Pingjing Yao

A combined genetic algorithm/simulated annealing algorithm for large scale system energy integration

Abstract A new algorithm named GA/SA (genetic algorithm/simulated annealing) is presented in this paper for solving a large scale system energy integration problem which is difficult to solve on the total process system level directly by traditional algorithm. The general GA has been improved by using OCX (orthogonal crossover) and EC (effective crowding) operators, and the improved GA is combined effectively with an SA algorithm to avoid the common defect of early convergence. Numerical calculation results show that the new algorithm can converge faster than either SA or GA algorithms alone, and has much more probability of locating a global optimum. The convergence proof of the new algorithm is also given. GA/SA has been used to solve a 167 streams problem. A good result is achieved for improving the total process retrofit efficiency.

Synthesis and optimization of heat integrated distillation systems using an improved genetic algorithm

Abstract Synthesis of heat integrated multi-component distillation systems is complex due to its huge search space for structural combination and optimization computation. To provide a systematic approach and tools for synthesis design and retrofits of distillation systems, an improved genetic algorithm (IGA) is proposed. The algorithm inherits main ideas in evolutionary computing, employs a distributed sub-population strategy to avoid local optima, and applies for a continuous variable space coding procedure, which is computational fast and stable in converging to global optima. This paper describes IGA-based optimization strategy for synthesis and optimization of separation sequences and their heat exchanger network. Two example are presented.

A systematic approach for synthesizing combined mass and heat exchange networks

Mass and heat are very important resources in the process industry. Numerous approaches have been proposed for the optimization of either mass or heat exchange networks. Since the process usage of mass and heat is typically intertwined, it is important to account for such interactions. The objective of this paper is to introduce a systematic method for the simultaneous synthesis of combined mass- and heat-exchange networks (CMAHENs). The proposed method is based on a novel approach that incorporates the mass pinch technology (MPT) for mass exchange networks (MENs) synthesis and the pseudo-T-H diagram approach (PTHDA) for the heat exchange networks (HENs) synthesis. New bypass streams are included in the structural representation of the problem to expand the search space. A combined optimization approach is applied to minimize the total annualized cost of the CMAHEN. Finally, two cases are solved to illustrate the application of the proposed method.

Simultaneous Optimization of Synthesis and Scheduling of Cleaning in Flexible Heat Exchanger Networks

Abstract A novel methodology is presented for simultaneously optimizing synthesis and cleaning schedule of flexible heat exchanger network (HEN) by genetic/simulated annealing algorithms (GA/SA). Through taking into account the effect of fouling process on optimal network topology, a preliminary network structure possessing twofold oversynthesis is obtained by means of pseudo-temperature enthalpy ( T - H ) diagram approach prior to simultaneous optimization. Thus, the computational complexity of this problem classified as NP (Non-deterministic Polynomial)-complete can be significantly reduced. The promising matches resulting from preliminary synthesis stage are further optimized in parallel with their heat exchange areas and cleaning schedule. In addition, a novel continuous time representation is introduced to subdivide the given time horizon into several variable-size intervals according to operating periods of heat exchangers, and then flexible HEN synthesis can be implemented in dynamic manner. A numerical example is provided to demonstrate that the presented strategy is feasible to decrease the total annual cost (TAC) and further improve network flexibility, but even more important, it may be applied to solve large-scale flexible HEN synthesis problems.

Modeling and simulation of high-pressure urea synthesis loop

In this paper, modeling and simulation of high-pressure urea synthesis loop has been studied. The extended electrolytic UNIQUAC equation has been used to describe the nonideality of liquid phase of NH3-CO2-H2O-urea system under high pressure and high temperature, and the perturbed-hard-sphere (PHS) equation of state has been used to predict the vapor fugacity coefficients. The vapor-liquid equilibrium has been treated simultaneously with chemical reactions due to the complex features of the urea synthesis system. A concrete procedure for simulation of the key units and flowsheet of the high-pressure urea synthesis loop has been developed. The equilibrium-stage model has been applied to modeling and simulation of the urea reactor, CO2 stripper and other key units in the high-pressure loop for urea synthesis, and the simultaneous-modular approach has been adopted to simulate the flowsheet. The simulation results have been found to be in good agreement with the design data from the industry-scale plant. This work provides an effective tool and significant guidance for design or retrofit of urea synthesis process. (c) 2004 Elsevier Ltd. All rights reserved.

