Acid Extraction with Benzene

[dipak2]

Hello..!! Guys..

I extract Organic e.g.Nitrobenzene,Benzene etc. from Dilute Spent Acid i.e.75 % Concentration H2SO4 approx. remaining water,HNO2 Etc. with the use of solvent Benzene at ambient around 28-30 C.Extraction Vessel Temperature is 40 C having Chilled water Jacket for better extraction.Still from the Extractor Benzene Vapour loss is more.How to minimize.Because Vapours of Benzene from Extraction vessel goes to Vaccum Blower to Scrubber/Atmosphereic Vent.

Any valuable suggestions/ideas will be useful for me.

Thanks

 

[sshep]

Hey Dipak2

This looks like it may be a solvable problem. 40C does not seem particularly chilled, and with the extractor running hotter, there is alot of benzene vapor for the vaccum blower and scrubber to pull on. A scrubber tower alone connected to an open vent from the extractor will suck on benzene vapors. Some direct connected scrubber systems will actually pull off extra vapors on their own- I once did some troubleshooting of an HCL vent scrubbed with water that had that pretty serious problem of that nature, and have also seen this on other occasions.

Answer please these questions:

1) Are you actually running the extractor under some vacuum?
2) What are the sources of non-condensibles entering the system (dissolved in wwtaer/solvent, deliberate purge of N2, etc)?
3) Is there pressure control on the extractor with an appropriately sized control valve between the extractor and the vapor recovery system (if not then this may be your problem as I understand your system)?
4) What are you scrubbing with and is there any trouble with benzene recovery in your scrubber (i.e. high temp rise across scrubber, benzene being vented to atmosphere)?

Thanks,
Sean

 

[dipak2]

Respected Mr.Sean,

Good morning.!!How are you Sir!!!First of all sincere thanks for keen intrest in my problem.

I have tried to Answers to your questions as follows :-
1.Actually, I don't have PT ,PC on extractor.It is dynamic type extractor with extraction vessel.I mean one is Spent Acid inlet & same way Benzene as solvent.There is vapour line in the extraction vessel.This line goes to Vaccum Blower suction.Yes,it is under minor vaccum as there is no PC ,Only Isolation valve in the vapour line from vessel to Blower which we keep 80 % open constant
2.Minor amount of Nox gases ,Spent Sulphuric Fumes,Sox,Acid fumes generating from Spent Sulphuric Acid --75 % (whatever present in Spent Sulphuric Acid +Nitrobenzene) etc.enteres in to this blower suction from the extraction vessel(which are not extracted but due to Vapour pressure goest to vapour line in the suction of blower).
3.No, I don't have Pressure Controle valve (PC) in the Vapour line of Extractor to vacum blower.
4.Yes, Benzene Emissions in Atmosphere is more.

Thanking You,
Yours sincerally,

 

[sshep]

dipak,
I think now maybe you are scrubbing acid gases with water rather than for benzene, and your benzene is just along for the ride.

Above 40C seems pretty warm for your mix. You seem like you could easily be in a situation where you are boiling off excess vapors just to satisfy the blower. The fact that the vapor line is throttled slightly indicates that in the past someone has tried to reduce the flow by increasing the system head/flow curve. Closing more than 80% may not have allowed for some upsets of temperature or more volatile material- this is where a control valve can help. Reducing blower speed can have a similar effect by reducing its head/flow curve, but will again lack the control functionality unless you can make variable speed.

Running the extractor colder will shift the blower back on its flow curve- the pressure in the extractor will run lower but so will the flow off the vessel.

best wishes,
sshep

 

[sshep]

I now understand the blower serves a common vent collection header, of which the extractor is the largest and most continuous load. Sized to accomidate design flows from all sources, the blower will be oversized for normal operation of the extractor alone.

Another common control system I have seen in such cases is to load the blower with air (or N2) right at the suction. If you can use air, this is sometimes done with a weighted pallet type vacuum breaker set at just a few inches of water vacuum.

-sshep