Research On The Characteristics Of Heat And Mass Transfer And Energy Conversion In Air Gap Diffusion Distillation

Amount of low grade heat energy(LGHE)below 100℃ can be released in the production processes of industrial enterprises,but it is hardly fully utilized by these exiting technologies.The LGHE comprehensive utilization technology based on reverse electrodialysis(RED)is a novel energy conservation technology,because lots of low temperature LGHE conversion processes can be realized by it,as:’heat to power’,’heat to hydrogen’ or ’heat to waste water treatment energy’.The working principle of this technology is described as:Firstly,the LGHE is converted to salinity energy(SGE)in thermal separation plant;and then,the SGE is converted to other forms of energy in RED stacks/reactor,like:power,hydrogen or waste water treatment energy.It is clearly that the structures,types or working performances of thermal separation plants play crucial roles in the energy conversion process of heat to SGE.However,the exiting thermal separation technology is hard to match with RED stacks/reactor,so a novel thermal separation technology operating at normal atmospheric pressure was proposed by our research team,called ’Air Gap Diffusion Distillation’(AGDD)technology.This technology not only retained these advantages of exiting thermal separation technologies,but also overcame part of their shortcomings.Therefore,this paper is focused on AGDD to carry out research works.Firstly,experimental research was adopted to verify the feasibility and working performances of AGDD.The experimental plant for one air-gap combination of AGDD system was built to test the heat and mass transfer performances in AGDD.The test results shown that not only the heat and mass transfer performances of AGDD system were excellent,but also the desalination rate was high(above 99.8%).Then,based on the experimental results,two numerical research models were respectively built from the perspectives of ’mass-energy conservation’ and ’effectiveness-numbers of transfer units’(ε-NTU),which were respectively called ’AGDD distributed parameter model’and ’AGDD lumped parameter model’.These models were verified by comparing with the previous experimental data and could be applied in the theoretical research of heat and mass transfer performances of AGDD.It could be found by comparing these two numerical research models,not only the experimental results could be precisely predicted by both of them,but also their prediction accuracies were nearly consistent.However,because the AGDD lumped parameter model could be performed without discretizing the computational domain,the computational efficiency of it was much higher than that of AGDD distributed parameter model.The effect of structure and operation parameters on heat and mass transfer performances of AGDD were investigated by these two proposed numerical research models,and the effect degrees of structure and opration parameters were also compared.Research results shown,comparing with the variations of air-gap thickness,inlet temperature and concentration of cold stream,the effect of air-gap height,inlet temperature of hot stream and inlet volume flow rate of cold stream variations on AGDD performance were more significant.Although the working process of AGDD was simple,due to the low solution concentration ratio for once-flowing AGDD system,the SGE feed by it was not enough to drive RED stacks/reactor unit.In order to increase the productive solution concentration ratio of AGDD,the working process of it was improved by proposing an AGDD solution recirculating separation working process(AGDD-SRS).Then,the starting characteristics and the separation performance under steady working state of AGDD-SRS system were respectively discussed by employing AGDD lumped parameter model.The simulated results shown,the heat and mass transfer performances of AGDD-SRS system could not be significantly changed with different solution cooling modes.The separation performance of AGDD-SRS system must be inferior to AGDD system under the same working condition.Therefore,the once-flowing AGDD process should be adopted as much as possible,when there was no solution concentration ratio requirement.Lastly,the indepth exploration of energy conversion performance for AGDD-SRS system was performed by employing the first and second laws of thermodynamics analysis methods.The discussion results indicated that the energy irreversible loss of AGDD-SRS system mainly sourced from the distiller,cooler and heater.In distiller,the total exergy loss rates of hot stream channel and air-gap accounted for about 95%of this unit.The energy conversion performance of ’heat to SGE’ for AGDD-SRS system could be improved by increasing the inlet temperature of hot stream,or by increasing the air-gap height,or by increasing the feed solution concentration properly.All the research works of this dissertation can lay the experimental and theoretical foundations for further studying AGDD thermal separation technology,indicate the improvement directions for enhancing the heat and mass transfer performances of solution thermal separation system operating at normal atmospheric pressure,provide the design method and guidance for LGHE comprehensive utilization technology on basis of RED in the future.