Ind. Eng. Chem. Process Des. Dev. 1986,2 5 , 828-834
828
X 1 2= concentration of tert-butyl perbenzoate, mol/L XM = concentration of monomer, mol/L XM,,= concentration of dead polymer with length n, mol/L Xpy = concentration of radical styrene polymer with length
.I, moVL Xpn+ = concentration of cation isobutylene polymer with length n, mol/L X , = jth moment of molecular weight distribution of the dead
styrene polymer, mol/L X E j= jth moment of molecular weight distribution of both
growing and dead isobutylene polymers, mol/L XAj = jth moment of molecular weight distribution of the
growing isobutylene or styrene polymer, mol/L y(k) = plant output y,(k) = secondary output z = Z-transform operator Greek Letters Wk) = plant input-output vector Y = kinetic chain length v(k) = adaptation error p = average reactant density, g/L
Superscripts in = inlet = estimated value * = desired value
Registry No. Isobutylene (homopolymer), 9003-27-4; isobutylene, 115-11-7;styrene (homopolymer),9003-53-6;styrene,
100-42-5.
Literature Cited Arnold, K.; Johnson, A. F.; Ramsay, J. Proceedings of 4th InternationalF e d eration of Automatic Control Conference on the Instrumentation and Automation in the Paper, Rubber, Plastlcs and Polymerization Industries, Ghent, Belgium, 1980, pp 359-367. Astrom, K. J. “Preprints, 8th Triennial World Congress of International Federation of Automatic Control”; Plenary Session: Kyoto, 1981; pp 28-39. Brosilow. C.; Joseph, B. AIChEJ. 1878, 24, 485-509. Goodwin, G. C.; Ramadge, P. J.; Caines, P. E. I€€€ Trans. 1880, Ac-25 (3), 449-456. Hoogendroorn, K.; Shaw, R. 4th International Federation of Automatic Control Conference on the Instrumentation and Automation in the Paper, Rubber, Plastics and Polymerization Industries, Ghent, Belglum, 1980, survey paper. Kiparissides, C.; Shah, S. L. Aotomatlce 1883. 79, 225-235. Landau, Y. D. “Adaptive Control”; Marcel-Dekker: New York, 1979. Lozano, R.; Landau, I . D. Int. J . Control 1981, 33 (2), 247-268. Macgregor, J. F.; Tidwell, P. W. Proc. Inst. Electr. Eng. 1977, 124 (8), 732-734. Monopoli, R. V. IEEE Trans. 1974, AC-19 (5). 474-484. Morari, M.; Stephanopouios, G. A I C E J . ISSO, 26, 247-259. Tolfo, F. “Computerized Control and Operation of Chemical Plants”; Verein Osterreichischer Chemiker: Vlenna, 1981. Whitaker, H. P.; Yamron, J.; Kerzer, A. Report R-164, Instrumentation Laboratory, MIT, Cambridge, MA, 1958.
Received for review May 13, 1985 Accepted September 3, 1985
A
On Interaction between Ethane and Propane in Simultaneous Pyrolysis and I t s Influence on Ethylene Selectivity Zou, Renlun Hebei Academy of Sciences, Shuiazhuang, Hebei Institute of Technology, Tianjin, The People’s Republic of China
Zou, Jlnt Hebei Institute of Technology, Tianjin, The People0 Republic of China
The interaction between ethane and propane in simultaneous pyrolysis will find expression in four aspects of two pairs of contradictions, i.e., acceleration and retardation of ethane upon propane and of propane upon ethane. Ethylene selectMty is one of the comprehenstve expressions of synergetic effects of these four aspects. I t depends upon the rate of ethylene formation as well as that of reactants disappearance. This present paper shows that the ethylene selectivity in simultaneous pyrolysis is definitely lower than its addltlve selectivity.
There are utterly different points of view in the problem of ethylene selectivity in simultaneous pyrolysis (the terms “simultaneous pyrolysis” and “individual pyrolysis” used in this paper are synonymous with “copyrolysis” and “separate pyrolysis”, respectively, in some references) of the ethane-propane mixture. Froment et al. (1979) held the view of negative deviation; i.e., the ethylene selectivity in simultaneous pyrolysis is lower than its additive select In
accord with the authors’ preference, their family names are listed first.
tivity. Mol (1981) held the contrary view. Goossens (1979) also did not agree with Froment’s view, but Hofmann (1980) has supported them. Zou et al. (1986) indicated that there is a negative deviation with respect to real selectivity and a positive deviation with respect to overall selectivity. The present paper analyzes and calculates the interaction between ethane and propane in simultaneous pyrolysis and further discusses ethylene selectivity.
