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.     

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.     

间歇萃取精馏操作过程的实验研究

采用复合式精馏塔实现了萃取精馏的间歇操作,塔顶产品物质的量分数达95%以上。以取得合格产品的量与时间之比作为目标函数,研究了回流比R、中间回流量Vm(中间罐向提馏段进料流量)、萃取剂用量S对萃取精馏过程的影响,在R=5～10,Vm=3.2～7.2mL/min,S=1.2～2.2mL/min范围内,随以上操作参数的增大,目标函数均呈先增大后减小的趋势。     

Influence of entrainer recycle for batch heteroazeotropic distillation

Dehydration of isopropanol applying batch heteroazeotropic distillation with toluene as entrainer (E) is investigated. The composition of the feed is near to that of the isopropanol (A)-water (B) azeotrope. The effects of recycling the entrainer and the off-cut are studied by dynamic simulation with a professional flow-sheet simulator. Three consecutive batches (one production cycle) is studied. Both operational modes (Mode I: decantation after distillation and Mode II: decantation during distillation) are simulated. For Mode II, calculations are performed both for Strategy A (distillate from the aqueous (E-lean) phase only) and Strategy B (partial withdrawal of the organic (Erich phase), as well). The E-rich phase, the final column hold-up and the off-cut (Mode II only) are recycled to the next batch. The influence of the following parameters are determined: quantity of entrainer, reflux ratios of the steps. The variations caused by the recycling in the 2^nd and 3^rd batches are also shown. The best results (lowest specific energy demand and highest recovery of A) are obtained by Mode II, Strategy A. Recycling increases the recovery, and drastically diminishes the entrainer consumption. However, it makes the production slower and decreases the quantity of fresh feed that can be processed.

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Heterogeneous extractive batch distillation of chloroform-methanol-water: Feasibility and experiments

A novel heterogeneous extractive distillation process is considered for separating the azeotropic mixture chloroform-methanol in a batch rectifying column, including for the first time an experimental validation of the process. Heterogeneous heavy entrainer water is selected inducing an unstable ternary heteroazeotrope and a saddle binary heteroazeotrope with chloroform (ternary diagram class 2.1-2b). Unlike the well-known heterogeneous azeotropic distillation process and thanks to continuous water feeding at the column top, the saddle binary heteroazeotrope chloroform-water is obtained at the column top, condensed and further split into the liquid-liquid decanter where the chloroform-rich phase is drawn as distillate. First, feasibility analysis is carried out by using a simplified differential model in the extractive section for determining the proper range of the entrainer flowrate and the reflux ratio. The operating conditions and reflux policy are validated by rigorous simulation with ProSim Batch Column^® where technical features of a bench scale distillation column have been described. Six reproducible experiments are run in the bench scale column matching the simulated operating conditions with two sequentially increasing reflux ratio values. Simulation and experiments agree well. With an average molar purity higher than 99%, more than 85% of recovery yield was obtained for chloroform and methanol.     

Optimal operation strategy of batch vacuum distillation for sulfuric acid recycling process

Vacuum distillation techniques are widely used in food, biological, pharmaceutical, and wastewater treatment industries. Because of its operation at low temperatures, vacuum distillation prevents the thermal decomposition of materials and alleviates corrosion processes; however, condenser size can be dramatically increased because of reductions in mean temperature differences under the vacuum operation. In batch vacuum distillation processes, vapor generation rate and mean temperature differences are changed with time. In view of these characteristics of batch operation, this paper suggests a novel methodology to minimize the condenser size in batch vacuum distillation processes. The target process is a sulfuric acid recycling system in semiconductor manufacturing plants. In this paper, an equation-oriented dynamic model is established and optimization problem is formulated. By solving the nonlinear programming problem, the condenser size is dramatically reduced when operation time is fixed. In contrast, operation time is greatly shortened when the installed condenser surface area is fixed.     

A note on the identification and control of batch distillation columns

A method for the identification and control of a batch distillation process is presented in this note. The proposed model consists of a first-order integrating process in composition with a high-frequency gain. The feedback controller is designed in the framework of robust nonlinear control with modeling error compensation techniques for the control of distillate composition via manipulations of the reflux ratio. The proposed identification and control procedures are illustrated via numerical simulations.     

间歇萃取精馏过程模拟计算

提出了一种可用于间歇萃取精馏过程模拟计算的快捷模型－准稳态模型，并对该方法的数学模型进行了推导。用此方法对间歇萃取精馏过程中塔顶、塔釜浓度，各塔板温度随时间的变化进行模拟，其结果与实验值吻合较好。它具有计算精度高、计算速度快等优点。     

New double column system for heteroazeotropic batch distillation

A new double column system (DCS) operated in closed mode is suggested for heterogeneous batch distillation. This configuration is investigated by feasibility studies based on the assumption of maximal separation and is compared with the traditional batch rectifier (BR). We study the configurations also by dynamic simulation based on a detailed model using a professional simulator. For the new configuration the minimal duration of the process is determined. The influence of the most important operational parameters is studied. The calculations and the simulations are performed for a binary (n-butanol–water) and for a ternary heteroazeotropic mixture (isopropanol–water + benzene as entrainer). One of the advantages of the DCS is that distillation of binary and ternary systems is performed in only one step. Furthermore the recovery of components is usually higher and the amount of byproducts is lower.     

