Recovery of Sulfur from Fuel Gases - Industrial & Engineering

Recovery J

smelter gases, in which the sulfur is present largely as sulfur dioxide. However, since they are not fuel gases, these developments will not be discussed here.

Recent Developments in Europe

of Sulfur from Fuel Gases ALFRED R. POWELL Koppers Company, Pittsburgh, Penna.

Up to the time of the World War the removal of hydrogen sulfide was accomplished almost universally by passage of the gas through boxes of iron hydroxide, and this process is still in extensive use throughout the world. The initial reaction is the formation of iron sulfide, which is then converted into regenerated iron hydroxide and elemental sulfur by oxygen; this conversion is either continuous by the addition of a small amount of air to the gas before i t enters the boxes, or periodical by the removal of the sulfided oxide and its exposure to the air before it is returned for further service. Finally the free sulfur content of the purifying material becomes so high that it is unsuitable for further use. For a great many years this ‘%pentoxide” has been a source of by-product sulfur in Europe, whereas in the United States it is usually discarded with no attempt a t recovery of the contained sulfur. Usually the sulfur is extracted with carbon

INCE the inception of the manufactured gas industry, hydrogen sulfide has been removed from the gas before it is distributed to consumers. This purification of city gas was necessary chiefly because combustion of most unpurified fuel gases leads to excessive production of sulfur dioxide or trioxide and this might be detrimental to the health, comfort, or property of those consumers burning the gas in flueless appliances. Until comparatively recent years no commercial attempt was made to recover the removed sulfur as a useful by-product in this country, although in Europe utilization of the recovered sulfur has been common practice for many years. However, within the last fifteen years much progress has been made in the United States and today the recovery of sulfur as a useful by-product from coke-oven gas, water gas, refinery-still gas, natural gas, and other fuel gases is a rapidly growing industry. The recovery and utilization of this by-product offsets a t least a portion of the expense of purification, and under some favorable conditions even leads to a net profit, the value of the recovered sulfur being greater than the total cost of purifying the gas. I n the fuel-gas industries the term “purification” is ordinarily limited to the removal of hydrogen sulfide from gas. It is so limited in this paper, and the sulfur recovered comes entirely from the hydrogen sulfide originally present in the unpurified gas. It is well known that fuel gases contain smaller quantities of other sulfur compounds, chiefly organic, but the removal or recovery of these . minor sulfur constituents is not commercially practiced except in rare and special instances. I n recent years there have been some interesting and practical developments in the recovery of sulfur as a by-product from flue and

