaft filter

3 meters in diameter, area 7 square meters

by mean5 of a screen; the coke passes over the screen and drops on top of the filter bed. The coke used was found t o have a settling rate in n7ater of 1 to 5 feet per second. The velocity of transporting water was 23 to 32 feet per second. Water for lifting the coke was added at the top of the bin and a t the tee on the bottom, about twice as much at the top as the bottom. Coke was removed from the filter bed on a schedule to prevent the gas-pressure drop through the bed from exceeding 300 mm. of water column. Figure 2 shows the results obtained from experiments made on pressure-drop determinations.

,

Operating data for the experimental shaft filter are given in Table I. It is claimed that the shaft filter can be employed wherever dust-containing gases must be treated without cooling the gases excessively. Experiments performed a t 750' t o 900 O F. were said to be successful. GUM FORMERS

Since vapor-phase gum is formed by the reaction of nitric oxide, oxygen, and dienes, while liquid-phase gum is the polymerization product of indene, styrene, and related hydrocarbons, i t follows that removal of either nitric oxide or dienes will prevent the formation of vapor-phase gum, while to prevent the formation of liquid-phase gum the monomers-indene, styrene, etc.-must be removed. Actually, most of the processes for the removal of dienes will also remove the monomers. Unless removal of nitric oxide, however, is accompanied by removal of the monomers, liquid phase gum may be formed. Processes emphasizing the removal of nitric oxide include those involving chemicals (10, 23, 24, 61), active carbon ( T O ) , modified dry-box operation (14 ) , treatment with electrical brush discharge (36), oxygen treatment (12, 69), and many others. Processes for removal of gum-forming hydrocarbons chiefly involve the use of solvents ( 5 @ , including wash oil ($9,4 7 ) , and adsorption on active carbon ( 3 7 ) . I n Germany either active carbon or wash-oil scrubbing was used. At Wintersehall, A. G. Lutzkendorf, where gas was made from pulverized brown coal, both wash oil and active carbon were used (38). At this plant, light oil amounting t o 0.054 gallon per 1000 cubic feet was recovered from the wash oil. This is as much as 5% of tho gasoline yield from Fischer-Tropsch synthesis. A t first, attempts were made to operate without either wash oil or active carbon, but gum trouble in the subsequent process for removal of organic sulfur required the installation of the wash-oil scrubbing equipment. Later continuance of trouble attributed to gum requiied the installation of active carbon treatment.

X unique method of operation of the active carbon unit was said to be used a t Lutzkendorf ( 1 ) . After becoming saturated x i t h impurities from the crude synthesis gas, exit gas from the Fischer-Tropsch units was passed through the carbon and the material used to recover the benzine vapors. Upon becoming saturated with this product, a single steaming freed the carbon of the product for recovery and eliminated impurities, so that the carbon could be re-used for purification of the synthesis gas. According to the patent covering the process (37),the benzine liberated a t the beginning of the steaming condenses in the still cold layers of active carbon, thus aiding in removal of elementary sulfur and resins. It would seem, however, that removal of those impurities from the carbon is aided a t the expense of contaminating the benzine. Active carbon (without wash-oil scrubbing) was used a t Essener-Steinkohle for removal of gum formers. IRON CARBONYL

rllthough not more than traces of iron carbonyl in synthesis gas are expected, the impurity is removed by active carbon (48). According to this process, when the carbon is saturated with iron carbonyl, air is blown through, and the carbonyl is catalytically oxidized. Subsequently, iron is extracted by means of dilute acids or in some cases by leaching with water. SULFUR COMPOUNDS

I n plants where complete removal of sulfur has been attempted, hydrogen sulfide has first been removed by one or more processes, followed by removal of organic sulfur. Hydrogen Sulfide. DRY-BOXPROCESS. Hydrogen sulfide may be completely removed by dry-box purification, using hydrated iron oxide mixed with a carrier material such as wood shavings. Dry-box purification was widely used in Germany, generally in the larger plants, for removing residual hydrogen sulfide not removed by a liquid-purification process but often as the single process for removing hydrogen sulfide. Luxmasse, a by-product of the refinement of bauxite containing approximately 45% ferric oxide, was extensively used for preparing the mixture. This same material, imported from Germany, was used inhhis country before the war. According to information received by one of the writers, i t is again being imported (2). Lautamasse and bog ore were also used in Germany. Sulfur was usually recovered from the spent material by solvent extraction, the material often being sent from many plants to a central plant for processing. I n German dry-box plants, towers are widely used as purifiers t o hold the oxide sponge instead of the conventional box design followed in this country. These towers, which havz been described by Gollmar (26), have the advantage of reducing ground space, time required for changing the material, and probably labor costs. Other than in the use of these towers, dry-box purificatior) practice does not appear to differ much from and be more efficient than that, used in the United States. LIQUIDPURIFICATION. For very large plants, particularly those dealing with fairly high concentrations of hydrogen sulfide, it is usually economical to remove most of the hydrogen sulfide by some liquid-purification process, following this with dry boxes t o remove the remaining small amount. For small plants, dry-box purification probably will give the lowest-cost removal of hydrogen sulfide. The choice between liquid purification and the drybox method must be determined for each plant as an individual case, as the factors to be considered will vary. In selecting processes for the Fiecher-Tropsch demonstration plant, the authors are governed by conditions that would obtain