A semicontinuous approach for heterogeneous azeotropic distillation processes

The separation of azeotropes has substantial energy and investment costs, and the available methods require high capital costs for reconstruction of process plants. As an alternative, a semicontinuous configuration that utilizes an existing plant with minor modifications has been explored. In this paper, a semicontinuous, heterogeneous azeotropic distillation process is proposed and acetic acid dehydration process is used as a case study. To carry out the simulation work, Aspen HYSYS^® simulation software is used along with MATLAB^® and an interface program to handle the mode-transition of the semicontinuous process. Sensitivity analyses on operating parameters are performed to identify the process limits. Comparisons are made to conventional heterogeneous azeotropic distillation, and dividing-wall distillation column on the annual cost. The results proved that the semicontinuous system is the best setup in terms of total annual costs and energy requirements.     

Dynamic parameter estimation and optimization for batch distillation

This work reviews a well-known methodology for batch distillation modeling, estimation, and optimization but adds a new case study with experimental validation. Use of nonlinear statistics and a sensitivity analysis provides valuable insight for model validation and optimization verification for batch columns. The application is a simple, batch column with a binary methanol–ethanol mixture. Dynamic parameter estimation with an ℓ₁-norm error, nonlinear confidence intervals, ranking of observable parameters, and efficient sensitivity analysis are used to refine the model and find the best parameter estimates for dynamic optimization implementation. The statistical and sensitivity analyses indicated there are only a subset of parameters that are observable. For the batch column, the optimized production rate increases by 14% while maintaining product purity requirements.     

Operation and control of batch extractive distillation for the separation of mixtures with minimum-boiling azeotrope

Batch distillation is commonly used in the fine chemicals, specialty polymer, biochemical, pharmaceutical, and food industries. For separating mixtures with minimum-boiling azeotrope, a heavy entrainer is frequently added to the top section of the batch column to aid in the separation. This process is called batch extractive distillation. Most of the papers in open literature have only studied the first operating step of the batch extractive distillation which is the recovery of the light component without mentioning the later steps for the recovery of the other component and entrainer. In this paper, two real chemical systems, one separating acetone and methanol using water as entrainer and the other separating isopropyl alcohol (IPA) and water using dimethyl sulfoxide (DMSO) as entrainer, are studied for the feasible operation of the complete batch distillation sequence. The operating variables, including the pre-load amount with the mixture, continuous feed rate of the entrainer, and reflux ratio at each operating step are determined in the operating sequence. The constant reflux ratio and constant entrainer feed rate operating policy and another policy to allow these two operating variables to be varied will be compared in order to further improve the batch operation. All dynamic simulations that are performed directly mimic industrial situations from an empty column using a rigorous dynamic simulator, Aspen Dynamics™.     

Dynamic composition estimation for a ternary batch distillation

A novel Kalman estimator has been proposed to provide the estimates of dynamic composition in a ternary batch distillation process operated in an optimal-reflux policy. The estimator is formulated based on a sequence of reduced-order process models representing a whole batch behavior. Therefore, the full-order models are first developed around different pseudo-steady-state operating conditions along batch optimal profiles. Then they reduce their orders to achieve all state observability and controllability by a balanced truncation method. In the estimator scheme, the reduced models as well as relevant covariance matrices of process noise are pre-scheduled and switched according to any desired periods. Four important issues have been studied including selection of a sensor frequency, effects of an integrating step size, a state initialization and a measurement noise. The performances of the reduced estimator have been investigated and compared with those of a conventional nonlinear estimator. Simulation results have demonstrated that the performances of the novel linear estimator are reasonably good and almost identical to the nonlinear estimator in all cases, though the linear estimator performs rather sensitively to the effect of high measurement noise. Nevertheless, it has been found to be applicable to implement in real plants with much lower computation effort, easier state initialization and unrequired a priori knowledge of thermodynamics.     

