CATALYTIC OXIDATION OF ORGANIC SULFUR COMPOUNDS IN COAL G A S . ROLAND H. GRlFFlTH North Thamer Gas Board, London,
T o reduce corrosion of gas-burning equipment and to extend the use of flueless appliances, a low sulfur content in the coal gas is desirable. A process is described for the oxidation of the organic sulfur compounds in coal gas, b y contact with a nickel subsulfide catalyst in the temperature range 990' to 370" C. A plant treating 1,500,000 cubic feet per day has been a t work for 14 years and i s operated by automatic air addition controlled from a magnetic oxygen analyzer. Thiophen i s not decomposed, but other sulfur compounds are reduced to about 2 grains per I 0 0 cubic feet. Small units using a similar catalyst produce furnace atmospheres with a low sulfur content. Equipment for periodic regeneration of the catalyst i s described. Proposals for a plant to treat 15,000,000 cubic feet per day are outlined, and an estimate shows that the process would add less than 2% to the cost of the distributed gas.
HE British gas industry has long been interested in t h e removal of organic sulfur compounds from coal gas. Although there is no statutory limit on the quantity permitted, it is considered t h a t the amounts below 10 grains of sulfur per 100 cubic feet of gas are desirable and t h a t even as little as 3 grains would be preferable (IS). Reduction to the limits necessary for synthesis gas is neither practicable nor advantageous. If low concentrations were attainable a t a reasonable cost, there would be a very appreciable increase in the scope of flueless space heating appliances and a striking reduction in the cost of maintenance of instantaheous water heaters, on which corrosion now imposes a significant charge. There is also a growing demand for reducing atmosphere units in the metallurgical industries; gas with a very low sulfur content is essential for many of these. The sulfur compounds which are normally present in coal gas are carbon disulfide, carbon oxysulfide, and thiophen; there may also be traces of thiols and other more complex bodies. The relative and absolute quantities depend on the kind of coal carbonized and on the plant used in the process; typical analyses are shown in Table I for gas produced in horizontal retorts or in continuous vertical retorts. With some coals now in use, the total sulfur content may be as high as 50 grains per 100 cubic feet. The possibility of a catalytic process for the decomposition of these compounds was established by Carpenter in 1913 ( I ) . The catalyst was shown by Evans and Stanier ( 4 ) t o be nickel subsulfide (Ni&), and it is interesting t o note that the use of a. sulfide catalyst was discovered at such a n early date in the gas industry. The first plant of the South Metropolitan Gas Co. of London had a capacity of 1,500,000 cubic feet per day, and a later installation treated 10,000,000 cubic feet of gas per day. The catalyst was prepared by soaking broken firebrick in nickel chloride solution, followed by heating t o 500" C. It operated a t about 430' C., the gas being raised t o this temperature in a coke-fired preheater, and led t o the conversion of the organic sulfur compounds t o hydrogen sulfide.
S.W. 6, England
This process was not widely adopted by the gas industry, largely because it was expensive in fuel and maintenance. Renewed attention was given t o the problem in 1936, and a modified process was developed in which the organic sulfur compounds were decomposed by catalytic oxidation ( I d ) . P R E P A R A T I O N AND PROPERTIES OF SUPPORTED NICKEL SUBSULFIDE CATALYST
China clay pellets, prepared by cutting a n extruded paste, were fired a t 900' C., and the finished material had a surface area of 15 square meters per gram. The pieces were cylinders about l/g inch in diameter and 1/4 inch long. They were impregnated with a boiling solution of nickel sulfate, then dried and reduced in coal gas at 400' C. (6). This gave a product in which about 8% of nickel subsulfide was uniformly distributed on a robust support, and the finished catalyst had a surfacearea of 18 square meters per gram.
T
May 1952
Table
I. Analysis of Sulfur Compounds in Coal Gas
csz
cos
Thiophen Other oompds. Total
Equivalent Grains of Sulfur/100 Cu. Ft. Vertical retort Horizontal Gas gas 20.1 21 . o 5.4 5.5 7.8 3.8 2.3 1.4 35.6 31.7
-
_ .