April 1948

INDUSTRIAL AND ENGINEERING CHEMISTRY

611

in a much larger plant. AlSHAFT FILTER TABLE I. OPERATINU DATAO F AN EXPERIMENTAL though the d e m o n s t r a t i o n (Filter area 5.1 square feet) plant will probably be limited Test No. 3 4 5 6 7 8 to 3,000,000 cubic feet of gas Temperature, O F. 158 158 158 158 86 158 per day, a full-size commercial 8,400 Gas per hour,a cu. ft. 9,300 9,400 8,050 8,050 ’ 8,250 10,900 10,900 12,300 12,700 8,800 10,600 Gas a n d water vapor, CU. ft./hour plant might produce 100,000,Gas and water vapor, cu. ft./sq. ft./hour 2,400 2,500 2,100 2,200 2,200 1,700 22.0 13.0 65-150 7-14 Carbon black content inlet, grains/100 cu, ft. 000 to 600,000,000 cubic feet 0 9-9.0 1 3-9.0 Carbon black content outlet, grains/100 ou. f t . 0.13-0.300.18-0.550.13-0.300.22-0.80 0.09-0.220.05-0.09 per day. Because the demonTest carried out, hours 144 192 240 154 290 120 Coke Coke Packing Pumice Pumice Coke Pumice stration plant is in the nature Size, inches 0.12-0.24 0.12-0 24 0.12-0.240 .OS-0.32 0 .OS-0.32 0 .OS-0.32 28 52 55 125 38 35 Total conRumption of packin cu ft of a large pilot plant for much 0 .04 0.017 0.024 0.021 0.106 Packing consumption, cu. ft.yM 0;. it. 0.032 larger commercial enterprises, Corrected to dry gas a t 30 inches H g absolute and 80’ F. and because gas purification bears such an essential relationship to the gasification and the phenolate process, employing a solution of sodium phenomocess and t o the synthesis, it is believed that purification late (76),are not believed to be applicable to gas having as high processes selected for demonstration should be of types apcarbon dioxide content as may be expected from the gasification of plicable to the much larger commercial plants of the future. coal with the use of oxygen. The Thylox process is used for purification of synthesis gas made from coke water gas. Hence, for the Bureau’s demonstration plant some type of liquidpurification process undoubtedly is indicated. The Girbotol process ( I I ) , employing ethanolamines, may be In general, liquid-purification processes, as normally operated, used for simultaneous removal of carbon dioxide and hydrogen will leave 5 to 10% of the original hydrogen sulfide content in the sulfide or for preferential removal of hydrogen sulfide with only gas, with final cleanup in dry boxes. partial removal of carbon dioxide. The spent solution is regenerWater scrubbing a t elevated pressures (generally around 300 to ated for further use by means of steam. Hydrogen sulfide may 400 pounds per square inch) may be considered where the subsebe recovered. According to the information supplied by the quent synthesis gas is to be operated at pressures appreciably Girdler Corporation (95) for gas of extremely high hydrogen sulabove atmospheric and hence can bear part of the gas-compresfide and carbon dioxide concentrations (600 grains per hundred sion charges. Pressure water scrubbing also reduces the carbon dioxide content simultaneously. I. G. Farbenindustrie at Leuna cubic feet and 30010, respectively), use of monoethanolamine (SO)treated gas after shift conversion which probably contained would be preferred for substantially complete removal of both 25 to 30% carbon dioxlde and reduced the content t o 1.5% by carbon dioxide and hydrogen sulfide. If partial removal of carbon water scrubbing under 25 atmospheres’ pressure. Here the gas dioxide and hydrogen sulfide is preferred, use of triethanolamine was to be used subsequently for ammonia synthesis, and the gas underwent three stages of compression before scrubbing. would reduce carbon dioxjde from 30 to 2770, with removal of I n the Seaboard process (80),which has found wide favor behydrogen sulfide from 600 grains per hundred cubic feet t o 50. cause of its low cost, gas is.scrubbed with 3.0 to 3.5% soda ash Amine solutions are known t o absorb carbon disulfide (and possolution. The foul solution is reactified for furtherpse in a n aerasibly other compounds) and hydrogen cyanide-forming comtion tower. Air from the aeration tower is blown elther to the atmosphere through a tall stack or to some units requiring air for pounds which cannot be converted back to the amine by means combustion. of steam, resulting in losses of the amine. Since the amines are The Koppers vacuum carbonate process (81) is a modification relatively expensive, i t appears that use of the Girbotol process of the Seaboard process, wherein the solution is actified under parwould require prior removal of organic sulfur and cyanogen comtial vacuum and steam is substituted for air as desorbing gas. Elimination of formatlon of thiosulfate, which occurs when actipounds. So far as the writers of this paper are informed, there has fying with air, results in lower soda consumption; conversion of been no industrial practice of removing organic sulfur before remore bicarbonate to carbonate results in greater hydrogen sulfidemoval of hydrogen sulfide. The Girdler Corporation, however, carrying power for the solution than with air activation; hydroadvises that, by means of a Girdler catalyst, which can be sigen sulfide and hydrogen cyanide may be recovered undiluted with air. Heat requirements for the process may in some cases multaneously used for the water-gas shift conversion, organic be supplied from waste heat. sulfur may be converted to hydrogen sulfide in the presence of as The Thylox process (27,68,46) employing an arsenical solution, much as 600 grains of hydrogen sulfide per hundred cubic feet. I n Germany a process similar to the Girbotol process, known as the Alkazid process, was used to remove hydrogen sulfide from synthesis gas. Instead of amines, this process employed either potassium-N-dimethyIgIycine (Dik) or potassium methyl alanine (M). A flow diagram reproduced from a captured German docu3M) ment (83)is shown in Figure 3. The Germans claimed that the Alkazid solutions had greater carrying power than the ethanolamines; the M solution was used E when absorption of both carbon dioxide and hydrogen sulfide was % desired, while the Dik solution absorbed hydrogen sulfide prefer5 200 entially. ti One of the more nijteworthy difficulties experienced with the -s5= Alkazid process was corrosion, which was particularly serious if the solution became diluted below a specific gravity of 1.16. Local dilution caused localized corrosion. Iron was used for con100 structing the cold parts, brick-lined iron for the regenerator, and aluminum for the indirect heater and hot pipes. Some parts of the system, particularly pumps, were made of Silumin (aluminumsilicon alloy, sometimes called Alpax), but corrosion was experienced with this alloy also. Sodium silicate was added to the solu0 1 2 3 4 THICKNESS OF BED, METERS tion to inhibit the corrosion of aluminum. Cyanogen compounds react with the Alkazid solution to form compounds not converted Figure 2. Pressure loss in shaft filter (1

