Synthesis of nonsharp distillation sequences via genetic programming

This paper addresses the application of Genetic Programming (GP) to the synthesis of multicomponent product nonsharp distillation sequences. Combined with the domain knowledge of chemical engineering, some evolutionary factors are improved, and a set of special encoding method and solving 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 simulated annealing algorithm simultaneously. Because GP has an automatic searching function, the optimal solution can be found including distillation, splitting, blending and bypassing operations automatically without any superstructures of nonsharp distillation sequences. Three illustrative examples are presented to demonstrate the effective computational strategies.     

A novel stochastic optimization method to efficiently synthesize large-scale nonsharp distillation systems

This study presents a novel stochastic optimization method for the efficient synthesis of large-scale nonsharp distillation systems, where heat integration and thermal coupling can be involved simultaneously. A new binary tree encoding method was developed to represent distillation sequences with no limits on the number of middle components in nonsharp splits to ensure a complete solution space. Thermally coupled structures were defined by 0–1 binary variables. Evolutionary rules were developed to generate neighboring distillation configurations randomly. Finally, an optimization framework was proposed, where simulated annealing (SA) algorithm was used to optimize distillation configurations; for a certain distillation configuration randomly generated by SA, its continuous variables were optimized using particle swarm optimization algorithm. Four cases—including the synthesis of six- and seven-component nonsharp heat integrated and thermally coupled distillation sequences—were studied to demonstrate that the proposed method was efficient and could obtain optimal and valuable suboptimal solutions with high probabilities.     

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.     

A systematic procedure for synthesis of intensified nonsharp distillation systems with fewer columns

Nonsharp separation sequences can intrinsically improve thermodynamic efficiency than that of sharp sequences for a multicomponent distillation. There are much more nonsharp sequences than that of sharp sequences. In this paper, the synthesis of new intensified distillation systems from nonsharp separation sequences is presented. Such intensified distillation systems have fewer columns and have the potential to reduce both energy and capital costs than conventional configurations. First, the representation of any nonsharp sequence with the simple column configuration is found to be the necessary starting point. Then, five strategies are introduced which present different mechanisms to change the structures of the distillation configurations. Third, a systematic procedure is formulated which can achieve the intensified distillation systems with fewer columns systematically. Finally, the systematic method has been applied to different examples of nonsharp separation sequences. It is demonstrated that the method can be used to generate the new intensified distillation systems from any nonsharp separation sequences. The method is easy to use and can explicitly derive all of the intensified distillation systems for the nonsharp separation sequences for a multicomponent distillation.     

改进的物料分配图用于非清晰精馏过程合成

本文提出一种改进的物料分配图(material allocation diagfam)表达分离过程。并在此基础上提出一种非清晰精馏过程合成方法。示例表明:本方法可很好地处理单一进料、只具有旁路的非清晰精馏过程合成问题,有时甚至可直接得最优解。     

基于带权有向图的非清晰复杂精馏序列的合成方法

在精馏系统中非清晰分离能够克服清晰分离所固有的返混效应,从而提高系统能效。基于数据结构理论,提出了非清晰精馏序列的合成与优化方法。对于N组元混合物的分离,合成的精馏序列由N-1个精馏塔构成,且其中的非清晰分离任务含有所允许的任意多个中间分配组分。为了合成上述分离序列,利用带权有向图建立了一种新的精馏分离序列模型,并基于“vector”动态数组,提出了一个同时拥有数组和链表优点的带权有向图的储存结构。序列合成过程定义了一系列的相关操作,且选择广度优先的策略以提高合成效率。算例证明了该方法在精馏序列合成和优化的高效性。算例结果表明该算例的最优分离序列可以有效减少设备投资和能耗。     

TCS-S热偶精馏过程的优化设计

建立了TCS－S热偶精馏过程优化设计的数学模型，给出非精晰分割精馏塔最小回流比的求解方法，并给出简捷计算和严格计算的求解过程，编制了计算机程序。用苯－甲苯－乙苯物系为例进行考核计算，与普通塔的两个分离序列进行比较，节能效果十分显著。     

最小有效能损耗的多组分精馏流程优化设计

提出了以有效能损耗最小为目标、同时又考虑热集成的多组分复杂精馏塔序列优化设计新策略。该复杂精馏塔模型:1股进料、2股出料,每块理论板上均可有中间冷凝器或再沸器。复杂精馏过程的设计步骤是:①根据过程有效能最小确定优化塔序列;②对每个塔优化设计出含中间换热器的复杂塔;③考虑多效且允许热集成的复杂精馏流程,以塔压为决策变量,以精馏过程有效能损耗最小为目标,建立并优化设计出一个热集成的复杂精馏流程。一个3组分精馏过程的例子表明所提策略简单有效,可用来指导多组分精馏过程的优化设计。     