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disulfide although in some cases the spent oxide is burned direct to produce sulfur dioxide for the manufacture of sulfuric acid. In England, especially, it is common practice today to sell the spent oxide to companies interested in the recovery of the by-product sulfur; some smaller gas companies in England are even “loaned” fresh iron oxide which is later to be returned to the sulfur manufacturer in a spent condition. I n recent years Germany has probably exceeded all other European countries in the rate of growth of sulfur recovery from fuel gas; this increase in tonnage has been especially marked during the last six years (14). This by-product sulfur has come mostly from coke-oven gas. From this source alone it amounted to 10,000 metric tons in 1927, slowly increased to 13,000 metric tons in 1932, and then rapidly increased to 32,000 metric tons in 1937; it is estimated that the 1938 production was about 50,000 metric tons. Along with this remarkable expansion in recovering sulfur from fuel gas in Germany, there has been much development work in processes for carrying it out. At the present time about 75 per cent of the recovered sulfur comes from the iron oxide dry-purification process combined with which is an extraction step usually involving the use of carbon disulfide as the solvent for the elemental sulfur. The remaining 25 per cent is recovered by various so-called liquid-purification processes, and there has been great activity along these lines for many years in Germany. More than twenty-five years ago processes were developed for the combined removal of hydrogen sulfide and ammonia fromcoke-oven gas in such a way that ammonium sulfate was recovered, thereby avoiding the necessity of purchasing sulfuric acid from outside. Among earlier processes of this type were those of Feld and Burkheiser. A more recent example was the so-called C. A. S. process. None of them proved successful commercially. A process for the combined removal of hydrogen sulfide and ammonia from coke-oven gas to give ammonium sulfate as the end product has been developed by the Gesellschaft fiir Kohlentechnik, and one plant is now in operation in the Ruhr district of Germany. After preliminary removal of the hydrocyanic acid from the gas, the gas is scrubbed with a suspension of ferric hydroxide; as a result the ammonia is absorbed by the water and the hydrogen sulfide is absorbed as ferrous sulfide. This foul liquor is passed continuously to an oxidizing tower, where it is treated with air and sulfur dioxide. The air oxidizes the ferrous sulfide to regenerated ferric hydroxide and free sulfur, and simultaneously this free sulfur, ammonia, and sulfur dioxide react to form ammonium thiosulfate. This solution carrying ferric hydroxide in suspension is then recirculated back to the scrubber to absorb further auantitigg of ammonia and hydrogen sulfide. After the solution has attained a sufficient concentration of ammonium thiosulfate, it is withdrawn from circulation and treated with sulfuric acid to produce ammonium sulfate and free sulfur. The separated sulfur is used to produce sulfur dioxide for the step mentioned above, while the solution of ammonium sulfate is concentrated by evaporation and crystalline ammonium sulfate is recovered. Another German development giving ammonium sulfate as the entl product is the recently announced Katasulf process. The coke-oven gas is passed through a heated catalytic chamber ztfter a small percentage of air is introduced into the gas stream. The nature of the catalyst and the temperature are such that a selective oxidation of the hydrogen sulfide to sulfur dioxide takes place, with little action on the other combustible constituents of the gas. The gas is then scrubbed in a tower so as to remove ammonia and the sulfur dioxide in the form of ammonium sulfite and bisulfite. When this solution of ammonium sulfite and bisulfite is heated under pres-

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sure, there is a conversion into ammonium sulfate and free sulfur. Crystalline ammonium sulfate is then recovered. Among liquid-purification processes which produce elemental sulfur as the end product, the one finding the widest use in Germany is the Thylox process, developed in the United States; it will be described in some detail later. Quite recently I. G. Farbenindustrie introduced the Alkazid process in Germany. The end product is a more or less pure hydrogen sulfide gas. The liquid used for absorption of the hydrogen sulfide is a water solutjon of salts of certain weak organic acids. Following the absorption step, the hydrogen sulfide is expelled by heat, thereby regenerating the solution for further use. Processes of this same general class will be described in some detail later. In addition to these late developments in liquid-purification processes for sulfur recovery, dry purification with iron oxide has also been a subject of recent improvements in Germany. An example is the Lenze-Bamag process for the purification of gas under pressure in specially constructed towers, followed by the usual extraction by solvents of the elemental sulfur from the spent oxide.

Recent Developments in the United States As stated previously, the extraction of free sulfur from spent iron oxide is not extensively practiced in the United States, although several large-scale attempts have been made, and one or two plants now do it. At present nearly all recovery of sulfur from fuel gases in the United States is secured by various liquid-purification processes. The processes for sulfur recovery in this country may conveniently be placed in two classes, depending on the end product. I n one class are those schemes producing solid elemental sulfur, and in the other are the processes giving gaseous hydrogen sulfide as the end product.

THYLOX-PROCESS PLANT OF

THE PORATION AT

HUDSON VALLEYFUELCOR-

TROY, N. Y.

JULY, 1939


The first commercial liquid-purification process introduced in this country was of a nonrecovery type. This was known as the Seaboard process (19). The absorbing liquid used is a solution of sodium carbonate (about 3 per cent) which contacts the gas in a tower and absorbs the hydrogen sulfide. The foul solution leaving the absorber is continuously regenerated by being passed through another “actifier” tower where it is contacted in countercurrent with a large volume of air which removes the hydrogen sulfide; the actified solution is then returned to the absorber for further use. The actifier air, containing only a small fraction of a per cent of hydrogen sulfide, is discharged through a tall chimney or is put under boilers or other equipment to furnish air for combustion. Despite the fact that the Seaboard process recovers no useful sulfur by-product, its relative simplicity has made it quite popular and about fifty plants are now in operation in the United States and Canada alone. Since the introduction of the Seaboard nonrecovery process, nearly all efforts a t development have been along the lines of processes that will recover sulfur as a by-product of gas purification. For a period of years more attention was given to systems that would produce free sulfur as the by-product, and later reference will be made to some of them. Within the last several years an increasing amount of development activity has been devoted to those processes giving hydrogen sulfide gas as the by-product, and several large installations are now in operation or in process of construction; they will also be referred to later.