The kinetics of the oxidation of carbon disulfide, and carbon osysulfide on nickel subsulfide were studied by Crawley and Griffith (3). Using nitrogen as the carrier gas, i t was necessary to inhibit the gas-phase oxidation of the sulfur compounds by adding traces of ethylene, and under these conditions the oxidation of cnrbon disulfide proved t o be of zero order with respect to carbon disulfide. By contrast, the oxidation of carbon oxysulfide was of the first order with respect t o t h a t substance but was retarded by the sulfur dioxide formed. These results with other work on the chemisorption of sulfur compounds by nickel subsulfide (10) showed t h a t chemisorbed carbon disulfide is oxidized by impact of oxygen on it but that carbon oxysulfide reacts by collison with chemisorbed oxygen. The carbon disulfide in coal gas would be completely oxidized by small amounts of oxygen in a gas-phase reaction if t h a t process were not retarded by the ethylene and hydrogen present. Many other reactions are catalyzed by nickel subsulfide and two of them are significant in the practical operation of the sulfuurremoval plant. These are:
+
2H2 0 2 e 2H20 SOz f 3H2 +HZS 2Hz0
+
(1)
(2) The first of these is important as i t provides a strongly exothermic reaction which can be controlled t o supply all the heat require-
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NICKEL-CATALYSTS o n both sides oi the tubes
Figure 1.
'
Catalytic Plant at Harrow
meiits of the process. At relatively low temperatures, the reaction is of zero order with respect to oxygen so that the temperature of the catalyst, bed cannot be raised by the addition of more oxygen if it had been allowed to fall below about 175' C. At the normal working temperature, the reaction rate is directly proportional t o the oxygen concentration. In spite of the ease with which the formation of \Taler occurs, and of the high concentration of hydrogen in the system, the sulfur compounds still undergo oxidation. Only when the oxygen concentration is low does the reaction between sulfur dioxide and hydrogen become appreciable. This reaction has been studied by Crawley and Griffith (9) who found that it was of the first order with respect l o sulfur dioxide and slightly retarded by excess of that reagent which was strongly adsorbed. Thiophen is not decomposed, either by Oxidation or by hydrogenation, It has been shown by Griffith, Marsh, and N e ~ l i n g ( 1 1 ) that thiophen can be hydrogenated on nickel subsulfide in pure hydrogen, only by interaction with metallic nickel produced from the catalyst. Its deconiposition is completely inhibited by substances present in coal gas which prevent the formation of free nickel. CATALYTIC PLANT FOR ORGANIC SULFUR REMOVAL
In 1938 a plant rated a t 1,500,000 cubic feet of coal gas per day was put to workat Harrow, 14 months after the catalytic oxidation reaction was first observed in the laboratory. The plant was designed as an effective operating unit which would provide fuller information for a much larger installation. The main features of the plant are shown in Figure 1. Gas from the horizontal retorts, having a calorific value of 560 B.t.u. per cubic foot, which has passed through the normal purification system plus a controlled amount of added air enters the heat exchanger, E, a t a pressure of 20 to 30 inches, travels over the outside of the tube bundle and leaves by the manifold, F . It is then distributed to three of the four catalyst vessels ( A , B, C, D ) , each of which holds 15 cubic feet of catalyst. The treated gas returns by manifold G to the tubes of the heat exchanger. The heat exchanger is a two-pass unit, each half containing 178 steel tubes (12 feet long, 1.25 inches outside diameter, and 1.09 inches inside diameter) arranged on 1.56-inch triangular pitch inside a cylindrical shell, 35.25 inches inside diameter. Gas flow
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15 parallel to their length. The usual floating head construction allows for expansion on heating, and movement of the shell is alloned by mounting on rollers. The catalyst vessels are 4 feet 6 inches high, 3 feet in diameter, and have conical grids with 45' ilope leading to a 2-inch dischaige pipe extending through the gas inlet connection. A charging point is provided in the top cover. The vesrels are suspended by slings so that they can move independently. All hot surfaces are well insulated. Treated gas leaving the heat exchanger enters a washer-cooler of conventional design for the removal of sulfur dioxide and water vapor. It is 30 feet high, 3 feet in diamcater, and packed for 18 feet 2 inches of its height with I/s-inch wooden boards a t '/,-inch spacing; liquor is distributed t o it from a