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Concept And Purpose Of Overflash Pump In Distillation Column


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#1 Venkat1989

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Posted 10 April 2020 - 11:56 AM

Dear All,

 

I am new to refinery operations, I started working in Diesel and  Vaccum gas oil hydro-treaters, in both of our plant fractionation columns, we have got over flash pump. What is the main role of this over flash pump in the column?. Kindly help me to understand the importance of this pump? What will happen, if this pump is not available?  Thank you.

 

The feed to fractionator column comes from the stripper operated at 10.5 kgf/cm2g and fractionation column pressure is 1 kgf/cm2g.

 

In our diesel hydro-treater unit, the fractionation column products are Naphtha at top, Kerosene at middle and Diesel at the bottom.

 

In Vaccum gas oil hydrotreater unit, the fractionation column products are Naphtha at top, diesel at middle and Vaccum gas oil at the bottom.


Edited by Chalapathi, 10 April 2020 - 12:11 PM.


#2 PingPong

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Posted 10 April 2020 - 04:55 PM

Upload a PFD of the fractionator with the pump.



#3 PingPong

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Posted 11 April 2020 - 06:17 AM

In the DHDT the pump 507-P-009 is useless and could be removed, including its FC and FCV, and the LC on chimney tray #7.

 

Such pump would only make sense if the liquid from stage #7 would have to pass through a distributor with a considerable pressure drop, such as a spray nozzle, to reach stage #6. Maybe in the beginning of the design it was considered to have packing for stages 1 thru 6 but as it looks now on the PFD there are simply 6 trays. You could verify that with the P&ID and fractionator detailed drawing. Note also NOTE 2 on the drawing.

 

In the VGOHDT it is not clear from the PFD from which tray number the pump gets its liquid. One would expect from the unnumbered chimney tray, but the LC is shown to be located much higher. Check with P&ID and fractionator drawing.

Again: as the stages below the pump discharge seem to be trays #1 thru #12, not packing with a spray distributor, the pump is useless and could be removed, including its FC and FCV and the LC on the chimney tray.

 

 

Sometimes a final design looks stupid because of philosophy changes during the design phase. Nowadays more often designs are stupid because the designers are not very smart. In the old days designs were made by experienced engineers with not only experience but also insight. Nowadays those old guys are retired and designs are done by young engineers that happen to be able to run a process simulator. They often have little or no insight or understanding of the process and unit operations (such as fractionation).

Nothing surprises me anymore, not even if it comes from a reputable licensor like Axens.

 

You could contact Axens and ask them why they designed it as it is. However you will probably get a BS answer as they are not likely to admit that the design is not optimal.


Edited by PingPong, 11 April 2020 - 06:26 AM.


#4 Venkat1989

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Posted 12 April 2020 - 03:17 PM

Dear Ping Pong,

 

Thanks for your response,

 

But, May I know the normal requirement and role of over flash pump in a fractionation column?

 

 

On what basis, we predict the requirement of over flash pump? Kindly help me.

 

Can you please provide any documents to understand the purpose of over flash pump?


Edited by Venkat1989, 12 April 2020 - 03:22 PM.


#5 PingPong

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Posted 13 April 2020 - 05:04 AM

May I know the normal requirement and role of over flash pump in a fractionation column?

Fractionation columns normally do not have an overflash pump, and there is no need for it in your units either. You could delete the pumps and modify the piping. If you like to measure the overflash liquid you could install a low pressure drop flow meter in the modified piping.

 

Normally however that is not done and the overflash simply falls from the tray above the flash zone on to the top stripping tray, together with the liquid part of the feed. To achieve that you would need to remove the chimney tray, or better: replace it with a new fractionating tray to slightly improve fractionation.

 

On what basis, we predict the requirement of over flash pump?

There is only a need for an overflash pump if the overflash is not intended to enter the stripping section of the fractionator, but is to be sent somewhere else. For example: if the overflash is to be sent to another process unit, or if it is to be recycled to the feed furnace of the fractionator. In a crude vacuum column the overflash is usually pumped to the inlet of the vacuum furnace.


Edited by PingPong, 13 April 2020 - 05:05 AM.





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