The HC gas upstream T,P is known and downstream P is know. A cooling effect is likely to occur since moving from a high pressure to low pressure across the control valve. One of the things I am not sure how to estimate is the Joule-Thomson coefficient.
Do you know the approximate composition of the LNG? In this case you can use the ThermoC software: http://thermoc.uni-koeln.de/
You can calculate Joule–Thomson coefficients (program xthN) or expansion curves (programs expandN and exp_lgN (the latter is for 2-phase expansions)).
Best use the predictive Peng–Robinson model (PR78/PPR78). The GERG equation is also available,
The decrease of the temperature of the gas is calculated with the relation:
DT= [(P1-P2)/Cp ] x [(2a/RT) - b]
In which: a, b - the coefficients of the Van der Waals equation for the respective gas; P1, 2 - the pressure from which the detention starts until the final pressure; Cp - the specific molar heat of the gas at the initial temperature of the gas that you will have to correct with the value of the general gas constant of 82.054 [cm3 x atm / mol * degree] or 0.082054 [l * atm / mol * degree].
You had to get a measurable result in - [degrees / atm] and shows with how many degrees the gas cools on 1 atm of pressure variation.
Attention! this equation is valid for not very high gas pressures; there are conditions for obtaining an inversion point in which the temperature of the gas at the detention will not decrease but may increase. Generally up to 250 bar most gases are cooled.
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