Miloš Bogataj

Design of non-isothermal process water networks

Abstract Despite the fact that many methods have been developed for the optimization of process water networks, solving the problem simultaneously considering heat recovery has rarely been addressed. This paper presents a new approach for the simultaneous synthesis and optimization of heat integrated water networks. The procedure is based on mixed integer non-linear mathematical programming (MINLP). A new superstructure for heat exchanger network (HEN) synthesis capable of exploiting unique features of water networks, like non-isothermal mixing of different streams, thus providing potentially more cost-effective solutions, is proposed. An example is presented to illustrate the synthesis of heat integrated water networks using the proposed approach.

Integrating renewables into multi-period heat exchanger network synthesis considering economics and environmental impact

Abstract This paper presents a further development of synthesis methods that considers economics and environmental impact in the integration of renewable energy into the optimisation of heat exchanger networks involving multiple periods of operations and multiple options of utilities. The multi-period process stream parameters, and those of the utility sources are integrated in a systematic approach using an expanded version of the simplified stage-wise superstructure multi-period model. Two examples were used to demonstrate the benefits of the expanded synthesis method and the quality of solutions obtained were judged by representation on a Pareto curve and by the use of a modified goal solution method. It was found that various combinations of utility sources were selected for use at various periods/seasons of operations, while utilities from solar photovoltaic were not selected for use at any of the periods/season of operation due to its relatively high cost and limited periods of availability.

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.

Data Reconciliation for Energy System Flowsheets

Abstract Data reconciliation is a key step of data extraction from existing plants. While there are many publications on data reconciliation generally, for Heat Integration Analysis (HIA) they are scarce. Reconciliation also determines the result. For HIA reconciling data for an individual heat exchanger is insufficient and incorrect. In this work complete heat exchanger networks are considered within the data reconciliation scope. Two methods are compared: i) iterative method using local NLP and ii) simultaneous method applying global NLP. The iterative method aims at reducing the complexity of a problem by decomposing the problem into two steps. In the first step the temperatures are reconciled and in the second the mass flow-rates are reconciled at fixed (reconciled) temperatures obtained during the first step. The simultaneous models reconcile the mass flow-rates and temperatures of the HEN within one step. The results indicated that the simultaneous approach achieves better accuracy and should be preferred when accuracy is the main focus but the iterative approach is simpler and still a feasible approach to be used when computational complexity is an issue.

Alternative mixed-integer reformulation of Generalized Disjunctive Programs

Abstract In this work, we propose an alternative mixed-integer (MI) reformulation of Generalized Disjunctive Programs (GDP) and compare its efficiency against the Big-M and hull reformulation (HR). We show that the reformulation yields a continuous relaxation equal to the HR for linear GDPs and comparable to the one obtained by HR for nonlinear GDPs. A particular advantage of the proposed reformulation is that the usage of the perspective function can be avoided when reformulating nonlinear GDPs. The reformulation is validated on a set of numerical examples using Branch and Bound algorithm. Results obtained thus far indicate that the proposed reformulation represents a promising alternative to the established ones.

Process Integration: HEN Synthesis, Exergy Opportunities

This chapter provides a brief description of thermodynamic analysis, the maximization of heat-recovery and power generation and the synthesis of IGCC’s heat exchanger network. Trade-offs between the income from power generation and utility costs plus the investment in HEN are studied with respect to the generation of different pressure-levels of steam and temperature driving force losses within the heat-recovery network. A combined pinch analysis/mathematical programming approach is applied and the optimization models are described for (i) the maximization of heat-recovery and power generation, and (ii) a synthesis of the heat-recovery and steam/power generation network. A sensitivity analysis for the synthesis is performed in order to show how optima are sensible for the expected increase in future electricity and utility prices, and the project’s lifetime. The results obtained from the studied IGCC process indicate that detailed optimization has to be performed during the network synthesis step, otherwise optimal trade-offs are missed that may result either in serious power generation losses or in obtained over-designed networks.