Research On Scheduling Problem Of Large-scale SWRO Desalination Plant And Its Application

The demand for fresh water is increasing quickly with the development of society and economy. The crisis of water resource has become one of the bottlenecks for the national sustainable development and the improvement of human’s life level.Seawater desalination technology is one of the most effective methods to alleviate the water crisis in coastal areas. Reverse osmosis (RO) is a main kind of seawater desalination technology. It has become the dominant method with the highly development of its core equipments, such as reverse osmosis membrane, high pressure pump and energy recovery device, and has obtained a widespread application.In recent years, with the scale expanding, a large-scale seawater RO (SWRO) plant usually contains numerous RO units and equipments. It is an effective mean to improve production efficiency and to reduce the total running cost that using the optimized technology to reasonable arrange for the units and equipments.Based on the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2009BAB47B06), the investigations are mainly focused on the modeling and optimal scheduling problem of SWRO process. The main contributions are as follows:(1) The solution-diffusion mechanism of RO process is described, and then a typical SWRO system is given. Based on the principle of thermodynamics, the mass transfer model is given. The spiral-wound components, which are widely used in desalination plant, are modeled using this mass transfer theory. The structure of large-scale parallel-unit SWRO process is analyzed and the full flowsheet model is formulated. The costs of production process are analyzed. All these models lay the theoretical foundation for the study of large-scale SWRO operational optimization.(2) A static optimization scheduling problem of large scale SWRO process is discussed in the thesis. According to the actual production practice, a lumped parameter model is carried on. The static scheduling problem is established as a mixed-integer nonlinear programming (MINLP) model. Because the MINLP is hard to be solved by analytical method, a two-stage differential evolution (TSDE) is proposed to computer the solution, and the parameters of TSDE are analyzed. The simulation results show that the proposed algorithm can solve this MINLP problem well.(3) In order to further reveal the impact of system parameters on the preferences in RO process, the operational optimization problem of whole SWRO desalination process with fresh water tanks is considered. A simultaneous solution technology, finite element collocation method, is used to transform the differential-algebraic optimization problems (DAOP) into a large-scale nonlinear programming problem. The selection strategy of finite element is studied. The production scheduling problems under different operation conditions are discussed in the thesis. A case study is then performed to demonstrate the efficiency of the proposed method, and the computing results show that the proposed approach can largely reduce the total operational cost of the SWRO system.(4) The robust scheduling problem under changing operating conditions is discussed. The robustness measurement indexes of production scheduling are given. A hybrid robust scheduling approach, a proactive scheduling combining with a reactive scheduling, is proposed to cope with the uncertainties in the SWRO process. In order to solve the rescheduling quickly, a rolling horizon approach (RHA) is proposed to decomposed the global optimizational scheduling problem into a series of local optimization ones. By reducing the dimension of problem, the calculation is sped up. Two kinds of typical uncertainties, machine faults and fluctuations of freshwater demand, are simulated, and the results show that the proposed hybrid scheduling scheme is satisfied robustness.(5) The implementation of optimal scheduling in large-scale SWRO process is discussed. The software structure is given, and the optimal scheduling software is developed with C#. The developed production scheduling software is suitable for large-scale SWRO system. By rational using of resources, the water production cost is reduced.At the end of the dissertation, contributions of this research are concluded and possible extensions in future research are discussed.