Separation Power and Energy Consumption

For the design of a distillation column, there is an economic trade-off between the use of an extremely tall distillation column that runs with minimum energy consumption and a shorter column that requires higher energy consumption. The energy consumption is for heat to the reboiler and cooling for the condenser. 

The separation of a feed mixture containing 50 wt% cyclohexane and 50 wt% w-heptane into a distillate of 99.9% cyclohexane and a bottoms of 0.1% cyclohexane requires a key impurity separation power of 1 million by Equation 3.4. With an infinite number of theoretical stages, a minimum reflux ratio of 2.40 is required to meet these product specifications. That is the lowest energy consumption possible for the desired separation by distillation. 

The highest energy consumption is at total reflux, that is, when all of the boilup is returned as reflux, and there is no feed to the column and no distillate or bottoms. A computer simulation for this separation at total reflux required a minimum number of 27.8 theoretical stages. This was accomplished by setting the reflux/distillate ratio to 10 million in a computer simulation. This is the shortest column that can achieve the desired separation. 

The desired separation can be achieved with 50 theoretical stages using a reflux ratio of 3.22, which is 1.34 times the minimum reflux ratio. This would meet upper specification limits (USLs) of 1,000 ppm n-heptane in the distillate and 1,000 ppm cyclohexane in the bottoms. However, the average key impurity concentrations in the products need to run with average impurity concentrations that are some distance below 

The distance between the average key impurity concentration and the upper spec limit is the subject of product quality performance in Chapter 7. If the standard deviation (o) is 10% of the upper specification limit, then the standard deviation for an upper specification limit of 1,000 ppm impurity is 100 ppm. A distillation column would need to produce an average key impurity concentration of 700 ppm impurity to keep the average DNS (distance from the nearest specification limit) at 3a. 

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