For your fractionating column, how about making it using two pieces of pipe, one that fits snugly inside the other, and about 10 washers that fit snugly inside the larger diameter pipe? you can take the small pipe and cut it into 2.5" lengths, and then stack them in the larger pipe, alternating pipe, washer, pipe, washer, etc. To make the washers drip properly, you canpress them into a convex shape before you slide them in. Once you have a column of your desired length, weld the inside pipes firmly in place.
As far as the gasket goes, teflon isn't going to give you the amount of compression you are going to need. I would say buy a sheet of 5/16" rubber gasket material and cut out several O-rings. Replace them periodically. Also, make sure to tighten your bolts down in a criss-cross pattern, or else you will never get a good seal.
Ideally, to get the best seal, one that will seal tighter when you apply vacuum, would be to use a countersunk design like so:
column
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to flask:
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