2 replies to this topic

[bitted]

I am a chemical engineer student. I must design a bio-diesel production process for my last year design course. I am using Aspen Plus.

The methanol coming out of the reactor is recovered using a distillation column (RadFrac) and recycled back to the reactor. Every process and simulation in the literature that i know of, is using a vacuum distillation. This is being done in order to keep the temperature in the reboiler below 150 degrees Celsius (this is the thermal decomposition temperature of glycerol). However, by manipulating the distillate rate and holding everything else constant, i have managed to keep the temperature in the reboiler below the 150 degrees limit. My question is this:

If it is so simple to keep the pressure at 1 bar why is everyone using vacuum conditions? Is there any variable that's changing, besides the temperature, that i am not taking into account? (i have checked and haven't found any). Is it just a numerical result without practical significance?

Edited by bitted, 27 October 2011 - 04:42 PM.

[Stubed]

I would suggest that it is sensible to keep the pressure below atmospheric in order to maintain the integrity of the system. Any small leaks will encourage air ingress rather than allow the methonal vapour to escape. I don't know whether there are any benefits to the quality of the product at reduced temperature though - wait until someone else posts for that response!

Why are you operating so close to the glycerol decomposition temperature anyhow? Methonal boils at around 65oC I think? Therefore surely something much less than 150oC would be sufficient?

On a side note, what is your knowledge on use of ethanol in bio-diesel production. It's much safer so would be my preference but I don't know whether it's more difficult to work with or produces a lower quality bio?

[hsfirmase]

Methanol distillation under vacuum condition is an open choice. You can actually recover hugh purity methanol at atmospheric distillation condition so long that you don't go much higher than 120C at the bottom and maintaining about 70C maximum at the top (you can lower that to 65C if your system permits). I have operated methanol columns at atmospheric condition with methanol-SW feed at varying conditions and they all work fine, with distillate at 98 to 99+% purity.