Study On The Optimization Design Of Water-Using Network

Scarcity of water is a life-and-death problem that human being is facing. Economically using water and improving the utilization efficiency of water resource is a important aspect of solving the problem of scarcity of water. The aggregation of the whole water-using processes in a business or community forms a water-using network, which is also a mass exchange network. For this sort of network, it is worthy of investigation arranging the water-using processes properly, i.e. optimizing water-using processes to reduce both fresh water consumption and wastewater production. In recent years, many scholars devoted themselves to the research of water-using network design, and presented many valuable ideas and methods, such as water pinch analysis and superstructure methods. However because of the complexity of the problem itself and the cognitive diversity towards the optimization problem of water-using network, the methods proposed either are only applied to the simple problem or are difficult to search the optimal solution for a large-scale problem. Basing on the existing achievements (such as water pinch analysis and superstructure methods), approaching from the characteristics of the variable water-using networks and taking the mathematical programming as a tool, the author makes a systematic and deep research on the problem of determining the minimum fresh water consumption and the problem of optimization design, and developed a set of sound methods. The main contents are extracted and listed below:1) This paper approaches reuse, regeneration recycling and regeneration reuse process for single contaminant, and addressed a mathematical programming method for determining minimum flowrate of fresh and regenerated water in water-using processes. The mathematical formulation of the synthesis problem leads to a linear programming for reuse and reuse recycling process of single contaminant. For the regeneration reuse process, the binary variable j is introduced to express the mass load supplied by the fresh water and regenerated water. Accordingly the design problem can be formulated as a mixed integer nonlinear programming (MINLP), and a two-step programming method is presented: In step 1, a programming with the objective of minfwx is used to determine the minimum flowrate of fresh water, then under the same constraints with step 1, a programming with the objective of minfreg in step 2 is used to determine the minimum flowrate of regenerated water corresponding to the minimum flowrate of fresh water.2) The relationship between the minimum fresh water flowrate required, fws, and inlet concentration to regeneration process, Cr, is investigated and a new conclusion is drawn,which differs from thaf'regeneration of water at pinch minimizes fresh water flowrate" derived from the literature: the minimum fresh water flowrate fws is a function of Cr, and there exist three relationships between fws, and Cr, which indicate three possibilities for Cb : below the pinch, above the pinch or at the pinch. Therefore, in some cases, regeneration at pinch concentration can't always ensure the minimum fresh water consumption. Three examples are solved to demonstrate the new conclusion.3) For the reuse process and the regeneration recycling process of single contaminant, the step-by-step programming method is presented: From the beginning, the processes are collated according to the limiting operating data, and then the matching between the rich streams and lean steams (water sources) is optimized step by step in accordance to the collated process sequence by solving the mathematical programming problem. For single contaminant, only one sequence needs to be solved, while for multiple contaminants, several sequence need to be solved and the design of minimum fresh water consumption is chosen among them by comparing the results of different operation sequence. For reuse process of single contaminant and multiple contaminants along with regeneration recycling process of single contaminant respectively, the step-by-step linea...