US 3409520 A
Description (OCR text may contain errors)
Nov. 5, 1968 P. w. BOLMER 3,409,520
REMOVAL OF HYDROGEN SULFIDE FROM A HYDROGEN SULFIDE-HYDROCARBON GAS MIXTURE BY ELECTROLYSIS Filed Sept. 23, 1965 PERCE W. BOLMER INVENTOR /M/ /MZ ATTORNEY pnitid es 7 BY ELECTROLYSIS Perce W. Bolmer, 'allas, Tex.,' assignor to Mobil Oil poration, a corporation of NewfYork Continuation-impart of application Ser. No. 434,533, Feb. 23','196'5.'This application Sept. 23,1?65, Ser. -.No. 489,548 1" Y I v:17 Claims. (Cl; 204101) Y r ,This application is a continuation-in-part application of copending. application Ser. No. 434,533,..filed Feb. 23, 1965,now.Patent No. 3,249,522.. I v I This .invention,relates ,to the electrolysis. of hydrogen sulfide .tq formfree, hydrogen and certain valuablesulfur products and particularly tetheremoval of-;-hydrogen sulfide from a hydrogensulfide-hydrocarbon gas mixture by electrolysis with the formation of free hydrogen and such sulfur products.
Hydrogen sulfide is present as a contaminant in many valuable fluids. For example, in the petroleum industry, hydrogen sulfide is a commonly found contaminant in production fluids such as oil and gas and in processing and product streams attendant to the refining of such production fluids. Perhaps the most troublesome occurrence of hydrogen sulfide is its presence in gaseous hydrocarbon mixtures produces from subterranean formations, either separately or concomitantly with liquid petroleum. Such gaseous hydrocarbon mixtures, commonly termed natural gas, areusually comprised predominantly of methane and ethane with trace amounts of'higher hydrocarbon vapors such as propane, butane, pentane, etc. In addition to hydrocarbons and hydrogen sulfide, natural gas often contains carbon dioxide which may range up to about 30 percent or even higher of the total gas volume.
Because of its corrosive nature and for various other Well-known reasons, it usually is necessary or at least preferable to remove hydrogen sulfide when it occurs in admixture with hydrocarbons. Also, where a market exists for certain materials of which hydrogen sulfide is a precursor, it usually will be advantageous for economic reasons to obtain, if possible, such materials as byproducts of the hydrogen sulfide-hydrocarbon gas separation process.
In accordance with the instant invention, there is provideda new and improved method and apparatus for the formation of free hydrogen and certain valuable sulfur products by electrolysis of hydrogen sulfide. In a preferred embodiment of the instant invention, these materials are obtained as byproducts of a novel process of separating hydrogen sulfide from a hydrocarbon gas mixture by electrolysis.
I The instant invention utilizes an electrolysis cell having an electrolyte therein and an anode and a cathode in. contact with'the electrolyte and connected to an external source of electric power. In carrying out the invention, a hydrogen sulfide-hydrocarbon gas mixture is introduced into the electrolysis cell and into contact with the electrolyte and the anode. Anfexternally generated electric current is passed through the electrolyte between the anode and cathode in order to electrolytically oxidize sulfide ions at the anode to a sulfur oxidation product of the sulfide ions. At the same time, the passage of the electric current results in the electrolytic reduction of hydrogen ions to free hydrogen at the cathode. The sulfur product then is withdrawn from the electrolysis cell. The hydrocarbon gas, which is unaffected by the electrolysis reaction at the anode, is withdrawn from the electrolysis cell separately from the sulfur product. The hydrogen gas produced by the cathode reaction also is withdrawn from the cell.
In one form of the invention, certain-polysulfides as well as free hydrogen may be produced by the electrolysis of hydrogen sulfide. In this aspect of the' inve ntionfhydrogen sulfide is contacted'with a-basic cation-furnishing electrolyte in order to neutralize at least a portionof the hydrogen sulfide and form a basic solution of sulfide ions. Theresulting solution is subjected to electrolysis in ac'ell ofthe type described above by passing anexternallygenerated electric current through the solution. Sulfideions at the anode are electrolytically oxidized to disulfide ions, and hydrogen'ions at the cathode are electrolytically reduced to free hydrogen, the free hydrogen beingfwithdrawn from the cell. The mixture of sulfide and dis'ulfide ions resulting from the primary oxidation of sulfide ions is subjected to further electrolytic oxidation 'to=form"trisufide' ions. The mixture of sulfide and trisulfide ions resulting from this step is further oxidized to'tetrasulfide ions. The resultant solution of polysulfides of the cation furnished by the electrolyte is withdrawn from the electrolysis cell. The formation of the polysulfide ions requires an electrolyte of a relatively high pH. Therefore, in a preferred embodiment of the invention, basic electrolyte is added to the electrolysis cell at a rate suflicient to maintain the pH of the electrolyte at a level sufiiciently high to permit the formation of tetrasulfide ions.
