260
Ind. Eng. Chem. Fundam., Vol. 17, No. 4, 1978
Absorption and Reaction of Acetylene in Cuprous Chloride Solutions' R. V. Chaudhari" and S. P. Gupte National Chemical Laboratory, Poona 4 11 008, India
The absorption of acetylene in an aqueous cuprous chloride solution in the presence of NH,CI, NaCI, and ammonia, was found to be in the fast reaction regime in the range of variables studied. The reaction was found to be second order with respect to cuprous chloride in all the three systems, first order with respect to acetylene for C2H2CuCI-NH,CI and C2H,-CuCI-NH,CI-NaCI systems, and zero order with respect to acetylene for the C2H2CuCI-aqueous NH, systems. The reaction rate constants for these systems have been obtained.
Introduction Unsaturated hydrocarbons such as ethylene or butadiene often contain small amounts of acetylene, the removal of which is sometimes essential for further use. As is evident from the literature reports (Mamalis, 1968; Booker et al., 1952), acetylene in small quantities is efficiently removed from the hydrocarbon streams by absorbing in cuprous chloride solutions. It has been found that the presence of alkali metal chlorides (NH,Cl, NaCl, etc.) or aqueous ammonia significantly enhances the acetylene absorption rates in cuprous chloride solutions and therefore could be very useful in an acetylene removal process. Acetylene reacts with cuprous chloride to produce copper(1)acetylene complexes, which are known to catalyze ethynylation (BIOS report 367) and dimerization reactions (Vestin and Lofman, 1953). This suggests that the reaction may be of considerable industrial importance and is also of theoretical interest in catalysis research. The literature on this reaction is very scanty and there are few reports on the kinetics and mechanism. Vestin and co-workers (1949, 1953) studied the reaction between acetylene and cuprous chloride in acidic medium, while Brameld et al. (1947) and Easterbrook and Erskine (1951) studied this reaction in the presence of alkali metal chlorides or aqueous ammonia solution. However, the investigations are of only qualitative nature and the results obtained are highly contradictary. It was therefore thought desirable to study the kinetics of absorption of acetylene in cuprous chloride solutions containing ammonium chloride, sodium chloride, or aqueous ammonia. Theory Gas absorption with chemical reaction has been discussed in detail by Astarita (1967) and Danckwerts (1970). Let us assume the following reaction scheme for the reaction of acetylene in cuprous chloride solutions CzHz(g) + CzHZ(1) (1) C,H,(l)
+ 2CuCl(1)
X
CzHz(CuC1.X,)z (complex)
(2)
where X represents NH,CI, NaCI, or NHB. Hikita and Asai (1964) have obtained an analytical solution (film theory) for gas absorption with pseudo mth-order irreversible chemical reaction (A + ZB products) and this could be applied for the absorption of acetylene in cuprous chloride solutions. If it is assumed that acetylene reacts within a short distance from the gas-liquid interface and its
-
NCL Communication No. 2199. 0019-7874/78/1017-0260$01.00/0
concentration in the bulk is zero and also that the concentration of cuprous chloride at the interface is practically the same as is in the bulk, the rate of absorption of acetylene would be given by the expression l
o
In order to justify the assumptions made above, the following conditions have to be satisfied (4) and (5) where D
1L
@ = -
kLA* and
M=
2DAkmnA*m-1Bon
[ m +l]kL2
(7)
It has also been assumed in the above analysis that the gas-side resistance is negligible and the absorption occurs under isothermal conditions. Experimental Section The apparatus used was a stirred cell, made of glass, in which the surface of the liquid could be renewed by means of a stirrer with four flat blades. The cell was 7.74 cm in diameter and the design was similar to that described by Danckwerts and Gillham (1966) and Zaveri and Sharma (1967). The cell was provided with an inlet and an outlet for the gaseous reactant and an outer jacket for circulating liquid a t constant temperature. The stirred cell was first flushed with acetylene gas and then a known amount of the cuprous chloride solution was introduced. The cell was again flushed with acetylene for a few minutes. Acetylene was saturated with water vapor a t the temperature of the experiment before it was introduced into the cell. The experiment was then started by adjusting the required stirring speed. All the experiments were carried out for a definite time and the samples were withdrawn for analysis of cuprous chloride before and after the experiment. The volumetric rates were recorded 0 1978 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 17, No. 4, 1978 261
(1955) equation. The values were corrected for the change in viscosities a t different concentrations of ions.
