Question

Is there a minimum reflux ratio for the batch distillation? If so, how can we determine it experimentally? Please help me with this. I'm not sure since batch operates like one stage but the slope of the rectifying section is the same? Is it the same way as finding Rmin for Continuous distillation? Thanks!

Answer 1

To get constant product composition from top, choose the top product composition, xd, and the still composition xo. Get the x-y graph.

With available Number of stages N, Choose an upper reflux ratio.

Draw another operating line using the same xd. Step off N stages to give xb, the final still composition. The still composition is changing – less and less of the product. R is increased with time but no. of stages is fixed so distillation must be stopped when bottoms composition drops to a minimum value  

An example graph is attached below

We get Reflux ratio range.

Since batch distillation is unsteady state process. Approach is

1)First Provide complete reflux till the column temperature profile reaches a steady state

2)Give Upper reflux ratio and collect product in distillate till Column Profile is not disturbing

3)After Temperature Profile in the column gets disturbed, again put column on complete reflux till temperature Profile again reaches steady state

4)Again give reflux ratio somewhat lesser than previous one and collect profile till Column Profile is not disturbing

5) Continue follow steps 2 to 4 till bottom Product composition reaches minimum value