Transport through a slab membrane governed by a concentration-dependent diffusion coefficient. Part I. The four time-lags: Some general considerations

被引:14
作者
Ash, R [1 ]
Espenhahn, SE [1 ]
机构
[1] Univ London Imperial Coll Sci Technol & Med, Phys Chem Labs, Dept Chem, London SW7 2AZ, England
关键词
diffusion; gas and vapour permeation; integral diffusion coefficients; theory; time-lags;
D O I
10.1016/S0376-7388(98)00286-5
中图分类号
TQ [化学工业];
学科分类号
0817 ;
摘要
Two (equivalent) sets of expressions have been derived for the four time-lags, L-l(a), L-0(d), L-l(d), L-0(a), associated with transport through a slab membrane under 'simple' boundary conditions and with the diffusion coefficient, D, a function of the concentration, C, of the disusing species. The individual sign and relative magnitudes of the time-lags have been determined. Order of magnitude has been derived for two particular classes of D(C): Class (A): D(C) a strictly-increasing function of C and Class (B): D(C) a strictly-decreasing function of C, thereby establishing the two time-lag sequences: (A) L-0(a) < L-l(d) < 0 < L-0(d) < L-l(a), (B) L-l(d) < L-0(a) < 0 < L-l(a) < L-0(d). The four time-lags have been employed to define four integral diffusion coefficients: (D) over tilde(L)(a), (D) over bar(L)(d), (D) over tilde(L)(d), (D) over bar(L)(a). these (time-lag) integral diffusion coefficients and the corresponding steady-state integral diffusion coefficient, (D) over bar, have been investigated. For the particular Classes (A) and (B) functions considered, the sequences of time-lag moduli are: (A) 2L(0)(d) < \L-l(d)\ < \L-0(a)\ < 2L(l)(a), (B) 2L(0)(d) > \L-l(d)\> \L-0(a)\ > 2L(l)(a). (C) 1999 Elseiver Science B. V. All rights reserved.
引用
收藏
页码:105 / 119
页数:15
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