| Desiccant solution dehumidification technology provides an energy-saving humidity control method,which can reduce moisture of the air to target working condition without cooling the air to dew point temperature,maximally lowering the demand of using traditional compression refrigeration method,so as to abandon the use of refrigerants that are harmful to the environment.The use of membrane-based dehumidifiers can prevent the solution droplets from entering the air,but the membrane material will bring additional mass transfer resistance.In order to eliminate or reduce the additional mass transfer resistance of membrane,this paper proposed a membrane-based dehumidification method using low-grade heat source to increase the air temperature so as to enhance the water vapor transfer performance by thermal diffusion,and built a solar heating-membrane-based dehumidification system in which the hydrophilic/hydrophobic composite membrane serving as the separation medium.Utilizing the structural characteristics of the asymmetric membrane: the hydrophilic layer can help achieving the thermal diffusion enhancement effect while the hydrophobic layer can prevent the solution from penetrating in,and matched the corresponding solar heatingmembrane-based dehumidification system which improves the latent heat efficiency of the dehumidification process,so that the dehumidification-regeneration cycle was performed at a lower solution concentration.An air-liquid dual-chamber experimental system was also built to explore the temperature distribution and concentration distribution of the thermal diffusion process,as well as the measurement of the diffusion coefficient.The work of this paper was based on the two experimental systems.The main research contents and conclusions are as follows:(1)The feasibility analysis and proof of the thermal diffusion process were carried out by using the non-equilibrium thermodynamics theory.According to the Onsager reciprocal relationship,the dual driving force expression of mass flux with respect to temperature gradient and chemical potential gradient was derived.The maximum point of the chemical potential gradient with respect to the temperature was obtained as well,and the minimum expression of the temperature gradient that makes the total entropy increase positive was obtained,thereby demonstrating the feasibility of the thermal diffusion effect to enhance the mass transfer process.(2)A two-dimensional unsteady state mathematical model was established for the heat and mass transfer process of the air-liquid dual-chamber experimental system,and an improved time lag method was proposed to measure the diffusion coefficient.The typical working condition groups and the control groups were simulated in FEATool Multiphysics.The simulation results showing the temperature distribution and concentration distribution in the air chamber verified the Dufour effect and Soret effect of the coupled heat and mass transfer process involved in the thermal diffusion enhancement process of membrane-based dehumidification.A two-dimensional unsteady state calculation model for heat and mass transfer of the internally cooled quasi-counterflow flat plate membrane dehumidifier was established.According to the simulation results,the maximum designing flow rate ratio in the membrane dehumidifier was 1m/s for the air flow and 0.1m/s for the solution flow.The experimental design of the internally cooled quasi-counterflow flat plate membrane dehumidifier was given,which includes three factors: the flow rate ratio of air and solution,the temperature gradient across the membrane in the thermal diffusion process and the average temperature of the membrane,and the mass fraction of the desiccant solution.(3)In order to explore the membrane-based dehumidification experiment with enhanced thermal diffusion,preliminary preparations were made,including the design and construction of an air-liquid dual-chamber experimental system and a solar heating-membrane-based dehumidification system,as well as the preparation of the membrane materials used in the experimental systems and detailed introduction of the chemical properties and characterizations.The test instruments used in the experiment process and its basic parameters were introduced in detail and the experimental system error analysis was carried out.The final total error of vapor diffusion resistance(!)was 2.45%,and the final total error of vapor diffusion flux(!)was 2.3 %,the errors of vapor flux measurement value of non-isothermal conditions were within ±5%,and the accuracy of the measurement error met the experimental requirements.The chemical structure and characterization of the three hydrophilic/hydrophobic composite membranes used in the experimental systems all met the design requirements.(4)Water vapor transfer mechanism of the thermal diffusion-enhanced membrane dehumidification process was analyzed,and the water vapor transfer process in the membrane dehumidifier was determined as combination of four transfer mechanisms,among which the dissolution diffusion mechanism plays a key role in the thermal diffusion process effect.The three factors that affect the heat and moisture exchange effect between humid air and desiccant solution across the membrane during the dehumidification process of the thermal diffusion enhanced membrane were analyzed and the conclusion can be drawn as: the faster the solution flow rate,the higher the surface renewal rate on solution side,and the membrane material is in a faster the desorption rate state;when the average temperature of the membrane is higher than the critical average temperature,the dehumidification rate per unit area will increase significantly,and at a certain temperature,the permeation activation energy of the membrane will drop to the corresponding threshold,causing reduce of water vapor transport resistance of membraned,and as the temperature continues to rise,a higher dehumidification rate per unit area can be achieved;the water vapor partial pressure difference across the membrane is still the main driving force for water vapor transfer,and the temperature gradient formed by Soret effect only serves as an auxiliary diffusion effect.Compared with the dehumidification rate gain effect from increasing the mass fraction of the solution,the increase of temperature gradient has a limited gain effect on the dehumidification rate.The latent heat efficiency of the thermal diffusion enhanced solution dehumidification process of the membrane dehumidifier made of two membrane materials was tested,and it was concluded that the thermal diffusion effect significantly improved the latent heat efficiency of the membrane solution dehumidification process.The Nusselt number and Sherwood number of the indirect heat and mass exchange membrane dehumidifier were fitted by the Gratz number and the mass transfer Gratz number,and the deviation of the fitted Nusselt number was within ±5%.The deviation of the Sherwood number was within ±12%,and the air-side operating conditions can be estimated more accurately.Finally,the proposed membrane dehumidification method for enhancing the water vapor transfer performance by thermal diffusion not only ensures that the solution droplets do not enter the air side,but also solves the problem that the additional mass transfer resistance of the membrane reduces the dehumidification performance.According to the mechanism of water vapor transmembrane transfer,the reason why the thermal diffusion process can improve the dehumidification efficiency of the membrane was pointed out: the low surface water vapor partial pressure provided by the dehumidifying solution on the downstream side is used to solve the problem of desorption and transfer capacity limitation of the hydrophilic layer.The transmembrane temperature gradient and the average temperature of the membrane can increase the desorption rate of the membrane material,so that the desorption rate on the solution side,the adsorption rate on the air side and the diffusion rate inside the membrane reach a dynamic balance,and the composite membrane has been transferred from a pure gas-liquid barrier into a temporary solid desiccant,forming a more efficient and faster adsorption-desorption cycle between the humid air and the dehumidifying solution. |
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