Pad Gas on fractionation plant feed drums

[chriswolff]

Our client has a number of pipeline feed surge drums at the inlet to their fractionation plant. The feed stock is allowed to settle out to its vapour pressure and is then pump into the de-ethaniser tower.

If the vapour pressure of the feed is below 400 kPag @ approx 24°C, then they will pad the drums with propane gas.
They can then raise the system pressure in the drum, but the propane vapour does not reach equilibrium with the feed stock, and the vapour pressure of the feed is not significantly increased.

I have modelled this in HYSYS, and if the vapour and liquid are in equilibrium, I will need about 8T/hr of propane vapour to raise the feed stock vapour pressure to 400 kPag. when they do need to pad the drums, they use several orders of magnitude less gas than this.

Is there a mass transfer effect limiting the amount of vapour being absorbed in the drums, which allows the system pressure to be increased, or some sort of stratification in the vapour space?

The problem is that the depropaniser reboilers are a bottleneck in my process and I need to get some idea of how much propane is likely to be absorbed, so I can resize the reboilers.

Any help would be appreciated.
Thanks
Chris

 

[epoisses]

Chris, assuming the plant flow reading is correct then propane absorbed equals propane added. If equilibrium is not reached, that would be difficult to model, but what would stop you from simply using the plant value and mixing it into the feed?
Why exactly is the propane added, by the way? (i.e. why the vapor pressure requirement?) Having no background knowledge at all, I would first try to debottleneck the depropaniser for free by challengeing (trying to get rid of) the propane injection.