A new bounding strategy for synthesizing distillation schemes with energy integration

An efficient method is developed for synthesizing distillation systems with energy integration. In order to reduce the search space a predictor-based ordered-search technique is used. The main feature of the two-level method is the utilization of the heat-cascade theory by setting up lower bounds for all feasible energy-integrated separation sequences. A separation structure can be built up from individual columns, two-column matches and higher-order integrated matches as well. The objective function of the optimal separation system is the total annual cost, which is a combination of capital and operating expenses. For comparison with previous works, in this study the pressures and reflux ratios of distillation columns are selected as design variables. The effectiveness of the synthesis method is demonstrated on the five-component example of Heaven. The algorithm, utilizing the heat-cascade theory, can be simply implemented on computer.

Scope—Chemical processes frequently apply distillation systems for separating multicomponent mixture into products with relatively pure species. The task of synthesizing optimal (or a small number of near-optimal) separation sequences with heat integrations often represents a huge combinatorial problem. The number of feasible separation schemes increases rapidly as the number of components to be separated increases. Because the computation time and cost for the analysis of separation processes are not negligible, it is desirable to have procedures for determining the optimal (or a small number of near-optimal) separation structures without examining all the possible schemes. In order to reduce the search space of the synthesis problem several systematic procedures have been developed.

Unlike previous methods, the predictor-based ordered-search method proposed here utilizes the heat-cascade theory [1, 2]. By applying the heat-cascade theory a minimum value can be predicted for the total annual cost of all heat-integrated sequences without detailed evaluation of all possible heat matches. The predicted minimum cost can be used as lower bounds for the objective function. The lower bounds are computed as the sum of the actual total annual cost of the unintegrated substructure and a prediction of the minimum cost for that part of the remaining substructure to be integrated. The structures are ordered according to the lower bounds and the heat matches are optimized starting with the structure of the lowest lower-bounding value. The optimized total annual cost of the best sequence is used as a continuously renewed upper bound. The search for a better candidate is complete when the lower bound of the forthcoming structure in the order exceeds the upper bound. In this manner, the predictor-based ordered-search strategy proposed here is capable of omitting a large number of feasible separation schemes and heat matches, and can save considerable computer time.

Conclusions and Significance—The method reported here for synthesizing heat-integrated distillation systems applies a predictor-based ordered-search technique. Using the heat-cascade theory a minimum value for the total annual cost of all the separation structures can be estimated. These minimum values are assigned to the corresponding structures and used as lower bounds of the objective function. The important feature of the method is that mathematical guarantees on the elimination of uneconomical solutions are maintained, hence the optimal solution for heat-integrated distillation schemes is not eliminated by the search reduction. The new bounding strategy could be combined, to include that integration, with the method proposed by Gomez and Seader [3]. The effectiveness of the method is demonstrated on a five-component hydrocarbon mixture where only 20% of the feasible separation structures had to be examined.     

Energy-efficient recovery of tetrahydrofuran and ethyl acetate by triple-column extractive distillation:entrainer design and process optimization

An energy-efficient triple-column extractive distillation process is developed for recovering tetrahydrofuran and ethyl acetate from industrial effluent.The process development follows a rigorous hierarchical design procedure that involves entrainer design,thermodynamic analysis,process design and optimization,and heat integration.The computer-aided molecular design method is firstly used to find promising entrainer candidates and the best one is determined via rigorous thermodynamic analysis.Subsequently,the direct and indirect triple-column extractive distillation processes are proposed in the conceptual design step.These two extractive distillation processes are then optimized by employing an improved genetic algorithm.Finally,heat integration is performed to further reduce the process energy consumption.The results indicate that the indirect extractive distillation process with heat integration shows the highest performance in terms of the process economics.     

能量集成蒸馏塔系列的合成

本文分两阶段合成最优热集成蒸馏塔网络:首先优化设计变量使一切可能的热集成产生最大的增益,然后通过合成热集成树在分离顺序空间中找出最小分离成本顺序.由于可行的换热塔对数目不多,而且采用了简捷方法确定最优蒸馏压力,大大减轻了计算量.以一个很通行的例子演示了合成过程.对不同方法合成结果的比较表明新方法对于能量和分离成本均实现了更大幅度的节省.     

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。     

A new approach to thermal integration of distillation sequences

This paper describes a new approach for synthesizing thermally integrated distillation sequences. There are two key developments in this research. First, the overall flowsheet is optimized simultaneously as opposed to the traditional technique of optimizing each tower sequentially followed by heat integration. Second, maximum energy recovery techniques are used to estimate the total energy requirements of the thermally integrated network, eliminating the need to synthesize a heat exchanger network on each function evaluation in the optimization routine. A frequently reported five component light alkane system (Rathore's Problem) was used to illustrate this approach. Optimized process flowsheets, energy savings and a comparison with literature values are presented.     

