Gas lift pipeline pressure after cooling down

[bouk]

Hi,

I have a 17 km gas lift pipeline (no water present in the gas as the gas has been dehydrated). It is at a temperature of 41 C and 220 barg.

The pipeline is closed in and then left until the temperature has cooled to seabed temperature of 8C. How do I work out what the new final pressure is?

Also, how do I work out what phase it will be (will it be all liquid phase or a mixture of gas and liquid)?

 

[quark]

Use Universal Gas Equation with volume as constant (check compressibility of gas at the given conditions and use it, if needed)

Check boiling temperature of the specific gas at the new pressure. If boiling temperature at new pressure > 8C then liquid else vapor.

BT > 8C, partial liquid initially and may be full liquid if you leave it for sufficient length of time.

 

[bouk]

Thanks, just to confirm:

V,R,n will be constant so cancel out of the universal gas equation.
P1 - 220 barg
T1 - 41 C
Z1 - 0.7784

P2 - unknown to solve
T2 - 8 C
Z2 - ?

(if I use compressibility in equiation, do I not now have too many unknowns - i.e. two unknowns Z2 and P2 and only one equation?)

 

[quark]

First do the calculation ignoring Z2. Get P2 and check corresponding Z2. Iterate few times.

 

[zdas04]

You said this is a GAS LIFT pipeline that you're filling with dehydrated gas. At 8C it is all going to be gas (n-Butane has a boiliing point of about 0C, and if you have anything heavier than that in your gas lift gas you are wasting a fair bit of money).

As Quark said, use Z(1) on both sides of the equation (so it falls out), calculate P(2), get the Z(2) that goes with P(2) and recalculate P(2). Now calculate the Z(2) for the second P(2) and see if it is close enough. Iterate until it is.

David

 

[bouk]

T2 = 8 C
P2 = 163 barg

All vapour. Thanks for your help guys.