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.     

Wirtschaftliche Einsatzbereiche von Wärmepumpen- und Mehrstufenschaltungen in der Rektifiziertechnik

Economic applications of heat pumps and of multi-column heat integrated systems in distillation processes . The influence of relevant parameters (e.g. relative volatility, flow rate, pressure) on the economics of distillation plants involving heat pumps and heat integrated columns is simulated by a computer program. The different structures of possible energy supply (cogeneration heat-power plant, direct heating) have been taken into account in the models employed. The areas of economic application of the various energy conserving systems are shown in diagrams. These allow a preliminary selection of the most economic variant. The application of heat pumps outside the field of cryogenic engineering is restricted to some special cases like the separation of closeboiling mixtures. In those cases, where a minimum plant size and steam price is exceeded heat integrated columns promise an economic advantage, provided that cheaper configurations of direct heat recovery are impossible.     

Flowsheet synthesis and complex distillation column design for separating ternary heterogeneous azeotropic mixtures

The boundary value method for design of distillation columns separating ternary heterogeneous azeotropic mixtures is extended to include complex configurations, i.e. columns with integrated decanters and with multiple heterogeneous stages; double-feed columns; columns with intermediate decanters. The methods can be used for establishing product feasibility in a column and evaluating a column design in terms of cost. Multiple feasible designs can be generated for a given set of product specifications, according to the reflux ratio, number of heterogeneous stages and liquid phase ratio; these designs may be evaluated with respect to operating and equipment costs. Case studies illustrate the design method.An algorithmic approach is presented for synthesising novel sequences for separating, by distillation and decanting, ternary heterogeneous azeotropic mixtures. Existing synthesis procedures that consider only simple single-feed columns with decanters and integrated decanters are extended to include more industrially relevant options such as columns with several heterogeneous stages, double-feed columns, columns with intermediate decanters, and those accepting a heterogeneous liquid feed. With these advanced column configurations included in the synthesis method, a wider range of sequences may be considered systematically, allowing sequences that are more economically attractive than conventional designs to be identified. A case study illustrates the approach.     

Heat‐integrated distillation columns: Vapor recompression or internal heat integration?

Internally, heat‐integrated distillation columns (HIDiC) and vapor recompression (VRC) constitute alternative design options to provide sustainable distillation processes. However, the design is often based on heuristic rules or the experience of the designer, as no systematic methodology driven by economics has been established so far. The increased complexity of heat‐integrated columns can hardly be dealt with using simulation studies but rather calls for a systematic design procedure. A new design methodology is presented here; it builds on a superstructure, mixed‐integer minimization of total annualized cost of operation and rigorous thermodynamic modeling. Optimal VRC and HIDiC designs are identified for the separation of binary, multicomponent, and nonideal mixtures and benchmarked against conventional distillation column designs. A small number of intermediate heat exchangers is optimal for these HIDiC configurations, eventually reducing to a single heat exchanger similar to VRC. Therefore, VRC designs are often more cost efficient due to simpler equipment. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Controllability analysis of modified Petlyuk structures

Fully thermally coupled distillation columns (Petlyuk‐type columns) represent an interesting alternative to conventional distillation sequences used in multicomponent mixture separation processes, due to potential savings in both energy and capital costs. However, possible operational difficulties have limited the industrial applications of Petlyuk systems. Some of the control challenges result from the transfer of vapour stream back and forth between columns. This means that those columns do not display a uniform lower or higher pressure with respect to the other. Recently, some alternative Petlyuk‐type schemes that might provide better operation properties than the traditional Petlyuk column have been proposed. In this work, the theoretical control properties of six alternative schemes to the Petlyuk system were obtained and compared. This was performed by using the singular value decomposition (SVD) technique in the frequency domain. Also, dynamic closed‐loop responses for set point tracking and disturbance rejection were obtained to support the theoretical control properties. The results showed that the reduction in the number of interconnections and the use of unidirectional flows affected the dynamic properties of the complex schemes leading to potential operational advantages in thermally coupled distillation sequences.     

