Trays determining

Select an Lm equation in accordance with the number of tray liquid passes, Eqs. (3.53) through (3.56). Execute the equation the same as for valve-type trays, determining Lm for the bubble cap tray. 

Downcomer length (to be fixed, thus the pressure drop of the tray determines the restrictor layer thickness and not the pressure drop of the downcomer) Dispersed phase velocity in the hole of diameter d ... 

In all the previous discussions on theoretical trays or stages in distillation, we assumed that the vapor leaving a tray was in equilibrium with the liquid leaving. However, if the time of contact and the degree of mixing on the tray is insufficient, the streams will not be in equilibrium. As a result the efficiency of the stage or tray is not 100%. This means that we must use more actual trays for a given separation than the theoretical number of trays determined by calculation. The discussions in this section apply to both absorption and distillation tray towers. 

Use the experimental data given in Exercise 4.20 to specify the operating conditions in the distillation column below. The column has nine trays. Determine operating conditions that will maximize L/V in the lower trays (stripping section) and minimize L/V in the upper trays (rectifying section). Also, specify the temperature of stream 1 so the vapor-liquid composition of the input matches that of the feed tray. 

Determination of the Temperature of the Evaporating Surface in Direct-Heat Tray Dryers When radiation and conduction are negligible, the temperature of the evaporating surface approaches the wet-bulb temperature and is readily obtained from the humidity and diy-bulb temperature. Frequently, however, radiation and conduction cause the temperature of the evaporating surface to exceed the wet-bulb temperature. When this occurs, the true surface temperature must be estimated. 

Calculate liquid densities, molar tray and condenser-reflux drum holdups, ana hquor and vapor enthalpies. Determine holdup and enthalpy derivatives with respect to time by forward difference approximations. 

After actual theoretical trays are determined (see Actual reflux and theoretical stages) one needs to estimate the actual physical number of trays required in the distillation column. This is usually done by dividing the actual theoretical trays by the overall average fractional tray efficiency. Then a few extra trays are normally added for offload conditions, such as a change in feed composition. 

Why not put new lyrics to an old tune This is an excellent idea, and many have done this very thing. Rice" started w ith the Smith-Brinkley raethod" used to calculate distillation, absorption, extraction, etc., overhead and bottoms compositions, and developed distillation equations for determining the liquid composition on any tray. This together with bubble point calculations yield a column temperature profile useful for column analysis. 

The design of a distillation column is based on information derived from the VLE diagram describing the mixtures to be separated. The vapor-liquid equilibrium characteristics are indicated by the characteristic shapes of the equilibrium curves. This is what determines the number of stages, and hence the number of trays needed for a separation. Although column designs are often proprietary, the classical method of McCabe-Thiele for binary columns is instructive on the principles of design. 

The tower operates in the same manner as a condensate stabilizer with reflux. The inlet liquid stream is heated by exchange with the gas to approximately 30 F and is injected in the tower at about the point in the tower where the temperature is 30 F. By adjusting the pressure, number of trays, and the amount of reboiler duty, the composition of the bottoms liquid can be determined. 

Distillation design is based on the theoretical consideration that heat and mass transfer from stage to stage (theoretical) are in equilibrium [225-229]. Actual columns with actual trays are designed by establishing column tray efficiencies, and applying these to the theoretical trays or stages determined by the calculation methods to be presented in later sections. 

To determine the proper operating pressure for a distillation system, whether trays or packed column, exam-... 

Example 8-4 Determine Minimum Number of Trays by Winn s Method (used by permission [99])... 

At conditions of feed tray, assume pressure is 15 psig as 1,533 mm Hg. Determine bubble point ... 

Figure 8-28 presents the usual determination of optimum or near optimum theoretical trays at actual reflux based on performance. It is not necessarily the point of least cost for all operating costs, febrication costs or types of trays. A cost study should be made to determine the merits of moving to one side or other of the so-called optimum point From the Figure 8-28 ...

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. 

To determine the column (with trays) diameter, an approach [130] is to (1) assume 0 hours (2) solve for V, Ib/hr vapor up the column at selected, calculated, or assumed temperature and pressure (3) calculate column diameter using an assumed reasonable vapor velocity for the type of column internals (see section in this volume on Mechanical Designs for Tray Performance ). 

For a plot of xp = 0.750, slope = 0.7875, read xyy at the equilibrium line for each theoretical tray and plot similar to Figure 8-38. Then determine the area under the curve between the selected x and the product x. Then ... 

Application may be (1) to determine a column diameter and number of plates or (2) to take an existing column and assume an operating reflux for the fixed trays and determine the time to separate a desired cut or product. 

Determine How much water is removed from the overhead accumulator and the intermediate dehydrator or water removal tray No water is removed from the bottoms due to the use of superheated steam. 

The minimum theoretical trays at total reflux can be determined by the Fenske relation as previously given... 

The method of Gilliland [23] (Figure S-24) is also used for multicomponent mixtures to determine theoretical trays at a particular operating reflux ratio, or at various ratios. 

Eduljee [107] evaluated published data and corrected relationships for determining the number of actual trays versus actual reflux with reasonably good agreement ... 

The minimum reflux ratio (L/D)min been determined to be 1.017. Using the Brown and Martin graph [9], evaluate the theoretical number of trays at an operating reflux of 1.5 times the minimum. The minimum number of stages was determined to be 22.1 including the reboiler. See Figure 8-49. 

From the theoretical trays at operating reflux the actual trays for installation are determined ... 

The actual number of trays in the rectifying section (Nact)r be determined by ... 

Determine top tray temperature for use in relative volatility calculations by running a dew point on the overhead rapor. For total condenser its composition is same as distillate product. For a partial condenser, run a dew point on the column overhead vapor composition as determined by a material balance around the partial condenser, reflux, and product. 

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