Analog Computation And Dynamic Analysis Of Distillation Column In Ammonia-Water Absorption Refrigeration Systems

Ammonia-water absorption refrigeration system is a kind of refrigerant circulation process based on consumption of thermal energy. The advantages of ammonia-water absorption refrigeration system are the use of low-grade heat, low power consumption, strong adaptability of energy and environmentally friendly refrigerants. Because the refrigerant ammonia have similar standard boiling point with water, the gas generated in the generating process contains some vapour which causes great influence on the COP of the ammonia-water absorption refrigeration system. That is why distillation column is used to purify the ammonia gas in the refrigerating system. The operation condition of ammonia-water absorption refrigeration system is not only related with the temperature of heat source and the refrigerating temperature, but also related to the environmental temperature. The operation condition varies with the changing environmental temperature. The distillation column often works under variable working condition and various disturbance. The rectification effect of the distillation column directly affects the performance of the refrigeration system. So the research of optimization of operating conditions in steady state and dynamic control method on distillation column should raise enough concern. Traditional distillation column is most common in current practice. In this paper, appropriate property equations of NH3-H2O phase equilibrium was selected and the mathematical model of distillation column in ammonia-water absorption refrigeration system was built. Some improvement measures are proposed for the existing distillation column through the analysis of the parameters. The conclusions obtained make active guidance meaning to further increase the COP of the ammonia-water absorption refrigeration system. Then dynamic analysis was carried out to build necessary control system for the distillation column. The main contributions are embodied in the following aspects:(1) Appropriate NH3 and H2O phase equilibrium property equation was determined. State equation model and activity coefficient model mentioned in references were analyzed and 5 property equations were selected and tested for 1MPa and 2MPa, respectively. The computed results were compared with data of NH3-H2O phase equilibrium put forward by JI.M. The data regression method in commercial software Aspen plus was used to amend binary interaction parameters of property equations in order to determine most appropriate property equation to describe NH3-H2O phase equilibrium properties.The result shows that the PR-BM equation can more accurately predic NH3-H2O phase equilibrium properties.(2)The different mathematical model of distillation column in ammonia-water absorption refrigeration system were built and analyzed, and then appropriate mathematical model was determined by comparison. Equilibrium stage model and two kinds of non-equilibrium stage models were built and then control equations were listed and solved. By comparing the calculation results with experimental data in the references deeply, the models which is most close to practical cases can be chosed so as to predict the effect of the operating conditions on distillation effenciency.(3) With the help of freedom analysis method, appropriate independent variables and dependent variables were chosen. Then the mathematical model was solved by iteration method. By analyzing the influence of reflux ratio, column diameter, height of downflow weir and plate number of rectifying and stripping section, some improvement measures were proposed for the existing distillation column. Then the influence of feeding concentration, generator heat load, feed position, operating pressure on distillation process were further analyzed based on the improved rectifying column. The conclusions obtained make active guidance meaning to further increase the COP of the ammonia-water absorption refrigeration system.(4) The open-loop response process and closed-loop response process of distillation column were simulated in a dynamic process, then appropriate control systems were built for the distillation column. The size of distillation column was calculated based on the steady simulation by Aspen plus. Then by giving the initial value needed in dynamic simulation and building the model of distillation column driven by flow, these dates were imported to the Aspen dynamic software. First, the LP control system and LPC control system were built for the distillation column. Then some improvements were made based on the above. The researches show that the improved LPCT control system can overcome feed rate and feed composition disturbance. It can effectively realize control target for the major parameters of distillation column in ammonia-water absorption refrigeration system.