Elvis Ahmetović

Water and energy integration: A comprehensive literature review of non-isothermal water network synthesis

Abstract Synthesis of non-isothermal water networks consisting of water-usage, wastewater treatment, and heat exchanger networks has been recognised as an active research field in process systems engineering. However, only brief overviews of this important field have so far been provided within the literature. This work presents a systematic and comprehensive review of papers published over the last two decades and highlights possible future directions within this field. This review can be useful for researchers and engineers interested in water and energy integration within process water networks using systematic methods based on pinch analysis, mathematical programming, and their combination. We believe that this research field will continue to be active in the near future due to the importance of simultaneous optimisation of process, water and energy integration for achieving profitability and sustainability within process industries.

Strategies for the Global Optimization of Integrated Process Water Networks

Abstract In this paper, we propose special strategies for obtaining the global or near global optimum solution from a general superstructure proposed recently by the authors for the design of integrated process water networks. The proposed model of the integrated water network is formulated as a Nonlinear Programming (NLP) and as a Mixed Integer Nonlinear Programming (MINLP) problem for the case when 0–1 variables are included to model the cost of piping and/or selection of technologies for treatment. The MINLP model can be used to find optimal network designs with different number of streams in the piping network. The proposed strategies rely on bounds on the variables that are derived as general equations obtained by physical inspection and using logic specifications needed for solving the model. The cut proposed in [1] and some variations of it are also used to significantly improve the strength of the lower bound for the global optimum. It is shown that the proposed strategies can effectively solve large-scale problems, and in most cases, to global optimality. Furthermore, the proposed strategies allow to readily obtain networks of varying degrees of complexity by limiting the number of piping connections

Synthesis of single and interplant non-isothermal water networks

This paper addresses the synthesis problem of non-isothermal water networks using a mathematical programming approach. A heat-integrated water network superstructure and its corresponding mixed integer nonlinear programming (MINLP) model is proposed for the synthesis of individual as well as interplant water networks. A new feature of the proposed model includes piping installation cost within the objective function minimising the total annual cost of the network. This introduces additional trade-offs between operating and investment costs that can impact a final network design. Three examples were solved in order to demonstrate the applicability and effectiveness of the proposed model and solution approach. The results show that additional saving in total annual cost can be achieved by enabling direct water integration between plants. Improved solutions were obtained compared to those reported in the literature considering freshwater and utilities consumption as well as total annual cost.

Mathematical programming synthesis of non-isothermal water networks by using a compact/reduced superstructure and an MINLP model

The synthesis problems of non-isothermal water networks, combining heat exchanger network and water network (WN), usually consist of a significant number of constraints and variables, namely, flow rates, contaminant concentrations, temperatures and a large number of non-linear terms. In most cases, solving medium and large-scale synthesis problems is computationally too expensive and challenging. In order to circumvent that problem, we propose a compact superstructure and mixed-integer non-linear programming model for the simultaneous synthesis of non-isothermal WNs. The proposed superstructure includes heat integration stages enabling direct and indirect heat exchanges with a manageable number of hot and cold streams. This reduces the models size enabling easier solutions of the synthesis problems using local solvers. In addition, a superstructure reduction strategy is proposed making the superstructure flexible and adaptable for different types of problems, namely, pinched and threshold, and providing additional reduction of connections within the proposed superstructure. The proposed model is solved using a two-step solution strategy including initialisation and design steps. The model is applied to the examples of different complexities including single and multiple contaminant problems, and water-using and wastewater treatment units. Using the proposed iterative strategy, the improved locally optimal solutions are identified for most examples, minimising the total annual cost of the overall network.

Synthesis of Water, Wastewater Treatment, and Heat-Exchanger Networks

Abstract This contribution describes a general methodology for the synthesis of water networks of different complexities, ranging from simple water networks up to combined water, wastewater treatment and heat exchanger networks. The overall network model is formulated as a mixed-integer nonlinear programming (MINLP) problem. The methodology is illustrated and implemented on a case study. In the first step we present a base case of water network design without water reuse and determine freshwater consumption. Then an optimal water network design with the minimum freshwater usage and wastewater generation is synthesized. In the next step, an integrated water-using and wastewater treatment network is synthesized. Finally, the water and wastewater networks are combined with the heat exchanger network (HEN), and solved simultaneously. The obtained results show that the methodology can be used both for the synthesis of isothermal and non-isothermal water and wastewater networks.

Optimisation of heat exchanger networks involving isothermal and non-isothermal mixing by global and local solvers

Abstract The synthesis of heat exchanger networks (HENs) has been an active research field over the last four decades (Klemes and Kravanja, 2013). Systematic methods based on pinch analysis, mathematical programming or their combinations have been successfully applied within this research field in order to reduce utility consumption and achieve sustainability within chemical processes. The HEN synthesis problems including isothermal (Yee and Grossmann, 1990) and non-isothermal (Bjork and Westerlund, 2002) mixing are formulated as Mixed-Integer Non-Linear Programming (MINLP) models. These problems are NP hard problems (Furman and Sahinidis, 2001), and combinatorial complexity, model size, number of non-convexities and hence, computational burden, increase dramatically by the number of hot and cold streams and stages. On the other hand, the computational speeds of computers and optimisation algorithms have been improved over the last two decades by several (6-7) orders of magnitude, which enable the solving of larger problems than those solved in past as well as solving smaller problems closer to global optima. This paper firstly presents an overview of the literature and problems of different complexities in order to show that over the recent period the medium and larger HEN synthesis problems have been successfully solved with local optimisation solvers, and that the obtained results are closer to their global optima. Also, a role of our research was to explore the possibilities of some currently available global and local optimisation solvers for solving the HEN synthesis problems of different complexities including isothermal and non-isothermal mixing. The obtained results of this work are in good agreements with the literature results, and in some cases the improved solutions have been identified.

Optimization model for the design of distributed wastewater treatment networks

In this paper we address the synthesis problem of distributed wastewater networks using mathematical programming approach based on the superstructure optimization. We present a generalized superstructure and optimization model for the design of the distributed wastewater treatment networks. The superstructure includes splitters, treatment units, mixers, with all feasible interconnections including water recirculation. Based on the superstructure the optimization model is presented. The optimization model is given as a nonlinear programming (NLP) problem where the objective function can be defined to minimize the total amount of wastewater treated in treatment operations or to minimize the total treatment costs. The NLP model is extended to a mixed integer nonlinear programming (MINLP) problem where binary variables are used for the selection of the wastewater treatment technologies. The bounds for all flowrates and concentrations in the wastewater network are specified as general equations. The proposed models are solved using the global optimization solvers (BARON and LINDOGlobal). The application of the proposed models is illustrated on the two wastewater network problems of different complexity. First one is formulated as the NLP and the second one as the MINLP. For the second one the parametric and structural optimization is performed at the same time where optimal flowrates, concentrations as well as optimal technologies for the wastewater treatment are selected. Using the proposed model both problems are solved to global optimality.