Simulation and analysis of extractive distillation process in a valve tray column using the rate based model

Valve trays are becoming popular in the chemical process industries owing to their flexibility to handle a wide range of vapor throughputs. Using the rigorous rate based model, the importance of the non-equilibrium approach is demonstrated for a typical extractive distillation process in a Glitsch V-1 valve tray column. Simulation results based on an in-house developed code indicated that the rate based model predictions for a valve tray column operation showed significant differences relative to the equilibrium model. Even small errors in product purities translated into nonoptimal feed stage locations and inaccurate number of stages required. The counter-intuitive effect of high reflux ratio on separation is explained.     

Multicomponent batch distillation with distillate receiver

Based on the assumption of adiabatic equilibrium stages, a rigorous calculation procedure applicable to a multicomponent batch distillation with a distillate receiver under total reflux condition was developed. Provided that the operating conditions including the desired product purity of the most volatile component in the receiver are specified, the proposed model estimates the volume of the distillate receiver, changes in compositions in the receiver and still, concentration profiles of the column, and the batch time required to complete the separation. In order to test the validity of the proposed model, experimental data for the separation of acetone-methanol-2-propanol mixture using a 10 cm I.D. column having six theoretical stages were compared with the simulation results for the two cases when the distillate receiver is initially empty and initially full.     

Design of a fully thermally coupled distillation column based on dynamic simulations

A dynamic simulation of a fully thermally coupled distillation column is conducted for the design of a possible operation scheme, and its performance is examined with an example process of butanol isomer ternary system. The outcome of the dynamic simulation indicates that the column can be operated by using a 3 × 3 control structure. The structure consists of three controlled variables of the compositions of overhead, bottom and side products and three manipulated variables of the flow rates of reflux and steam and liquid split ratio between a main column and a prefractionator. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

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.     

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.     

Design of a fully thermally coupled distillation column for hexane process using a semi-rigorous model

An industrial scale hexane process is designed for the implementation of a fully thermally coupled distillation column (FTCDC). A semi-rigorous material balance and Peng-Robinson equilibrium relation are utilized in the structural design. The operational design is conducted with a commercial design program, the HYSYS. The design outcome of the structural design indicates it to be comparable with the practical system of a conventional two-column arrangement in field operation, which shows the effectiveness of the design procedure implemented here. The procedure is good for the system of many components found from actual field applications. In addition, an investigation of the energy requirement of the FTCDC and a conventional system shows that an energy saving of 34.1% is available with the FTCDC.     

Control and dynamic analysis of a packed distillation column based on modeling approaches

In this work, dynamic analysis and control of a packed distillation column have been utilized theoretically and experimentally. In theoretical studies, two types of mathematical models stagewise (Frank model) and partial differential approaches (back-mixing model), were used. Packed distillation uses 1400 mm packing height, and packing type is rashing ring with 20-15 mm diameter. The reboiler was made from a 13 L glass container. Reflux ratio was adjusted by an on-line computer. The system temperature was measured with six thermocouples. For control studies, the reflux ratio and the reboiler heat dutywere chosen as manipulated variables. Perturbation in feed composition was utilized as the disturbance. Decoupling multivariable dynamic matrix control (DDMC) and Nondecoupling multivariable dynamic matrix control (NDMC) of overhead and bottom compositions were applied for control studies. Performance of the control system was tested by using an integral absolute error (IAE) criterion and it was also compared with decoupling multivariable PID control (DPID) and Nondecoupling multivariable PID control (NDPID).     

Application of a thermally coupled distillation column with separated main columns to gas concentration process

A modified fully thermally coupled distillation column (FTCDC) for operability improvement is utilized in a gas concentration process. The column consists of a prefractionator and two separated main columns having high distillation efficiency and flexible control structure. The operability of the proposed column is evaluated by examining the open-loop dynamic responses of step input variations with the HYSYS simulation. The simulation result indicates that the modified system can give better control than the original FTCDC. The energy saving and reduction of construction cost are discussed, and the ease of vapor flow manipulation and the elimination of a compressor in the vapor transfer are also evaluated as possible improvements.     

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.     

Measurement of concentration profiles in a liquid-liquid extraction column

A novel experimental technique for withdrawing uncontaminated samples of each phase from a highly agitated two liquid phase system (primary dispersion) is presented. The technique has been applied in the study of the continuous and dispersed phase axial mixing characteristic of a mechanically agitated liquid Scheibel extraction column operating under different conditions treating the chemical system acetone-toluene-water. The column mixing compartments were separated by a mixed stainless steel-polypropylene knitted mesh packed bed which was completely ‘wetted’ by the organic dispersed phase. Several concentration profiles are presented and the non-ideal flow parameters as well as the mass transfer coefficients for the column and system under study are reported.     

