3 replies to this topic

[dwoodard]

What are the typical feed conditions for a propane-propylene splitter? The feed is 30% C3 and 70% C3=. Where could I get an equilibrium curve for these conditions?

[Zauberberg]

There are no "typical" process conditions for separation between propane and propylene. It all depends on available overhead condensation temperature, available reboiling medium temperature, maximum operating pressure of a column (if it is an existing one) etc.

First, you have to know at which pressure/temperature levels you want to separate C3s. By knowing this, equilibrium curve can be calculated/plotted. You can consider propane/propylene system as an ideal mixture where Dalton/Raoult laws can be applied. Therefore, Ki = yi/xi = component vapor pressure/system pressure (at given temperatures). Relative volatility is, of course, Alpha (i,j) = Ki/Kj. And there is your McCabe-Thiele chart.
Fenske-Underwood-Gilliland correlations will guide you regarding minimum number of theoretical plates, minimum reflux ratio, and optimum feed stage location.

Feed conditions should be known to you, if you are the one who is going to design fractionation tower.

[Zeeshan]

There are no "typical" process conditions for separation between propane and propylene. It all depends on available overhead condensation temperature, available reboiling medium temperature, maximum operating pressure of a column (if it is an existing one) etc.

Hi Zauberberg,

A small clarification is needed here (may look silly or might have been discussed elsewhere in the forum).

How do we fix the maximum operating pressure of column. Is it governed by available cooling medium condition or other factors, such as reboiler duty, vapour loading (in turn Column diameter) come into play?

If available cooling water temperature limits the saturation pressure of overhead vapours, a refrigeration loop may be considered, if proved cost-effective?

I may be missing other inportant parameters, need your comments please. Reference to any literature or attachments will also be helpful.

Thanking in anticipation.

......Zeeshan

[Zauberberg]

Zeeshan,

For new designs, operating pressure is determined based on maximum available condensing medium temperature, with a reasonable +10C (cooling water) or +20C (ambient air) temperature approach. Refrigeration loop can be considered as well - this will cut your reboiler temperature level - but on the other hand it will probably call for a larger tower diameter, due to increased volumetric flow of vapor at lower pressures. In any case, all technically feasible options should be thoroughly evaluated: both from hydraulic and mass transfer points of view.

For an existing tower, you are limited by maximum allowable operating pressure - and that is your constraint, regardless of available condensing medium temperature. In case the pressure is not sufficient to provide 100% overhead condensation, a sort of chilling loop is mandatory in order to prevent overhead product losses.

Refer to any ChE textbook related to distillation equipment design and operation, such are: Kister, Ludwig, GPSA, Lieberman, Walas etc.