2 replies to this topic

[sugato]

Hi
Firstly Im working at a Polypropylene Copolymer plant.

A little bit about the process:
Now propylene which is our main raw material is dissolved in n-hexane (which is the inert medium for the polymerization to happen in) with some catalysts in the reactor at high pressure (7-11 Atm (Absolute))

Now after the reaction happens in the reactor, the slurry (hexane + formed polymer powder + unreacted polypropylene) from the reactor is moved to a degassing vessel, where by exposing the mixture to low pressures (1.3-1.4 Atm) and high temp (60-65 deg C) propylene solublity in hexane is reduced, causing it to come out of solution. This propylene is recycled back to our reactors for consumption.

Now for my question. This is about the degassing portion.

I am told that the degassing process is in fact a Flashing operation. Is this possible ? Since propylene is in a vapour-dissolved-in-liquid state!

Secondly and more importantly, the degasser vessel is of a particular volume. designed for a particular flowrate, giving the slurry a certain amount of residence time inside it.
Is it possible that the removal of propylene from hexane is dependant not only on the temp and pressure, but also on the residence time in the vessel ?

Additionaly, i've looked on NIST but they have no solubility data (wrt Temp, Pressure) for the propylene, n-hexane binary system which would also be helpful to me.

Thanks in advance
Sugato

[siretb]

It is indeed a flash operation. So yopu have to calculate it in terms of K values, like a flash.

The K value at 60°C is around 12.5 K= y molefrac / x molefrac and is around 13.7 at 65°C. It is not pressure dependant (at least in your range of possible pressures)

It involves a mass transfer operation. In itself it is not depending on the residence time, but but certainly be depending on actual agitation, or flow circulation conditions.

[sugato]

Thanks a lot for your reply.

I agree its a mass transfer operation... but then having said that, wouldnt a higher pressure cause an increase in partial pressure of the propylene in the vapour space above liquid and cause mass transfer rate to drop ? I'm guessing the MTR is dependant upon the partial pressure of the component in vapour and concentration in liquid.