[afd]

If I have a vessel containing ethylene at 21 barA and -18°C and I isolate it and begin depressurizing it to flare.
What is the approximate temperature the gas in the vessel can reach when the pressure becomes 2 bar A?

Thanking you all in advance for your information.
Afd

[fallah]

afd, on 19 March 2010 - 11:41 PM, said:

If I have a vessel containing ethylene at 21 barA and -18°C and I isolate it and begin depressurizing it to flare.
What is the approximate temperature the gas in the vessel can reach when the pressure becomes 2 bar A?

Thanking you all in advance for your information.
Afd

Is the case a cold depressurizing (no heat input)?

[afd]

Yes the vessel in question is insulated by cold insulation(Poly Urethane Foam) and depressurizing is done by opening a manual vent valve to the flare. What will the approximate temperature be of the ethylene gas in the vessel if the manual vent valve is closed at around 2 barg.

Afd

[fallah]

afd, on 20 March 2010 - 06:17 AM, said:

Yes the vessel in question is insulated by cold insulation(Poly Urethane Foam) and depressurizing is done by opening a manual vent valve to the flare. What will the approximate temperature be of the ethylene gas in the vessel if the manual vent valve is closed at around 2 barg.

Afd

You need to do dynamic simulation with proper software,e.g. PRO/II.

Prior to writing PRO/II input,the following calculation should be performed:
-Total volume of the circuit to be depressurized
-Total liquid volume
-Total gas volume

The circuit that is assumed as a hypothetical vessel,is considered as filled with the actual amount of liquid and vapor at operating pressure and temperature.

Initial pressure would be design pressure of the circuit (here vessel)
Final pressure would be 1 barg
Initial temperature should be equal to -18 C
Final temperature of the gas would be obtained from fluid T and P vs time plot as an output of the software

This post has been edited by fallah: 20 March 2010 - 09:16 AM

[prasad54]

Hello,

You need to first decide how to model the depressurization themodynamically using a proper equation of state. For ethylene using Benzler-Koch diagarm describing its VLE and considering an Isentrpoic depresization, you can model it in ASPEN PLUS as a heater block setting equal entropy at inlet and oulet and varying the temperature. This would give you the temperature of gas.
As an intial guess you will get around -100°C as you are at very low temparature. The material of the vessel should be SS.

[go-fish]

prasad54, on 23 March 2010 - 12:21 PM, said:

Hello,

You need to first decide how to model the depressurization themodynamically using a proper equation of state. For ethylene using Benzler-Koch diagarm describing its VLE and considering an Isentrpoic depresization, you can model it in ASPEN PLUS as a heater block setting equal entropy at inlet and oulet and varying the temperature. This would give you the temperature of gas.
As an intial guess you will get around -100°C as you are at very low temparature. The material of the vessel should be SS.

Based on the above approach, it should be -90 C and vapor fraction is 0.44 after isentropic throttling.However, these calcs are based on depressurization from the 21 bara, -18 C to 2 bara. Could be different if you use your starting point as design pressure of equipment which could be higher than 21 bara.

@ fallah, could you please elaborate on why would this be a time dependent case? I am not highly experienced, could certainly use more knowledge.