1601
Ind. Eng. Chem. Res. 1991,30,1601-1608 Conference,San Fianckco; TAPPI Press: Atlanta, GA, 1984, pp 1-12.
Same, D.A.; Shadman, F. Mechanism of potassium-catalyzed carbon/C02 reaction. AIChE J. 1986,32(7), 1132-1137. Shadman, F.; Same, D. A,; Punjak, W.A. Significanceof the reduction of alkali carbonate in catalytic carbon gasification. Fuel 1987, 66(12), 1658-1663. Smith, J. M.; Van Ness, H. C. Introduction to Chemical Engineering Thermodymmics, 3rd ed.; McGraw-Hill: New York, 1982; pp 300.
tion of coal char. Fuel 1978,57, 194. Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G.Gas reactions of carbon. Adv. Catal. 1959,11,133-221. Wigmans, T. Catalytic gasification of carbon; A mechanistic study. Ph.D. Dissertation, University of Amsterdam, The Netherlands, 1982.
Received for review December 17, 1990 Accepted January 23, 1991
Veraa, M. J.; Bell, A. T. Effect of alkali metal catalysts on gasifica-
Hydrogen Production from Hydrogen Sulfide by the Fe-CI Hybrid Process Susumu Mizuta,* Wakichi Kondo, and Kinjiro Fujiit Materials Research Division, National Chemical Laboratory for Industry, Tsukuba, Zbaraki 305, Japan
Hiroshi Iida, Shingo Isshiki, Hiroshi Noguchi, Tohru Kikuchi, and Harufusa Sue Central Research Laboratories, Idemitsu Kosan Company, Ltd., 1280 Kami-izumi, Sodegaura-machi, Kimitsu-gun, Chiba 229-02, Japan
Koshiro Sakai Ebara Company, 1-6-27 Kohnan, Minato-ku, Tokyo 108, Japan
The Fe-C1 hybrid process consisting of the absorption of H2S gas by FeC& aqueous solution and subsequent electrolysis of FeClz aqueous solution was proposed as a process for decomposing H2S to H2 and S. From the fundamental study, H2S absorption with almost 100% yield a t 75 O C and satisfactorily low electrolysis voltage (0.7V) a t a current density of 100 mA cm-2 at 70 O C were obtained. In the chemical engineering study for process development, a bubble-column H a absorber with a rotating-cup type gas feeder and a bipolar type electrolysis cell (5 or 20 cells with an electrode area of 432 cm2) were developed. A bench-scale plant for the whole Fe-C1 hybrid process with a production capacity of 2.1 N ms of H2 day-' and 3 kg of S day-' was constructed, and intermittent operation of the plant was successfully performed for 1000 h.
1. Introduction Industrial treatment of hydrogen sulfide has so far been carried out mainly by two processes: the Claus process and the wet absorption process. At present, almost all of the H a generated by desulfurization of fossil fuels is treated by the Claus process, where H2Sis converted to elemental sulfur and water through a partial oxidation reaction. The amount of sulfur produced by the Claus process currently runs up to one million tons per year in Japan alone, which corresponds to treatment of 700 million N m3 H2S. This process has been successfully used for many years; however, there are still several disadvantages, as follows. (i) Since the hydrogen component of H2S is oxidized to water, the H2 introduced to the hydrodesulfurization process is consumed and therefore unrecoverable. (ii) Additional tail gas treatment such as the SCOT process is necessary because the total conversion of the Claus process is not sufficiently high (90-98%). (iii) As the refinery acid gas generated from the hydrodesulfurization process is a mixture of hydrocarbon, H2, and Ha,separation of the mixed gases (amine absorption process) is required prior to the Claus process. (iv) Because of the difficulties in controlling the very high temperature gas reaction and maintenance of the catalysts, the Claw process is considered to be not flexible enough to immediately adjust to changes in the load. *Towhom correspondence should be addressed. 'Deceased on May 6, 1989.
