Heat-integrated Water Networks Research Articles

Synthesis of heat-integrated water networks considering detailed heat exchanger design

This paper introduces a mathematical programming approach for synthesising heat-integrated water networks (HIWNs) with a detailed heat exchanger design. A common assumption of constant heat capacities of water streams is relaxed. The specific heat capacity of water is now considered as a nonlinear function of temperature to avoid errors in calculating energy balances. In addition, most previous works have assumed constant individual heat transfer coefficients for water streams and utilities when estimating heat transfer areas. This assumption can lead to significant over- or under-estimation of heat transfer areas. In this research, individual and overall heat transfer coefficients for heat exchangers are calculated based on the detailed design of plate heat exchangers for heat recovery. The objective function of the proposed mixed integer nonlinear programming (MINLP) model is to minimise the total annualised cost (TAC). The TAC includes operating costs for freshwater and utilities, pumping costs for overcoming pressure drops in heat exchangers, and investment costs for heat exchangers. Three examples are solved in this work to verify the proposed model.

Efficient simultaneous optimization approach for the synthesis of large-scale heat-integrated water networks

This study develops a generalized superstructure and a compact mixed-integer nonlinear programming (MINLP) model for achieving efficient energy and water utilization in designing integrated process water systems with large sizes. Utilizing non-isothermal mixing reduces the need for many heat exchangers for indirect heat recovery. Hence, an exchanger-centric modeling strategy for heat integration is embraced over a hot–cold streams matching-based approach. A novel two-step approach is proposed to address the nonconvex and nonlinear challenges and ensure the efficiency of getting high-quality solutions. Initially, a targeting model incorporating linear heat integration is adopted to reduce the superstructure and partially initialize the design model. Subsequently, in the synthesizing stage, a two-level global search procedure is devised, integrating adaptive exchanger adjustment with multi-start local optimization. The robustness and efficiency of the proposed approach are confirmed through the resolution of four large-scale problems within one hour, yielding improved results compared to the best-known existing solutions.

Synthesis of heat-integrated water networks: Integration of heat pumps

A typical heat-integrated water network (HIWN) synthesis problem aims to minimise freshwater and utility consumption while establishing trade-offs between operating and investment costs by exploring different water and heat integration opportunities. Heat integration between hot and cold water streams is performed to save external heating and cooling utilities required to achieve the target temperatures of these streams. This paper proposes a mathematical programming approach to integrate water-to-water heat pumps into the HIWNs. A simple cascade heat pump system with isobutane as a natural working fluid enables a larger temperature lift between the heat pump source and the sink. The objective function of the proposed mixed-integer nonlinear programming (MINLP) model is to minimise the total annualised cost (TAC). The TAC includes operating costs for freshwater, heating/cooling utilities, electricity, and investment in heat exchangers and heat pumps. A sensitivity analysis is performed to evaluate the economic feasibility of the system by varying the heating/electricity cost ratio and depreciation period for the capital investment. The results show that additional utility savings can be obtained by integrating heat pumps into the HIWNs. For threshold problems, a heat pump is feasible for heating/electricity price ratios from 1.175, and heating utility is replaced by electricity for a 1:1 ratio. However, for pinched problems, the feasibility of the heat pump is shifted to lower heating/electricity price ratios, from 0.3 to 0.5, depending on the annualisation factor for the investment.

Synthesis of Heat-Integrated Water Networks Using a Modified Heat Exchanger Network Superstructure

This work presents the synthesis of heat-integrated water networks (HIWNs) by using mathematical programming. A new superstructure is synthesised by combining a water network and a modified heat exchanger network. Based on the proposed superstructure, a mixed-integer nonlinear programming (MINLP) model is developed. The model is solved by using a one-step solution strategy enabling different initialisations and the generation of multiple solutions, from which the best one is chosen. The results show that the proposed model can be effectively used for solving HIWN problems of different complexities, including large-scale problems.

