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distillation at low temperatures 1
- Thread starter Dahb
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Theoretically, yes. Distillation is done lots of time at temperatures below 0C.
There are a few things you'll need to think about. I'm assuming you are talking about relatively a high purity split between the i-C4 and the C3 in the following since you're pretty short on details:
If the -1C is in the bottom of the column, you are talking about 7 or 8 psig operating pressure. Low operating pressures increase the selectivity of the two components (good) but means larger diameter columns to handle the vapor flow rate (bad). That also implies a condensing temperature of your C3 overhead product of about -33C so you need likely need a propane refrigeration compressor loop to give those temperatures.
If the -1C is in the overhead, your column pressure would be about 55 psig. You still need a refrigeration source for this temperature though but not as low. The bottoms would be close to 35C.
In both cases, you want the feed to be dry (free of water) to avoid freezing problems. You might not have a problem with hydrates because of the low pressures but you'd need to check that.
Conventionally, you run higher pressures for this separation. Typically, your column pressure is set by your condensing medium, usually cooling water in this case. Cooling water is available at about 70F, assuming a 10F approach in a shell and tube exchanger that gives a condensing temperature of 80F or about 130 psig operating. The bottoms temperature for the i-C4 product then sets your reboiling medium, commonly low pressure steam.
There are a few things you'll need to think about. I'm assuming you are talking about relatively a high purity split between the i-C4 and the C3 in the following since you're pretty short on details:
If the -1C is in the bottom of the column, you are talking about 7 or 8 psig operating pressure. Low operating pressures increase the selectivity of the two components (good) but means larger diameter columns to handle the vapor flow rate (bad). That also implies a condensing temperature of your C3 overhead product of about -33C so you need likely need a propane refrigeration compressor loop to give those temperatures.
If the -1C is in the overhead, your column pressure would be about 55 psig. You still need a refrigeration source for this temperature though but not as low. The bottoms would be close to 35C.
In both cases, you want the feed to be dry (free of water) to avoid freezing problems. You might not have a problem with hydrates because of the low pressures but you'd need to check that.
Conventionally, you run higher pressures for this separation. Typically, your column pressure is set by your condensing medium, usually cooling water in this case. Cooling water is available at about 70F, assuming a 10F approach in a shell and tube exchanger that gives a condensing temperature of 80F or about 130 psig operating. The bottoms temperature for the i-C4 product then sets your reboiling medium, commonly low pressure steam.
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