Internal mass transfer

Analogously to external mass transfer, internal mass transfer can be the rate-limiting step. In that case, it appears that the observed activation energy is related to the true activation energy as follows ... 

Managing all complexity Calculation of the overall reaction rate combining external mass transfer, internal mass transfer, and reaction... 

Brauner, N., and Maron, D. M. (1982) Characteristics of Inclined Thin Films, Waviness and the Associated Mass Transfer, International Journal of Heat and Mass Transfer, Vol. 25(1), pp. 99-110. Burdukov, A. P., Bufetov, N. S., Deriy, N. P., Dorokhov, A. R., and Kazakov, V. I. (1980) Experimental Study of the Absorption of Water Vapor by Thin Films of Aqueous Lithium Bromide., Heat Transfer -... 

Comini, G. and Lewis, R.W., 1976, A numerical solution of two-dimensional problems involving heat and mass transfer, International Journal of Heat and Mass Transfer, 19, 1387-1392. 

As this ratio usually has values in the range 10-500 for gas-soM systems, it may be concluded that ACjn, and AT must be very high. In other words, heat transfer would be the controlling resistance externally and mass transfer internally. This can be understood by considering a... 

Separation trays and structured and random packings are the prevailing mass transfer internals at choice for rectification columns and, in first deciding between them, a comparative performance design needs to be prepared including examinations on capacity limitations, pressure drop and separation performance. 

In most chemical reactors, ext al mass transfer to die catalyst particles cannot be neglected. For describing the combined effect of external mass transfer, internal diffusion and chemical reaction (for the limitations indicated above) the following expresssion can be used ... 

Sherwood, T. K., R. L. Pigford and C. R. Wilke. Mass Transfer, Internal Student Edition. McGraw-HUl, Kogakusha, Tokyo, 1975. 

Imaishi N, Suzuki Y, Hozawa M, Fujinawa K (1982) Interfacial turbulence in gas-liquid mass transfer. Intern Chem Eng 22 659-665... 

In the fluid-solid systems, the reaction takes place on the catalyst surface. Prior to this, the reactant molecules have to reach the catalyst surface (Figure 2.9) and therefore the rate of mass transfer is an important operational parameter. Two types of mass transfer need to be considered external and internal mass transfer. Internal mass transfer can be avoided by limiting the thickness of the porous catalytic layer, dcat- Internal mass transfer has a negligible influence on the overall transformation rate, if the following conditions are fulfilled [25] ... 

Pervaporation Selectivity High product enrichment Easy to sterilize Low mass transfer Internal or external... 

In many cases in the Hterature, when experimental data are reported for the influence of the reaction rate or TOP on the cluster size the impact of mass transfer, internal diffosion is not discussed in particular. Intuitively, it can be anticipated that when the experimental observations are influenced by mass transfer, the reaction kinetics and thus cluster-size dependence should be less prominent. For the two-step sequence it is, however, worth to consider such dependence in a quantitative way. As discussed in the previous section, for the case of Langmuir adsorption and subsequent transformation of the adsorbed species, the plots for the catalyst effectiveness factor dependence of the Thiele modulus are available when the rate expression is given by v = kKC/ + KC) and the Thiele modulus rp is rp = L kKCs/ D 1 + KQ)), where Dg is the effective diffosion coefficient, Q is the concentration on the surface and L is the grain characteristic length (ratio of volume to surface area). When 3, the effectiveness factor is inversely proportional to the Thiele modulus and thus to the particle size. 

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