NRB: Recycle Example

Recycle Example

Benzene and Toluene are to be separated in a distillation column. The feed is composed of a mixture of the two with 50 wt% benzene and a total flow of 160kg/hr. The material taken off from the "bottoms" is found to flow at a total rate of 60kg/hr and contains 50kg/hr benzene and 10kg/hr toluene. The "tops" has a total flow of 100kg/hr and a composition of 0.3 mass fraction of benzene.

The "new guy" on the job has gotten the analysis this far (she wrote-up the flow chart), but for scale up purposes you need to know the reflux ratio in the column (the ratio of the mass recycled from the condenser (R) to the mass collected in the "tops"). Unfortunately the only measurement that you can easily make is that the flow rate into the condenser is 150kg/hr. You tell the young ChE not to fear because you can take it from there. So.... take it from there!

If the condensation is complete (in other words, the condensed liquid has the same composition as the incoming gas), can you determine the composition in the recycle (reflux) stream?


How many Unknowns are there?
How many different Species (material balances) are there?
How many Process Specifications are we given?
How many Physical Constraints are there?
The total number of Degrees of Freedom is