Synthesis of non-isothermal heat integrated water networks in chemical processes

This paper presents a new approach for the simultaneous synthesis and optimization of heat integrated water networks. A new superstructure for heat exchanger network (HEN) synthesis is proposed. The procedure is based on mixed integer non-linear mathematical programming (MINLP). Four relevant examples are presented to illustrate various aspects of the proposed approach.     

A systematic approach for synthesizing a low-temperature distillation system

In this paper, by combining a stochastic optimization method with a refrigeration shaft work targeting method, an approach for the synthesis of a heat integrated complex distillation system in a low-temperature process is presented. The synthesis problem is formulated as a mixed-integer nonlinear programming (MINLP) problem, which is solved by simulated annealing algorithm under a random procedure to explore the optimal operating parameters and the distillation sequence structure. The shaft work targeting method is used to evaluate the minimum energy cost of the corresponding separation system during the optimization without any need for a detailed design for the heat exchanger network (HEN) and the refrigeration system (RS). The method presented in the paper can dramatical y reduce the scale and complexity of the problem. A case study of ethylene cold-end separation is used to il ustrate the application of the approach. Compared with the original industrial scheme, the result is encouraging.     

Mathematical programming approaches to the synthesis of chemical process systems

This paper presents a review of advances that have taken place in the mathematical programming approach to process design and synthesis. A review is first presented on the algorithms that are available for solving MINLP problems, and its most recent variant, Generalized Disjunctive Programming models. The formulation of superstructures, models and solution strategies is also discussed for the effective solution of the corresponding optimization problems. The rest of the paper is devoted to reviewing recent mathematical programming models for the synthesis of reactor networks, distillation sequences, heat exchanger networks, mass exchanger networks, utility plants, and total flowsheets. As will be seen from this review, the progress that has been achieved in this area over the last decade is very significant.     

过程系统能量集成同步最优综合法

将换热网络超结构混合整数非线性规划多目标同步最优综合方法进一步扩展到与过程系统的联合优化,提出了改进的过程热集成同步综合方法,并以反应分离过程与换热网络能量集成为例,建立了同步优化超结构模型,采用混合整数非线性规划的遗传算法求解,可同时得到热集成系统最优的流程结构和操作条件。     

考虑中间换热器的能量集成精馏序列合成

针对考虑中间换热器（IHE）的精馏序列合成问题,提出基于随机优化策略的能量集成非清晰精馏序列（IHE-HIDSs）合成方法。通过对精馏序列分离任务合并处引入二元0/1变量表示是否存在IHE,以精馏序列的年总成本（TAC）为优化目标,建立了该合成问题的隐式混合整数非线性规划模型（MINLP）,通过模拟退火和粒子群优化（SA-PSO）混合随机优化算法进行求解。为验证在精馏序列合成中同时考虑IHE的必要性以及所提出合成方法的有效性,对五组分醇类混合物和五组分烷烃类混合物两个算例的精馏序列合成问题进行了研究。结果表明,相比同时考虑热耦合和能量集成的精馏序列,IHE-HIDS具有更低的TAC。此外,所提出的方法可以在合理的计算时间内以高概率获得多个分离序列方案。     

Simultaneous MINLP synthesis of heat exchanger networks comprising different exchanger types

This paper describes the simultaneous MINLP synthesis of heat integrated heat exchanger networks comprising different heat exchanger types. The stage-wise superstructure of heat exchanger networks (HEN) by Yee and Grossmann (Comput. Chem. Eng. 14 (1990) p. 1165) is extended to alternative exchanger types. The selection of the types is modeled by disjunctions based on operating limitations and the required heat transfer area. Since different types of heat exchangers involve different design geometries, which influences the inlet and outlet temperatures of heat exchangers, additional constraints are specified to provide a feasible temperature distribution in HEN. The consideration of different exchanger types drastically increases the combinatorics, size and computation effort needed to solve the problem. The integer-infeasible path MINLP approach has been applied to perform an efficient initialization scheme and to halve CPU times for solving MILP master problem of the modified OA/ER algorithm. A special multilevel MINLP procedure in reduced integer space has been proposed to solve medium size HEN problems (20 streams) comprising 10³ and more binary variables.     

