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

 

[orenda1168]

Why not use a dissolved air flotation system with an appropriate demulsifier to maximize the oil/water separation?

Orenda

 

[EnvOp]

Orenda,

I appreciate you taking the time to reply. We currently do utilize a DAF system. The problem is that when we do have carry under from the Desalting system, due to the minimal capacity of the canals, and the weir that is supposed to keep the oil layer from carrying into the water side,the oil becomes too much for even the DAF tank, and CPI (corrugated plate intercepter)to handle. Any thoughts on using height vs. length. 1500bbl cylindrical tank equipped with heat tracing vs. 1000bbl horizontal canal system 4.5' deep.

 

[orenda1168]

EnvOp:

Though I've been away from crude desalting operations for a number of years, your problem seems to be more related to desalter operational issues, leading to excessive oil carryunder with the desalter wash water discharge. If this is so, then I would be more inclined to search out the desalter issues, maybe with the assistance of such firms as Petreco, Baker-Hughes, etc. and minimize the oil loading from that point, rather than trying to treat the symptoms of the problem by modifying the oily water discharge/separation/treatment system.

Orenda

 

[bimr]

The biggest problem that I have experienced with O/W separators is varying hydraulic flows. If you have a variable water level, it is also difficult to skim oil.

O/W separators work well under steady state conditions. Unfortunately, it is rare to get steady state conditions in industrial wastewater.

I actually had a system like you are describing many years ago. The O/W separator was not working properly because of varying hydraulic conditions. The tank in the system was originally intended to be an equalization tank, but the operators turned off the mixer and allowed the oil to accumulate.

If you have a bottom outlet on the tank, the long retention time in a tank will enable most of the oil to separate.

 

[EnvOp]

I appreciate you both replying. First, one thing that we are looking at is improving our desalter operation, but we're a coastal refinery that processes many different crude types, making it difficult to have good desalter operations at all times, so that's still in the works. So what I have to work on right now is the emulsified mess we have out there now, and what we're going to do when we do have carry under. On top of our desalter effluent stream there's the rest of the refinery waste water that combines with this going into a canal system that just re-emulsifies the problem. This is why I'm leaning towards having a primary separation tank that utilize heat to better separate the oil/water. Thoughts? Thanks

 

[bimr]

The system that you are proposing may be the correct alternative for your site. Why don't you do a bench pilot test to confirm the operating parameters.

 

[orenda1168]

I would still recommend that you address the carryunder problem at it's source...the desalter, whose operation appears to be periodically faulty, allowing an excess of crude in the discharge wash water. You have some expert help available to you via desalter manufacturers, e.g. Petreco, etc, as well as refinery chemical additive specialists, e.g. Baker-Huges, GE Betz, Nalco, etc., a combination of whom can identify and solve the the problem.

Anyway you cut it, this approach will most likely be a less expensive direction to proceed in than a revamp of your oily water discharge system, both in terms of Capex and Opex.

Orenda

 

[elguero]

At the Wilmington (CA) refinery, Unocal obtained instant separation of phases by filtering though a filter precoated with diatomaceous earth. (This was around 1990, when Unocal still owned the Wilmington refinery.) The filtration removed the many micron-sized solids that kept the emulsion from separating.

This worked, but was expensive.

I devised a flow sheet for using a classic evaporator to separate water from the oil phase, then blending the oil phase back into the refinery feed and using the water as "plant water."

A test was performed by Swenson Evaporator. The results are available in a California Department of Health Services report.

The work went no further, as Unocal became engrosed in business dealings that eventually led to the sale of their California assets to other firms.

I have further info, if this seems to hold promise for you.

 

[EnvOp]

Thank you again for all your input. Elguero, I am interested in learning more on your findings from CA. I've read a little on the diatomaceous filtering but heard that it was expensive, and wasn't sure @ what rate that would have to be replenished or replaced. Sounds like a clay tower filtration system.
I agree with you Orenda that the desalters are the root cause of our carry under, but even beyond that we've outgrown our water treatment system, and are looking to improve our effluent before our those restrictions are imposed on us. I will check out some of those contractors to see what they have to say about helping with our desalters. Does anyone have experience with a heater treater? Maybe preheating and treating the crude oil before with water and chemical before the exchanger train and desalters to get a better mix? Anybody know of a de-emulsifier that works well with various sweet and sour blends of crude oil? Thank you all again.