Table I. Range of Variables Covered in the Absorption Experiments ~
C, H,-CuClNH,Cl-NaCl 3 x 10-3 concentration of g-mol/cm3 NH,Cl _.3 x 10-3 concentration of g-mol/cm3 NaCl 5.022 x 10-4to 4.760 x to concentration of 1.600 X cuprous chloride 2.289 X partial pressure 0.21-1.0 atm 0.4-1.0 a t m of acetylene 40-100 rpm 40-100 rpm stirring speed C,H,-CuClNH,C1 4 x 10-3 g-mol/cm3
system
using soap-bubble flovvmeters. The rates of absorption were calculated from the volumetric rates and also from the analysis of cuprousj chloride solutions. It was found that the rates calculated by both methods were in good agreement. Constant temperature within f0.05 "C was maintained during each experiment using a thermostat. The materials used (cuprous chloride, ferric chloride, potassium dichromate, ammonium chloride, sodium chloride, and aqueous ammonia, etc.) were of more than 99% purity. Acetylene from the cylinder was passed through a silica gel trap to remove traces of acetone and other impurities. The purity of acetylene (>99.5%) was tested in a gas chromatograph, while the cuprous chloride analysis was carried out by a method described by Vogel (1975). Physicochemical Properties Solubilities and diffusivities are the most important properties required in the analysis of gas-liquid reactions. The solubility of acetylene in water, A*, is expressed by Henry's law as
The values of He' for the acetylene-water system were obtained from the data reported by Miller (1965). The data could not be used directly in the present study, as the solubility values change substantially in the presence of ions. However, the data on the acetylene-water system could be corrected for the presence of ions, using the following relationship suggested earlier by van Krevelen and Hoftizer (1948) log [He/Hl?'] = hlIl + hz12+ ... (9) The corrected values of solubilities using eq 9 are presented in Tables 11, 111, and IV. Diffusion coefficients for acetylene-water and CuClwater systems were calculated from the Wilke-Chang
Results and Discussion Absorption of Acetylene in Aqueous Cuprous Chloride Solution Containing Ammonium Chloride and Sodium Chloride. A series of preliminary experiments on the absorption of acetylene in cuprous chloride solution containing ammonium chloride and sodium chloride indicated that the absorption occurs in the fast reaction regime under certain conditions. Absorption experiments for the two systems C2H2-CuC1--NH4C1 and C2H2-CuC1-NH4C1-NaC1, were carried out a t 27 "C in the stirred cell described earlier. The range of variables covered for these systems is presented in Table I. The stirring speed showed no effect on the rate of absorption of acetylene in 4 M NH4Cl and 2 M CuC1, thus suggesting that the absorption occurs in the fast reaction regime. In the presence of NaCl also the rate was found to be independent of the stirring speed. In order to prove that all the data points conform to the fast reaction regime, conditions given by eq 4 and 5 have to be satisfied. Application of condition 5 requires knowledge of the kinetics, but condition 4 can be used without a prior knowledge of kinetics to verify the regime , of control. This requires an approximate value of' k ~ the physical mass transfer coefficient. Zaveri and Sharma (1967) have reported a value of k, = 2.68 X cm/s a t 60 rpm in equipment similar to that used in the present work, which suggests that a reasonable value of hL a t 50 rpm, the speed employed in this work, would be approximately 2 x cm/s. The enhancement factors, 4, calculated from the observed rates, the assumed value of kI,,and the solubility of acetylene are presented in Tables 11and 111,from which it can be seen that most of the data satisfy condition 4 for the fast reaction regime. After this the kinetic parameters (reaction orders and the rate constants) were evaluated for both systems using eq 3 for the rate of absorption of acetylene in the fast reaction regime. In order to obtain the reaction order with respect to acetylene, log-log plots were prepared of the rate of absorption vs. partial pressure of acetylene. As can be seen from Figure 1, the slope of these plots [which gives (rn + 1)/2] is unity for both systems, suggesting a firstorder reaction with respect to acetylene. Similarly, log-log plots were prepared of RIA* vs. cuprous chloride concentration (Figure 2) and it was found that the slope, which gives n / 2 in this case (eq 3), is unity. This suggests second-order reaction with respect to cuprous chloride. The third-order rate constants a t 27 "C were then cal-
Table 11. ExDerimental Data on the AbsorDtion of Acetylene in Cuurous Chloride Solutionsa
run no.
