Tray Efficiency Research Articles

Effect of mixing on efficiencies for reactive tray contactors

The relationship between point efficiencies and Murphree tray efficiencies was studied for systems where reactions take place in the liquid within a tray column. For nonreactive systems, several well-known mixing models are available for predicting Murphree tray efficiency from point efficiencies. For reactive systems, the magnitude and nature of the effect of mixing on tray efficiency are different. An analytical representation is developed of the mixed-pool and plug-flow mixing models for reactive systems with first-order kinetics, and there are different regimes, depending on the reaction kinetics. For each of these regimes, the effect of mixing on tray efficiency differs in magnitude and arises from different mechanisms. The resulting tray efficiencies predicted for reactive systems using the methods may differ by an order of magnitude or more from the values predicted when standard methods for nonreactive trays are applied to reactive trays.

Distillation Sieve Trays without Downcomers: Prediction of Performance Characteristics

A large amount of performance data on larger-scale trays without downcomers has become available recently, and the data have been examined with a view toward understanding and modeling the contacting mechanisms under distillation conditions. The objective of this paper is to describe new predictive models for efficiency, pressure drop, and flooding of trays without downcomers and to show how they can be applied to distillation separations. Such an effort has not been reported previously. The contacting devices are assigned the generic name of dual-flow trays. The models are shown to give reasonable estimates of the performance of these trays in larger columns with open areas in the range of 15−25%, hole diameters in the range of 12−25 mm, and tray spacings in the range of 0.3−0.6 m.

Simulation and experimental study of intermediate heat exchange in a sieve tray distillation column

The aim of this study was to combine a simulation study with an experimental in the investigation of intermediate heat exchange as a method to change the distribution of the driving forces in the column where the number of trays is kept constant. It was applied to a pilot plant sieve tray distillation column for a binary separation of ethanol from n-propanol. Intermediate heat exchange has been accomplished experimentally, both in the stripping section and in the rectifying section. When comparing experimental results to simulation results the inclusion of the tray efficiency is of interest. The results show that intermediate heat exchange can be used to improve the recovery, the separation capacity and to decrease the entropy production if the points of sidestream withdrawal and sidestream return are chosen properly. It is important that the heat exchanged liquid stream returned as vapour enters at a position in the column where the vapour phase has the same composition.

Hydraulics and efficiency of screen trays for distillation

Screen trays are proprietary devices similar to sieve trays which are used in distillation and absorption applications. The pressure drop, liquid holdup and entrainment are measured for screen trays in an air/water column and compared to results obtained with sieve trays. The effciencies of both types of trays were compared in a distillation column using systems of various physical properties (methanol/water, acetic acid/water and cyclohexane/n-heptane mixtures). It was found that the screen tray has a lower dry pressure drop, higher liquid holdup, lower entrainment and weeping, and higher jet flooding capacity than the sieve tray. However, the effciencies of both trays are similar for the systems studied. A model was developed for predicting total pressure drop for screen trays using an air/water column. The model was tested using results for methanol/water, cyclohexane/n-heptane and acetic acid/water systems at total reflux. The deviations are within +/- 20% for 90% of the data points. The effect of a bed of mesh packing on the screen tray was also studied. It was found that the packing led to a higher tray effciency, but resulted in a higher tray pressure drop.

Mixing pool model for prediction of distillation tray efficiency with liquid entrainment

In order to make clear the effect of liquid entrainment on the distillation tray efficiency, a new type of mixing pool model is developed in this paper to calculate the apparent distillation tray efficiencies when the liquid entrainment happens for the counter current tray with vapour unmixing between trays and liquid partially mixing on trays. The analytical solutions are obtained and compared with the calculated results of Bennett’s experimental correlations. The mean relative error between them is 5.6% for the range of 0≤ Pe≤1000, 0.5≤ λ≤3.0 and 0.4≤ E≤1.0. The result shows that the model presented in this paper is reliable.

Three‐phase fluidized distillation

AbstractSeparation efficiency in distillation operations can be improved by modifying the characteristics of the dispersions formed on the trays. The present work reports on the hydrodynamic and mass transfer characteristics of liquid‐solid‐vapour dispersions formed on sieve trays without downcomers of a distillation column operating under total reflux conditions. Murphree efficiency and the operating limits of distillation using the ethanol‐n‐butanol system are analyzed for a large range of vapour velocities and liquid mixture compositions, utilising wettable PVC particles and non‐wettable silicone, PE, and Teflonr̀ particles. It was verified that wettable particles show a drastic reduction in the upper operating limit of vapour velocity, but this does not occur for non‐wettable particles. Tray efficiency can be increased when non‐wettable particles are used, mainly for high vapour velocity operations.

