Design Of Distillation Columns Research Articles

Design and control analysis of the side-stream extractive distillation column with low concentration intermediate-boiling entrainer

Side-stream extractive distillation (SSED) column has attracted increasing attentions due to its simple structure for separating the binary azeotropes using intermediate-boiling entrainer. However, the high-concentration intermediate-boiling entrainer may lead to a disadvantage of high energy consumption. To address this issue, an improved design of the SSED column with low-concentration intermediate-boiling entrainer is proposed to improve economic performances without adversely affecting its controllability for separating binary azeotropes. Economic optimization and sensitivity analysis are employed for designing SSED column using low-concentration intermediate-boiling entrainer for separating the methanol/toluene azeotrope, in which the advantages of cost reduction and energy saving have been observed. Following that, multiple case studies have been conducted to demonstrate the superiority of low-concentration intermediate-boiling entrainer in energy saving. Finally, dynamic control analysis illustrates that the SSED column using low-concentration intermediate-boiling entrainer can react effectively against the feed flowrates fluctuations and compositions disturbances by adopting control structures with composition controller.

Determination of the Minimum Catalyst Amount in the Design of Catalytic Distillation Columns

This paper presents a method to determine the minimum amount of a catalyst needed to operate a heterogeneously catalyzed reactive distillation column based on kinetic and VLE data only, avoiding rigorous column simulations. Good initial estimates for catalyst loadings are rarely discussed, and shortcut methods are lacking. The described method determines the highest possible reaction rate using known kinetics and column stage temperatures obtained from ternary diagrams of the column top and bottom sections, neglecting the highest or lowest boiler of the column, respectively. The method was validated against several literature cases. From these cases, it could be determined that a conservative estimate for the amount of the catalyst in the early design phase is six times the minimum catalyst found using the method described. Additionally, the presented method provides information regarding the operability of the column, for example, related to catalyst deactivation.

Application and Performance Evaluation of Model Predictive Controllers on Driving Force Based Reactive Distillation Processes

In this work, integrated design and control of Reactive Distillation (RD) column operating under Model Predictive Controller (MPC) is presented. At first, a design target is defined, and the RD column is designed based on the element concept at the maximum driving force. After that, the steady state simulation is carried out in ASPEN PLUS that satisfies the design target. Next, the flow driven dynamic system is linearized in ASPEN PLUS Dynamics and the state space matrices are obtained. Using the matrices, MPC closed-loop simulation is carried out in MATLAB. To quantify both the steady state and dynamic operation, a multi objective function is defined that takes into account both the design (total energy consumption, total carbon footprint), and the control performance (Integral Absolute Error, total variation of input, relative gain array, Niederlinski Index). For further verification, an alternative design based on non-maximal driving force is also obtained, and the corresponding steady state and dynamic simulation is performed. The closed-loop performance of MPC controller under feed disturbance shows that the process that is designed at the maximum driving force exhibit better control performance compared to the design alternative that does not take into account the maximum available driving force.
 Chemical Engineering Research Bulletin 21(2020) 49-53

A graphical method for the preliminary design of ternary simple distillation columns at finite reflux

The present study proposes a graphical method for the design of ternary distillation systems at finite reflux. Here key design parameters are determined, in particular feed stage location and the number of theoretical stages required for separation at finite reflux for ideal and nonideal systems in simple columns. The procedure provides designers with a visual tool to determine the design parameters simultaneously and no additional calculations are required unlike current approaches. The proposed graphical method correlated to the Fenske-Underwood-Gilliland (FUG) method for ideal systems. The parameters obtained from the proposed graphical allow for implementation in rigorous simulators as starting values to achieve a converged column with the first simulation and less computational difficulty.

Feasible separation regions for mixtures with S-shaped distillation lines

This paper presents an approach for determining feasible separation regions for S-shaped distillation lines. There are three main assumptions: (1) the feed is a mixture of vapor and liquid in equilibrium; (2) the column has a partial or total condenser and a partial or total reboiler; and (3) the border of the feasible separation region is defined by the extreme operating modes of the distillation process. The separation region itself was constructed using a geometric model of the column, while accounting for the shape of the distillation lines. Contrary to well-known cases for C- and U-shaped distillation lines, the border of the feasible separation region for S-shaped lines is formed from a portion of a product composition multitude (i.e., the set of distillate and bottoms composition points for a column with an infinite number of stages operating at total reflux; part of a distillation limit, which demarcates the sloppy splits from the area that is not accessible by distillation; and additional segments). The feasible separation region includes additional product composition areas, which enable the design of distillation columns with fewer theoretical stages, operating under a lower reflux ratio, as demonstrated for a mixture of methyl ethyl ketone, benzene, and toluene.

