Feed trays

In addition, one other feature of the prefractionator arrangement is important in reducing mixing effects. Losses occur in distillation operations due to mismatches between the composition of the column feed and the composition on the feed tray. Because the prefractionator distributes component B top and bottom, this allows greater freedom to match the feed composition with one of the trays in the column to reduce mixing losses at the feed tray. 

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... 

The thermal quality of the solvent feed has no effect on the value of (S/F)mjn, but does affect the minimum reflux to some extent, especially as the (S/F) ratio increases. R nax occurs at higher values of the reflux ratio as the upper-feed quality decreases a subcooled upper feed provides additional refluxing capacity and less external reflux is required for the same separation. It is also sometimes advantageous to introduce the primary feed to the extractive distillation column as a vapor to help maintain a higher solvent concentration on the feed tray and the trays immediately below... 

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 feed tray is kept matched in composition to the feed (which means the feed tray moves with feed composition changes). 

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. 

The condition of the feed as it enters the column has an effect on the number of trays, reflux requirements and heat duties for a given separation. Figure 8-15 illustrates the possible situations, i.e., sub-cooled liquid feed, feed at the boiling point of the column feed tray, part vapor and part liquid, all vapor but not superheated, and superheated vapor. The thermal condition is designated as q, and... 

For a condition of overall total trays allowance is to be made for feed tray effect, then add one more theoretical tray to the total. As demonstrated in the tabulation to follow, allowance should be made for the reboiler and condenser. 

Because the feed tray is essentially non-effective it is suggested that an additional theoretical tray be added to allow for this. This can be conveniently solved by the nomographs [21] of Figures 8-16 and 17. If the minimum number of trays in the rectifying section are needed, the)t can be calculated by the Fenske equation substituting the limits of xpi for x jj and x i, and the stripping section can be calculated by difference. 

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

The a should be for the feed tray. However, the value of a = 2.8 should be accepted for feed tray conditions (not 158°F). It would not be if this were predominantly a rectifying or a stripping operation. 

Oh = relative volatility of components heavier than hea y key at feed tray temperature... 

Hengstebeck [224] presents a technique for locating the feed tray by plotting. 

The relation is solved for Sr/Sj. The results are not exact, because the feed tray composition is very seldom the same as the feed which is the assumption in this relation. Actually, the feed point or correct location for the feed may be off by two or three theoretical trays. This will vary with the system. It does mean, however, that when this approach is used for feed plate location, alternate feed nozzles should be installed on the column to allow for experimental location of the best feed point. These extra nozzles are usually placed on alternate trays (or more) both above and below the calculated location. A minimum of three alternate nozzles should be available. 

Maas [108] presents a useful analysis for selecting the feed tray in a multicomponent column. For accuracy it involves the use of a tray-by-tray computer calculation. 

Kirkbride s [174] method for estimating the ratio of theoretical trays above and below the feed tray allows estimation of the feed tray location ... 

Continue step-wise calculations until the ratio of light to heavy key on a tray equals (or nearly so) that ratio in the liquid portion of the feed. This is then considered the feed tray. 

If there are components in the feed and bottoms which do not appear in the overhead product, they must gradually be introduced into the calculations. The estimated position above the feed tray to start introducing these components is determined by ... 

The total of theoretical trays in the column is the sum of those obtained from the rectifying calculations, plus those of the stripping calculations, plus one for the feed tray. This does not include the reboiler or partial condenser as trays in the column. 

Calculated bubble point of feed = 266°F at assumed feed tray pressure of 15 psig. 

Tray No. 11 should be used as feed tray (counting down from the top). Note that since the relative volatility did not change much from top to feed, the same value was satisfactory for the range. 

Continuation of the calculations gives an approximate match of ratio of keys in feed to those on plate 10. Then feed tray is number 10 from bottom and this is also number 11 from top. 

Total theoretical trays = 11 + 10-1 (common feed tray count)... 

From tray-by-tray calculations, feed tray is 10/0.496 = 20.1 trays from bottom, use 20. 

Nn = Number of theoretical trays above feed, or reference plate, n, but not including n Nm = Number of theoretical trays before feed tray Njra = Mols of immiscible liquid No = Mols of non-volatile material present or, number of theoretical trays/stages in column only, not reboiler or condenser Ng = Mols of steam... 

Ol = Relative volatility of components lighter than light key at feed tray temperature P = Constant of fixed pressure in Winn s relative volatility. Equation 8-43... 

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