Higher energy saving with new heat integration arrangement in heat‐integrated distillation column

In conventional heat‐integrated distillation columns (HIDiCs), the internal heat exchange is executed between the pressurized rectifying section and the stripping section, which are located at the same elevation. In such a structure, the amount of heat exchanged between two sections depends on the temperature profile of both sections. The resulting enthalpy profile inside the column departs from that in reversible distillation, which is the ideal distillation operation in view of energy conservation. More energy saving may be achieved by providing appropriate arrangement of heat exchanges between sections. The interactive graphical design method to determine the appropriate heat exchange arrangement in a previous paper was developed for a binary system. The design method was extended and applied to a multicomponent system by adopting the idea of a quasi‐binary system. Also, a new HIDiC structure that can realize the outcomes of the proposed design method was developed. The economics of the proposed structure was precisely evaluated through a case study of a commercial scale column. It demonstrated that the proposed structure has attractive economics. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3479–3488, 2015     

无冷凝器及再沸器的热集成蒸馏塔技术进展

蒸馏塔内部热集成技术系指同一塔内通过精馏段和提馏段之间的热量集成。介绍了内部热集成蒸馏塔的结构、特点、节能原理以及发展概况。该项技术被认为是最具节能潜力的新型蒸馏技术之一,与传统蒸馏塔相比,内部热集成蒸馏塔节能可达到30%—60%。近年来,塔内部热集成技术研究呈现加速发展的势态,国外学者已成功地完成了中试,并开始工业化应用的研究。     

Distillation Control Structure Selection for Energy‐Efficient Operations

Few studies have been reported concerning the energy efficiency of various distillation column control structures. The choice of an energy‐efficient control configuration by incorporating thermodynamics second law in the selection criteria is described. In addition to a relative gain array for assessing control loop interactions, a relative exergy array is used in evaluating the energy efficiency of various control structures. The preferred control structure should have both good operability and good energy efficiency as distillation columns are the major energy consumer in the chemical industry. The performance of the control structures in the dynamic mode is analyzed in detail. The proposed method is demonstrated on two binary distillation columns: methanol‐water separation and benzene‐toluene separation. Dynamic simulation results prove the efficiency of the proposed distillation control structure selection method.     

Rate-based analysis of reactive distillation sequences with different degrees of integration

In this work, reactive distillation sequences with different degrees of integration are investigated with the aim to identify the optimal configuration. Such integration includes the combination of both unit operations, reaction and distillation, as well as the combination of distillation columns within one unit. For rapid determination of required design parameters, a special two-step approach based on the decomposition method is developed.As a test system, the transesterification of carbonates is chosen, and three promising configurations with increasing integration degree are identified. These configurations comprise two different combinations of single non-reactive and reactive distillation columns as well as a highly integrated reactive dividing wall column. Their analysis shows that the unit integration leads to significant savings in energy consumption and production costs.     

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.     

物系和分离要求对最优三组元精馏结构选择的影响

针对三组分混合物的分离,选取6种不同分离指数的物系,根据多种进料组成和分离要求,对传统的直接分离序列和间接分离序列及其热集成方式、部分热耦合精馏中的侧线精馏和侧线提馏、完全热耦合结构（即隔板塔）进行了模拟与优化设计,并以年度总费用最低为目标选出各种情况下的最优精馏结构。结果表明,分离指数、进料组成及分离要求都对精馏结构的优化选择都有显著影响,热集成精馏、部分热耦合以及完全热耦合精馏结构分别在不同条件下各具有优势。根据分析结果,对影响最优三组元分离精馏结构的因素进行归纳。     

Rigorous design of distillation columns using surrogate models based on Kriging interpolation

The economic design of a distillation column or distillation sequences is a challenging problem that has been addressed by superstructure approaches. However, these methods have not been widely used because they lead to mixed‐integer nonlinear programs that are hard to solve, and require complex initialization procedures. In this article, we propose to address this challenging problem by substituting the distillation columns by Kriging‐based surrogate models generated via state of the art distillation models. We study different columns with increasing difficulty, and show that it is possible to get accurate Kriging‐based surrogate models. The optimization strategy ensures that convergence to a local optimum is guaranteed for numerical noise‐free models. For distillation columns (slightly noisy systems), Karush–Kuhn–Tucker optimality conditions cannot be tested directly on the actual model, but still we can guarantee a local minimum in a trust region of the surrogate model that contains the actual local minimum. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2169–2187, 2015     

Energy Efficiency of an Indirect Thermally Coupled Distillation Sequence

Several thermally coupled distillation sequences have been proposed to improve the thermal inefficiency of conventional distillation sequences. Particularly, for the separation of ternary mixtures, structures that perform a lateral extraction in one of the columns of the integrated arrangement have been shown to provide significant energy savings. The structure of existing sequences, based on conventional distillation columns, might provide the basis for alternate thermally coupled designs. In this paper, it is shown how a thermally coupled system derived from an indirect conventional sequence can provide energy savings through a proper optimization of the interconnecting streams.     

