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Indirect hydrogen sulfide conversion—I. An acidic electrochemical process
Int. J. Hydrogen Energy, Vol. 10, No. 3, pp. 157-162. 1985. Printed in Great Britain.
0360-3199/85 $3.00 + 0.00 Pergamon Press Ltd. ~) 1985 International Association for Hydrogen Energy.
INDIRECT H Y D R O G E N SULFIDE CONVERSION--I. AN ACIDIC ELECTROCHEMICAL PROCESS D. W. KALINA and E. T.
MAAS,
JR.
Standard Oil Company (Indiana), Amoco Research Center, P.O. Box 400, Naperville, IL 60566, U.S.A. (Received 10 October 1984) indirect hydrogen sulfide conversion process was explored which ultimately results in the production of elemental hydrogen and sulfur. The process is based on the electrochemical oxidation of iodide in aqueous hydriodic acid at high current densities and current efficiencies. Hydrogen gas is produced concurrently with soluble triiodide. Hydrogen sulfide gas was reacted with electrolyte solutions containing triiodide to yield an impure sulfur product recovered in an unusual sticky plastic form. Recrystallization of this product from toluene yielded sulfur of comparable purity to that presently produced. Abstract--An
can be coupled with the hydrogen evolution reaction to produce a quantity of hydrogen gas equivalent to that chemically contained in the hydrogen sulfide treated. This paper summarizes our work in developing a low pH process.
INTRODUCTION Hydrogen sulfide represents, in a less-than-agreeable form, the source of two elements which individually have significant economic value. In 1982, conversion of hydrogen sulfide, obtained from petroleum treating and natural gas sweetening processes, yielded 4.2 × 106 long tons of sulfur or approximately 50% of all the elemental sulfur produced in the United States that year [1]. The amount of hydrogen stoichiometrically associated with this amount of sulfur in the form of H2S is 1.1 × 1011SCF. Presently, only sulfur is recovered from hydrogen sulfide using, primarily, established Claus technology wherein the H2S is partially oxidized to yield water and sulfur [2]. A portion of the heating value of the hydrogen is recovered as steam from the exothermic Claus reaction. Recovery of elemental hydrogen to utilize its chemical value would represent a significant supplement to the hydrogen requirements of heavy crude oil upgrading and coal liquifaction and gasification. Various approaches to hydrogen recovery from hydrogen sulfide have been considered [3]. Direct oxidation of sulfide ions at an electrode formed the basis of an electrochemical process explored by Bolmer [4]. In that process, production of elemental sulfur was reported to result in deposition of a blocking sulfur layer on the electrode, which could be removed using a vaporizing solvent extraction technique. Indirect electrochemical conversion processes have also been investigated [5, 6]. One of these, utilizing soluble halogen oxidants in buffered solutions at pH = 7-8.5, has been commercialized with little apparent economic success
PROCESS CONCEPTS Iodine oxidants are ideally suited for H2S conversion. In acidic solutions, iodine, as triiodide ions (Iff), will oxidize sulfide only to elemental sulfur. Aqueous chlorine or bromine reacts with dissolved sulfide to yield soluble species containing sulfur in positive oxidation states. The iodide/triiodide couple fulfills other process requirements, namely: (1) stability under anticipated operating conditions, (2) no unwanted side reactions with H2S, (3) high solubility of both the oxidized and reduced forms, and (4) uncomplicated and well-documented electrochemistry. A generalized process flow sheet is shown in Fig. 1. This flow sheet is applicable to either large-scale gas sweetening processes or tail gas clean-up. The process depicted consists of two main unit operations, namely, electrochemical conversion and chemical reaction. In the electrochemical section, the soluble oxidant (Iff) is generated from acidic iodide solution. Concurrent with this oxidation is the reduction of protons to yield hydrogen gas. These two electrochemical reactions, the anodic reaction (equation 1) and the cathodic reaction (equation 2), performed simultaneously at their respective electrodes in the electrochemical cell, make up the overall electrochemical process (equation 3).
[7]. We have chosen to explore indirect electrochemical conversions of hydrogen sulfide using iodine oxidants in acid solution ( p H = 0 - 1 ) and in basic solutions (pH = 13-14) [8]. In these pH regions oxidized iodine species can be electrochemically generated in very high concentrations. These species, I~ in acid and I O ; in base, react with hydrogen sulfide yielding elemental sulfur. Electrochemical regeneration of spent oxidant - -
31- ~ I~ + 2e-
(1)
2H + + 2e- --~ H2
(2)
3I- + 2H" ---, I~ + H2.
(3)
In the chemical reaction section, a gas stream rich in H2S is contacted with triiodide solution, and sulfur is generated according to the reaction in equation (4). I2(as I~) + H2S ~ 2H ÷ + 21- + S. 157
(4)
D. W. KALINA AND E. T. MAAS, JR.
158
Oxidant-rich anolyte
H2S-poor gas stream
"1 Anolyte reservoir
Chemical reactor
r
Electrolysis cell
I I H2S-rich gas stream
Oxidant-rich slip stream
Catholyte reservoir
I Oxidant-depleted anolyte
Fig. 1. Schematic flowsheet of electrochemical H2S conversion process.
This reaction is utilized in standard quantitative analytical determinations of sulfide [9]. We believe that three critical technological components control the viability of the overall process. These are: (1) the anode reaction; (2) the H2S absorption in acid; and (3) sulfur production. For successful operation, the anode reaction (i.e. the oxidation of acidic iodide) must proceed at high current density and efficiency while requiring low overvoltage. Likewise, the anode material must exhibit long-term stability during the electrochemical process. The use of an acidic scrubbing solution to absorb an acid gas (in this case, H2S) is contrary to most preconceived ideas and demonstrated processes, which usually employ caustic scrubbers to absorb acid gases. In this acidic approach, the driving force for H2S removal from the gas phase is the thermodynamics associated with the redox reaction in equation (4). In order to compete favorably with other H2S treating processes, this acidic process must yield a sulfur product of comparable quality to that of, for example, the Claus process. Commodity sulfur is produced typically at a 99.95% purity level with no individual contaminant being present at greater than the 100 ppm level. EXPERIMENTAL The electrochemical unit operation was based on the commercially available MP ('Multipurpose') electrolytic cell developed and marketed by the Swedish National Development Corporation. The MP cell employs a parallel plate electrode configuration with an anode-cathode separation of c a 13 mm and an electrode surface area of 100 cm 2. This cell allows delivery of separate anolyte and catholyte feed streams and maintains electrolyte isolation inside the cell via a separator or ionexchange membrane sandwiched between the electrodes.