Synthesis of Heat-Integrated Water Networks: Case Study with Sensitivity Analysis

This work addresses the synthesis of heat-integrated water networks (HIWNs) by using a superstructure optimisation approach. A recently developed mixed-integer nonlinear programming (MINLP) model and an iterative solution strategy are applied in this work to a case study of HIWN. The objective function of the MINLP model is to minimise the network total annualised cost (TAC) comprising operating and investment costs. As there are trade-offs between operating and investment costs, good solutions can be obtained if the TAC is minimised by simultaneously exploring all water and heat integration opportunities within the network.A case study with sensitivity analysis is solved by analysing the impact of freshwater and utility costs on the network design and key performance indicators. The results indicate that in cases of low freshwater cost increased freshwater usage is forced and thus lowering wastewater regeneration/recycling and wastewater treatment cost. Increased freshwater flowrate is related to an increase of HEN investment. The high cost of freshwater could produce solutions with lower freshwater consumption compared to base case depending on utility cost and wastewater treatment cost. However, a decrease in freshwater consumption increases wastewater regeneration costs.

Heat Integrated Water Regeneration Network Synthesis via Graph-Theoretic Sequential Method

The integration of multiple resources conservation networks is necessary to attain the ever-stringent sustainable goals. This work takes initiatives to develop a heat integrated water network via a proposed P-graph-based sequential methodology. In the first step, a set of feasible water regeneration networks is generated using the conventional P-graph framework. Then, the obtained feasible networks will be used as the inputs in the second stage which aims to generate various sets of feasible heat exchanger networks. It is worth noting that the second model is solved by an extended P-graph framework (P-HENS) for combinatorial process network optimization. The solutions are then ranked based on the total network cost. To demonstrate the effectiveness of the proposed method, a typical water regeneration network (three sources and three sinks) with multi-contaminants is used. The results show a total of 103 feasible water network structures (water network cost ranging from 0.76 M$/y to 1.18 M$/y). Thereafter, a list of feasible HIWRN can be determined using P-HENS. The top four HIWRNs which offer similar total network cost (~1.639 M$/y) are demonstrated. This proposed method provides valuable insights that allow decision-makers to further select the optimal solution which may be more beneficial as compared to the one obtained via conventional methods.

Simultaneous optimisation of large-scale problems of heat-integrated water networks

This work presents a simultaneous mixed-integer nonlinear programming (MINLP) optimisation model and an efficient iterative solution strategy that can be successfully applied to various heat-integrated water networks (HIWNs), including large-scale problems with a large number of water streams. These problems are highly nonlinear, non-convex and combinatorial. To circumvent such difficulties, including network complexity as well as identifying the roles of water streams in the heat exchanger network (HEN) whether they are hot or cold, a modified convex hull formulation proposed by Ahmetović and Kravanja [1] is applied. The overall model combines the water network (WN), wastewater treatment network (WTN), heat integration (HI), and heat exchanger network synthesis (HENS) models. This model is iteratively solved in three steps including targeting and design steps. The proposed model and solution strategy are tested on large-scale problems. To the best of our knowledge, the results obtained for all the problems in this paper are better than those reported in the current literature.

Optimum Integration of Regeneration in Heat-Integrated Water Networks Through a Hybrid Approach

Optimum Integration of Regeneration in Heat-Integrated Water Networks Through a Hybrid Approach

Simultaneous synthesis of heat-integrated water networks by a nonlinear program: Considering the wastewater regeneration reuse

Heat-integrated water network synthesis (HIWNS) has received considerable attention for the advantages of reducing water and energy consumptions. HIWNS is effective in water and energy sustainability. Mixed integer non-linear programming (MINLP) is usually applied in HIWNS. In this work, a novel nonlinear programming (NLP) was proposed for HIWNS by considering wastewater reuse and wastewater regeneration reuse. Integer variables are changed to non-linear equation by the methods for identifying stream roles and denoting the existence of process matches. The model is tested by examples with single and multiple regeneration unit problems. The testing results showed that the NLP is an alternative method for HIWNS with wastewater reuse and regeneration reuse.

