Ethyline pipeline operating near triple point

[Lynx]

We need to uprate the carrying capacity of an ethylene pipeline 140 kilometer long and 14" diameter. It is proposed to operate the supply side of the pipeline near the triple point of ethylene, MAOP of 52.8 bar absolute at 10°C reducing to 20 bar at the outlet of the pipeline.
There is a concern about possible two phase conditions in the pipeline. The client reports that the ground temperature is 10°C the length of the pipeline. Ethylene flow is currently 42000 Kg/h maximum and the uprated flow will be 81500 Kg/h.

 

[GGW]

At 52.8 bara the saturation temperature of ethylene (C2H4) is 11.3 C. At 10 C, 52.8 bara you are dealing with a slightly subcooled liquid. At 20 bara the saturation temperature of ethylene is –28.8 C. At 10 C, 20 bara you are dealing with a superheated vapor. The freeze point of ethylene is 104 K (-169.15 C, 0.00163 bara) . At the conditions given you are a long way from the triple point. Assuming this is pure ethylene that you are dealing with you are going to need reliable thermodynamic data before you can do any substantive analysis. One set of such reliable data can be found from ASHRAE’s Fundamentals Handbook or ASHRAE’s Thermodynamic Properties of Refrigerants (THPRSI). The Fundamentals Handbook contains saturation properties for both transport and thermodynamic properties. ASHRAE’s THPRSI contains only thermodynamic data but includes both compressed liquid and superheated regions. Either of these books can be found and ordered from the ASHRAE online site.

 

[Lynx]

The point at which it is proposed to operate the pipeline is the critical point. Thanks for the correction. The problem still stands on whether there are any problems associated with operating the pipeline near the critical point.

 

[GGW]

Well, I goofed here also. The critical pressure and temperature for ethylene is 50.75 bara, 282.65 K. I said in my earlier response that at 52 bara, 10 C you were in a slightly subcooled state. Actually, you are in a compressed liquid state. At that condition which is above the critical state I calculate a density of 266.67 kg/m3. Now, if the flow is basically going to be isothermal not falling below 10 C then as the pressure drops to 20 bara you are going to enter the two phase region which I am sure you do not wish to do. I guess my suggestion would be, would you be able to raise the entering temperature of the ethylene to at least 15 C @ 52 bara? This would put you in the superheated region with a gas density of 125.19 kg/m3. Of course as the pressure drops to 20 bara and the environmental temperature as indicated does not drop below 10 C you obviously stay in the superheated region.

I would just like to point out that my pressure, temperature and density numbers given above come from ASHRAE’s THPRSI that I have found very reliable.