旋转填料床间歇萃取精馏性能研究

为了寻找强化间歇萃取精馏过程的有效途径,在装填2mm×2mm三角形螺旋填料的旋转床中,以乙醇-水为实验物系,乙二醇为萃取剂,通过考察馏出液组成随时间的变化情况,产品的纯度和回收率随转速、回流比和溶剂比的变化情况,研究了旋转填料床间歇萃取精馏的性能。结果表明,旋转填料床中强大的离心力和高效填料的协同作用极大地强化了间歇萃取精馏过程,具有分离效果好、操作时间短、节能、小设备大生产能力等突出优点;存在最佳转速使产品的纯度和回收率最大;增加溶剂比和回流比均能使产品的纯度和回收率得到提高,但增加溶剂比的效果更显著。旋转填料床是强化间歇萃取精馏过程的有效途径。     

Feasible separations and entrainer selection rules for heteroazeotropic batch distillation

A feasibility analysis is presented for the separation of close-boiling and azeotropic (minimum- and maximum-boiling) binary mixtures into pure components by the addition of an entrainer introducing a heterogeneous azeotrope. The analysis is done for both the conventional batch rectifier and the multivessel batch column. The analysis is theoretical and based on the assumptions of total reflux/reboil ratios and infinite number of stages. Two feasibility conditions are formulated that make it possible to investigate feasibility based on information coming solely from the distillation line map along with the binodal curve of the ternary mixture. Serafimov's classification is used for classifying the azeotropic phase diagrams. The feasibility analysis provides the necessary background and information for formulating rules for entrainer selection for the process. Two simple rules are then proposed, which make it possible to “screen” entrainers for heteroazeotropic batch distillation with minimum efforts.     

Nonlinear state estimation and control of a batch reactive distillation

This paper deals with the advanced adaptive control of a batch reactive distillation (RD) column for the production of ethyl acetate. The nonlinear adaptive control law consists of the generic model controller (GMC) and an adaptive state estimator (ASE). In the first part of the present work, the design approach of the ASE scheme in two different forms, namely ASE1 and ASE2, has been addressed for a batch reactive rectifier. The predictor model of both the ASE estimators includes only a component mole balance equation around the condenser-reflux drum system and an extra state equation having no dynamics, and therefore, there is a large process/predictor mismatch. In presence of this structural discrepancy, the adaptive estimation schemes compute the imprecisely known parameters quite accurately based on the measured distillate composition under initialization error, disturbance and uncertainty. In the subsequent part, the adaptive GMC–ASE1 control structure has been formulated for the sample reactive column. This nonlinear control strategy shows comparatively better closed-loop performance than the gain-scheduled proportional integral (GSPI) controller due to the exponential error convergence capability of the estimation scheme and the high-quality control of the GMC law.     

Use of glycerol as entrainer in the dehydration of bioethanol using extractive batch distillation: Simulation and experimental studies

In this paper the dehydration of bioethanol via extractive batch distillation using glycerol as entrainer is studied. Simulation and experimental tests were carried out in order to use glycerol to produce bioethanol with a composition higher than that of the azeotropic point. The results are compared to those reported using ethylene glycol and ionic liquids as entrainers for the same separation. The simulation and experimental results indicate that it is possible to produce high purity bioethanol that can be used as a fuel oxygenate. Among the entrainers used in the experimental tests, the glycerol presented the best performance in terms of the purity in the distillate. Also, it is important to highlight that glycerol has a lower cost in comparison to ethylene glycol and ionic liquids and that is considered a by-product in the biodiesel production.     

Composition estimation of multicomponent reactive batch distillation with optimal sensor configuration

An inferential state estimation scheme based on extended Kalman filter (EKF) with optimal selection of sensor locations using principal component analysis (PCA) is presented for composition estimation in multicomponent reactive batch distillation. The properties of PCA are exploited to provide the most sensitive dynamic temperature measurement information of the process to the estimator for accurate estimation of compositions. The state estimator is supported by a simplified dynamic model of reactive batch distillation that includes component balance equations together with thermodynamic relations and reaction kinetics. The performance of the proposed scheme is evaluated by applying it for composition estimation on all trays, reboiler, reflux drum and products of a reactive batch distillation column, in which ethyl acetate is produced through an esterification reaction between acetic acid and ethanol. This quaternary system with azeotropism is highly nonlinear and typically suited for implementation of the proposed scheme. The results demonstrate that the proposed EKF estimation scheme with optimal temperature sensor configuration is effective for inferential estimation of compositions in multicomponent reactive batch distillation.     

Residue curve maps for reactive distillation systems with liquid-phase splitting

A model has been developed to study the effects of chemical kinetics on the residue curve maps (RCM) for reactive distillation systems with liquid phase splitting. In the model, chemical reaction can occur in both or only one of the two liquid phases. The heating policy V/V₀=H/H₀ is applied so that the kinetic effect can be described by a single parameter, the Damköhler number Da. The effects of reaction kinetics on pseudohomogeneous and heterogeneous mixtures have been compared. The properties of their RCMs are the same outside, but are fully different inside the liquid-liquid (L-L) region if they have different chemical equilibrium curves. Inside the L-L region, the chemical equilibrium curve coincides to a unique reactive liquid-liquid tie line in case that the pseudohomogeneous chemical equilibrium curve intersects with the L-L envelope. When the reaction occurs in only one of the two liquid phases, the residue curves inside the L-L region are strongly affected by the L-L envelope, especially at high Da. In the present paper, first an illustrative arbitrary reaction system, and then the reaction of cyclohexene with water to cyclohexanol are analysed with respect to their RCMs.