Design of multi-loop control systems for distillation columns: review of past and recent mathematical tools

Abstract Design of a control structure in distillation columns involves selecting proper sets of manipulated and controlled variables (often including tray temperatures for inferential control of product compositions) and one-to-one pairing between the two sets. In this paper, various mathematical tools for achieving this goal are reviewed. First, traditional methods such as Singular Value Decomposition (SVD) and Relative Gain Array (RGA) that build upon a simplified steady-state or dynamic model of the column are explored. The role of optimization in systematizing the control design procedures is also investigated. Then, more recent inferential control techniques that rely on statistical methods such as Principal Component Regression (PCR), Partial Least Squares Regression (PLSR), and other machine learning techniques such as Artificial Neural Networks (ANN) and Support Vector Machine Regression (SVMR) are discussed extensively. The discussions include newer distillation technologies with complex configurations such as dividing-wall columns. Finally, the use of process simulators in aiding the control structure design of distillation columns is surveyed.

Anhydrous tert-butanol production via extractive distillation using glycerol as an entrainer: technical performances simulation

tert-Butanol is widely used as a main solvent in the industries. The high purity of tert-butanol is required to fulfil the industrial standard for the solvent. Unfortunately, tert-butanol separation from water required a special distillation named extractive distillation due to the azeotropic point in the mixture. Thus, entrainer as a third component should be used to break the azeotropic point. Glycerol is one of the potential green entrainer used in the extractive distillation. The simulation of the extractive distillation using glycerol as entrainer for the separation of tert-butanol/water mixture was conducted in this study. The Aspen Plus Process Simulator V.10 was used to simulate the feasibility of the glycerol as an entrainer and determine the optimum operating condition of the extractive distillation and recovery column in the anhydrous tert-butanol production. The results show that the optimum configuration for the extractive distillation column design consists of the number of stages of 25, binary feed stage of 21, entrainer feed stage of 3, reflux ratio of 0.6, mixed feed temperature of 25°C, and an entrainer feed temperature of 45°C. The condenser and reboiler duties are -1171.04 kW and 1744.48 kW, respectively. Moreover, the tert-butanol purity with this configuration can be achieved up to 0.996 of mole fraction.

Optimal synthesis and design of catalytic distillation columns: A rate-based modeling approach

This work presents the optimal synthesis and design of a rigorous catalytic distillation (CD) column that explicitly considers the multiscale and multiphase nature of this intensification process. A rate-based model that couples micro and macroscale events taking place inside the CD column is explicitly considered. The direct solution of such intensive optimization problems is challenging due to nonlinearities introduced by heterogeneous reactions, transport phenomena, and interactions between discrete and continuous variables. Also, combinatorial complexity involved in the selection of multiple structural decisions complicates the problem, e.g., distribution of reactive stages along the column, location of feed stages, and total number of stages. A discrete-steepest descent-based optimization framework recently developed is used to address the optimal synthesis and design of rate-based CD columns. A case study involving the production of ethyl tert-butyl ether (ETBE) has been considered. The results show that the multiscale events occurring in this process intensification (PI) unit cannot be ignored since they produce process designs different from those obtained with an equilibrium-based CD model. The results also show that neglecting multiscale phenomena may result in infeasible CD designs. The outcomes gained through this study illustrate the critical need to systematically consider multiscale and multiphase events in CD columns for the optimal design of realistic, cost-effective, and attractive process intensification (PI) systems.

Molecular tracking: A concept for side‐draw distillation column design

AbstractA simple approach is introduced to locate a side‐draw tray for ternary and multi‐component mixtures with middle boiling component(s) present in the system at trace levels. The concept is based on a probability function defined by the thermodynamic properties of the system. The advantage of this method over existing methods is the ability to quickly and efficiently provide a feasible configuration of the distillation unit without relying on rigorous optimization or trial and error approaches. Moreover, it provides an intuitive understanding of the movements of the middle boiling components in the column.

Optimal design of flexible heat-integrated crude oil distillation units using surrogate models

The design of distillation columns often considers a given fixed feedstock and nominal operating conditions. Here we present an optimization-based approach for the optimal design of these units considering a flexible operation under a range of potential feedstocks. Our method combines an artificial neural network with a support vector machine to model the crude oil distillation unit. The artificial neural network model predicts the performance of the distillation unit for a given crude oil feedstock whilst the support vector machine classifier filters out infeasible design alternatives from the solution space (i.e., designs that are unlikely to converge when simulated using a rigorous model). The inputs to the artificial neural network include the column structural variables and operating conditions, whilst the outputs are process variables linked to the column performance. The artificial neural network models and support vector machines constructed for different crude oil feedstocks are integrated into a two-stage optimization framework in order to optimize the column structural variables and operating conditions, where the minimum utility demand is estimated using the pinch analysis. An effective solution strategy that combines stochastic and deterministic optimization algorithms is applied to search for economically viable and flexible design alternatives that can operate over a given range of crude oil feedstocks while satisfying the product quality specifications. The capabilities of the proposed approach are illustrated using an industrially-relevant case study, where we clearly show that the proposed approach can identify design alternatives capable of handling various feedstocks effectively.