The process of the instant invention may be carried out to produce a sulfur product comprising free sulfur, certain polysulfides, or mixtures thereof. Regardless of the sulfur species desired, it has been found that a tendency exists for some free sulfur to be formed in the electrolyte and deposited on the anode. Such sulur de-' position will lead to concentration polarization of the anode which ultimately will render the process ineffective or at least impractical. Therefore, in a preferred embodiment of the invention, deposition of free sulfur on the anode is restricted by introducing into the electrolysis cell and into contact with the anode a solvent for free sulfur. Preferably, the solvent is introduced at a temperature lower than its boiling point while maintaining the anode at a temperature at least as great as the boiling point of the solvent so that the solvent is vaporized from the anode.
For a better understanding of the instant invention, reference may be had to the following detailed descrip tion and the accompanying drawing which is an illustration partly in section of a system for carrying out the present invention.
With reference to the drawing, there is shown an electrolysis cell 10 which is comprised of an outer fluidtight shell or container 11 having top closure plate 11a. Con-' tainer 11 is divided into an anode compartment 12 and a cathode compartment 14 by a suitable diffusion barrier 15 and a gas impermeable baffle 16. Diffusion barrier 15, which will be described in detail hereinafter, restricts, i.e., impedes or completely prevents, the diffusion of fluid from one electrode compartment to the other while permitting at least some ionic transference.
Disposed in compartment 12 is an anode 17 comprising a porous and permeable cylindrical member 18 having an inner passage 19. Anode member 18 is supported on a tube 21 formed of a suitable electrically conductive material such as steel which extends exteriorly of the electrolysis cell through an insulator 22 of rubber or other suitable dielectric material. Electrode tube 21 is connected to an inlet pipe 23 by means of a nonconductive collar 24 formed of a suitable dielectric such as plastic. Disposed in compartment 14 is a cathode 25- ,central; passageway whichxopens at;one end thereof to provide .for the introduction of gas into the passage .so
that .it diffuses laterally through the wall ofthe anode rnembeuIt, will be. understood, however,.that the anode member .may take. variousrshapes including such as provided-;for byspaced discs or plates. For example, the anodearnembermay be formed of two spaced permeable plates-extending transversely of the anode compartment such that gas-introduced into the area between the plates diffuses-laterally through the plates. While an" anode of the flow-through type, i.e.,,one having a permeable memberthrough which the injected gas diffuses, is preferred, it will be recognized that the anode may be formed 'of a single plate in .whichcase the injected gas merely flows along-the surfaces of theplate or -is dissolved in the electrolyte and then carried to the anode by the solution. -The-anode member 18 may be formed of any-suitable material. Porous carbon isa preferred materiaL-although thevanode may be formed of other structures such as screens, gauzes, and the like of materials such as titanium, nickel, stainless steel, etc. The anode member 18 may have disposed on or impregnated therein a suitable catalyst such as platinum.
The cathode 25 is shown as comprising a single plate, although it will be recognized that numerous other electrode structures may be employed. For example, the cathode may comprise a porous structure or a plate having numerous fins extending laterally thereof in order to increase the surface area thereof. The cathode maybe formed of any materials such as those noted above with regard to the anode. Also, it will be recognized that the cathode may be provided with a suitable catalyst such as platinum for the electrolytic reduction reaction.
' The electrolysis cell is provided with an electrolyte 30 in contact with the anode and cathode. In most cases, the electrolyte in anode compartment 12 (the anolyte) and the electrolyte in cathode compartment 14 (the catholyte) Will be the same. However, different electrolytes may 'be used. For example, the anolyte may bean aqueous solu-' tion of sodium hydroxide and the catholyte an aqueous solution of ammonium carbonate, or vice versa.
