Elvis Ahmetović, Ignacio E. Grossmann , Zdravko Kravanja, Nidret Ibrić University of Tuzla, Faculty of Technology, Univerzitetska 8, 75000 Tuzla, Bosnia and Herzegovina; elvis.ahmetovic@untz.ba, nidret.ibric@untz.ba Carnegie Mellon University, Department of Chemical Engineering, 5000 Forbes Avenue, Pittsburgh 15213, Pennsylvania, grossmann@cmu.edu University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova ulica 17, 2000 Maribor, Slovenia, zdravko.kravanja@um.si

This paper addresses the synthesis problem of non-isothermal interplant water networks by using a mathematical programming approach based on superstructure optimisation. A recently proposed compact superstructure (Ibric et al., 2015) was used and the mixed integer nonlinear programming (MINLP) model modified in order to identify the existence of process water using units, wastewater treatment units, and hot/cold streams within different plants, as well as optimal water flow and heat transfer between the plants. The superstructure includes direct and indirect heat exchange opportunities, with a manageable number of hot and cold streams enabling the control of heat exchanger network (HEN) complexity. In addition, a compact superstructure reduction strategy is employed in order to reduce the model size. By using those parameters having values of 1 or 0, the existence of superstructure elements of the different plants can be addressed simply, without introducing additional variables. The proposed model is solved by using a two‒step iterative solution strategy (Ibric et al., 2015). The first step in the proposed strategy provides initialisation and rigorous bounds on water and utilities consumption. In the second step, an MINLP model is solved, simultaneously minimising the total annual cost of the network. Different case scenarios were considered, analysing water and heat integration within and between the plants. The solutions obtained show that the model can be successfully used for the synthesis of interplant non-isothermal water networks, thus minimising the total annual cost of the overall network. The results show that additional saving in total annual costs can be achieved by enabling direct water and heat integration between plants.

Evaporation processes are used within the process industries in order to produce concentrated products by evaporating part of water from different feeds-diluted water solutions. Concentrated products can represent final products (fruit and vegetable juices) or intermediate products in cases that crystalized (salt, sugar) or dried (milk powder) final products should be produced. Large amounts of steam and cooling water are consumed in these processes. In order to reduce energy and water consumption within evaporation processes different systems can be applied, namely, multiple-effect evaporation, vapor recompression (thermal and mechanical) or their combinations. Additionally, these processes can be integrated with other process subsystems in order to achieve improved energy and water integration. To address these issues different computer-aided tools have been proposed. However, most studies have focused on analysis and simulation of evaporation processes. Some of the initial studies [1, 2] considered the synthesis of evaporation processes in order to develop tools for computing the minimum utility use for a multiple-effect evaporation system, which was heat-integrated with process hot and cold streams. These studies were based on a modified grand composite curve and heat-path diagram. Also, the focus of the recent works have been on multiple-effect evaporation systems [3] and their energy integration with the background processes in order to minimize the energy consumption within the overall system [4]. These studies have motivated us to further expand research in this direction, by applying mathematical programming approach for the analysis of existing and the design of new evaporation systems as well as their heat integration with other process subsystems or process streams. The main goal of this paper is to develop models based on mathematical programming that can be applied for the analysis, synthesis and optimization of multiple-effect evaporation systems. The proposed models will be developed in General Algebraic Modeling System (GAMS). The developed models will enable examination of different scenarios of multiple-effect evaporation in order to address the analysis of existing, retrofit and/or design new evaporation process. Within the proposed framework, a network consisting of a multiple-effect evaporation system and heat exchanger network will be investigated in order to achieve the improved heat integration within the overall system. Two strategies will be considered to achieve this task, namely, sequential and simultaneous. The developed models will be tested on several examples, and also applied to different feed streams. New results are expected to be obtained within this field. Keywords: multiple-effect evaporation, analysis, synthesis, optimization, mathematical programming. Acknowledgment The authors are grateful to the Swiss National Science Foundation (SNSF) and the Swiss Agency for Development and Cooperation (SDC) for providing financial support within the SCOPES 2013â??2016 (Scientific Co-operation between Eastern Europe and Switzerland) joint research project (CAPEâ??EWWR: IZ73Z0_152622/1). References: [1] Hillenbrand JJB, Westerberg AW. The synthesis of multiple-effect evaporator systems using minimum utility insightsâ??I. A cascaded heat representation. Computers & Chemical Engineering. 1988;12:611. [2] Westerberg AW, Hillenbrand JJB. The synthesis of multiple-effect evaporator systems using minimum utility insightsâ??II. liquid flowpattern selection. Computers & Chemical Engineering. 1988;12:625. [3] Khanam S, Mohanty B. Energy reduction schemes for multiple effect evaporator systems. Applied Energy. 2010;87:1102. [4] Sharan P, Bandyopadhyay S. Energy Integration of Multiple Effect Evaporators with Background Process and Appropriate Temperature Selection. Industrial & Engineering Chemistry Research. 2016;55:1630.

