Depressurization scenario in FLUENT
 

I am attempting to carry out a depressurization scenario in FLUENT. A long 36" pipeline is to be depressurized to atmospheric conditions through a 12" vent system. The main pipe is filled with natural gas at 80Bar while the vent system is initially filled with air at atmospheric conditions. I am attempting to model the initial purging of air from the vent by the high speed flow of the methane gas once the depressurization process begins. I have separated the vent and main pipe into 2 different zones, set each zone medium as air & methane respectively, and patched the pressures accordingly after initialization. By using this scheme, I find that the 2 zones do not really mix and that the air is not being purged. How should I go about to set this up successfully? I am not very familiar yet with either the multiphase or species transport models. Any help would be appreciated

Ok, I've solved the previous problem by defining the vent line as a seperate continuum and patching the pressure after initialization. Now I'm back with another question. Based on data I have, the density of the gas within the main pipeline is supposed to be around 69kg/m3 with a Z factor of around 0.83. However, using the ideal gas model in FLUENT, i am getting a maximum density of 59kg/m3, which is about consistent with a Z factor of 1 (perfect ideal gas). Correct me if I'm wrong but this would lead to an incorrect result as the FLUENT system would have less total gas than the actual system. How would it be possible for me to incorporate the initial compressibility effect into the system?