Overhead product composition

For a binary, let s denote as V the fractional molar split of the feed into overhead product and as L the fractional split into bottom product. Calculate compositions of the flash separation of feed into vapor v and liquid 1 to give v/1 = V/L. The resulting vapor can be regarded as being composed of a portion d of the overhead product composition and a portion r of the flash liquid composition. 

A distillation column uses a partial condenser as shown in Figure 9.19. Assume that the reflux ratio and the overhead product composition and flowrate and the operating pressure are known and that the behavior of the liquid and vapor phases in the column is ideal (i.e. Raoult s Law holds). How can the flowrate and composition of the vapor feed to the condenser and its liquid products be estimated, given the vapor pressure data for the pure components. Set up the equations that need to be solved. 

The tray temperatures in our preflash tower, shown in Fig. 4.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows down the column. The greater the temperature drop per tray, the greater the evaporation of internal reflux. It is not unusual for 80 to 90 percent of the reflux to evaporate between the top and bottom trays in the absorption section of many towers. We say that the lower trays, in the absorption section of such a tower, are drying out. The separation efficiency of trays operating with extremely low liquid flows over their weirs will be very low. This problem is commonly encountered for towers with low reflux ratios, and a multicomponent overhead product composition. 

A typical arrangement is illustrated in Fig. 7.68 where the variations in feed composition are measured by a suitable composition analyser (An) (see Section 6.8). The signal from the analyser is fed directly to the feed-forward controller, the output of which is cascaded on to the set point of the reflux flow controller (see also Section 7.13). If the transfer functions relating feed composition Xp, reflux flowrate R and overhead product composition xD are known, then we can write ... 

The two possible control configurations for a system with two inputs and two outputs are shown in Fig. 7.77. One example of this is illustrated in Fig. 7.73 where the overhead and bottoms product compositions of a distillation process are controlled using the reflux and steam-to-reboiler flowrates respectively as the manipulated variables. Theoretically, we could employ the reflux flowrate to control the bottoms product composition and the steam-to-reboiler flowrate to control the overhead product composition. It is possible to determine which configuration produces the least interaction by forming the system relative gain array A, where ... 

Figure 13 shows a McCabe-Thiele diagram, which can be used when the mixture to be distilled consists of only two components or can be represented by two components. Starting at the required overhead product composition x/), an upper-section operating line is drawn hav-... 

Adjust the reflux ratio so as to maintain the required overhead-product composition. It is... 

F igure 13-33 also shows the intersections of the operating lines with the diagonal construction line. The top operating line must always intersect the diagonal at the overhead-product composition Xp. This... 

Batch Distillation With Fractionation li-ays—Constant Overhead Product Composition, Multicomponent and Binary... 

In the second variation, the reflux ratio is continuously adjusted to maintain a constant overhead product composition. This variation is preferred for fractional separation of binary mixtures. 

In many cases, shortcut calculations can fill in the gaps. An example used in Kenney s book (Kenny, 1984) gives good illustration for how to do it Consider the tower in Figure 13.2. As for many plants, cooling water rates are not measured and overhead product comes off on level control. However, since feed rate and composition and overhead product composition are known, much of the missing data can be derived by energy and mass balances. 

The concentration profile of the rectifying section under reflux R and an overhead product composition Xi d will be represented by broken lines, the lengths of which come through points Xij, which are found by means of solving Eqs. (2.3) and (2.5). 

Adjust the reflux ratio so as to maintain the required overhead-product composition. It is necessary to raise the reflux ratio in order to keep the concentration of dye intermediate in the overhead product high enough. In this case, assume that an additional 1400 lb intermediate can be recovered in the remaining 3 h while maintaining the original vapor rate of 4200 Ib/h. Then D for this latter 3 h is 1400/3 = 467 Ib/h L is (4200 — 467) = 3733 Ib/h and the reflux ratio L/D for this portion of operation must be 3733/467 = 8. 

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