791

ess is in much more extensive use than any other liquidpurification process producing elemental sulfur as the recovered product. Approximately a dozen plants, some of them quite large, are in operation in the United States, and several large factories are producing in Germany and Japan. The Thylox process has been rather thoroughly described in the past and will be only briefly referred to here. The liquid used for the absorption of hydrogen sulfide from the gas is essentially a solution of sodium or ammonium thioarsenate. The chief reactions that occur, according to Gollmar (8), are as follows:

+

Absorption: Na4As2S602 HzS = Na4AszSs0f HzO Regeneration: Na4AszSe0f 0 = Na4As2Ss02 f S The concentration of arsenic in the solution is usually about 7 grams per liter of arsenous oxide, and the solution is maintained slightly alkaline, usually a t a pH of 7.6 to 8.0. , JULF

I= JLURRY

I

t

t

PRUJURt THIOHIZER

Processes for Recovery as Elemental Sulfur

’

Several different processes of liquid purification of fuel gas have been developed and put into commercial operation in the United States which produce elemental sulfur as the end product. I n general, these processes, like the Seaboard process referred to above, operate in two steps, the solution cycling continuously between the steps. I n the absorption step the solution absorbs hydrogen sulfide from the gas, and in the actification step the solution is regenerated for further use. However, in the actification step these processes differ decidedly from that of the nonrecovery Seaboard process. Whereas the Seaboard actifier secures a physical removal of hydrogen sulfide from the solution by the sweep action of a large volume of air, the sulfur-recovery processes of this class involve the thorough contacting of the solution with a relatively small volume of air. Oxidation takes place and results in the release of finely divided free sulfur in the solution undergoing actification or regeneration. The air supplied to the actifiers (or thionizers as they are usually called in this type) not only furnishes oxygen for this chemical reaction but also serves as a medium to float the fine sulfur particles to the top of the solution where the sulfur is withdrawn as a liquid slurry from which the sulfur is recovered by a filter. The chief difference between the various processes included in this class is in the nature of the solution used for absorption of hydrogen sulfide. The Ferrox process (20) employs a solution of soda ash containing ferric hydroxide in suspension. This process is now in use at only two or three plants in this country. Although purification efficiency is usually very good, the sulfur produced is inclined to be rather impure, owing to the inclusion of ferric hydroxide. The Nickel process (6) uses a nickel catalyst suspended in a solution of sodium carbonate. The resultant elemental sulfur is quite pure and has found a market for certain purposes, as will be mentioned later. The Nickel process is being successfully employed in several gas plants in this country. One of the most recent American developmenks is the Thylox process of liquid purification (6,8,11, 12). This proc-

FIGURE1. FLOW DIAGRAM OF THYLOX PROCESS ?

A simple flow diagram of this process is shown in Figure 1. The fuel gas being purified flows up through the absorber tower and is contacted countercurrently by a downflowing stream of actified Thylox solution. The foul solution, which has absorbed the hydrogen sulfide from the gas, is pumped into the bottom of a tall tower, known as the pressure thionizer, and here the solution is regenerated and the elemental sulfur is released. Compressed air is also introduced at the bottom of this tower and bubbles up through the solution, finally being released to the atmosphere a t the top. The oxygen of the air causes the above regeneration reaction to occur, and the air itself also serves the physical purpose of floating the released sulfur to the top. At the top of the pressure thionizer the solution and sulfur slurry are separated, and the regenerated solution flows to the top of the absorber to begin another cycle of operation, while the sulfur slurry flows into a storage tank. From this storage tank the slurry is withdrawn and filtered through a continuous filter. The filter cake contains elemental sulfur in a very finely divided form associated with enough water (about 50 per cent of the weight of the cake) to constitute a firm paste. Mention will be made later of the further processing and uses of this sulfur from the Thylox process. The Thylox process plant may be so designed that it will secure substantially complete removal of the hydrogen sulfide from fuel gas, provided the gas is free of certain con-

