Characterisation of irreversibility in the heat and mass transfer process involving liquid absorbents: An alternative approach based on entransy theory

The absorber and generator are the key components of the absorption system, where heat transfer and mass transfer is coupled along with the concentration change of the solution mixtures. Quantifying the irreversible dissipation in the transfer processes is of guiding significance for further optimisation. As there is no heat-work conversion in the processes, it is appropriate to characterise the irreversibility using entransy dissipation. However, the current entransy analysis is not fully developed when dealing with concentration change of liquid absorbents. The mass transfer irreversibility between the vapour and the absorbent was not considered and the entransy was not balanced. In such case, the role played by the absorbent is analysed and the “concentration entransy” is defined in this paper based on the saturation temperature of the absorbent. It associates vapour transfer with latent heat transfer and enables entransy dissipation to characterise mass transfer irreversibility. Owing to the unified dimension, the entransy balance equation is derived by taking into account the concentration entransy change and the entransy dissipation in the mass transfer process. Then, an integral method and a simplified method based on the entransy balance equation to calculate entransy dissipation are presented. According to a calculation example, the results can tell whether the heat transfer or the mass transfer is more crucial to strengthen. Further, the entransy balance equation in the dehumidifier involving liquid desiccants is deduced based on the new concept of concentration entransy. The present work provides an alternative view of the simultaneous heat and mass transfer processes and lays the foundation for the future entransy analysis of systems concerning liquid absorbents or desiccants.