On the other hand, the wet absorption process has no such disadvantages as (ii), (iii), or (iv) described above since absorption of H2Sby an aqueous solution of an alkaline or ferric compound is very fast and selective. In practice, an aqueous solution of sodium carbonate or ferric complexed compounds has been used under neutral or weakly alkaline conditions (Craggs and Arnold, 1947;Riesenfeld and Kohl,1974). However, it is still hydrogen-consuming and available only when the amount or concentration of H2S gas is small or low because the rate of regeneration of alkaline or ferric compound by oxidation with air is quite slow. Besides these industrialized hydrogen-consuming processes, decomposition of H2S into H2 and S has been widely studied by pyrolytic, electrolytic, thermochemical (Raymont, 1979),and photolytic (Borgarello and Graetzel, 1982)processes. A hybrid system of the wet absorption of H2S and electrolysis has also been studied for Ha decomposition by use of a redox system such as Fen(CN)6-Fem(CN)6(Fischer, 1932)or 12-1- (Kalina and Maas, 1985a,b). The Fischer procesa was specially developed into a bench-scale plant (Muller, 1931;Thau, 1932);however, it was finally considered less economical or commercially unfeasible because of ita high electrolysis voltage (1.S2.2 V at a current density of 2W30 mA cm"). 2. Proposal of the Fe-Cl Hybrid Process
To solve such problems as described above, the present authors (Fujii et al., 1982;Mizuta et al., 1984;Kondo et al., 1984) proposed the Fe-Cl hybrid process, which is suitable even for treatment involving a large amount of highly concentrated H2S. As shown in Figure 1, this
0888-6886/91/2630-1601$02.60/00 - 1991 American Chemical Societv
1602 Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991
( H i l
Table 1. Experimental Conditionr for the Abwrption of HA by FeC12-FeCls-HCl Solution HCl concnl symbolin run (mol (kg liquid bubble Figure 2 no. temp/'C of H*O)-') height/cm diam/mm
t
0
1
0 A
2 3 4
0
5
A
6
75 35 75 75 75 75
5.0 5.0 2.5 8.0 5.0 5.0
1.2 1.2 1.2 1.2 1.2
26
26 26 26 52 26
0.4
100
@
i
( H+ydHr;nrbon)
(Sulfur)
)
[Elecrrolyler]
=
e n ::
Figure 1. Outline of the Fe-Cl hybrid process.
process consists of the absorption of H2S gas by FeC1, aqueous solution and subsequent electrolysis of FeC1, aqueous solution, totally decomposing HzS to Hz and S: H,S(g) + 2FeC13(aq) 2FeCl,(aq) + 2HCl(aq) + S(c) (R1)
-
-
2FeCl2(aq) + 2HCl(aq)
electrolysis
HAg) + 2FeCb(aq) (R2) 033) net: HZSk) HZk) + S(c) This process is carried out under strongly acidic conditions by using a large excess of HC1 in contrast with the other conventional processes. In the reaction R1, H2S gas reacts smoothly with aqueous FeCl,, where solid sulfur and aqueous HC1 are produced together with a reduction of FeC1, to FeC1,. Since the standard Gibbs free energy change (AGO) of (Rl) is sufficiently negative, -27.6 kcal mol-', the extent of the H2S absorption reaches more than 99% at 70 "C, even in the presence of excess HC1. After the particles of precipitated sulfur are filtered, the resultant filtrate of the FeC1, and HCl aqueous solution is electrolyzed, where Hz is evolved at the cathode while FeC13 is recovered at the anode by oxidation of FeCl,, as shown in reaction R2. In this electrolysis, the present authors observed that the electrolysis voltage decreased a great deal by addition of excess HCl due to the abnormal increase in the activity of HC1. The theoretical electrolysis voltage of (R2) is 0.77 V under standard-state conditions (25 "C, 1atm). However, when 5.0 mol of HCl (kg of H2O)-' was added to the electrolyte, the actual electrolysis voltage was found to decrease to 0.7 V at 70 "C, even at a current density of 100 mA cm-,. Though such strongly acidic conditions are obviously unfavorable to (Rl), it was considered still advantageous for the whole Fe-C1 hybrid process by virtue of the very large decrease in electrolysis voltage in (R2). The features of this process are suIllIIlLvlzed as follows: (1) Hydrogen can be recovered. (2) The rate and yield of H a absorption by FeC13solution is sufficiently high, so further treatment for tail-gas clean-up, which has been needed in the conventional Claus method, is not required. (3) The hydrogen sulfide containing hydrocarbons can be directly introduced into the process. Thus, a gas separation proceas such as that using an amine absorber column is unnecessary. After a fundamental study (proposal and laboratory-scale experiment) was performed at the National Chemical Laboratory for Industry (NCLI) (1982-1984), further studies on the process development directed toward commercializationwere done by Idemitau Kosan Co., Ltd. (IKC) and NCLI (1985-1988), where a bench-scale test
a
cn N z
90
80 70 60
"
01 in
3
025 05 075 [ Fe3'l / ( [ Fe2' 1 t [ Fe" 1 1
10
Figure 2. Extent of H2S absorption versus Fe9+fraction (m)of the solution. (Experimental conditions are shown in Table I.)