Simultaneous water and energy integration with isothermal and non-isothermal mixing – A P-graph approach

Heat-integrated water network (HIWN) is an active area of research in the past decade. It focuses on simultaneous saving of energy and water, which are both important resources to enhance sustainability for the process plants. This problem has been previously approached using insight-based pinch analysis and mathematical programming techniques. In this work, an alternative optimization tool, process graph (P-graph) framework is utilized to solve for water and energy integration. P-graph is a graph-theoretic approach to process synthesis, for which the developed algorithms can reduce the complexity of the mathematical programming algorithms efficiently. P-graph methodology enables the effective generation of sub-optimal network structures that allow wider exploration of alternatives through network topology. This paper also introduces some transformation techniques to convert the non-linear programming (NLP) model in the non-isothermal HIWN problem into model which is solvable using P-graph. Two examples with different water recovery schemes are used to elucidate the proposed approach, covering both direct recycle/reuse and regeneration networks; for both cases, isothermal and non-isothermal mixing problems are solved.

Optimal Synthesis of Heat-Integrated Water Regeneration Network

Optimal synthesis of a heat-integrated water network (HIWN) accounts for the simultaneous conservation of energy and water resources. A significant amount of research work, ranging from complex non...

Design of heat integrated water networks with multiple contaminants

The heat integrated water networks have the features of complex network structure and high costs. To overcome the problems, this paper presents a new design method for heat integrated water networks with simple structure and low costs. The proposed method includes two steps: the water-using network is designed first by considering the concentration specifications of water-using processes, which is reflected by the values of concentration potentials. Then, the heat exchanger network can be designed based on the water network obtained in the first step. Self heating strategy is adopted in the design of heat exchanger network to reduce the complexity of the network. Examples are illustrated to demonstrate the effectiveness of the proposed method. The network structures obtained by using the proposed design method are simple, and the total annual costs calculated in this paper are comparable to that of literature results.

A Mathematical Programming Method for Optimizing the Single-Contaminant Regeneration Heat-Integrated Water Networks

A superstructure-based mathematical model is established for single-contaminant heat-integrated water networks (HIWN), and the water loss of both wastewater regeneration recycling and regeneration reuse is considered. Furthermore, a sequential optimization procedure is established to achieve multi-objective optimization. The loss rate of water is taken into the mathematical model as there must have water loss in regeneration process, so that the optimization would be more realistic. Two cases are optimized using proposed model with regeneration recycling and regeneration reuse considered, respectively; the results show the differences between these two modes. Compared with the optimization method in the literature, the proposed method is more realistic.

Energy and Water Management for Industrial Large-Scale Water Networks: A Systematic Simultaneous Optimization Approach

Water and energy management issues in process industries are generally related to the synthesis of heat integrated water networks (HIWN), because water and energy are inextricably intertwined in process water networks. The aim of this study is to present a novel mathematical programming model for HIWN synthesis problems and develop an efficient solution strategy. This research is an extension of our previous work, where the targeting of heat integrated water-using networks (HIWUN) has been addressed. In this model, wastewater treatment units and multiple contaminants are embedded, and the total annual cost (TAC) is optimized rather than targeting a simplified TAC. What is more, a three-step solution strategy is proposed to guarantee obtaining promising solutions. The freshwater consumption, the relaxed TAC (rTAC), and the TAC are optimized successively. Good initial points for the rigorous HIWN model in the last step can be generated by the minimizing the rTAC. Four examples including two large-scale exam...

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.

Sequential synthesis of heat integrated water networks: A new approach and its application to small and medium sized examples

This paper presents a new step-wise sequential solution strategy for the synthesis of heat integrated water networks (HIWNs) comprising of two mathematical models. The first model minimizes water and energy costs. The optimal freshwater and water flow rates within the HIWN are determined from this model. Using these optimal flow rates, different configurations of HIWN are evaluated for the purpose of heat integration. The second model is the stage-wise heat exchanger network (Yee and Grossmann, 1990), which is solved for each of the evaluated network configuration in a sequential manner. Using this proposed strategy, a set of locally optimal HIWNs are produced and the best one is chosen based on the minimum total annual cost (TAC). The proposed sequential strategy is applied to small and medium sized examples in the literature. The results show that the proposed new sequential solution strategy can be successfully applied to small and medium sized HIWN problems.