Calculation of Free-Radical Concentration The present paper calculates and compares three systems: ethane pyrolysis, propane pyrolysis, and ethane-
0196-4305/86/1125-0828$Ol.50/0 @ 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986 829 Table I. Arrhenius Parameters of Ethane Pyrolysis (Froment et al.. 1979) ~~
reaction
i A, or L.mo1-l.s-l E, kcal-mol-'
1 1.3 X 10l6 86
2 4.0 X 10le 87.5
3 1.0 X 10" 9.7
4 3.8 X 10" 16.5
5 3.2 X 10ls 40
8
6
7
1.0 X lo8
4.0 X 1O'O
1.3
9.2
0
0
X
1O'O
propane simultaneous pyrolysis. Froment's simplified schemes (Froment et al., 1979) and their relevant kinetic parameters are adopted. A. Ethane Pyrolysis. The reaction scheme of ethane pyrolysis (model I)
Figure 1. Free-radical concentration pattern of ethane pyrolysis at 1073 K.
Reaction 6 was neglected in free-radical balances for the individual pyrolysis because the propylene weight yield of ethane pyrolysis is less than 1% (Froment et al., 1979) and the CzH5. concentration is low in the propane pyrolysis system (Froment et al., (1979). However, this reaction was considered in ethane-propane simultaneous pyrolysis. When the free radicals are balanced in the ethane pyrolysis system, the following differential equation set is obtained d[H.l -
-
dt kdC2H61 - k3[C2H6I[Ho1+ ~ ~ [ C Z H- ~~7[CZH5'l[H'] 'I
k4[C2H61[CH3'1 - k5[C2&.I - k7[C2H6'1 LH.1 When the steady-state assumption was used, the differential equation set could be transformed to a nonlinear equation set as follows: kl[C&6I - k3[C2H6I[H'l + ~ ~ [ C ~ H-S ' I ~~[C~HE,*I[H*I = 0 (9) 2k,[C,H,] - k4[C&6][CH3.] - ka[CH3.]' = 0 (10) h[C2H61
+ ~ ~ [ C Z H I ~ I+[ Hk4[C2Hd[CH3*] *] -
k5[CZH5*]- k7[C2H5.][H*] = 0 (11)
When this set was solved by a Newton-Raphson method, the free-radical concentrations corresponding to different [C2H6]were calculated. In light of the Arrhenius parameters listed in Table I, the free-radical concentration
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
830
Table 11. Arrhenius Parameters of ProDane Psrolssis (Froment et al.. 1979) reaction i 1' 2a' 2b' 3a' 3b 4' A, 8-l or 2.0 x 1016 1.2 x 109 8.0 x io* 3.4 x 1010 4.0 x io9 4.0 x L-mol-'.s-' E, kcal.mol-' 84.5 12.6 10.4 11.5 10.1 32.6 ~~
~
~
pattern of propane pyrolysis at 1073 K could be obtained as shown in Figure 2. C. Ethane-Propane SimultaneousPyrolysis. The reaction scheme of the ethane-mopane simultaneous py-
(CH. *] s 10 4.0
3.0
-
2.0 x
ioi3
38.7
'
9.7
0
8.3
[24, H,-]xlO'
T = 1073 K
- 15.0
[ H .)xlO'
-
14.0
-
12.0
-
10.0
2.0
6a' 6b' 7' 1.0 x 10" 9.0 x iolo 1.3 x ioio
5'
ioi3
-
-
8.0
-
-4.0
1 .o
-
6.0 -
-
- 2.0
L
-
- 5.0
[CzHj.)
