Synthesis of refrigeration system based on generalized disjunctive programming model

Refrigeration system holds an important role in process industries. The optimal synthesis cannot only reduce the energy consumption, but also save the production costs. In this study, a general methodology is developed for the optimal design of refrigeration cycle and heat exchanger network (HEN) simultaneously. Taking the heat integration between the external heat sources/sinks and the refrigeration cycle into consideration, a superstructure with sub-coolers is developed. Through defining logical variables that indicate the relative temperature positions of refrigerant streams after sub-coolers, the synthesis is formulated as a Generalized Disjunctive Programming (GDP) problem based on LP transshipment model, with the target of minimizing the total compressor shaft work in the refrigeration system. The GDP model is then reformulated as a Mixed Integer Nonlinear Programming (MINLP) problem with the aid of binary variables and Big-M Constraint Method. The efficacy of the process synthesis model is demonstrated by a case study of ethylene refrigeration system. The result shows that the optimization can significantly reduce the exergy loss as well as the total compression shaft work.     

Simultaneous synthesis of distillation sequences in overall process schemes using an improved minlp approach

This paper describes the synthesis of distillation sequences simultaneously with the synthesis of other process subsystems. An improved MINLP approach is proposed for the synthesis of complex problems. The improvements were made on three major MINLP levels. By postulating the superstructures we propose to use smaller and more compact superstructures of the distillation sequences included in the flowsheet superstructure rather than the usual tree and network representations. In the modelling step a higher degree of simultaneity has been accomplished by the use of simultaneous models of the distillation column and aggregated models of other process units. To facilitate the optimization phase the modified OA/ER algorithm [Kravanja and Grossmann, Ind. Engng. Chem. Res.26, 1869–1880 (1994)], implemented in the computerized synthesizer PROSYN-MINLP, has been used to reduce the effect of nonconvexities. Also, a special initialization and linearization scheme for simultaneous synthesis of HEN has been proposed. The improved method is illustrated by the synthesis of the separation subsystem followed by the simultaneous synthesis of the heat integrated separation system and its heat exchanger network (HEN). Finally, the optimization of the overall process and the simultaneous synthesis of its distillation system and heat integrated HEN is presented.     

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。     

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.     

A Two-step Design Method for Shaft Work Targeting on Low-temperature Process

In low-temperature processes, there are interactions between heat exchanger network （HEN） and refrig-eration system. The modification on HEN of the chilling train for increasing energy recovery does not always coor-dinate with the minimum shaft work consumption of the corresponding refrigeration system. In this paper, a sys-tematic approach for optimizing low-temperature system is presented through mathematical method and exergy analysis. The possibility of＂pockets＂, which appears as right nose section in the grand composite curve （EGCC） of the process, is first optimized. The EGCC with the pockets cutting down is designed as a separate part. A case study is used to illustrate the application of the approach for a HEN of a chilling train with propylene and ethylene refrig-erant system in an ethylene production process.     

Process synthesis for cascade refrigeration system based on exergy analysis

Refrigeration system holds an important role in chemical/petrochemical processes. The traditional cascade refrigeration system (CRS) used in ethylene plants contains multiple refrigerants working at multiple temperature/pressure levels. In this study, a general methodology is developed for the optimal process synthesis of a CRS based on exergy analysis. This procedure involves four stages: (1) refrigeration system exergetic analysis; (2) optimization model development for simultaneous synthesis of refrigeration system and heat exchanger network (HEN); (3) HEN configuration; and (4) final solution validation. The exergy–temperature chart is used to comprehensively analyze a CRS. A mathematical model is presented to minimize total compressor shaft work of the HEN‐considered CRS, where multiple recycling loops satisfying all cooling/heating demands are simultaneously addressed. The optimal solution is examined by rigorous simulations to verify its feasibility and consistency. The efficacy of the developed methodology is demonstrated by a case study of a propylene CRS in an ethylene plant. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2471–2488, 2015     

Optimal design of cryogenic distillation columns with side heat pumps for the propylene/propane separation