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The principal difference between various processes of the hot-actification type lies in the nature of the absorbing solution used. The Espenhahn process (7) uses a solution of sodium carbonate; it is operating commercially and is fairly satisfactory except that the steam consumption is rather high. Solutions of ammonia have been tried out in large installations for the purpose of absorbing hydrogen sulfide from gas, but a t present no commercial plant in this country uses such a solution. Suspensions of magnesium hydroxide in water (22)have shown promising results, although they are not yet adopted on a large commercial scale. The Borate process (3) calls for the use of a solution of potassium or sodium borate. The Girdler process ( 2 , d J ) uses solutions of certain organic amines, such as the various ethanolamines, to absorb hydrogen sulfide from fuel gases. Usually these solutions contain a rather high concentration of the amines dissolved in water. Several plants in this country are now using the Girdler process for the removal of hydrogen sulfide from refinery-still and natural gas. The Girdler process also finds use in the recovery of carbon dioxide from gases, and several plants are now operating to produce a carbon dioxide end product. Quite recently the Phosphate process (15) has been introduced. The absorbing solution contains 40 to 50 per cent of tripotassium phosphate. Several plants have been constructed for the removal of hydrogen sulfide from refinerystill gas.

THYLOX-PROCESS PLANT OF E. I. DU PONTDE NEMOURS & COMPANY, INC.,AT BELLE,W. VA.

stituents that affect the efficiency of removal to some extent. Especially by the use of the two-stage Thylox process ( I d ) substantially complete removal of hydrogen sulfide may be secured. Organic sulfur constituents in the fuel gas are not removed by the Thylox solution. The Thylox process is now in operation for the removal and recovery of sulfur from various kinds of fuel gases, including coke-oven gas, blue water gas, carbureted water gas, oil gas, etc. In general, the Thylox process, as well as the previously mentioned Ferrox and Nickel processes, are best adapted to the removal and recovery of sulfur from fuel gases that are at ordinary pressure and that contain small or moderate quantities of hydrogen sulfide. I n this field these processes have been successfully applied. Typical installations of the Thylox process are illustrated on this page and on page 790.

Processes for Recovery as Hydrogen Sulfide This type of process involves the evolution of the sulfur in the regeneration step as gaseous hydrogen sulfide. Whereas in the class of processes described previously oxidation was caused to take place in order to release elemental sulfur, in the present type oxidation is avoided and regeneration is secured by releasing the absorbed hydrogen sulfide by contacting the solution countercurrently with a flow of steam. After condensing the steam, a more or less pure hydrogen sulfide gas remains as the recovered by-product. Usually the steam for these processes is generated from the solution itself by heating with indirect steam or even by direct fire *in a reboiler located beneath a stripping column down which the solution flows countercurrently to the upflowing steam. Because heat is used for the regeneration of the solution in this type of process, it is sometimes referred to as a hotactification process.

FIGURE 2. PARTIAL PRESSURE OF HYDROGEN SULFIDE FROM SODIUM PHENOLATE SOLUTION AT 25' C. AND AT THE BOILING POINT

The Phenolate process (4, 13,17)uses a fairly concentrated solution of sodium (or potassium) phenolate. However, depending on conditions, the ratio of sodium to phenolate radical need not necessarily be the stoichiometrical ratio of one to one, but may vary between fairly wide limits. The chemical reaction involved in the operation of the Phenolate process is :

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T-IUDUATIffi MEPMOMETCP (L -QCCOEDlNOTHE9rY)METE9 P PPCIJUPE M- QECOPDIN(U FLOWMETER W-THEPMOWCTEP WELL

