Dividing Wall Distillation Column Research Articles

Conventional Proportional–Integral (PI) Control of Dividing Wall Distillation Columns: Systematic Tuning

This work addresses the systematic tuning of proportional–integral (PI) controllers for dividing wall distillation columns. By following an approach of stable pole assignment to a linear dynamics that approximately describes the control system convergence, a technique results that forces the gains of controllers to be dependent on well-known parameters for each control loop: a static gain and a time constant that characterize the open-loop response of the output with respect to the control input, and a damping factor and a response velocity that outline the path of the closed-loop response. Then, it becomes possible to tune all the controllers in a simultaneous way, substantially reducing trial-and-error activities on tuning the entire control system. Via simulation, the control system performance is illustrated for disturbance rejection and set-point tracking in a representative dividing wall distillation column, showing that this tuning technique is an effective choice.

Divided wall distillation column: rationalization of degree of freedom analysis

The present work attempts to rationalize the degrees of freedom analysis of a divided wall distillation column for steady-state simulation using detailed mathematical model, which contains the MESHD equations (conventional MESH equation plus the pressure drop equation). In a divided wall distillation column, pressure difference plays an important role in deciding vapour and liquid flow rates across trays in the divided section. It is observed that for a given divided wall distillation column or Petlyuk column operating under steady state conditions with known feed and condenser pressure, the degrees of freedom is four. This is one higher than that for a conventional distillation column with three product streams (distillate, bottoms and the intermediate product). If this extra degrees of freedom is used to specify liquid split, then no extra degrees of freedom is left to specify vapour split ratio.

Importance of pressure drop in divided wall distillation column

Pressure drop calculations are extensively used for finding tray spacing at the time of designing a column, but these are rarely performed for the simulation purposes as a minor change in pressure (from condenser to reboiler) has insignificant effect on the vapour–liquid equilibrium. However, pressure difference plays an important role in deciding vapour and liquid flow rate across a tray (or trays), particularly in case of interconnected or divided wall distillation column (DWC). In case of a DWC, the feasible vapour split ratio below the divided wall section is naturally adjusted in such a way that the pressure drops in the two parallel branches of the DWC are equal. In the present work, simulation of a DWC using the MultiFrac model of aspen plusTM (Aspen Technology, Inc., Burlington, MA, USA) was carried out to study the feasible vapour split as a function of reflux ratio and liquid split ratio. The results are interesting in that they show a little effect of liquid split ratio on the feasible vapour split ratio and significantly different pressure profiles in two sides of the DWC. It was also found that the variation in vapour flow rate leads to a large variation in the concentration profile. These results indicate that the pressure drop may be one of the key parameters that should be considered for the optimal design and control of a DWC. © 2012 Curtin University of Technology and John Wiley & Sons, Ltd.

Optimization of structural and operational variables for the energy efficiency of a divided wall distillation column

In the present work, the optimization of different structural and process parameters of a divided wall column (DWC) for the energy efficiency has been presented. The optimal design and operation of divided wall distillation columns involve a number of variables and is a non-linear problem. Rigorous simulation of a DWC was carried out using Multifrac model of ASPEN Plus software. Box–Behnken design (BBD) under response surface methodology (RSM) was used for the optimization of the parameters and to evaluate the effects of these parameters and their interactions on the energy efficiency of a DWC. The process variables were found to have significant effect on the energy efficiency of a DWC as compared to the effect of structural variables, and the predictions from the BBD optimization agree well with the results of the rigorous simulation.

Reactive Divided Wall Distillation Column: Simulation and Experiment of Methyl Acetate Hydrolysis

As a combination of divided wall column (DWC) and reactive-distillation column, the reactive divided wall distillation column is a highly complex technology that reaction and separation can occur simultaneously, which can reduce the energy consumption and decrease the costs of captial and operation. This new process was simulated with PROⅡ software and mini plant experiments were implemented. In addition, we investigated the influences of reflux ratio, liquid distribution ratio and molar ratio of ester in water on the conversion rate of methyl acetate and the purity of the product respectively. It could be seen that the trend from experiments was suitable with simulation results.

From simulation studies to experimental tests in a reactive dividing wall distillation column

The design and control of thermally coupled distillation sequences have been studied since many years, but the real implementation occurred until middle of the 1980s using a single shell divided by a wall named dividing wall distillation column. In this work, experimental results for the production of ethyl acetate in a reactive dividing wall distillation column are presented for first time. The experimental results are in agreement with those obtained using steady state simulations with AspenONE Aspen plus. These experimental results are important since it is possible to validate most of the previous results generated using simulations.