Electrolyte reservoirs, centrifugal pumps, and associated plumbing (chemically resistant polyvinylidene fluoride or polypropylene throughout) complete the electrolytic section. Reservoirs have been employed for both electrolyte solutions in order to aid in the removal and metering of hydrogen gas from the catholyte and to insure the supply of an oxidant-rich anolyte slipstream to the H2S reactor. The slip-stream/reservoir approach allows the use of flow rates through the MP cell meeting equipment specifications (1-151 min -l); flow rates far below specifications would be required in a design for the electrochemical generation of oxidant-rich anolyte from an extremely depleted anolyte in a single pass through the cell. Electrolyte temperatures and electrode voltages were monitored in-line via thermocouples (inside mercury-filled ceramic wells) and standard calomel reference electrodes (SCE) immersed in each electrolyte flow downstream from the MP cell. Electrode voltages vs SCE downstream from the cell agreed well with voltages vs Ag/AgC1 micro reference electrodes inserted inside the cell itself. Use of the micro reference electrodes has caused leakage problems, however, and measurements with SCE downstream were generally recorded. Hydrogen production was measured with a calibrated wet-test meter attached to the catholyte reservoir. The iodine content of the anolyte was determined by thiosulfate titration using a starch/visual endpoint indicator [10]. The electrochemical section has been operated on a continuous basis with the anolyte and catholyte each being circulated in closed loops through the appropriate sides of the electrochemical cell. Power was supplied to the cell by an external d.c. source capable of passing 0-100 A at 0-10 V. A typical experiment consisted of a continuous 5 h run with the current being controlled at 10, 20, 30, 40 and 50 A for 1 h each. The reaction between the triiodide solution and H2S was carried out in a cylindrical glass reactor approxi-
159
INDIRECT HYDROGEN SULFIDE CONVERSION--I mately 30 cm tall and 5 cm in diameter. The triiodide solution was the continuous phase and the H2S-containing gas (25%H2S/75%N2) was injected into the liquid phase through a sintered-glass gas dispersion tube. Liquid electrolyte was pumped into the reactor through a polypropylene tube. Both the gas and liquid inlets were located near the bottom of the reactor. The chemical reactor was stirred at the bottom using a magnetic stirring apparatus. The gas bubbles flowed upward in the same direction as the electrolyte flow and exited out the chemical reactor through a tube passing through the top cover of the reactor. The electrolyte spilled over through a sidearm attached near the top of the reactor. The liquid formed a gas trap at this point to insure isolation of all of the H:S-containing gas in the chemical reactor. At the termination of each experimental run, the sulfur product was recovered manually from the chemical reactor where it had been deposited in the form of a sticky, plastic mass (vide infra). This crude product was recrystallized from toluene to yield purified sulfur. The residual iodine content of the sulfur was determined by X-ray fluorescence. The off-gas from the chemical reactor was directed into a caustic scrubber. Hydrogen sulfide absorbed in this scrubber was determined by potentiometric titrations using standardized Pb 2. solutions. The ability of acid triiodide solutions to remove H:S from a gas was quantified by a series of closed-vessel equilibrium reactions in which measured amounts of H2S gas were injected over measured volumes of scrubber solutions (both acid triiodide and monoethanolamine). After equilibrium was attained, samples of the gas phase were analyzed for H2S content.
RESULTS
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The cathode material utilized in the MP cell for our process was platinized platinum on titanium. Variation in acid identity and concentration (Table 2) indicated that 3 M HCI minimizes both cathode overpotential ( - 0 . 5 4 V vs SCE at 500 mA cm -2) and resistive losses of the aqueous electrolyte and membrane cell separator. Three proton conductive membranes were evaluated in the MP cell experiments. These were (1) Nation ® 390, a fabric-reinforced membrarie 0.30 mm thick; (2) Nation ® 117, an unsupported membrane 0.19mm thick; and (3) Raipore ® R1010, an unsupported membrane 0.05 mm thick. All three gave reasonable performance in this process. As expected, the voltage drops associated with the resistances of the membranes followed their thicknesses with the Raipore ® R1010 requiting only 0.03V to overcome its resistance at 500 m A cm -2.
Electrochemistry Oxidation of acidic iodide, to form soluble triiodide, together with the reduction of protons to yield hydrogen gas, was carried out at commercially interesting current densities of 1 0 0 - 5 0 0 m A c m - : using the MP electrochemical cell. The anode of choice for the iodide oxidation was graphite. No adverse effects were noted for these anodes in 1.5 M I: + 5.5 M HI anolyte. High density graphite ( 1 . 8 g c m -3) was employed to minimize leakage of the electrolyte through the electrode. Typical current-voltage behavior for such graphite anodes in the above-noted electrolvte is illustrated in Fig. 2. Very high current densities (500mAcro -2) were attainable at 0.71 V vs SCE with no indication of any concentration polarizations. Measured current efficiencies f o r I ; generation were essentially 100%. Not unexpectedly, lower initial I: concentrations in the anolyte resulted in lower anode potentials as indicated in Table 1. This trend reflects the availability of I- for oxidation at the anode based on the equilibrium [11] in equation (5). I- + I : ~ - I r K = 700.
