Development of a New Absorption-Demixing Heat Transformer: Cycle and Working Mixture Optimization

Abstract

The recovery of waste heat generated by many processes is one of the measures to reduce energy consumption. Heat pump is a machine which upgrades waste heat to make it useful. This study focuses on the development of an Absorption Demixing Heat Transformer (ADHT), a new type of Absorption Heat Transformer (AHT). An ADHT, using a working mixture exhibiting a miscibility gap at low temperature, can use low temperature heat to produce useful thermal energy at a higher temperature. The distillation column used to separate the components of the working mixture used in classical AHT is replaced by a settler where the separation is energetically free. The aim of this study is the cycle and working mixture optimization to find suitable working mixtures for an industrial use (targeted temperature lift (?Ti) ~40°C). In previous studies, the machine was operated successfully with the n-heptane/N,N dimethyl formamide (DMF) mixture, but has not led to a significant temperature lift (less than 8°C). A simulation code, written in FORTRAN 95, is developed to evaluate the performance of ADHT (temperature lift and thermal yield). This code includes a simplified thermodynamic model to represent the properties (enthalpy, equilibrium phase, etc.) of the mixture. The influence on the performances of operating parameters (column feed ratio and number of theoretical stages, etc.) is studied. The main parameters affecting the performance of ADHT are identified. The influence on the performances of each individual thermo-physical property of the mixture components (heat capacity, enthalpy of vaporization, etc.) is also studied. Key properties are identified including the enthalpy of vaporisation of components. A pilot ADHT unit will be constructed in order to prove the reliability of the machine.