1 reply to this topic

[picusz]

Dear All,

 

I’d like to ask you to help with one issue coming from HAZOP – propylene flashing during depressurization could cause freezing of cooling water inside of heat exchanger:

Shell side: liquid propylene (45°C @ 5.0 MPa g) - depressurization to 0.11 MPa g (-45°C)

Tube side: cooling water (20°C @ 0.55 MPa g)

 

How to check/calculate this?

 

I have an idea to compare evaporation enthalpy of propylene vs. enthalpy of water (20°C -> 0°C) with respect to hold-up.

 

Thanks!

[Art Montemayor]

Picusz:

 

I don’t see your problem in recognizing that you have a HAZOP issue: the liquid propylene (propene) will evaporate when de-pressurized from 5 to 0.11 MPag and it will result in -45 ^oC liquid propylene remaining in the heat exchanger.

 

This certainly poses the hazard of freezing any cooling water that happens to be static in the exchanger tubes or is flowing at a slow flow rate.  There is no hold up involved, in my opinion.  All you have to ensure is that you can ensure  sufficient water flow occurs in the tube side such that it will be a secure heat sink for the cold propylene on the shell side.  You have to calculate the worst case scenario for the heat transfer: a clean over all “U” and the coldest entering cooling water temperature.  If you start to approach 0 ^oC, you start to get into trouble with water crystallization starting to form in the tubes.

 

I don’t see any difficulty in identifying the hazard - unless I misunderstand the situation.