(5)
H2S
Scrubbing
The ability of acidic triiodide solutions to remove H2S from a gas was demonstrated by a series of closedcontainer equilibrium reactions in which measured amounts of H2S gas were injected over measured volumes of acidic triiodide solutions. After equilibrium was attained, samples of the gas were analyzed for H2S content. Similar reactions were carried out using the traditional H2S scrubber, monoethanolamine dissolved
Table 1. Anode potentials at 500 mA crn-2 for graphite plate in 5.5 M HI anolyte with variable I2 concentration 12 Concentration (M)
Anode potential vs SCE (V)
1.0 1.8 2.0 2.4
0.70 0.83 0.85 0.90
D. W. KALINA AND E. T. MAAS, JR.
160
Table 2. Cathode potentials and electrolyte + membrane potential drop for platinized platinum on titanium in various inorganic acid catholytes
Catholyte
Cathode potential vs SCE (V)
Electrolyte* + membrane potential drop (V)
-0.54 -0.54 -1.00 --0.64 -0.83
0.70 0.97 1.1 0.75 1.4
3 M HCI 6 M HC1 1.5 M H2SO4 3 M HESO4 9 M H2SO4
Sulfur recovery
* Includes I2 + HI anolyte potential drop. in water. The results of these comparative experiments are shown in Fig. 3. These equilibrium data indicate that on a per-mole-of-scrubber basis, the reaction of H2S with I~ leaves more H2S in the gas phase than does the traditional HaS scrubber, monoethanolamine. This difference is not large enough to preclude the use of the triiodide solutions as H2S scrubbers. However, the data indicate that in order to provide for equivalent H2S removal, larger gas contactors will have to be provided for a triiodide scrubbing process. In the process experiments, the triiodide electrolyte was delivered into the chemical reactor at the rate of ca 5.8 x 10-2 mole I= min -l. This should provide for 5.8 x 10-2moleH2Smin -1 capacity. Gas flow rate to the reactor was adjusted to follow the current level in the electrochemical cell, with 220ccmin -1 of gas injected at 10 A ranging to 1100 cc min -~ of gas at 50 A. The gas employed was H2S in nitrogen with a typical analysis of 25% H2S. These flow rates corresponded to molar flow rates of c a 2 . 3 x 10-3-1.1 x 10-2mole H2S per minute. The ratio of I2/H:S ranged from 25 to 5. In most experiments, there was no indication of any H2S breakthrough from the scrubbing solution. But in a few random experiments and for no apparent reason, there was some H2S breakthrough even though the
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I2/H2S ratio remained high. This can presumably be attributed to a variation in gas bubble size and dispersion caused by build-up of sulfur deposits in critical places in the reactor. It was virtually impossible to observe anything like this visually while the reaction was proceeding due to the opaque coloration of the triiodide solution.
2
Molar ratio (H2S / scrubber)
Fig. 3. Comparative equilibrium H2S contents in the gas phase over various scrubber solutions as a function of initial H2S/ scrubber ratios.
Sulfur produced by the reaction of H2S and acidic triiodide is of a different physical and possibly chemical form from that normally encountered. Whereas sulfur is usually observed as a microcrystalline, yellow, freely flowing solid, the triiodide-produced sulfur is deposited in the reaction vessel as a reddish-brown, sticky mass. This gum-like product converts slowly (of the order of hours) into a hard solid, still of the same color. Analysis of this 'acid' sulfur (as we have termed it) indicates that it contains 90-92% (by weight) recoverable sulfur. The other 8--10% is presumably entrained electrolyte as well as excess iodine. Assuming, as the worst possible case, that the entire non-sulfur component of acid sulfur consists only of iodine in some form, we can calculate empirical formulations for acid sulfur ranging from $36I to $46I. Experimentally, we have found that virtually all the acid sulfur (99.3%) produced remained in the reaction vessel in which it was originally made. Very small amounts were carried over as entrained particles into subsequent process vessels. In the reaction vessel, the acid sulfur exhibited a definite preference for the type of surface to which it adheres. In all of the experiments performed, the acid sulfur was found adhering to components made from both polypropylene and Teflon ®. Glass surfaces in extremely close proximity to these organic polymer components showed no evidence of being coated with this rather strange material. Even though the acid sulfur formed a solid deposit on the plastic components in the reaction media, the product could be separated from these components rather easily by manually peeling it off or else by heating it to liquify the acid sulfur as described below. While the acid sulfur is a sticky viscous mass when fresh and hardens with aging, the material can be transformed into a freely flowing liquid by heating it to 95-105 °C. This melting results in release of some iodine and transformation of the 'acid sulfur' into a more conventional form of sulfur. Even after melting, however, this sulfur still contains significant quantities of iodine, and further treatment such as recrystallization is needed to yield an acceptable product. We have demonstrated that the melting and recrystallization steps can be combined. Treatment of the solid acid sulfur with boiling toluene initially results in the melting of the acid sulfur in contact with the organic, yielding a two-phase system. Continued reflux of the organic solvent effects the dissolution of the liquid sulfur phase into the organic solvent. Cooling the now single-phase system results in the precipitation of crystalline elemen-
INDIRECT HYDROGEN SULFIDE CONVERSION--I tal sulfur containing routinely 100-150 ppm iodine as an impurity.
