18 ◾  Distillation Control, Optimization, and Tuning
reflux ratio. A design of 4.0 times the minimum number of
theoretical stages may use only 1.01 to 1.10 times the minimum reflux ratio.
3.3 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% n-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