A direct method for incorporation of tray-efficiency matrix in simulation of multicomponent separation processes

Both experimental and estimated multicomponent tray efficiencies are known to become unbounded and exhibit bizarre variation from tray to tray in a distillation column even for ideal systems. Besides lacking the physical significance, they pose numerical difficulties in computation of the component tray efficiencies from the point efficiencies whenever the latter assume negative values (Lockett, M. J. (1986). Distillation tray fundamentals. Cambridge: Cambridge University Press). The use of the Stefan–Maxwell approach for estimation of point efficiency leads to a matrix form. Recently, the tray-efficiency matrix for well-mixed, dispersed and plug flow of liquid has been obtained in terms of point-efficiency matrix similar to binary systems. However, there are no method to use directly the tray-efficiency matrix in simulation. Here, we present a method for direct incorporation of the efficiency matrix in the well-known Naphtali–Sandholm method. We have studied the convergence characteristics of the modified and the original methods for distillation, absorption and extraction columns. The results show the modified one is superior to the original method.

A reactive distillation process for a cascade and azeotropic reaction system: Carbonylation of ethanol with dimethyl carbonate

Modelling and simulation of the reactive distillation column have been carried out for the carbonylation process of ethanol with dimethyl carbonate (DMC) producing diethyl carbonate (DEC). As it includes three azeotropes and two cascade reversible reactions with an undesired intermediate, methyl-and-ethyl carbonate (MEC), the reactive distillation is ideally appropriate. Calculations via a robust transient method reveal that a complete DMC conversion, a sound DEC selectivity over 99.5%, a perfect overcoming of azeotropic limitation, as well as a thorough separation of the alcohols from carbonates can be accomplished in the studied column. The model reliability is analyzed by assuming several values of Murphree tray efficiency, showing no considerable difference between the equilibrium and non-equilibrium models for this system. Moreover, effects of the feed locations of the two reactants and the reflux ratio on the performance of the column are discussed, too.

Effect of number of fractionating trays on reactive distillation performance

AbstractSneesby et al. recently suggested that adding trays in the stripping and rectifying sections of a reactive distillation column can degrade performance. This effect, if true, is not only counterintuitive, but very disturbing because it suggests that the design of reactive distillation columns cannot use conservative estimates of tray numbers, that is, we cannot simply add excess trays, as in conventional distillation. The problem is compounded by the uncertainty in vapor–liquid equilibrium data and tray efficiencies. This implies that developing reactive distillation columns would require extensive experimental work at the pilot‐plant and plant stages to find the numbers of stages offering the best performance. Such a scenario would mean long and expensive development programs. This article explores the effect of the number of trays in the rectifying and/or stripping sections of reactive (catalytic) distillation columns. Three reactive distillation systems are used: an ideal hypothetical system, the ETBE system, and the methyl acetate system. Contrary to the published results, it is demonstrated that additional trays do not degrade performance. Two degrees of freedom available in all cases must be carefully chosen for fair comparisons.

On-line optimization system of pilot scale multi-effect batch distillation system

The operation system, which generates a dynamic optimal operation procedure, is developed for a multi-effect batch distillation system (MEBAD) and is installed in a pilot scale MEBAD. It is said that in the MEBAD, the separation performance can significantly be increased by optimally operating the reflux flow rate as a function of time. However, because of the disturbances and modeling errors, the optimal operation profile calculated off-line is not always optimal when it is applied to the real plant. In the proposed system, by executing the optimization repeatedly within a batch of operation, the deviation from the optimal trajectory is minimized. The proposed system consists of four subsystems: In the composition measuring subsystem, the compositions of the products in all vessels are measured on-line by the near-infrared analyzer (NIR). In the composition estimation and model update subsystem, the unmeasurable compositions on all trays and tray efficiency are updated periodically so as to minimize the difference between the measured and estimated composition profiles of the products. In the optimization subsystem, the optimal composition profiles in the reboiler, the middle vessel, and the reflux drum are calculated so as to maximize the separation performance. The optimal profiles are sent to the control subsystem, and the process is controlled so that the product compositions follow the given trajectories. The proposed system is applied to the problem of separating ternary mixture. Using the pilot plant, it is experimentally verified that the energy consumption of the MEBAD can considerably be reduced when the optimal operation scheme is adopted.

