automatic particle counter studies where the upper size is well bounded and the instrument set to give N(L) directly. The conclusion that B = NT/T is well known for MSMPR crystallizers. This equation results directly from equating sources to sinks in the over-all size domain and is already
appreciated by researchers in the field. Department of Chemical Engineering University of Arizona Tucson, Arizona 85721
A. D. Randolph
A Comment on the Chlorine-Sulfur Dioxide Reaction in Aqueous Solution Sir: In a recent paper (Sadek et al., 1977), the use of the chlorine-sulfur dioxide reaction in the aqueous phase was discussed in connection with the control of sulfur dioxide emission from the Hargreaves process. Interestingly enough, the same reaction can be used for the control of unwanted chlorine emissions which occur in power plant cooling water streams (Berker and Whitaker, 1977). In order to keep condenser tubes free from slime, chlorine is periodically dissolved in the cooling water stream yielding hypochlorous and hydrochloric acid. H20
+ Cl2 ~1HClO + HCl
(1)
The former is a powerful biocide while the latter simply increases the chloride ion concentration in the well buffered water supplies used for power plant cooling. After having worked its wonders in the condenser tubes the hypochlorous acid is returned to the local stream or estuary where it is a threat to aquatic life. This threat can be removed by injecting sulfurous acid into the cooling water stream after it passes through the condenser, leading to the oxidationreduction reaction H2SO3
+ HClO
-+
H2S04
+ HCl
(2)
In a well buffered stream this process does nothing more than increase the sulfate and chloride ion concentration. The design of dechlorination units requires knowledge of the kinetics indicated by eq 2, as does the control of sulfur dioxide emission discussed by Sadek. Although apparently
Sir: In Professor Whitaker’s comment on the paper by Sadek et al. (1977) on the simultaneous absorption of Cl2 and SO2, reference is made to the work of Lister and Rosenblum (1963). These investigators studied the rate of reaction of OC1- and S032- in alkaline solution. However, in the Sadek work the absorbent liquids were concentrated mixed acid solutions ( N 7 g-equiv/L) in which the hydrolysis of Cl2 and SO2 is suppressed. Because of this, together with the small and dissociation constants for hypochlorous acid ( K = it is expected that the concentrations bisulfite ion ( K N of OC1- and SO3*- in these solutions will be very low and will not contribute significantly to the overall oxidation-reduction rate in the absorbers. Such will probably not be the case in the
unknown in the chemical engineering literature, the kinetics have been studied by Lister and Rosenblum (1963). Their studies indicated that the reaction takes place between the hypochlorite and sulphite ions and is second order; thus the reaction mechanism is expressed as
c10- + so32-
-
so42-
+ c1-
(3)
rather than eq 2. The rate of reaction is given by (rate of reaction) = k(C10-)(S032-)
(4)
where the rate constant can be expressed as k = A,-AH/RT
(5)
The parameters in eq 5 are given by A = 2.7 X 10g/(g-mol/ L)(s) and AH = 7.5 kcal/g-mol. Lister and Rosenblum indicate that theirs was the first study of the kinetics of this important reaction, and a computerized search of the literature by the author indicates that it is currently the only such study.
Literature Cited Berker, A., Whitaker, S., submitted to J. Wafer Pollut. Control fed., 1977. Lister, M. W., Rosenblum, P., Can. J. Chem., 41,3013 (1963). Sadek, S.E.. Nawrocki, D. A., Sterbis, E. E., Vivian, J. E., Ind. Eng. Chem.,
Fundam., 16,36 (1977).
Department o f Chemical Engineering University of California Dauis, California 95626
Stephen Whitaker
interesting application suggested by Professor Whitaker for which the very fast ionic reaction reduces the problem to one of mixing rather than kinetics.
Literature Cited Lister, M. W., Rosenblum. F., Can. J. Chem., 41,3013 (1963). Sadek, S.E., Nawrocki. D. A,, Sterbis, E. E., Vivian, J. E., hd. Eng. Chem., fundam., 16,36(1977).
Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts 02139
J. Edward Vivian
ind. Eng. Chem., Fundam., Vol. 16, No. 3, 1 9 7 7
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