Heat and mass transfer in a bubble plate absorber with NH3/LiNO3 and NH3/(LiNO3 + H2O) mixtures

An experimental study of bubble absorption in a plate heat exchanger using ammonia/lithium nitrate and ammonia/(lithium nitrate + water) mixtures has been carried at operating conditions of air-cooled absorption systems driven by low temperature heat sources. An experimental test has been layout and set-up for the absorber characterization at different operation conditions. Experiments have been performed at a nominal system pressure of 510 kPa absolute using a corrugated plate heat exchanger formed by three channels in which absorption takes place in the central one. A sensitive study of the main operating conditions such the weak solution inlet concentration and flow rate, and cooling-water inlet temperature and flow rate on the absorber efficiency parameters has been performed. For both binary and ternary mixtures, the mass absorption flux, heat transfer coefficient, subcooling and mass transfer coefficient increase as the solution flow rate increases. The mass absorption flux achieved with the binary mixture is enhanced as the cooling-water inlet temperature decreases. This trend is reversed for the solution-side heat transfer coefficient. This is attributed to a limiting heat transfer process in the absorber at lower cooling-water inlet temperatures. Increasing the concentration of ammonia in the binary mixture by 3% by weight significantly reduces the mixture's capacity to absorb ammonia. The mass absorption flux and the solution heat transfer coefficient achieved with the ternary mixture are around 1.3-1.6 and 1.4 times higher, respectively, than those of the binary mixture under similar operating conditions. This is due mainly to the lower viscosity of the ternary mixture and the high affinity of ammonia for water. Empirical correlations for the solution Nusselt and Sherwood numbers are proposed on the basis of the experimental data presented here for the absorption of ammonia vapor by ammonia/lithium nitrate mixture in a plate heat exchanger. (C) 2012 Elsevier Masson SAS. All rights reserved.