The anolyte is maintained at a level above the uppermost;point of egress of gas from the anode member 18 such that all of the gas diffusing therethrough contacts and passes through the electrolyte. This is accomplished most expeditiously by maintaining the top of the anolyte well above the top of the permeable anode member 18, as shown. Also, electrolyte is maintained above the bottom of baffie 16, thus preventing direct gaseous communication between compartments 12 and 14.
The operation of the electrolysis cell and the attendant system is as follows. A hydrogen sulfide-hydrocarbon gas mixture is conducted from a suitable source such as a gas-oil separator (not shown) through line 23,
collar 24, and tube 21 into the porous anode member 18. The hydrogen sulfide-hydrocarbon gas mixture flows into the passageway 19 of the anode member and diffuses through the porous walls thereof into the electrolyte 30. As the gas mixture diffuses through the anode, a three-phase contact zone is formed which comprises the anode surfaces, the adjacent electrolyte, and the gas itself. As the hydrogen sulfide contacts the electrolyte,
it becomes ionized to form a solution of hydrogen and sulfide ions in the electrolyte. Sulfide ions are electrolytically oxidized to a sulfur oxidation product of the sulfide ions which may include free sulfur and/ or one or more ionic sulfur species, as described in detail hereinafter, and hydrogen ions are electrolytically reduced at the cathode to form free hydro-gen. The sulfur product is withdawn from the anode compartment of the cell through an outlet 31 and free hydrogen gas is withdrawn from the cathode compartment through an outlet 32. "The cathode compartment also is" provided'with"arsuitable inlet line 33, for the introduction of such make-up catholyte as may be necessary. The hydrocarbon gases in the mixture pass upwardly through the anolyte and are withdrawn from theanodecompartment through a sec nd fofu le't'ca ther in. tiv'vLiItt e anuerstod' thatfas Hatfie 16 "extends transversely of]; e sp'acei aboyefejlectroy e30 i t rm n lin t h r iza n sa l rd agen'will not occur..." Q i The anode reactionga d therefore the, nature; of the sulfide oxidation, PllOdllGttiOiIHltid, depends upon the type of electrolyte used. In the broadest aspect of the'invention, the electrolyte'may' beeithe'r ah'acid such as one molar sulfuric or hydrochloric acid or a base. While the use of aiifacid electrolyte would in theory'requii'e a mini mum decomposition potentiaL'it ap ea s lead to severe activation polarization of the anode. In addition, the only sulfur oxidation product obtainable'by electrolysis'ofhydrogensulfide in air acid lectrolyte' is free s'ulfur'asindi cated by the'anodereacti'on: I 2 e- I (1).
Free hydrogen is formed at the cathode by theelectro-' lytic reduction of hydrogen ions as indicated by the cathode reaction: 2H++2e- H (2) With the use of a basicelectrolyte, on the other hand, a wider selection of sulfur products may be obtained and the anode is not subject tothe severe activation polarizav tion which is encountered with the use of an acidelectrolyte. As a practical matter, therefore, and, in.the pre-,
ferred embodiment of the invention, a basic electrolyte is used.
Suitable basic electrolytes for carrying out the present invention include aqueous solutions of compounds such as sodium sulfide, sodium carbonate, sodium phosphate, sodium monoor di-hydrogen phosphate, sodium hydroxide, sodium bicarbonate, sodium bisulfide, and the like. Ammonium or potassium compounds corresponding to the foregoingvsodium compounds are also of value. While numerous other basic electrolytes may be used, either aqueous or nonaqueous, it usually will be preferred to use electrolytes which furnish sodium, potassium, or ammonium ions, since it usually will be desired. to form polysulfides of these cations. It is preferred to.- utilize an aqueous solution of a hydroxide with the cation;
being selected from the class consisting of sodium, potassium, and ammonium ions. 1 v I A number of products maygbe recovered at the anode,
depending upon the alkalinity of the electrolyte. With an electrolyte of relatively-low alkalinity, e.g., a pH on the order of 7.5 to 8, free sulfur is produced at the. anode. In this case, hydrogen sulfide is ionized. in the basic electrolyte to form bisulfide ions according to the following reaction:
The bisulfide ions are in turn electrolytically oxidized to free sulfur, the anode reaction being:
above and/or by thereduction of water according to the following reaction: p
2 H 0+2e- H +2oH- cathode. When it follows reaction (5), the current flow will be by migration of hydroxide ions to the anode. l, l
Free hydrogen is formed at the cathode by reaction 2' When, as in the case of electrolysis of -hydrogen;sul-fide in a mildly alkaline electrolyte, free sulfur is formed as a product, it is likely that the finely divided precipitated sulfur'may enter the pores'of the porous anode member 18, causingan increase in anode polarization and an increasein the resistance across the anode-electrolyte interfaceaIn orderto restrict such depositiomand in accordancewith a-preferred embodiment ofthe instant invenf tion, a solvent for free sulfur is introduced into the electrolysis celljandinto'contact with theanode.