concn of rate of partial cuprous absorption pressure chloride (av), of C,H,, of C,H,, Bo X l o 3 R X lo7 atm g-mol/cm' g-mol/cm2s
_________________ 1 2 3 4 5 6 7 8 9 10
1.0 1.0 1.0 1.0 1.0 1.0 0.21 0.38 0.50 0.55
0.502 0.727 1.128 0.998 2.148 2.231 2.289 2.259 2.242 2.235
0.941 1.127 1.654 1.767 2.834 3.389 0.664 1.329 1.728 1.860
Henry's law reaction rate constant, constant, He x h,, x enhance(atm cm3)/ ( ~ m ~ / g ; m o l ) ment ~ g-mol Sfactor, Q
-
3.728 3.846 4.061 3.990 4.666 4.714 4.755 4.735 4.723 4.723
2.584 1.880 1.876 2.641 2.005 2.714 2.289 2.847 2.797 2.723
1.75 2.16 3.36 3.53 6.61 7.99 7.53 8.29 8.15 8.07
-
ab
R,/ZA*
1.70 2.46 3.82 3.38 7.28 7.56 7.76 7.66 7.60 7.58
9.35 13.98 22.90 19.91 50.11 52.59 259.5 140.8 105.7 96.33
a System, C,H,-CuCl-NH,Cl; temperature, 27 "C; volume of liquid, 1 2 6 cm3; concentration of NH,CI, 4 x g-mol/ ~ m -stirring ~ ; speed, 50 rpm; duration of each run, 20 min; cross sectional area of the cell, 47.0 cmz. Calculated using an average value of k,, 2.435 x l o 6 ( ~ m ~ / g - m os -l '). ~
262
Ind. Eng. Chern. Fundarn., Vol. 17, No. 4, 1978
Table 111. Experimental Data on the Absorption of Acetylene in Cuprous Chloride Solutionsa
run no.
1 2 3 4 5 6
concn of rate of Henry's law reaction rate partial cuprous absorption constant, constant, pressure chloride (av), of C,H,, He x h,, x enhanceof C,H,, B, x l o 3 ment R x lo' (atm cm3)/ (cm3/g;mol)' factor, @ gmol/cm3 gmol/cm2 s atm g-mol S1.0 1.0 1.0 1.0 0.7 0.4
0.476 0.930 1.342 1.600 1.546 1.570
0.59 1.02 1.42 1.62 1.20 0.664
6.544 7.342 7.770 8.045 7.993 8.013
4.950 4.875 5.082 4.992 5.916 5.400
1.93 3.74 5.51 6.52 6.85 6.65
a
b
1.97 3.86 5.58 6.65 6.42 6.52
B,/ZA* 15.57 34.14 52.13 64.36 88.34 157.31
a System, C,H,-CuCl-NH,Cl-NaCl; temperature, 27 "C;volume of liquid, 126 cm3;stirring speed, 50 rpm; duration of g-mol/cm3;concentration of each run, 20 min; cross sectional area of the cell, 47.0 cm2;concentration of NH4Cl, 3 x NaCl, 3 X lo-' gmol/cm3. Calculated using an average value of k,, = 5.202 x l o 6 ( ~ m ~ / g m os-l. l)~
Table IV. Experimental Data on the Absorption of Acetylene in Cuprous Chloride Solutionsa
run no.
reaction concn of rate of Henry's law rate partial cuprous absorption constant, constant, pressure chloride (av), of C,H,, He X lo-, k,, X enhanceof C,H,, B, x l o 3 R x 10' (atm cm3)/ (~m~/g-?ol)~ ment gmol/cm3 g-mol/cm2 s gmol atm Sfactor, @
ab
B,/ZA* 1 0.24 0.487 8.69 9.29 38.65 1.096 3.809 2.28 47.44 14.04 13.63 2 0.36 0.853 2.525 4.003 2.27 3 0.43 1.354 21.39 20.48 67.49 4.292 4.284 2.84 28.51 27.84 4 0.24 1.375 3.200 4.284 2.75 122.76 5 0.74 1.335 14.76 15.39 38.66 5.100 4.284 2.40 6 0.15 1.928 50.03 51.32 297.53 3.242 4.631 2.48 55.50 49.55 264.22 7 0.18 2.024 4.252 4.693 3.27 a System, C,H,-CuC1-aqueous NH,; temperature, 27 "C;volume of liquid, 126 cm3;concentration of NH,, 2 X lo-, gmol/cm3;stirring speed, 50 rpm; duration of each run, 10 min; cross sectional area of the cell, 47.0 cmz. b Calculated using an average value of k,, = 2.615 X 10, (cm3/g-mol)s - l . 10.0 culated from eq 3 and the average values for C2H2-CuC1-NH4Cl and C2H2-CuC1-NH4C1-NaCl systems were 2.435 X lo6 ( ~ m ~ / g - m os-ll )and ~ 5.202 X lo6 ( ~ m ~ / g - m o l ) ~ s-l, respectively. 5.0 It was interesting to observe similar reaction orders and regimes of absorption in both systems. The addition of NaCl retards the rate of absorption, but the regime of control is unchanged. This is due to the ionic effect of NaCl on the solubility of acetylene. It can be seen from 2.0 Table 111 that the solubility of acetylene is significantly reduced in the presence of NaCl and though lower rates % have been observed in the presence of NaC1, the rates per E unit acetylene concentration are about the same for both $ ,o systems. Since the regime of absorption depends more on ; m the enhancement factor, 4, (eq 41, than the specific rate of absorption, the controlling regime was unaffected by the addition of NaCl. 0.5 K Absorption of Acetylene in Ammoniacal Cuprous Chloride Solution. Absorption rates of acetylene in CpH2-CuCL -NH,CL-NaCL SYSTEM ammoniacal cuprous chloride solutions were measured in CONCENTRATION OF NH4CL: 3 . 