Short-cut methods for the optimal design of simple and complex distillation columns

New short-cut methods providing optimal design parameters for distillation columns with simple and complex configurations including two-feed and one-feed-one-side-stream columns are presented. The methods assume constant relative volatilities and constant molar flow rates within each distillation section. The design equations are based on the Underwood equations for the calculation of minimum reflux (reboil) ratio, the analytical formulations of the distillation line, the Eigenfunction and the number of theoretical stages for each mass transfer section of the column. Furthermore, the geometrical properties of a given separation are considered. Optimization algorithms based on the minimization of the total number of theoretical stages of the column with taking into account the mass balance at each feed section have been elaborated. In comparison to the boundary value method the new short-cut methods require a minimum number of specifications; they do not need any graphical support, and provide a lower total number of theoretical stages particularly for complex configurations. The new short-cut methods have been extended to the design of columns separating azeotropic mixtures by approximating the latter by appropriate pseudo-ideal mixtures. Several separation examples for azeotropic mixtures, including different types of splits as well as columns with simple and complex configurations were tested and show a very good agreement with the simulation results obtained with Radfrac (Aspenplus).     

用夹紧区原理确定多元恒回流比分批精馏最小回流比

夹紧区是精馏塔内出现的浓度几乎不变的区域,对于三元物系来说,根据夹紧区在塔内出现的位置可分为：上、中和下加紧区.在无穷板数和任一瞬时釜浓下选择不同的回流比可使分批精馏处于不同夹紧区下操作.文中讨论了三元恒回流比分批精馏过程中夹紧区的演变,在Rayleigh方程的基础上建立了应用夹紧区原理确定多元恒回流比分批精馏最小回流比的方法.该方法并不要求相对挥发度为常数,并且能准确计算顶浓,克服了通常所采用的Underwood公式法的缺陷,为多元分批精馏的简捷设计奠定了基础.     

Effect of Solvent Content on the Separation and the Energy Consumption of Extractive Distillation Columns

This article sets out to evaluate the effect of solvent content in the extractive section on the separation efficiency and energy consumption of extractive distillation columns. Contrary to the classical approach, the proposed approach enables a simultaneous evaluation of the effect of the major decision variables (reflux ratio, solvent flow rate, and the number of stages of the extractive section [NSE]). The procedure allows calculating the minimum solvent flow rate for the separation and the minimum specific energy consumption. The results show that the minimum specific energy consumption is obtained for the minimum reflux ratio and not for the minimum solvent flow rate. Moreover, the results show that it is not always the case that a larger NSE results in lower energy consumption. Due to its industrial importance, the dehydration of aqueous mixtures of ethanol using ethylene glycol as solvent has been chosen as a case study.     

Minimum energy demand of distillation columns with multiple feeds

Distillation columns with multiple feeds are often used in processes for the separation of multicomponent mixtures. The minimum energy demand of such columns is determined via the minimum reflux and reboil, respectively. A novel method for directly calculating the minimum reflux ratio is presented in this paper. Applicable to ideal, nonideal and azeotropic mixtures the method is based merely on the knowledge of vapor-liquid equilibrium of the feed mixtures. This knowledge allows the determination of the internal concentration profile near the feed entry and, in turn, the calculation of both minimum reflux and optimal sequence of feeds. This sequence of feeds depends on feed compositions and product compositions, as well. Surprisingly, sometimes a multiple feed entry requires a higher energy demand than premixing the feed streams.     

Optimization of the Design and Operation of Extractive Distillation Processes

The achievement of the optimal operating point of extractive distillation systems involves determining the values of the process variables, such as the solvent flowrate, the reflux ratio of the extractive, and recovery columns. From the point of view of design, the optimum involves defining the number of stages of extractive and recovery columns, as well as the feed stage positions of these columns. The above-mentioned columns are coupled through a recycle stream, which makes obtaining the optimal operating and design points a more complex task. This study arose from a new procedure for the analysis of extractive distillation columns, in which the solvent mole fraction in the solvent feed stage is the primary variable to be analyzed. The procedure allows for determining the values of the process and design variables that provide the global minimum for the total annual cost and the specific energy consumption of the extractive distillation processes (extractive and recovery columns). Furthermore, it is possible to determine the minimum solvent flowrate and the minimum reflux ratio for separation. Obtaining anhydrous ethanol using ethylene glycol as solvent is the case study of this work.     

