Wastewater Minimization in a Chlor-Alkali Complex

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Water network integration is one of the most efficient technologies for wastewater reduction [1]. During the past two decades, both the water pinch technology and the mathematical programming method have been frequently discussed and widely applied in the industry. The water pinch technology divides the water network integration into two steps: targeting and design. This technology was initiated by Wang and Smith [2] in 1994. They treat the water using operation as a mass transfer unit and use concentration vs mass load coordinate to obtain the minimum freshwater consumption of the whole system. Based on this coordinate system, the targets of wastewater reuse and regeneration reuse are established. The methods of Wang and Smith [2, 3] have been well supplemented by many authors in recent years. The first supplement is on the model of the water using operation. It is obvious that not all the water using operations are the mass transfer type. Typical water using units like cooling tower, boiler and reactor are not this kind. Actually, these units are flow rate fixed operations. To treat operations in this category, targeting methods in different coordinates were developed. Dhole et al.[5] obtained the composite curve in concentration vs flow rate coordinate, which has been supplemented by several works [6-9]. Hallale [6] introduced a water surplus diagram and obtained the real target. El-Halwagi et al. [8] and Prakash and Shenoy[9] developed a mass load vs flow rate composite by analogy to the heat integration system. In addition, Agrawal and Shenoy[10] achieved the freshwater target in the concentration vs mass load coordinate; Bandyopadhyay et al.[11, 12] calculated the wastewater target in the same coordinate. Recently, Pillai and Bandyopadhyay [13] established a simple and more effective algebraic method for wastewater targeting. The above mentioned four methods are the most efficient methods for fixed flow rate operations, and they can be extended to cases of multiple water sources [14-16]. The targeting concept is also applicable to process changes [11, 17] and threshold problems [18]. The second supplement is on the regeneration target. The regeneration has two cases: regeneration reuse and regeneration recycle. For regeneration reuse, Wang and Smith [2] proposed that the regeneration concentration should be at the pinch concentration. Latter, Mann and Liu [19] pointed that the optimum regeneration concentration can be above the pinch. Feng et al. [20] introduced a targeting method for regeneration recycle, which has been extended to the regeneration reuse system [21] and the zero discharge system [22]. On the other hand, the regeneration problem for fixed flow rate problems are more complicated than the fixed mass load problems, because the regeneration flow rate is constrained by