-c
4.0
-
-
C.O
Figure 2. Free-radical concentration pattern of propane pyrolysis at 1073 K. T = 1073 K H,O 8 CsE'
t
CSE,
40
8
50
t
50
8.0
12.5
10.0
7.5
6.0
4.0
5.0
2.0
2.5
0.0 0 . 0
5.66
0.2
0.4
0.6
0.8 0.0 propane conversion
5.09
4.30
3.30
2.16 (C,H6)x10'
Figure 3. Free-radical concentration pattern of ethane-propane simultaneous pyrolysis a t 1073 K (the relationship among [C,H,], [C,H,], and propane conversion in the abscissa is taken from the experimental results of Sundaram and Froment (1977)).
could be obtained as shown in Figure 3.
Interaction between Reaction Components in Ethane-Propane Simultaneous Pyrolysis From the above-mentioned reaction schemes (models I, 11, and 1111, we have the expressions for the ethylene formation rate in the three pyrolysis systems as follows: ethane individual pyrolysis . FI
= k5[C$&.]
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986 831
from eq 11, we have 8.0
propane individual pyrolysis
6.0
rII = k,[l-C3H7.] from eq 25, we get
6.0
(33)
2.0
ethane-propane simultaneous pyrolysis 0.0
I
2.0
from eq 29, we have
I
5.0
l
l
4.0
l
l 5.0
I
I 6.0
[C,%]*lo'
Figure 4. Difference between [C2H,.] in ethane individual pyrolysis and that in ethane-propane simultaneous pyrolysis.
(34) and from eq 30, we get
Second, the rate constant of the chain-initiation step (1073 K)of ethane individual pyrolysis is compared with that of propane individual pyrolysis.
(35) When the data listed in Tables I and I1 are used, the free-radical concentrations obtained by calculation are combined, and the order of magnitude of each of the terms in eq 32-35 is compared, some terms, which are of a very small order of magnitude, could be neglected and the simplified expressions, therefore, could be obtained: for ethane pyrolysis
for propane pyrolysis
for ethane-propane pyrolysis
(35) When eq 36 is compared with eq 38, it will be seen that there are two terms in the denominator of eq 38 more than in that of eq 36 due to the fact that the radical C2H5., which is formed from ethane in the simultaneous pyrolysis of the ethane-propane mixture, reacts with propane via reaction 13 and 14. Owing to the fact mentioned above, [C2H5-] tends to decrease and then it leads to the reduction of the value for rII1. This is the one aspect of retardation of propane to ethane pyrolysis. Froment et al. (1979) have considered only this aspect but did not consider the aspect of acceleration of propane on ethane pyrolysis. There are actually two aspects, as analyzed below. First, the radical CH3. is formed in the chain-initiation step and then disappears by reaction 4 but does not come into the cycle by the chain-propagation step of ethane pyrolysis. It does come into the cycle in the chain-propagation step of propane pyrolysis.
-
1.226 X lo-' i 2 kl + k2 3.944 x 10-3 6.005 x 10-2 It indicates that the chain-initiation rate of propane pyrolysis is twice as fast as that of ethane in the same initial concentration. The chain-initiation step is the slowest one in the chain reaction. Therefore, propane is more easily pyrolyzed than ethane at the same condition. The above analysis leads to the following conclusion:
--kll
+
[cH3.11< [CHS'IIII< [CHB'III This conclusion is verified by calculating the concentration of each free radical. It can be seen clearly by comparing the [CH,.] in Figures 1, 2, and 3. During ethane-propane simultaneous pyrolysis, a part of CH3.,which is formed from propane, reacts with ethane via reaction 4 and then it is transformed into C2H5.. So [C2H5.]i s tending to increase and then it leads to enlargement of the value for rIIp That is the one aspect of acceleration of propane to ethane pyrolysis. As a result of the contradiction between the retardation and acceleration in the present calculated condition, the following relationship is tenable: [c2H5'1111> [C2H5'11 The relationship is also shown in Figure 4. This result is contrary to that of the theoretical analysis by Froment et al. (1979). Because the radical Ha w ill come into the chain cycle in the propagation step, respectively, of the ethane and propane individual pyrolysis, it is certain that the [H-] is impossible to remain equal in the three pyrolysis systems. See Figures 1-3. When eq 35 is compared with eq 37, there i s the term k2,[C2H5.]in the numerator of eq 35 which is not eq 37. The order of magnitude of this term in eq 37 is too small, hence it has been neglected. During ethane-propane simultaneous pyrolysis, a part of C2H5., which is formed from ethane, reacts with propene via reaction 13 and then it is transformed into 1-C3H7'. So the [1-C3H7-]tends to increase, which leads to an increase in rIIp That is the one aspect of the acceleration of ethane on propane pyrolysis. Similarly, Froment et al. (1979) have considered only this aspect but did not consider the aspect of retardation of ethane on propane pyrolysis. There is actually this aspect of retardation.