INDUSTRIAL AND ENGINEERING CHEMISTRY

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Vol. 40, No. 4

gas per hour through a 10,000-bushel dry box-a typical rate in plant operation for final removal of low concentrations.

I Figure 3.

~

1 Heat exchanger

I

Flow sheet of alkazid scrubbing plant

back to the original absorbent by means of steam. A similarly detrimental effect results i f thiosulfates are formed by contact of the foul solution with oxygen. A review of many technical data concerning the Alkazid process in the captured German documents leads to the conclusion that the process has no advantage over existing American processes. One factor to be considered in connection with some of the liquid-purification processes is the possibility of recovering sulfur. I n some localities, the discharge of hydrogen sulfide desorbed from the solution to the atmosphere is objectionable, and although in some cases this may be disposed of by burning in some combustion equipment, this is not always practicable. Conversion of the hydrogen sulfide to sulfur dioxide for manufacture of sulfuric acid and other chemical purposes, or partial combustion to elementary sulfur in a Clam kiln, affords a means of disposing of the hydrogen sulfide without venting large quantities to the atmosphere. Recovery of sulfur may yield appreciable credits to purification costs under certain conditions, particularly i n plants where the quantity of recoverable sulfur is very large. ilt the I. G. Farbenindustrie A. G. Leuna works (SO), some of the synthesis gas was purified by oxidation on active carbon. The carbon reduced the hydrogen sulfide from 130 to 175 grains down to 0.4 to 0.8 grain per hundred cubic feet. It was not considered possible to use the active carbon oxidation process on Winkler gas containing 650 to 850 grains per hundred cubic feet because the high content of hydrogen sulfide resulted in an excessive rise in temperature. It was said that the process could not be used on coke-oven gas because gum formers interfered. The process is used extensively in Upper Silesian plants. Elementary sulfur was recovered from the active carbon by extraction with solvent. MORGAX'TOWN EXPERIMENTS ON REhfOVING TRACES O F HYDROGEN SULFIDE. The authors' laboratory was interested in determining the effectiveness of oxide treatment for removing traces of hydrogen sulfide. One of the writers of this paper investigated thisquestion some years before in connection with the evaluation of purifying materials (60). I n that work, however, trace removal was determined with basic lead acetate paper. It was decided to determine the ability of a commercial dry-box material to remove traces by applying the more sensitive methylene blue test. Nitrogen containing 12 grains of hydrogen sulfide per hundred cubic feet was passed through a tube containing a commercial iron oxide mixed with sawdust at a space velocity of 40 per hour. Although this flow rate is low compared with space velocities in many catalytic operations, it is equivalent to 500,000 cubic feet of