反置式间歇蒸馏塔的设计程序(Ⅰ)多组分理想物系

Inverted batch distillation column(stripper) is opposed to a conventional batch distillation column(rectifier). It has a storage vessel at the top and products leave the column at the bottom. The batch stripper is favourable to separate mixtures with a small amount of light components by removing the heavy components as bottom products. In this paper, we are presenting a shortcut procedure based on our earlier work for design and simulation of the inverted batch distillation column, which is equivalent to the Fenske-Underwood-Gilliland procedure for continuous distillation. Given a separation task, we propose to compute the minimum number of stages(Nbmin) and the minimum reboil ratio(Rbmin) required in a batch stripper,which are the stages and reboil ratio required in a hypothetical inverted batch distillation column operating in total reboil ratio or having an infinite number of stages,respectively. Then, it is shown that the performance of inverted batch columns with a finite number of stages and reboil ratios could be correlated in Gilliland coordinates with the minimum stages Nbmln and the minimum reboil ratio Rbmin.     

Shortcut Procedure for Inverted Batch Distillation Column (Ⅰ) Multicomponent Ideal System

Inverted batch distillation colunm(stripper) is opposed to a conventional batch distillation column(rectifier). It has a storage vessel at the top and products leave the column at the bottom. The batch stripper is favourable to separate mixtures with a small amount of light components by removing the heavy components as bottom products. In this paper, we are presenting a shortcut procedure based on our earlier work for design and simulation of the inverted batch distillation column, which is equivalent to the Fenske-Underwood-Gilliland procedure for continuous distillation. Given a separation task, we propose to compute the minimum number of stages(Nbmin)and the minimum reboil ratio(Rbmin) required in a batch stripper,which are the stages and reboil ratio required in a hypothetical inverted batch distillation colnmn operating in total reboil ratio or having an infinite number of stages,respectively. Then, it is shown that the performance of inverted batch columns with a finite number of stages and reboil ratios could be correlated in Gilliland coordinates with the minimum stages Nbmin and the minimum reboil ratio Rbmin.     

Identification and analysis of possible splits for azeotropic mixtures—1. Method for column sections

The identification of possible splits at the finite reflux has been the main challenge for the creation of efficient flowsheets for the separation of azeotropic mixtures. We show in this paper that this problem can be solved by analyzing the mode of infinitely sharp splits, for which trajectories of distillation and pinch lines have special characteristics. The feasibility of sharp splits depends on the relationship of phase equilibrium constants K of components at certain points of the concentration simplex. Pinch branches and bundles of trajectories can arise only when certain relationships of K are satisfied on edges of the concentration simplex. Bundles of trajectories of both sections of column can intersect each other only at complementary relationships of these values at points of components and azeotropes. In this case, there are profiles of concentrations of both sections, which together connect the points of products with each other, i.e., a split is possible. The theory of infinitely sharp split mode determines the mentioned relationships of K. The method of infinitely sharp split mode has been developed on the base of this theory. This method includes the calculation of K at certain points of the concentration simplex. It is not iterative. Therefore, it is very fast, reliable, and can be easily programmed. For the first time, this method covers all n-component azeotropic mixtures, and all types of sharp and half-sharp splits, at which both or one product of column is sharp. This method can find unknown energy-efficient splits for different azeotropic mixtures because it is based on fundamental regularities of distillation. The automatic identification of possible splits is the basis for the synthesis of flowsheets for distillation units and for the conceptual designing of distillation columns. The theory of infinitely sharp split, the method of delimitation of product regions on elements of the concentration simplex for each section of simple columns, and the method of identification of types, characteristics, and terminals of pinch branches are considered in this article.     

Optimal programming of ideal and extractive batch distillation: single vessel operations

This work addresses the problem of optimal programming of multi-component batch distillation columns with a single vessel (batch rectifier, batch stripper, middle vessel and extractive middle vessel) so as to maximize an annualized profit function. A smooth price function is formulated for product valuation, allowing to release traditional purity constraints. The solution is the optimal batch policy of top/bottom withdrawals. A simplified cascade model is developed for separation calculations, offering several operational patterns. Ideal and non-ideal mixtures can be handled with this model that is proposed as a substitute for traditionally used Fenske–Underwood–Gilliland cascades. Batch separations of ideal quaternary feeds are optimally programmed for the first three operations aforementioned. The extractive middle vessel column was optimally programmed for production of anhydrous ethanol from hydrated, nearly azeotropic, alcohol with ethylene–glycol as entrainer. All applications considered fixed number of stages, heat duty, and, in the extractive system, fixed pump-around rate of entrainer.     

Shortcut design of complex distillation: Part 1. Basic theory and applications to multi-feed columns

In this study, we have formulated a general geometric based shortcut method that can be used for multi-feed multi-component columns. Simulated annealing algorithm is used to obtain the solution. This method can be used to find the minimum reflux ratio, minimum number of stages at total reflux, and number of stages at given reflux ratio, as well as the reflux ratio with given number of stages. Reliable solutions were obtained for different types of splits in both single-feed columns and double-feed columns. This method serves as the basis for developing a general method for estimating minimum reflux for other more complex columns.     

