Conceptual design of distillation processes for mixtures with distillation boundaries. II. Optimization of recycle policies

Geometric design methods for the conceptual design of azeotropic distillation processes are fast and efficient tools for the economic screening of different process alternatives. The second article of this two‐part series presents a novel optimization‐based conceptual design framework for azeotropic distillation processes, which allows a rapid screening of the different process alternatives with respect to feasibility and economic incentive. The design framework is based on the economic assessment of distillation columns by the rectification body method. The feasibility limits imposed by the azeotropes are incorporated using the split feasibility test introduced in the first part of this series. The application of the framework is highlighted with several ternary and quaternary process alternatives for the production of high‐purity alcohols. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Modelling and simulation of a hybrid process (pervaporation–distillation) for the separation of azeotropic mixtures of alcohol–ether

This work reports the modelling and simulation of a hybrid process, based on the combination of distillation and pervaporation, for the separation of azeotropic mixtures of alcohol–ether. After having selected the separation of methanol‐2‐metoxi‐2,2‐dimethyl ethane (ETHER) as a motivating example the mathematical modelling of the distillation column was achieved and used together with a mass transfer model previously reported for the pervaporation operation in order to simulate the behaviour of the hybrid process for different compositions of the feed stream (case 1: 3.2 wt% methanol, 55.4 wt% C₄, 41.4 wt% ETHER, and case 2: 5.2 wt% methanol, 42 wt% C₄, 52.8 wt% ETHER). Simulation tasks were carried out with the process modelling system gPROMS and the results of alternative process configurations that result from the relative location of the separation technologies have been compared on the basis of the required membrane area. Finally, the design of the pervaporation unit including the overall processing costs is reported. Copyright © 2001 Society of Chemical Industry     

Extractive distillation: Advances in conceptual design,solvent selection,and separation strategies

Extractive distillation(ED) is one of the most promising approaches for the separation of the azeotropic or closeboiling mixtures in the chemical industry. The purpose of this paper is to provide a broad overview of the recent development of key aspects in the ED process involving conceptual design, solvent selection, and separation strategies. To obtain the minimum entrainer feed flow rate and reflux ratio for the ED process, the conceptual design of azeotropic mixture separation based on a topological analysis via thermodynamic feasibility insights involving residue curve maps, univolatility lines, and unidistribution curves is presented. The method is applicable to arbitrary multicomponent mixtures and allows direct screening of design alternatives. The determination of a suitable solvent is one of the key steps to ensure an effective and economical ED process. Candidate entrainers can be obtained from heuristics or literature studies while computer aided molecular design(CAMD) has superiority in efficiency and reliability. To achieve optimized extractive distillation systems, a brief review of evaluation method for both entrainer design and selection through CAMD is presented. Extractive distillation can be operated either in continuous extractive distillation(CED) or batch extractive distillation(BED), and both modes have been well-studied depending on the advantages in flexibility and low capital costs. To improve the energy efficiency, several configurations and technological alternatives can be used for both CED and BED depending on strategies and main azeotropic feeds. The challenge and chance of the further ED development involving screening the best potential solvents and exploring the energy-intensive separation strategies are discussed aiming at promoting the industrial application of this environmentally friendly separation technique.     

Enhancing mass and energy integration by external recycle in reactive distillation columns

The synthesis and design of reactive distillation columns separating reacting mixtures with the most unfavorable relative volatilities (i.e., the reactants are the heaviest and lightest components with the products being the intermediate ones) are described. The unfavorable thermodynamics poses great difficulties in combining the reaction operation and the separation operation involved and limits severely the potential of reactive distillation columns in the reduction of capital investment (CI) and operating cost. To remove the limitation, we propose two strategies for facilitating the synthesis and design of this kind of reactive distillation columns in this article. One is to arrange prudentially the reactive section so as to strengthen internal energy integration between the reaction operation and the separation operation involved; that is, while the reactive section should be placed at the bottom of the reactive distillation columns separating exothermic reactions, it should be at the top of the reactive distillation columns separating endothermic reactions. The other is to introduce an external recycle flow between the two ends of the reactive distillation columns to reinforce internal mass integration and internal energy integration between the reaction operation and the separation operation involved; that is, whereas the external recycle flow should be directed from the top to bottom of the reactive distillation columns separating exothermic reactions, it should be from the bottom to top of the reactive distillation columns separating endothermic reactions. Separation of four hypothetical ideal (i.e., two quaternary and two ternary systems, respectively) and two real nonideal (i.e., two quaternary systems) reacting mixtures is chosen to evaluate the proposed strategies. The results show that they can considerably lower energy requirement besides a further reduction in CI. © 2012 American Institute of Chemical Engineers AIChE J, 59: 2015–2032, 2013     

