A good day to all of you. I am working on an assignment about the recovery of propionaldehyde. The feed contains 4.89% CO2, 4.14% water, 6.02% ethane, 84.09% propionaldehyde, and balance is trace of H2, CO and ethylene (all in molar basis). The feed is at 30 degree C and 620 kPa. I use Aspen Hysys for simulation and the fluid package is UNIQUAC.
My task is to recover as much propionaldehyde as possible, and also, get rid of all the light gases from propionaldehyde. First, I tried to use a flash column. However, I could not get rid of CO2 and ethane from propionaldehyde.
Next I tried to use a reboiled absorber. I ended up with 93% recovery of propionaldehyde, but there were still CO2 and ethane in propionaldehyde.
I then tried to use a distillation column with a partial condenser. I ended up with 98% recovery, and the bottom product only had propionaldehyde (more than 95 mol%) and water. This is very good and is what I want. However, I have a conceptual question here. What does this distillation column separate? What is the cut? 98% of the water and propionaldehyde from the feed go to the bottom products. The light gases go to the overhead vapour stream. As for the condensate stream, it contains propionaldehyde (87.68 mol%), water, CO2 and ethane. It seems to me that propionaldehyde is the major component in both distillate and bottom product, which does not seem right. It is as if I were separating propionaldehyde from itself.
Can you please advise me on this issue? Even though I can do the assignment, I feel that there may be something wrong with my understanding. Many thanks in advance.
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Edited by clifford.v, 22 September 2020 - 02:51 PM.