3 replies to this topic

[ongwm]

I am currently learning about petroleum refining. I have some doubts about how the flow rate of pumparounds used in crude distillation unit and vacuum distillation unit affect the cut points in the column.

 

For example, in a particular distillation column the boiling range of naphtha is 0-140 degree Celsius, kerosene is  130-250   degree Celsius and gas oil is 240-400 degree Celsius. There is a pumparound system for the kerosene fraction. What will happen to the cut points of naphtha, kerosene & gas oil if I increase the flow rate of the pumparound, vice versa?

 

Any inputs is appreciated. Thanks 

[PingPong]

I am not sure whether you understand what TBP cutpoints are. They are not the same as boiling ranges.

 

In general: TBP cutpoints are determined by product drawoff rates.

It depends on the specific location of PA's and the type of product drawoff trays whether PA's will affect the amount of product that can be drawn off at various points in the column.

Assuming product drawoff trays are not total drawoff trays (which they should not be in a properly designed column), and as long as the pumparound duties in a column are not so high that there is not enough vapor passing through to sections above to be turned into products, PA's usually have no impact on possible product drawoff rates. They do however have a big impact on the fractionation between products as they affect the V/L ratios in the column sections. This impact will be visible in the product boiling ranges and in the 95% - 5 % ASTM gaps and overlaps between adjacent fractions. As such they also have impact on other product properties such as smoke point, cetane index, density, viscosity and pour point. To avoid that these properties do not meet specifications it may be necessary for the operator to adjust product drawoff rates, when PA's are changed.

Edited by PingPong, 25 August 2014 - 01:17 PM.

[sheiko]

Hello ongwm,

Theoretically, as you increase pumparound flowrate, you extract more heat from the column. Doing so, the temperature will decrease inside the column from the draw-off tray to the return tray.
But, after that, the behaviour of the column will depend on the control loops and panel operator decisions...

Edited by sheiko, 24 September 2014 - 07:06 AM.

[sheiko]

I have re-edited my answer.