Optimal design of extractive distillation columns—A systematic procedure using a process simulator

The separation of binary mixtures which form azeotropes is not possible through conventional distillation and they are usually separated by extractive or azeotropic distillation. The optimization of extractive distillation columns is usually performed using a process simulator; however, normally, the result is only obtained after several simulations and the simultaneous analysis of several graphs. This paper sets out to present a systematic procedure, using a process simulator (Aspen Plus^®), in order to obtain the optimum condition for extractive distillation columns. The optimization achieved is characterized by the fact that it is not necessary to perform various simulations, and it also avoids the simultaneous analysis of dozens of curves. The dehydration of aqueous mixtures of ethanol using ethylene glycol as solvent was the system chosen as a case study.     

精馏塔分段正交配置建模方法

针对具有多股进料及中间出料的复杂精馏塔，提出一种新颖的简化建模方法。通过对精馏塔合理分段以及运用多项式正交配置对每一塔段进行简化，可使精馏塔数学模型的维数显著降低，既合理地解决了由进料及中间出料引起的变量分布不连续问题，又有效地克服了全局正交配置方法的缺陷。通过对某一四元物系的丙烯腈萃取精馏塔的计算机仿真，证明该法的优越性．     

The mathematical representation of a light hydrocarbon refining network

The block-relaxation method, system-tH equations and a direct sequential calculation have been combined to solve the non-linear algebraic equations describing the steady-state behavior of a multi-recycle network of five distillation towers and two reactors. Approximately 2,000 such equations are solved in 30 minutes of IBM 7040 computation time. The existence of optimum tolerances on individual column calculations and on individual recycle calculations is demonstrated. The application of the block-relaxation method to a network of distillation columns is numerically shown to simultaneously converge the network equations and the individual column equations to the final solution.     

Calculation of countercurrent separation processes—III. Computation of two interlinked columns.

The relaxation procedure is used for solution of two interlinked distillation columns. Both approaches-sequential iterative method and the block relaxation technique are compared. A new algorithm making use of simultaneous solution of all equations is proposed.     

Feasible separation regions for distillation II: A generalized distillation limit

The boundary of the feasible separation region consists of several curves that are related to specific types of operating modes of the column. Some of these curves create a well‐known product composition multitude, whereas other curves form a generalized distillation limit. The generalized distillation limit demarcates the sloppy splits (i.e., separations in which the composition of at least one product lies inside the composition space) from regions not accessible by distillation and depends on the thermodynamic state of the feed (a mixture of vapor and liquid in equilibrium or saturated vapor/liquid) and column equipment (total/partial condenser and total/partial reboiler). The mathematical equations describing the generalized distillation limit are obtained based on the relationships between the curves (which form the generalized distillation limit) and specific types of operating modes of the column as well as the material balances for the enriching and stripping columns. Furthermore, the vapor and liquid pinch‐point curves, which go through the feed composition point, are not dependent on the thermodynamic state of the feed and column equipment. In addition, an algorithm for determining the generalized distillation limit is obtained. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

Application of factor method for minimum reflux: Multiple feed distillation columns

Analytical equations for determining the minimum reflux ratio of multiple feed distillation columns are presented by using a factor method. For the method, a factor which converts feed flow to minimum reflux ratio is used. The factor is a function of the feed concentration and thermal condition. The calculation steps for determining minimum reflux ratio of multiple feed columns are summarized. A computer program is available (microcomputer or mainframe). Application of the factor method for minimum reflux ratio is illustrated by several examples.     

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).     

A method for steady-state simulation of multistage separation columns involving nonideal systems

A new algorithm is developed for steady-state simulation of multistage separation columns. The algorithm decouples the model equations into two groups. The component-material balance and summation equations are solved simultaneously by the Newton-Raphson method for temperatures and component flow rates in the inner loop. The energy-balance equations are solved in the outer loop to obtain total flow rates. The K-factor is separated into three parts, each representing separately the effect of temperature, composition and apparent volatility. This makes the analytical calculation of the partial derivatives of the K-factors in the Jacobian matrix a simple task. The stability and efficiency of the algorithm is illustrated by solving a variety of problems for distillation, absorber and reboiled absorber columns. The algorithm does not require large storage space.     

Distillation of Multicomponent Mixtures of Higher Aliphatic Acids in Thermally Coupled Distillation Systems

A method is proposed to calculate the characteristics of distillation of multicomponent mixtures of higher aliphatic acids in distillation systems with reversible mixing of flows and coupled heat flows. The method is based on the use of the Underwood equations and the Thiele–Geddes method for independently determining the concentrations. Application of the method is illustrated by the example of calculating the characteristics of distillation of a mixture of coconut oil fatty acids into four fractions (C₆–C₁₀, C₁₂, C₁₄, and C₁₆–C₁₈ acids) in a system containing three columns, one reflux condenser, and two reboilers.     