Auslegung und Optimierung von hybriden Trennverfahren

Design and Optimization of Hybrid Separation Processes Hybrid separation processes are defined as the combination of at least two different unit operations in different apparatus which contribute to the separation task. Hybrid processes are used for difficult separations, e.g., close‐boiling mixtures and azeotropes, if a single unit operation, e.g., distillation, membranes, extraction, crystallization or chromatography, is not efficient or even not feasible. Because of the structure of a hybrid process which implies two or more unit operations and recycle streams, the design is not straightforward and therefore subject of today's research. In this work general criteria for such a consistent design method are described and a design approach for hybrid separation processes is presented. It bases on rigorous modeling of the unit operations and simultaneous multivariable optimization. The approach feasibility is demonstrated by the separation of an isomer mixture.     

Energy‐efficient complex distillation sequences: Control properties

Four thermally coupled distillation systems were designed for the separation of five‐component mixtures (the light‐ends separation section of a crude distillation plant); their steady‐state design was obtained by starting from a conventional distillation sequence and then optimizing for minimum energy consumption. The thermally coupled distillation systems were compared to sequence based on conventional columns design. Comparison was based on controllability properties under open and closed loop operation, following the dynamic behaviour after common industrial operating disturbances. Simulation results were analyzed by the singular value decomposition technique and with the performance examination of elimination of feed disturbances using PI controllers. It was found that thermally coupled distillation systems are controllable and, sometimes, they exhibit dynamic responses that are easier to manage than in the case of conventional distillation sequences; this result is innovative in the study of this kind of systems.     

Conceptual design of single-feed kinetically controlled reactive distillation columns

Reactive distillation, simultaneous reaction and separation within a single unit, represents an exciting alternative to conventional processes, leading to significantly reduced in capital and operating costs. Process design for reactive distillation is facilitated by fast and effective methods for synthesis and conceptual design that take into account reactions that do not instantaneously reach equilibrium. This work presents a new methodology for synthesis and design of single-feed kinetically controlled reactive distillation columns. The design method allows rapid and relatively simple screening of different reactive distillation column configurations. Feasibility is assessed and operating conditions are determined using an extension of boundary-value methods. The approach is limited to systems with four or fewer chemical species. Both fully reactive and hybrid columns are considered. The methodology is illustrated for a metathesis reaction and for MTBE production.     

Systematic design of an extractive distillation for maximum‐boiling azeotropes with heavy entrainers

Extractive distillation is one of the most attractive approaches for separating azeotropic mixtures. Few contributions have been reported to design an extractive distillation for separating maximum‐boiling azeotropes and no systematic approaches for entrainer screening have been presented. A systematic approach to design of two‐column extractive distillation for separating azeotropes with heavy entrainers has been proposed. A thermodynamic feasibility analysis for azeotropes with potential heavy entrainers was first conducted. Then, five important properties are selected for entrainer evaluation. Fuzzy logic and develop membership functions to calculate attribute values of selected properties have been used. An overall indicator for entrainer evaluation is proposed and a ranking list is generated. Finally, the top five entrainers from the ranking list have been selected and use process optimization techniques to further evaluate selected entrainers and generate an optimal design. The capability of the proposed method is illustrated using the separation of acetone–chloroform azeotropes with five potential entrainers. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3898–3910, 2015     

反应精馏过程耦合强化技术基础与应用研究述评

反应精馏技术是过程强化概念在化学工业成功应用的典范。由于反应与精馏耦合的高度复杂性和非线性,反应-精馏相互影响机制、耦合方式及其过程能效调控与优化成为制约该技术广泛应用的关键科学问题。综述了近二十年来国内外反应精馏技术从基础研究到工业应用的概况,包括反应精馏过程的可行性分析与概念设计方法、稳态模拟过程优化与动态模拟控制策略设计、兼具催化反应与气液传质的高效内构件开发以及反应精馏在各领域的应用。探讨了目前制约反应精馏广泛应用关键问题的解决方法与途径,阐明了普适性的反应精馏过程开发方法,总结了基于反应精馏过程耦合的新型过程强化技术,指出了反应精馏技术的发展趋势。     

