Feed tray location

Particularly when the number of trays is small, the location of the feed tray has a marked effect on the separation in the column. An estimate of the optimum location can be made with the Under-wood-Fenske equation (13.116), by applying it twice, between the overhead and the feed and between the feed and the bottoms. The ratio of the numbers of rectifying Nr and stripping Ns trays is 

Occasionally the minimum reflux calculated by this method comes out a negative number. That, of course, is a signal that some other method should be tried, or it may mean that the separation between feed and overhead can be accomplished in less than one equilibrium stage. 

An improved relation that, however, requires more information is due to Akashah, Erbar, and Maddox [Chem. Eng. Commun. 3, 461 (1979)]. It is 

As discussed briefly in Section 13.4, the operating reflux is an amount in excess of the minimum that ultimately should be established by an economic balance between operating and capital costs for the operation. In many cases, however, as stated there the assumptions R = 1.2Rm often is close to the optimum and is used without further study unless the installation is quite a large one. 

An early observation by Underwood (Trans. Inst. Chem. Eng. 10, pp. 112-152,1932) of the plate-reflux relation was 

The calculations made thus far are of theoretical trays, that is, trays on which vapor-liquid equilibrium is attained for aU components. Actual tray efficiencies vary widely with the kind of system, the flow rates, and the tray construction. The range can be from less than 10% to more than 100% and constitutes perhaps the greatest uncertainty in the design of distillation equipment. For hydrocarbon fractionation a commonly used efficiency is about 60%. Section 13.14 discusses this topic more fidly. 

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

Multicomponent distillation, 393 absorption factor method, 398 azeotropic, 420-426 bubblepoint (BP) method, 406-409 computer program references. 404 concentration profiles, 394 distribution of non-kevs. 395 Edmister method, 398,399 extractive, 412, 417-422 feed tray location, 397 free variables, number of 395 Lewis-Matheson method 404 MESH eauations. 405-407 molecular, 425-427 nomenclature, 405 number of theoretical trays, 397 packed towers, 433-439 petroleum, 411-415 reflux, minimum, 397 reflux, operating, 397 SC (simultaneous correction) method, 408-411... 

Example 13.10 shows how theoretical stages and feed tray location are determined by the foregoing method. 

Distillation towers feed-tray location for, 10 optimum reflux ratio for, 371-376 specifications for, 16 (See also Bubble-cap contactors, Packed towers. Sieve trays, and Valve trays) Distribution costs, 194, 196, 207, 211 Distribution in statistical analyses, 745-746 Dividends, tax exemptions for, 259 Documentation, 137-149,452-476 Double-entry bookkeeping, 143-144 Downcomers in tray columns, 684-686 Drives, cost of 532-533 Dryers, cost of 713-716... 

Federal environment regulations, 75-78 Feed-tray location in distillation towers, lo Fiberglass reinforced plastics (FRP), 436-437 Fibonacci search, 407 Fifo method for materials accounting, 148... 

Once one has proposed alternative configurations for systems of separation devices to effect a desired separation, one must then design these devices so the various alternatives may be compared. For a distillation column, the first set of design decisions is to choose the number of trays, the feed tray location, and the reflux ratio at which to operate it. For a binary separation, the McCabe-Thiele diagram (or the concepts behind it) is an indispensable aid in making these decisions. 

Underwood-GUliland Method) 395 Minimum Trays 395 Distribution of Nonkeys 395 Minimum Reflux 397 Operating Reflux 397 Actual Number of Theoretical Trays 397 Feed Tray Location 397... 

To redistribute the stages in the remaining sections, a shortcut simulation is used to find out the required number of trays, the feed tray location and the minimum reflux ratio for each column in the sequence. To make use of the existing column with the same number of trays (24 trays) iterations are required to adjust the sum of the rectifying sections in each column equal to 24 (number of trays in the main column). Finally, the sequence of the simple columns is merged into a complex column. The main column is not changed, but the side strippers and pump arounds need to be relocated or adjusted. 

Once the operating lines are plotted, the equilibrium stages are stepped off between the operating line and equilibrium curve as described in Section 5.1.3. The optimum feed tray location is at the step that crosses from one operating line to the other over their intersection point. If the number of stages is fixed, the operating lines are determined by trial and error to match the specified number of stages. 

FIGURE 5.12 Effect of feed tray location on the separation. 

If the feed is placed on the fourth tray from the top, there would be only four rectifying trays and, therefore, a higher reflux ratio, calculated at 2.0, would be required to obtain the same rectification. The stripping section becomes over-trayed, with several trays showing little change in the separation parameter. If the feed tray is too low, the rectifying section becomes over-trayed. Figure 7.11 is a schematic plot (not to scale) of the separation parameters for the different feed tray locations. 

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