Analytical investigation of two different absorption modes: falling film and bubble types

The objectives of this paper are to analyze a combined heat and mass transfer for an ammonia–water absorption process, and to carry out the parametric analysis to evaluate the effects of important variables such as heat and mass transfer areas on the absorption rate for two different absorption modes — falling film and bubble modes. A plate heat exchanger with an offset strip fin (OSF) in the coolant side was used to design the falling film and the bubble absorber. It was found that the local absorption rate of the bubble mode was always higher than that of the falling film model leading to about 48.7% smaller size of the heat exchanger than the falling film mode. For the falling film absorption mode, mass transfer resistance was dominant in the liquid flow while both heat and mass transfer resistances were considerable in the vapor flow. For the bubble absorption mode, mass transfer resistance was dominant in the liquid flow while heat transfer resistance was dominant in the vapor region. Heat transfer coefficients had a more significant effect on the heat exchanger size (absorption rate) in the falling film mode than in the bubble mode, while mass transfer coefficients had a more significant effect in the bubble mode than in the falling film mode.