plant with a treatment size of 3 kg of sulfur day-l was constructed and successfully operated for lo00 h (Iida et al., 1990). This paper gives a comprehensive report of the Fe-Cl hybrid process, including a fundamental study and process development by the bench-scale test plant. 3. Fundamental Study 3.1. Absorption of HzS by FeCl,-FeCIS-HCl Solution. The absorption reaction of H a by FeC12-FeC13-HC1 solution was examined. Commercially available diluted H2S (30% H2S + 70% Ar) was used as starting gas since the average concentration of H a contained in the refinery acid gas was assumed to be 30%. The starting HzS gas was fed into the FeClz-FeCl3-HC1 solution in a glase bottle through a glass ball type sparger with a flow rate of 200 cm3 min-' for a period of 1-4 min. The amount of the unabsorbed H2S collected in the 12/KI solution was measured by iodometry. Experimental conditions were varied as follows: (i) absorption temperature (35 or 75 "C), (ii) fraction of Fe3+(m= [Fe3+]/([Fe2+]+ [Fe3+])= 0.1-LO), (iii) concentration of the excess HC1 (CHa = 2.5-8.0 mol (kg of H,O)-') of the starting solution, (iv) liquid height (26 or 52 cm), and (v) bubble size (diameter = 0.4-1.2 mm) of the HzS gas (see Table I). The total concentration of Fez+and Fe3+of the FeClpFeC13-HC1 solution was set at 1.0 mol (kg of H,O)-' for adaptation of the subsequent electrolysis. Experimental results are shown in Figure 2. Absorption of HzS was nearly 100% at 75 "C with m L 0.25, even in the presence of HC1 at a concentration of 5.0 mol (kg of H,O)-'. In this case, the bubble size was 0.4 mm or the liquid height was 52 cm. 3.2. Size, Shape, and State of Agglomeration of Sulfur Particles. The size, shape, and state of agglomeration of the sulfur particles produced by the absorption of H2S by the FeC12-FeC13-HC1 solution were examined. The plane gas-liquid contact method and bubble blowing methods, corresponding to a wetted-wall column and a bubble column, respectively, were tried and compared with each other. In the plane gas-liquid contact method, 70 cm3 of the starting solution with a composition of FeCl,/FeCl,/HCl = 1.5/0.5/4.0 mol (kg of HzO)-' was put into the glassmade cylindrical bottle (45" inner diameter X 90-mm
Ind. Eng. Chem. Rea., Vol. 30,Nc 7,1991 1603 b
d
e
Figure 3. Optical micrographs of the sulfur particles p r o d u d by the absorption of H a with FeC12-FeC18-HCl solution (bar indicates 100 pm): (a) plane contact at 60 "C; (b) plane contact at 73 "C; (c) plane contact at 84 O C ; (d) bubble blowing at 60 O C ; (e) bubble blowing at 73 "C.
height) and stirred slowly by a glass-covered magnet. The starting gas containing Ha (30%)was fed over the solution with a flow rate of 240 cm3min" for a period of 10-40 min. In the bubble blowing method, the m e starting gas was fed into the solution with the same composition as used in the plane gas-liquid contact method. The gas was fed through the glass ball type sparger (average bubble size is 1.2 mm) with a flow rate of 240 cm3 min-l for a period of 20-40 min; the solution was vigorously stirred throughout. After the absorption, the sulfur particles produced were separated by filtration and observed by optical microscope. The results of the plane gas-liquid contact method a t 60, 73, and 84 "C are shown in Figure 3, parts a-c, respectively, while those of the bubble blowing method at
60 and 73 "C are shown in parts d and e, respectively. In the case of the plane contact method, the sulfur particles deposited on the surface of the solution gradually agglomerated into a film and finally diffused into the solution, where the stirring rate of the solution and absorption time (10-40 min) hardly affected the agglomeration state of the sulfur particles. By comparison of Figure 3a-c, it is found that the size of the primary particles became larger with an increase in absorption temperature and most of them form secondary particles with the size of 10-50 pm by agglomeration. By comparison of parts a and d or b and e of Figure 3, it was found that agglomeration was greatly enhanced by bubble blowing. By comparison of parts d and e of Figure 3, it was also shown that agglomeration in the bubble blowing case increases with absorption temperature.
