392d

In analyses of local rates of absorption with chemical reaction in packed columns - e.g., based on Film Theory - it is generally assumed that reaction equilibrium (or, in the case of irreversible reaction, zero concentration of dissolved gas) is closely approached in bulk liquid. This paper outlines an analysis that identifies circumstances under which this assumption breaks down and estimates the consequent errors in design calculations. The assumption of bulk liquid equilibrium is typically justified by the relative magnitudes of: (a) h, the liquid holdup, and (b) aδ, the product of interfacial area per unit volume and the thickness of the liquid film. However, the assumption must break down when reaction is sufficiently slow, and relative reaction kinetics underlie absorption-based separations of considerable practical importance. For example, the H2S/CO2 selectivity of alkaline scrubbing solutions is attributable to the effectively instantaneous dissociation of dissolved H2S and slow CO2 hydrolysis. In such cases, design errors are introduced either by assuming the slow reaction attains equilibrium in bulk liquid or by neglecting it altogether. To estimate the magnitudes of such errors, an analysis of absorption of gas A accompanied by the reversible reaction A + B = C was undertaken in the context of steady-state Film Theory. Nonlinear ordinary differential equations governing diffusion and reaction in the liquid film were coupled to nonequilibrium reaction in well-stirred bulk liquid. The aim was to calculate rates of mass transfer at the liquid film interfaces with gas and bulk liquid, and thereby to relate the extent of absorption to column height. The equations were solved using regular perturbation methods: local liquid film concentration profiles and the rate of change of [A] in bulk liquid with column height, were expressed as power series in the parameter ε defined as the ratio of the characteristic times of liquid-phase diffusion and reaction. The solution to second order in ε makes clear the range of conditions over which the first-order solution is exact. The latter solution, in turn, facilitates identification of conditions in which the assumptions of bulk liquid equilibrium or negligible bulk liquid reaction introduce substantial errors in design calculations.

See more of #392 - Advances in Distillation Modeling and Processes II (02A04)

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See more of The 2007 Annual Meeting

See more of Separations Division

See more of The 2007 Annual Meeting