Irreversibility characterization and analysis of coupled heat and mass transfer processes in an absorption system

Abstract

Irreversibility analyses identify system inefficiencies and the potential for improving each component. Absorption systems do not involve heat-work conversion, so the irreversibilities can be characterized by the entransy dissipation. However, previous entransy analyses were not developed for coupled heat and mass transfer processes involving concentration changes of the liquid absorbent. Thus, a “concentration entransy” is defined in this paper based on the saturation temperature of the absorbent. The concentration entransy associates the vapor transfer with the latent heat transfer so that entransy dissipation can be used to characterize the mass transfer irreversibility. An entransy balance equation is then derived by taking into account the concentration entransy change and the entransy dissipation in the vapor transfer process. In addition, an equivalent T-Q diagram for the vapor absorption process is introduced to intuitively analyze the main factors causing the irreversibilities. An entransy analysis for an absorption chiller is then compared with conventional entropy and exergy analyses, which shows that these methods are generally consistent in identifying the key source of the irreversibility with the entransy analysis supplying more information and more detailed guidance. This analysis provides insights into the internal processes in the absorber and explicitly indicates how the flow rates should be adjusted and whether the heat transfer or mass transfer should be enhanced.