1604 Ind. Eng. Chem. Res., Vol. 30,No. 7, 1991 Table 11. Settling Characteristics of the Sulfur Particles Produced at 76 OC proportion of blowing stationary unsettled sulfur particles/ I no. time/min time/min 1 2 3 4 5
2 2 8 25 25
5 120 5 5 100
0.8 -
' 0.7*>
15.4 5.1 4.5 2.2 0.38
In the case of the bubble blowing method, the enhancement of agglomeration may be attributed not only to an increase in the collision times among the primary particles by the stirring of the solution but also to an increase in the gas-liquid contact area. On the other hand, below 55 OC the gummy sulfur was produced and it adhered to the inside of the glass bottle. This phenomenon is considered to be unfavorable from the viewpoint of sulfur recovery. 3.3. Settling Characteristics of Sulfur Particles. The settling characteristics of the sulfur particles produced by the bubble blowing method were examined. The starting gas and solution were the same as the ones used in section 3.2. The volume of the solution, liquid height, blowing rate, and reaction temperature were set at 230 an3, 20 cm, 720 cm3 min-', and 75 OC, respectively. Blowing times of 2,8,and 25 min were used. The product slurry was allowed to stand in a thermally controlled bath for 5, 100,and 120 min, and then the sulfur particles that had settled were recovered by decantation. The unsettled particles contained in the supernatant were finally agglomerated after standing for a long time and then completely recovered by filtration. The ratio of the unsettled sulfur particles in the supernatant to the total recovered is shown in Table 11. It is clear that the amount of sulfur particles settled increased with increases in blowing time and stationary time. 3.4. Electrolysis of FeC12-FeC13-HCl Solution. The theoretical electrolysis voltage of (R2)at 25 "C, denoted as Et, is expressed as follows Et = 0.77 + 0.059 log [ ~ F , C I ~ ) H ~ ~ / ~ / ~ F (1) ~ C ~ where u F ~ u, F ~ QH", , and pHz are the activities of FeCl,, FeC12, and kCl and the partial pressure of H2,respectively. When the concentrations of FeC12and FeC13are nearly equal and pHncan be set at 1atm, eq 1 can be written as (2) Et = 0.77 - 0.059 log UHCl Thermochemical data of HC1 activity for pure HC1 soluand those in the presence of (1 mol (kg of tions (aHC1) H20)" F&12 or F&13 (uH"*) obtained by the measurement of HCl vapor pressure have been reported (Lewis and Randall, 1961;Rhode, 1963). Thus, values of Et and Et* can be calculated and plotted as shown in Figure 4. The strong effects of both excess HCl and coexisting FeC12or FeC13on the decrease in Et are immediately evident. On the basis of such theoretical considerations, hydrogen evolution by the electrolytic oxidation of aqueous FeC12was experimentally confirmed by using a fuel cell type small monopolar electrolysis cell with gas diffusion electrodes. The electrolysis cell was constructed, as illustrated in Figure 1, with the following parts: (1)two graphite collectors (95 X 110 X 22 mm thick), each provided with 26 ditches (2X 50 X 1 mm depth) to allow the solution or gas to pass from inlet to outlet; (2)two gas diffusion type graphite cloth electrodes (18X 45 X 0.5 mm thick) (3.5mg of Pt cm-2 is loaded on the cathode); (3)PTFE spacers (0.3-0.5 mm thick); (4) ion-exchange membrane. The electrolytic solution (FeC12-FeC13-HC1) preheated at 70
W
k
E, (calculated by
aHc, 1
06W
0.5
~
u 12
0.40
2
4
6
8
HCI Concentration / mol ( k g H20)
Figure 4. Theoretical voltage (Et or Et*) for electrolysis of FeCh aqueous solution as a function of HCl concentration (calculated by eq 2; Et, for pure HCI solution, and Et*. for the effect of the presence of FeClz or FeCl,). 200 -
i80160-
'5 4
E
140-
120-
\
.-F
ioo-
v)
80-
E
c
60-
L
5 4020-
Figure 6. Cell voltage-current density characteristica for a variety of electrolyte compositions at 70 OC for a monopolar electrolysiscell. (The flow rate of the electrolyk was 20 cm3mi&. The electrolyte composition (FeC&/FeC18/HCl(mol (kg of H20)-')) were 1.2/0/4.8 (m = 0) (open double circle), 0.6/0.6/4.8 ( m = 0.5) (open single circle), and 0.3/0.9/4.8 (m = 0.75) (closed circle).
introduced to the anode side, while Ar or H2 as a carrier gas was supplied into the cathode side at a flow rate of 80 cm3 min-'. To prevent drying of the ion-exchange membrane, the carrier gas was saturated with water vapor at 80 O C . As a result of preliminary experiments, optimum conditions for electrolysis temperature and feeding rate of the electrolytic solution were found to be 70 OC and 20 cm3 mi&, respectively. The cell voltage-current density characteristics were examined for various conditions: (i) fraction of Fe3+(m= [Fe3+]/([Fe2*]+ [Fe3+])= 0.00,0.50, and 0.75); (ii) concentration of exceses HC1 (CH" = 2.4-10.0 mol (kg of H20)-') in the electrolytic solution; (iii) ionexchange membranes (Nafion, Neosepta, K-101);and (iv) carrier gas (Ar or H,)in the cathode. The results of the cell voltage-current density characteristics are shown in Figures 5 and 6. A satisfactorily low voltage of 0.7V was found at a current density of 100 mA cm-2 under the conditions of m I0.5 and C H C ~> 5.0 mol (kg of HzOl-', where the total concentration of Few and FeS+was 1.2 mol (kg of H,O)-'. No significant differences were observed among the membranes or between the carrier gases (H2 and Ar). Thus, product hydrogen with 100% purity was found to be obtainable since hydrogen itaelf can be used as a carrier gas for the electrolysis. 3.5. Effects of Ammonium Chloride on Electrolysis. Since coal and crude oil contain nitrogen compounds, as well as sulfur compounds, H2S produced by the hydro-
~ ~ OHC~was I ]
Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991 1605
t
0.9
h
0.4;
2'
1012 HC I Concentration / mol I kg 4
6
8
I
Figure 6. Cell voltage versus HCl concentration at current densities of 60 and 100 mA cm-l at 70 OC for a monopolar electrolysis cell. (The flow rate of the electrolyte was 20 cms mi&. The electrolyte compositions (FeCIP/FeCls/HCl(mol (kg of H20)-l)) were 0.6/0.6/ 2.4,0.6/0.6/4.8,0.6/0.6/8.0,and 0.6/0.6/10.0.)