Sulfur yield In a typical experiment, 2.68moles of H2S were passed into the reaction vessel over a 5-h period. Afterward, 92.47 g of acid sulfur were recovered from the system. After treatment with 1.21 of hot toluene, this acid sulfur yielded 85.35 g (2.66 moles) of sulfur for a 99.3% sulfur recovery based on the H2S added. The reaction efficiency, that is, the amount of iodine consumed compared to the amount of sulfur recovered, was found to be essentially 100%. DISCUSSION Hydrogen sulfide, a waste product from petroleum refining and natural gas production, represents an untapped resource for significant quantities of hydrogen. We have demonstrated a process that goes beyond presently utilized hydrogen sulfide treating technology and results in the conversion of H:S into elemental hydrogen and sulfur. This process is based on the concurrent electrogeneration of hydrogen and iodine from acid solutions. Subsequently, the iodine, solubilized as triiodide in aqueous hydriodic acid, is reacted with H2S to yield elemental sulfur. The utility of this process rests on the successful demonstration of technology in three critical areas, namely: (1) the electrochemical oxidation of iodide in aqueous HI solution; (2) the efficient scrubbing of H_,S gas with acid triiodide solutions; and (3) the production of elemental sulfur of comparable quality to that presently recovered from petroleum and natural gas sources. We have shown that in an electrolyte consisting of 1.5 M I: in 5.5 M HI, iodide can be electrochemically converted into iodine at very high current densities and current efficiencies. The overpotential for this reaction on a graphite electrode amounts to 0.11V per 100 m A cm-'-. Under the conditions of high current density ( 5 0 0 m A c m - 2 ) , graphite electrodes have exhibited good stability for runs on the order of 5 h. Obviously, much longer-term qualification would be required before process implementation could be considered. We have concentrated our efforts on the anode reaction and have done little to elucidate the optimum cathode material and catholyte. Our choice of platinized platinum on titanium was one of convenience, and subsequent economic evaluations will probably dictate a less costly cathode material. Once this is defined, the nature of the catholyte will be more easily optimized. It should be noted that a very significant voltage penalty is paid to compensate for the resistance of both electrolytes and the ion-conductive membrane. The majority of the voltage drop arises from the electrolyte resistivity. This can be initially addressed and minimized by appropriate design of the electrochemical cell. While the cell we used in our experiments has an inter-elec-
161
trode spacing of ca 13 mm, another similar commercial cell also marketed by the same supplier has an interelectrode spacing of approximately half this value and could be advantageously utilized. While the design of our chemical reaction vessel, in which H2S-containing gas is contacted with the acidic triiodide solution, has allowed for virtually complete absorption and reaction of the H2S, the equilibrium studies performed in a closed vessel have indicated that acidic triiodide solutions are not as effective as the traditional scrubbing solution, monoethanolamine, for removing H2S from the gas phase. This will have an adverse impact on the utilization of this technology in that larger than usual gas contactors will have to be employed. This would increase the capital expense of process implementation. Selected applications could still present economic opportunities. In our laboratory demonstration, the sulfur product isolated from the reaction vessel represented 99.3% of the sulfur injected as hydrogen sulfide. Evidence for the remaining 0.7% was found throughout the system indicating the need to provide for more efficient product confinement. The product isolated from the reaction vessel was a reddish-brown sticky mass containing 90-92% elemental sulfur. Treatment of this plastic mass by dissolution in toluene with heating released the sulfur for subsequent recovery. During this recrystallization procedure, no evidence for any solid-phase contaminants was found, and it was concluded that the 8-10% (by weight) constituent was elemental iodine and entrained electrolyte. Assuming that this 8--10% represented only iodine, empirical formulations of 5361 to $46I can be calculated. These formulas indicate that an approx. 2-3 atom percent iodine contamination is sufficient to alter the properties of elemental sulfur to those observed for the product of the reaction between acid triiodide and hydrogen sulfide. While the chemistry of binary compounds of sulfur and iodine is quite sketchy, some allusions to iodine-terminated polymeric sulfur chains of the form I-Sx-I have been made [12]. These types of polymers could, in fact, be similar to the material recovered from our acidic solutions. To adhere to the general bi-capped unbranched polymer formulation, our materials would be considered on the average to be 57212 tO 59212. The properties of the acid sulfur also are reminiscent of those of amorphous sulfur which is made in elementary chemistry experiments by pouring molten sulfur into cold water. This type of sulfur contains long helical polymers and immediately upon forming exhibits plastic properties such as the ability to be drawn into fibers. Amorphous sulfur is also insoluble in carbon disulfide and slowly hardens into a rock-like mass, both properties similar to our acid sulfur. Amorphous sulfur slowly transforms upon standing to a crystalline form. The toluene recrystallization procedure is capable of transforming acid sulfur into elemental sulfur of low iodine content (100-150ppm) and has been included in our overall process flow sheet. This recrystallization
D. W. KALINA AND E. T. MAAS, JR.
162
is straightforward and employs a reagent which is usually readily available in petroleum refineries. Other factors, which we have not directly addressed, must be considered for adapting the electrolytic acidic iodine process for potential applications. Acidic triiodide solutions are quite corrosive, and reaction vessels must be constructed of materials which will retain their integrity in contact with them. Likewise, the partial pressure of iodine over acidic triiodide solutions is significant, and means must be provided to minimize the migration and loss of iodine vapor. Attempts to accomplish this have been made by others [13]. Finally, carbon dioxide, another acid gas often found with H2S in gas streams and removed simultaneously with it by amine scrubbing, will not interfere with the acidic process. Carbon dioxide is virtually insoluble in acid solutions and will pass through with other inert gases. In fact, the acidic triiodide process can be viewed as a means of removing H2S from CO2.
offers a source of hydrogen competitive with other presently utilized sources. REFERENCES
1. Chemical Week, 17 August 1983, pp. 24-25. 2. A. Kohl and F. Riesenfeld, Gas Purification, pp. 410--421, Gulf Publishing Company, Houston, Texas, U.S.A. (1979). 3. R. W. Bartlet, D. Cubicciotti. D. L. Hildenbrand, D. D. Macdonald, K. Semran and M. E. D. Raymont, Preliminary evaluation of processes for recovering hydrogen from hydrogen sulfide, Final Report, SRI Project No. 8030, JPL Contract No. 955272, SRI International, Menlo Park, California, U.S.A. (July 1979). 4. P. W. Bolmer, U.S. Patent No. 3,409,520 (1968). 5. F. Fischer, U.S. Patent No. 1,891,974 (1932). 6. H. F. Keller, Jr., U.S. Patent No. 3,401,101 (1968). 7. Ref. 2, p. 495. 8. E. T. Maas and D. W. Kalina, U.S. Patent Application No. 604,460. 9. J. Basset, R. C. Denney, G. H. Jeffery and J. Mendhan.
Vogel's Textbook of Quantitative Inorganic Analysis, p. CONCLUSIONS We have demonstrated an acidic process to convert hydrogen sulfide into hydrogen and sulfur. While we have addressed and optimized a number of critical components of the overall process, other technological questions remain. These wiU-be answered when the perceived process economics indicate that this approach
10. 11.
12. 13.
384. (4th edn) Longman, London (1978). Ref. 9, p. 377, 386. A. S. Downs and C. J. Adams in Comprehensive Inorganic Chemistry, Vol. 2, (J. C. Bailer, Jr., H. J. Emeleus, R. Nyholm and A. F. Trotman-Dickenson, eds.), p. 1540. Pergamon Press, Oxford (1973). R. Stuedet and H. J. Maensle, Angew. Chem. Int. Ed. Eng. 18, 152 (1979). C. K. Deem, U.S. Patent No. 4,220,505 (1980).