Computationally efficient optimization models for preliminary distillation column design and separation energy targeting

We propose versatile shortcut distillation column and separation energy targeting models that are well-suited for superstructure-based process synthesis. The models are based on a novel reformulation of the Underwood equations to address systems where the components that are present in the feed can vary due to zero flow rates of some components. Also, we propose valid constraints, resulting in a significant enhancement of the computational performance of the models. The proposed distillation column model can automatically identify adequate key components and the energy requirement of a desired separation task, while considering a wide range of types of separations including non-sharp/sloppy splits. Also, the proposed separation energy targeting model can be used to estimate an energy requirement target for the separation of a mixture without finding detail network configurations. Due to their versatility and computational efficiency, the proposed models can be readily used for and expand the scope of superstructure-based process synthesis approaches.

A method to circumvent the combinatorial difficulties in the optimal design of distillation columns

A new method is presented for the optimal design of distillation columns with structural and operational decisions. This has been a challenging issue because of difficulties such as complexity of problem formulation, numerical instabilities and lengthy computational times. The new approach presented here solves combinatorial optimization problems by a pattern search method. In the method, the best combination of integer values, each representing the number of stages placed in a column section of a distillation unit, is identified by comparing the objective values evaluated by the embedded NLP solver, while derivatives are not used explicitly. The reliability of the implemented algorithm is numerically confirmed by solving DWC (dividing wall column) problems. This reliability is due to the use of globally convergent pattern search in combination with a full-space Hessian SQP algorithm where global convergence in large and highly nonlinear flowsheeting problems is ensured (Ishii & Otto, 2018).

Acetone-Methanol Mixture Separation by Extractive Distillation Using 1-Allyl-3-Methylimidazolium Chloride

Acetone with methanol form a minimum boiling point azeotrope. To separate the mixture components, extractive distillation in the presence of new ionic liquid, 1-allyl-3-methylimidazolium chloride ([Amim]Cl), was assumed. The aim of the present study was the design of an extractive distillation column (number of theoretical stages, position of input stages, reflux ratio) in which acetone with the purity of 99.5 mole % is obtained at the minimum heat duty of the column reboiler. Higher acetone–methanol separation efficiency of [Amim]Cl in comparison with e.g. 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [Emim][triflate] IL was reflected in the resulting parameters of the optimised extractive distillation column: for solvent to feed ratio of 0.42, the number of theoretical stages of the extractive distillation column was 13 and the reflux ratio was 0.1. Heat demand in the reboiler of the extractive distillation column was about four times lower than that in the extractive distillation column employing [Emim][triflate] solvent. Based on the obtained results, [Amim]Cl seems to be a very good solvent for the acetone–methanol azeotropic mixture separation.

Equation-Oriented Optimization of a Distillation Column Considering Stage Hydraulics

Accurate prediction of pressure drops around trays or packing sections is crucial for the optimal design of distillation columns. However, simultaneous optimization of all operating and structural ...

Optimal design of superstructures for placing units and streams with multiple and ordered available locations. Part II: Rigorous design of catalytic distillation columns

This work addresses the optimal design of catalytic distillation (CD) columns taking into account discrete design variables, i.e., the number of stages and location of feed stages and reactive stages. This optimization problem is challenging due to the combinatorial complexity introduced by these discrete decisions. In this work, the binary variables of the superstructure are expressed as a function of a reduced variable set: the external variables, which allows the use of a local algorithm that solves a series of master and primal sub-problems introduced in our previous study (Part I). The master problem is constructed from the reduced variable set and solved with a Discrete-Steepest Descent Algorithm (D-SDA). The primal sub-problem is a nonlinear programming problem obtained by fixing the binary terms in accordance with the reduced variable set. This strategy avoids finding multiple local minimizers, thus making this methodology more practical and efficient than existing Mixed-Integer Nonlinear Programming (MINLP) local and global optimization solvers. The proposed approach was evaluated on a catalytic column used for the production of ethyl tert-butyl ether (ETBE). It was found that an alternating distribution pattern of the reactive trays, the optimization of the location of feed stages, and the optimization of the total number of trays results in a more cost-effective CD column.

Modeling, Simulation, and Optimization 4.0 for a Distillation Column

AbstractSimulation is besides experimentation a key technology in design, analysis and operation of distillation columns. The reliability of simulations to answer questions concerning the real process strongly depends on model quality, which needs to be reliable and predictive. Usually available plant data can be used to validate and if necessary, to improve the model. In this contribution challenges and opportunities of a continuous model improvement in a flowsheet simulator with emphasis on new possibilities triggered by digitalization will be discussed.