Extractive distillation with ionic liquids: A review

Extractive distillation is commonly used for the separation of azeotropic or close‐boiling mixtures in the chemical industry. During the past decade, the use of ionic liquids (ILs) as entrainers has received considerable attention due to their unique advantages when applied in extractive distillation. This work is devoted to providing an easy‐to‐read and comprehensive review on the recent progress made by chemical engineers, focusing on the issues of predictive thermodynamic models, structure‐property relations, separation mechanisms, and process simulation and optimization. This review spans from the molecular level to the industrial scale, to provide a theoretical insight into the molecular interactions between ILs and the components to be separated. Moreover, a comprehensive database on the vapor–liquid equilibria and activity coefficients at infinite dilution concerning ILs is provided as Supporting Information. Concluding remarks are made on the unsolved scientific issues with respect to this promising special distillation technology. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3312–3329, 2014     

外部环流反应精馏塔的分散控制方案设计

对于具有最不利相对挥发度排序（即反应物为最轻和最重组分，生成物为中间组分）的四元反应分离物系而言，在反应精馏塔的顶部和底部之间引入外部环流能够提高系统的反应分离效率，从而大幅度地降低系统的能量消耗和固定投资成本。以理想四元可逆放热反应的分离为例，研究了外部环流反应精馏塔的分散控制方案的设计问题。结果表明外部环流的引入提高了系统的反应速率，使得外部环流反应精馏塔的闭环控制效果更好（与传统反应精馏塔相比），对干扰的处理能力更强。另外，由于外部环流反应精馏塔比常规反应精馏塔有更多的操作变量（即外部环流流量），利用该变量对出料浓度进行控制，可以进一步提高系统的闭环控制效果。     

Batch extractive distillation as a hybrid process: separation of minimum boiling azeotropes

The batch extractive distillation is compared with the hybrid process (absorption+distillation) by feasibility studies and rigorous simulation. A new method is presented for the assessment of feasibility of the hybrid batch extractive distillation. The limiting values of the operational parameters are determined. Calculations are presented for the separation of the minimum boiling azeotropic mixtures of acetone-methanol and ethanol-water by the application of water and ethylene glycol as heavy solvents, respectively.     

Thermodynamic efficiency of membrane-assisted distillation processes

Membrane processes are considered as comparably mild separation processes offering the potential for significant energy savings compared with azeotropic distillation processes. Despite higher investment and material costs, they are of particular interest for improving the energy efficiency in the chemical industry. However, energy savings of more than 20%–30% are rarely reported and even a general superiority can be disputed. To further elucidate this controversial, the current study pursues a quantitative assessment of the thermodynamic efficiency of pervaporation and vapor permeation processes with stand-alone distillation and hybrid membrane-assisted distillation processes for the separation of azeotropic mixtures. The results confirm the case-dependent potential of distillation processes to outperform membrane-assisted separations in terms of energy efficiency, considering proper heat integration. Although energy efficiency is becoming significantly important, it should be considered in the context of economic performance to determine an optimal trade-off and to select the best process alternative during conceptual process design.     

Synthesis,experiments and simulation of heterogeneous batch distillation processes

The presence of azeotropes in multicomponent mixtures complicates the design of batch distillation separation processes widely used in pharmaceutical and speciality chemical industries. Most of those processes include the use of a homogeneous entrainer to ease the separation. We describe novel methods to break azeotropes using an entrainer that is partially miscible with one of the initial binary mixture components. We depict some of the advantages of heterogeneous batch distillation processes: more design alternatives for the separation of an azeotropic binary mixture than with homogeneous batch distillation, batch distillation boundary crossing thanks to a controlled reflux of the entrainer-rich phase, simplified distillation sequences as a consequence of less distillation tasks. Three examples based on the separation of non-ideal azeotropic or close boiling point binary mixtures through heterogeneous batch distillation are simulated using a commercial batch distillation package. Experiments validate the simulated separation of a minimum boiling azeotropic mixture.     