-

WATER-

-

WATER-

HzS VAPOE

T P

+WATER

FIGURE3. FLOW DIAGRAM OF SINGLE-STAGE ‘PHENOLATE PROCESS

NaOCsHb

+ H2Se NaSH + CcHsOH

During the absorption stage of the process the above reaction moves from left to right; that is, hydrogen sulfide is absorbed from the gas and reacts with the sodium phenolate to form sodium hydrosulfide and free phenol. During the succeeding regeneration stage the solution is heated to boiling, and the reaction moves in the reverse direction, the steam serving to sweep out the hydrogen sulfide. The function of the phenol in this process is to balance the above reversible reaction so that it will proceed easily in either direction under the conditions existing in the absorption and the regeneration stages. For example, if sodium hydroxide d o n e were used, the absorption would take place easily and completely, but regeneration of the foul solution would be impossible by any simple or inexpensive means. The presence of phenol in the solution makes regeneration possible by simple heating and with only a moderate expenditure of steam. The choice of phenol or other tar acids (such as cresols, etc.) as the auxiliary acidic constituent was made after much study and experimentation with various other acids. Phenol is a weaker acid than hydrogen sulfide, and this allows the hydrogen sulfide to replace the phenol during the absorption stage. Another important property of phenol is that its electrolytic dissociation constant or “acidity” increases markedly with temperature. Hydrogen sulfide also has this property, and there is a tendency for hydrogen sulfide to be held in chemical combination with the alkali more firmly a t the boiling temperature of the regeneration stage; this is just the point in the process where it is desired to release it. The phenol, which also becomes a stronger acid a t the high temperature of the regeneration stage than it was a t the lower temperature of the absorption stage, counteracts this tendency and serves to “crowd out” the hydrogen sulfide a t the boiling temperature of the regenerator or actifier. The curves in Figure 2 show the partial pressure of hydro-

gen sulfide for varying contents of hydrogen sulfide in solution and at two temperatures; 25’ C. represents the normal absorption temperature, and the boiling point a t atmospheric pressure represents the regeneration temperature. These curves show only one combination of sodium hydroxide and phenol concentrations in the solution; as stated before, they may be varied between fairly wide limits. Figure 3 is a flow diagram of the ordinary single-stage Phenolate process. The gas and the solution are contacted by a series of bubble-cap trays in the absorber in the usual manner. The foul solution flows from the base of the absorber, is then preheated in a heat exchanger, and finally flows into the top of the actifier or regenerator. The solution then flows down over the bubble-cap trays of the actifier where it is contacted with the steam coming from the reboiler in the base and finally flows into this reboiler. Here i t is heated to boiling with indirect steam through heating coils. From the reboiler the actified solution flows back through the heat exchanger, where it gives up part of its heat to the foul solution, and thence through a cooler; it is then ready to begin another cycle of operation by being pumped into the top of the absorber. The mixture of steam, phenol vapor, and hydrogen sulfide leaves the top of the actifier to pass through a dephlegmator or reflux condenser, where the steam and phenol vapor are condensed and returned to the top of the actifier as a liquid, The cool hydrogen sulfide gas leaves the dephlegmator as the end product and may be conducted to a sulfuric acid plant or for other uses to be described later. As the equilibrium partial-pressure curves of Figure 2 show, the absorption of hydrogen sulfide by phenolate solution departs considerably from Henry’s law, since otherwise these equilibrium curves would be straight lines. Advantage has been taken of this fact in devising a two-stage Phenolate process, which shows a considerable saving in steam requirement as compared to the single-stage process.


794

T

P

M

VOL. 31. NO. 7

-- PTHEWlOPlkTEIL R W U R € GAUGt - FLOW METER

!

