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[alex.gutu]

Dear Gents,

I have built a PFD of Maleic Anhydride (MA) production plant unsing HYSYS. The raw material for MA is benzene (BZ). After reaction section, reactor product which consist of MA, Quinone (QN), CO2, H2O and a small amount of unreacted BZ is sent to separation section. Also I want to mention that the reactor product is a vapor phase with temperature about of 600 C.

So here I got some problems with separation section. I have cooled reactor product about 100 C in order to get liquid phase and to do a flash separation. Basically I wanted to remove the largest amount of CO2 and H2O in flash separation and then to separate unreacted BZ, MA and QN in other 2 columns.
Unfortunately after the chiller I realized that the phase of the stream remained as vapor phase. Then I tried to make a extractive distillation (tower 1) to separate gases (CO2 and H2O and if it possible QN). As a solvent I used Dibuty-Phthalate (DPH) and I succeeded to do this separation but there appeared some questions.

After the simulation had finished I tried to make a kind of optimization and to look for the best amount of solvent (DPH) for extractive distillation (TW-1) that I have to use, but I have noticed that in my simulation it doesn't matter what amount of DPH I use, because the result of extractive distillation are basically the same, either I use DPH or no. I set a flow rate of 500 kmols/h and 10 kmols/s and the results are very closely. It seems that the solvent doesn't have any effect on this unit operation. I also set DPH flow 0 and the distilltion occurs even better than I use solvent.

I want to ask you if there is something wrong. Could anybody tell me if I made something wrong, or extractive distillation is an operation that can't be performed in HYSYS.

As fluid package I use UNIQUAQ for main flowsheet and for extractive distillation column I used Extended NRTL fluid package.

I'll kindly appreciate if anybody could share his experience on this problem with me.
What solvent should I use for extractive distillation?
Which fluid packed should I choose to obtain real behavior of the system

Thank you in advance,
Alex