Mass-transfer resistance

As the interface offers no resistance, mass transfer between phases can be regarded as the transfer of a component from one bulk phase to another through two films in contact, each characterized by a mass-transfer coefficient. This is the two-film theory and the simplest of the theories of interfacial mass transfer. For the transfer of a component from a gas to a liquid, the theory is described in Fig. 6B. Across the gas film, the concentration, expressed as partial pressure, falls from a bulk concentration Fas to an interfacial concentration Ai- In the liquid, the concentration falls from an interfacial value Cai to bulk value Cai-... 

Solution Ohmic Resistance Kinetic Resistance Mass-Transfer Resistance... 

This model is applicable to the reactions of nonporous pellets and to porous pellets when the global rate is controlled by pore diffusion. Reaction is limited to a surface separating the solid reactant at the core of the pellet surrounded by a porous layer of solid product. It occurs initially on the external surface of the pellet, and the thickness of the product layer increases as the reaction proceeds, as illustrated in Fig. 1. The global reaction rate is determined by three resistances— mass transfer from bulk gas to particle surface, diffusion... 

A tank reactor and separator (Fig. 12-6) are used to study the heterogeneous reaction between pure liquid A (phase 1) and reactant B dissolved in phase 2 (also liquid). The solvent in phase 2, reactant B, and the products of reactior are all insoluble in liquid A. No reaction occurs in the separator. The reactor operates isothermally at 25°C, and at this temperature A has a limited solubility in phase 2, the value being 2.7 x 10 g mole/liter. Phase 2 is dispersed aj bubbles in continuous phase 1, which is recycled. There is excellent stirring in the reactor, but the fluid motion within the bubbles of phase 2 is insufficien to prevent some mass-transfer resistance. From independent measurements it is estimated that at average conditions the reaction resistance within the bubbles is 75% of the total resistance (mass-transfer plus reaction resistance) (n) Derive a relationship between the concentration of reactant B entering the reactor in phase 2 and the concentration leaving the separator. 

Some implications of this resistance term can be observed immediately. First, the larger the value of k, the smaller the resistance. Second, the value of each resistance can be different. When one resistance is significantly larger than the other (or the mass transfer coefficient for one phase is significantly smaller), it is dominant and is termed the controlling resistance. Mass transfer across both films is controlled (limited) by the dominant resistance. Third, the larger the value of m, the larger the liquid-phase resistance (can you see why physically ). 

Ratio of the internal species transfer resistance to the boundary layer species transfer resistance Mass transfer between fluid and solid... 

We have already mentioned that, even in his early work, Golay proposed [30-32] that in order to increase the column capacity ratio one should deposit a stationary liquid phase layer not onto the smooth inner walls of capillary columns but rather onto a porous layer of the solid carrier located on capillary walls. If this is done, the stationary liquid phase film thickness on the separate solid support particles can remain equal to that in the case of a smooth capillary wall (and, hence, resistance mass transfer of liquid phase layer remains practically unchanged), but the amount of stationary liquid phase per unit column length significantly... 

McKay, G and Bino, M.J., Application of two-resistance mass transfer model to adsorption systems, Chem. Eng. Res. Des., 63, 168-174 (1985). 

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