Energy optimization in heat integrated water allocation networks Gopal Chandra Sahu, Santanu Bandyopadhyay n Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India article info Article history: Received 12 September 2011 Received in revised form 22 October 2011 Accepted 24 October 2011 Available online 29 October 2011 Keywords: Energy Heat transfer Mathematical modeling Optimization Process integration Heat integrated water systems abstract In this paper, linear programming formulations, complemented by concept based pinch analysis results, are developed to target the minimum energy requirements in a heat integrated fixed flow rate water allocation networks. These formulations can be applied for the cases of heat integration through isothermal and non-isothermal mixing in water allocation networks involving single as well as multiple contaminants. The earlier reported approaches are based on linear programming formulation for the case of isothermal mixing and either mixed integer non-linear programming or discontinuous non- linear programming formulations for the case of non-isothermal mixing. It has been observed that the earlier reported methodologies produce sub-optimal results for the case of non-isothermal mixing. However, the proposed methodologies produce the optimum results because of the rigorously proved linear formulation. Utility requirements for isothermal as well as for non-isothermal mixing cases are compared over a range of minimum approach temperatures, to evaluate the energy performance using illustrative examples. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Chemical process industries require enormous amount of fresh water and fossil fuel based energy sources for their process operations. Rapid depletion of energy resources, scarcity of fresh water as well as stricter environmental regulations necessitate for rationalized use of both water and energy. This is why process industries are constantly innovating ways to conserve both water and energy. The advantages of conservation of water and energy are twofold: reduction in operating costs as well as protection of the environment. During the last four decades various studies have been carried out to minimize energy and water consumption in process plants. Most of these studies are focused either on minimization of energy (Linnhoff et al., 1982; Papoulias and Grossmann, 1983; Furman and Sahinidis, 2002) or on minimiza- tion of fresh water requirement (Takama et al., 1980; Wang and Smith, 1994; Hallale, 2002; Prakash and Shenoy, 2005; Foo et al., 2006; Bandyopadhyay, 2006; Bandyopadhyay et al., 2006; Pillai and Bandyopadhyay, 2007). These have resulted in an efficient design of stand-alone heat exchanger networks (HEN) and inde- pendent water allocation networks (WAN). However, it is impor- tant to note that in a chemical process plant with heat integrated water systems both energy and water systems interact with each other. It is found that the minimization of industrial fresh water consumption not only results in minimization of waste water discharge but also reduces the amount of energy required for heating and cooling processes (Bagajewicz et al., 2002). Therefore, water and energy management issues in a process plant need to be considered simultaneously, rather than independently. The importance of simultaneous minimization of water and energy in heat integrated water system has been addressed by Savelski and Bagajewicz (1997), followed by Savulescu and Smith (1998). Further, both mathematical and conceptual methods of process integration have been attempted to analyze the problem. Sequential and simultaneous are the two distinct approaches, which are often used in mathematical methods. However, only sequential approach is used for conceptual methods. In sequential approach, fresh water requirement is minimized first. It may be noted that for a single contaminant case, the minimum fresh water can be targeted using any of the estab- lished techniques: limiting composite curve (Wang and Smith, 1994; Agrawal and Shenoy, 2006), water surplus diagram (Hallale, 2002), material recovery pinch diagram (El-Halwagi et al., 2003; Prakash and Shenoy, 2005), water cascade analysis (Manan et al., 2004; Foo et al., 2006), source composite curve (Bandyopadhyay, 2006; Bandyopadhyay et al., 2006), analytical method (Pillai and Bandyopadhyay, 2007), etc. It may be noted that for an optimum fresh water requirement, multiple WANs are possible to design (Pillai and Bandyopadhyay, 2007; Das et al., 2009) and different WANs have different utility requirements (Sahu and Bandyopadhyay, 2010). Hence, exhaustive search of the entire solution space has to be performed to determine the minimum energy requirement. The conceptual design tools based on sequential approach, such as, two dimensional grid diagram as Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.10.054 n Corresponding author. Tel.: þ91 22 25767894; fax: þ91 22 25726875. E-mail address: santanub@iitb.ac.in (S. Bandyopadhyay). Chemical Engineering Science 69 (2012) 352–364