Location of Feed Tray

Two different relative volatility rankings for a ternary decomposition reaction were explored in this chapter. The behavior of systems with an intermediate-boiling reactant is similar to the quaternary system with intermediate-boiling reactants. Systems with heavy reactant are quite different. Two possible flowsheets (two-column and one-column configurations) were discussed. The two-column flowsheet is workable for all positive chemical equilibrium constants. The one-column configuration is feasible for systems with higher chemical equilibrium constants. Economical comparisons of these two flowsheets were also given. 

STEADY-STATE DESIGN OF REAL CHEMICAL SYSTEMS 

---

Large amounts of lost work due to mixing and mass transfer mainly occur around the feed trays. The mixing may take place between streams with widely different compositions. From the thermodynamic perspective, we may adjust the location of feed tray to counterbalance the lost work. 

FIGURE 5.12 Effect of feed tray location on the separation. 

Figure 11.4-8. Method of stepping off number of theoretical trays and location offeed plate (a) improper location of feed on tray 4, (b) proper location of feed on tray 2 to give minimum number of steps.

Next, shortcut methods are applied to find the minimum number of trays (Fenske equation) for distillation columns, locate the feed tray location (Kirkbride equation), and size the... 

Effect of Feed Tray Location on Reboiler Duty and Ethane Recovery (Example 9.1)... 

When both streams are fed to the same column, the above considerations still apply, but their relative importance may vary. If one of the streams is predicted to have a higher optimum feed tray than the other on account of both its composition and thermal conditions, then the optimum feed trays in the two-feed column would correspond to the same relative locations as with the streams fed separately. However, if the two factors are contradictory, as in this example, the optimum feed locations in the two-feed column may or may not correspond to the relative single-feed optimum locations. In the two-feed toluene-xylene column, the reboUer duty is minimized when feed 1 is higher than feed 2, which is contrary to the single-feed optimum feed trays. With the two-feed column, the compositional effect on the feed location outweighs the phase effect Table 9.11 lists reboiler duties for several combinations of feed trays. The optimum corresponds to feed 1 on tray 15 and feed 2 on tray 22. 

Make a McCabe-Thiele analysis for the design separation to check the number of trays required and to locate the feed tray. 

Estimate the number of theoretical stages required at the operating reflux ratio. Applying a suitable tray efficiency, set the number of actual trays to be provided. Make a McCabe-Thiele analysis to check the number of theoretical stages and to locate the feed tray. 

The lower right graph in Figure 6.19 shows that there is an optimal feed tray location. Figure 6.21 shows the effect of feed tray locations on temperature and composition profiles. This optimum location is the top of the reactive section. Above this point, the effect of the rectifying section to keep the reactant in the reactive sections is lessened, and below this point there is an effect similar to reducing the number of reactive trays. Either position will increase the vapor boHup. 

EFFECTS OF FEED TRAY LOCATIONS ON DESIGN AND CONTROL OF REACTIVE DISTILLATION... 

For the rectifying tray number Nr we use the liquid composition right above the reactive tray for xr, and for Ns we use the vapor phase composition right below the reactive zone as xd- This leaves us with the feed tray locations as the design variables. Because all tray numbers are determined, the effects of feed tray locations can be compared by simply looking at the energy consumption (i.e., vapor boilup rate). 

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. 

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

Establish the location of the feed, bottoms and overhead compositions on the graph. Draw in the operating lines as for a distillation in a tray column. 

Devise a superstructure for a distillation design involving a single feed, two products, a reboiler and a condenser that will allow the number of plates in the column itself to be varied between 3 and 10 and at the same time vary the location of the feed tray. 

Example The location of the best temperature-control tray in a distillation column is a popular subject in the process-control literature. Ideally, the best location for controlling distillate composition xa with reflux flow by using a tray temperature would be at the top of the column for a binary system. See Fig. 8.9o. This is desirable dynamically because it keeps the measurement lags as small as possible. It is also desirable from a steadystate standpoint because it keeps the distillate composition constant at steadystate in a constant pressure, binary system. Holding a temperature on a tray farther down in the column does not guarantee that x will be constant, particularly when feed composition changes occur. 

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... 

---