用于热集成精馏序列综合的改进模拟退火算法

提出一个用于多组分热集成精馏系统综合问题的改进模拟退火算法.将优化问题表示为混合整数非线性规划（MINLP）问题,将算法设计同研究问题的特征相结合,运用本文方法可在同一退火策略下实现流程结构和操作参数的同步优化,并能达到计算精度和计算速度的更好均衡.通过对大规模算例（产品数≥10）的计算表明,该方法对求解此类问题非常有效.     

Design of flexible heat exchanger network for multi-period operation

Heat exchanger networks (HENs) increase heat recovery from industrial processes by matching hot and cold streams to exchange heat and reducing utility consumption. The design of HENs is a very complex task which generally involves mixed-integer non-linear programming (MINLP).This work evaluates and critically compares existing HEN design methods. It then presents a systematic methodology in the design of HENs under multiple periods of operation. The model presented in this work is a superstructure-based MINLP model which minimises the total annualised cost containing heat exchanger area cost and utility costs. The model is based on the superstructure by Yee and Grossmann [1990. Simultaneous optimisation models for heat integration—II, heat exchanger network synthesis. Computer & Chemical Engineering 14(10), 1165-1184], which was later formulated for multiple periods by Aaltola [2002. Simultaneous synthesis of flexible heat exchanger network. Applied Thermal Engineering 22, 907-918]. It includes a multi-period simultaneous MINLP model to design the HEN structure, and an NLP model to improve the solution and allow for non-isothermal mixing. Modifications to Aaltola's model include the use of maximum area per period in the area cost calculation of the MINLP objective function, and the removal of slack variables and weighed parameters from the existing NLP improvement model.The new model has been applied to one industrial case study, demonstrating that the new combined MINLP-NLP model can obtain better solutions by not relying on the average area assumption in the MINLP stage.     

Simultaneous MINLP synthesis of heat and power integrated heat exchanger networks

This paper describes the simultaneous MINLP synthesis of heat and power integrated heat exchanger networks. The objective of this work is to obtain feasible networks with optimal tradeoffs between investment, utility and power consumption caused by pressure drops in heat exchangers, and to use a detailed design model for exchangers in the synthesis in order to represent pressure drops and heat transfer coefficients as functions of exchanger design variables. In this way, the simultaneous synthesis of HEN and the detailed exchanger design become completely consistent and optimal.     

A simple synthesis method for studying thermally integrated distillation sequences

In this paper an improved method for studying processes for separation of multicomponent mixtures is presented. A thermodynamically oriented procedure was developed for the analysis and synthesis of the best heat‐integrated distillation system. Systematic ranking of possible schemes was used in order to verify the best structure of a heat‐integrated distillation system. An integrability criterion was introduced for the selection of the most promising heat‐integrated sequences. Distillation columns were studied according to their ability for heat integration. The proposed selection of possible distillation columns sequences using rising integrability criteria gave a good approximation of the best heat‐integrated distillation column sequences according to their total annual cost. Computational simulation of the process and pinch analysis were used for thermodynamic optimization of energy consumption. The method is illustrated with five‐ and six‐component example problems.     