desulfurization includes some amount of NH3. This NH3 accumulates as NH4Cl in the highly acidic iron chloride solution. We examined how the electrolysis was affected by the presence of accumulated NH4Clin the iron chloride solution. The electrolysis was carried out at 70 OC by using the small monopolar type electrolysis cell. The concentration of electrolyte was FeC12/FeC13/HC1/NH4C1 = 0.6/0.6/4.8/1.0 mol (kg of H,O)-l. The results were found to be almost the same as those without NH4Cl (Figure 5). Thus, addition of NH4Cl to the electrolyte was observed not to affect the electrolysis at all. Electrolysis of a solution of 1.0 mol of NH4Cl (kg of H,O)-' and 4.8 mol of HCl (kg of H,O)-' without iron chloride was also tried; however, no current was observed below 0.9 V. A small amount of current and hydrogen were observed above 0.9 V. Thus, any influence of the NH3 in HzS on the electrolysis can be disregarded. As described above, feasibility of the Fe-C1 hybrid process was confirmed, and results of the fundamental study are summarized as follows. The extent of the H2S absorption reached almost 100% at 75 OC, and an acceptably low voltage of 0.7 V was obtained at a current density of 100 mA cm-, at 70 OC, where the most suitable composition of the solution FeC1,/FeCl3/HC1 (mol (kg of H20)-') was found to be 0.4-0.6/0.3-0.6/5.0 (corresponding to 0.25 < m < 0.5). Agglomeration of the sulfur particles and their highly separability were also confirmed. As will be mentioned below, however, in the actual operation of the bench-scale test plant, the temperatures for the H2S absorption and electrolysis were lowered to 60 and 50 OC, respectively, in order to prevent deterioration of equipment materials. Also the total concentration of FeC1, and FeCl, of the solution was increased to 1.7 or 2.0 mol (kg of H,O)-l to reduce the amount of the circulating solution and size of the equipment. 4. Chemical Engineering Study and Process Development by Bench-Scale Test Plant 4.1. Evaluation of the Overall Volumetric Coefficient and Determination of Absorber Type. In an effort to reduce the absorber volume, a packed bubble column was tried for the absorption of HzS gas by FeC12-FeC13-HCI solution. This was done because packed bubble columns have a large contact area between gas and liquid. However, the sulfur particles that were produced adhered to the packing; therefore, it was difficult to discharge them from the absorber. Thus, use of this type of column was abandoned. A simple bubble column without packing was then studied. A cylindrical absorber (0.04-m inner diameter X 1.0-m height) was made of glass and a solution with a compo-
i-= l
/
0.2
' 3 4 5
0.11
'
1
'
I
,
L
'
10
20 30 40 50
0
Gas Superficial Velocity / m h-'
Figure 7. Relationship between overall volumetric coefficient (Koa) and gas superficial velocity UGfor both simple bubble column and improved bubble column at 60 "C. (Hfi concentration was 28%, and solution composition (FeC12/FeCls/HC1 (mol (kg of H 2 0 ) 9 ) was 0.8/1.215.0.)
sition of FeCl3/FeClZ/HC1= 1.2/0.8/5.0 mol (kg of H,O)-' (m= 0.4) was fed to the absorber using a controlled flow rate pump. A mixture of gases (28% HzS, 72% H,) was also fed to the absorber at a rate controlled by a mass flow control valve. The absorption reaction was carried out at 60 "C, and the sulfur particles produced were recovered smoothly from the absorber. To evaluata the absorption capacity of the simple bubble column, the overall volumetric coefficient was examined from a chemical engineering point of view. The overall volumetric coefficient (Koa (mol mJ h-' atm-')) is defined by the following equation KGu = 103UG(22.4ZP)-' In [Pi,/Pout]
(3)
where U, is the gas superficial velocity (m h-'), 2 is the liquid height (m), P is the total gas pressure (atm), Pi,is the H$3 partial pressure at the inlet of the absorber (atm), and Poutis the H2S partial pressure at the outlet of the absorber (atm). Experimental conditions were set as follows: Z = 0.76 m, P = 1.0 atm, Pi,= 0.28 atm, and UG = 10.8 or 27.0 m h-'. Poutwas measured, and KGu was calculated. As shown in Figure 7, the values of Koa were found to be too low; thus,we tried to make the bubble size smaller by reducing the size of the dispersing nozzle to increase the value of Koa. As the dispersing nozzle became smaller, higher values of Koa were obtained; however, clogging of the dispersing nozzle by sulfur particles took place, and such a small dispersing nozzle was considered unsuitable for this case. Next, we tried to use a rotating cup at the outlet of the gas feeder in order to make bubbles smaller by utilizing the shear force of the rotation. For this experiment, a cylindrical absorber (0.2-m inner diameter X 0.7-m height) was constructed with FRP (fiber reinforced plastics), and a rotating cup (50-mm inner diameter X 60-mm height) was placed on top of the H a feeder pipe. H,S-containing gas was continuously fed into the cup so that the gas was overflown below the cup. The rotation speed was set at 4300 rpm. This improved bubble column was operated successfully. No clogging was observed, and the sulfur particles produced were discharged
1606 Ind. Eng. Chem. Res., Vol. 30,No.7,1991
0.6
I
1
I
0.5
0.4
0.3
m = [Fe%I / ( C F P I
I
I
I
0.1
0.2
+ [FeS*l
0
1
Figure 8. Effect of Few fraction (m)of the solution on the overall volumetric coefficient for a variety of gas superficial velocities (Vo) in the improved bubble column at 60 "C ( H a concentration,2896, liquid height, 30 cm;gas superficial velocity (Uo) 23.4 m h-l (denoted by open triangle) and 46.8 m h-l (denoted by open circle)). (HCl concentration 5.0 mol (kg of HZO)'' was constant.)