832
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
Table 111. Instantaneous Ethylene Selectivity in Ethane-Propane Simultaneous Pyrolysis 0.2
0.3
0.4
0.5
0.666
0.677
0.688
0.701
ethylene selectivity is evaluated at 1073 K, 1.5 atm, and H20/C3H8 = 0.4:l (weight ratio) and under different conversions. Using eq 41, SiI(C2HJ
(c, H~
lxi 0 3
Figure 5. Difference between [1-C6H7.]in propane individual pyrolysis and that in ethane-propane simultaneous pyrolysis.
Because [CH3.] in ethane-propane simultaneous pyrolysis is smaller than in propane individual pyrolysis, i.e., [CH3.lIII< [CH3.III. It will be seen from eq 35 that [lC3H7.]tends to decrease, which results in a reduction of PIII. As a result of the contradiction between acceleration and retardation in the present calculated condition, the following relationship is tenable: [1-c3H7'1111< [1-C3H7'111 This relationship is also shown in Figure 5. This result is contrary to that of the theoretical analysis by Froment et al. (1979).
Variation of Ethylene Selectivity In the simultaneous pyrolysis of a binary mixture of A and B, the selectivity for the product P is defined (Froment et al., 1977) as
YAM(P)XAMJ/A + YBM(P)XBMIC/B (39) XAMIC/A + XBMJ/B When A and B are pyrolyzed individually, each to the same conversion as in the simultaneous pyrolysis, the total selectivity for product P is obtained which is called the additive selectivity (Froment et al., 1979)S(P): S(P) = YA(P)J/A+ YB(P)$B (40) For convenience, we define the instantaneous selectivity SX-,(P) as the ratio of the formation rate of P to the disappearance rate of the feedstock, i.e., SA-B(P) =
rP
Sk-B(P) = -
c
i=l
(41)
WLPL
A. Instantaneous Ethylene Selectivity in Ethane Individual Pyrolysis. The value of instantaneous ethylene selectivity is evaluated by using eq 41 Si(C2H4) = r5/(rl + r2 + r3 + r4 - r7 - rd = ~ ~ [ C ~ H ~ . I+/ k2 W+ I kdH.1 + k4[CH3'11[C2H61 - k7[H'l[C2H5'l ka[CH3.l2/2) At 1073 K, 1.5 atm, and H20/C2H,= 0.41 (weight ratio) and under different conversions, we have Si(C2H4) = 0.997
B, Instantaneous Ethylene Selectivity in Propane Individual Pyrolysis. The value of instantaneous
=
0.661
From the calculated value, it is clear that S;(C2H4)and SiI(C2H4)are nearly independent of conversion. C. Instantaneous Ethylene Selectivity in EthanePropane Simultaneous Pyrolysis. The value of instantaneous ethylene selectivity, S;II(C2H4),at 1073 K, 1.5 atm, and H20/C2H6/C3H8 = 0.4:0.5:0.5 (weight ratio) and under propane conversion 0-0.5,was evaluated by using eq 41 and listed in Table 111. This case is obviously different from the first two cases; the instantaneous ethylene selectivity in ethane-propane simultaneous pyrolysis is dependent upon propane conversion. It is caused by the variation of propane conversion with the relative content of ethane and propane. Specifically, because propane conversion is larger than ethane conversion in simultaneous pyrolysis, the relative content of ethane to propane increased as the propane conversion increases. Besides, instantaneous selectivity of ethylene related to ethane is larger than that related to propane, so the instantaneous ethylene selectivity is increased as the propane conversion increases. From the above analysis, the law of variation of ethylene selectivity is one of the comprehensive manifestation of interaction between acceleration and retardation of ethane and propane. The size of the ethylene selectivity and its variation are dependent upon not only the formation rate of ethylene but also the disappearance rate of feedstocks. Therefore, the formation rate of ethylene as well as the disappearance rate of feedstocks all must be considered. Froment et al. (1979)have compared the formation rate of ethylene but did not consider the disappearance rate of feedstocks in discussing the problem. Let us discuss the influence of the interaction between ethane and propane on the ethylene selectivity by way of comparing the concrete data of these two rates in the three pyrolysis systems as shown in Table IV. Comparing the data listed in Table IV, it can be seen that the ethylene formation rate in the simultaneous pyrolysis is greater than the sum of the ethylene formation rates in the individual pyrolysis as below:
'
rIII TI + TI1 In addition, the disappearance rate of feedstocks is also larger than the sum of the disappearance rate of the feedstock in the individual pyrolysis as below: r;
+ ri1
There is a fact mentioned above that ethane both accelerates and retards the propane pyrolysis; meanwhile propane both accelerates and retards the ethane pyrolysis, too. Under the reaction condition of Table IV, the global manifestation is that the overall formation rate of C2H4 during simultaneous pyrolysis of the ethane-propane mixture is larger than the additive formation rate of C2H4 during the individual pyrolysis of ethane and propane, and the overall disappearance rate of feedstocks during the simultaneous pyrolysis of the ethane-propane mixture is larger than the additive disappearance rate of feedstocks during the individual pyrolysis of ethane and propane.
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986 833
Table IV. Reaction Rates and Instantaneous Ethylene Selectivity" pyrolysis system formation rate of CzH4 from CzHs., mol.L%-' formation rate of CzH4 from 1-CsH7-,mol-L-'d overall formation rate of C2H,, mol.L%-' disappearance rate of Cz&, mol.L-'-s-' disappearance rate of CsHB,mol.L-'.s-' overall disappearance rate of feedstocks, mo1.L-l.s-l instantaneous selectivity of C2H4/C2H6 instantaneous selectivity of C2HI/C3H8 total instantaneous selectivity of C2Hl
ethane 0.1001 0.1001 0.1004 0.1004 0.9970 0.9970
propane 0.1278 0.1278 0.1932 0.1932 0.6615 0.6615
ethane-propane 0.1447 0.09556 0.2403 0.1445 0.2159 0.3604 1.001 0.4426 0.6665
"Reaction conditions: 1073 K, 1.5 atm, H20/C2&:C3H8 = 0.4,0.5,0.5 (weight ratio), propane conversion 0.2, [CzH,] = 5.092 X = 2.882 X
Furthermore, the increment range of riII is larger than that of rIn. Hence, the instantaneous ethylene selectivity in simultaneous pyrolysis is smaller than the additive corresponding selectivities in the individual pyrolysis as following: S;II(CZH4) < $lS;(CZH4) + $ZS;I(CZH4) From Figure 5 and Table IV, in the case of simultaneous pyrolysis, the following relationship is tenable: [~-C&']III < [1-C3H7'111 So, the ethylene formation rate from 1-C3H7.in simultaneous pyrolysis is smaller than the corresponding rate in propane pyrolysis, while the propane disappearance rate in simultaneous pyrolysis is greater than the corresponding rate in propane pyrolysis. Thus, more propane was consumed to produce 2-C3H7-and then propylene. Consequently, the ethylene selectivity related to propane in simultaneous pyrolysis is smaller than the ethylene selectivity in propane individual pyrolysis. Besides, from Figure 4 and Table IV,the formation rate of ethylene from C2H5. in simultaneous pyrolysis is larger than the corresponding rate in ethane pyrolysis and the disappearance rate of ethane in simultaneous pyrolysis is also greater than the corresponding rate in ethane pyrolysis. So the selectivities of ethylene related to ethane in these two pyrolysis systems are almost identical and all close to 1. In virtue of these two things, the ethylene selectivity in simultaneous pyrolysis is smaller than the corresponding additive selectivity. Finally, because the ethylene yield is the ratio of the amount of ethylene formation to that of the initial reaction of feedstocks, so that the ethylene yield depends upon the ethylene formation rate and independent of the disappearance rate of the feedstocks, clearly speaking, the faster the ethylene formation rate, the higher its yield. It will be expected from Table IV that the ethylene yield in ethane-propane simultaneous pyrolysis can be greater than the additive yield in ethane and propane separate pyrolysis. These conclusions confirm the results of Zou et al. (1978, 1986).