Propylene is one of the most important products in the petrochemical industry, which is used as raw material for a wide variety of products. The propylene/propane separation is a very energy-intensive process because their boiling points are quite similar. In addition, at atmospheric conditions, their boiling points are −47.6 °C and −42.1 °C, respectively. To separate this mixture conventional columns which operate at high pressure and cryogenic distillation columns which operate at low pressure have been used, however, this approaches are still energy-intensive. This work presents energy-efficient and intensified distillation columns which are adiabatic such as the vapor recompression column (VRC) or diabatic such as columns with heat-integrated stages. A design and optimization procedure, which minimizes the energy consumption in the propylene/propane separation is presented. Conceptual design, superstructure representation, rigorous simulations and mathematical programming techniques are effectively combined to assess all the candidate distillation structures, refrigeration cycles, and heat integration possibilities simultaneously. Results showed that VRC and diabatic distillation columns with heat-integrated stages can reduce the energy consumption between 58 and 75% when compared with conventional distillation at high pressure. Furthermore, the proposed synthesis procedure derived simplified optimal distillation structures with few heat-integrated stages and still attained important energy savings.     

低温蒸馏塔与冷冻系统的热泵流程集成

本文复合夹点分析方法和　分析原理,建立了低温蒸馏塔与冷冻系统热泵流程集成的优化方法。其中首次构造了低温过程系统热泵流程集成的　总组合曲线,用以目标化低温蒸馏塔与冷冻系统的热泵流程集成;并提出低温过程系统热泵流程集成的　效能系数,预测冷冻系统通过热泵流程集成所得到的轴功节省目标。所提出的方法已成功地应用于乙烯精馏塔与冷冻系统联合解瓶颈工程实例,经济效益显著。     

Optimum design of Petlyuk and divided-wall distillation systems using a shortcut model

An optimization approach for fully thermally coupled distillation systems for the separation of ternary mixtures is presented. The thermally coupled schemes considered here can be implemented in the form of the original Petlyuk arrangement (with an external prefractionator) or the modified Kaibel arrangement (in the form of a divided-wall column). The approach uses a shortcut design method that allows the system to be modeled as a nonlinear programming problem. Given the practical interest in divided-wall column arrangements, special attention is given in this work to this type of structure. Several cases of study are presented to show the applicability of the proposed approach.     

多流股换热器网络综合方法在原油预热系统中的应用

以原油常减压蒸馏装置原油预热网络为研究对象,进行了多流股换热器网络的实例综合. 基于超结构物理模型建立了改进的多流股换热器网络综合数学模型,提出将该网络综合问题由混合整数非线性规划问题转化为简单的非线性规划问题的求解策略,并利用改进的遗传/模拟退火新算法进行了原油预热网络的综合. 与Hextran软件的综合结果以及现场换热网络的对比表明,本模型和求解策略可以应用于工业规模的多流股换热器网络综合,有可能取得较好的经济效益.     

模拟退火方法用于带有中间换热器精馏塔的优化综合

This article presents a simulated annealing-based approach to the optimal synthesis of distillation column considering intermediate heat exchangers arrangements. T-he number of intermediate condensers and/or intermediate reboilers, the placement locations, the.operating pressure of column, and the heat duties of intermediate heat exchangers are treated as optimization variables. A novel coding procedure making use of an integer number series is proposed to represent and manipulate the structure of system and a stage-to-stage method is used for column design and cost calculation. With the representation procedure, the synthesis problem is formulated as a mixed integer nonlinear programming （MINLP） problem, which can then be solved with an improved simulated annealing algorithm. Two examples are illustrated to show the effectiveness of the suggested approach.     

Synthesis of Heat Integrated Complex Distillation Systems via Stochastic Optimization Approaches

This paper addresses the application of stochastic optimization approaches to the synthesis of heat integrated complex distillation system, which is characterized by large-scale combinatorial feature. Conventional and complex columns, thermally coupled (linked) side strippers and side rectifiers as well as heat integration between the different columns are simultaneously considered. The problem is formulated as an MINLP (mixed-integer nonlinear programming) problem. A simulated annealing algorithm is proposed to deal with the MINLP problem and a shortcut method is applied to evaluate all required design parameters as well as the total cost function. Two illustrating examples are presented.     

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。     

Stochastic GP synthesis of heat integrated nonsharp distillation sequences

Genetic programming (GP) is used to solve the synthesis problem of heat integration nonsharp distillation sequences and the optimization objective of it is to seek the optimal heat integration nonsharp flow based on minimizing the annually total cost. Meanwhile, the major technological parameters for these important equipments are given. Combining with the domain knowledge of chemical engineering, some evolutionary factors are improved, and a set of special encoding method and heat integration strategy is proposed to deal with this kind of problem. The system structural variable is optimized by GP and the continuous variable is optimized by the complex algorithm simultaneously. Because GP has the automatically searching function, the optimal heat integration solution can be found automatically without any superstructures of nonsharp distillation sequences. Three illustrating examples are presented to demonstrate the effective computational strategies.     

