Ethylene tank dry out

[Barhp]

Hi,

I am working in an Ethylene plant. The Ethylene cryo tank was opened for inspection and now we have to put it service.

To put in service we have to first get ride of the Oxygen and Humidity. To get ride of the humidity I was told to use dry nitrogen. We will use to nitrogen evaporator. I am going to install a dewpoint at the evaporator outlet and a the tank vent outlet. When both dewpoint (inlet and outlet) are similar then we can say we eliminate the water. It is the normal procedure? Does anybody has done this before?

 

[cheute79]

Barhp:

I have never worked on a cryo tank in an ethylene plant, but a common first thing to do to clean/purge a tank (a tank like a cryo tank that is rated for full vacuum) is to draw a vacuum on it, then bleed in dry nitrogen back to 1 atm or more, and repeat 2 more times. Depending on how much vacuum you pull, you can remove well in excess of 99% (maybe even 99.99+%) of all O2 and similar for water vapor in 3 cycles. It will also remove the majority of condensed moisture. The evacuation/purge process is faster and uses a lot less N2, than to just purge through... do the math.

 

[dcasto]

be careful with vacumms, they can suck. I mean suck pressure tanks in. You say humidity or was their free water in the tank from say a hydrotest? If so, you will have to use a purge and not a pressure up and release.

In ethylene we used the flowing purge method because it is the best way to assure you have moved the O2 out. I can give you war stories at a PE plant. a few ppm O2 ant the reaction runs away and there is a HUGE release and fire from the safety valves.

Another caution, upon refilling with ethylene, you can use a heater to take liquid ethylene and drop the pressure and heat it at about 3 atm and 20C to purge out the N@. If you can have N2, becareful raising the pressure on the vessel with N2 in it too quickly, you could start a decomposition reaction and disaster.