Energy optimization of bioethanol production via gasification of switchgrass

In this article, we address the conceptual design of the bioethanol process from switchgrass via gasification. A superstructure is postulated for optimizing energy use that embeds direct or indirect gasification, followed by steam reforming or partial oxidation. Next, the gas composition is adjusted with membrane‐PSA or water gas shift. Membrane separation, absorption with ethanol‐amines and PSA are considered for the removal of sour gases. Finally, two synthetic paths are considered, high alcohols catalytic process with two possible distillation sequences, and syngas fermentation with distillation, corn grits, molecular sieves and pervaporation as alternative dehydration processes. The optimization of the superstructure is formulated as an mixed‐integer nonlinear programming problem using short‐cut models, and solved through a special decomposition scheme that is followed by heat integration. The optimal process consists of direct gasification followed by steam reforming, removal of the excess of hydrogen and catalytic synthesis, yielding a potential operating cost of $0.41/gal. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

基于双塔精馏的甲醇-碳酸二甲酯分离工艺

基于甲醇和碳酸二甲酯体系的共沸特性,应用双效热集成和分割式热泵两种双塔精馏工艺进行该体系的分离研究。利用文献报道的实验数据对选用的Wilson物性方程中的二元交互作用参数进行回归修正。利用Aspen Plus模拟软件,以年运行总费用最小为目标函数,分别对提出的两种双塔精馏工艺进行模拟与优化,得到合适的工艺参数。模拟结果表明,两种双塔精馏工艺均比单塔加压精馏工艺其能耗更低、年运行总费用更少。就两种双塔精馏工艺而言,分割式热泵精馏工艺较双效热集成精馏工艺,可节约设备投资费用约2.69%,年运行总费用节约17.33%。     

Optimizations and heat integrations on the separation of toluene and 1-butanol azeotropic mixture by pressure swing distillation

In this study, optimizations on separation of Toluene and 1-Butanol mixture is carried out by pressure swing distillation process. New heat integration methods are applied and compared to conventional PSD process by using vapor recompression technique combined with pressure swing distillation process. New heat integration methods show positive results to decrease the utilities used in the process. A simulator Aspen HYSYS V8.4 is used for simulating all processes. Cost analysis is also performed and Aspen HYSYS V8.4 optimizer program is used for optimizing profit for the process. Pressure is also optimized for minimum heat duty requirements in the process and the ultimate concern is to minimize the total reboiler heat duties and overall operating cost for the system which results into increasing profit.     

基于有效能分析的复杂精馏塔优化设计

提出了以有效能消耗最小为目标的复杂精馏塔优化设计新方法。给出了精馏塔优化设计模型及最佳进料位置、适宜理论板数NT、塔内换热器简约的确定方法,并以两组分正庚烷和乙苯的混合物为例进行设计与讨论。优化后的精馏塔含有中间换热器,与传统的简单塔有本质的区别,平衡线和操作线均处于相对平均的位置,更节能。     

关于分批蒸馏投料量的研究

本文利用半隐含龙格－库塔方法（ＳＩＲＫ）求解有持液多元分批蒸馏过程,以产品收率最大为目标得出了二元分批蒸馏过程的适宜投料量,并分析了各种因素对适宜投料量的影响。     

Retrofitting of the Heat Exchanger Network with Steam Generation in a Crude Oil Distillation Unit

Steam generation through hot streams has an important impact on the utility consumption of a crude oil distillation unit. Retrofitting of the heat exchanger network with steam generation in a crude oil distillation unit is studied with regard to efficient energy usage. The grand composite curve is employed to provide insights into the steam generation problem, and a mixed‐integer linear programming model, presented previously for heat integration through hot discharges/feeds and steam generation, is used to obtain the optimal parameters for steam generation. Three heuristic rules are then proposed to determine suitable hot streams for steam generation. Finally, the heat exchanger network is modified based on pinch technology. After the retrofit, the hot and cold utility decrease.     

Efficient generalized shortcut distillation model with improved accuracy for superstructure-based process synthesis

We propose a general shortcut distillation column model that can address a wide range of types of separations including nonsharp/sloppy splits. The model can automatically identify key components and estimate the energy requirement of a desired separation in the presence, potentially, of components with zero flow rates. To obtain more practical column height and vapor flow rates, we augment the model with constraints inspired by the Fenske equation. We also propose valid constraints to enhance the computational performance of the proposed model. Due to its flexibility and computational efficiency, the proposed model can be readily used for superstructure-based process synthesis.