Overflas is liquid entering flesh zone from tray above, or internal reflux flowing from first tray above flash zone to flash zone.
Internal reflux in this part of fractionator provide fractionation between AGO and atmospheric residue.

Tray section between flash zone and AGO draw tray is also called wash section with purpose to wash entrained (black) residue from flash zone.

Higher overflash - better fractionation between AGO and AR, it means better color and lower distillation end point (for same cut point)of AGO.

Quantity of overflash can be adjusted by AGO drawing, if you draw  more AGO you will have less overflash or by changing heater coil outlet temperature, if you increase COT, for same drawing of products, you will increase overflash.

Design value for overflash is usually 3-5 vol% to crude oil processed.

On some units overflash is drawn from chimney tray as external stream and routed to FZ, in that case it is possible to directly measure quantity of overflash.

Regards,

Milutin

Dear Milutin

Thank you for your kind reply.
So, you said the original fuction of overflash is refulx for fractionation between AGO and residue, didn't you?
If there is no over-flash system, could we loose a theoretical separation stage?

Best Regards,

The excess heat supplied in the heater is removed by reflux (generally by pumparound). The main purpose, as I see it, is to generate sufficient liquid to scrub or wash down non volatiles carried upward by the vapors into the AGO stream.

In general, AGO has to be maximized (sometimes even w/o an end point specification) while minimizing the presence of coke precursors, which can be measured by color. In the end, the amount of overflash (as Milutin indicated) is dictated by economics.

Dear Mr. Lion,

You are right, if there is not overflash in column it will result in no fractionation between AGO and AR, and as result color of AGO will be  black.

Regards,

Milutin

To my grasping, the small amount of overflash wouldn't serve to get true fractionation effects but rather a mechanical wash down of carried over non-volatiles.

Hi 25362,

Increasing overflash (it also means increasing internal reflux in wash section) can significantly improve fractionation between AGO and AR. In one occasion we increased heater COT and improved AGO end point without dropping AGO to AR.

Regards,

Milutin

Milutin, only in one occasion ? Does it mean the economics didn't justify a continued operation ?  

Sory I wasnt clear in previous post, we continued to run in such mode with increased COT.
We have higher fuel consumption on heater, that is the price,  but produced AGO is now much vluable then earlier with low overflash - low COT mode.

Noted.

Milutin/25362,

From the control standpoint, how to control the overflash if we are running different kind of crude (crude switch). The heater COT cannot be always fixed. Control the AGO draw and endpoint might not be the only solution. Can we calculate overflash? from H&M balance? or there is a formula?

Regards,

Dagur

dagur777. You may try changing the partial pressure at the flash zone, by adjusting the top pressure and/or changing the amount of bottom stripping steam.

An example calculation can be found in D.S.J.Jones' Elements of Petroleum Processing, WILEY.

Overflash will change if:
Switch to diferent crude
Change of unit throughput
Change of stripping steam quantity (as 25362 noted)
Change of COT
Change of distillate draw
Change of column pressure


Overflas flow is seldom metered/controled dyrectly. You can monitor AGO color (visually) and adjust AGO draw.

Yes, There is a formulate, You can calculate Online Overflash also,If you have ASPEN Steady state simulation model than also you can calculate amount of overflash % (ideally it should be between 1 to 3 Wt % , If you know the amount of Vapour leaving the flash zone towards upper side,Amount of stripping steam you are putting & amount of RCO  that is going from the bottom of the column & amount of crude oil intake to CDU then you can easily calculate overflash %.Even online in DCS you can put soft tag also to calculate overflash %.

Thanks & Regards,
 

Petro0707,

Could you give an example and formula based on your description for overflash calculation? Thanks


Regards

Here is the formulae :

Overflash formulae =

A= Vapour Flow leaving the flash zone towards top (MT/hr) –   Stripping steam input to the column (MT/hr)

B= ( Total Crudemix intake MT/hr – Amount of RCO leaving  the last tray of the column MT/hr)

C = A-B

Overlflash % = [ C / (Amount of crudemix intake MT/hr)]*100

Don't forget that by making such changes on different crude oils, some adjustments would have to be done to H&M balances by, for example, by modifying circulating (pumparound) refluxes.

I am really interested in, if  anybody have calculated overflash value on DCS screen, if yes, is it value used to adjust parameters on CDU?

Regards,

Milutin