smoothly from the column. Experimental conditions were set as follows: 2 = 0.3 m, P = 1.0 atm, Ph= 0.28 atm, and Uc= 5.4-64.8m h-l. The values of KGafor this improved bubble column were found to be much higher than those for the simple bubble column, as shown in Figure 7.
The effect of the solution composition on the overall volumetric coefficient (Kea) was also examined for the improved bubble column. The compition of the solution (FeCl2/FeCl3/HC1[mol (kg of H,O)-l]) was varied between 0.8/1.2/5.0(m= 0.6)and 1.8/0.2/5.0(m= O.l), while Uc was set at 23.4 or 46.8 m h-l. As shown in Figure 8,the KCa values were found to be stable and almost constant for m values higher than 0.3 when UGwas set at 46.8 m h-? 4.2. Development of the Bipolar Type Electrolysis Cell. It was difficult to stack or scale up the monopolar type electrolysis cell used in the fundamental study because the current collector was made of graphite block, which is bulky and expensive. For cost reduction and convenience in handling, various materials were surveyed for use as spacer, bipolar plate, and electrode. Then the structure, stacking, and pressing of the whole cell were also studied. The structure and a photograph of the bipolar electrolysis cell are shown in Figures 9 and 10,respectively. End plates and spacers were made of a heat-resisting poly(viny1 chloride) (PVC)resin. Current collectors and bipolar plates were made of copper and resin-bound graphite powders, respectively. Electrodes were made of _End-Plate_(PVC)
SpacerJP)/VC_! Membrane
Collector(Copper) Term ina I
-
\
7 !,
7 ' \
I
-Cathode-(Carbon-Felr) LAnode-(Carbon-Felt) Bipolar_Plate_(RBG P!
-EndPlate - ____ (PVC) , Co!ector(Coppec Figure 9. Structure of the bipolar electrolysis cell. (PVC is poly(viny1 chloride) resin, and RBGP is resin-bound graphite powders.)
'i Figure 10. Photographs of the bipolar electrolysis cell.
Ind. Eng. Chem. Res., Vol. 30, No. 7,1991 1607 N2
140 m = 0.2
H2
120 -
carrier gas
ms0.4 mg0.6
(Y
5
Flow controlled solutlon pump tonk
-t absorber
Mu flow control volvc
100-
I I
I
E
\
P"
c
.-a c v)
80-
O
60-
c
$
E
Electrolysis cell
Flow eontrolled
4
T
n
x
HCI solution
Electrolyte tank
S
3 40-
Figure 12. Simplified flow diagram of the bench-scale plant.
20 "65
0:6
017
0:8
Cell Voltage / V / Cell
0:s
'
Figure 11. Cell wltqe-cumnt density characteristics at 50 "C for the bipolar electrolysis cell (20 cells). The flowrate of the electrolyte was 200 cma min-I cell-'. FeC12/FeClS/HC1(mol (kg of H20)-' is 1.6/0.4/5.0 (m = 0.2) (open circles), 1.2/0.8/5.0 (m = 0.4) (open aquares), and 0.8/1.2/5.0 (m= 0.6) (open triangles).