FIGURE4. FLOW DIAGRAM OF TWO-STAGE PHENOLATE PROCESS

The flow diagram of this two-stage process is shown in Figure 4. The principle used is that of split solution flow, and both the absorber and the actifier are divided into two parts, as far as the flow of solution is concerned. The total flow of foul aolution enters the top of the actifier and is in a normally actified condition after passing through the top half. The larger part of this normally actified solution is withdrawn a t this point to be returned to the absorber, but a minor portion continues to flow down through the lower half of the actifier where it is intensively actified by the steam from the reboiler in the base. The normally actified solution enters a t a point halfway u p the absorber and serves to absorb the major part of the hydrogen sulfide from the foul gas entering a t the bottom. The smaller quantity of superactified solution enters a t the top of the absorber and serves to remove most of the residual hydrogen s a d e that remains in the gas after it has passed through the lower half of the absorber, Space does not permit a full discussion of the principles involved in the operation of the two-stage process, but they have been described in some detail elsewhere (4, 18). I n addition to better heat economy, the two-stage process has several other advantages over the single-stage process, such as ability to secure a more complete removal of hydrogen sulfide from the gas. The chief item of operating expense in the Phenolate process, as in other processes of the hot-actification type, is the steam used for regeneration of the solution. Many variables affect the quantity of steam required, such as the hydrogen sulfide content of the gas, the percentage of hydrogen sulfide removed, the gas pressure and temperature, the number of absorption and stripping stages, and various design features of the plant. Since there are almost an infinite number of combinations of these variables, it is impossible to express even an approximate figure that would be generally applicable. The following example applies only to the specific conditions stated and has no relation to other sets of conditions.

Assume that the gas to be treated contains 1200 grains of hydrogen sulfide per 100 cubic feet of gas (27.5 grams per cubic meter) when the gas is measured under standard conditions of temperature and pressure. The temperature of the gas is 90' F. (32' C.), and the pressure is 300 pounds per square inch gage (21.4 atmospheres total pressure). The composition of the sodium phenolate solution used for treatment is 4 gram moles per liter of sodium hydroxide equivalent and 1 gram mole per liter of mixed tar acids (60 per cent phenol, 40 per cent mixed cresols). When the single-stage Phenolate process is used and the plant is operated so as to remove from the gas only 90 per cent of the hydrogen sulfide, the steam consumption will be 3.5 pounds of steam per pound of hydrogen sulfide recovered (3.5 kg. of steam per kg. of hydrogen sulfide). However, if it is desired to remove 99.9 per cent of the hydrogen sulfide from the gas, the single-stage Phenolate process will consume 12.0 pounds of steam per pound of hydrogen sulfide recovered (12.0 kg. of steam per kg. of hydrogen sulfide). In this latter case it is much more desirable to use the two-stage Phenolate process, since the steam consumption for 99.9 per cent removal of hydrogen sulfide will be only 3.5 pounds per pound of hydrogen sulfide (3.5 kg. of steam per kg. of hydrogen sulfide). One of the interesting characteristics of the phenolate solution is its selectivity in absorbing hydrogen sulfide in preference to carbon dioxide when both compounds are present in the fuel gas being purified. Despite the fact that carbon dioxide is a somewhat stronger acid than hydrogen sulfide, the rate of absorption of the former by phenolate solution is decidedly less than for hydrogen sulfide. When carbon dioxide in the fuel gas is quite low, the hydrogen sulfide produced will be 98-99 per cent purity. Even in cases where the fuel gas contains about 3 moles of carbon dioxide per mole of hydrogen sulfide, the composition of the by-product gas will be just about the reverse-that is, 3 moles of hydrogen sulfide

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795

per mole of carbon dioxide, corresponding to 75 per cent by volume of hydrogen sulfide. Typical Phenolate process plants are shown in the photographs on page 789. Plants now in operation have an aggregate capacity of 55,000,000 cubic feet of fuel gas per day. The total sulfur-recovery capacity of these plants is 26,000 tons per year, corresponding to approximately 75,000 tons of 66" BB. sulfuric acid. At present substantially all of the hydrogen sulfide produced by Phenolate-process plants is being converted into sulfuric acid. Up to the present time, all plants using this class of sulfurrecovery process-that is, recovery as hydrogen sulfide-are operating on refinery-still gas and natural gas, and usually these gases are under high pressure. However, investigation has indicated that the Phenolate process, at least, is adapted to the removal and recovery of sulfur from low-pressure, lowsulfur gases, such as coke-oven gas, and commercial application to such gases may be made in the near future.