0360-3199/85 $3.00 + 0.00 Pergamon Press Ltd. ~) 1985 International Association for Hydrogen Energy.
INDIRECT H Y D R O G E N SULFIDE CONVERSION--I. AN ACIDIC ELECTROCHEMICAL PROCESS D. W. KALINA and E. T.
MAAS,
JR.
Standard Oil Company (Indiana), Amoco Research Center, P.O. Box 400, Naperville, IL 60566, U.S.A. (Received 10 October 1984) indirect hydrogen sulfide conversion process was explored which ultimately results in the production of elemental hydrogen and sulfur. The process is based on the electrochemical oxidation of iodide in aqueous hydriodic acid at high current densities and current efficiencies. Hydrogen gas is produced concurrently with soluble triiodide. Hydrogen sulfide gas was reacted with electrolyte solutions containing triiodide to yield an impure sulfur product recovered in an unusual sticky plastic form. Recrystallization of this product from toluene yielded sulfur of comparable purity to that presently produced. Abstract--An
can be coupled with the hydrogen evolution reaction to produce a quantity of hydrogen gas equivalent to that chemically contained in the hydrogen sulfide treated. This paper summarizes our work in developing a low pH process.
INTRODUCTION Hydrogen sulfide represents, in a less-than-agreeable form, the source of two elements which individually have significant economic value. In 1982, conversion of hydrogen sulfide, obtained from petroleum treating and natural gas sweetening processes, yielded 4.2 × 106 long tons of sulfur or approximately 50% of all the elemental sulfur produced in the United States that year [1]. The amount of hydrogen stoichiometrically associated with this amount of sulfur in the form of H2S is 1.1 × 1011SCF. Presently, only sulfur is recovered from hydrogen sulfide using, primarily, established Claus technology wherein the H2S is partially oxidized to yield water and sulfur [2]. A portion of the heating value of the hydrogen is recovered as steam from the exothermic Claus reaction. Recovery of elemental hydrogen to utilize its chemical value would represent a significant supplement to the hydrogen requirements of heavy crude oil upgrading and coal liquifaction and gasification. Various approaches to hydrogen recovery from hydrogen sulfide have been considered [3]. Direct oxidation of sulfide ions at an electrode formed the basis of an electrochemical process explored by Bolmer [4]. In that process, production of elemental sulfur was reported to result in deposition of a blocking sulfur layer on the electrode, which could be removed using a vaporizing solvent extraction technique. Indirect electrochemical conversion processes have also been investigated [5, 6]. One of these, utilizing soluble halogen oxidants in buffered solutions at pH = 7-8.5, has been commercialized with little apparent economic success
PROCESS CONCEPTS Iodine oxidants are ideally suited for H2S conversion. In acidic solutions, iodine, as triiodide ions (Iff), will oxidize sulfide only to elemental sulfur. Aqueous chlorine or bromine reacts with dissolved sulfide to yield soluble species containing sulfur in positive oxidation states. The iodide/triiodide couple fulfills other process requirements, namely: (1) stability under anticipated operating conditions, (2) no unwanted side reactions with H2S, (3) high solubility of both the oxidized and reduced forms, and (4) uncomplicated and well-documented electrochemistry. A generalized process flow sheet is shown in Fig. 1. This flow sheet is applicable to either large-scale gas sweetening processes or tail gas clean-up. The process depicted consists of two main unit operations, namely, electrochemical conversion and chemical reaction. In the electrochemical section, the soluble oxidant (Iff) is generated from acidic iodide solution. Concurrent with this oxidation is the reduction of protons to yield hydrogen gas. These two electrochemical reactions, the anodic reaction (equation 1) and the cathodic reaction (equation 2), performed simultaneously at their respective electrodes in the electrochemical cell, make up the overall electrochemical process (equation 3).
[7]. We have chosen to explore indirect electrochemical conversions of hydrogen sulfide using iodine oxidants in acid solution ( p H = 0 - 1 ) and in basic solutions (pH = 13-14) [8]. In these pH regions oxidized iodine species can be electrochemically generated in very high concentrations. These species, I~ in acid and I O ; in base, react with hydrogen sulfide yielding elemental sulfur. Electrochemical regeneration of spent oxidant - -
31- ~ I~ + 2e-
(1)
2H + + 2e- --~ H2
(2)
3I- + 2H" ---, I~ + H2.
(3)
In the chemical reaction section, a gas stream rich in H2S is contacted with triiodide solution, and sulfur is generated according to the reaction in equation (4). I2(as I~) + H2S ~ 2H ÷ + 21- + S. 157
(4)
D. W. KALINA AND E. T. MAAS, JR.
158
Oxidant-rich anolyte
H2S-poor gas stream
"1 Anolyte reservoir
Chemical reactor
r
Electrolysis cell
I I H2S-rich gas stream
Oxidant-rich slip stream
Catholyte reservoir
I Oxidant-depleted anolyte
Fig. 1. Schematic flowsheet of electrochemical H2S conversion process.
This reaction is utilized in standard quantitative analytical determinations of sulfide [9]. We believe that three critical technological components control the viability of the overall process. These are: (1) the anode reaction; (2) the H2S absorption in acid; and (3) sulfur production. For successful operation, the anode reaction (i.e. the oxidation of acidic iodide) must proceed at high current density and efficiency while requiring low overvoltage. Likewise, the anode material must exhibit long-term stability during the electrochemical process. The use of an acidic scrubbing solution to absorb an acid gas (in this case, H2S) is contrary to most preconceived ideas and demonstrated processes, which usually employ caustic scrubbers to absorb acid gases. In this acidic approach, the driving force for H2S removal from the gas phase is the thermodynamics associated with the redox reaction in equation (4). In order to compete favorably with other H2S treating processes, this acidic process must yield a sulfur product of comparable quality to that of, for example, the Claus process. Commodity sulfur is produced typically at a 99.95% purity level with no individual contaminant being present at greater than the 100 ppm level. EXPERIMENTAL The electrochemical unit operation was based on the commercially available MP ('Multipurpose') electrolytic cell developed and marketed by the Swedish National Development Corporation. The MP cell employs a parallel plate electrode configuration with an anode-cathode separation of c a 13 mm and an electrode surface area of 100 cm 2. This cell allows delivery of separate anolyte and catholyte feed streams and maintains electrolyte isolation inside the cell via a separator or ionexchange membrane sandwiched between the electrodes.