multitrough system. The liquor, prepared from a dilute sodium carbonate solution, is delivered by a centrifugal pump from a storage tank through a rack-cooler at about 18" C. The washed gas still contains traces of hydrogen sulfide (generally 2 to 3 grains per 100 cubic feet) which have to be removed by iron oxide. At Harrow, this is done by passage through a single box of conventional design, which was already available. As no other external source of heat is provided, an air burner is necessaiy for starting from cold. This is shown in Figure 2 and is fitted into the upper trunk main a t G. Air is fed to the nozzle, A , and draws in gas through chamber E in an amount controlled by valve J . Ignition is by sparking plug B, and the flame can be observed through a window. It is important to operate this burner with a smokeless flame to pievent deposition of soot in the heat exchanger A self-supporting reaction can be attained in 5 to 6 how5 from cold, and the air buinei is then shut off. Working Conditions and Performance. The precise condition of the plant dependJ on the age of the catalyst and on the volume of gas being treated, but the average temperatures are as follows: gas entering the catalyst, 220" C.; maximum in catalyst, 370" C.; inlet heat exchanger, 355" C.; and treated gas t o washer, 145' C. This needs an inlet oxygen concentration of about 1.2% which is almost completely consumed. The washer-cooler liquor has an average concentration equivalent to 1.8% of sodium carbonate and is circulated a t 30 gallons per 1000 cubic feet of gas. Three times daily an addition of fresh sodium carbonate is made, and some spent liquor is rejected to compensate for water accumulated by condensation and to keep the p H above 7.5 so that sulfur oxides are properly removed. Table I1 gives typical data for two weeks in 1943. Of the residual sulfur, all but 1.5 to 2.0 grains per 100 cubic feet were thiophen, and when the treated gas had passed through a benzene recovery plant its final sulfur content was about 3 grains per 100 cubic feet. More complete analytical data and reference to the behavior of individual sulfur compounds were publish.ed in 1944 (12). Of the organic sulfur decomposed, 15 to 20% appears as hydrogen sulfide, 2 to 3% as sulfur trioxide and the rest as sulfur dioxide. Control of the Plant. TThen first installed the plant was controlled by hand setting of air addition to the ingoing gas as both the rate of gas flow and its oxygen content may vary widely. This needed the attention of one man twice hourly and was
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NICKEL-CATALYSTS Table II. Total Sulfur Content of Treated and Untreated Gas (Mean spaoe velocity, about 1000 standard cu. ft./hour/cu. ft. of catalyst) Total Sulfur, Grains/100 Cu. f t . Date Heat Exohanger In out 10.9 26 3880 c. July 6 7 8 9 12 16 16
19
3 88 399 393 397 393 391 387
26 28.7 29.1 29.3 30.6 32.4 31.5
7.9 8.9 9.2 9.9 8.6
9.1
10.2
continued during the war. Automatic control systems have recently been examined with particular reference to their suitability for plants of larger capacity and i t has become evident t h a t the addition of air can be controlled from a sensitive oxygen meter t o give a constant Qxygen concentration at the inlet t o the plant. A magnetic instrument supplied by George Kent Ltd., Luton, England, has proved suitable for this purpose, and the system is described in a paper by the author (9). Regeneration of Catalyst. A slow deposition of fouling material takes place on the catalyst, the actual rate depending on the temperature and on the type of gas treated. Two kinds of fouling agent Figure 2. Air Burner for Starting the have. been recogCatalytic Plant nized(l4). Atrelativelv low temperatures small amounts of hydrocarbon polymers of medium molecular weight obscure the active surface; as the temperature rises to 320' t o 380' these polymers are decomposed t o a less objectionable and more porous deposit. But even this deposit seriously affects the interaction of hydrogen and oxygen on the catalyst when it amounts t o about 6% by weight of the catalyst. Regeneration of the catalyst is effected by controlled oxidation of the carbonaceous material at the lowest practicable temperature, followed by reactivation of the nickel oxide-nickel sulfate mixture by contact with hot coal gas at 400' C. If the temperaturein t h e oxidation stage is allowed t o exceed 600' C., interaction between china clay and nickel oxide sets in and leads t o permanent damage to the catalyst; a process has therefore been developed which uses gas with a low oxygen content t o burn the impurities. Owing t o the need for this careful control, it was necessary t o carry out t h e regeneration in a separate vessel.