Point efficiency of sieve trays at elevated liquid viscosities

Liquid viscosity plays a major role in determining the mass transfer efficiency in distillation through its contribution to the liquid-phase mass transfer coefficient, interfacial area of mass transfer, and vapor and liquid phase residence times. Most of the distillation efficiency correlations in the literature were developed or validated based on efficiency data at liquid viscosities less than 1.5mPas. However, distillation at liquid viscosities greater than 1.5mPas is encountered in practice in the chemical process industry. Therefore, distillation efficiency data at liquid viscosities greater than 1.5mPas are required to accurately quantify the effect of liquid viscosity on distillation efficiency and to develop improved efficiency correlations. Point efficiency defines the most fundamental mass transfer efficiency for distillation trays upon which tray and column efficiencies are calculated. In this paper, the effect of liquid viscosity on point efficiency of sieve trays is reported with new data for four different test systems at liquid viscosities ranging from 0.2 to 4.2mPas. These new data are critical for evaluating the validity of the existing tray and point efficiency correlations and for extending the existing correlations or developing new correlations for distillation column design at elevated liquid viscosities.

Evaluation of the validity of tray and point efficiency correlations at elevated liquid viscosities and proposal of an improved point efficiency correlation

The accuracy of the distillation efficiency correlations plays a critical role in cost-effectively designing distillation columns. Liquid viscosity is considered to be a major factor that influences distillation efficiency through its contribution to the liquid-phase mass transfer coefficient, interfacial area of mass transfer, and vapor and liquid phase residence times. Nearly all of the distillation efficiency correlations in the literature were developed or validated on the basis of efficiency data at liquid viscosities less than 1.5mPas. However, distillation at liquid viscosities greater than 1.5mPas is encountered in several cases in the chemical process industry. In this paper, the evaluation of three different tray and point efficiency correlations using Oldershaw column data at liquid viscosities from 0.2 to 4.2mPas is presented. In addition, we propose an improved point efficiency correlation, based on the Duss and Taylor correlation, with a modified liquid viscosity exponent for the liquid-phase mass transfer coefficient. The proposed correlation was validated with commercial sieve tray efficiency data and Oldershaw column data. This proposed correlation will aid in the cost-effective design of distillation columns, especially at elevated liquid viscosities.

A multi-objective reactive distillation optimization model for Fischer–Tropsch synthesis

In the design of a reactive distillation column, aspects such as column configuration, catalyst loading, tray temperature, and side extraction rates should be well considered. Though preferences in Fischer–Tropsch (FT) synthesis may vary, it is acknowledged that the final product contains a wide range of hydrocarbons including fuel gas, gasoline, diesel, and linear wax. Zhang et al. (2018) previously developed an equation-oriented framework for optimal synthesis of integrated reactive distillation systems for FT processes.Here, we extend the mass, equilibrium, summation, and heat equations to a mathematical program with complementarity constraints to deal with possible dry trays in the non-reactive sections. The purpose of describing disappearing phases is to avoid infeasibilities due to multiple bilinear terms in the model for complicated model structures. The model is implemented by solving initialization steps and a sequence of nonlinear programming problems to determine an optimal structure and operating conditions.Design specifications for multiple products could be set as individual objectives to determine design limits. Moreover, a balance of multiple objectives could be reached by formulating the reactive distillation model as a multi-objective optimization problem. In this work, we employ the augmented ϵ-constraint method. The results show that significant design insights can be gained from the Pareto-optimal front regarding acceptable trade-offs among various objectives.

Design and control of a novel side-stream extractive distillation column for separating methanol-toluene binary azeotrope with intermediate boiling entrainer

It is innovative and important to perform complex binary azeotrope separation via single distillation column. In the article, design and control of a novel and simple side-stream extractive distillation (SSED) column with extraction and solvent recovery function is investigated for the separation of methanol and toluene binary azeotrope with intermediate boiling triethylamine (Et3N) as solvent. The steady-state design parameters of the novel SSED column are optimized and obtained with the objective of minimal total annual cost (TAC) via conventional sequential iterative procedure. On the basis of steady-state design, four single-end temperature control structures (STCS), four dual-temperature control structures (DTCS) and four dual-composition control structures (DCCS) are developed and established successively to overcome the operation and control challenges caused by the novel scheme. To seek the robust control, the dynamic performance analysis on account of the above control structures is conducted for the SSED column, which is evaluated for feed flowrates and composition disturbances. The results show that the control performance of DCCS is significantly superior to that of STCS and DTCS while the integral squared error (ISE) and energy-consuming effect are equivalent, and can take shorter time to bring the product purities back to the original steady state in coping with specified feed disturbances.