Implementation and Operation of a Dividing‐Wall Distillation Column

AbstractThe design and construction of a prototype of a dividing‐wall distillation column was possible by integrating previous knowledge in process intensification, energy savings, theoretical control properties, and closed‐loop dynamics of thermally coupled distillation sequences. In order to achieve the predicted energy savings for this class of complex distillation column, a dividing wall and a side tank were implemented in order to manipulate the internal flows associated with energy consumption. The reaction between ethanol and acetic acid was conducted within the prototype, and the experimental results indicate that a heterogeneous mixture of ethyl acetate and water is obtained as the top product. The temperature profile measured during the experimental run can be used for controlling the batch distillation column in cyclic operation mode.

Time-Optimal Control of Dividing-Wall Distillation Columns

Time-optimal control of startup traditional distillation columns by iterative programming proposed by Woinaroschy for ideal [Ind. Eng. Chem. Res. 2008, 47, 4158] and nonideal mixtures [Ind. Eng. Chem. Res. 2009, 48, 3873] is extended to the case of dividing-wall distillation columns. The minimization of distillation startup time is performed by iterative dynamic programming employing randomly chosen candidates for admissible control. The control variables are the reflux ratio, the reboiler heat duty, and the side-draw flow rate. The dynamic distillation model proposed by the author in the previous papers is applied. Two illustrative case studies for the separation in a dividing-wall column with sieve trays and lateral downcomers are presented as follows: the separation of an ideal benzene−toluene−ethylbenzene ternary mixture and the separation of a nonideal methanol−ethanol−1-propanol mixture. In another case study, a conventional two-column system is presented in comparison to the dividing-wall column. As in the cases of traditional distillation columns, the startup time decrease and the corresponding reboiler energy savings are significant for each of the control variables.

Extractive Dividing Wall Column: Design and Optimization

This paper proposes a novel extractive dividing wall distillation column, which has been designed using a constrained stochastic multiobjective optimization technique. The approach is based on the use of genetic algorithms to determine the design that minimizes energy consumption and total annualized cost. Several case studies are used to show the feasibility of performing extractive separations in dividing wall distillation columns. The simulation results show the effect of the main variables on the complex extractive distillation process.

Experimental study on pressure drops in a dividing wall distillation column

Previous studies in the fields of process design and process control [1] have shown the potential benefits that can be achieved through the implementation of thermally coupled distillation sequences, in particular, the dividing wall distillation column. The dividing wall distillation column meets important goals of process intensification, including energy savings, reduction in carbon dioxide emissions and miniaturization. In this paper, an experimental study on the hydrodynamic behavior of a dividing wall distillation column is presented. Several different values for gas and liquid velocities were tested in order to measure pressure drops and identify operational regions; the air/water system was used as the basis for the experimental setup. Results regarding pressure drops (fitted to the model of Stichlmair et al.) provide operational limits for the operation of the packed dividing wall distillation column. According to the results, the experimental dividing wall column can be operated at turbulent regime that is associated to proper mass transfer.

Process intensification in the petrochemicals industry: Drivers and hurdles for commercial implementation

The process intensification technologies, reactive distillation, dividing wall column distillation (DWC) and reverse flow reactors (RFR) have been implemented at commercial scale in the petrochemical industry each more than 100 times. These technologies have been analysed with four drivers for innovation in the chemical process industry: feedstock cost reduction, capital expenditure reduction, energy reduction and safety risk reduction and with four hurdles for innovation: risk of failure by combining novel aspects, scale-up knowledge uncertainty, equipment unreliability and higher Safety, Health, Environmental risks compared to conventional technologies. The analysis shows that reactive distillation, DWC and RFR all have significant capital cost reduction over conventional technologies and the first two also have energy reductions, while all hurdles for innovation are low. The preliminary conclusion is that process intensification technologies will probably be rapidly implemented in commercial scale operation when at least one of the mentioned drivers is existent and when all mentioned hurdles are low.

