And tray efficiency

When the dry tray pressure drop is significantly less than the hydraulic tray pressure drop, then the tray will start to leak or weep, and tray efficiency will be adversely affected. 

The net result of this unpleasant scenario is loss of both vapor-liquid contacting and tray efficiency. Note how the mechanical problems (i.e., levelness) of tray 1 ruins the tray efficiency of both trays 1 and 2. 

At pressures exceeding 10 to 20 bar (150 to 300 psia), and especially at high liquid rates, vapor entrainment into the downcomer liquid becomes important, and tray efficiency decreases with further increases in pressure [Zuiderweg, Int. Chem. Eng. 26(1), 1 (1986)]. 

Special considerations In multipass trays. In a multipass tray, vapor distribution between the passes is largely determined by the hole area, while liquid distribution is largely a function of the weir height and length. If the geometry of the passes is perfectly identical, the distribution of vapor and liquid is the same for each pass, and tray efficiency is uniform. This is readily achievable in two-pass trays, where the design of each pass is identical to the other, but not so when a... 

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. 

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. 

The internal flow of liquid and vapor must be re-evaluated from the standpoint of column capacity, both in the design and performance studies of columns. The physical dimensions of a column can handle only limited ranges of vapor and liquid flow rates. The objective of this chapter is to evaluate the hydraulic aspects of fluid flow in trayed columns. The column performance is examined with regard to factors such as flooding, entrainment, pressure drop, mass transfer, and tray efficiency. 

A major assumption made in the column models of Chapters 3 through 13 was the equilibrium stage. Tray hydraulics provides additional information essential for applying mass transfer theories to evaluate the column performance with a rate-based approach. This analysis provides a basis for calculating the tray efficiency associated with an equilibrium stage. The topics of rate-based analysis and tray efficiency are also discussed in this chapter. 

Tray Hydraulics, Rate-Based Analysis, and Tray Efficiency... 

The froth on the tray is a turbulent mass of usually liquid-continuous fluid with vapor dispersed in the form of small bubbles. The various designs of tray vapor openings attempt to maximize vapor dispersion by generating the smallest possible bubbles. The froth is where mass transfer takes place between the vapor and the liquid. Mass transfer and tray efficiency are enhanced by creating the largest possible interfacial area. 

An understanding of column hydraulics in both trayed and packed columns is essential for a complete performance analysis and design of such devices. The reader will find instructional coverage of these topics, as well as rate-based methods and tray efficiency, in subsequent chapters. 

The mass transfer efficiency of contacting trays is often expressed in several ways, but here only two efficiencies will be nsed point efficiency and tray efficiency. The former deals with the approach to equilibrium at some point on the tray and cannot be greater than 1.0 (100%). Clearly, the equilibrium can vary across the tray as liquid composition varies thus, there are a number of different values of point efficiency when the tray liquid is not completely mixed (the normal case). 

In another study, Grayson examined the effect of K-values on bubble-point, dew-point, equilibrium flash, distillation, and tray efficiency calculations. He noted a wide range of sensitivity of design calculations to variations in K-values. 

At the present state of the art the mass transfer on a tray is expressed by tray efficiencies. Two definitions of tray efficiencies have to be distinguished, i.e., point efficiency qg and tray efficiency qgm-... 

In case of plug flow of the liquid the following relationship between point and tray efficiency holds (Lewis 1936) ... 

TRAYS allows the design of sieve-trays, sieve-valve, tunnel-valve, Baycr-valves, bubble trays, Vario-Flex-valve - including all hydraulics parameters and tray efficiency for absorption, desorption and rectification. 

The second step is to determine feed tray position. Theoretical stages should be used in simulating a column. If tray efficiency is known, the feed tray in terms of theoretical stage can be determined from the actual feed tray and tray efficiency. 

Figure 11.5-1. Vapor and liquid compositions on a sieve Cray and tray efficiency.

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