0 M CONCENTRATION OF NaCt : 3.OM the stirred cell a t 27 "C. Initial experiments a t 1.0 atm CONCENTRATION OF CuCL : j 3 7 M of acetylene and low cuprous chloride concentration in0.2 dicated that the absorption was followed by an instan0 C#~-CUCL -Aq. N H l SYSTEM CONCENTRATION OF NH, : 2 . 0 M taneous reaction. But the kinetics of fast reactions cannot CONCENTRATION OF U l C l : f , 35 M be inferred in the instantaneous reaction regime; therefore, 0.4 experiments were carried out a t lower acetylene pressures 0.1 0.2 0.5 1.0 2.0 4.0 and higher cuprous chloride concentrations so as to satisfy PARTIAL PRESSURE OF C2H2, o t m condition 4 for the fast reaction regime. Cuprous chloride Figure 1. Effect of partial pressure of acetylene on the rate of concentration was varied between 4.870 x 10-4 and 2,024 x g-mol/cm3 and the partial pressure of acetylene absorption' between 0.15 and 0.75 atm, keeping the concentration of can be seen from Figure 1, the value of the slope [which g-mol/cm3. The data dissolved NH3 constant at 2.0 X gives (rn 1)/2] is 0.5, thus suggesting zero order with reported in Table IV satisfy conditions 4 and 5 for the fast vs. conrespect to acetylene. A log-log plot of (R/A*0.5) reaction regime. In order to obtain reaction order with centration of cuprous chloride (Figure 2) indicates secrespect to acetylene, the rate of absorption was plotted ond-order behavior with respect to cuprous chloride. The against the partial pressure of acetylene a t constant average value of the second-order rate constant a t 27 " C cuprous chloride concentration (1.3 X g-mol/cm3). As was found to be 2.615 X lo2 (cm3/g-mol) s-l. Note from
,
bi
+
Ind. Eng.
,D C2H2-CuC(-NH,CL
-NaCL SYSTEM CONCENTRATION OF NH,CI. 3 0 M CONCENTRATION OF NaCl 3 0M
I 0.2
1 1 1 1I
13 C2H2-CuCI-Aq
0.5
NHS SYSTEM
CONCENTRATION OF NHS :2.0M
I I i 4.0
i 2.0
1
1
0.1 5.0
CONCENTRATION OF CUPROUS CHLORIDE, (g m o ~ c / c m 3 )x 103
Figure 2. Effect of cuprous chloride concentration on the rate of absorption.
Table IV that these data also satisfy condition 5 for the fast reaction regime. This is an interesting case of fast, pseudo-zero-order reaction. Reaction Mechanism. The mechanism of a reaction between acetylene and cuprous chloride in the presence of NHICl or NH3 has not been investigated thoroughly, but formation of the complexes of the type CzHz(CuCl),.X, has been reported (Klebanskii e t al., 1963; Vestin and co-workers, 1949,1953) the values of m and n depending on the reaction conditions. A systematic analysis of the complexes has not been carried out due to the explosive nature of these complexes. Under the reaction conditions employed in the present work, a few experiments were carried out to investigate the stoichiometry. It bas been found that in all three systems, one mole of acetylene consumed two moles of cuprous chloride. From1 these observations, the following reaction scheme is proposed 2CuC1+ 2nX =+2[CuC1Xn] C2H2 + ~[CUC:~.X,] C2H2[CuC1.Xnj'2 +
(10) (11)
resulting in the overall reaction given earlier (eq 2 ) . The zero order with respect to acetylene in the reaction in ammoniacal cuprous chloride solution is interesting. It probably suggests that the first step (formation of CuC1.NH3 complex) is rate determining. On the other hand, the first order with respect to acetylene in the reaction in the presence of NHICl and NaCl suggests that the complex formation is fast and the second stage, eq 11, is rate determining. Conclusion Absorption of acetylene in cuprous chloride solutions, which is a useful system in acetylene removal processes,
Chem. Fundam., Vol.