carbon fiber cloth (felt), and a platinum catalyst was supported on the cathode. The electrode area was approximately 432 cm2 (18cm X 24 cm). Cation-exchange membranes were used. Cell voltagecurrent density characteristics of a bipolar unit containing 20 electrolysis cells were first examined for various conditions of (i) electrolyte composition (FeC12/ FeC13/HC1 [mol (kg of H2O)''] = 1.6/0.4/5.0(m= 0.2), 1.2/0.8/5.0(m= 0.4),and 0.8/1.2/5.0(m= 0.6))and (ii) flow rate of the electrolyte (50-300an3min" cell-'). The electrolytic solution (FeCl2-FeCl3-HCl), reserved in the charge tank, was fed into the anode side of the electrolysis cell, while hydrogen containing a small amount of HC1 solution (4.5mol (kg of H2O)" was introduced into the cathode as a carrier gas. The whole electrolysis cell was kept at 50 "C in a thermally controlled bath. Results of cell voltage-current density characteristics for a flow rate of 200 cm3 min" cell" are shown in Figure 11. These voltage-current density characteristics are satisfactorily stable, and the influence of the electrolyte flow rate on current density was observed to be very small. Changes of the current densities between 50 and 300 an3min-' cell-' were found to be only several percent, and the optimum flow rate was found to be 200 cm3 min" cell-'. Since the increase in voltage over every 4 unit cells along the entire 20 unit cells was found to be almost constant, good voltage distribution due to homogeneous electrolyte flow among the 20 cells was confirmed. A bipolar unit with 5 cells was also examined to compare with the 20 unit cells under the same electrolysis conditions. Current density obtained in the 5 cells was found to be nearly 10% higher than that obtained in the 20 cells. The resultsobtained in the 5 cells were deemed satisfactory since they are nearly equal to or slightly better than those obtained in the monopolar cell (Figure 5). Thus, a bipolar unit with 5 cells was chosen for use in the bench-scale plant. A possible reason for the poorer performance of the 20 cells may be that the pressing of the 20 cells was insufficient, which might have caused an increase in the contact resistance among the bipolar plate, electrode, and ion-exchange membrane. 4.3. Construction and Operation of the BenchScale Plant. On the basis of the individual experimental results of absorption, separation, and electrolysis, a bench-scale plant was designed and constructed. The simplified flow diagram and photograph of the bench scale
Figure 13. Overview of the bench-scale plant.
plant are shown in Figures 12 and 13,respectively. This plant has a production capacity of 2.1 N m3 of H2 day-', which corresponds to a recovery of 3 kg of S day-'. In order to overcome the corrosive atmosphere due to the highly concentrated HC1, heat-resistant vinyl chloride polymer was used for all the pipelines and the absorber was made of stainless steel lined with Teflon. The absorption unit was operated under the following conditions: (a) absorption temperature 60-80"C; (b) flow rate of solution 1.0 dm3min-l; (c) changes in concentration of the solution during absorption (before after) FeC12/FeC13/HC1= 0.96 1.1/0.74 0.6/4.36 4.5 mol dm-3, respectively (m= 0.43 0.35);(d) flow rate of gas, 10 dm3 min-'; (e) concentration of feed gas, 15% for H$ and 85% for NP The extent of H a absorption achieved was 98.5-99.5%. In the separation unit, the sulfur slurry discharged from the absorber was concentrated, and then solid-liquid separation (pressed filtration) was carried out to recover the sulfur. Since a rotating cup was used in the improved bubble column, sufficiently agglomerated sulfur powder was obtained after absorption. It was very easy to discharge the powder from the absorber and separate it from the solution. The wet slurry was washed with water (10fold excess) and then dried. The purity of the sulfur particles was over 99.9%. For the electrolysis unit, a bipolar unit with 5 cells was used. Operation conditions were (a) electrolysis temperature 50 "C, (b) flow rate of electrolyte 1.0 dm3 min", (c) concentration of electrolyte (before after electrolysis) FeC12/FeC13/HC1= 1.1 0.96/0.6 0.74/4.5 4.36 mol dm-3 (m= 0.35 0.43),and (d) electrolysis current 44 A (current density = 102 mA cm-2). Although the voltage of each cell was as low as 0.7 V during the first 150 h on stream, the voltage of each cell gradually increased with time, and the total cell voltage reached 3.7-3.8 V (which corresponds to 0.74-0.76 V for each cell) after lo00 h on stream. The unit cell nearest the cathode showed a slightly
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1608 Ind. Eng. Chem. Res., Vol. 30,No. 7, 1991
higher voltage than the others. Intermittent operation of the bench-scale plant was successfully carried out for lo00 h. Further studies on the cost estimation and demonstration by use of the real refinery acid gas generated from the hydrodesulfurization process will be published elsewhere. 5. Conclusion
(1)The Fe-Cl hybrid process consisting of the absorption of H2S gas by FeC13 aqueous solution and subsequent electrolysis of FeC1, aqueous solution was proposed as a process for decomposing H2S to H2 and S. The whole process was carried out under strongly acidic conditions by using a large excess of HC1. (2) The fundamental study confmed the feasibility of the procees. The extent of H+3 absorption reached almost 100% at 75 OC, and an acceptably low voltage (0.7V)was obtained at a current density of 100 mA cmW2at 70 O C . The suitable compoeition of the solution (FeC&/FeCl,/HCl (mol (kg of H20)-') was found to be 0.9-0.6/0.3-0.6/5.0 (0.25< m < 0.5). Agglomeration of the sulfur particles and their high separability were also confirmed. (3) A chemical engineering study for process development was performed. An improved bubble column absorber was developed by using a rotating cup at the gas feeder outlet, and a satisfactorily high overall volumetric coefficient was obtained at 60 OC. The sulfur particles that were produced were recovered smoothly. A bipolar type electrolysis cell (5or 20 cells) with an electrode area of 432 cm2was developed, and good performance was confirmed at 50 OC. (4)A bench-scale plant, consisting of the absorption unit, sulfur separation unit, and electrolysis unit, with a production capacity of 2.1 N m3 of H2 day-' and 3 kg of S day-' was constructed for the whole Fe-Cl hybrid process. Intermittent operation of the plant was performed successfully for lo00 h.