The synthesis problems of non‒isothermal water networks have received considerable attention throughout academia and industry over the last two decades because of the importance of simultaneously minimising water and energy consumption [1]. Most papers have addressed this issue only by considering heat integration between hot and cold water streams. In this study, the scope of heat integration is expanded by enabling heat integration of process streams (such as waste gas streams and reactor effluent streams) together with the water network’s hot and cold streams. This approach integrates the non‒isothermal water network synthesis problem with the classical heat exchanger networks (HENs) synthesis problem by considering them simultaneously as a unified network. A recently proposed superstructure [2] for the synthesis of non‒isothermal process water networks is extended to enable additional heat integration options between hot/cold water streams and hot/cold process streams. Within a unified network, heat capacity flow rates and inlet and outlet temperatures are fixed for process streams, and variable for water streams. The complexity of the overall synthesis problem increases significantly when compared to the syntheses of both networks separately. Therefore, solving this types of problem is more challenging. The objective function of the proposed mixed integer nonlinear programming (MINLP) model accounts for operating costs (including fresh water and utilities) and investment costs for heat exchangers and treatment units. The results indicate that by solving a unified network, additional savings in utilities consumption and total annual cost can be obtained, compared to the sequential solution obtained by solving both sub‒networks separately. Thus, more efficient water networks can be designed.

Processes industries consume large amounts of natural resources and generate large amounts of waste/emissions into the environment. Consequently, important issues and challenges within the process industries are rational use of raw materials, water and energy, pollution prevention, minimization of waste generation, and achieving profitability and sustainability of industrial processes [1, 2]. In order to successfully address those challenges systematic methods [3], and computer aided tools can be applied during synthesis and operation of the industrial processes. Over the last several decades there have been an increasing number of applications of systematic methods based on pinch analysis and mathematical programming in order to minimize water/energy usage, and wastewater generation within a manufacturing sector. In early studies these methods have been only applied for heat or water integration. However, in recent years water and heat integration within the process water networks have been performed simultaneously [4]. This paper presents recent advancements and applications of pinch analysis and mathematical programming methods for synthesizing of non-isothermal water networks through illustrative case studies. Case studies of non-isothermal water networks reported in the literature are of different complexities, including a network of water-using units, a network of wastewater treatment units, an integrated network of process water-using and wastewater treatment units, single and multiple contaminants, pinched and threshold problems, etc. [5]. Those problems have been solved using different synthesis concepts, tools and solution strategies. The main goal of this paper is to present and discuss the current state of the art of pinch analysis and mathematical programming methods for solving the synthesis problems of non-isothermal water networks of different complexities, and highlight challenges and possible further directions in this field.

The syntheses of water network systems are usually performed by minimizing the total annual cost. In this contribution, Mixed Integer Nonlinear Programming (MINLP) syntheses of water networks are performed by using various economic objectives, in order to investigate their effects on the structural, environmental, and economic characteristics of optimal water networks. Batch-semicontinuous and isothermal continuous water networks were analyzed during this study. Significant differences between optimal networks were obtained when using different economic objectives. Minimization of freshwater costs produced highly integrated designs with high levels of water reuse, regeneration reuse or recycling, but low profitability. In contrast, maximization of the internal rate of return resulted in highly profitable designs with low investment and a low level of water integration. Either minimization of the total annual cost, maximization of the net present value, or maximization of the annual profit produced designs with intermediate or high levels of integration between water using operations, and modest profitability. These criteria produced compromise solutions with proper trade-offs between the profitabilities and sustainabilities of water network designs.