This paper introduces a simultaneous optimization approach to synthesizing work and heat exchange networks (WHENs). The proposed work and heat integration (WHI) superstructure enables different thermodynamic paths of pressure and temperature-changing streams. The superstructure is connected to a heat exchanger network (HEN) superstructure, enabling the heat integration of hot and cold streams identified within the WHI superstructure. A two-step solution strategy is proposed, consisting of initialization and design steps. In the first step, a thermodynamic path model based on the WHI superstructure is combined with a model for simultaneous optimization and heat integration. This nonlinear programming (NLP) model aims to minimize operating expenditures and provide an initial solution for the second optimization step. In addition, hot and cold streams are identified, enabling additional model reduction. In the second step of the proposed solution approach, a thermodynamic path model is combined with the modified HEN model to minimize the network’s total annualized cost (TAC). The proposed mixed integer nonlinear programming (MINLP) model is validated by several examples, exploring the impact of the equipment costing and annualization factor on the optimal network design. The results from these case studies clearly indicate that the new synthesis approach proposed in this paper produces solutions that are consistently similar to or better than the designs presented in the literature using other methodologies.

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.

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.

In recent work, a general superstructure and a Non-Linear Programming (NLP) model were presented for Multiple-Effect Evaporation Systems (MEESs). This NLP model was combined with a Heat Exchanger Network (HEN) model in order to simultaneously perform optimisation and heat integration of the overall system. The results of a forward-feed evaporation system integrated with hot and cold streams of the evaporation system as well as with the background process were presented. In this paper, the superstructure is extended by including multi-stage flash vessels for improving energy efficiency within the overall system. Additionally, various flow-patterns of heat-integrated MEES are studied. Also, trade-offs between energy and investment costs of heat-integrated MEES are explored for different numbers of evaporation effects in order to determine the optimum number of effects. The proposed Mixed-Integer Non-Linear Programming (MINLP) model of the combined MEES-HEN networks is implemented in a General Algebraic Modelling System (GAMS) and solved simultaneously using a two-step solution strategy. In the first step of the strategy, the NLP model of MEES is solved, providing an initialisation point for solving the MINLP model of the combined MEES-HEN network within the second step. A case study of a milk concentration process is used to illustrate the method. The results show that the forward feed flow-pattern with three evaporation effects is totally integrated with hot and cold process streams from the background process, and the system exhibits the minimum Total Annualised Cost (TAC).