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Solving Problems with Vacuum Distillation Systems

Q: In our vacuum distillation system, we are getting 0.4 Torr vacuum while in reactor we are getting 8 Torr. Our client requirement is 1 Torr. Do you think there is any problem from the vacuum system side?

System volume is 5,000 liters; air leakage is 1kg/hour; and carry over load of 1kg/hr (MW 137). Vacuum system of 570 ACFM (Mech booster backed by Dry Vacuum pump)supplied. We have 4-inch pipe line.

A:

This is a bit out of my area, but here are my thoughts:

1.) If I did the calculation correct, I calculate the unit is generating 1,108 ACFM (air + carry-over) at 60F and 0.4 Torr.  (This ACFM calculation needs confirmed.)  The calculated ACFM is greater than the stated capacity of the vacuum system given as 570 ACFM.  If the temperature is lower than 60 F, the volume would be less but I doubt it is low enough to reduce the ACFM to half and if the temperature is greater than 60 F the ACFM would be higher.  If the gas/vapor load on the vacuum system is not the problem or the whole problem, and it may not be since the pressure is 0.4 Torr at the pump, then in addition to vacuum system load evaluation the other equipment in the system needs its pressure drop evaluated.

2.) I don't know the equipment arrangement, but if I assume that the vacuum is being drawn through the overhead condenser and that the distillation column is between the reactor and vacuum system then the pressure drop of various components need to be evaluated.   Some of the components may be a packed or trayed tower, overhead condenser and the connecting piping.

If you already considered these possibilities, you may want to contact a distillation consultant. 

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