Feed plate

As fibers in the feed mat pass between the feed toU and feed plate, they ate separated by metallic wine teeth on the lickerin toU and carried to an air venturi where they ate stripped and tumbled until they strike a moving, perforated collection surface. At the collection surface, the airborne fibers foUow paths of least resistance and accumulate in a self-leveling manner while the air passes through perforations. Fiber orientation in the web is isotropic in layers corresponding to the number of fibers transferred from the wine teeth to the air-transportation 2one, the intensity of the air, and the speed of the collection surface. 

Condition of Feed (q Fine). The q line, which marks the transition from rectifying to stripping operating lines, is determined by mass and enthalpy balances around the feed plate. These balances are detailed in distillation texts (15). 

FIG. 13-29 Material-balance envelope which contains two external streams D and S, where S represents a sidestream product withdrawn above the feed plate,... 

Example 3 Calculation of TG Method The TG method will he demonstrated hy using the same example problem that was used above for the approximate methods. The example column was analyzed previously and found to have C -I- 2N + 9 design variables. The specifications to be used in this example were also hstedat that time and included the total number of stages (N = 10), the feed-plate location (M = 5), the reflux temperature (corresponding to saturated liquid), the distillate rate (D = 48.9), and the top vapor rate (V = 175). As before, the pressure is uniform at 827 kPa (120 psia), but a pressure gradient could be easily handled if desired. 

Stripping or desorption is the transfer of gas, dissolved in a liquid, into a gas stream. The term is also applied to that section of a Fractionating column below the feed plate. 

The slope of a line from the intersection point of the feed composition, xp with the 45° line on Figure 8-2 is given by q/(q - 1) = - q/(l - q). Physically this gives a good approximation of the mols of saturated liquid that will form on the feed plate by the introduction of the feed, keeping in mind that tmder some thermal conditions the feed may vaporize liquid on the feed plate rather than condense any. 

Note This is not too accurate due to switched operating line equations before the feed compositions were reached, yet, one more calculation on the stripping line would have placed us below the feed plate composition. Hence a change in reflux ratio is necessary in order to split right at the feed composition. 

Now substitute this value x = 0.88 into the equation and calculate the vapor coming up from the first plate below the top (t - 1). Thus, if x = top plate, yn + i = vapor from plate below top. Now, read equilibrium curve aty(j i) and get X(n +1) or Xt 1 which is liquid on plate below top. Then using Xf 1, calculate yt- 2 (second plate below top, etc.). Then, read equilibrium curve to get corresponding liquid X( 2- Continue until feed plate composition is reached, then switch to equation of stripping section and continue as before until desired bottoms composition is reached. Operating line of stripping section ... 

Use this equation as described above following down from the feed plate cross-over from the rectifying equation to the stripping equation. 

This represents no change in overflow from the feed plate, and the increase in vapor flow is equal to the mols of feed. 

This represents feed as a superheated vapor, and there is a decrease in liquid overflow from feed plate. 

Binary system pinch zone adjacent to feed plate... 

Calculate estimated ratio of key components on feed plate based on the liquid portion of the feed. 

The feed plate location, for either rectifying or stripping sections ... 

The approximate location can be determined by the ratio of the total number of theoretical stages above and below the feed plate from the Fenske total reflux relation ... 

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. 

Hengstebeck, R J., Finding Feed Plates from Plots, Chem. w -.july 29 (1968), p. 143. 

The relationships for the section of column above the feed plate, i.e., the enriching section of the column, are exactly the same as those derived previously for the case of the batch distillation column. 

The mass balance relationships for the feed plate, the plates in the stripping section, of the column and for the reboiler must, however, be modified, owing to the continuous feed to the column and the continuous withdrawal of bottom product from the reboiler. The feed is defined by its mass flow rate, F, its composition xp and the thermal quality or q-factor, q. The column bottom product is defined by its mass flow rate, W, and composition, xw and is controlled to maintain constant liquid level in the reboiler. 

Assuming constant liquid holdup on the plate, which is equivalent to neglecting the plate hydraulic variations, a component balance for the feed plate is given by... 

Figure 5.227. The feed plate showing the streams and components i.

Where a side-stream is withdrawn from the column the plate design must be modified to provide a liquid seal at the take-off pipe. A typical design is shown in Figure 11.25a. When the feed stream is liquid it will be normally introduced into the downcomer leading to the feed plate, and the plate spacing increased at this point Figure 11.25A... 

Assuming that the mixture is fed to the column as a boiling liquid and using a reflux ratio of twice the minimum value possible, determine the location of the feed plate and the rate at which the mixture can be separated. 

This column contains the equivalent of (10 x 0.5), that is 5 theoretical plates, so that these may be stepped off from the point (xd, xd) to give the feed plate as the third from the top. 

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