Synthesis of Multi-component Mass-exchange Networks

Abstract This paper presents a superstructure-based formulation for the synthesis of mass-exchange networks (MENs) considering multiple components. The superstructure is simplified by directly using the mass separation agents (MSA) from their sources, and therefore the automatic synthesis of the multi-component system involved in the MENs can be achieved without choosing a ‘key-component’ either for the whole process or the mass exchangers. A mathematical model is proposed to carry out the optimization process. The concentrations, flow rates, matches and unit operation displayed in the obtained network constitute the exact representation of the mass exchange process in terms of all species in the system. An example is used to illustrate and demonstrate the application of the proposed method.

Strategy for Synthesis of Flexible Heat Exchanger Networks Embedded with System Reliability Analysis

Abstract System reliability can produce a strong influence on the performance of the heat exchanger network (HEN). In this paper, an optimization method with system reliability analysis for flexible HEN by genetic/simulated annealing algorithms (GA/SA) is presented. Initial flexible arrangements of HEN is received by pseudo-temperature enthalpy diagram. For determining system reliability of HEN, the connections of heat exchangers(HEXs) and independent subsystems in the HEN are analyzed by the connection sequence matrix(CSM), and the system reliability is measured by the independent subsystem including maximum number of HEXs in the HEN. As for the HEN that did not meet system reliability, HEN decoupling is applied and the independent subsystems in the HEN are changed by removing decoupling HEX, and thus the system reliability is elevated. After that, heat duty redistribution based on the relevant elements of the heat load loops and HEX areas are optimized in GA/SA. Then, the favorable network configuration, which matches both the most economical cost and system reliability criterion, is located. Moreover, particular features belonging to suitable decoupling HEX are extracted from calculations. Corresponding numerical example is presented to verify that the proposed strategy is effective to formulate optimal flexible HEN with system reliability measurement.

Synthesis of waste interception and allocation networks using genetic-alopex algorithm

Abstract In this paper, the hybrid genetic-alopex algorithm (GAA) is proposed to solve the problem of synthesis of waste interception and allocation networks (WINs). Algorithm of pattern extraction (alopex) adopts ‘noises’ generated randomly to cast off local optima and enhances the capacity of hillclimbing. Genetic algorithms (GA) is good at keeping evolving of total population and parallel computing. The combination of GA and alopex enhances the algorithm in that it converges at the global optimum easily and accelerates the convergence to some extent. In addition, several advanced genetic operators are adopted to protect diversity of the population. Results of testing functions and a case study show that GAA is stable and can converge to global optimum well for the problem of high dimensions.

New model and new algorithm for optimal synthesis of large scale heat exchanger networks without stream splitting

Abstract A new superstructure rigorous model for mixed-integer nonlinear programming is proposed for optimal synthesis for large scale heat exchanger network without stream splitting (number of streams is over 50). The model is based on a new superstructure, comprises more feasible structures, and deals with the problem of “combination explosion” successfully. At the same time, a new improved genetic algorithm is proposed for solving the model. One industrial case of retrofit of Ethylene plants HEN is reported as a case study.

A simultaneous synthesis method for combined heat and mass exchange networks

Abstract Mass-exchange operations are highly impacted by heating and cooling. Therefore, there is a natural coupling between the problems of synthesizing mass exchange networks (MENs) and heat exchange networks (HENs). The objective of this paper is to introduce a systematic method for the simultaneous synthesis of combined mass- and heat- exchange networks (CM&HEN). The proposed method is based on a novel approach that integrates the pseudo-T-H diagram approach (PTHDA) for HEN synthesis, with the mass pinch technology (MPT) for MEN synthesis. The applicability of the proposed approach is shown through an example problem, where a significant reduction in the total annual cost is obtained.