夹点计算及其在稳健精馏设计中的作用(英文)

Rising energy costs and growing environmental awareness motivate a critical revision of the design of distillation units. Systematic design techniques, such as the rectification body, column profile map, and temperature collocation methods, require exact knowledge of all pinch points in a particular system, because these stationary points delineate the possible composition trajectories realizable in separation columns. This paper demonstrates novel methods for rigorously determining all pinch points for the...     

Pilot-scale experimental studies on ethanol purification by cyclic stripping

Cyclic distillation is a proven process intensification method for enhanced separation of various mixtures. It uses an alternative operating mode based on separate phase movement which leads to important practical advantages (vs conventional mode) such as increased column throughput, lower equipment cost (using much less trays at the same reflux ratio) and reduced energy requirements by 20–35% (smaller reflux ratio at the same number of stages), and better separation performance (up to three times). However, if the impurities to be separated are in very low amounts in the feed then distillation is not favorable from an energy use viewpoint. This article is the first to report the practical performance of a continuous process for ethanol purification by air stripping using a cyclic mode of operation, a novel process that avoids the costs of distillation. The purification of ethanol food grade (96.4 vol%) from volatile impurities (0.5 vol%) such as esters, aldehydes, and alcohols, is carried out in a hydro-selective column with five stages. The lightweight impurities are removed from a stream that is the head fraction of a distillation column. This is usually a waste stream amounting to 3–6% of the plant production rate. By concentrating the stream with impurities, more ethanol is produced such that the losses are reduced to only 1–1.5% of the plant capacity. Based on the experimental results presented in this work, a process consisting of two air stripping columns using cyclic operation is proposed for industrial implementation.     

Modeling and Analysis of Internal Heat Integrated Distillation Columns

A generalized steady-state model is being developed for an internal heat integrated distillation column (IHIDiC). A procedure incorporating the Newton-Rapheon method is devised for solving the model equations. Separation of an ethanol-water binary mixture is simulated and analyzed with the model, Two pinch points are found within the process, making the separation ex-tremely difllcult and expensive. Two sharp fronts in the temp-erature and the composition profiles are being observed. With the introduction of heat integration, satisfactory separation may be obtained in a limited number of stages with lower reflux ratios. Increasing the pressure difference between the rectifylng and the strlpping sectlons, however, would bring about a reduced relative volatility between the two components involved, creating adverse separation performances. It is obvious that optimigation of the IHIDiC is of prime importance.     

An enhanced pseudo-binary-mixture (PBM) algorithm for multistage separation processes

A scale-shift approach is introduced to further refine the pseudo-binary-mixture (PBM) simultaneous correction procedure for the solution of multicomponent, multistage separation problems [Ishii, Y., & Otto, F. D. (2001). An efficient simultaneous correction procedure for multicomponent, multistage separation calculations for non-ideal systems. Computers and Chemical Engineering, 25, 1285–1298; & Ishii, Y., & Otto, F. D. (2003a). A method to extend the domain of convergence for difficult multicomponent, multistage separation problems. Computers and Chemical Engineering, 27, 855–868]. The scale-shift approach takes advantage of the fact that a solution to a stage-wise separation problem is more easily obtained the fewer the number of stages. In the approach the column specified in a given problem is initially downsized to a configuration having a small number of stages and then scaled up to the original one in a multiple-step manner. A solution is obtained at each scale-shift step and the results are used as a set of initial assumptions to solve the subsequent scaled-up problem. Combining the scale-shift approach along with the gradual non-ideal enhancing method with the PBM algorithm provides a more robust and efficient algorithm for the simulation and solution of difficult multicomponent, multistage separation problems including those involving non-ideal mixtures that are highly sensitive to the quality of initial values. A geometric consideration for the gradual non-ideal enhancing method and details of the unique procedure for the algebraic inversion of matrices employed in the algorithm are also presented.     

Combining extractive heterogeneous-azeotropic distillation and hydrophilic pervaporation for enhanced separation of non-ideal ternary mixtures

The separation of non-ideal mixtures using distillation can be an extremely complex process and there continues to be a need to further improve these techniques. A new method which combines extractive heterogeneous-azeotropic distillation (EHAD) and hydrophilic pervaporation (HPV) for the separation of non-ideal ternary mixtures is demonstrated. This improved distillation method combines the benefits of heterogeneous-azeotropic and extractive distillations in one column but no added materials are needed as is usually the case with pervaporation. The separation of water-methanol-ethyl acetate and water-methanol-isopropyl acetate mixtures were investigated to demonstrate the accuracy of the combined EHAD/HPV technique. There is not currently an established treatment strategy for the separation of the second mixtures in the literature. These separation processes were rigorously modelled and optimized using a professional flowsheet. The objective functions were total cost and energy consumption and heat integration was also investigated. The verification of the process modelling was carried out using laboratory-scale measurements. Extractive heterogeneous-distillation combined with methanol dehydration was found to be more efficient than conventional distillation for the separation of these highly non-ideal mixtures.     