' With; reference .to the drawing, this. leaching of the anode; surfaces with the-sulfur solvent maybe accomplished inthe followingfmanner. The incoming stream in line 23--is diverted to. asolvent reservoirs-3 6.by C1081 ing or. partially closing a valve 38 in line 23 andopeninga valveififlin a bypass line 40.1eading to the reservoir Reservoirf'36 contains a suitable. solvent for-free sulfur.
such-,as benzene; As the gas stream-.flows through reservoir.u36',- it:- picksuup-the benzene or other solvent :and
forms an-aerated mixture of gas and benzene, the benzene being: incthe form of dispersed, liquid-phase droplets which'may have .somehydrogen sulfide and hydrocarbon di'ssolved'th'erein; The resulting mixture leaves reservoir 36 and flows through a line 41 to a heater-43 where it is heated to. a temperature less than the boiling-point of the solvent. Thereafter, the mixture flows through a line 45 andan opened valve 46 therein into inlet line 23 and tube 21 and thence into the passage 19 of anode member 18.
' The anode and the electrolyte adjacent thereto are maintained at a temperature at least as great as the boiling 'p'oint'of the solvent but suitable temperature control means. As shown in the drawing, this temperature control means preferably takes the form of a heat ex changer 47 immersed in the electrolyte near the anode.
Heat "exchanger 47-may be. simply a conventional electric heater provided with suitable thermostatic means (hot shown) for automatically regulating the temperature. It'will beunderstood, however, that other temperature control means could beused. .For. example, the
temperature'ofthe anolyt'e and anode could be regulated by continuously introducing anolyte at a desired tem-' perature into the anode compartment.
As the gas-solvent mixture enters the pores of anode. member 18 and diffuses therethrough, the solvent is vaporizedib'y the heat of the anode and boils off from the outer-surfaces thereof into the adjacent electrolyte, concomitantlyretarding the deposition of sulfur on the anode and removing any sulfur so deposited. The solvent is una'ffected by the electrochemical oxidation reaction at the anode. However, any hydrogen sulfide which may have been entrained in the liquid solvent is evolved therefrom and undergoes electrochemicaloxidation as the solvent vaporizes. The free sulfur dissolves in the electrolyte or precipitates out, depending upon the saturation of the electrolyte, and the solvent vapors thus free of sulfur bubble up through the anolyte and are withdrawn with the hydrocarbon. gas from 'the anode compartment through outlet line 34. With a valve 48' in line 34 closed, the hydrocarbon gas-solvent effluent is diverted through a line 50 and an opened valve 51 therein to a gas-solvent separation chamber 53. The solvent is separated from the gas as by cooling and condensation and-the condensed solvent is recirculated-through a line 55 an'd'an opened valve 56 therein to the-solvent reservoir 36. Such make-up solvent as may be necessary is introduced into the reservoir by way of aline 57. The
gaseousefiiuent from separator 53 is passed through a 2 line-58 and an opened valve 59 therein into line- 34. In
the unlikely event any free sulfur iscntrained in the efiiuent withdrawn from the anode compartment, such sulfur may be separated from the gaseous solvent and hydrocarbons, for example,.by filtration, prior to condensation of the solvent..
.As. n t d above, th an de s a t ne t a. tempe ature at least equal to; the boiling point of the solvent. Preferably, the temperature of the anode will be maintained at a temperature higher than the boiling point-of the solvent in order to insure rapid and relatively com' plete vaporization of the solvent as it leaves theanode, Where benzene, havinga boiling pointfof about- 'C., is ,utilized as a solvent, the anode and the .adjacentelectrolyte may be maintained ata temperature .Qf.- about C.
