Number of Stages versus Reflux Ratio

If the reflux ratio is increased, the slope of the rectifying section operating line increases, the operating lines move farther away from the equilibrium curve, and 

If the number of stages is fixed, the problem is to calculate the reflux ratio required to meet the specified separation. In this situation, two cases may arise—the feed location is fixed or is to be determined. 

The H-X diagram technique for determining the number of stages and optimum feed location for a specified separation was described in Section 5.3.3. It was implied that the reflux ratio was also specified since the condenser and reboiler duties were given. If the number of stages is specified, but the condenser and reboiler duties are allowed to vary, it becomes necessary to repeat the above procedure with different pivot points (corresponding to different condenser and reboiler duties) until the number of stages is matched. 

EXAMPLE 6.3 BENZENE-TOLUENE COLUMN—SEPARATION SPECIFIED 

It is required to separate the benzene-toluene mixture of Example 6.1 into a benzene-rich distillate with 0.80 mole fraction benzene and a toluene-rich bottoms with 0.05 mole fraction benzene. The separation is to be made using a distillation column with 15 theoretical stages that include a partial condenser and a partial reboiler. Calculate the reflux ratio required to achieve the specified separation and determine the optimum feed location. What effect would lowering the number of stages to ten have on the reflux ratio and the optimum feed location  

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