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.     

基于乙醇压缩的超重力热泵精馏热集成系统性能分析

将超重力精馏塔、单螺杆压缩机与新型热泵工艺技术创新性地相结合,提出超重力热泵精馏的概念。设计并搭建一套处理量为300 kg·h^-1的超重力热泵精馏热集成系统。以乙醇-水溶液为研究对象,使用单螺杆压缩机直接压缩乙醇蒸气。通过对超重力精馏塔不同转动频率的全回流实验和工业条件下系统的不同进料位置、不同回流比实验的研究,综合分析系统各影响参数的变化情况、节能特性和经济效益。结果表明,f_HG在40 Hz下运转时系统性能最佳;进料位置下移或增加回流比,都可提高y_D;如果仅从y_D值大小考虑,F_L03位置最佳;相比于低浓度乙醇-水溶液,处理高浓度优势更大;系统节能及经济效益显著,可为超重力热泵精馏在乙醇精馏的工艺流程选择、设计和应用方面提供理论指导。     

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.     

带有中间热集成的精馏塔序列及其性能

提出一种带有中间热集成的精馏塔序列(IHISDC)的流程,针对三组元混合物分离的简单塔直接序列,对该流程进行了分析。与传统热集成精馏序列(HISDC)相比,提出的IHISDC通过中间换热器将高压塔的精馏段与低压塔的提馏段进行局部热集成,使能量集成精馏塔之间的压力差更小,进而使能耗费用下降。同时发现,IHISDC中的高压塔再沸器热负荷和低压塔冷凝器热负荷增加,由于换热器数量的增加,IHISDC的投资费用较大。为了进一步降低IHISDC的年度总费用,需要对其设计参数进行优化。     

Structural flexibility for heat integrated distillation columns—I. Analysis

In two related papers we define the concept of structural flexibility and apply it specifically to a class of heat integrated distillation processes. A process is structurally feasible if the type of equipment it contains and how it is currently configured allow it to meet specified design requirements. It is structurally infeasible if it does not. Size of equipment is not at issue.The first paper develops a simple method to permit an estimate to be made of a structural flexibility index for a heat integrated distillation process with a fixed configuration. Multieffect columns are permitted. This work shows that processes that differ only in their heat integration can have very different indices for structural flexibility.The second paper presents a synthesis methodology to discover the heat integrated arrangements which maximize the structural flexibility index for the process. Only the simpler arrangements of such columns are considered where they are placed one above the other and without multieffecting.The papers present algorithms by example. Insights are stressed.     

Thermal analysis as an aid to monomer plant design

The thermal data needed for the design of heat transfer equipment and distillation columns for the manufacture of a new vinyl monomer, diacetone acrylamide, were rapidly and effectively provided by differential scanning calorimetry. A method has been developed for determining the effects of inhibitors and metals on the polymerization of the vinyl monomer. The latter was an aid in the selection of materials of construction. DSC was used to determine the total inhibitor concentration in the monomer. Selection of inhibitors was facilitated by information derived from thermal analysis.     

Conceptual Design of Double‐Feed Reactive Distillation Columns

The feasibility and feasible range of operating parameters for double‐feed reactive distillation columns are evaluated, based on the combination of pinch point map analysis for the middle‐section in the compositional space and the feed angle method as an efficient shortcut design method. Limiting bounds for operating parameters are determined where the properties of singular points change. The existence and values of such bounds may vary in double‐feed reactive distillation columns depending on the nature of the system under study. The methodology is illustrated by production of methyl acetate and ethyl acetate. An efficient method is described to identify the most promising candidates of double‐feed reactive distillation columns and to study the design flexibility in terms of operating parameters.