As noted previously, the instant invention, may .be carriedout to: produce. polysulfides. Inthis'form ofthe invention, a strongly alkaline electroylte, e.g., one hav, ing a; pH on the order of. 1-4, is used.- In this case, .a feed gas comprising-hydrogengsulfide is ,introduceed'into the electrolysis cell in the 'manner described beforehand. The hydrogen sulfide is neutralized in the strongly alkaline electrolyte to form 'a basic. solution-of I sulfide ionswhich are electrolytically oxidizedby the..passage gof current through .the electrolyte to dis ulfide, ions. The mixture-3f disulfide ions ,and sulfide ions. resultingvfrorn-the ,aboveoxidation stepvand the continuous-introduction of hydro. gen sulfide may -.be further electrolytically oxidized to form trisulfide ions which may in turn be oxidized to form tetrasulfide ions; In addition, free sulfur formed at the anode will dissolve in the solution to form polysulfides. Depending upon the residence time of the anionic sulfur species in the cell and upon the current flowing through the anode, electrolytic oxidation at the anode may be carried out to produce disulfide, trisulfide, or tetrasulfide ions, or mixtures thereof, or even free sulfur. Also, in some instances as, for example, where the anolyte comprises a potassium compound, the product formed at the anode may include pentasulfide ions. Generally, an increase in the residence time of the ions within the cell or an increase in current density will tend to;
produce a predominant amount of the higherpolysulfides and ultimately free sulfur. The cathode reaction in any case will be the reduction of'hydrogen ions to' free hydrogen as indicated by reactions (2) and/or 5) and the hydrogen will be withdrawn through outlet 32. a Y The degree of oxidation permitted to take place at the anode will of course depend upon the desired sulfur product. One valuable product obtainable by the process of the present invention is sodium tetrasulfide; In the production of this material, a preferred electrolyte is an aqueous sodium hydroxide solution having'a molar concentration in the range of 1 to 10 and maintained at a pH of about 14 or greater. By Way of illustration, the
reactions at the anode attendant to the production of'sodium tetrasulfide by the'electrolysis of hydrogen sulfide in an aqueous sodium hydroxide solution are as follows:'
actions. However, it will be recognized that in the actual operation of the processvarious reactions will occur simultaneously. For example, while reaction (9) is taking place, neutralized sulfide ions will be undergoing electrolytic oxidation in accordance with reaction (7).
In addition to the reactions noted above, free sulfur I will be formed in the anolyte. For example, some bisul-' fide ions may be formed, some of which may in turn be oxidized to free sulfur in accordance with reaction (4).
Also, free sulfur may result directly from the electrolytic oxidation of some sulfide ions. Unless the anolyte is saturated, the free sulfur will tend to dissolve therein to form 7. polysulfides. For example, free sulfur will dissolve in a sodium sulfide solution to form sodium polysulfides which in turn may be converted to higher polysulfides by further combination with additional free sulfur or by electrolytic oxidation.
In the utilization of the instant invention to produce sodium tetrasulfide (or otherpolysulfides), the process preferably is carried out as a continuous operation. That is, anolyte containing the desired polysulfide is withdrawn through line 31 while at the same time hydrogen sulfide feed is'introduced through tube 21 and make-up sodium hydroxide (or other cation-furnishing electrolyte) is introduced through an inlet line 64. The make-up electrolyte will be introduced in a concentration and at a rate sufiicient to maintain the pH of the anolyte at the desired level.
As noted above, the particular sulfur product formed will depend upon the residence time of the oxidized sulfide products in the anode compartment and the current density at the anode as well as of course the rate of hydrogen sulfide feed to the cell. The desired product can be obtained by testing-the efiluent from line 31 and adjusting one or more of the above factors accordingly. For example, should a test of the anolyte withdrawn show a predominance of sulfides, disulfides, and trisulfides with only a small amount of tetrasulfide, the residence time in the cell can be increased (while holding the current density and hydrogen sulfide feed constant) in order to obtain a greater proportion of the higher polysulfides. The residence time can be increased simply by decreasing the rate of withdrawal through line 31 and correspondingly decreasing the input of electrolyte through line 64 in order to achieve a balance with withdrawal and maintain a relatively constant amount of electrolyte in the anode compartment. Alternatively, the current density at the anode can be increased or the rate of hydrogen sulfide feed to the anode decreased while holding the residence time unchanged.