 

[orenda1168]

EnvOP:

I can't imagine desalting crude of any variety without suitable preheat....this is part of the specific support that the desalter contractors will provide, for the individual or blended crudes being run.

Demulsifier selection is case dependent, and though many of these formulatiuons are wide-ranging in their performance, they still require suitable bottle testing to pick and choose the appropriate product(s) for your system and crude blends. The vendors of these products, e.g. Baker-Hughes, Nalco, GE Betz, et al are expert at this.

Orenda

Orenda

 

[EnvOp]

I may have left it out, but we do utilize a preheat train of exchangers before the crude goes into the desalters, so that the temp is almost always between 230 and 290, but I was thinking more along the lines of treating it before it gets to our charge tank so that much of the bs&w is knocked out before it gets there. Thoughts?

 

[elguero]

The report sponsored by the California Department of Health Services is described as follows:

***** Start Here:

DEWATERING OF REFINERY SLUDGES
USING A FORCED CIRCULATION EVAPORATOR

DRAFT REPORT
DOHS CONTRACT NO. 89-T0120

DECEMBER 1990

California Department of Health Services
Alternative Technologies Division
Toxic Substances Control Section

******* End Here



I note that your phase separations are done at elevated temperatures, so some of the necessary heating capability is already in place. The NaCl portion of the aqueous phase will stay with the heavy bottom product, but that is OK, as, ultimately, it will leave the refinery with the heaviest tar products.

 

[refeng]

I am currently dealing with this problem. The emulsion generated by the desalter is so tight that it makes a mess out of the waste water treatment plant. I do not believe an ordinary gravity separator will touch this stuff. We have about 50 frac tanks of this stuff paying $1000 per month for rental and I have the job of figuring out what to do with it. It has set there for months and has not separated. This what I have learned so far. The emulsion is generally created by the desalter which is the smallest and runs the highest crude rate so I believe desalter residence time could be playing a role. We have Baker/Petrolite managing our desalters and they do a reasonably good job but for whatever reason there is periods when we make this stuff and there seems to be nothing anyone can do. Mix valve pressure does play a role and sometimes backing off on that helps but desalting suffers. I found someone who uses membrane techology to separate desalter rag layer slip streams and I'm evaluating that. It's called a VSEP and I can provide a website if you are interested. I am considering processing the water stream from the problem desalter before it is sent to the waste water system. The other solution I am looking at is buying a new larger desalter and use it as a second stage. This has benefits in better desalting such as fouling in the crude tower and ovhd and corrosion. I could then take the old smaller one and use it to reprocess the slop oil neat providing heat and extremely long residence times along with electrostatic grid should break out the fine oil droplets that are the cause of the problem.

 

[elguero]

Sounds like my evaporation scheme is what you need.

 

[refeng]

Wouldn't evaporation be very energy intensive? I guess there isn't that much slop so maybe it would be ok. Do you have any details?

 

[elguero]

What is the ratio of organic to water? I roughly guess that the cost would be acceptable at ratios 1 to 100 and upward. At 50 to 100, it's a no-brainer.

 

[refeng]

As little as 10% oil to as much as 90% oil is what I have seen and but usually around 40% to 50%. I can see one problem with evaporation. I need to desalt this material so it can by-pass the desalter and I don't see evaporation doing that. If you run it back through the desalter it just makes more emulsion. If you run salt to a tower you salt it off and get corrosion.

 

[elguero]

Just envision the evaporator as follows.

The entering stream consisists of water, salt and crude petroleum. Two streams leave the evaporator. The light (overhead) stream is water with some light organic low molecular weight material (say pentane on up through C18). The heavy (bottoms) stream consists of asphaltic materials plus the salt.

The light end is condensed (at, say 30 degC). And the water is separated from the organic by decanting.

The heavy end can be blended with other heavy material destined for sale as asphalt. The salt leaves your refinery as part of your asphalt product.

Operation of the evaporator requires some design features that are hard to describe without drawings. Get a copy of the report issued by the California DOH (see above).

Buena suerte.

 

[refeng]

That sounds like fractionation to me. I guess if you do it at low enough temperatures that you don't break down the chlorides it might work. In a normal crude tower you would get the chlorides breaking out going ovhd and salting off and corroding everything. What temperature does it operate at? Have you tested it?