natural gas processing

[10some]

Hello,

I have been reading about natural gas processing and I am really confused about Hydrocarbon and water dew point. I clearly understand what they mean but these are my issues:

1) When we pass the gas say into a glycol unit to remove water content and lets say the water dew point is -15 C, are we basically cooling the gas to -15 C?

2) Similarly when gas comes out of separation unit for Hydrocarbons removal by cooling is the gas temperature equals Hydro carbon dew point temperature?

 

[zdas04]

Both "temperatures" are a calculation of the temperature where condensation would occur at a given pressure. So when we take 1000 psig natural gas through a TEG dehydrator and it goes in at a water content of 120 lbm/MMCF and comes out at 7 lbm/MMCF the new water content is very much sub saturated at flowing temperature and pressure, but if you cooled it (at constant pressure) to below 32[°]F you would get condensation--the dew point is now 32[°]F even though the gas is at 80[°]F (for example).

Same with hydrocarbon dew point, you have done something to the stream to remove the things that would condense (at expected pressures) at temperatures above the required dew point.

David

 

[maddocks]

1.) Not necessarily - removal of the water will lower the dewpoint - a glycol dehydration process does no cooling but extracts the water.

2.) If the process operates at a perfect equilibrium, the dewpoint of the gas leaving the unit has a hydrocarbon dewpoint that matches the conditions in the low temp separator. Most commercial units have a dewpoint slightly higher due to minor amounts of hydrocarbon carryover.

Get yourself a copy of the GPSA data book - its an excellent reference and you can work through these issues. As well, refer to publications by John Campbell for some good stuff.

 

[10some]

thanks for the reply.

 

[MortenA]

A thing to remember when considering cold processing of hc flows:

The phase invelope may mean that at your operating pressure - the HC dewpoint may not be the highest. This means that condensation could occur if the pressure changed somewhere but the temperature remained constant.

Also, retrograde condensation may occur if you are operating your LT process near the "top" of the phase envelope. The ctrical point is often somewhere a little down the boiling point curve. This means that the HC dewpoint could in fact increase even if you lowered your operating temp.

Bets regards

Morten

 

[JLSeagull]

Most (not all) gas processing plants in my experience were cryogenic gas plants. The low temperature around the demethanizer can be below -100 degree C (-150 deg F). For the cryogenic gas plant dehydration process the glycol system lacks any capability.

These type units separate methane from C2 and higher compounds except permit just enough stuff to increase the mostly methane gas BTU within the requirements. The C2 and related stuff can be mixed or separated and sold for other products.