Experimental characterization of heat and mass transfer in a horizontal tube falling film absorber using aqueous (lithium, potassium, sodium) nitrate solution as a working pair

This paper deals with an experimental study which was carried out to investigate the absorption process with the aqueous solution of nitrates (Li, K, Na) in salt mass compositions of 53, 28 and 19%, respectively, used as a working fluid in a horizontal tube falling film absorber. This working fluid was proposed for use in the last stage of triple-effect absorption cooling cycles driven by high temperature heat sources (up to 260 °C). Taking into account these operating conditions, an experimental mini-absorber was designed and built. In a previous study, the optimal configuration of the solution distributor was determined based on flow visualisations to ensure a uniform distribution of the solution on the external surface of the tubes. A sensitivity study of the operating variables of the absorber was then carried out to evaluate a series of efficiency parameters for the absorber. At the operating conditions established in the experimental tests, the absorption rate ranged from 2.83 to 6.55 g·m−2·s−1, the heat transfer coefficient of the falling film varied between 631.9 and 1,715.8 W·m−2·°C−1, while the mass transfer coefficient was between 2.1 × 10−5 and 6.0 × 10−5 m·s−1.