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I notice that Graham condensers tend to come in three main sizes: 200mm, 300mm and 400mm. What are the relative merits of each size?
My typical application is vacuum distillation of organic solvents like ethanol and acetone with the goal of a single pass distillation.
A longer condenser presents a greater surface area and longer path length that vapors must get past in order to escape into the atmosphere. A short condenser will work fine if you are refluxing or distilling a high-boiling liquid, or if you are not concerned about losing some of the material as vapor. A longer condenser is preferable for a more volatile liquid. Vacuum distillation of an already low boiling liquid like ethanol or acetone (why use vacuum?) would likely require a long condenser.
You can get better efficiency from any condenser if you use a colder circulating liquid in the jacket (such as ethylene glycol with a circulating chiller). Alternatively, consider using a cold-finger condenser with a dry ice-acetone mix as the coolant.
Vacuum distillation of ethanol/acetone isn't wise. Setup time is longer, changing flasks is more difficult, the apparatus is bulkier and a lot of ethanol will go to the pump. Try normal pressure distillation with an efficient distillation column.
Longer condenser means: less material will fly through, but more material will sit on condenser. For large scale distillation use larger condensers. For small scale - use shorter one.
Vacuum distillation makes sense for liquids like DMSO with boiling point much higher than 100C.