热集成复杂精馏系统综合的研究

将夹点分析法应用于热集成复杂精馏系统综合的换热网络设计中,避免了以换热网络结构作为独立变量,建立了一个以预分馏塔组分回收率、回流比及操作压力为连续变量,以分离序列和热耦合方式为离散变量的混合整数非线性规划(MINLP)模型。该模型用改进的模拟退火算法求解,可同时得到优化的流程结构和操作参数。对多个五组分混合物分离问题进行了求解,并对不同优化方案的优化结果及其经济性作了比较和分析,结果表明采用热集成复杂精馏塔流程可以显著地降低系统的总费用,还表明该方法是求解热集成复杂精馏系统综合问题的有效方法。     

New developments and capabilities in prosyn—An automated topology and parameter process synthesizer

This paper describes an improved, equation oriented user friendly version of the modular computer package PROSYN—a mixed-integer nonlinear programming (MINLP) process synthesizer. A number of strategies are described and implemented for the outer approximation and equality relaxation algorithm (OA/ER) in order to reduce the undesirable effects of nonstructured nonconvexities in the master problem. These strategies include use of valid outer-approximations for splitters and mixers and various ways of handling the linearizations. An NLP initializer, model generator and a comprehensive library of models for basic process units and interconnection nodes, and a comprehensive library of basic physical properties have been developed and built in the PROSYN package. In this way PROSYN can be used to solve problems at different levels of complexity: from the simple NLP flowsheeting up to the MINLP synthesis through the modeling and decomposition (M/D) strategy and simultaneous heat integration including HEN costs. Applications with PROSYN are demonstrated with several example problems.     

考虑非等温混合的能量集成多杂质水网络优化设计

提出了一种分步综合多杂质体系水网络和换热网络的新方法。对于水网络,考虑进入操作单元的新鲜水与回用水之间的非等温混合,确保流股间的直接热回收;对于换热网络,可以只考虑水网络中的新鲜水和回用水流股之间的换热匹配。采用无进化次数的改进粒子群算法对本文建立的多杂质体系水网络非线性模型和换热网络混合整数非线性模型进行求解。实例表明,与不考虑非等温混合时比较,考虑非等温混合时的最优网络结构更简单,且年度总费用要节省2.4%。因此,本文提出的方法在实际生产中有较好的可行性和有效性。     

Simultaneous process synthesis,heat, power,and water integration of thermochemical hybrid biomass,coal, and natural gas facilities

A comprehensive wastewater network is introduced into a thermochemical based process superstructure that will convert biomass, coal, and natural gas to liquid (CBGTL) transportation fuels. The mixed-integer nonlinear optimization (MINLP) model includes simultaneous heat, power, and water integration that utilizes heat engines to recover electricity from waste heat and several treatment units to process and recycle wastewater. A total of 108 case studies are analyzed which consist of combinations of six coal feedstocks, three biomass feedstocks, three plant capacities, and two process superstructures. This study discusses important process topological differences between the case studies and illustrates each component of the process synthesis framework using the two medium-sized capacity case studies that have low-volatile bituminous coal and biomass feedstocks.     

多周期操作工况下换热网络的弹性设计

提出了基于专家系统的多周期换热网络的超结构形式及弹性设计方法。首先通过专家系统确定各周期操作工况下物流的分流及匹配禁止情况,在此基础上建立网络多周期操作工况下的超结构模型;然后用遗传算法对上述模型进行求解;最后用分解协调法对网络结构和匹配单元同时进行优化。该方法不仅能自动地合成多周期操作工况的换热网络最优流程结构,而且能同时满足网络在各操作工况下的各种工艺限制条件。最后,通过某个换热网络的最优合成设计,说明本方法的有效性和应用前景。     

换热器网络同步最优综合方法──改进的MINLP转运模型法

针对现有的换热器网络最优综合方法的局限性，采用分级超结构转运模型和物流吸、放热潜力的概念，提出了非等温混合线性约束的换热器网络同步最优综合混合整数非线性规划（MINLP）的改进模型．该模型不仅可以对公用工程费用、换热面积、换热设备台数及物流的匹配选择进行多目标同步优化，而且在线性约束的条件下消除了等温混合的不合理假设，只需求解一次MINLP问题就可得到包括分流情况在内的最优网络结构．算例表明，该模型优于以往的几种同步优化模型．     

Prosyn—an MINLP process synthesizer

This paper reports the development of the program PROSYN, an automated implementation of the modelling/decomposition (M/D) strategy by Kocis and Grossmann (Computers chem. Engng 13, 797–819, 1989) for the MINLP optimization of process flowsheets. A systematic procedure is first presented for the decomposition of general superstructures of process flowsheets that avoids the optimization of units with zero flow. A model for simultaneous optimization and heat integration is also proposed that can account for area and energy considerations. Finally, a computer implementation is described that is suitable for the complex logic involved in the M/D strategy for the synthesis and heat integration of process flowsheets. Examples are presented to illustrate each of these points, as well as the performance of PROSYN.     

