Tray point

FIGURE 12.62 Comparison of tray point efficiencies for different column sizes. Cyclohexane/ -heptane system, 0.20-0.27 atm pressure, total reflux. (Separations Research Program, Univ. of Texas at Austin.)... 

With a more mechanistic model for predicting tray point efficiency, Garcia and Fair showed a better fit to a large database than did the older Chan-Fair model. A parity plot for the Garcia-Fair work is given in Figure 12.64. The newer method is more complex, however, and requires a fairly elaborate computer program. 

A method to estimate the overall volumetric mass-transfer coefficient at any point over the tray, Kya, is needed in order to use equation (4-52). Alternatively, Bennett et al. (1997) proposed the following correlation for estimating sieve-tray point efficiency ... 

The steps drawn between the section operating lines and the equilibrium curve illustrate the compositions on the trays points on the operating lines correspond according to Eq. (2.5) to the composition of liquid from the /th tray, which meets the composition of vapor from the tray below and points on the equilibrium curve correspond to the compositions of liquid and vapor leaving the/th tray. Figure 2.2a allows a number of important conclusions ... 

With the operating and equilibrium lines drawn on the chart as indicated, it can be seen that about 1.5 theoretical stages are required. Assuming an individual Murphree tray efficiency of 40%, the number of actual trays can be estimated by using vertical steps on the tray diagram that extend 40% of the distance from the operating line to the equilibrium line at each tray point. The use of this procedure indicates that at least six actual trays should be used. The... 

A well known example of capillary-buoyancy equilibrium is the experiment in which a number of glass tubes of varying diameter are placed into a tray of water. The water level rises up the tubes, reaching its highest point in the narrowest of the tubes. The same observation would be made if the fluids in the system were oil and water rather than air and water. 

In the use of temperature measurement for control of the separation in a distillation column, repeatability is crucial but accuracy is not. Composition control for the overhead product would be based on a measurement of the temperature on one of the trays in the rectifying section. A target would be provided for this temperature. However, at periodic intervals, a sample of the overhead product is analyzed in the laboratory and the information provided to the process operator. Should this analysis be outside acceptable limits, the operator would adjust the set point for the temperature. This procedure effectively compensates for an inaccurate temperature measurement however, the success of this approach requires good repeatability from the temperature measurement. 

By viriue of its vertical construction, the turbo-type tray dryer has a stack effect, the resulting draft being frequently sufficient to operate the dryer with natural draft. Pressure at points within the dryer is maintained close to atmospheric, as low as 0.1, usually less than 0.5 mm of water. Most of the roof area is used as a breeching, lowering the exhaust velocity to settle dust back into the dryer. 

The feed material, whicdi is to be separated into fractious, is introduced at one or more points along the column shell. Because of the difference in gravity between vapor and liquid phases, liquid runs down the column, cascading from tray to tray, while vapor flows up the column, contacting hquid at each tray. 

Operating Lines The McCabe-Thiele method is based upon representation of the material-balance equations as operating lines on the y-x diagram. The lines are made straight (and the need for the energy balance obviated) by the assumption of constant molar overflow. The liqmd-phase flow rate is assumed to be constant from tray to tray in each sec tiou of the column between addition (feed) and withdrawal (produc t) points. If the liquid rate is constant, the vapor rate must also be constant. 

Example 1 Calculation of FUG Method A large hutane-pentane splitter is to he shut down for repairs. Some of its feed will he diverted temporarily to an avadahle smaller column, which has only 11 trays plus a partial rehoiler. The feed enters on the middle tray. Past experience on similar feeds indicates that the 11 trays plus the rehoiler are roughly equivalent to 10 equilih-rium stages and that the column has a maximum top-vapor capacity of 1.75 times the feed rate on a mole basis. The column will operate at a condenser pressure of 827.4 kPa (120 psia). The feed will he at its huhhle point (q = 1.0) at the feed-tray conditions and has the following composition on the basis of... 

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. 

Tbe best-established theoretical method for predicting E is that of tbe AlCbE [Buhhle-Tray Design Manual, American Institute of Chemical Engineers, New York, 1958). It is based on tbe sequential prediction of point efficiency, Murpbree efficiency, and overall column efficiency ... 

Example 12 Estimation of Plate Efficiency For the conditions of Examples 9 and 11, estimate the point efficiency of the tray. Additional property data ... 

The reason for this simple relationship is that the concept of minimum reflux implies an infinite number of stages and thus no change in composition from stage to stage for an infinite number of stages each way from the pinch point (the point where the McCabe-Thiele operating lines intersect at the vapor curve for a well-behaved system, this is the feed zone). The liquid refluxed to the feed tray from the tray above is thus the same composition as the flash liquid. 

The thermal condition of the feed is designated as q, and is approximately the amount of heat required to vaporize one mole of feed at the feed tray conditions, divided by the latent heat of vaporization of the feed. One point on the q line is on the 45° line at Xp. 

Column Bottom Temperature. The bottom temperature is often controlled on the reboiler outlet line with a control valve in the heating medium line. The control point can also be on a bottom section tray. Care must be exercised in location of the temperature control point. It is recommended, especially for large columns, that a cascade arrangement be used. The recommended scheme has a complete flow recorder/controller (FRC) in the heating medium line including orifice and control valve. The set point of this FRC is manipulated by the temperature recorder/controller (TRC). This eliminates the TRC from manipulating the control valve directly (recall that temperature is the most difficult parameter to control). This makes for smoother control for normal operations. Also, it is handy for startup to be able to uncouple the TRC and run the reboiler on FRC for a period. 

Horizontal natl. circ. Ease of maintenance Lower skirt height than vertical Less pressure drop than vertical Longer tubes possible Less cost than kettle No theoretical tray Extra space and piping as compared to vertical Fouls easier as compared to vertical Accumulation of higher boiling point components in feed line, i.e., temperature may be slightly higher than tower bottom... 

Kister points out that, in the final analysis, design comparisons of packing versus trays must be evaluated... 

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. 

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