Optimal Heat Exchanger Network Research Articles

Heat exchanger network simulation, data reconciliation & optimization

This work introduces new heat exchanger network (HEN) model for networks containing single phase and two phase exchangers. Rigorous simulation of HEN performance requires sequential solutions of two sets of linear equations: mass balances and energy balances which are linear in temperatures. This enables data reconciliation via QR factorization as sequential solution of linear mass and energy balances. Hence, reconciliation of energy balances can be added as a follow-on step to material balance reconciliation, thereby greatly simplifying data reconciliation in process plants. If simultaneous reconciliation of flows, temperatures and heat transfer coefficients is required, the convergence is attained in a handful of successive substitution iteration via QR factorization. Similarly, optimization of the HENs based on the proposed model converges in two three to five successive linear programming iterations. Excellent convergence of the proposed HEN model makes it suitable for use in large scale site-wide energy models.

Design of optimum flexible heat exchanger networks for multiperiod process

Due to the rising of energy prices, energy saving became very important. Optimum design of Heat Exchanger Networks (HEN) is a successful way to minimize energy consumption. The present work discusses the design of optimal flexible heat exchanger networks that adapt with changes in streams’ start and target temperatures and heat capacity flowrates. For a process consisting of n periods, multiperiod LP and MILP models were used to determine the target utility requirements and the heat exchanger network configuration that achieves the minimum number of units and remain flexible to ensure minimum utility requirements at each period of operation. Applying these models on a multiperiod literature problem resulted in different solutions corresponding to different iteration runs. The optimum solution that realizes the least exchangers’ cost was compared with literature results for the same problem.

Modeling, Simulation, and Optimization of Postcombustion CO2 Capture for Variable Feed Concentration and Flow Rate. 1. Chemical Absorption and Membrane Processes

Studies on leading technologies for industrial CO2 capture are performed. Each technology includes flue gas dehydration, capture of at least 90% of CO2 from the feed, and compression to almost pure CO2 for sequestration at 150 bar. This paper presents the modeling, simulation, optimization, and energy integration of a monoethanolamine (MEA)-based chemical absorption process and a multistage membrane process over a range of feed compositions (1–70% CO2, 5.5–15% H2O, 5.5% O2, and the balance N2) and flow rates (0.1, 1, 5, and 10 kmol/s). A superstructure of process alternatives is developed to select the optimum dehydration strategy for the feed to each process. A rigorous simulation-based optimization model is proposed to determine the minimum annualized cost of the MEA-absorption process. The MEA-absorption process is energy integrated through heat exchanger network optimization. A novel mathematical model is developed for the optimization of multistage and multicomponent separation of CO2 using membranes...

Optimal heat exchanger network synthesis: A case study comparison

Heat exchanger network synthesis (HENS) is one of the most extensively studied synthesis/design problem in chemical engineering. This is attributed to the importance of determining energy costs and improving the energy recovery in chemical processes. Several synthesis procedures for heat exchanger networks have been published in the literature. However, only a modest number can be implemented as computer tools for the full automatic generation of network configurations. In spite of the computational methods evolution, some difficulties still need to be overcome. For instance: computational time, and problem convergence. In this work, the main approaches to solve the problem of heat exchanger network synthesis, sequential and simultaneous, were presented and implemented using the General Algebraic Modeling System (GAMS). Initialization strategies for generating feasible starting points are proposed. Five case studies with different dimensions, from four to 39 streams, are used as background to compare, and analyze the different approaches. The results showed the efficiency of the initialization strategies. Furthermore, the suitable solvers are also analyzed through solving each subproblem. Simultaneous approaches presented better performance than the sequential one. Finally, the main challenges involved in using different approaches is discussed to synthesize heat exchangers networks.

Using convex nonlinear relaxations in the global optimization of nonconvex generalized disjunctive programs

In this paper we present a framework to generate tight convex relaxations for nonconvex generalized disjunctive programs. The proposed methodology builds on our recent work on bilinear and concave generalized disjunctive programs for which tight linear relaxations can be generated, and extends its application to nonlinear relaxations. This is particularly important for those cases in which the convex envelopes of the nonconvex functions arising in the formulations are nonlinear (e.g. linear fractional terms). This extension is now possible by using the latest developments in disjunctive convex programming. We test the performance of the method in three typical process systems engineering problems, namely, the optimization of process networks, reactor networks and heat exchanger networks.

