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I think I might have an answer to your question. I started a topic to answer it and joined this forum as well to that end. I am now a retired ME but early in my carrer I was the engineering and maintenance manager for two large processing facilities. We required 8.7 megawatts to run these factories simultaneously but our steam driven generator was only capable of making 5.2 megawatts as I recall. I assume that you are using a barometric condenser to pull your vacuum. What I think is happening is that because you have an open system, you are pulling more heat away than is possibly necessary. Also, once you have established your boiling point, energy is being removed from the boil thus requiring more energy input. When your solvent or ethyl alcohol is condensed, it indicates an open system. You might want to look up designing a barometric condenser. Later in my carrer, I was in upper management. When I interviewed some engineers for a Vice Presidents job I would ask them why specific volume was important to have in the steam tables by Keenan and Keys provided my spelling of their names is correct the steam tables are amazing pieces of work. i hope this helps to answer your question.

I was very recently reading a question regarding the extraction of solvents using negative pressure and why more heat was neccesary. If I’m correct regarding the question, I may have an answer. I’m now a retired mechanical engineer but early in my career I was the chief engineer in two large co-gen processing facilities. I assume that you are using a barometric condenser to establish the vacuum. What can happen is that the vacuum itself will pull more energy than what is required to maintain the lowered boiling point. Hence, more energy is required to maintain the boiling point. If you were dealing with a closed system, this phenomenon wouldn’t be a problem but that is not your case. When the solvent is condensed, it then enters an open system. i hope that I have the question and set-up correct and that whomever posted it gets this response.