Electrolyte reservoirs, centrifugal pumps, and associated plumbing (chemically resistant polyvinylidene fluoride or polypropylene throughout) complete the electrolytic section. Reservoirs have been employed for both electrolyte solutions in order to aid in the removal and metering of hydrogen gas from the catholyte and to insure the supply of an oxidant-rich anolyte slipstream to the H2S reactor. The slip-stream/reservoir approach allows the use of flow rates through the MP cell meeting equipment specifications (1-151 min -l); flow rates far below specifications would be required in a design for the electrochemical generation of oxidant-rich anolyte from an extremely depleted anolyte in a single pass through the cell. Electrolyte temperatures and electrode voltages were monitored in-line via thermocouples (inside mercury-filled ceramic wells) and standard calomel reference electrodes (SCE) immersed in each electrolyte flow downstream from the MP cell. Electrode voltages vs SCE downstream from the cell agreed well with voltages vs Ag/AgC1 micro reference electrodes inserted inside the cell itself. Use of the micro reference electrodes has caused leakage problems, however, and measurements with SCE downstream were generally recorded. Hydrogen production was measured with a calibrated wet-test meter attached to the catholyte reservoir. The iodine content of the anolyte was determined by thiosulfate titration using a starch/visual endpoint indicator [10]. The electrochemical section has been operated on a continuous basis with the anolyte and catholyte each being circulated in closed loops through the appropriate sides of the electrochemical cell. Power was supplied to the cell by an external d.c. source capable of passing 0-100 A at 0-10 V. A typical experiment consisted of a continuous 5 h run with the current being controlled at 10, 20, 30, 40 and 50 A for 1 h each. The reaction between the triiodide solution and H2S was carried out in a cylindrical glass reactor approxi-
159
INDIRECT HYDROGEN SULFIDE CONVERSION--I mately 30 cm tall and 5 cm in diameter. The triiodide solution was the continuous phase and the H2S-containing gas (25%H2S/75%N2) was injected into the liquid phase through a sintered-glass gas dispersion tube. Liquid electrolyte was pumped into the reactor through a polypropylene tube. Both the gas and liquid inlets were located near the bottom of the reactor. The chemical reactor was stirred at the bottom using a magnetic stirring apparatus. The gas bubbles flowed upward in the same direction as the electrolyte flow and exited out the chemical reactor through a tube passing through the top cover of the reactor. The electrolyte spilled over through a sidearm attached near the top of the reactor. The liquid formed a gas trap at this point to insure isolation of all of the H:S-containing gas in the chemical reactor. At the termination of each experimental run, the sulfur product was recovered manually from the chemical reactor where it had been deposited in the form of a sticky, plastic mass (vide infra). This crude product was recrystallized from toluene to yield purified sulfur. The residual iodine content of the sulfur was determined by X-ray fluorescence. The off-gas from the chemical reactor was directed into a caustic scrubber. Hydrogen sulfide absorbed in this scrubber was determined by potentiometric titrations using standardized Pb 2. solutions. The ability of acid triiodide solutions to remove H:S from a gas was quantified by a series of closed-vessel equilibrium reactions in which measured amounts of H2S gas were injected over measured volumes of scrubber solutions (both acid triiodide and monoethanolamine). After equilibrium was attained, samples of the gas phase were analyzed for H2S content.
RESULTS
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The cathode material utilized in the MP cell for our process was platinized platinum on titanium. Variation in acid identity and concentration (Table 2) indicated that 3 M HCI minimizes both cathode overpotential ( - 0 . 5 4 V vs SCE at 500 mA cm -2) and resistive losses of the aqueous electrolyte and membrane cell separator. Three proton conductive membranes were evaluated in the MP cell experiments. These were (1) Nation ® 390, a fabric-reinforced membrarie 0.30 mm thick; (2) Nation ® 117, an unsupported membrane 0.19mm thick; and (3) Raipore ® R1010, an unsupported membrane 0.05 mm thick. All three gave reasonable performance in this process. As expected, the voltage drops associated with the resistances of the membranes followed their thicknesses with the Raipore ® R1010 requiting only 0.03V to overcome its resistance at 500 m A cm -2.
Electrochemistry Oxidation of acidic iodide, to form soluble triiodide, together with the reduction of protons to yield hydrogen gas, was carried out at commercially interesting current densities of 1 0 0 - 5 0 0 m A c m - : using the MP electrochemical cell. The anode of choice for the iodide oxidation was graphite. No adverse effects were noted for these anodes in 1.5 M I: + 5.5 M HI anolyte. High density graphite ( 1 . 8 g c m -3) was employed to minimize leakage of the electrolyte through the electrode. Typical current-voltage behavior for such graphite anodes in the above-noted electrolvte is illustrated in Fig. 2. Very high current densities (500mAcro -2) were attainable at 0.71 V vs SCE with no indication of any concentration polarizations. Measured current efficiencies f o r I ; generation were essentially 100%. Not unexpectedly, lower initial I: concentrations in the anolyte resulted in lower anode potentials as indicated in Table 1. This trend reflects the availability of I- for oxidation at the anode based on the equilibrium [11] in equation (5). I- + I : ~ - I r K = 700.
