Turbo Expander and Slug Catcher 2

[Ossareh1352]

Dear Friends,

I am working on the Gas Treating Plant. The feed of our plant comes from two NGL sources and in the inlet of our plant we have considered one Slug Catcher. Nowadays we informed that Turboexpander in two NGL will be in the services so the voulme of our slug catcher will be decrease. i don't know the relation between Turbo Expander and Slug Catcher. How the Turbo Expander decrease the liquid in the gas ?

I will be highly appreciated if someone help me in this regard.

 

[EmmanuelTop]

If you have turboexpander upstream of gas treating unit, it means the gas is already conditioned. By turboexpansion unit, the quality of gas is adjusted to meet export (usually pipeline or downstream units) requirements.

By almost isentropic expansion you have two-phase fluid at the outlet of turboexpander: gas, and liquid hydrocarbons which are removed from the system. This type of facility is usually employed for meeting target gas parameters such are cricondetherm temperature, dew point etc. There should be no liquids in gas treating unit inlet scrubber.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[JLSeagull]

Normally the gas is dehydrated - water removed upstream of the turbo expander to prevent freezing problems. The inlet of the turbo-expander-compressor turbine is high pressure gas. The gas pressure is reduced by turning an expander. This refrigerates the gas causing liquids to drop out of the gas. The other end of the shaft is a compressor - causing additional "work" by the expander. This adds to the amount of regrigeration as compared to expanding via a choke or JT valve. A cold separator and demethanizer separate the ethane and heavier components leaving mostly methane that is on the outlet - the inlet to the recompressor end of the turbo expander. The liquids may be sold for further downstream processing or sent to a fractionation facility for separation into the C2, C3 and C4 components.

As most of the condensate is removed the slug catcher volume requirement is decreased.

 

[dcasto]

Decreased? After a turbo expander plant there will be no liquids, no slugs, while the expander is running.

 

[JLSeagull]

OK, dry -- and -- Perhaps the natural gas is always dehydrated upstream of the expander too, right? Perhaps during a bad liquids market slugs of low molecular weight liquids (C2) could exist (search for ethane rejection around 2nd Q 1983).

Advanced controls limit the ethane in residue gas to just barely make the BTU contract during a good liquids market -- and adds heat to boil the ethane off during a bad liquids market.

 

[dcasto]

You stack up 25% ethane with 75% methane and you won't get liquids in most areas I'm familiar with. Its the c4's and heavier that give the problems.

In plants you end up at dew point in the deethanizer/demethanizer overhead and they operate well below -40, so it'll take a -40 or colder to make liquids.