Ha
Acknowledgment We thank Drs. K. Nozaki and T. Ozawa of Electrotechnical Laboratory for helpful discussions and encouragement. Registry No. PVC, 9002-86-2;Fe, 7439-89-6; Ha,7783-06-4; FeCIS,7705-08-0;FeClZ,7758-94-3;Hz,1333-74-0;S,7704-34-9
HCl, 7647-01-0; NH,Cl, 12125-02-9; stainless steel, 12597-68-1; Teflon, 9002-84-0.
Literature Cited Borgarello, E.; Graetzel, M. Visible Light Induced Generation of H2 from Water and H a in Colloidal Semiconductor Dispersion. In Hydrogen Energy 4, Proceedings of the 4th World Hydrogen Energy Conference, Pasadena, CA, June 13-17,1982;Veziroglue, T. N.,Van Vorst, W. D., Kelly, J. H., Eds.; Pergamon: New York, 1982;Chapter 4,p 739. Craggs, H. C.; Aronold M. H. M. Hydrogen Sulfide Removal by Ammoniacal Iron Ammonium Ferrocyanide Liquors. Chem. Znd. 1947, 59, 571. Fischer, F. Method of Purifying Gases. US. Patent 1891974,1932 (applied in Germany, 1927). Fujii, K.; Kondo, W.; Mizuta, S.; Oosawa, Y.; Kumagai, T. Recovery of Sulfur and Hydrogen from Hydrogen Sulfide. Japan Patent 1567378, Tokugan-sho 57-64962,1982; Tokkai-sho 58-181706, 1983;Tokko-hei 1-53201,1989. Iida, H.; Ieshiki, S.; Noguchi, H.; Kikuchi, T.; Sue, H. A New Process for Treatment of Hydrogen Sulfide. In Process Technology Proceedings 8, Gas Separation Technology; Vansant, E. F., Dewolfs, R., Eds.; Elsevier: Amsterdam-Oxford, U.K.-New YorkTokyo, 1990,pp 531. Kalina, D. W.; Maas, E. T., Jr. Indirect Hydrogen Sulfide Conversion I-An Acidic Electrochemical Process. Znt. J. Hydrogen Energy 1986a, 10, 157. Kalina, D. W.; Maas, E. T., Jr. Indirect Hydrogen Sulfide Conversion 11-A Basic Electrochemical Process. Znt. J. Hydrogen Energy 1985b,10, 163. Kondo, W.; Mizuta, S.;Fujii, K. Hydrogen Production from Hydrogen Sulfide by the Fe-Cl Hybrid Process 11. Hydrogen Evolution by the Electrolytic Oxidation of Aqueous FeC12. Denki Kagaku 1984, 52, 693. Lewis, G. N.;Randall, M. Thermodynamics, 2nd ed.; McGraw-Hill Book Co.: New York, 1961; p 317. Mizuta, S.;Kondo, W.; Fujii, K. Hydrogen Production from Hydrogen Sulfide by the Fe-Cl Hybrid Proceee I. Proposal and Absorption of H a with Aqueous FeCg. D e d i Kagaku 1984,52,688. Muller, H. Die Nasse Schwefelwasche der Hamburger Gaswerke G.m. b.H. GWF,Gas-Wasserfach1931, 74 (28),653. Raymont, M. E. D. Hydrogen from Fuel Desulfurization; ACS Symposium Series 116;American Chemical Society: Washingbn, DC, 1979;p 333. Rieenfeld, F. C.; Kohl, A. L. Liquid Phase Oxidation Proceseee for Hydrogen Sulfide Removal. In Gas Purification, 2nd ed.; Gulf Publishing Co., Book Publishing Division: Houston, TX, 1974; Chapter 9,pp 398. Rhode, N. G. Some Thermodynamic Properties of Aqueous Ferrous Chloride or Ferric Chloride-Hydrochloric Acid Solutions. PbD. Thesis, Oklahoma State University, Stillwater, OK, May 1963,p 58. Thau, A. Wet Purification for the Removal of Sulphur from Gas. Gas World 1932, 97, 144.
Received for reuiew March 22, 1990 Revised manuscript receiued February 18,1991 Accepted February 27,1991
