Reflux minimum plates

Fig. 2.3.2-9 Balance lines and staircase construction, a) Minimum reflux ratio, infinite plate number, b) Finite reflux ratio, resp. finite plate number, c) Total reflux, minimum plate number.

For an overview approximation and dimensioning, the relationships of Fenske and Underwood can be used to determine the minimum plate number N j and the minimum reflux ratio (Equations 2.3.2-26 and 2.3.2-27) ... 

Economic designs result for plate numbers that are about 1.3 times the minimum plate number or 1.2-1.3 times the minimum thermal power. With regard to controllability and the minimum trickle density, the reflux ratio should not be less than about 0.3-0.5. 

Find minimum plates, minimum reflux, and ideal stages at 1.2 times minimum reflux. 

In general the same type of information given by the constant 0/V method can be obtained by the use of the Ponchon and Savarit method. For example, the cases of total reflux, minimum reflux ratio, and optimum feed-plate location can be easily solved. 

Thus, one has only to compute these two extreme conditions and by working with the ratios of actual vapor (F) to minimum vapor (F ) and the corresponding ratio for number of plates (Table 16-11), the entire economic range of reflux and plates can be explored in a few hours. Bach system behaves a little differently, but Table 16-11 enables the establishment... 

Favorable Vapoi Liquid Equilibria. The suitabiHty of distiUation as a separation method is strongly dependent on favorable vapor—Hquid equiHbria. The absolute value of the key relative volatiHties direcdy determines the ease and economics of a distillation. The energy requirements and the number of plates required for any given separation increase rapidly as the relative volatiHty becomes lower and approaches unity. For example given an ideal binary mixture having a 50 mol % feed and a distillate and bottoms requirement of 99.8% purity each, the minimum reflux and minimum number of theoretical plates for assumed relative volatiHties of 1.1,1.5, and 4, are... 

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

Gilliland" tells how his famous correlation was developed for relating actual and minimum reflux to actual and minimum theoretical stages for a fractionating column. Numerous plate-to-plate calculations were made and the results plotted using his well-known correlating parameters. The best curve was then drawn through points. 

To obtain a low flash zone pressure, the number of plates in the upper section of the vacuum pipe still is reduced to the minimum necessary to provide adequate heat transfer for condensing the distillate with the pumparound streams. A section of plates is included just above the flash zone. Here the vapors rising from the flash zone are contacted with reflux from the product drawoff plate. This part of the tower, called the wash section, serves to remove droplets of pitch entrained in the flash zone and also provides a moderate amount of fractionation. The flash zone operates at an absolute pressure of 60-90 mm Hg. 

The conditions of total liquid reflux in a column also represent the minimum number of plates required for a given separation. Under such conditions the column has zero production of product, and infinite heat requirements, and Lj/Vs = 1.0 as shown in Figure 8-15. This is the limiting condition for the number of trays and is a convenient measure of the complexity or difficulty of separation. 

Figure 8-17. Minimum reflux at infinite theoretical plates. Used by permission. The American Chemical Society, Smoker, E. H., Ind. Eng. Chem V. 34 (1942), p. 510, all rights reserved.

This graphical representation is easier to use for nonideal systems than the calculation method. This is another limiting condition for column operation, i.e., below this ratio the specified separation cannot be made even with infinite plates. This minimum reflux ratio can be determined graphically from Figure 8-23, as the line with smallest slope from xp intersecting the equilibrium line at the same point as the q line for mixture following Raoul t s Law. 

Example 8-9 Using Figure 8-24B to Solve Gilliland s Equation for Determining Minimum Theoretical Plates for Setting Actual Reflux (used by permission [122])... 

If the minimum reflux ratio is 2.0 and the minimum number of theoretical plates is 20, how many plates will be required if a reflux ratio 1.5 times the minimum is used ... 

Smin = minimum number of theoretical plates R = any reflux ratio Rmin = minimum reflux ratio... 

Using the same operating reflux (same fraction times the minimum) as was used in Example 8-10, calculate the theoretical plates required for feed of the following thermal conditions Use Figure 8-27. 

Example 8-12 Minimum Theoretical Trays/Plates/Stages at Total Reflux... 

Assume actual reflux ratios of 1.2, 1.8, 2.25, 3.0 times the minimum and plot the effect on theoretical plates using Gilliland plot... 

Using Figure 8-33 the separation from Xq, initial kettle volatile material to X3 as the distillate of more volatile overhead requires three theoretical plates/stages at total reflux. Using finite reflux R4, and four theoretical plates the same separation can be achieved with infinite theoretical plates and the minimum reflux ratio, Rmin- The values of reflux ratio, R, can be determined from the graph with the operating line equation as,... 

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