I understand that there are many different technologies out there for the purification of ethanol/water mixtures from 95% to >99.5%. I most recently worked with one of our clients on a molecular sieve skid and back in my college days did a project comparing extractive distillation to pervaporation. I know people have used low pressure (vacuum) distillation for this process, but it is not commonly used due to energy requirements (to pull the vacuum and to sub-cool the condenser water). That being said, even though it may not be the best technology, I am investigating the feasibility of using vacuum distillation to purify an ethanol/water mixture.
A little background: A 10wt% ethanol water mixture will be feed to an atmospheric column purifying the stream to ~95wt%. This stream will enter the vacuum distillation column and be separated to 99+%. I’m looking to do this on a relatively small scale continuous process (5 gpm feed rate). Everything was going along smoothly until I got to designing the vacuum distillation column. I was under the impression that the azeotrope shifts to the right on a VLE diagram (towards 100%) as the overall pressure decreases. However, this was not the case for the many different sources/calculators/correlations I found on the internet. Many of them showed a negative dip (below the equilibrium line), but the azeotrope stayed at ~95.5wt% in all case.
My main questions: Am I just finding bad VLE data? Or does this separation require a “pressure swing” where the concentration would “jump” over the azeotrope and I would then separate the water from ethanol? From my literature search it looks this purification has been done before with just standard vacuum distillation.
Any help or direction would be extremely appreciated. And don’t hesitate to ask questions if I’ve left out any data.