The embodiment of the instant invention utilizing a strongly alkaline electrolyte may be carried out to form free sulfur in the electrolysis cell. This may be accomplished by increasing the residence time in the cell past the point at which the predominant product is a tetrasulfide. The sulfur thus formed initially will dissolve in the anolyte to form polysulfides as indicated above, but as the anolyte becomes saturated, sulfur will begin to precipitate out of the solution. The precipitated sulfur may be recovered through line 31 as is the case with the other sulfur products produced in accordance with the instant invention.
Regardless of the product desired, in carrying out the invention with a strongly alkaline electrolyte it is likely that some sulfur will form and tend to be deposited on the anode. Therefore, in accordance with this embodiment of the invention, as will as where a relatively weak base is used, the invention preferably is carried out utilizing solvent extraction at the anode as described above.
To summarize briefly, the nature of the sulfur product will depend upon the basicity or acidity of the anolyte. In an electrolyte that is an acid or a relatively weak base, the product will be free sulfur while in a strongly alkaline electrolyte the product will comprise free sulfur or certain polysulfides, or mixtures thereof. It is preferred in carrying out the invention to utilize a basic electrolyte in view of the above-noted disadvantages attendant to the use of an acid.
In the production of free sulfur, the electrolyte preferably will be at a pH in the range of about 7.5 to 8 with a pH of about 7.7 being particularly preferred, it having been found that operations at this value give very good results. However, the production of free sulfur by the electrolytic oxidation of bisulfide ions may be carried out in a basic electrolyte having a pH up to about 12. It will be recognized that as the pH of the electrolyte approaches 12, there will be an increasing tendency toward the pro- 8 duction of polysulfides in accordance with reactions (6), and
In order to form polysulfides, the process of the instant invention is carried out in an electrolyte having a pH preferably of 14 or greater and, in any case, at least greater than 12. 4
As noted previously, free sulfur can be formed by utilizing a strongly alkaline electrolyte. However, this embodiment of the invention cannot as a practical :matter tolerate feedstreams having material amounts, i.e., greater than about two percent, of carbon dioxide therein since the carbon dioxide makes it difiicult to maintain the pH of the electrolyte at the desired level. Therefore, when the feedstream contains appreciable carbon dioxide, as is many times the case with natural gas, the instant invention preferably will be carried out in the lower pH range with the predominant byproduct being free sulfur.
In instances where the feedstream contains little or no carbon dioxide, free sulfur can be obtained by carrying out the invention utilizing a strongly alkaline electrolyte to form polysulfide in the anode compartment, withdrawing the polysulfide from the cell, and decomposing it to obtain a product including free sulfur. In this embodiment of the invention, an ammonium compound such as an aqueous solution of ammonium hydroxide is used as the anolyte and the residence time in the cell is controlled to produce a product comprised predominantly. of ammonium polysulfide and including as little free sulfur as practical. The product then is withdrawn from the cell and heated in order to decompose the ammonium polysulfide to form free sulfur. This embodiment of the invention offers the advantage of keeping the formation of free sulfur within the cell at a minimum, thus lessening the tendency toward polarization of the anode due to deposition of sulfur thereon.
The effiuent from anode compartment 12 is passed through line 31 to a suitable separation system 70. Regardless of the particular material or materials desired as a byproduct of the electrolysis process, the effluent through line 31 will in all likelihood contain some free sulfur. Therefore, as a first step in the separation process, the anolyte withdrawn from the anode compartment is passed to a suitable sulfur-separation zone 72. Separation zone 72 may conveniently comprise a filtration chamber. The sulfur is filtered out of the withdrawn anolyte and dispensed through line 74 to a suitable terminal zone (not shown) and the filtrate is withdrawn through line 76. If the sulfur thus separated is the only desired product, the fluid in line 76 may be recycled to the anode compartment through a line 78, an opened valve 79 therein, and line 64. Where a soluble sulfur product such as sodium tetrasulfide is desired, the anolyte may be passed to a suitable terminal station (not shown) by closing valve 79 and opening a valve 81 in line 76. a
In the above-described embodiment of the invention in which free sulfur is formed from ammonium polysulfide, the filtrate from chamber 72 is passed through a line 84 and an opened valve 86 therein (valves 79 and 81 being closed) to a suitable heating zone 88. In zone 88, the
withdrawn anolyte containing ammonium polysulfide is heated to a temperature sufficient to decompose the polysulfides and precipitate sulfur and evolve hydrogen sulfide and ammonia. The sulfur is Withdrawn from zone 88 through line 90. The ammonia and hydrogen sulfide are withdrawn through line 92 and may be passed to inlet line 23 as shown. 1 i