Thermodynamic analysis and hydrodynamic behavior of a reactive dividing wall distillation column

A complete thermodynamic analysis of a reactive dividing wall distillation column and an equilibrium reactor followed by a dividing wall distillation column was conducted for several equilibrium reactions using data of a real pilot plant for the distillation column. In addition, several aspects related to the hydrodynamic behavior of the implemented reactive dividing wall distillation column were analyzed in order to prevent operation problems with regard to hydraulics. Results indicate that the reactive dividing wall column presented both higher thermodynamic efficiencies and lower exergy losses than those obtained in the classical configurations of a reactor plus a distillation column. The reactive dividing wall distillation column also required lower energy consumption compared to that required by classical processes. These facts confirm the higher energy efficiency of reactive dividing wall designs. Results also indicate that the reactive dividing wall column meets process intensification goals: i) it requires lower energy consumption, which can be translated into lower carbon dioxide emissions, and ii) the reduction in energy consumption can be associated with lower traffic of liquid in the column and reduction in column diameter (miniaturization). Finally, it was observed that proper collection of the liquid in a side tank and an adequate split to both sides of the dividing wall play an important role in hydraulics. The manipulation of this split enables minimum energy consumption and high thermodynamic efficiency.

Energy Reduction in a Divided Wall Distillation Column

The paper presents the analysis of the thermal efficiency of a dividing wall distillation column and the influence of the feed composition on the reduction of energy consumption compared to a classical scheme of multicomponent mixture separation. The study relays on rigorous simulations in HYSYSTM using thermodynamically equivalent schemes. A case study is presented for the separation of a hydrocarbon mixture: benzene, toluene, ethylbenzene, o-xylene separated in three products in a dividing wall column. The dividing wall column solution led to about 40 % energy saving. The efficiency of a dividing wall column increases when the middle component is in large amount in the feed.

Reactive dividing wall distillation columns: Simulation and implementation in a pilot plant

We performed steady state and dynamic simulations of a reactive Petlyuk column through an equivalent reactive dividing wall column (RDWDC). In the case of the reaction between ethanol and acetic acid catalyzed by sulfuric acid to produce ethyl acetate and water, we have found that the reactive Petlyuk column can achieve set point changes in two control loops of temperature. Also, for load rejection, the control loops can eliminate the effect of the disturbances in the feed composition. These results and previous knowledge reported about thermally coupled distillation columns and reactive distillation were considered to design and implement a RDWDC.

Plant-wide design and control of acetic acid dehydration system via heterogeneous azeotropic distillation and divided wall distillation

Energy-saving plant-wide design and plant-wide control of an acetic acid dehydration system with the feed containing methyl acetate and p-xylene are investigated in the study. A heterogeneous azeotropic distillation using isobutyl acetate as an entrainer is designed to obtain high-purity acetic acid at the column bottom and to keep a small acetic acid loss through the top aqueous draw. The accumulation of p-xylene in the column is avoided by adding a side product stream. The mixture in the aqueous phase of decanter, containing mostly water, methyl acetate, and isobutyl acetate is separated using a divided wall distillation column. The whole acetic acid dehydration system includes a heterogeneous azeotropic distillation column and a divided wall distillation column. The control strategies using temperature loops are proposed for this acetic acid dehydration system. For the heterogeneous azeotropic distillation column, the requirements for acetic acid compositions in both the aqueous phase of the decanter and the column bottom can be satisfied by designing entrainer inventory temperature control and cascade temperature control simultaneously. The stages of controlled temperatures are chosen by singular value decomposition and closed-loop analysis methods based on the criteria of minimum entrainer makeup. For the divided wall distillation column, steady-state analysis methods are used for the selection of proper controlled and manipulated variables and the determination of their pairings. Dynamic simulation results demonstrate that the proposed plant-wide control strategy can maintain product purities and reject external disturbances in feed flow and composition changes as well as internal disturbances such as changes in liquid and vapor splits.

Exergy loss analysis of heat transfer across the wall of the dividing-wall distillation column

The dividing-wall distillation column is thermodynamically equivalent to the Petlyuk column on the condition that no heat transfer is allowed across the dividing wall. However, better energy efficiency of the column may be obtained if heat transfer occurs within a certain part of the wall. The effects of heat transfer across the dividing wall can be analyzed by using the column grand composite curve (CGCC). The heat transfer potential across the wall can be observed by looking at the CGCC of both column sections alongside the dividing wall. However, the possibility of whether heat should be added or rejected at any stage is not clearly known ahead of the CGCC. Consequently, in this work, the exergy analysis is applied to the dividing-wall column in order to determine whether heat should be added or rejected at any particular stage. Also, the minimum exergy loss value in the column is set as the criterion for determining the heat load targets at any stage. These load targets can then be plotted as a T–H profile similar to the CGCC. This methodology was reported to successfully apply to the column with multiple feeds and products. After having identified the locations and quantities of the feasible heat transfer across the dividing wall, the benefits are discussed via three case studies.