Simultaneous optimization of heat-integrated water networks by a nonlinear program

Considerable attention has been paid to the heat-integrated water network synthesis (HIWNS) because it has the advantages of reducing water consumption, energy consumption and total cost. In this work, a revised superstructure of Ahmetović and Kravanja (2013a Energy 57, 236–250) for simultaneous HIWNS is developed by changing the position of the heaters and coolers. Based on the superstructure, a nonlinear programming (NLP) model is developed for the HIWNS. We develop a method to denote the existence of a process match, which is generally addressed using discrete variables. In addition, the temperature difference terms are incorporated into the objective function using a reformed approximated equation for the logarithmic mean temperature difference (LMTD); therefore, the temperature approach variables are not necessary in the NLP model. A method to identify the stream roles as hot or cold streams is proposed with no discrete variables. The number and type of variables are reduced by these strategies. Branch-And-Reduce Optimization Navigator (BARON) solver is used to solve the NLP model with the default initial point (the lower bounds) by GAMS. The model was tested on single- and multiple-contaminant problems using seven cases, where one case has an identical total annual cost (TAC) to the literature value and the other six cases have smaller TAC than the literature reported values.

Optimal design for heat-integrated water-using and wastewater treatment networks

This work proposes a novel general superstructure and a simultaneous optimisation model for the designing of a heat-integrated water-using and wastewater treatment network (HIWTN) by combining a water-using network (WN), a wastewater treatment network (WTN), and a heat exchanger network (HEN). The proposed work is an extension of our previous studies that considered only heat-integrated water networks (HIWNs) or combined WN and HEN without WTN. The new proposed superstructure of this work combines water integration (water-usage, wastewater treatment, and recycling) and heat integration (direct and indirect heat exchanges) within an overall network. The simultaneous optimisation model of the proposed superstructure is formulated as a non-convex mixed integer non-linear programming (MINLP) problem for minimising the total annual network cost (TAC). This model enables appropriate trade-offs between freshwater usage, hot and cold utilities consumption, and capital cost of heat exchangers (HEs) and wastewater treatment units (TUs). Three literature examples are used to test the proposed model. The improved results of the first two examples are given whilst for the third modified example a novel network design is presented in order to include wastewater treatment.

Two-step mathematical programming synthesis of pinched and threshold heat-integrated water networks

This contribution describes a novel simultaneous mathematical programming model for the synthesis of pinched and threshold heat-integrated water networks (HIWNs). A two-step solution strategy is proposed consisting of a subsequent solution using two models. Firstly, the nonlinear programming (NLP) targeting model is solved by assuming a given value for the heat recovery approach temperature (HRAT), with the objective of minimising the operating cost of the network. This model provides a good initialisation point and rigorous bounds (freshwater and utilities consumption) for the mixed integer nonlinear programming (MINLP) model solved during the second synthesis step. The solution procedure is repeated for a range of HRAT values providing different initialisations and constraints identifying a set of good locally-optimal solutions. In this study we considered and presented solutions for three types of HIWN problems, namely, pinched in which both utilities are required, threshold in which a hot utility is required, and threshold requiring a cold utility. The obtained results indicate that the proposed mathematical programming model and the solution strategy can be successfully applied to the synthesis of pinched and threshold HIWNs.

Simultaneous optimization of heat-integrated water networks involving process-to-process streams for heat integration

This paper presents an extension of our recent work, in which we addressed the simultaneous synthesis of heat-integrated water networks. The novelty and goal of this work is the development of an extended superstructure and simultaneous optimization model of heat-integrated water networks now involving process-to-process streams, and other streams within the overall network, for heat integration. Those heat-integration opportunities have not yet been fully taken into account in most existing models of heat-integrated water networks. In this study, we presented two strategies for heat integration of process-to-process streams. The first one includes the placement of heat exchangers on each hot and cold process-to-process stream. The second allows for the cooling and splitting of hot streams, and heating and splitting of cold streams. This extended model was formulated as a non-convex mixed-integer non-linear programming (MINLP) problem. The objective was to minimize the total annual network cost. Two examples with single and multiple contaminants are used in order to demonstrate that involving process-to-process streams for heat integration, novel and improved solutions can be obtained compared to those reported in the literature.