Heat-work conversion optimization of one-stream heat exchanger networks

Heat exchanger networks (HENs) are widely used in industry. This paper extends the entransy theory to heat-work conversion optimization in one-stream HENs. The expression for entransy loss is set up for the open system with work output. The entransy loss, as well as entropy generation, is used to analyze the one-stream HENs with the Carnot engines. The fluid friction is ignored in the analyses. For the cases discussed in this paper, the dimensional and non-dimensional parameters analyses show that the applicability of the minimum entropy generation principle is conditional. The minimum entropy generation does not always correspond to the maximum output power, while the minimum entropy generation number and the minimum revised entropy generation number do not always correspond to the maximum heat-work conversion efficiency. On the other hand, greater power output always corresponds to larger entransy loss and larger entransy loss coefficient always corresponds to larger heat-work conversion efficiency for all the cases discussed in this paper.

Heuristic Approach to Incorporate Timesharing Schemes in Multiperiod Heat Exchanger Network Designs

Heat exchanger network (HEN) synthesis has been widely recognized as an effective design method for significant energy saving in industrial processes. Although traditionally its sole objective is to minimize the total annual cost, there are additional implementation issues which must be addressed in realistic multiperiod applications. In particular, if a conventional programming-based procedure is adopted for generating the optimal HEN structure, each embedded match inevitably calls for different heat-transfer areas in different periods. Since the usual practice is to select the largest unit, its operation in one (or more) period may be grossly inefficient because the corresponding overdesign level is simply too high. To circumvent this drawback, a modified mathematical model and a set of timesharing heuristics have been developed in this work to generate practical network configurations that can handle all heat duties. On the basis of the extensive case studies performed so far, it can be observed that this proposed approach is especially effective for multiperiod HEN design problems in which the process conditions vary significantly.

Multi-period design of heat exchanger networks

Heat exchanger networks are an integral part of chemical processes as they recover available heat and reduce utility consumption, thereby improving the overall economics of an industrial plant. This paper focuses on heat exchanger network design for multi-period operation wherein the operating conditions of a process may vary with time. A typical example is the hydrotreating process in petroleum refineries where the operators increase reactor temperature to compensate for catalyst deactivation. Superstructure based multi-period models for heat exchanger network design have been proposed previously employing deterministic optimisation algorithms, e.g. (Aaltola, 2002; Verheyen and Zhang, 2006). Stochastic optimisation algorithms have also been applied for the design of flexible heat exchanger networks recently (Ma et al., 2007, 2008). The present work develops an optimisation approach using simulated annealing for design of heat exchanger networks for multi-period operation. A comparison of the new optimisation approach with previous deterministic optimisation based design approaches is presented to illustrate the utilisation of simulated annealing in design of optimal heat exchanger network configurations for multi-period operation.

An Improved Mathematical Programming Model for Generating Optimal Heat-Exchanger Network (HEN) Designs

The traditional model-based approach to synthesize the heat-exchanger networks (HENs) was developed on the basis of several unrealistic assumptions. An improved mixed-integer nonlinear programming (MINLP) model is thus developed in this work to circumvent these drawbacks. Specifically, a well-established empirical relation is introduced in the improved model formulation to account for the variation in heat-transfer coefficient with respect to flow rate, and, furthermore, the concept of heat-exchanger efficiency is incorporated to produce an accurate estimate of the heat-transfer area in each exchanger. As a result, it is possible to produce more cost-effective HEN designs with this model. The optimization results obtained in various case studies also show that the process conditions of individual exchangers in the optimal network can generally be tuned simultaneously to achieve a proper compromise between high efficiency and low irreversibility.

Modular design of heat exchanger networks

Heat exchanger networks (HENs) are essential for energy recovery in process plants. Current HEN design methods use approximate models to improve convergence. This work presents a novel linear rigorous algorithm for computation of mass and energy balances which enables rapid synthesis of optimal HEN structure by using modular size heat exchangers. HEN non-iterative simulation algorithm: 1) solves mass balance equations for the network; 2) computes heat exchanger energy transfer factor which depends only on the current flows through the exchanger and conditions for a typical counter-current heat exchanger; 3) solves linear energy balance given results from 1 and 2. This once-through rapid computation enables network design via differential evolution algorithm by using heat exchangers in modular sizes. Several examples illustrate the algorithm capability to determine the best structures. Proposed method enables design of lower capital cost networks that are as efficient as the networks with custom exchanger sizes.