(5)
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The ability of acidic triiodide solutions to remove H2S from a gas was demonstrated by a series of closedcontainer equilibrium reactions in which measured amounts of H2S gas were injected over measured volumes of acidic triiodide solutions. After equilibrium was attained, samples of the gas were analyzed for H2S content. Similar reactions were carried out using the traditional H2S scrubber, monoethanolamine dissolved
Table 1. Anode potentials at 500 mA crn-2 for graphite plate in 5.5 M HI anolyte with variable I2 concentration 12 Concentration (M)
Anode potential vs SCE (V)
1.0 1.8 2.0 2.4
0.70 0.83 0.85 0.90
D. W. KALINA AND E. T. MAAS, JR.
160
Table 2. Cathode potentials and electrolyte + membrane potential drop for platinized platinum on titanium in various inorganic acid catholytes
Catholyte
Cathode potential vs SCE (V)
Electrolyte* + membrane potential drop (V)
-0.54 -0.54 -1.00 --0.64 -0.83
0.70 0.97 1.1 0.75 1.4
3 M HCI 6 M HC1 1.5 M H2SO4 3 M HESO4 9 M H2SO4
Sulfur recovery
* Includes I2 + HI anolyte potential drop. in water. The results of these comparative experiments are shown in Fig. 3. These equilibrium data indicate that on a per-mole-of-scrubber basis, the reaction of H2S with I~ leaves more H2S in the gas phase than does the traditional HaS scrubber, monoethanolamine. This difference is not large enough to preclude the use of the triiodide solutions as H2S scrubbers. However, the data indicate that in order to provide for equivalent H2S removal, larger gas contactors will have to be provided for a triiodide scrubbing process. In the process experiments, the triiodide electrolyte was delivered into the chemical reactor at the rate of ca 5.8 x 10-2 mole I= min -l. This should provide for 5.8 x 10-2moleH2Smin -1 capacity. Gas flow rate to the reactor was adjusted to follow the current level in the electrochemical cell, with 220ccmin -1 of gas injected at 10 A ranging to 1100 cc min -~ of gas at 50 A. The gas employed was H2S in nitrogen with a typical analysis of 25% H2S. These flow rates corresponded to molar flow rates of c a 2 . 3 x 10-3-1.1 x 10-2mole H2S per minute. The ratio of I2/H:S ranged from 25 to 5. In most experiments, there was no indication of any H2S breakthrough from the scrubbing solution. But in a few random experiments and for no apparent reason, there was some H2S breakthrough even though the
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I2/H2S ratio remained high. This can presumably be attributed to a variation in gas bubble size and dispersion caused by build-up of sulfur deposits in critical places in the reactor. It was virtually impossible to observe anything like this visually while the reaction was proceeding due to the opaque coloration of the triiodide solution.
2
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Fig. 3. Comparative equilibrium H2S contents in the gas phase over various scrubber solutions as a function of initial H2S/ scrubber ratios.
Sulfur produced by the reaction of H2S and acidic triiodide is of a different physical and possibly chemical form from that normally encountered. Whereas sulfur is usually observed as a microcrystalline, yellow, freely flowing solid, the triiodide-produced sulfur is deposited in the reaction vessel as a reddish-brown, sticky mass. This gum-like product converts slowly (of the order of hours) into a hard solid, still of the same color. Analysis of this 'acid' sulfur (as we have termed it) indicates that it contains 90-92% (by weight) recoverable sulfur. The other 8--10% is presumably entrained electrolyte as well as excess iodine. Assuming, as the worst possible case, that the entire non-sulfur component of acid sulfur consists only of iodine in some form, we can calculate empirical formulations for acid sulfur ranging from $36I to $46I. Experimentally, we have found that virtually all the acid sulfur (99.3%) produced remained in the reaction vessel in which it was originally made. Very small amounts were carried over as entrained particles into subsequent process vessels. In the reaction vessel, the acid sulfur exhibited a definite preference for the type of surface to which it adheres. In all of the experiments performed, the acid sulfur was found adhering to components made from both polypropylene and Teflon ®. Glass surfaces in extremely close proximity to these organic polymer components showed no evidence of being coated with this rather strange material. Even though the acid sulfur formed a solid deposit on the plastic components in the reaction media, the product could be separated from these components rather easily by manually peeling it off or else by heating it to liquify the acid sulfur as described below. While the acid sulfur is a sticky viscous mass when fresh and hardens with aging, the material can be transformed into a freely flowing liquid by heating it to 95-105 °C. This melting results in release of some iodine and transformation of the 'acid sulfur' into a more conventional form of sulfur. Even after melting, however, this sulfur still contains significant quantities of iodine, and further treatment such as recrystallization is needed to yield an acceptable product. We have demonstrated that the melting and recrystallization steps can be combined. Treatment of the solid acid sulfur with boiling toluene initially results in the melting of the acid sulfur in contact with the organic, yielding a two-phase system. Continued reflux of the organic solvent effects the dissolution of the liquid sulfur phase into the organic solvent. Cooling the now single-phase system results in the precipitation of crystalline elemen-
INDIRECT HYDROGEN SULFIDE CONVERSION--I tal sulfur containing routinely 100-150 ppm iodine as an impurity.
