Tray efficiency in distillation

AiCbE Researcb Committee, Tray Efficiency in Distillation Columns, final report. University of Delaware, Newark, 1958. 

AIChE Research Committee, Tray Efficiencies in Distillation Columns, Final report from the University of Delaware, 1958. 

The calculated point efficiency must be converted to overall column efficiency, which will lower its value and make it closer to the O Connell prediction. The calculated value of Eog is slightly higher than obtained experimentally (Eog = 0.83-0.92) at the University of Delaware for bubblecap trays (Annual Progress Report of Research Committee, Tray Efficiencies in Distillation Columns, AIChE, New York, 1955). 

J. A. Gerster, A. B. Hill, M. N. Hochgraf, and D. N. Robinson, Tray Efficiencies in Distillation Columns—Final Report from the University of Delaware to the AIChE Committee, 345 East 47 Street, New York, American Institute of Chemical Engineers, 1958. 

The results of simulation models are strongly affected by the particular assumptions made in the plant model and by the model parameters. This is the reason why these parameters should be fitted to reliable data describing simplified systems, for example, binary mixtures. Regressing parameters to observations on the complex target system is usually not feasible, as the influence of the assumptions made on the performance of the equipment (e.g., tray efficiency in distillation) is not sufficiently isolated from the influence of the model parameters. The experimental data for the isolated subsystems are the key for a successful process model. They contain all the information about the process model, the model parameters are only a representation. If new data become available, the model can be upgraded by extending the database and perform a new regression. 

Here 2A,eff,2,z = A,eft,2,z- This equation is the basis of the AIChE model (AIChE, 1958) for tray efficiency in distillation. 

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

Weir Height Taller weirs raise the liquid level on the tray in the froth and emulsion regimes. This increases interfacial area and vapor contact time, which should theoretically enhance efficiency. In the spray regime, weir height affects neither liquid level nor efficiency. In distillation systems, the improvement of tray efficiency due to taller weirs is small, often marginal. 

The absorber and stripper shown in Fig. 9 can be combined into a single reboiled absorption column however water condensed internally will cause bottom column corrosion and plugging, whereas in the scheme of Fig. 9, the water is removed at the feed drum. Tray efficiencies in absorption columns are much lower than in distillation columns because of the presence of noncondensible gases. Furthermore, there is a tendency to... 

In fact, through use of matrix models of mass transfer in multicomponent systems (as opposed to effective diffusivity methods) it is possible to develop methods for estimating point and tray efficiencies in multicomponent systems that, when combined with an equilibrium stage model, overcome some of the limitations of conventional design methods. The purpose of this chapter is to develop these methods. We look briefly at ways of solving the set of equations that model an entire distillation column and close with a review of experimental and simulation studies that have been carried out with a view to testing multicomponent efficiency models. 

Example 13.3.3 Tray Efficiency in the Distillation of the Methanol-l-Propanol-Water System... 

Estimate the tray efficiency in the distillation of the ternary system methanol(l)-l-pro-panol(2)-water(3) considered in Examples 12.2.1 and 13.3.1. 

Chapter 12 presents models of mass transfer on distillation trays. This material is used to develop procedures for the estimation of point and tray efficiencies in multicomponent distillation in Chapter 13. Chapter 14 uses the material of Chapter 12 in quite a different way in an alternative approach to the simulation and design of distillation and absorption columns that has been termed the nonequilibrium stage model. This model is applicable to liquid-liquid extraction with very little modification. Chapter 15 considers the design of mixed vapor condensers. 

Two empirical correlations for absorption and distillation overall tray efficiencies in commercial towers are available for standard tray designs (OI). For hydrocarbon distillation these values range from about 50 to 85% and for hydrocarbon absorption from about 10 to 50%. These correlations should only be used for approximate estimates. 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

The rate-based model gave a distillate with 0.023 mol % ethylbenzene and 0.0003 mol % styrene, and a bottoms product with essentially no methanol and 0.008 mol % toluene. Miirphree tray efficiencies for toluene, styrene, and ethylbenzene varied somewhat from tray to tray, but were confined mainly between 86 and 93 percent. Methanol tray efficiencies varied widely, mainly from 19 to 105 percent, with high values in the rectifying section and low values in the stripping section. Temperature differences between vapor and liquid phases leaving a tray were not larger than 5 F. 

---