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Ind. Eng. Chem. Res. 1999, 38, 2160-2162
Enhancement of the Absorption of CO2 in Alkaline Buffer Solutions: Joint Action of Two Enhancers Gonzalo Va´ zquez,* Francisco Chenlo, Gerardo Pereira, and Pilar Va´ zquez Department of Chemical Engineering, University of Santiago de Compostela, Avda. das Ciencias, s/n, E-15706 Santiago (A Corun˜ a), Spain
We measured the absorption of CO2 in alkaline 0.5 M/0.5 M sodium carbonate/bicarbonate buffers containing either saccharose and sodium arsenite or saccharose and formaldehyde. Absorption enhancement increased upon increasing the concentration of either of the catalysts, but the joint action of the two was always less than the sum of their individual effects, the difference being a function of the acidities and concentrations of the catalysts and the pH of the carbonate/ bicarbonate buffer solution. Introduction In our earlier study of CO2 absorption in wetted-wall column by alkaline 0.5 M/0.5 M sodium carbonate/bicarbonate buffers containing an absorption catalyser, sodium arsenite, formaldehyde, glycerin, methanol, ethanol, saccharose, glucose, or fructose,1 we found that kcat, the rate constant for the reaction involving the catalyst, depended on two characteristics of the catalyst, its degree of dissociation, δ ) Ka/(Ka + [H+]), and its mass density of OH groups, β ) mNOH/M, where NOH is the number of OH groups per molecule of catalyst, M is its molecular mass, and m is the molecular mass of the OH group:
kcat ) 9.83δ0.35β-3.95
(1)
Since the effects of two catalysts on CO2 absorption are sometimes additive and sometimes synergistic when both are used together,2 we have now investigated the use of binary combinations of the catalysts that proved most effective in the earlier study: sodium arsenite, saccharose, and formaldehyde. Theory and Experimental Design The absorption of CO2 in alkaline sodium carbonate/ bicarbonate buffer can be treated as a pseudo-first-order reaction that can be modeled by the equations that follow:3,4
Q)
Nte ) A
x [(
Ce
1 D k t + erf (xkcte) + kc c e 2
)
x
]
kcte exp(-kcte) (2) π
Once the gas-liquid exposure time te and the total contact area A have been calculated as described previously,1 the overall pseudo-first-order rate constant kc can be obtained by fitting eq 2 to the experimental data, and the enhancement factor E and Hatta number Ha can be calculated:
x
2
E ) Q π/4Ce Dte
Ha ) xπkcte/4
(3)
* To whom correspondence should be addressed. Phone: 3481-563100. Fax: 34-81-595012. E-mail:
[email protected];
[email protected].
Table 1. Rate Constants k0 and kcat, Degree of Dissociation, and Molecular OH Density for Each Catalyst1 solute
k0 (1/s)
kcat (m3/(kmol‚s))
δ × 102
β
saccharose arsenite formaldehyde
0.83 0.87 0.86
40.84 310 28.20
0.18 70.1 41.4
0.397 0.405 0.708
When just a single catalyst is present, kc is related to kcat by
kc ) k0 + kcat[cat]
(4)
where k0 is the pseudo-first-order rate constant for the uncatalyzed reaction. The previously determined1 values of the parameters of eqs 1 and 4 for catalysis by sodium arsenite, saccharose, and formaldehyde are listed in Table 1. In this work we studied the effects of two binary combinations of these catalysts, saccharose + arsenite and saccharose + formaldehyde, on absorption of CO2 by 0.5 M/0.5 M sodium carbonate/bicarbonate buffers. The concentrations used, which for each individual catalyst are within the range used in the previous study, are listed in Table 2 together with the corresponding values of relevant concentration-dependent physical properties. The experimental procedure and the reactor, a wetted-wall column, were exactly as reported previously.1 Since it has been reported that the effects of two catalysts on CO2 absorption are sometimes additive and sometimes multiplicative when both are used together,2 the overall pseudo-first-order rate constant in these experiments was assumed to satisfy the following expression:
kc ) k0 + kcat1[cat1] + kcat2[cat2] + akcat1[cat1] kcat2[cat2] (5) where the parameter a was obtained by fitting eq 5 to the experimental results. Results and Discussion For both catalyst combinations, the rate of CO2 absorption increased upon increasing the concentration of either of the catalysts; Figure 1 shows the results obtained with 0.219 M saccharose and various concen-
10.1021/ie980722n CCC: $18.00 © 1999 American Chemical Society Published on Web 03/30/1999
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 2161 Table 2. Physical Properties of a Solution of CO2 Absorption Enhancers in 0.5 M Sodium Carbonate/0.5 M Sodium Bicarbonate Buffer, with the Corresponding Values of kc, the Overall Rate Constant for the Reaction of CO2, at 298.1 K [saccharose] (kmol/m3)
[arsenite] (kmol/m3)
0.073 0.073 0.073 0.146 0.146 0.146 0.146 0.146 0.219 0.219 0.219 0.094 0.146 0.146 0.146 0.205
0.005 0.010 0.015 0.005 0.0075 0.010 0.0125 0.015 0.005 0.010 0.015
[formaldehyde] (kmol/m3)
F (kg/m3)
µ ×103 (kg/(m‚s))
Ce × 103 (kmol/m3)
D × 109 (m2/s)
kc (1/s)
0.27 0.18 0.27 0.36 0.27
1081.9 1081.9 1081.9 1091.8 1091.8 1091.8 1091.8 1091.8 1102.1 1102.1 1102.1 1093.7 1091.8 1100.9 1108.9 1107.1
1.403 1.403 1.403 1.533 1.533 1.533 1.533 1.533 1.668 1.668 1.668 1.490 1.534 1.575 1.601 1.679
18.78 18.78 18.78 18.21 18.21 18.21 18.21 18.21 17.75 17.75 17.75 18.60 18.21 18.21 18.21 17.83
1.47 1.47 1.47 1.37 1.37 1.37 1.37 1.37 1.27 1.27 1.27 1.40 1.36 1.33 1.32 1.27
5.35 6.87 8.39 8.31 9.05 9.79 10.53 11.28 11.25 12.71 14.16 10.46 10.00 11.51 13.13 12.80
Figure 1. Rates of CO2 absorption by buffer solutions containing 0.219 M saccharose, without and with various concentrations of sodium arsenite, plotted against the liquid flow rate.