On the Relation between Ideal and Real Mixtures in Multicomponent Distillation

A complete mathematical solution exists for the distillation of ideal multicomponent mixtures, i.e., mixtures with constant relative volatilities, allowing to answer all questions arising in the distillation of such mixtures by relatively simple algebraic equations. As shown in this paper, the distillation of a real mixture of any complexity may – at least qualitatively – be reduced to the distillation of an ideal mixture. The understanding of the distillation behavior of complex real mixtures is thus greatly simplified and useful both in the design as well as the operation of multicomponent distillation columns.     

Dynamic simulation of multicomponent distillation

A program has been developed for the dynamic simulation of large scale chemical processes. The simulator has a highly modular structure and an explicit integration routine which is capable of handling large systems of stiff ordinary differential equations. The use of an explicit method reduces computer storage requirements and greatly simplifies the writing of new modules. Unit subroutines have been written which are capable of describing multicomponent distillation columns without resorting to conventional thermodynamic equilibrium calculations. The absence of these iterative routines reduces computation time and allows tray efficiencies to be handled in a more natural manner than is possible with Murphee efficiencies. The program also includes an advanced thermodynamic property package based on the Peng Robinson equation of state. An industrial process consisting of two distillation columns with a combined total of seventy-five mass transfer stages, has been successfully simulated and the computed results have been compared with dynamic data collected from plant tests.     

多元精馏过程的非平衡级动态模型

提出了-种通用的多元精馏过程非平衡级动态模型。模型中采用传质、传热速率方程表征实际塔板上的传递过程,避免了级效率的计算。通过引入“分离效率函数”提出一种简洁适用的仿真模型和求解算法,分析了速率模型与“平衡级一级效率”模型之间的内在联系。在两个工业精馏塔上进行了仿真计算和实验检验,结果表明,非平衡级动态模型能够准确预测实际塔板上的动态行为。     

A flexible solution method for generalized equilibrium stage columns

This paper presents a flexible solution method for the process design and simulation of generalized equilibrium stage absorption or distillation columns for solving a wide range of multistage, multicomponent separation problems in the petroleum or petrochemical industries. Although the model was developed to increase the control flexibility of product components in a solution, it was found that it also increases the efficiency of convergence at the expense of greater core usage. The mathematical model includes overall material balances, component material balances, energy balances, summation equations and specification equations. These nonlinear equations are solved simultaneously via the matrix partitioning technique together with the popular Newton-Raphson iterative algorithm. The method is applicable for both absorption and distillation columns with multiple feeds and sidedraws. This model will offer more flexible choices of the column specifications such as tray temperature, overhead product rate, reflux ratio, boilup ratio, tray vapor/liquid flow and product purity/recovery. The composition dependent equilibrium and enthalpy correlations such as Chao-Seader, Grayson-Streed, and Soave-Redlich-Kwong are incorporated into the mathematical model. Most problems can be easily converged with less than ten iterations.     

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.     

催化精馏塔通用数学模型及其求解方法

建立了醋酸甲酯水解反应精馏实验流程,并探讨了反应精馏的工艺特点和可行性。建立了催化精馏塔通用数学模型,并给出了求解方法。该模型对于化学反应和精馏分离同时进行及交替进行两种形式的催化精馏塔均能适用,且可用于催化精馏塔的动态分析。     

催化精馏塔通用数学模型及其求解方法

建立了醋酸甲酯水解反应精馏实验流程,并探讨了反应精馏的工艺特点和可行性。建立了催化精馏塔通用数学模型,并给出了求解方法。该模型对于化学反应和精馏分离同时进行及交替进行两种形式的催化精馏塔均能适用,且可用于催化精馏塔的动态分析。     

Effects of Liquid Holdup in Condensers on the Start‐Up of Reactive Distillation Columns

Compared with start‐ups in conventional distillation columns, those in reactive distillation (RD) columns are much more time and energy consuming, and generate a large amount of by‐products which are not easy to deal with together. For several years, researchers have been trying out different methods to shorten the time required to lower the cost of the start‐up. In this work, a rigorous dynamic model in the ChemCAD simulator is applied to model the start‐up process for the esterification of ethyl acetate in a reactive distillation column. In the model, two sets of equations are employed: one for the fill‐up and heating stage and the other for the equilibration process which follows. In the fill‐up and heating stage, fluctuation curves of the reboiler temperatures with respect to time which are similar to those for conventional distillation columns are observed, while in the equilibration process it is found that the increase of the liquid holdup volume in the condenser reduces the time required to reach steady state for the reactive column and decreases the liquid holdup volume in the reboiler at the equilibrium state. This shows that the liquid holdup volume in the condenser has an important effect on the start‐up of reactive distillation columns.     

Systematic Modeling of the (13C) Isotope Cryogenic Distillation Process

Isotopic enrichment by low-temperature distillation of carbon monoxide is difficult to handle, the experiments providing with the necessary data are time-consuming and difficult to perform, so the mathematical modeling of the process is an appropriate approach to study and to further control the column. This paper develops a mathematical model for the (¹³C) isotope cryogenic distillation process using two subsystems of the plant: the hydrodynamic part and the isotope-separation part. The model is validated using experimental data from the pilot column from the National Institute for Research and Development of Isotopic and Molecular Technologies Cluj-Napoca. The proposed model is suitable for various cryogenic distillation columns, using the proper parameters.