Effects of operation variables on the recovery of lactic acid in a batch distillation process with chemical reactions

In this study, the feasibility of recovery of lactic acid by batch reactive distillation using cation exchange resin as a catalyst was investigated. For the recovery of lactic acid, two reactions, esterification and hydrolysis, are involved and hence, an apparatus with two distillation columns was developed and operated in a batch mode to ensure enough residence time in the reboiler and column. The effects of operation variables such as catalyst loading, reactant mole ratio, feed concentration, type of alcohols and partial condenser temperature on the yield were studied. In this study, the reaction products of the esterification (methyl lactate and water) were distilled to the hydrolysis part to be recovered into pure lactic acid. The yield of lactic acid increased as catalyst loading in the esterification part increased and reactant mole ratio and feed lactic acid concentration decreased. Methanol as a reactant gave higher yield than any other alcohols. The yield of recovered lactic acid was as high as 90%. The yield of lactic acid was closely related to the boiling temperature of the reaction mixture in the esterification part     

Approximate estimation of operational variables in fully thermally coupled distillation columns using pseudo-pinch points

An approximate procedure for the estimation of operational variables in fully thermally coupled distillation columns (FTCDCs) using two pseudo-pinch points of the feed and side draw trays is proposed, and its performance is examined with two example processes. The estimates from the proposed procedure show some 20% error compared with the results of rigorous simulation using commercial design software, the HYSYS. In addition, the relation between vapor flow rate and composition at one stage above the feed tray-required in the estimation of operational variables of the FTCDC-is analyzed to give information for the selection of the feed tray composition. A preliminary evaluation of operational variables helps to screen unrealizable design obtained often from iterative trial procedures employing the mathematical programming.     

Rapid and reliable tuning of dynamic distillation column models on experimental data

A systematic procedure is presented to assist the calibration of tray distillation column models over plant data. Practical suggestions are provided that contribute to reduce the time needed to check the closure of the experimental material and energy balances. Typical modeling inaccuracies that may have a strong impact in the model performance are highlighted. A set of "cold" and "hot" test runs is suggested to minimize the time that is needed to reliably tune the model in view of using it for control system design and testing. The procedure is discussed with reference to the experimental results obtained in a pilot-plant column.     

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.     

Observer-based control algorithms for a distillation column

This paper studies the synthesis of nonlinear observer-based globally linearizing control (GLC) algorithms for a multivariable distillation column. Two closed-loop observers/estimators, namely extended Kalman filter (EKF) and adaptive state observer (ASO), have been designed within the GLC framework to estimate the state variables along with the poorly known parameters. Exactly same basic model structure was used for developing the observers. The model structure is so simple that the estimator design was performed based on only two component balance equations around the condenser-reflux drum and the reboiler-column base systems of the distillation column. To construct these observers, the poorly known parameters, namely component vapor flow rate leaving top tray, component liquid flow rate leaving bottom tray and distribution coefficient in the reboiler, were considered as extra states with no dynamics. The comparative study has been carried out between the proposed GLC in conjunction with ASO (GLC-ASO) and that coupled with EKF (GLC-EKF). The GLC-ASO control scheme showed comparatively better performance in terms of set point tracking and disturbance as well as noise rejections. The control performance of GLC-ASO and a dual-loop proportional integral derivative (PID) controller was also compared under set point step changes and modeling uncertainty. The proposed GLC-ASO structure provided better closed-loop response than the PID controller.     

Dynamic analysis of thermally coupled distillation sequences with undirectional flows for the separation of ternary mixtures

The Petlyuk distillation system has been considered with special interest because of the high energy savings it can provide with respect to the operation of sequences based on conventional columns. The original design of the Petlyuk structure, however, shows two interconnections that seem to affect its operational and controllability properties. To overcome this problem, two alternate structures have been suggested that use unidirectional flows of the vapor or liquid interconnecting streams. In this work, a comparative analysis of the control properties of the Petlyuk column and the alternate arrangements with unidirectional interconnecting flows is presented. Through a singular value decomposition analysis, it is shown that the alternate schemes provide better theoretical controllability properties than the Petlyuk system. Closed loop tests using proportional-integral controllers were also carried out, and the results showed that, in most of the cases considered, the alternate arrangements improved the dynamic responses of the Petlyuk column. Such arrangements, therefore, show promising perspectives for its practical consideration.     

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。     

Approximate design of fully thermally coupled distillation columns

An approximate design procedure for fully thermally coupled distillation columns (FTCDCs) is proposed and applied to example ternary systems. The procedure gives a fast solution of structural and operation design for a preliminary study of the FTCDC. The structural information resolves the design difficulty, caused from the interlinking streams of the column, which is encountered when a conventional design procedure is implemented. The design outcome explains that how the thermodynamic efficiency of the FTCDC is higher than that of a conventional two-column system and how the system of a separate prefractionator is different from a dividing wall structure. From the design result of three example systems with three different feed compositions, the useful performance of the proposed scheme is proved. In addition, the structural design of the FTCDC gives better understanding of the system and leads to high efficiency design of the column.