17, No. 4, 1978
263
was studied. It has been observed that the absorption was followed by fast reaction in the presence of NH,C1, NaCl, and aqueous ammonia. The reaction was found to be second order with respect to cuprous chloride in all three systems. The order with respect to acetylene was one in the presence of NH&l and NaCl, but a zero-order dependence was found for the ammoniacal cuprous chloride system. The corresponding rate constants obtained a t 27 "C for the three systems are: C2H2-CuC1-NH4C1 system: 2.435 X lo6 ( ~ m ~ / g - m os-l; l ) ~C2H2-CuC1-NH4C1-NaCl l ) ~and C2H2-CuC1system: 5.202 X lo6 ( ~ m ~ / g - m os-l; aqueous NHBsystem: 2.615 X lo2 (cm3/g-mol) s-l. Nomenclature A* = concentration of acetylene at the gas-liquid interface, g-mol/cm3 Bo = concentration of cuprous chloride in the bulk liquid, g-mol/cm3 DA = molecular diffusion coefficient of dissolved acetylene, cm2/s DB = molecular diffusion coefficient of dissolved cuprous chloride, cm2/s He = Henry's law constant in solution, atm cm3/g-mol He' = Henry's law constant in water, atm cm3/g-mol hl,hz = solubility factor for ionic species, L/g-ion I J 2 = ionic strength, g-ion/l k,, = m, nth-order reaction rate constant, ( ~ m ~ / g - r n o l ) ~ + ~ - ~ S-1
kL = liquid-film mass transfer coefficient, cm/s M = [2DAkmnA*m-1Bon]/[(m + 1)kL2],defined by eq 7 m = order of reaction with respect to acetylene n = order of reaction with respect to cuprous chloride Pi = partial pressure of acetylene at the gas-liquid interface,
atm R = rate of absorption of acetylene, g-mol/cm2 s 2 = number of moles of cuprous chloride reacting with each mole of acetylene Greek Letters 4 = enhancement factor, defined by eq 6
Literature Cited Astarka, G., "Mass Transfer with Chemical Reaction", Elsevier Publishing Co., Amsterdam, 1967. Booker, J. D., Harp, W. M., Wadley, E. F., U.S.Patent, 2604485 (July 1952). Brameld. V. F., Clark, M. T., Seyfange, A. P., J . SOC. Chem. Ind., 66, 346 (1947). Danckwerts, P. V., Gillham, A. J., Trans. Inst. Chem. Eng., 44, T42 (1966). Danckwerts, P. V., "Gas-Liquid Reactions", McGraw-Hill, London, 1970. Easterbrook, W. C., Erskine, J. W.. J. Appl. Chem. I., Suppl. No. 1. 853 (1951). Hikita, H., Asai. S.,Int. Chern. Eng., 4, 332 (1964). Klebanskil, A. L., Dolgopalskii, I.M., Dobler, 2. F., Zh. Obshch. Khim., 33[3], 761 (1963). Mamalis, P., British Patent 1 120452 (July 1968). Miller, S. A., "Acetylene, Its Properties and Uses",Vd. I Ernest Benn Ltd., London, 1965. van Krevelen, D. W., Hoftizer. P. J., "Chimie et Industrie: Numero Speciale du XXI e congress International de Chimie Industrialle", Bruxelles, p 168. 1948. Vestin, R., Lofman, C., Acta Chem. Scand., 7, 398 (1953). Vestin, R. Ralf, E., Acta Chem. Scand., 3, 101 (1949). Vestin, R., Somersale, A., Acta Chem. Scand., 3, 125 (1949). Vestin. R., Acta Chem. Scand., 3, 650 (1949). Vestin. R., Somersalo, A., Muller, B., Acta Chem. Scand., 7, 745 (1953). Vogel, I.,"Quantitative Inorganic Analysis", 3rd ed, Longmans, Green and Co., New York, N.Y., 1975. Wilke, C. R., Chang, P., AIChE J., 1, 264 (1955). Zaveri, A. S.,Sharma, M. M., Chem. Eng. Sci., 22 1 (1967).
Received for review October 17, 1977 Accepted July 6, 1978
