5 replies to this topic

[process705]

Hi I have a question regarding Two phase flow.
when we have gas and liquid in the seperation vessel we say that liquid and vapors are in equilibrium. Can we also say same thing about Two phase flow. when you have gas and liquid inside the pipe. can we say that liquid and gas are in equilibrium?
Thank you for your help in advance.

Edited by process705, 28 September 2010 - 03:29 PM.

[Zauberberg]

Process,

That's quite difficult question, and certainly without a single, universal answer.

When dealing with equilibrium concept in a steady state operation, we normally assume that both phases (liquid and vapor) are in equilibrium, which is the case - from a practical standpoint. Any change of process conditions inside the vessel that is sufficiently fast (e.g. closing the vapor outlet valve) will cause subcooling of liquid at the vessel bottom, while the surface liquid will be in equilibrium with vapor phase.

What is the difference between these two cases? The answer is: dynamics. In the real multiphase flow applications, vapor and liquid phase may travel through the pipe at quite different velocities, depending on fluid regime (e.g. stratified, mist, annular, or slug flow). Different velocities (and consequences arising from that - J/T cooling effects for each phase, evaporation along the pipeline, heat transfer with ambient etc.) influence deviation from ideal equilibrium. Obviously, for different cases and different flow regimes, the deviation from equilibrium conditions differs as well.

So, the true answer on your question would be: for what purposes you are trying to evaluate this phenomenon? What is the impact of predicting or not predicting accurate phase distribution? If you are interested in tracking compositional profiles and phase distribution along the pipeline, I believe using transient flow simulation software would give you satisfactory results. If you do it manually by using available flow correlations, and dividing the pipeline into several sufficiently shorts segment which obey to the adiabatic flow principles, you can evaluate pressure drop and ambient heat transfer effects on the overall phase distribution but certainly this cannot tell you how far you are from theoretical equilibrium inside the real pipeline.

[Zauberberg]

To give a short update on the subject: I spoke to our Flow Assurance engineer this morning and he says that the temperature difference between phases in the pipeline (35 km long) can reach several degrees Celsius, due to different flow regimes/velocities.

[chem101]

Thank you very much for your help.
I ask this question because one of my co-workers mentions that 2-phase flow should be in equilibrium with each other. I heard this first time. When I ask him to explain me he was un-able to explain me. So I thought someone from Che forum will help me to understand this phenomenon.
Thank you for your help.

[Zauberberg]

The Flow Assurance engineer says that even for long-distance pipelines we assume perfect equilibrium in the line, although it is not the case in reality. The reason: not so much influence on the pipeline operation and management. For shorter lines (and process piping) this assumption is applicable even more.

Thanks goes to you as well, since I have found something new that I didn't know from before.

[Padmakar Katre]

Dear,
Just addition to the above discussions, one point I want share with you about equilibrium in case of the two phase flow through pipes. If we take an example of the heater transfer lines in case of CDU/VDU heaters the liquid and vapors are not in equilibrium at the flash zone of the main fractionation column. The coking is always there on the last bed of VDU fractionation column and i.e. mainly due the superheat of the vapor. To have mass transfer we need the saturated vapors and liquid, in this case part of the liquid falling on the wash bed or last bed(just above the flash zone in fractionation column) gets vaporized due to the degree of superheat of the vapors rising up. The basic evidence is the coking issue with this bed. So certainly I can say that in case of the two phase pipelines, due temperature and pressure profile transients the vapor and liquid inside may or may not be in equilibrium, but surely when we have vapor-liquid flash at feed tray or in the vessel we have vapor and liquid in equilibrium.(Here I assume the two phase occurs downstream of the Control Valve which is just near to the column/vessel and the length of two phase is not considerable.)