Conclusion The interaction between ethane and propane in simultaneous pyrolysis will find manifestation in four aspects of two pairs of contradictions, Le., acceleration and retardation of ethane upon propane and of propane upon ethane. Froment et al. (1979) have considered only one aspect of the retardation of propane on ethane pyrolysis but did not consider the other aspect of the acceleration of propane on ethane pyrolysis. Otherwise, the present paper considers the two aspects of retardation and acceleration a t the same time. The relationship, which is contrary to Froment et al. (1979), is followed as [CZHSIIII> [CzH5.11
[C3H8]
Froment et al. (1979) have considered only one aspect of the acceleration of ethane on propane pyrolysis but did not consider the other aspect of the retardation of ethane on propane pyrolysis. The present paper considers the two aspects of both retardation and acceleration. The relationship, which is contrary to Froment et al. (19791, is followed as [~-C&']III < [1-C3H7*111 The present paper proves the following results: there is a negative deviation of ethylene selectivity in ethanepropane simultaneous pyrolysis from the additive ethylene selectivity in individual ethane and propane pyrolysis, whereas there i s positive deviation of the ethylene yield in simultaneous pyrolysis from the additive ethylene yield in individual pyrolysis. These results agree with those of Zou et al. (1978, 1986) and Froment et al. (1979).
Nomenclature A = frequency factor, s-l or L.mol-'d E = energy of activation, kcal mol-' k = reaction rate constant, s-l or L-mo1-l.s-l n = number of the reaction in its system rI, rII, rIII= formation rates of ethylene in models I, 11, and 111, respectively r;, rII, rIII= disappearance rates of the feedstock in models I, 11, and 111, respectively rp = formation rate of product P in reaction system ri = rate of reaction i in its system SA-B(P)= selectivity of product P in A-B pyrolysis system S(P) = additive selectivity of product P 5'A-B(P).= instantaneous selectivity of product P in A-B pyrolysis system SI(C~H~), S&HJ, Sm(C2H4)= ethylene selectivityof models I, 11, and 111, respectively S;(CzH4), S;(CzH4), S;II(C2H4) = instantaneous selectivity of ethylene in models I, 11, and 111, respectively T = reaction temperature wi = reaction coefficient of reaction i, i = 1, 2, ...,n;w i= 1, if feedstock acts as reactant; wi = -1, if feedstock acts as product; wi = 0, if the reaction of feedstock does not take place x = conversion x m , xBM = conversion of feedstocks A and B in simultaneous pyrolysis, respectively YAM(P), YBM(P) = yields of product P from A and B in simultaneous pyrolysis, respectively YA(P), YB(P) = yields of product P corresponding to conversions lcm and xBM of A and B in individual pyrolysis, respectively I, 11, I11 = the ordinal numbers of pyrolysis models of ethane individual pyrolysis, propane pyrolysis, and ethane-propane simultaneous pyrolysis, respectively +A, = mole fractions of constituent A and B in A-B mixture, respectively +1, +z = mole fraction of ethane and propane in ethane-propane simultaneous pyrolysis, respectively G] = mole concentration of component j , mo1.L-' Registry No. CzHB,74-84-0; C3H8, 74-98-6; C2H4,74-85-1.
Ind. Eng. Chem. Process Des. Dev. 1986, 25, 834-836
834
Literature Cited
Mol A. Hydrocarbon Process. 1981, 60 (2), 129.
Froment, G. F.; Van de Steene, B. 0.; Vanden Berghe, P. J.; Gossens, A. G. AIChE J . 1977, 23 (I), 93. Froment, G. F.; Van de Steene, B. 0.; Sumedha, 0. OilGas J . 1979, 77(16). 87 -. . Goossens, A. G. Private correspondence (provide Goossenapos work affiliation), Aug 14, 1979. Hofmann, H. Private correspondence (provide Hofmann's work affiliation), Nov 25, 1980,
zOu, Sundaram. R~~~~~K.sCl. M.; Froment, G.Ed.) F. Chem. 1979, 22 Eng. (6),Sci. 637,1977, 32(6), 601. Zou, Renjun; Qiangkun, Lou; Blngchang, Zhang; Hongwu, Cui; Zhushan, Guo; Xiaorui, Song Ind. Eng. Chem. Process Des. Dev. 1986, 25. 12-17.
