Mixing tray liquid

Sakata [180] evaluates the degree of mixing of the liquid as it flows across a tray and its effect on the tray efficiency, Figure 8-30. For plug flow the liquid flows across the tray with no mixing, while for partial or spot mixing as it flow s over the tray, an improved tray efficiency can be expected. For a completely mixed tray liquid, the point efficiency for a small element of the tray, Eog> tray efficiency, E V, are equal. 

A question to be resolved in predicting efficiency concerns the liquid-flow pattern. It is usual practice to assume that the vapour is fully mixed, but there is a diversity of treatments of the liquid phase. The two limiting cases are completely-mixed-liquid and plug-flow-liquid. Achieved efficiencies on well designed trays usually fall between these cases. The assumption of a well-mixed tray liquid is only valid for the smallest trays (pilot scale). 

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. 

Trays operate within a hydraulic envelope. At excessively high vapor rates, liquid is carried upward from one tray to the next (essentially back mixing the liquid phase in the tower). For valve trays and sieve trays,. i capacity limit can be reached at low vapor rates when liquid falls through the rray floor rather than being forced across the active area into tlic downcomers. Because the liquid does not flow across the trays, it rass.scs contact with the vapor, and the separation efficiency drops dramatically. ... 

From Figure 8-31 the effect of mixed and unmixed pools of liquid can be noted. For a completely mixed tray, there is no concentration gradient from inlet to outlet, and therefore the entire tray has a uniform composition. The degree of mixing across the tray as determined by the number of discrete mixing pools on the tray has an effect on the relationship between Eqg E y as a function of X. 

The dry Murphree efficiency calculated thus far takes into account the vapor and liquid resistances and the vapor-liquid contact patterns, but is uncorrected for the effects of entrainment and weeping. This correction converts the dry efficiency to a "wet or actual Murphree tray efficiency. Colburn [Eq. (14-98), under "Entrainment ] incorporated the effect of entrainment on efficiency, assuming perfect mixing of liquid on the tray. 

Egg yolks, pigments (see below), powdered or liquid tempera (poster paint) in assorted colors, and distilled water, or egg tempera paint prepared in Activities 2.3-2.5 gesso grounds prepared in Activity 3.2 narrow-width paintbrushes with soft bristles paper towels mixing trays water containers pencils. 

The feed stream mixes with the feed-stage fluids prior to any separation. This assumption is good for a single-phase feed, but less satisfactory for a partially vaporized feed (11). A partially vaporized feed splits prior to mixing the feed liquid then mixes with liquid of the tray below, while vapor mixes with vapor of the tray above. Ledanois and Olivera-Fuentes (11) derived a simple correction to the x-y diagram construction to alleviate the inaccuracy. Their correction is valid where tray efficiency is high (i,e,f above 60 to 70 percent) at lower tray efficiencies, the inaccuracy is more difficult to quantify. 

The above derivation assumes that vapor flows upward in plug flow and that there is no horizontal vapor mixing, while liquid flows horizontally in plug flow and there is no vertical mixing. Lockett and Uddin (12,122) and Standart (123,124) showed that these liquid flow assumptions are poor, unnecessary, and lead to incorrect implications regarding tray efficiency. By modifying the definition of NL, Lockett derived a fundamentally superior equation analogous to Eq. (7.13). Most theoretical models, however, use Eq. (7.13). Equation (7.13) is also the equation used for packed columns, but for packed columns, it is based on sounder assumptions (12). 

Liquid from the tray above descends a downcomer to the tray below. In the downcomer, the vapor in the liquid leaving the active area de-entrains. The downcomer inlet panel deflects the liquid sideways to the active area. Mass-transfer contacting devices on the active area intimately mix the liquid and rising vapor. The aerated liquid leaves the tray by entering the downcomer. In the space above the active area, the entrained liquid falls out of the vapors and returns to the tray deck. Fig. 1 shows the function of the tray zones. 

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). 

The best known and most used empirical method is that of O Connell (Figure 12.60), for distillation columns and absorbers. The curves are based on plant data for several bubble-cap columns plus a few pilot-scale units. Efficiency is related to two properties of the feed mixture liquid viscosity and relative volatility a. Higher values of the p a product indicate larger liquid-side mass transfer resistance and hence a lower efficiency. For a vapor feed or a mixed vapor-liquid feed, the correlating viscosity should be that of the feed tray liquid. 

A schematic diagram of a tray is shown in Figure 16-10. The column is operating at steady state. A mass balance will be done for the mass balance envelope indicated by Ae dashed outline. The vapor above the trays is assumed to be well mixed thus, the inlet vapor mole frac V does not depend on the position along the tray, The vapor leaving the balance envelope has not yet had a chance to be mixed and its corrposition is a function of position G. The rising vapor bubbles are assumed to perfectly mix the liquid vertically. Thus, x does not depend upon the vertical position z, but the vapor fraction y does depend on z. The liquid mole frac can be a function of the distance G along the tray measured from the start of the active... 

The relationship between and can then be derived by integrating the local E y s over the surface of the tray. Clearly, if all the gas entering were uniformly mixed and fed uniformly to the entire tray cross section, and if the mechanical contacting of gas and liquid were everywhere uniform, the uniformity of concentration of exit gasy ., would then depend on the uniformity of liquid concentration on the tray. Liquid on the tray is splashed about by the action of the gas, some of it even being thrown backward in the direction from which it enters the tray (back mixing). The two extreme cases which might be visualized are ... 

As presented in Fig. 72, ftie construction does not allow fiill mixing of the liquid phase coming fk>m the packii above. Tliat is wly- a parate tray to collect and mix the liquid is proposed [11] to be put over the collector distributor VS. 

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