FCC Fractionator OVHD Temp vs Naphtha End Point

[dwwgsl]

At our FCC Fractionator, the overhead temperature runs at about 270F. This overhead stream eventually gets condensed and fractionated into dry gas, LPG, and naphtha. The ASTM 3710 (Sim Dist) end point of the heaviest naphtha is about 440F. Question is why would the naphtha that have a 440F EP would boil overhead in the Fractionator at 270F? What am I missing? Thanks.

 

[EmmanuelTop]

Hi dwwgsl,


The presence of steam reduces the partial pressure of hydrocarbons and heavier fractions are carried overhead. Secondly, there is no real tower in the field that can fractionate perfectly between adjacent fractions (components). That is why you will always find lower boiling fractions in your first side-cut, LCO, instead of having them in the overhead products.

http://antwrp.gsfc.nasa.gov/apod/astropix.html