Hi all,
I would like to know if anyone has ever been able to analize the reliability of Hysys' calculation of pressure drop for two phase vertical lines. In the case I'm working, I have a line that goes up 40 meters, and in the top part of the line, the mass vapour fraction is as low as 5% (propane in liquid-vapour equilibrium). My first guess was that given the low mass fraction, the pressure drop given by height change would be similar to the one that a line full of liquid would have, which was about 2 kg/cm2, so I was expecting a pressure drop of at least that amount (without even considering friction pressure drop that would add even more to it). To my bewilderment, Hysys calculated a total pressure drop of about 0,4 kg/cm2g. Trying to understand what was happening, I was surprised to see that in spite of the low mass fraction, the actual volumetric fraction of the product at the top of the line was about 90%. In hindsight, I guess that is expectable given the difference between both phases' density (570 kg/m3 against about 3 kg/m3). The problem is, it doesn't seem right to me for the potential energy changes to drop to almost nothing with such a little mass of propane evaporated. Even if the volumetric fraction of the vapour phase is so high, the amount of area occupied by it would be dependent on the velocity of the phase. Furthermore, the flow pattern Hysys estimates is dispersed bubble which, I gather, means that most of the cross-sectional flow would be occupied by the liquid.
Looking into the pipe segment calculations, velocity phases calculations don't seem to account for the actual fraction of the cross-sectional area occupied by the phase, rather, it calculates the superficial velocity (which would be volumetric flow / total cross-sectional area).
To give further information, the model I'm using is the Tulsa 2-phase. I tried other models though, and all of them calculate almost the same pressure drop in the vertical pipe.
If anyone has any information that could help me understand this better or convince me that Hysys calculation is correct, I'd be really grateful.