Mathematical Efficiency Evaluation of Modern Sieve Trays

Calculating the flow efficiency of modern sieve trays is the focus of this paper. Using experimental studies, the authors have achieved good conformity between measured and the calculated values, which support the validity of the scientific model.

Modern Sieve Trays Minimal Vapor Velocity

The known negative influences of weeping – the simultaneous decrease of tray efficiency and the turndown ratio – is opposed by the minimal vapor velocity (Fmin). By evaluating the literature, the authors were able to draw conclusions about the construction requirements for trays and their effects on Fmin.

Entrainment effect on tray efficiency

The AIChE model for Murphree tray efficiency and Colburn's equation for apparent efficiency have become standardized procedures for tray efficiency estimation, although derived under different assumptions. In this paper one-parametric diffusion model has been improved in order to evaluate the entrainment effect and mass transfer in the settling zone. In the case when mass transfer in the settling zone is negligible, the analytical solution obtained for the first time gives the same results as in Locketts numerical solution. It has been found that the presented model improves the accuracy of calculations of the mass transfer parameters in presence of entrainment (overall point efficiency, Peclet number and Murphree tray efficiency) and consequently, of the apparent tray efficiency as well. Derived equations can be converted to the classical AIChE model when the entrainment is neglected.

A Fundamental Model for the Prediction of Distillation Sieve Tray Efficiency. 2. Model Development and Validation

A mechanistic model has been developed for the prediction of sieve tray point efficiency for both aqueous and hydrocarbon systems. A forerunner of the model was developed from fundamental relationships by Prado and Fair in 1990 and was based on gas−liquid transfer in systems of predominantly air and water. The earlier model has now been modified and extended. Contacting on a sieve tray is considered analogous to gas−liquid contacting in a mechanically agitated vessel, and supporting research has been adapted to the tray-contacting case. Studies in bubble columns have provided information on bubble size distribution and bubble stability at high pressure; these studies coupled with isotropic flow theory have formed the basis for correlating bubble size distribution as well as the fraction of small bubbles in the sieve tray froth. New tray efficiency data taken on a semi-industrial scale have been combined with published data as well as new data released by Fractionation Research, Inc. to form a large database suitable for model testing. The development of the database is described in part 1 of this series. In part 2, details of the new model are presented along with a comparison of predicted and measured efficiencies. Predicted efficiencies were found to be within ±25% of the measured (or deduced) values of the same efficiency. Comparisons are included, showing the fit of the same data bank to the model of Chan and Fair published in 1984.

A Fundamental Model for the Prediction of Distillation Sieve Tray Efficiency. 1. Database Development

A mechanistic model has been developed for the prediction of sieve tray point efficiency for both aqueous and hydrocarbon systems. A forerunner of the model was developed from fundamental relationships by Prado and Fair (Ind. Eng. Chem. Res. 1990, 29, 1031) and was based on gas−liquid transfer in systems predominantly air and water. The earlier model has now been modified and extended. Contacting on a sieve tray is considered analogous to gas−liquid contacting in a mechanically agitated vessel, and supporting research has been adapted to the tray-contacting case. Studies in bubble columns have provided information on bubble size distribution and bubble stability at high pressure; these studies coupled with isotropic flow theory have formed the basis for correlating bubble size distribution as well as the fraction of small bubbles in the sieve tray froth. New tray efficiency data taken on a semi-industrial scale have been combined with published data as well as new data released by Fractionation Research, ...

Influence of Tray Geometry on Scaling Up Distillation Efficiency from Laboratory Data

This paper studies the effect of tray geometry (especially hole diameter) and liquid tray composition on tray efficiency in a bench-scale distillation column. The results of this study are used for scaling up tray efficiency. Two binary systems, ethanol/water and cyclohexane/n-heptane, were considered. The operating conditions were atmospheric pressure and total reflux. For each one, two different hole diameters (small and large) were also tested. Kirschbaum's industrial data (1962) for the ethanol/water system and of Yanagi and Sakata's (1982) for the cyclohexane/n-heptane system were considered as reference values. The results show the importance of reproducing the hole diameter and liquid tray composition in small trays for using laboratory data to predict large tray efficiency.