certain amount of flexibility in this matter is desirable for any specific plant. The general appearance of the elemental sulfur as it is taken from the drum of the filter press is shown on this page. In this particular plant the Thylox process is used and the filter cake consists of about 50 per cent sulfur and 50 per cent water, together with small quantities of soluble salts, etc. This paste varies in color from white to a light cream color. One of the most interesting and significant properties of the sulfur produced by this class of sulfur-recovery processes is the extremely fine particle size. Ninety-five per cent of the particles are not over 3 microns in diameter, and a considerable percentage approaches colloidal dimensions. This fine particle size has made this form of by-product sulfur particularly valuable for agricultural use, especially as a fungicide (16). Recent investigations in plant pathology have indicated that high toxicity of sulfur toward fungus diseases is determined to a large extent by particle size, and large amounts of this flotation sulfur have been successfally used for this purpose. In addition, the sulfur has been utilized Miscellaneous Processes for other agricultural purposes, such as a component of inSeveral processes other than those mentioned in the two secticides and for the conditioning of alkali soils. classifications above have been developed or used for the For fungicidal purposes this flotation sulfur is prepared in recovery of sulfur from fuel gases. Sperr and Hall (22) emthree different forms. The simplest form is the washed paste, ployed a solution of sodium carbonate which is regenerated just as it comes from the filter drum or rewashed if necessary. by a combination of heat and vacuum. Hultman (IO) proThe customer applies this form of sulfur by agitating it in posed regeneration of the sodium carbonate solution by water and thereby secures the proper suspension to be used for vacuum a t ordinary temperatures. In both of these processes, orchard spraying, etc. Another form is prepared by carefully hydrogen sulfide gas is the end product. Thau (63)described drying the sulfur paste so that no melting of the sulfur occurs; a process developed in Germany which utilized a solution of the product is a fine, free-flowing sulfur dust for dusting orpotassium ferrocyanide and potassium bicarbonate, which is chards or fields. The third form is wettable sulfur dust preregenerated by electrolysis and produces elemental sulfur as pared by adding a wetting agent to the sulfur paste before it is the end product. Recently the Houdry process (9) was andried to a dust; this product easily goes into suspension in nounced as being in the pilot-plant stage but ready for comwater to give a spraying composition. I n addition to the use of mercial application. the flotation sulfur The gas at 750" F. alone, it may also be (400" C.) is passed combined with oil over nickel oxide. emulsions, Bordeaux Periodically t h i s mixture, lime-sulmaterial is regenerfur, etc. ated by air, and the Another interests u l f u r d i o x i d e so ing application of produced m a y be the sulfur from the passed direct into Thylox process is its a sulfuric a c i d incorporation into a plant. special sulfur soap. This soap has shown e x c e l l e n t results: F i n a l Processwhen used for cering and Uses tain types of skin, for Recovered disorders which apSulfur parently respond to The end products the action of the of elemental sulfur finely divided sulfur paste coming from contained in it. t h e f i l t e r s of t h e Since the market first class of procfor agricultural sulesses d e s c r i b e d fur is decidedly seaabove, or the more sonal, most plants SULFUR PASTECOMING OFF A CONTINUOUS FILTERIN THE THYLOX-PROCESS recovering elemental or less pure hydroAT TROY, N. Y. sulfur from fuel, PLANTOF THE HUDSONVALLEYFUELCORPORATION gen sulfide gas produced by the second gases p r o v i d e o p class of processes tional means to melt may be marketed in their original form with no further this sulfur down to brimstone when market conditions dictate. processing a t the point of production, or they be subjected With the sulfur paste from the Ferrox and Nickel processes to further processing to change the physical or chemical this has not been very successful, but the Thylox process sulfur nature of the product to be sold or used. The nature of melts down easily when heated with steam in an autoclave or further processing, if any, is determined by economic and pressure melter. The molten sulfur settles to the bottom and marketing conditions, and, since these are changeable, a is withdrawn and cast into blocks ready for shipment. )