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.     

Systematic exploration of the applicability of the idiomatic vapor balance rule to distillation column consolidation

For N-component distillation(N ≥ 3), consolidation between different column sections is an inevitable manipulation when synthesizing complex distillation configurations. In the consolidation, the idiomatic vapor balance(IVB) rule, in which the larger vapor flowrate in the two columns before consolidation will be chosen as the balanced vapor flowrate at the consolidation point, has been widely used. However, the applicability of the IVB rule has not been verified, which is of essential importance to the accuracy of the distillation configuration synthesis.In the present study, the applicability of the IVB rule to distillation column consolidation was systematically explored by rigorous method for the first time. First, the separation of ideal and non-ideal three-component mixtures with variable compositions was studied, and the optimized configurations before and after consolidation were determined by a rigorous method. The results indicated that for the separation of an ideal mixture, the IVB rule was applicable for the whole composition range, while for the separation of a non-ideal mixture, the IVB rule was only applicable for very limited composition range. Finally, two cases of synthesizing distillation configurations for the separation of non-ideal mixtures were studied to verify the remarkable deviations the IVB rule may cause. The results indicated that the applicability of the shortcut method using the IVB rule to the distillation configuration synthesis depended on the composition of the non-ideal mixture, and a remarkable error might result and the truly optimal configuration might be missed if the IVB rule is applied to a non-ideal mixture.     

丙烷-异丁烷体系的稳态精馏模拟和节能优化

首先对丙烷和异丁烷的精馏分离使用Fensake公式和Underwood公式分别确定理论级数和最小回流比,并与使用Aspen Plus中的DISTWU模块计算出的数值进行比较。在此基础上,文章将使用Distl模块,利用灵敏度等分析工具通过确定塔底塔顶的产物摩尔分率以寻找到合适的馏出物和回流比,并对最佳的进料位置,精馏塔的塔径进行优化模拟。最后,还从节能的角度对丙烷-异丁烷分离精馏塔进行经济优化。模拟所得结果,可为精馏塔设计及工业生产提供参考依据。     

带共沸的乙醇/乙酸乙酯/2-丁酮三元物系变压精馏分离过程及其参数优化

提出了一种带两股循环的三塔变压精馏结构用于分离乙醇（C₂H₅OH）/乙酸乙酯（C₄H₈O₂-3）/2-丁酮（C₄H₈O-3）三元混合物。由该三元混合物的剩余曲线图（RCM）可知,在大气压下该混合物的每一对组元均形成二元最低共沸物,且乙醇/乙酸乙酯二元共沸物组成随压力变化敏感。三个最低共沸物的同时存在形成了精馏边界线夹紧点现象,而通常压力下的精馏塔无法跨越,以至于传统变压精馏无法应用。通过对精馏塔压力的最优化克服了这一困难,提出了新的分离流程,并对过程进行严格稳态模拟。针对初分塔（T1）塔压不同的6种流程,采用序贯迭代法对各塔的塔板数、进料板位置、回流比等参数进行了优化。通过对比6种流程的经济评价和比较,得到了T1塔的最优压力,并经过能量集成使得过程的年度总费用降低了14.88%。     

Systematic exploration of the applicability of the idiomatic vapor balance rule to distillation column consolidation

For N-component distillation (N ≥ 3), consolidation between different column sections is an inevitable manipulation when synthesizing complex distillation configurations. In the consolidation, the idiomatic vapor balance (IVB) rule, in which the larger vapor flowrate in the two columns before consolidation will be chosen as the balanced vapor flowrate at the consolidation point, has been widely used. However, the applicability of the IVB rule has not been verified, which is of essential importance to the accuracy of the distillation configuration synthesis. In the present study, the applicability of the IVB rule to distillation column consolidation was systematically explored by rigorous method for the first time. First, the separation of ideal and non-ideal three-component mixtures with variable compositions was studied, and the optimized configurations before and after consolidation were determined by a rigorous method. The results indicated that for the separation of an ideal mixture, the IVB rule was applicable for the whole composition range, while for the separation of a non-ideal mixture, the IVB rule was only applicable for very limited composition range. Finally, two cases of synthesizing distillation configurations for the separation of non-ideal mixtures were studied to verify the remarkable deviations the IVB rule may cause. The results indicated that the applicability of the shortcut method using the IVB rule to the distillation configuration synthesis depended on the composition of the non-ideal mixture, and a remarkable error might result and the truly optimal configuration might be missed if the IVB rule is applied to a non-ideal mixture.