May 1952
Figure 3.
Catalyst Regenerator
The normal life of catalyst a t Harrow is 3 to 4 months, so t h a t the volume to be regenerated each year is small. A regenerator unit with a capacity many times as great as would be needed for this has been installed so t h a t it will handle material from several larger plants. Catalyst Regenerator. The removal of the fouling material from spent catalyst by controlled combustion is carried out in the regenerator shown in Figure 3. Each 7-cubic foot batch is transported in a single full-aperture drum to which attachments are made so that it may be lifted and inverted to form a cone-based feed hopper over the kiln, A . Inside the insulated refractory lining of the kiln on a double discharge cone of special sheet steel is the underbed of 31/2 cubic feet of oxidized catalyst from the previous operation onto which the 7 cubic feet of unburnt material are fed. About 10,000 cubic feet per hour of inert gas containing 2% oxygen is then passed upwards through the charge at a n entry temperature of 420' t o 440' C. The fouling material is removed in about 6 hours without the temperature exceeding 580" C. in any part of the charge. The oxygen-free waste gas travels upward through the 16-inch diameter cast-iron washer-cooler D to be cooled and washed free from sulfur oxides; the &foot packing of 1-inch earthenware rings is irrigated with hard water. A centrifugal booster, F , provides pressure for recirculation of clean, cold waste gas and draws in, through a Venturi proportioner, enough air t o restore the oxygen concentration t o 2%. Excess gas is then blown from t h e circuit through a relief governor. The remaining cold waste gas, about 8000 cubic feet per hour, is recycled to the kiln through the chamber, B, in which it is raised t o 420' to 440' C. by admixture with about 2000 cubic feet per hour of gases from a n air-blast burner. Thermostatic control of air flow from blower E t o t h e burner gives a constant temperature at the kiln entry. Control of the gas to the burner by the Venturi proportioner ensures t h a t the hot products from
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NICKEL-CATALYSTS The treat'ed gas passes t o t h e aashcr-cooler, C, which is packed with 1-inch ceramic rings to a depth of 5 feet and irrigated by hard water flowing countercurrent to the gas. The water run3 to waste after one pass; it removes sulfur dioxide and cools the g:j3 to a temperature and dew point below atmospheric. The catch purifier, D,contains 5 cubic feet of active iron oxide spheres (?) from which small amounts of t h e packing can be rcmoved periodically, neF material being added a t the top. h y hydrogen sulfide prment in the gas is thus removed and it is r ~ a t l y t o go t o t h e burners. Flowmeters, safety devices, pressure controls, and means for working on reduced load or against back pressure are incorporated in t h e completed unit. The plant can normally be started fro111 cold in about 2 to 3 hours and will run for long periods with very little supervision. The rate of fouling of the catalyst is generally much lower when running on t o m gas than that experienccd on t'he large unit on coal gas, and a life of 2 years has been reached i n many insta1lat)ions. DESIGN OF FUTURE {NSTALLATIONS
Figure 4.