One-step approach for heat exchanger network retrofitting using integrated differential evolution

Heat exchanger network (HEN) retrofitting is more important and challenging than HEN synthesis since it involves modifying existing network for improved energy efficiency. Additional factors to be considered include spatial constraints, relocation and re-piping costs, reassignment and effective use of existing heat exchanger areas. The previous studies using stochastic global optimization algorithms are mainly focused on two-level approach: the first level uses a stochastic algorithm for optimizing structure, and the second level uses either a stochastic or a deterministic algorithm for optimizing continuous variables. In this study, we propose and test one-step approach where a stochastic global optimization method, namely, integrated differential evolution (IDE), handles both discrete and continuous variables together. Thus, HEN structure and retrofitting model parameters are simultaneously optimized by IDE, which avoids the algorithm trapping at a local optimum and also improves the computational efficiency. Results on HEN applications show that the proposed approach gives better solutions.     

Synthesis of flexible multi-stream heat exchanger networks based on stream pseudo-temperature with genetic/simulated annealing algorithms

The synthesis of flexible heat exchanger network (HEN) is generally regarded as an over-design of process units over a specified range of deviations in process parameters from their nominal values. The HEN obtained is more costly because of the over-design of HEN. The global solution to flexible design problems cannot be guaranteed because of the resulting non-differentiable, non-convex, max–min–max constraint of mixed integer nonlinear programming (MINLP) models. In this paper a new simultaneous two-stage strategy for synthesizing flexible multi-stream HEN (FMSHEN), optimized by genetic/simulated annealing algorithm (GA/SA), is presented. First, based on the pseudo-temperature enthalpy (T–H) diagram method, a new nonlinear programming (NLP) formulation involving all of the vertices of the polyhedral uncertainty region in the space of process parameters is proposed, with the supposition that the feasible region defined by the reduced inequality constraints is convex. An over-design FMSHEN is obtained by optimizing the stream heat transfer temperature difference contribution. Secondly, the optimal structure of the over-design FMSHEN is retained and each heat exchanger area is modified in order to make the FMSHEN less costly. The total annual cost of MSHEN, obtained from the simulation of MSHEN according to the vertices of the polyhedral uncertain region, is regarded as an objective function, and GA/SA is adopted for optimizing the heat exchanger areas. The remarkable feature of the strategy is that the size and the complexity of the problem are reduced significantly and with more probability of locating the global solution. Finally, two examples are illustrated to demonstrate the performance of the strategy for the synthesis of flexible multi-stream heat exchanger networks.     

Optimization of a High Temperature PEMFC micro‐CHP System by Formulation and Application of a Process Integration Methodology

A 1 kWe micro combined heat and power (CHP) system based on high temperature proton exchange membrane fuel cell (PEMFC) technology is modeled and optimized by formulation and application of a process integration methodology. The system can provide heat and electricity for a single‐family household. It consists of a fuel cell stack, a fuel processing subsystem, heat exchangers, and balance‐of‐plant components. The optimization methodology involves system optimization attempting to maximize the net electrical efficiency, and then by use of a mixed integer nonlinear programming (MINLP) problem formulation, the heat exchange network (HEN) annual cost is minimized. The results show the high potential of the proposed model since high efficiencies are accomplished. The net electrical efficiency and total system efficiency, based on lower heating value (LHV), are 35.2% and 91.1%, respectively. The minimized total annual cost of the HEN is $8,147 year^–1.