Sulfur yield In a typical experiment, 2.68moles of H2S were passed into the reaction vessel over a 5-h period. Afterward, 92.47 g of acid sulfur were recovered from the system. After treatment with 1.21 of hot toluene, this acid sulfur yielded 85.35 g (2.66 moles) of sulfur for a 99.3% sulfur recovery based on the H2S added. The reaction efficiency, that is, the amount of iodine consumed compared to the amount of sulfur recovered, was found to be essentially 100%. DISCUSSION Hydrogen sulfide, a waste product from petroleum refining and natural gas production, represents an untapped resource for significant quantities of hydrogen. We have demonstrated a process that goes beyond presently utilized hydrogen sulfide treating technology and results in the conversion of H:S into elemental hydrogen and sulfur. This process is based on the concurrent electrogeneration of hydrogen and iodine from acid solutions. Subsequently, the iodine, solubilized as triiodide in aqueous hydriodic acid, is reacted with H2S to yield elemental sulfur. The utility of this process rests on the successful demonstration of technology in three critical areas, namely: (1) the electrochemical oxidation of iodide in aqueous HI solution; (2) the efficient scrubbing of H_,S gas with acid triiodide solutions; and (3) the production of elemental sulfur of comparable quality to that presently recovered from petroleum and natural gas sources. We have shown that in an electrolyte consisting of 1.5 M I: in 5.5 M HI, iodide can be electrochemically converted into iodine at very high current densities and current efficiencies. The overpotential for this reaction on a graphite electrode amounts to 0.11V per 100 m A cm-'-. Under the conditions of high current density ( 5 0 0 m A c m - 2 ) , graphite electrodes have exhibited good stability for runs on the order of 5 h. Obviously, much longer-term qualification would be required before process implementation could be considered. We have concentrated our efforts on the anode reaction and have done little to elucidate the optimum cathode material and catholyte. Our choice of platinized platinum on titanium was one of convenience, and subsequent economic evaluations will probably dictate a less costly cathode material. Once this is defined, the nature of the catholyte will be more easily optimized. It should be noted that a very significant voltage penalty is paid to compensate for the resistance of both electrolytes and the ion-conductive membrane. The majority of the voltage drop arises from the electrolyte resistivity. This can be initially addressed and minimized by appropriate design of the electrochemical cell. While the cell we used in our experiments has an inter-elec-
161
trode spacing of ca 13 mm, another similar commercial cell also marketed by the same supplier has an interelectrode spacing of approximately half this value and could be advantageously utilized. While the design of our chemical reaction vessel, in which H2S-containing gas is contacted with the acidic triiodide solution, has allowed for virtually complete absorption and reaction of the H2S, the equilibrium studies performed in a closed vessel have indicated that acidic triiodide solutions are not as effective as the traditional scrubbing solution, monoethanolamine, for removing H2S from the gas phase. This will have an adverse impact on the utilization of this technology in that larger than usual gas contactors will have to be employed. This would increase the capital expense of process implementation. Selected applications could still present economic opportunities. In our laboratory demonstration, the sulfur product isolated from the reaction vessel represented 99.3% of the sulfur injected as hydrogen sulfide. Evidence for the remaining 0.7% was found throughout the system indicating the need to provide for more efficient product confinement. The product isolated from the reaction vessel was a reddish-brown sticky mass containing 90-92% elemental sulfur. Treatment of this plastic mass by dissolution in toluene with heating released the sulfur for subsequent recovery. During this recrystallization procedure, no evidence for any solid-phase contaminants was found, and it was concluded that the 8-10% (by weight) constituent was elemental iodine and entrained electrolyte. Assuming that this 8--10% represented only iodine, empirical formulations of 5361 to $46I can be calculated. These formulas indicate that an approx. 2-3 atom percent iodine contamination is sufficient to alter the properties of elemental sulfur to those observed for the product of the reaction between acid triiodide and hydrogen sulfide. While the chemistry of binary compounds of sulfur and iodine is quite sketchy, some allusions to iodine-terminated polymeric sulfur chains of the form I-Sx-I have been made [12]. These types of polymers could, in fact, be similar to the material recovered from our acidic solutions. To adhere to the general bi-capped unbranched polymer formulation, our materials would be considered on the average to be 57212 tO 59212. The properties of the acid sulfur also are reminiscent of those of amorphous sulfur which is made in elementary chemistry experiments by pouring molten sulfur into cold water. This type of sulfur contains long helical polymers and immediately upon forming exhibits plastic properties such as the ability to be drawn into fibers. Amorphous sulfur is also insoluble in carbon disulfide and slowly hardens into a rock-like mass, both properties similar to our acid sulfur. Amorphous sulfur slowly transforms upon standing to a crystalline form. The toluene recrystallization procedure is capable of transforming acid sulfur into elemental sulfur of low iodine content (100-150ppm) and has been included in our overall process flow sheet. This recrystallization
D. W. KALINA AND E. T. MAAS, JR.
162
is straightforward and employs a reagent which is usually readily available in petroleum refineries. Other factors, which we have not directly addressed, must be considered for adapting the electrolytic acidic iodine process for potential applications. Acidic triiodide solutions are quite corrosive, and reaction vessels must be constructed of materials which will retain their integrity in contact with them. Likewise, the partial pressure of iodine over acidic triiodide solutions is significant, and means must be provided to minimize the migration and loss of iodine vapor. Attempts to accomplish this have been made by others [13]. Finally, carbon dioxide, another acid gas often found with H2S in gas streams and removed simultaneously with it by amine scrubbing, will not interfere with the acidic process. Carbon dioxide is virtually insoluble in acid solutions and will pass through with other inert gases. In fact, the acidic triiodide process can be viewed as a means of removing H2S from CO2.
offers a source of hydrogen competitive with other presently utilized sources. REFERENCES
1. Chemical Week, 17 August 1983, pp. 24-25. 2. A. Kohl and F. Riesenfeld, Gas Purification, pp. 410--421, Gulf Publishing Company, Houston, Texas, U.S.A. (1979). 3. R. W. Bartlet, D. Cubicciotti. D. L. Hildenbrand, D. D. Macdonald, K. Semran and M. E. D. Raymont, Preliminary evaluation of processes for recovering hydrogen from hydrogen sulfide, Final Report, SRI Project No. 8030, JPL Contract No. 955272, SRI International, Menlo Park, California, U.S.A. (July 1979). 4. P. W. Bolmer, U.S. Patent No. 3,409,520 (1968). 5. F. Fischer, U.S. Patent No. 1,891,974 (1932). 6. H. F. Keller, Jr., U.S. Patent No. 3,401,101 (1968). 7. Ref. 2, p. 495. 8. E. T. Maas and D. W. Kalina, U.S. Patent Application No. 604,460. 9. J. Basset, R. C. Denney, G. H. Jeffery and J. Mendhan.
Vogel's Textbook of Quantitative Inorganic Analysis, p. CONCLUSIONS We have demonstrated an acidic process to convert hydrogen sulfide into hydrogen and sulfur. While we have addressed and optimized a number of critical components of the overall process, other technological questions remain. These wiU-be answered when the perceived process economics indicate that this approach
10. 11.
12. 13.
384. (4th edn) Longman, London (1978). Ref. 9, p. 377, 386. A. S. Downs and C. J. Adams in Comprehensive Inorganic Chemistry, Vol. 2, (J. C. Bailer, Jr., H. J. Emeleus, R. Nyholm and A. F. Trotman-Dickenson, eds.), p. 1540. Pergamon Press, Oxford (1973). R. Stuedet and H. J. Maensle, Angew. Chem. Int. Ed. Eng. 18, 152 (1979). C. K. Deem, U.S. Patent No. 4,220,505 (1980).