trations of sodium arsenite. This shows that, in both combinations, both the catalysts are involved in CO2 absorption. However, the absorption rate does not depend linearly on the concentration of any of the catalysts, especially in the case of formaldehyde (Figure 2) and the difference is too large to be attributable to changes in the physical properties of the gas-liquid system. Simplex optimization of the fit between eq 2 and the experimental results afforded the values of kc listed in Table 2. The corresponding values of the enhancement factor and Hatta number are greater than those obtained with the individual catalysts1 for the same exposure times and concentrations (Figure 3). Fitting eq 5 to the kc, [cat1] and [cat2] data, including the single-catalyst data obtained in the earlier study,1 with k0 ) 0.86 s-1 and kcat1 and kcat2 as in Table 1, afforded the values a ) -7.03 × 10-3 s for saccharose + arsenite and a ) -6.39 × 10-2 s for saccharose + formaldehyde, with satisfactory agreement between the experimental kc values and those given by the fitted equations (Figure 4). The negative values of parameter a show that the overall pseudo-first-order rate constant are always less than those corresponding to the additive contribution of enhancers. Thus, the joint catalytic action of solutes are minor, as to be expected, and the enhancement factors and Hatta numbers are between 1 and 15% less
Figure 2. Rates of CO2 absorption by buffer solutions containing 0.146 M saccharose against various concentrations of formaldehyde.
Figure 3. CO2 absorption enhancement factors E plotted against the Hatta number.
than the theoretical values obtained as the sum of their individual effects (see Figure 3). In the earlier study1 it was concluded that, in buffer solutions with a catalyst, the catalytic efficiency depends on the acidity of the catalyst, its mass density of OH groups, and the pH of the buffer solution, in accordance with eq 1. Assuming that the same variables are
2162 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 Ce ) solubility of CO2 in the liquid phase, kmol/m3 D ) diffusivity of CO2 in the liquid phase, m2/s E ) CO2 absorption enhancement factor Ha ) Hatta number kc ) overall pseudo-first-order rate constant, 1/s kcat ) individual rate constant of the catalytic reaction, m3/ (kmol‚s) k0 ) rate constant for the uncatalyzed reaction in buffer solution, 1/s N ) CO2 absorption rate, kmol/s Q ) quantity of CO2 absorbed per unit contact area in exposure time, kmol/m2 q ) flow rate of the liquid phase, m3/s te ) gas-liquid exposure time, s [ ] ) concentration in the liquid phase, kmol/m3 β ) molecular OH density, on a molecular mass basis δ ) degree of dissociation, KA/(KA + [H+]) µ ) viscosity of the liquid phase, kg/(m‚s) F ) density of the liquid phase, kg/m3 Figure 4. Values of kc given by eq 5 plotted against the experimental values, for the reaction of CO2 in buffer solutions containing sodium arsenite, saccharose, formaldehyde, sodium arsenite + saccharose, or saccharose + formaldehyde mixtures.
involved in the antisynergism observed in this study of binary catalyst combinations, it is found that, with good accuracy,
a ) -9.6(β1β2)3.95
(6)
which implies that the coefficient of the quadratic term in eq 5, the term responsible for antisynergism between the catalysts, depends only on the degree of dissociation of the latter, which in turn depends on the acidity of the catalyst and the pH of the solution. Our results therefore reinforce the conclusion1,5,6 that the catalysis of CO2 absorption in alkaline solutions via a shuttle mechanism depends largely on an acid/base reaction that removes a proton from the catalyst. Nomenclature a ) optimized parameter in eq 5, s A ) gas-liquid transfer area, m2
Literature Cited (1) Va´zquez, G.; Chenlo, F.; Pereira, G. Enhancement of the absorption of CO2 in alkaline buffers by organic solutes: relation with degree of dissociation and molecular OH density. Ind. Eng. Chem. Res. 1997, 36, 2353. (2) Alper, E. Kinetics of absorption of CO2 into buffer solutions containing arsenite: effects of buffer composition. Chim. Acta Turcica 1981, 9, 447. (3) Danckwerts, P. V. Gas-liquid reactions; McGraw-Hill Inc: New York, 1970. (4) Charpentier, J. C. Mass-transfer rates in gas-liquid absorbers and reactors. In Advances in Chemical Engineering; Drew, T., Cokelet, G. R., Hoopes, J. W., Vermeulen, T., Eds.; Academic Press Inc.: New York, 1981. (5) Astarita, G.; Savage, D. W.; Longo, J. M. Promotion of CO2 mass transfer in carbonate solutions. Chem. Eng. Sci. 1981, 36, 581. (6) Astarita, G.; Savage, D. W.; Longo, J. M. Promotion of CO2 mass transfer in carbonate solutions. Chem. Eng. Sci. 1982, 37, 1593.
Received for review November 17, 1998 Revised manuscript received February 16, 1999 Accepted March 2, 1999 IE980722N
