电渗析分离NH3-H20-LiBr三元溶液中LiBr的机理及其对氨吸收式制冷性能的影响

Based on the research about NH3-H2O-LiBr ammonia absorption refrigeration cycle of other experts, electrodialysis method was proposed to separate LiBr from the NH3-H2O-LiBr ternary solution stream entering absorber from generator, so as to strengthen its positive effect in generation process and reduce or eliminate the inhibition of LiBr in absorption process. After theoretical analysis of heat and mass transfer mechanism on LiBr separation by electrodialysis from the ternary solution, A mathematical model of mass transfer will be established to describe the LiBr separation effectivity from NH3-H2O-LiBr ternary solution under different membrane specification (conductivity, ion transport number ,diffusivity) solution parameters(temperature, pressure, concentration, flow rate) and electrodialysis parameters(voltage, current). An electrodialysis experimental system for LiBr separation will be built, the characteristic data of LiBr separation by electrodialysis under working condition of ammonia absorption refrigeration cycle will be obtained in order to verify and optimize the heat and mass transfer model of LiBr separation by electrodialysis. And then the best ion exchange membranes for LiBr separation under different working conditions will be selected. The heat and mass transfer models of LiBr separation in electrodialysis device will be integrated with the thermodynamic model of NH3-H2O-LiBr ternary working fluid ammonia absorption refrigeration cycle to study the performance of the NH3-H2O-LiBr ammonia absorption refrigeration system with electrodialysis device and the influence of LiBr separation by electrodialysis device on the performance of NH3-H2O-LiBr ammonia absorption refrigeration will be concluded. This research provides a theoretical fundamental for analyzing the influence of LiBr separation by electrodialysis on improving the performance of NH3-H2O-LiBr ammonia absorption refrigeration system.

基于氨-水-溴化锂三元工质氨吸收制冷的已有研究，提出电渗析方法分离氨-水-溴化锂三元溶液中溴化锂的设想，使溴化锂在溶液循环中保留在发生器，尽量少进入吸收器，保持其对发生过程的强化，减弱或消除对吸收过程的抑制，进一步提升三元工质氨吸收制冷的性能系数。通过理论分析电渗析分离三元溶液中溴化锂的传热传质规律，建立溴化锂分离的传质数学模型，阐明膜参数(电导率，迁移数、扩散系数)、溶液参数(温度、压力、浓度、流速)及电参数(电压、电流)对分离特性的影响机理；搭建电渗析分离实验装置，获得吸收式制冷工况下溴化锂分离的特性数据，验证并完善电渗析分离溴化锂的传质计算模型；优选三元工质氨吸收制冷高温高压工况下具有高溴化锂分离效率的离子交换膜，将其电渗析分离模型与三元工质氨吸收式制冷的热力学模型结合，分析电渗析分离溴化锂对制冷性能的影响规律，为明确电渗析分离溴化锂对三元工质氨吸收式制冷性能提升的机理提供理论依据。

为了进一步提升氨-水-溴化锂三元工质氨吸收式制冷性能，提出电渗析方法分离氨-水-溴化锂三元溶液中溴化锂的设想。首先实验研究氨-水-溴化锂三元工质对氨吸收式制冷性能的影响特性，发现15%是最佳溴化锂浓度，COP可直接提升8.7%，因为溴化锂提高了发生热利用效率并降低精馏冷负荷，但当其浓度达到20%时循环倍率增长了121%导致COP降低。为了探索电渗析降低吸收器中溴化锂浓度提高三元工质系统性能的机理，采用总传质方程法建立了电渗析分离氨-水-溴化锂三元溶液中溴化锂的传质计算模型，模型中考虑了电参数(电压、电流)、膜参数(电导率，迁移数、扩散系数)和溶液参数(流量、浓度、压力)对传质的影响，计算了溴化锂、氨和溶液在电和浓差驱动下的跨膜传质；计算结果表明溴化锂的分离率可达98%。根据模型搭建了电渗析分离实验台，改进并建立了氨和溴的联合滴定法，提升氨和溴化锂浓度的测量的精度和可靠性。最后根据三元系统性能实验确定溶液和电参数确定实验工况，完成不同工况下电渗析分离三元溶液中溴化锂的实验，对电渗析分离传质计算模型进行了验证。结果首先证明了实验结果与理论模型吻合，电渗析可以高效完成工质中溴化锂的分离；其次大电流适用于溴化锂分离，减少分离时间同时提高分离总物质中溴化锂的比例，实现高速且高效的溴化锂分离传质；分离终点溴化锂浓度为3%到4%最优，浓度过高会降低对吸收传质的改善，浓度过低时溶液电阻升高导致电压急速上升损伤电渗析设备。基于欧姆定律建立了电渗析能耗计算模型并分析其对三元工质系统的影响，通过实验和溶液电导率数据对能耗计算模型进行了验证，结果表明三元溶液电阻远小于氨水，所以电渗析能耗低且在氨吸收式制冷系统总能耗中占比极少，依据实验中溶液物性数据对模型进行了完善，提高了能耗计算模型预测精度。三元工质具有提高氨发生和精馏和阻碍氨吸收的特性，利用其特性在吸收-再吸收热泵，建立模型计算发现性能可以提升9%；使用超声波和纳米流体提升氨吸收传质，建立降膜吸收计算模型和进行三元工质那纳米系统性能实验，初步分析可以提升三元溶液的氨吸收能力，但能耗和纳米稳定性有待深入研究。