Mathematical model and experiment investigation on electrodialysis separation process of LiBr from NH3-H2O-LiBr ternary solution in ammonia absorption refrigeration system

Abstract

The utilization of NH3-H2O-LiBr ternary working fluid is effective to improve the COP of the ammonia absorption refrigeration system (AARS). However, LiBr impairs the absorption of NH3 in the absorber and consequently limits the COP promotion. To address this limitation, electrodialysis (ED) is introduced into AARS to reduce the LiBr concentration in the absorber because it has been widely applied in ion separation from industrial waste solutions. In this study, a simulation model for the NH3-H2O-LiBr ternary solution ED separation process was developed based on the overall mass transfer equation. Experiments were conducted to measure and verify the coefficients in the model. The results showed that the NH3 transport was hard to be separated by ED. For the LiBr, the overall transport number and the overall electro-osmotic permeability were −3.49 × 10−3 kg min−1 A−1 and -1.33 × 10−4 L min−1 A−1. The overall diffusion permeability, overall osmosis permeability, and average resistance were dependent on the LiBr concentrations in the solutions. Moreover, the influences of ED parameters on the LiBr separation were studied using the model and the energy efficiency was applied to evaluate the separation performance. The energy efficiencies were 0.0325 and 0.323 when the membrane pair exceeded 10 and the membrane effective area (Aeff) surpassed 0.0325 m2. The energy efficiency increased with the decrease of current density and the increase of LiBr concentration difference between the two solutions. Moreover, an effective method was built to obtain a set of suitable ED parameters that meet high ED energy efficiency and the LiBr separation requirements in AARS.