As noted previously, the diffusion barrier 15 restricts the diffusion of fluid from one electrode compartment to the other. While such a barrier is not strictly a necessity and the invention may be carried out without a ditfusion barrier separating the anolyte and catholyte, one usually will be preferred since it tends to prevent the intermingling of the anolyte with the catholyte with the resulting migration of sulfide and polysulfide ions to a location remote= from the anode. As such, the diffusion barrier may take a number ,of forms. Thedilfusion barrier may be comprised f; ay ti nel ticngsele tive membr n .h.p mitsrl lestr-ansferenceof cations therethroughbut restricts as itr m s ns an n .s a m mb ne. may be formed froma cationexchange resin bypastingthe resin into the form of a sheet or membraneorpy incorporating granules of the,. resin in amatrix comprisinga binder such as polyethylene. The cation exchange i.membrane may hav ,a .permselec.tivity;. o f ;abut;0.9 and a .condi1ctivity,of
'ed .that. these yalu es are innsions therethrou'gh' underthe influence of-the electricalbias produced at the anode andcathodebut-which prevents or at least strongly impedes fluid movement between the anode and cathode compartments. The term diffusion barrier as used herein and in the appended claims therefore designates a barrier which extends transversely of the electrolysis cell between the anode .and cathode and restricts, i.e., prevents or at least impedes, diffusion of electrolyte from one electrode compartment to the other while permitting at least some ionic transference.
Having described specific embodiments of the instant invention, it will be understood that further modifications thereof may be suggested to those skilled in the art, and it is intended to cover all such modifications as fall within the scope of the appended claims.
What is claimed is:
1. In the removal of hydrogen sulfide from a hydrogen sulfide-hydrocarbon gas mixture with the formation of sulfur product and free hydrogen by electrolysis in an electrolysis cell having an electrolyte therein and an anode and a cathode in contact with said electrolyte and connected to an external source of electric power, the method of:
(a) introducing a hydrogen sulfide-hydrocarbon gas mixture into said electrolysis cell and into contact with said electrolyte and said anode;
(b) passing an externally generated electric current through said electrolyte between said anode and said cathode to electrolytically oxidize at said anode sulfide ions to a sulfur oxidation product of said sulfide ions and electrolytically reduce at said cathode hydrogen ions to free hydrogen;
(c) separately withdrawing said hydrocarbon gas and at least a portion of said sulfur product from said electrolysis cell; and
(d) withdrawing said hydrogen gas from said electrolysis cell.
2. The method of claim 1 wherein said sulfur product includes free sulfur which tends to deposit on said anode and further comprising introducing into said electrolysis cell and into contact with said anode a solvent for free sulfur whereby deposition of free sulfur is restricted.
3. In the removal of hydrogen sulfide from a hydrogen sulfide-hydrocarbon gas mixture with the formation of sulfur product and free hydrogen by electrolysis in an electrolysis cell having a basic electrolyte therein and an anode and a cathode in contact with said electrolyte and connected to an external source of electric power, said anode including a gas-permeable member, the method of:
(a) introducing a hydrogen sulfide-hydrocarbon gas mixture into said permeable member and diffusing said mixture through said permeable member and Q0- 9. ms .pen, squ re e t et em fe,
eo l w r els u ma b l tth .is taat nY fn n- ,said electrolyte a. solvent for fre tion of free sulfur is restricted.
, ,-i n,tot contactwith said-electrolyte whereby a basic l ionp mQQiQnSi -fqrmed ats idanodem (b). passing .an; externally genera tec l electric. current through said electrolyte between said anode andsaid cathode to electrolytically oxidize atesaid anodeisulfide ions to a sulfur oxidation product of saidsulfide ions and. electrolytically reduce at said cathode hyo n nsto f hydrQg n;:- f
1 (c) separately .withdrawing hydrocarbon gas and eelec 1 trolyte. containing .apsulfufi product- -.:from.-:said elec-v -(d.) withdrawing-hydrogen gasfrom said electrolysis;
....: 1 d= multaneously:withstep 0, 6
introducing basic. electrolyte into. said electrolysis cells-51 a 4:- ,'l?he.,r n introdueedat a rate .suflicient, tomaintainthe level of the electrolyte adjacent said anode above the uppermostipoint of-egress ,of: said mixturefrom saidzpermeable member. 5. .The method ;of;claim 3 wherein said -;s ulfur3 product includes free sulfur which tends to, deposit onsaid permeable member and further comprising introducing into said permeable member and diffusing therethroughinto e sulfur whereby deposi- 6. The method of claim 5 wherein said solvent is introduced into said permeable member at a temperature lower than its boiling point and said permeable member is maintained at a temperature at least as great as the boiling point of said solvent whereby said solvent is vaporized from said anode.