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     

γ-戊酮酸丁酯反应精馏过程的逐步优化方案设计

Butyl-levulinate has been identified as a promising fuel candidate with high oxygen content. Its combustion in diesel engines yields very low soot and NOx emissions. It can be produced by the esterification of butanol and levulinic acid, which themselves are platform chemicals in a biorenewables-based chemical supply chain. Since the equilibrium of esterification limits the conversion in a conventional reactor, reactive distillation can be applied to overcome this limitation. The presence of the high-boiling catalyst sulfuric acid requires a further separation step downstream of the reactive distillation column to recover the catalyst for recycle. Optimal design specifications and an optimal operating point are determined using rigorous flowsheet optimization. The challenging optimization problem is solved by a favorable initialization strategy and continuous reformulation. The design identified has the potential to produce a renewable transportation fuel at reasonable cost.     

Feasibility of new pressure swing batch distillation methods

The pressure swing distillation in different batch column configurations is investigated by feasibility study and rigorous simulation calculations. Besides studying the well known batch configurations (rectifier, stripper, middle vessel column) we also suggest two novel configurations such as double column batch rectifier (DCBR) and double column batch stripper (DCBS). The alternate application of a batch rectifier and a batch stripper is also studied. The feasibility method is based on the assumption of maximal separation. The results of the feasibility studies are verified by rigorous simulations based on less simplifying assumptions. The calculations are made by a professional dynamic flow-sheet simulator for the separation of a minimum (ethanol–toluene) and a maximum boiling (water–ethylene-diamine) azeotropic mixture. The different column configurations are compared. The DCBS (for the separation of the minimum azeotrope) and the DCBR (for the maximum azeotrope) showed several advantages (e.g. only one production step without pressure change, lower energy consumption) compared with the other configurations.     

Pilot‐scale studies of process intensification by cyclic distillation

Process intensification in distillation systems receives much attention with the aim of increasing both energy and separation efficiency. Several technologies have been investigated and developed, as for example: dividing‐wall column, HiGee distillation, or internal heat‐integrated distillation. Cyclic distillation is a different method based on separate phase movement—achievable with specific internals and a periodic operation mode—that leads to key advantages: increased column throughput, reduced energy requirements, and better separation performance. This article is the first to report the performance of a pilot‐scale distillation column for ethanol‐water separation, operated in a cyclic mode. A comparative study is made between a pilot‐scale cyclic distillation column and an existing industrial beer column used to concentrate ethanol. Using specially designed trays that truly allow separate phase movement, the practical operation confirmed that 2.6 times fewer trays and energy savings of about 30% are possible as compared with classic distillation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2581–2591, 2015     

三组元全热耦合精馏过程的模拟计算

本文利用Aspen软件对三组分进料全热耦合精馏的分离流程进行设计和模拟,建立了相应的具体计算步骤。首先利用三塔模型把热耦合精馏过程简化为3个单独的简单清晰分割塔,通过简捷法设计和模拟得到塔板数和回流比等初值后进行严格法模拟,得到热耦合精馏的数据和操作条件。然后采用Aspen软件中的RadFrac模型,将三塔模型的模拟初值代入全热耦合模块进行严格模拟。结果表明,采用全热耦合精馏分离C4三组分比传统精馏的直接序列和间接序列节能约为20%。     

A new technique for recovering energy in thermally coupled distillation using vapor recompression cycles

Even though it has been proved that a fully thermally coupled distillation (TCD) system minimizes the energy used by a sequence of columns, it is well‐known that vapor/liquid transfers between different sections produce an unavoidable excess of vapor (liquid) in some of them, increasing both the investment and operating costs. It is proposed here to take advantage of this situation by extracting the extra vapor/liquid and subjecting it to a direct/reverse vapor compression cycle. This new arrangement restores the optimal operating conditions of some of the affected sections with energy savings of around 20–30% compared with conventional TCD columns. Various examples, including the direct and reverse vapor recompression cycles, are presented. Furthermore, in each example, all possible modes of distillation (direct, indirect and Petlyuk distillation) with and without vapor recompression cycles (VRC) are compared to ensure that this approach delivers the best results. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3767–3781, 2013