Numerical Analysis of Ammonia Absorption in a Falling Film

Abstract

A numerical analysis of a counter current ammonia-water falling film absorption process in a vertical plate absorber is performed. Simultaneous steady state heat and mass transfer processes occurring during the absorption process are considered. In order to withdraw the heat released near the liquid-vapour interface during the absorption process and thus control the absorption temperature, the plates are supposed to be water cooled. A mathematical model of the process has been built up on the basis of mass and energy balances and heat and mass transfer rate equations. Empirical correlations in combination with the Chilton-Colburn analogy are used to predict the heat and mass transfer coefficients near the liquid-vapour interface. The mathematical model results in a system of differential equations, the numerical integration of which is performed after spatial discretization. The algebraic non linear equations system thus obtained is solved using a Newton-Raphson FORTRAN coded method in combination with an optimization procedure. For a given geometry and operating conditions the model predicts the behaviour of the absorber. Evolution along the absorber length of several parameters is computed such as: temperature and concentration of the vapour and liquid phases, temperature and liquid and vapour compositions at the interface, coolant temperature as well as the ammonia and water molar fluxes through the liquid-vapour interface. It was found that the liquid side mass transfer resistance controls the overall absorption process. The resistance in the vapour phase is negligible. On the other hand, heat transfer resistance is dominant in the vapour phase. The simulation reveals the existence of water desorption phenomenon occurring near the bottom of the absorber: While ammonia vapour is absorbed into the liquid, liquid water evaporated first into the vapour flow.
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