Propylene moisture (FCC)

[July77]

Hello,
Does anybody have a problem with a propylene moiusturize immediately after start of the unit?
I had that problem last few years. I am not sure is there problem with a start up procedure or with something else.
First start up procedure was:
- steaming all GCU
- gasification with fuel gas to depropanizer section and intertization of depropanizer, C3 Drier and Propylene spliter section
We change our start up procedure:
- steaming to C3 Drier section
- intertization of Debutanizer, Depropanizer, C3 Drier and Propylene spliter section ( the first two columns and naphtha splitter were not opened)
- intertization of LPG Merox unit
In both case Propylene moisture is appeared and after a few days it droped belows 10ppm.

What is your oppinion? What can be a problem?

Thanks.

 

[EmmanuelTop]

Definitely there is residual moisture in the system prior to startup, as you are seeing that water content gets reduced to 10ppm after few days of operation - meaning that Propylene picks up the water that was sitting inside the system after steaming and inertization.

You need to use dry medium for prolonged time in order to remove/absorb the residual moisture. Gassing up just takes 30min or so, which is definitely not enough to flush all liquid water from the system and to dry up the piping and equipment.

Dejan IVANOVIC
Process Engineer, MSChE

 

[July77]

Which dry medium do you recommend? Thank you.

 

[EmmanuelTop]

Ideally this should be a warm (or hot) stream introduced at low pressure, since high temperature and low pressure of the gas medium favor absorption of water. The best candidate is dry N2. However, the plants usually do not have sufficient Nitrogen for this kind of drying operations, so dry process gas or dry fuel gas are employed instead. The best practice is to introduce this gas at multiple points and ensure continuous sweeping towards the discharge point(s), likely to flare for safety reasons. Measure moisture content of the system at convenient vent points by using portable dew point meters.

If you can't find suitable dry stream/medium, then a heating source would be required to heat up the (wet) medium and increase affinity for absorption of moisture from piping and equipment. See if you can utilize process heaters for that purpose - just remember to perform a dedicated risk assessment for this activity and address all possible issues. This can sometimes be a very hazardous operation.

Dejan IVANOVIC
Process Engineer, MSChE

 

[July77]

We did intertization by N2, like I wrote, but wu did not measure moisture content of the system. Thank you ET.

Does anybody have a same problem? Or some other recommendation for start-up procedure?

 

[EmmanuelTop]

Inertization is normally not sufficient for dry out. As its title says, it is performed in order to ensure inert environment inside the equipment/piping, and for maintaining positive pressure so as to prevent from air ingress.

Try to obtain more effective control over residual moisture removal activity. Perhaps repeat inertization/purge steps till dew point improves, if there is not enough gas for continuous purge.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

For air separations plant ( which operates at -160 to -190degC), we use hot dry air from downstream of the mole sieves adsorbers, heated up to 70degC for startup plant defrosting and drying out operations. And after several of purging with this gas, with gas let out at various instrument tapping bleed points, we check water dewpoint with a portable MSA water dewpoint meter.

In your plant, think the column overheads water cooled condensors ( deC3, deC4 etc) should also be completely drained out and dried also to prevent water ingress into the process side through small leaks in the tube to tubesheet roller expanded joints. There may be other sources of water ingress also.