近沸点工质膜分离过程热质耦合特性与非平衡级机理研究

In the refrigeration system, ammonia does not cause the ozone depletion and greenhouse effect. Ammonia-water absorption refrigeration system can effectively utilize waste heat or renewable energy to achieve the temperature below 0℃. However, the ammonia absorption refrigeration system has not widely applied in industrial applications yet, partly due to the small difference in the standard boiling point of ammonia and water of 133.4℃. The generated ammonia contains water, which decreases the efficiency and narrows the working range of the ammonia-water absorption refrigeration system. Thererfore, a distillation equipment has been utlized for the separation of ammonia and water. However, the normally used distillation collum, using plate equilibrium model to design, is bulky in relating to the working environment, because the research of purification in distillation collum is not very clear..This proposal investigate the application of the membrane contactor (MC) in place of distillation collum. We aim at providing a theoretical basis for regulating the cooling capacity and control strategy of MC and obtain a quantitative relationship between heat and mass transfer in MC in order to clearly describe the Non-equilibrium stage of the ammonia water membrane distillation process.The differences of MC and distillation collum in ammonia distillation process will be analyzed; the micrcopic analysis of the mass transfer area, heat and mass transfer coefficient under different gas-liquid contact will be conducted to establish a Non-equilibrium model to obtain optimal parameters such as the pressure, temperature and concentration in membrane distillation process; both simulation and experimental research of ammonia concentration's impact to the refrigeration system performance will be carried out, and the experimental and the impact properties such as the material status of MC inlet, the occurrence of temperature and other parameters will be optimized. The research will create a solid basis for the efficient separation of the nearly boiling point medium, especially have an important practical influence on the optimum design of the distillation equipment and dimension of the absorption refrigeration system.

氨吸收制冷系统可利用余热驱动制取0℃以下低温。工质氨不破坏大气臭氧层，不产生温室效应，但氨与水标准沸点相差较近（133.4℃），分离产生的共沸现象会影响制冷剂纯度。含水分的氨蒸汽在蒸发器中聚集引起分压力下降，影响系统效率和工作范围，蒸发温度越低影响越大，需在系统中设置精馏设备提高制冷剂纯度。.目前，由于不能明确精馏塔传递过程机理，导致设备尺寸庞大，为增大接触面积、减小不利影响，将膜接触器应用于精馏过程是值得开发的新技术。本项目选择氨水分离过程进行研究，在膜材料和结构特性优化的基础上，从气液两相在膜界面接触微观角度出发，结合工质物性探讨膜接触器中气相、膜和液相间阻力的分布规律；探讨边界条件对热质传递驱动力的影响及驱动力内在分布之间的关系；以非平衡级理论为基础建立具有普遍应用意义的近沸点分离热质耦合模型。研究成果对优化近沸点工质分离设备设计，推动吸收式制冷系统实用化，拓宽应用场合具有重要意义。

与传统分离单元操作相比，膜接触器能提供更大传质比表面积和流体相对独立运行场所，将膜蒸馏应用于近沸点工质分离是值得开发的新技术。本项目采用数值模拟和实验研究的方法开展以下研究。利用等离子体改性疏水膜的制备方法，研究了膜材料结构参数对传递过程的影响。结果表明：采用氩气、四氟化碳等离子气体对疏水膜进行改性，可得到更适合气液分离的超疏水蒸馏膜。在不影响蒸馏膜结构参数的前提下，在膜表面产生多维表面粗糙度，从而获得超疏水蒸馏膜。采用更为精确的膜表面温度与溶液进出口平均温度平均值作为主体温度，通过实验测定膜两侧料液温度、流量对渗透通量的影响，考虑了浓差极化及两侧流体密度、热容、粘度和导热系数随浓度及温度的变化关系。通过比较理论模型与实验数据，得到了具有预测结果的直接接触式膜蒸馏传递过程模型。经实验验证结果对模型修正后，该膜分离过程传质系数值为7E-7kg/m2sPa，符合文献报导数据。通过对氨吸收制冷系统中主要部件建立稳态集总参数模型，分析了精馏后氨气浓度对制冷系统性能的影响。随着制冷剂中氨气浓度的升高，制冷剂中水分含量的降低会使系统的蒸发压力升高。在相同的压力下，氨制冷剂纯度越高，对应的饱和温度就越低，当氨的浓度降低时，将导致实际蒸发温度比浓度较高时氨作为制冷剂的蒸发温度高。在相同蒸发温度下，其蒸发压力相比浓度较高的氨作为制冷剂时的蒸发压力要低。搭建了真空膜蒸馏与直接接触式膜蒸馏一体化实验装置。采用直接接触式膜蒸馏过程对氨水溶液进行分离，分离后的渗透液电导率快速上升，渗透液碱性增强，有大量的NH3蒸发到了渗透侧，改性膜的通量要优于未改性膜。采用30wt%的乙醇用于求证将膜蒸馏过程进行近沸点工质分离时的性能，实验同样证明膜蒸馏过程实现了近沸点工质间的分离，且改性膜的通量下降速度明显小于未改性膜。膜组件结构设计和优化对分离性能也有重要的影响。采用Fluent软件研究中空纤维膜组件在不同情况下壳程内部流动状态的影响规律。研究结果表明：进入膜组件壳程的液体流量存在最优值，既要保证流体在膜丝间有足够大的流速，还要降低流体流入时动能对膜丝的影响。膜组件的出口位置尽量设置在膜组件的终端，以减少膜丝间滞留区域。项目研究成果将对优化近沸点工质分离设备设计奠定技术基础。目前本项目已在国内外期刊发表研究论文5篇，申请专利2项，协助培养硕士研究生2名。