Received for review February 14, 1985 Revised manuscript received October 7, 1985 Accepted October 25, 1985
COMMUNICATIONS Continuous Solvent Extraction of Sulfur from the Electrochemical Oxidation of a Basic Sultide Solution in the CSTER System
A continuous system was designed to remove the anode-adhered sulfur in basic sulfide solutions and, as a result, to regenerate the deactivated anode. The system used was a CSTER with introduction of an appropriate amount of organic solvent to extract the desired product. The sulfur yield was 40-80%, depending on the volume flow rate (10-66 mL/h), temperature (20-60 "C),applied voltage (1.8-4.8 V), sulfide concentration, and electrolyte concentration. This system demonstrates the feasibility of electrochemical splitting hydrogen sulfide to produce hydrogen (cathode) and sulfur (anode).
Electrochemical oxidation of basic sulfide solutions (S2-) has been investigated for many decades as an alternative process to the Claus process which is used to oxidize hydrogen sulfide to sulfur (Borgarells et al., 1983; Gregory et al., 1980; Kameyama et al., 1981). However, it is also observed that the Pt-anodic oxidation of the sulfide solution generates a nonconducting thin film of sulfur coated on the anode, while the cathode generates hydrogen (Allen and Hickling, 1957; Binder et al., 1967; Farooque and Fahidy, 1977). The accumulation of nonconducting sulfur on the anode surface deactivates the reaction as shown in Figure 1and, consequently, prohibits further reaction to yield any more sulfur. On the other hand, further oxidation of the adhered sulfur can occur to produce sulfate (SO:-) or thiosulfates Sz02-) (Bard and Lund, 1975). In other words, though eq 1can be written stochiometrically, the reaction is generally inhibited.
The foregoing phenomena constitute the bottleneck to the complete oxidation of basic sulfide solutions to sulfur either in batch or continuous reactors. Some methods were proposed to overcome this problem, including the adoption of rotating-type electrode (Nadebaum and Fahidy, 1975) and the solvent stripping process (Bolmer, 1968) but all without much success due to mechanical leakage or inconvenience.
Principle Here, we propose and demonstrate a feasible method to remove continuously the produced sulfur and, as a result, to regenerate the electrode from deactivation by introducing an appropriate amount of organic solvent such as toluene or benzene to the agitated anode compartment. This method is generally known as the extractive reaction 0196-4305/86/1125-0834$01.50/0
to remove the desired product from the immiscible phases (Levenspiel, 1972). The resulting aqueous-organic mixture, though possessing a slightly higher cell resistance than the aqueous system alone due to the nonconducting nature of the organic solvent, could successfuUy remove the product sulfur from the electrode. The simultaneous oxidation of the organic solvent on the anode could be prevented by a suitable choice of operating conditions. therefore, an extensive, even complete, oxidation of sulfide to sulfur could be accomplished. Experiments The system used for the present research was a continuous-stirred tank electrochemical reactor (CSTER) with Pt wire as the electrodes (as shown in Figure 2). An appropriate volume ratio of toluene and basic sulfide solution (15) was continuously pumped into the anode compartment of the cationic-membrane-separated cell. Typically, 2-13 mL/h of toluene and 10-66 mL/h of 0.1 M sodium sulfide solution in 5 M NaOH electrolyte were introduced simultaneously at ambient temperature. Voltages of 1.8-4.8 V were applied by a potentiostat. The two immiscible liquids were agitated mechanically to allow a suitable mixing during the reaction. It is reported that sulfur is quite soluble in toluene (William, 1954) and, meanwhile, sulfide ion is insoluble to any extent. During the reaction, the product mixture was continuously overflowed out from the cell and was separated by decantation to yield sulfur-containingtoluene solution and sulfide-containing aqueous solution with coproduction of polysulfide ions. The sulfur-containing toluene solution was evaporated to generate sulfur powder whose weight purity was subsequently determined. The sulfide-containing solution was also used to check the conversion. 0 1986 American Chemical Society