热泵热集成精馏流程的优化设计与综合

从综合的角度详细讨论了热泵精馏的原理,提炼出了用于综合的热泵精馏规则,并开发了综合所用的热泵精馏简捷经济评价方法,提出了一个两水平综合策略,给出了热泵精馏综合步骤。实例研究表明,本文开发的算法及提出的规则用于热泵热集成精馏流程的综合,能够得出最优的絷泵精馏流程。     

基于遗传规划的复杂精馏系统综合

提出了基于遗传规划的复杂精馏系统的综合方法.针对复杂精馏系统特点并结合化工领域知识,对遗传算子进行了改进,给出了一套独特的编码方法和求解策略.算法利用层次化结构来直观地表达复杂分离流程,同时解决了其他随机算法编码在表达精馏过程时的歧意性问题.在不必给定精馏系统超结构的情况下,能够完成可行域的自动搜寻.实例计算表明,基于遗传规划的综合策略可以用于求解复杂精馏系统的最优综合问题,实现了包括侧线塔、完全热偶塔及同时考虑热集成的各种复杂精馏结构的自动搜索寻优.     

Optimization of a reactive distillation process with intermediate condensers for silane production

This work presents a reactive distillation column for the catalytic disproportionation of trichlorosilane to silane which includes three consecutive reversible reactions. This reaction system is however characterized by a large distinction in the boiling points of the components, which make the reactive distillation extremely favored. Nevertheless, the normal reactive distillation column possesses the shortage of high refrigeration requirement. By removing heat at temperature higher than that at the condenser a superstructure representation, rigorous simulations, and optimization problems were combined to derive optimal reactive distillation columns which can realize heat integration between stages and utilities at several refrigeration conditions. An iterative simulation-optimization procedure was proposed to consider temperature changes in stages due to heat integration. The results showed that the installation of two inter-condensers results in the best option with economic savings up to 56%.     

Optimal synthesis of compression refrigeration system using a novel MINLP approach

The optimal design of a compression refrigeration system (CRS) with multiple temperature levels is very important to chemical process industries and also represents considerable challenges in process systems engineering. In this paper, a general methodology for the optimal synthesis of the CRS, which simultaneously integrates CRS and Heat Exchanger Networks (HEN) to minimize the total compressor shaft work consumption based on an MINLP model, has been proposed. The major contribution of this method is in addressing the optimal design of refrigeration cycle with variable refrigeration temperature levels. The method can be used to make major decisions in the CRS design, such as the number of levels, temperature levels, and heat transfer duties. The performance of the developed methodology has been illustrated with a case study of an ethylene CRS in an industrial ethylene plant, and the optimal solution has been examined by rigorous simulations in Aspen Plus to verify its feasibility and consistency.     

碳酸二甲酯的合成及换热网络的优化

节能减排越来越受到人们的重视,夹点技术是目前应用最广泛的热集成技术之一。文章利用夹点技术对碳酸二甲酯合成工艺进行了系统用能分析,找出了用能状况不尽合理之处。将碳酸二甲酯合成及精馏四个单元作为一个系统整体考虑,提出了碳酸二甲酯合成及精馏单元的换热网络优化方案。优化后整个系统可以节约低压蒸汽33.7%,节约循环冷却水93.6%。     

乙烯分离与复叠制冷系统用能的综合优化

乙烯装置的分离过程要在低温下进行,由乙烯制冷系统提供所需冷量。乙烯制冷系统为封闭式循环,独立于分离单元之外。将乙烯分离单元与制冷系统同时优化,能有效提高装置用能效率。复叠式制冷级数是当前乙烯工业中使用最为广泛的制冷技术。本文针对乙烯分离过程和配套的复叠制冷系统,采用Aspen Hysys进行模拟并进行(火用)分析,发现系统主要的(火用)损失发生在换热与压缩两部分,其占总(火用)损失的83%,为节能的重点。进而通过夹点技术对冷剂配置进行分析,发现-56℃以上各温位的冷量配置不合理,远超过理论最小值,-56℃以下各温位的冷量基本达到理论最小值。提出了采用多股流换热器的换热网络理论设计方法,并对冷剂进行重新配置,该理论方案可以降低丙烯制冷压缩机约30%的功耗,并节约部分乙烯制冷压缩机功耗,显著降低了乙烯深冷分离能耗。