Study On The Optimization Design Of Seawater Desalination Processes By Reverse Osmosis Membrane Method

The fresh water is an important resource to human. It is becoming increasingly scarcity. Therefore, the problem of fresh water resource should be solved for the sustainable develop in China. One of the more promising alternative is desalination of seawater and brackish water. In the last few year, the reverse osmosis (RO) desalination process have found widespread application in the field of water resource protect and develop. The rapid growth of RO process is because it is able to produce fresh water with lower cost . The other attractive feature of RO process is that the plant design and operation is simplicity and modularity. At present, the technical economical indices of RO don't reach the public desired value, and the cost of desalination is on the high side. It shows that the current RO system can be improved in some aspects-the RO technology, the engineering design, and the operation etc.In this paper, the optimization method based on process synthesis has been applied to design the RO system. The reverse osmosis desalination process with multiple-feed and multiple-product required has been studied. The adoption of this approach can provide an economically attractive desalination scheme. It can lead to significant energy and raw-materials saving and provide income from the multiple product water sales. The main contents are extracted and listed below:1. The simulation and design of RO desalination process has been investigated. From the beginning, the detailed mathematical models of each process units are given, and the relevant economic models to the RO desalination process are introduced. Then the interdependence of the above mathematical models via some corresponding variables constitutes a system model. Single-stage and two-stage processed are considered. The membrane module can be classed four type, i.e. higher salt-rejection membrane, high-flux membrane, normal membrane, lower salt-rejection membrane. Finally, the optimum design problem can be solved as a mixed-integer non-linear programming (MINLP), which incorporates thermodynamic, technical and flexibility constraints and minimizes the total annualized cost. The different optimal design options are obtained for different design specification and feed water quality and the corresponding design rule are derived. Two examples in different cases are solved to illustrate the advantage and effectiveness of the new method.2. The design of various multistage RO systems under different feed concentration and product specification is presented in this work. An optimization method using the process synthesis approach to design an RO system has been developed. First, a simplified superstructure that contains all the feasible design in present desalination process has been presented. It offers extensive flexibility towards optimizing various types of RO system and thus may be used for the selection of the optimal structural and operating schemes. A pressure vessel model that takes into account the pressure drop and concentration changes in the membrane channel has also been given to simulate multi-element performance in the pressure vessel. The solution to the problem includes optimal arrangement of the RO modules, pumps, energy recovery devices, the optimal operating conditions, and the optimal selection of types and number of membrane elements.3. The reverse osmosis (RO) desalination process with multiple-feed and multiple-product required has been studied in this work. The optimization method based on process synthesis has been applied to design the RO system. The adoption of this approach can provide an economically attractive desalination scheme. Membrane separation units employing the spiral wound reverse osmosis elements were described by using the pressure vessel model presented in this paper, which takes into account the pressure drop and concentration changes in the membrane channel. A simplified superstructure that contains all the feasible design for this desalination problem has also been presented. In this structural representation, the stream split ratios and logical expressions of stream mixing were employed which can make the mathematical model to be easily handled. The solution of the problem includes the optimal system structure and operating conditions, and the optimal streams distribution. The design method could also be used for the optimal selection of the types of membrane elements in each stages and the optimal number of membrane elements in each pressure vessel.4. In the reverse osmosis desalination process, the membrane fouling and scaling is a big problem that must be solved. The regular cleaning is one of main measures that prevent and cure the membrane fouling and scaling. Cleaning can eliminate the surfacial contamination of membranes and recover the water flux. On the condition that the water production is ensured, the optimal cleaning frequency and the minimum cleaning cost can be obtained by choosing the appropriate cleaning strategy and time. This paper makes a more deep research on the problem of membrane module cleaning and replacing base on the previous study, and addresses the new fouling model and the criterion of cleaning. The mathematical formulation of the optimum design problem is a mixed-integer non-linear programming (MINLP), which targets the minimum total operation cost during the whole project period. One example was solved and the corresponding optimal time and cleaning strategy was acquired during the five-year project period, which illustrates the effectiveness of the new method for the optimal maintenance cost of the 3-5 year life-span membrane modules.5. Based on the studies of technology and operation optimization, this paper made a more deep research into optimization design of reverse osmosis seawater desalination system. In the design process, the optimal cleaning and replacing schedule of membrane module during the five-year project period is consided simultaneously. RO system process design is carried out in the following steps: 1) The mathematical model of each operation unit is given and used to simulate and predict the effects in different conditions of operation; 2) RO system model is built up to describe the overall RO process; 3) RO system model describing the pollution of membrane module is built up and the criterion of membrane washing and repairing are presented; 4) The economic model of RO system is given, which correlates the investment and operation cost via some design variables; 5) Based on the above 4 steps, the RO process is designed using the mathematical programming method. Two examples in different cases are solved to illustrate the advantage and effectiveness of the new method.