7. The method of claim 3 wherein said electrolyte is an aqueous solution of a base having a cation selected from the class consisting of sodium, potassium, and ammonium ions.
8. The method of claim 7 wherein said droxide.
9. In the formation of sulfur products and free hydrogen by electrolysis of hydrogen sulfide, the method of:
(a) contacting hydrogen sulfide with a basic, cationfurnishing electrolyte to neutralize at least a portion of said hydrogen sulfide and form a basic solution of sulfide ions;
(b) in an electrolysis cell having a cathode and an anode therein, passing an externally generated electric current through said solution between said cathode and anode to electrolytically oxidize at said anode a portion of said sulfide ions to disulfide ions and electrolytically reduce at said anode hydrogen ions to free hydrogen;
(c) continuing to pass said electric current through said solution to electrolytically oxidize at said anode a mixture of sulfide and disulfide ions resulting from step (b) to trisulfide ions;
((1) continuing to pass said electric current through said solution and oxidizing at said anode a mixture of sulfide and trisulfide ions resulting from step (c) to tetrasulfide ions;
(e) continuously repeating steps (a), (b), (c), and
(d) while withdrawing free hydrogen from said electrolysis cell at the cathode and withdrawing a solution of polysulfides of the cation furnished by the basic electrolyte from said electrolysis cell at said anode; and
(f) simultaneously with steps (a), (b), (c), (d), and (e) adding basic electrolyte to said electrolysis cell at a rate and in a concentration sufficient to maintain the basicity of said electrolyte in said cell at a level sufiiciently high to permit the formation of tetrasulfide ions.
10. The method of claim 9 wherein the electrolyte in said cell is maintained at a pH greater than 12.
11. The method of claim 9 wherein the electrolyte in said cell is maintained at a pH of at least 14.
base is a hygemi113 is wherein "said 1 1 12. Thc' method of claim9 wherein said electrolyte is an aqueous solution-of a base having'a cation selected from the class consistingof sodium, potassium, and ammoniumionsi- 1 3. The method of claim 12 wherein said base is a hydroxide;
-14IThe'method of 'claim 9 whereinsaid cation furnished by said basic electrolyte is ammonium and comprisingflthe 1 further steps of withdrawing a solution of ammonium polysulfides; from said electrolysis cell and heating said solution to a temperature sufiicient to decompose saidammonium polysulfides and. form free sulfur. 15. The method of claim 14 wherein said electrolyte is'an' a'queous solution of ammonium hydroxide.
"16. Inthc formation of sulfur-products and free hydrogen by electrolysis of hydrogen sulfide, the method of:
(a-) contactinghydrogen sulfide with a liquid electrolyte'to form 'a' solution of sulfide ions in" said electrolyte;
(b) in an'electrolysis cell 'having a cathode and an -anode therein, passing an externally generatedelec- -tric current-through said solution between said 'cath ode and anode to electrolytically reduce at said cathode hydrogen ions to free hydrogenand to electrolytically oxidize at said anode sulfide ions to a sulfur oxidation product of said sulfide ions including free sulfur which tends to deposit on'said anode;
(c)-"introducing into said electrolysis cell and into contact with said anode a solvent for free sulfur whercb'y' deposition of'free sulfur on'said' anodeis restricted;
' (d) withdrawing free hydrogen from'said electrolysis cell at said cathode; and
(e) withdrawing sulfur product frorn'said electrolysis cell at said anode.
' 17. The method of claim 16 wherein said solvent is introduced into said electrolysis cell at a temperature: lower than its boiling point and 'said anode is r'naintained" at a" temperature at least 'asgreat 'as the'boilin'g 'point'of said solvent whereby thesolvent is vaporized fromsaid' Juda 204'-1.06 XR HOWARD s1 WILLIAM Prin zdryiExaniin'fl f i D. R.'JORDAN,/issistalzt Examiner.- V I