A Short Note on Control Structures for Thermally Coupled Distillation Sequences for Four-Component Mixtures

Thermally coupled distillation represents an alternative to conventional distillation to separate multicomponent mixtures. Industrial implementation of those systems can be achieved by using the divided-wall distillation column. It has been proven that a savings of up to 30% in the total annual costs can be obtained with these complex distillation sequences. In a previous work by Cardenas et al. (Cardenas, J. C.; Hernandez, S.; Gudino-Mares, I. R.; Esparza-Hernandez, F.; Irianda-Araraujo, C. Y.; Dominguez-Lira, L. M. Analysis of Control Properties of Thermally Coupled Distillation Sequences for Four-Component Mixtures. Ind. Eng. Chem. Res. 2005, 44, 391−399), it was found that thermally coupled distillation sequences not only can provide savings in the total annual costs but also can present good dynamic responses under closed-loop operation in contrast to distillation sequences based on conventional columns. In the previous study, pairings in the control loops were established according to practical cons...

분리벽형 증류탑의 최적 설계를 위한 내부 순환량 분포와 전열 특성 연구

This paper addresses the optimal design of dividing wall distillation column which is rapidly applied in a variety of chemical processes over recent several years because of its high energy saving efficiency. A general dividing wall column model which can cope with the heat transfer through the dividing wall is developed using rigorous computer simulation. Based on the simulation model, the effects of the internal recycle flow distribution around the dividing wall and the heat transfer across the dividing wall on overall system performance are investigated. An improved column design method is suggested to utilize the heat transfer through the wall. The suggested method is compared with the existing method via simulation study in which the proposed design shows improved energy saving result.

Development of Dividing Wall Distillation Column Design Space for a Specified Separation

The dividing wall distillation column has a larger number of design variables than a conventional column. For design of the column, it will be desirable to define a priori the feasible space over which all the designs lie. An attempt has been made in this paper to address this problem through a graphical representation of all the possible dividing wall column (DWC) designs for a specified separation of a ternary feed. The development of the theory is based on splitting the dividing wall column into three simple columns (a prefractionator and two downstream columns) and applying the shortcut methods of Fenske, Underwood, Gilliland and Kirkbride. For specified terminal product compositions, it is shown that the design space can be constructed on a 3-dimensional plot, the axes being the flow rates of two of the components in the ’net distillate’ from the prefractionator (dividing wall column being representable as a Petlyuk system), and the effective reflux ratio of the prefractionator. For ease of graphical representation, the designs will be projected on to a 2 dimensional space of prefractionator output flow rate variables for a fixed prefractionator reflux ratio. Constraints related to the availability of feed components to downstream columns, infeasible reflux ratio and imbalance in plate assignment on either side of the wall are also placed on the 2 dimensional design space to generate a feasible design space. On this design space, enveloped by various constraints, various equi-parameter curves are drawn depicting locus of points on which the chosen parameter has a constant value. The parameter chosen can be either the total number of column plates or the number of plates above/below the dividing wall, reboiler duty, or the cost. The design space proposed in this paper, even though it uses the shortcut methods, provides the designer with a broad view of what all designs are available, out of which some attractive options may be explored further. The location of equi-cost or equi-energy curves assist the designer in identifying design changes which could lead to either decreased cost or decreased energy.

Design and Optimization of Fully Thermally Coupled Distillation Columns: Part 1: Preliminary Design and Optimization Methodology

The design of a fully thermally coupled distillation column, or its thermodynamically equivalent arrangement, the dividing wall distillation column, is more complex than conventional arrangements because of the greater number of degrees of freedom. All of these degrees of freedom must be initialized before rigorous simulation can be performed. The distribution of stages in the various sections of the column, the reflux ratio, vapour and liquid splits on either side of the fully thermally coupled columns and feed condition must all be initialized. Yet these are important degrees of freedom that all interact with each other in the design. A new approach to the design of fully thermally coupled columns is proposed in this paper. The procedure uses the equilibrium stage composition concept developed for the design of azeotropic distillation systems1. The method is semi-rigorous in nature, providing an initial design that is very close to the results of rigorous simulation. The approach then allows the degrees of freedom to be optimized simultaneously and an optimized initial design established for rigorous simulation. A case study has been used to demonstrate the application of the new method.