Small Catalytic Unit for Treating I000 Cubic Feet of Gas per Hour
the burner contain 2% oxygen and practically no combustible gas. A carbon dioxide indicator on the recycle gas shows when the oxidation of the charge is complete, Seven cubic feet of material are then discharged through a special valve and adaptor hood into a full aperture drum on a bogie; after cooling this batch is screened t o remove dust and is then ready for reactivation in hot coal gas. The plant is brought into operation by three press-buttons, mounted on panel C, for the booster starter, the air-blower starter, and the sparking-plug ignition of t h e pilot air-blast burner. An electrode in the pilot flame then opens the magnetic valves for the main burner. Safety is ensured by sequence interlocking of the starters and the ignition, by the relay system operated from the flame electrode, and by safety switches which operate if the cooling water fails or if there is inadequate flow of recycled waste gas. Small Scale Sulfur Removal Units. For use on consumers' premises where low sulfur gas is not available, small catalytic units have proved very useful, notably in the preparation of reducing atmospheres for metallurgical purposes. They have ranged in size from 500 to 12,000 cubic feet of gas treated per hour, the standard space velocity being 1000 vol. per hour. A unit t o treat 1000 cubic feet of gas per hour which contains 1 cubic foot of catalyst is shown in Figure 4 described below. On this scale of operation, heat exchange b e h e e n treated and untreated gas is not practicable, and a gas-fired preheater, A , is therefore provided. I n this, the ingoing gas passes downward inside the stainless steel tubes mounted in a lagged cylindrical shell through which the hot gases pass upward from the burner. T h e heating gas is controlled by a direct-acting thermostat. The catalyst vessel, B, is a lagged steel cylinder holding a bed 16 inches deep on a flat grid, through which gas preheated t o 300" C. flows downward. The maximum temperature reached is about 380" C., and this is controlled by thermostatic adjustment of air added t o inlet gas.
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Owing to the outbreak of Korld War 11, the construction of larger plants for organic sulfur removal was not possible in 10:39, and subsequently a general reconstruction program has still delayed such work. But plans have now been completed for a nwv plant having t'en times the capacity of that a t Harrow and a f c ~ v of its chief characteristics have been described ( 1 4 ) . No change is contemplated in the chemical principles already demonstrated, but improvements have been made in the design of catalyst vessels and heat exchanger to give economy in wpital and running cost's. A relatively larger heat exchanger of ne%!-design (8) will be provided so that the exit gas t,eniperature is 80" C. The catalyst will be placed in a single vessel, through which gas will flow horizontally. This vc-ill be subdivided into compartments by vertical grids so that sections of the catalyst can be rcmoved separately for regeneration (6). By the improved heat exchange and reduced heat losses, the consumption of osygrn burned to provide the heat for the process will be lowered from about 1.25 to about 0.8% by volume. Direct, cooling of the liquor supplied to the washer-cooler will also obviate the installation of rack-coolers, which were convenient a t Harrow because a plentiful supply of cold wat'er was available, The decrease in the running cost will partly offset the consideiable increase in the capital cost of any new plant. Although any estimate of expenditure is now very approximate, Plant, and Newling ( 1 4 ) gave detailed figures for 1948 conditions whirh amounted tc 0.189 pence (0.32 cent) per therm of gas treated. It is thought t h a t in fut,ure the process m-ill still add less than 2y0to the cost of the diqtributed gas. LITERATURE CITED
Carpenter, C., J. Gas Lighting,122,1010 (1913); 123,30 (1913). Crawley, B., and Griffith, R. H., J . Chem. SOC.,1938, p. 720. Zbid., p. 2037. Evans, E. V., and Stanier, H., Proc. Roy. Soc., 105A, 626 (1924). Gas Light and Coke Co., Griffith, R. H., and Plant, J. H. G . , Brit. Patent 529,711 (1940); U. 8. Patent 2,295,653 (1942). (6) Gas Light and Coke Co., Griffith, R. H., and Plant, J. H. G . , Brit. Patent 597,501 (1948). (7) GasLightandCoke Co., and Hopton, G. U., Ibid., 567,231 (1046). (8) Gas Light & Coke Co., and Newling, W. B. S., Zbid.,619,555 (1947). (9) Griffith, R. H., Znst. Gas Engrs., P u b . 395 (1981). (10) Griffith, R. H., and Hill, S. G., J . Chem. Soc., 1938,p. 717. (11) Griffith, R. H., Marsh, J. D. F., and Wewling, W. B. S., Proc. Roy. Soc., 197A, 194 (1949). (12) Griffith, R. H., and Plant, J. H. G., Gas J . 244, 48 (1944). (13) Institution of Gas Engineers, Rept. on Removal of Organic Sulphur Compounds from Town Gas, Comm. 146 (1936). (14) Plant, J. H. G., and Newling, W. B. S., Znst. Gas Engrs., Picb. 344/157 (1949). RECEIVED for review October 17, 1951. ACCEPTED February 2 5 , 1952. (1) (2) (3) (4) (5)
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