Oily water separation. 1

[EnvOp]

Hi folks,
I'm trying to get information on upgrading our wastewater system. We currently process up to around 5000bbls/day of waste water and are looking to revamp the whole system. One question I have is if height of a tank and volume outweigh surface area when trying to separate oil from water. Our current system uses box style canals that are only 5' deep but probably have a total volume of 1000bbls capacity. I'd like to make our primary separation a cylindrical tank with a height of 25' and a total volume of 2500 bbl cap., though it would probably be operated around the 15' level so 1500 bbls. Heat is another factor that is possible using this tank that we wouldn't have in the canal design. Thanks

 

[elguero]

Evaporation is one, single stage. Fractionation is multiple stages. The Swenson evaporator test was at about 300 degF. This was not optimum. A recycle of blended light and heavy organics is used to control the fluidity of the material in the evaporator. The Swenson test did not contemplate an optimum blend.

Are you sure that significant amounts of chlorides will go overhead with the water vapor?

 

[refeng]

At the temperature found in a crude column (600 F range )much if not all the chlorides will disassociate and go overhead as single chloride ions. They become HCl in the ovhd. I am not sure what temperature the disassociation occurs at. You are talking about 1/2 the temperature so the chlorides may stay with the bottoms in your case. A vacuum evaporator would be even better in that regard, lower temperature and you get more of the goodies out. Asphalt is a lowered valued product, you hate to downgrade the crude oil to asphalt. I would prefer to physically separate the oil and leave the salt with the water, then what's left has crude value or it could be sent the FCC and cracked and have even more value. We checked the metals and they were no worse than DAO and it's a small stream. But just getting rid of it now it the prime mover as the rental on the frac tanks are eating our lunch.

 

[elguero]

The stream I have been referring to is the DAF (dissolved air flotation) overflow at the Wilmington (California) refinery. Typically it consists of 5% crude, but on a good day can be 50%, and on a bad day 1% --- the balance is water. I would not attempt to do this on the entire refinery feed stream.

The source of the feed to the DAF was all the watery sludges, including washdown, spill, desalter, etc.

The overflow from the DAF was stored in a pair of tandem tanks that were used as phase separators. Sometimes the separation was good giving 50/50 organic/aqueous mix. On other occasions the separation was lousy, <5% organic.

 

[refeng]

We don't have a DAF but some sort of separator and the results are similar. I think it is basically the same stuff. I did just find this website and I'd like to know if anyone reading this ever tried one of these separators?

http://www.pall.com/datasheet_chemical_40422.asp?sectionid=performance

 

[elguero]

It seems to work by causing the coalescing of each phase. None-the-less there will be a certain amount of uncoalesced material - - a mix of oil and water droplets stabilized by the solid particles. That's why, when Unocal filtered their sludge, the filtrate separated without a hitch. There were no solids to stabilize the emulsion.

It looks to me as though the Pall unit will do a good job on "easy" emulsions, but will fail with really stable emulsions. Maybe you can send a sample to Pall and have them run a test. Also ask them for references at petroleum refineries who are presently using the units on streams similar to yours. Then call the references. Find out what the real operating experience has be with a unit in a similar service.

 

[refeng]

That sounds like sound advice, I hope to do that. So far I have not been able to make contact with anyone. I sent an email to them requesting a contact so I'm sure they will sooner or later. I tried calling but was not able to get anyone. They must be really busy.