How to determine the HC dew point temp in such sale gas contract.

producer · May 28, 2002

One company as natural gas producer got a problem of sale gas contract agreement with it's customer. Customer and producer has agreed that sale gas must have hydrocarbon dew point temperature not exceed 60 degree F at any pressure. So, producer has installed expander to control dew point temperature and design temperature of outlet expander at 50 degree F. Dew point temperature has reported by using expander outlet temperature for 15 years or we know RTD method. This year customer want to use the new method to determine the hydrocarbon dew point temperature, calculation or on-line analysis method. And force the company to accept the new method. Engineers in the company have been reviewed the contract agreement and found that the hydrocarbon dew point that customer required is the maximum point along P-T diagram, well known cricondentherm. Some engineer suggested that it very hard to control cricondentherm because it very sensitive with gas composition, both on-line analysis and calculation method. The question is "how to solve this problem"

grt12 · May 28, 2002

If it is a natural gas then the composition should not chnage that much and the control at the cricondentherm temperature should work ok - this is fairly common in gas systems.

ggordonstewart · Jun 2, 2002

Because of the retrograde nature of natural gas, the dewpoint temperature at a given pressure is not the same for other pressures. For most natural gases, the peak (maximum) temperature occurs between 400 and 800 psia, normally around 600 psia. If the pressure after the expander is around 600 psia, then the temperature will be near the maximum. If you have a process simulator such as Hysys, you can calculate the phase envelope for your particular gas composition and satisfy all parties regarding the HCDP. G. Gordon Stewart, P.Eng.
Gas & Oil Process Engineering Consultant

http://www.ggordonstewart.com/

ggstewar@telusplanet.net

producer · Jun 19, 2002

Thank you for answer especailly Mr. G Gordon Stewart. One thing that concern about maximum point is HC composition in sale gas. If composition of C7+ change or attach with sale gas stream, even we can control expander outlet temperature belower 60 degree F with pressure around 600 psia. We might face with maximum point exceed 60 degree F. So, I think reduce temperature of outlet expander as much as possible should be the best way to do this. Yes I did but form the test result, It's cricondentherm still higher than 60 degree F although we control temperature at 52 degree F.
I think composition is the problem. Turbo expander can't completely remove C7+.Or we may face with problem of liquid carry over after entering cold separator.

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How to determine the HC dew point temp in such sale gas contract.

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