Overall Infinite Reflux

FIGURE 7.6 Superimposed CPMs with die associated region of double int sections widi Xa =Xac= [0.201, 0.031], with Rac(red) and Ras= 10(blue) with a = [5,1,2]. 

FIGURE 7.7 A possible design for the infinite reflux Petlyuk in Hgure 7.2 colunui (with CS and CSc at finite reflux) achieving 90% intermediate boiling component in the sidestream. 

This section has thus presented a quick synthesis and analysis method for two simplified infinite reflux cases. Just like simple columns, it can be generally stated that if a design is considered feasible at infinite reflux conditions, then a feasible design can be found at finite reflux too. This fact is particularly useful for nonideal systems. An illustration of a more complex infinite Petlyuk column example is given in the following example for the azeotropic acetone/benzene/chloroform system. 

Example 7 1 Determine whether it is feasible to obtain a sidestream product of 90% chloroform for the azeotropic acetone/benzene/chloroform system in an infinite reflux Petlyuk column with / ac= = 

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This effect is best explained by a simple illustration. Suppose we feed a column with 50 mol/h of A and 50 moVh of B, and A is the more volatile component. Suppose the distillate contains 49 mol/h of A and 1 mol of B, and the bottoms contains 1 mol/h of A and 49 mol/h of B, Thus the distillate flowrate is D = 50 mol/h and the purity of the distillate is xDA = 0.98. Now we attempt to fix the distillate flowrate at 50 mol/h and also hold the distillate composition at 0.98 mole fraction A. Suppose the feed composition changes to 40 mol/h of A and 60 mol/ h of B. The distillate will now contain almost all of the A in the feed (40 mol/h), but the rest of it (10 mol/h) must be components. Therefore the purity of the distillate can never be greater than xD A = 40/50 = 0.80 mole fraction A. The overall component balance makes it impossible to maintain the desired distillate composition of 0.98. We can go to infinite reflux ratio and add an infinite number of trays, and distillate composition will never be better than 0.80. 

The second inhnite reflux condition in the Petlyuk column is where CSj and CSe operate at infinite reflux, but we do not specify that CS2-5 operate under these conditions, that is, the vapor and liquid flowrates in 82-5 are finite values and are not equal to each other. Since the overall reflux is infinite, there is still no effect from feed addition or product removal on the column. Therefore, the CS breakdown for this structure is equivalent to the one shown in Figure 7.2. We can again apply the mass balance around the thermally coupled junction at the top of the column as in Equation 7.4. In this case, we again have the condition that Va = La and x = y , but in this case Vb 7 Lb, Vc Lc- Under these conditions, we find that Equation 7.4 reduces to... 

If this process is carried out in a distillation column, the minimum energy required may be determined from the heat Qk supplied in the reboiler/gmol of feed at Tg if we may assume that the total heat supplied at the reboiler is equal to that withdrawn in the condenser (i.e. Qc) at Tc-Further, this minimum will occur at the minimum reflux ratio, which means that there will be an infinite number of plates. Following Humphrey and Keller (1997), we aissume the fallowing complete separation of feed into two pure products constant relative volatility i2 constant molar overflow feed at bubble point minimum reflux ratio single reboiler and condenser liquid feed at bubble point. Consider now the distillation column shown in Figure 10.1.5(a). The overall and component material balance equations are ... 

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