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[Niru1234#]

I have a question about the material balance of a distillation column. When I used radfrac distillation model to simulate the distillation of a ternary mixture using three columns. The first column gives the top product having benzene and 50% recovery of toluene and the bottoms containing 50% toluene and xylene. The two of them act as feeds to two other distillation columns giving the products benzene, toluene and xylene ( recovery of 99 %) . I got the reflux rate as 149 lbmol/hr and boil-up rate as 263 lbmol/hr. The feed flow rate is 150 lbmol/hr. I mean that sounds unconvincing because the feed itself is only 150 lbmol/hr and we get reflux rate and boil-up rate as these values. What happens during the start-up of a distllation column? Don't these relux rate and boil-up affect the steady state material balance of the column?

[Pilesar]

Reflux and boilup can be larger than the feed. Draw a material balance envelope around the column and the only streams intersecting the envelope are the feed and product streams. The required reflux and reboil are linked in that they increase or decrease together depending on the number of distillation stages and the product compositions . With 150 lbmol/hr liquid from the feed and 149 lbmol/hr liquid from the reflux, the boilup must be less than 300 lbmol/hr. As the reflux rate increases, then more liquid must be boiled up from the bottom of the column to maintain the same product flow rate.

 

So what happens to the column operation when there is no feed flow? As long as there is material in the column and no product flow, the column operates just fine! If the column has a total condenser, at startup the column may be brought up to near normal operating conditions without sending any product forward. Then the product is allowed to leave the column to make room for more feed. The feed rate does not in itself tell you anything about the rate of reflux or the size of the column. All the feed rate tells you is the total product rate at steady state.