PUMPAROUNDS , ATMOSPHERIC DISTILLATION 3

Dear Mr. Padovano

When you refer “The reflux ratio for the pumparound for the kerosene/light diesel fractionation section of the atmospheric distillation column is about 0.7” do you have on mind ratio of pamparound (P/A) to crude oil (feed)?

Anyway, you can change (P/A) flow as you wish until you satisfy column heat balance.
If you, for example, decrease kero/Lt diesel P/A flow, column top reflux flow (if top reflux flow is on cascade mode with tower top temperature) will automatically increase.
If you have one more P/A you can change ratio between that two P/A and keep top reflux flow the same.

Regards,

Milutin

What I have in mind is actually the ratio of pumparound to ncrude oil feed.
Yes I have three pumparounds. This one I am talking of is the one inbetween the higher and the lower pumparound. I would like to increase the present (P/A) of this pumparound.Is it possible to increse it without destabilising the column or the functioning of the other two pumparounds?

Answer is yes.
Increase middle P/A flow (“one inbetween the higher and the lower pumparound”), observe top tray reflux flow, it would decrease. After that, if you want top tray reflux flow the same as before middle P/A change, you can decrease one of two other P/A.

When you make changes, make it in small steps, if you want stable operation of column.

Question: do you have three P/A and top tray reflux (liquid from accumulator drum) or just three P/A?

Regards,

Milutin

Balancing between different pumparounds implies significant changes in fractionation efficiency, almost in linear fashion.
For example, by increasing HGO pumparound rate, the amount of lighter (LGO) material which ends up in HGO becomes higher - due to reduced vapor flow from HGO pumparound return tray, and reduced internal reflux from LGO draw-off tray. This is equivalent to reduced reboiling and reflux rates in simple columns.
On the other hand, "extracting" heat on higher temperature level (HGO>LGO>Kerosene) from the main fractionator, improves heat transfer in P/A exchangers (higher LMTD). So, you will always have to swing between column heat balance and distillate yields (ratios of yields between Kerosene, LGO and HGO). It is best to maximize pumparound rates where separation efficiency between adjacent products has very high margin, while maintaining as low as possible yields of lower grade products, such is HGO.

Top reflux from the overhead receiver has similar drawbacks: besides corrosion issues (when refluxing overhead naphtha mixed with salt water back to the column), increasing top reflux at the expence of top pumparound causes further separation efficiency reuction between Naphtha and Kerosene. This happens because the amount of heat removed from the system is caused by evaporating relatively small amounts of overhead naphtha, compared to top P/A sensible heat change (higher equivalent mass flow required), which affects internal reflux.

There are just three P/A attached to the column no top tray reflux. Thanks

Correction of what was written above. There are three P/A pumparounds with a top tray reflux

In my opinion, the only way to determine the full effect of changes in pumparound rates is to use a calibrated column simulation model. The calibration must be done against averaged on-line data (gathered over at least a few hours of steady operation, with lab samples gathered for product quality). Then, running the base model against various cases and simple plotting of the pertinent variables provides great insight.

You would also need to run pumparound exchanger rating calculations because, beyond a certain point, increasing circulation does not increase duty appreciably.

It's a lot of work, but that's what process engineers should do.