Replacement for Benzene as Entrainer in Ethanol Dehydration

[sshep]

My Friends,

Due to a shortage of benzene caused by a process upset, we have the opportunity to test a replacement ethanol dehydration entrainer once the trays in the tower are replaced. I am sure you can envision the "technical details" of the upset and loss of solvent.

The process is a simple azeotropic distillation, with feed coming into a decanter, and the ethanol and water phases sent to the top of the dehydrator and solvent recovery towers respectively, the towers share a common condenser which returns to the decanter. Candidate solvents include diisopropyl ether, toluene, hexene, heptene, plus any streams from the associated refinery.

Any advice is appreciated. Some ternary (LLE and VLLE, residue map) resources of a candidate would be nice.

best wishes,
sshep

 

[katmar]

Hi Sshep, I do not have any experience with the entrainers that you have listed here, but I have had experience using cyclohexane and came across a potential problem that you should be aware of while evaluating the other candidates.

I know that using a common condenser for the two columns works when using benzene, but because of the very different slope of the tie lines on the LLE diagram for cyclohexane/water/ethanol I could not get it to work - on paper that is as I have never tried it on the real plant. With cyclohexane a higher proportion of the ethanol goes into the water layer (than when using benzene) so if the tops of the recovery column go back to the decanter the ethanol can never get back to the dehydrator.

Cyclohexane works very well with two separate condensers and the condensate from the top of the recovery column being recycled directly back to the dehydrator (i.e. not via the decanter).

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[sshep]

Hey Katmar,

Thanks my friend. You always give good advice.

We actually got some experience using a blend of cyclohexane and benzene as a result of the upset which happened in two parts. First we blew some trays (eventually found 19 trays down) out of a 40 tray tower, and lost the benzene to the downstream hydrotreater unit. That product was recycled back to the dehydration unit after the upset, and the benzene was "recovered" as cyclohexane. We limped along for several weeks with a cyclohexane-benzene mixture until the wheels finally fell off- i.e. the solvent inventory became too low to continue to run; at which time we opened up and found the full extent of the damage. We already knew the tray damage to be extensive due to low column section dP's via pressure survey.

best wishes,
sshep