No indication of hydrogen sulfide was found. Since 60 cubic feet of gas were passed, with the known sensitivity of the test, the amount of hydrogen sulfide escaping removal, if any, was certainly less than 0.0005 grain per hundred cubic feet. Admittedly, other factors in plant operation may be encountered which can lessen the efficiency of removal; the authors were determining only the inherent activity of the material itself. Such factors as channeling and decreasing efficiency due to fouling of the material can be controlled by careful plant operation. Many detrimental factors in the usual gas-plant operation of dry boxescyanides, tar, etc.-will be absent from gas entering dry boxes in the demonstration plant. Organic Sulfur. Wash-oil scrubbing and active carbon-adsorption methods which remove gum formers and light oils will also remove part of the organic sulfur content of the gas. Thus, in addition t o improving removal of organic sulfur by the soda-iron catalyst (if this process is used) through elimination of gum formers, wash-oil or active carbon treatment will also remove thiophene, which is only partly, if a t all, removed by the sodairon cat,alyst. Carbon disulfide is also removed by active carbon or wash oil, but this removal is not so important as the removal of thiophene, as carbon disulfide is removed much more easily by catalytic processes, whereas thiophene is chemically and thermally resistant. Pressure scrubbing with 5% caustic soda solution has been used in some synthetic ammonia plants to remove simultaneously small amounts of carbon dioxide escaping removal by water scrubbing and may also reduce t'he organic sulfur to a low valueperhaps 0.1 grain per hundred cubic feet. The organic sulfur compounds were those occurring in water gas made from coke, however., SODA-IROK PROCESS. Perhaps the most intaresting method for removing organic sulfur compounds is the German soda-iron process (84, which served to keep the German synthetic fuel plants operating during the late war. It appears that the soda-iron oxide catalyst promotes the oxidation of organic sulfur compounds to oxides of sulfur while, by virtue of the soda content, these oxides are retained in the catalyst, as sodium sulfate. About 0.27, oxygen in the inlet gas is sufficient to effect the oxidation. Figure 4 is a flow diagram of this process as practiced by Ruhrchemie A. G. Oberhausen-Holten. Here, gas is purified by five units of two towers each, each unit treating 55,000 to 65,000 cubic feet per hour of water gas, free of hydrogen sulfide. $iter passing through the five units in parallel, all t'he gas goes through a sixth unit of two towers provided as a safety measure. Tracing the flow through a single unit,, gas enters the heat exchanger and then the heater, where the gas is heated to reaction temperature. Reaction temperature in the first tower usually ranges from 390" to 500' F., material in this tower being more nearly exhausted than that in the second. Leaving the first tower, exit gas passes through a heat exchanger, where the gas is partly cooled by the inlet gas. The heat exchanger is intended to lower the temperature to a range of 325 to 400 O F. This lower temperature range is desired because the material in the second tower is fresher. Temperatures in the system are controlled by automatic recording temperature controllers. Temperatures in the second tower depend on those in the first; temperatures in the system are progressively raised through the ranges indicated as the material becomes more nearly spent. When the temperature in the first tower has been raised to 500 O F. and purification begins to fail (after about 2 days a t this temperature) the material in the first tower is dumped, the second tower is placed in first position, and the other tower is refilled with

613

INDUSTRIAL AND ENGINEERING CHEMISTRY

April 1948

Heat exchanger Heater

Purifier

Purifier

Final purifiers

C

.-

E

8 Water gas from crude purification 16-18,000 cubic meters per hour per system with 0.2.0.310 0;

System 1 to 5 2 purifying towers each

.$ J

System 6 2 final purifiers

5

6

e

Contents of purifying mass Towers with tubs 60 tons per tower Towers with screens 70 tons per tower

S-content in grams per 100 cubic meters

~ lBefore final purification

org S

2After tower 1 ~

5-10

Sample location

18-22 -

3 After tower 2

0.05-0.5

4 After final puriflcatton