An alternative strategy for global optimization of heat exchanger networks

The HEN synthesis problem is one amongst many engineering problems which can be characterized as highly combinatorial, nonlinear and nonconvex, all contributing to computational difficulties shown either in a form of long computational times and/or in identifying poor locally optimal solutions. In this work, a new strategy for global optimization of heat exchanger networks (HENs) is presented. We first introduce a concept of stage-wise superstructure augmented by an aggregated substructure. On this basis, the HEN synthesis problem is formulated as a mixed integer nonlinear program (MINLP). The strategy for providing globally optimal solutions relies on solving a single convex MINLP which incorporates piecewise linear and nonlinear convex underestimators of the nonconvex linear fractional terms present in the nonconvex MINLP. It is shown that the optimal solution of the convex MINLP can provide a lower bound tight enough that the gap between the upper and lower bound falls below 1%. In addition, an algorithm for identifying good locally optimal solutions is presented. The approach was tested on two examples, showing that currently we are able to solve small HEN synthesis problems to global optimality with reasonable computational effort, while good locally optimal solutions can be identified for larger problems.

Optimal Heat Exchanger Network Synthesis via Particle Swarm Optimization

On the basis of superstructure of heat exchanger network (HEN), we established a particle swarm optimization (PSO) model of HEN with no splits, with the target of minimizing investment and operation cost. A typical HEN was solved via a modified particle swarm optimization (PSO). Through comparative of the optimization result, we could know that this method could reach better solution accuracy.

Entransy-dissipation-based thermal resistance analysis of heat exchanger networks

Heat exchanger network optimization has an important role in high-efficiency energy utilization and energy conservation. The thermal resistance of a heat exchanger network is defined based on its entransy dissipation. In two-stream heat exchanger networks, only heat exchanges between hot and cold fluids are considered. Thermal resistance analysis indicates that the maximum heat transfer rate between two fluids corresponds to the minimum entransy-dissipation-based thermal resistance; i.e. the minimum thermal resistance principle can be exploited in optimizing heat exchanger networks.

A SIMPLE FIVE-STEP METHODOLOGY TO DETERMINE CONTROLLABILITY OF HEAT EXCHANGER NETWORKS USING STEADY-STATE INFORMATION

Process integration design methodologies have been developed and introduced to synthesise an optimum heat exchanger network (HEN) arrangement. However, controllability issues are often overlooked during the early stages of a plant design. In this paper we present a five-step procedure that involves the use of multivariable disturbance and control analyses based solely on steady-state information and with the purpose to assess process design developments and to propose control strategy alternatives appropriate and suitable for a HEN.

Thermal analysis and new insights to support decision making in retrofit and relaxation of heat exchanger networks

Pinch analysis offers a rational framework for identifying energy saving targets and designing efficient heat recovery networks, especially in process industry. Several scientists have contributed to improve and automate the original pinch method over the last decades, increasing its capability to deal with a number of specific issues; the expertise of the analyst, however, remains determinant in achieving optimal results. In this paper a procedure for retrofit of existing networks is proposed, based on an integrate use of several techniques (either existing or innovative). The diagnosis of the existing network and of a “Minimum Energy Requirement” configuration emerges as a useful preliminary instrument for the retrofit study. Then, an innovative spider-type diagram is presented to identify a hierarchic order among a set of retrofit topologies and the most promising relaxation paths for each network topology. The procedure is aimed at offering a conceptual-interpretative approach to energy analysts, to identify preferential routes in networks’ retrofit (and exclude the least promising improvement directions of the existing network); it should be therefore conceived as alternative to algorithms for automatic optimization of heat exchanger networks. The focus is mainly put on the energetic performance of the different schemes (evaluated by thermal analysis of the involved heat exchangers), but the methodology finally enables the energy analyst to identify solutions achieving near-minimum total costs.