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Sizable amounts of this brimstone have been utilized by the paper industry. Although the by-product brimstone has not been used for the manufacture of sulfuric acid, this is quite feasible, and it would be possible for coke-oven plants to make their own sulfuric acid to be employed for the recovery of ammonia as ammonium sulfate from the gas (6). The hydrogen sulfide gas resulting as the end product of the second class of sulfur-recovery processes described above is not so easily shipped from the point of production without conversion to some other form. It is apparently possible to

VOL. 31, NO. 7

35,000 tons of sulfur in the form of hydrogen sulfide are being recovered from fuel gases annually in this country, and that the rate is increasing rather rapidly. For example, when plants now under construction are completed, the production of the sulfur as hydrogen sulfide will be about 40,000 tons annually. This means that within the next few months the total production of sulfur in all forms recovered from fuel gases will be approximately 50,000 tons per year. This is slightly less than the present rate of sulfur recovery from coke-oven gas alone in Germany.

FIGURE5. FLOWDIAGRAM OF “PURIFICATION TYPE”OF CONTACT ACIDPLANT

send hydrogen sulfide as st liquid in special tank cars, but up to now this has not been done, a t least in the case of the product obtained from fuel gases. At present the best utilization of the hydrogen sulfide appears to be its conversion into sulfuric acid, and this is now being done on a large scale, as indicated by the previous figures given in connection with the description of the Phenolate process. The sulfuric acid plant need not be immediately adjacent to the sulfur-recovery plant, since the hydrogen sulfide gas may be piped for a considerable distance through ordinary steel pipe lines. At one plant a large amount of hydrogen sulfide gas is carried over a mile through a pipe line t o the sulfuric acid plant, and a t another plant the distance is approximately one mile. Figure 5 is a flow diagram of a contact acid plant as arranged for the utilization of hydrogen sulfide as the raw material (4). Since the combustion of hydrogen sulfide leads to the formation of a considerable amount of water vapor, the plant is designed to remove i t ; present American practice calls for dry gas entering the converter. More recently Lurgi in Germany has developed a process for the manufacture of sulfuric acid from hydrogen sulfide which eliminates the step of removing the water vapor from the combustion gas (1, 2.4). The hydrogen sulfide is burned with an excess of air, and the products of combustion are passed directly over the contact mass without any removal of water vapor; the sulfur dioxide is thus converted to trioxide. The gas is then cooled by a condenser and the sulfur trioxide and water vapor combine and condense as sulfuric acid without the formation of any mist. This process is now in successful operation in several coke-oven plants in Germany and England, and the hydrogen sulfide used is that extracted from coke-oven gas. The hydrogen sulfide gas may also be converted into elemental sulfur by means of the Claus kiIn or some modified form of this old and well-known process. Recently a modified form of the Claus process has been installed in several cokeoven plants in Germany, and it is understood that satisfactory results are being obtained. Up to the present time none of the by-product hydrogen sulfide being produced in the United States has been converted into elemental suIfur on a commercial scale. Unfortunately there are no accurate and up-to-date statistics on the quantity of sulfur now being recovered from fuel gases in the United States. However, a rough survey indicated that about 8000 tons of elemental sulfur and about

Kot all of this recovered sulfur is being sold or utilized a t present. Some of the smaller plants that recover hydrogen sulfide as a by-product dispose of it by burning. Since the product is a gas, it cannot be easily stored or shipped, and in some cases the production is too small to justify an acid plant. Of the approximately 35,000 tons of by-product hydrogen sulfide being produced annually, probably almost 25 per cent is not being converted into salable form. However, considerable attention is now being given to this matter and it is hoped that a feasible means will be found in the near future to convert the hydrogen sulfide from small plants into some useful form. The present rate of sulfur recovery from fuel gases is only a small percentage of the potential field. Since this type of operation achieves two desirable objectives-namely, the benefication of the gas by removing the sulfur and the simultaneous recovery of a useful and salable product-it is almost certain that the application of these processes will constantly expand for many years to come.

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Recovery of Sulfur from Fuel Gases - Industrial & Engineering

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