When gas absorption is performed in an irrigated packed tower, the following conditions may be assumed:
i) Flowing characteristics of liquid are specified by the mean flow velocity, u, and the apparent mixing coefficient, E.
ii) The rate of mass transfer across the interphase is proportional to the difference between the bulk concentration in the gas and that in the liquid stream, and the capacity coefficient is constant throughout the tower.
iii) The composition of the gas is uniform in the tower.
Eq. (1) is the basic differential equation governing the concentration gradient in the flowing liquid, where, in the boundary conditions, Eq. (2) or Eq. (3) is to be applied. Thus, when the mixing coefficient, E, in the tower, is small, Eq. (5), and when large, Eq. (4) is derived from Eq. (1). Eq. (5) may be replaced by the approximation, Eq. (6). The degree of absorption calculated by means of Eq. (5) is illustrated in Fig. 1, as a function of dimensionless groups (kLa/Ht)θ and M. kLa is the modified iquid-side capacity coefficient, obtained, from Eq. (4) or Eq. (5), by taking into account the mixing characteristics of liquid flowing through the tower.
Fig. 2 shows the experimental apparatus for determining kLa. The total holdup Ht and the mixing coefficient E of the packed tower used in this study have been measured, in our preliminary experiments, by the direct weighing method and the residence time curve method, which have determined the correlations of Ht and E to u, as shown by Eqs. (9) and (10). (Cf. our previous paper
3)).
In this tower, water flows countercurrently to CO
2 gas which is transferred at 15°C.
Fig. 4 shows the results of these experiments, as well as kLa calculated from the experimental data obtained by Hikita et al1), and Onda et al
2), as shown in the table, by making use of Eq. (11) or Eq. (12) and general correlation of Ht and E to u, Eqs., (14), (15) and (16). The correlation of kLa to Reynolds numbcr, (dpuρ/μ), is expressed by Eq. (17).
The results of this study indicate that the absorption of CO
2 in water is rapid and that when a short packed tower is used, the error due to the longitudinal mixing of liquid has a serious effect on the measuring of kLa of this system, and when a long packed tower is used, the same is true of the error due to the high degree of absorption, as suggested by Eq. (5) or Fig. 1. For determining kLa or predicting the degree of absorption, it is more reasonable to use Eq. (5) than Eq. (7).
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