Modeling, synthesis and optimization of heat exchanger networks. Application to fuel processing systems for PEM fuel cells

The development of biofuels has gained much attention in recent years. Thermodynamic analyses to obtain energy from biofuels using fuel cells were addressed in previous works for a variety of processes. In those processes, the determination of the best conditions to achieve high efficiency values in the conversion of chemical energy into electrical power is a critical issue from the net global energy efficiency point of view. In this regard, a main aspect is to address the energy integration of the whole process. In a previous paper, the authors dealt with energy integration studies for glycerin- and ethanol-based processors coupled to PEM fuel cells resorting on the “multi-stream heat exchanger” feature provided by the simulation tool HYSYS. In that work, the aim was to maximize the energy recovery from the process streams that renders the maximum achievable net global efficiency. In this paper, the aim is to synthesize and design the optimal heat exchangers network (i.e. determination of the process configuration and units sizes) while maintaining the net global efficiency of the whole system at its achievable value.Three modifications to the original SYNHEAT model developed in 1990 by Yee and Grossmann for synthesizing heat exchanger networks are proposed in this work aiming at a better problem description, and consequently searching for best problem solutions.First, a modification in computing the minimum approach temperature difference is proposed. Second, the called “operation line method” is coupled to the SYNHEAT model to built-up the network superstructure to be optimized. Finally, the SYNHEAT model’s hypothesis of constant cp value for modeling heat exchange between process streams is improved by considering enthalpy variable instead of temperature variable, which is convenient when latent heat is transferred.The model variables number involved in the heat exchanger network synthesis problems solved has been reduced to less than a half by applying the operation line method.The proposed methodology and modifications made are of general application and not just for the specific cases addressed in this work.

Synthesis of Large-Scale Heat Exchanger Network Based on Sub-Networks

The optimization of heat exchanger networks (HEN) is a typical MINLP problem. For large-scale HEN, it is difficult to solve this problem globally. After optimization, the large-scale HEN is divided into several independent sub-networks automatically. The sub-network is defined as a part of the HEN in which the streams have no heat transfer with the streams outside the sub-network. If a HEN can be divided into two or more sub-networks, then, these sub-networks are independent from each other. Based on optimization of sub-networks, a new method which can solve large-scale HEN problem efficiently is proposed.

HEN optimization for efficient retrofitting of coal-fired power plants with post-combustion carbon capture

This study aims at reducing the energy penalty burdened by integration of pulverized coal-fired power plants with solvent-based post-combustion carbon capture (PCC) processes via heat exchanger network (HEN) optimization. The base case used is a 300 MWe coal-fired power plant burning pulverized black coal and emitting 256 tonnes/h of CO 2. Integration of the base case with PCC showed that achieving 90% CO 2 capture with purity of 99% using 30 wt% monoethanolamine (MEA) solvent admits an energy penalty of 19.4% to the overall plant output. Pinch analysis showed that a reduction in the mentioned energy penalty down to 15.9% can be achieved via integration.

Optimization of heat exchanger network

Abstract In this paper, a new method is presented for optimization of heat exchanger networks making use of genetic algorithm and Sequential Quadratic Programming. The optimization problem is solved in the following two levels: 1- Structure of the optimized network is distinguished through genetic algorithm, and 2- The optimized thermal load of exchangers is determined through Sequential Quadratic Programming. Genetic algorithm uses these values for the determination of the fitness. For assuring the authenticity of the newly presented method, two standard heat exchanger networks are solved numerically. For representing the efficiency and applicability of this method for the industrial issues, an actual industrial optimization problem i.e. Aromatic Unit of Bandar Imam Petrochemistry in Iran is verified. The results indicate that the proposed multistage optimization algorithm of heat exchanger networks is better in all cases than those obtained using traditional optimization methods such as Pinch Analysis Method and Mathematical Optimization Method.

The Use of Pinch Technology to Reduce Utility Consumption in a Natural Gas Processing Plant

The present work aims at investigating opportunities for energy conservation in Western Desert Gas Complex (WDGC) through modification of the heat exchanger train. Process simulation of the plant was performed using HYSYS steady-state simulation program. This step was necessary to furnish stream property data requested for heat analysis as well as condenser and reboiler duties. Adopting the pinch design technology, minimum heating and cooling energy requirements were calculated for different values of minimum approach temperature (ΔTmin). Two methods were used, the problem table algorithm of the pinch method and the linear programming method. Then designing the heat exchanger network (HEN) took place according to the pinch design method to achieve target utilities for each ΔTmin. The optimum HEN is that which achieves considerable heat recovery with least annualized total cost. The optimum HEN design was found to correspond to ΔTmin of 10°C and achieved saving of hot and cold utilities as 42 and 21%, respectively, compared to the actual utilities consumption of the existing plant. These savings can be realized by adding only two heat exchangers to the existing network with a payback period of ≈1 year.