A comparison of the equilibrium and nonequilibrium models for a multicomponent reactive distillation column

Apropriate models and computational algorithms were developed and a series of simulations for a reactive distillation tray column were performed in order to compare the equilibrium and nonequilibrium model. The homogeneous liquid phase reversible reaction between acetic acid and ethanol in production of ethyl acetate was employed as a test chemical reaction. Both the Newton–Raphson and homotopy-continuation methods were applied to solve the set of model equations. The homotopy-continuation method was found superior to the Newton–Raphson method in guaranteeing the desired solution for the nonequilibrium model. The comparison between the equilibrium and the nonequilibrium model reveals that the nonequilibrium model is to be preferred for the simulation of a tray column for reactive distillation because of the difficulty in the accurate prediction of tray efficiency for the equilibrium model. Close agreement in the prediction of the two models arises only when the tray efficiency can be correctly guessed for the equilibrium model. The effects of reaction rate and reflux ratio on the column performance were illustrated using the nonequilibrium model. The use of the heat generated by the exothermic reaction as an internal heat source in the column is discussed.

Limitations and Dynamics Imposed on Multicomponent Batch Distillation by Tray Hydraulics

Batch distillation models incorporating a complete set of hydraulic relations, energy balances, and tray efficiencies and simpler models based upon constant molar holdup and zero holdup (the Rayleigh equation) were used to obtain distillation curves and optimal reflux policies. The approach to hydraulic limitations was monitored in the complete hydraulic model. The results indicate that during a multicomponent batch distillation the combination of the changes in the liquid composition and liquid flow rates causes the molar holdup on the trays to vary with time and, consequently, the characteristic time constant of the trays changes. The predicted composition profiles for descriptions which incorporate tray hydraulics differ significantly from those obtained from simpler models. Optimal reflux policies generated from the complete model which includes tray hydraulics differ significantly from those obtained from simple models and may lead to lower product yield. The optimum reflux ratios obtained from the m...

Effect of two‐pass trays on distillation efficiencies

AbstractTwo important structural factors that affect distillation efficiencies, the outlet weir heigh and the liquid flowpath length, are investigated. Performance and efficiency data of an industrial scale i‐butane/n‐butane distillation column equipped with two‐pass trays are used as a basis for the calculations. A mathematical development for a new method for predicting the numbers of vapor and liquid phase transfer units is given. This method together with some other NTU calculation methods is used to assess the effect of the outlet weir height on efficiencies. The effect of outlet weir height on the Murphree tray efficiencies is investigated using the observed point efficiencies and different point efficiency to the Murphree tray efficiency relation methods. The effect of varying liquid flowpath lengths on efficiencies is studied by calculating the Murphree tray efficiencies for one‐pass and two‐pass trays. The results obtained using the NTU calculation method presented in this study show that a certain outlet weir height point efficiency reaches its maximum. Most of the other methods give opposite results giving a minimum for point efficiency at a certain outlet weir length. The results also show that the Murphree tray efficiencies for one‐pass trays are higher than for two‐pass trays. Obviously, this is caused by the longer liquid flowpath length of one‐pass trays. The Murphree tray efficiencies are also calculated for an industrial‐scale MTBE purification column. The column is equipped with two‐pass trays in the stripping section and with one‐pass trays in the rectifying section. The Murphree tray efficiencies of one‐pass trays are considerably higher than the two‐pass tray Murphree tray efficiencies.

New correlation for sieve‐tray point efficiency, entrainmnt, and section efficiency

AbstractA comprehensive composite database for distillation sieve‐tray efficiency is used to develop point efficiency and entrainment correlations based on a model that considers the fluid on the distillation tray to be contained in a liquid‐continuous region near the tray deck and a vapor‐continuous region on top of the liquid‐continuous region. This model allows estimates of the portion of the mass transfer that occurs in each region and the mass‐transfer resistance that occurs on the liquid side and vapor side of the interface. For most cases, most of the mass transfer occurs within the liquid‐continuous region. The liquid side resistance is often significant.The entrainment correlation is consistent with the work of Bennett et al., which relates entrainment to the ratios of the liquid to vapor density and the froth height to the tray spacing. A simple liquid continuous‐only mass‐transfer model containing only four empirical parameters correlates the point efficiency data to within 6.4%. Despite a twofold change in vapor Schmidt number, no dependency on vapor Schmidt number is seen. Important dimensionless groupings are the Reynolds number based on the hole velocity, effective froth density, ratio of the liquid inventory to the perforation diameter, and fraction of the tray area perforated. Mathematically simple and accurate methods allow the prediction of the section efficiency for trays operating in cross or parallel flow. They address vapor and liquid mixing, entrainment and a criterion to avoid significant degradation of the tray efficiency due to weeping.