Rotating pipe/drum skimmers for API separators

[petrotrinsmerv]

At our refinery we have an API separator that skims separated oil using slotted pipes. In looking into rotating pipe/drum skimmers, I have seen certain claims for the oil recovered being <10 v/v % or even <5 v/v %water. Oil extracted using the slotted pipes generally has >50 v/v % water, unless the operator is very careful. In any case you expect a minimum of about 25 v/v % water.
What has been the experience with these rotating skimmers in actual use? We have limited storage and handling facilities for slop oil and we would like to minimize the volume recovered from our API separator.

 

[chicopee]

I have seen similar rotating skimmers in steam degreasers and they seem to work very well.

 

[bimr]

The sole purpose of the skimmer is to remove the oil from the water, not to concentrate the skimmings. The skimmers work adequately for that purpose.

 

[petrotrinsmerv]

I understand that the removal of the oil is the main objective. However, the ammount of water removed along with the oil has implications for downstream processing, especially for equipment/vessel sizing. That's why the manufacturers make the claims they do. Also, there are alternative technologies (pipe/drum, brush, tube, disc etc. or hydrocyclones) each with strengths and weaknesses. These things matter when contaminated water effluent is a fact of life but investments must yield the best return. While most manufacturers do not make blatantly false claims, they certainly push the envelope in trying to 'make the sale'. I was hoping to get info on the pitfalls and/or shortfalls encountered using this technology, or if and why other facilities opted for alternative technologies.

 

[bimr]

It is somewhat difficult to entertain the pro/cons of an application when you have provided very little information on your application. People also don't normally discuss wastewater treatment as an investment. Wastewater treatment is an expense and it is typical that corporations want to spend as little as possible to get by. Oil removal also tends to be a low tech labor intensive process.

API separators are intended for coarse oil removal. The API units are typically in the ground since pumping of oil tends to emulsify the oil, and you do not want to emulsify the oil.

Note that the API units should also be considered a fire hazard since the units are typically open top with oil floating on the surface. You may also encounter environmental issues from the air emissions.

If your application consists of volumes of oil, then the API unit is the correct type of oil separator. If you have only have emulsified oil, then you probably want to consider the use of plate type oil separator. If you have a tough emulsification, then you would be interested in chemical treatment.

It is common to follow the API separator with additional wastewater treatment like a dissolved air flotation unit to polish off the removal of oil.

The Abanaki type tubular or belt units work well, but would be overwhelmed with an application with large volumes of oil.

Recovered oil from an API type of oil separator would typically be taken to a small vertical decant tank, where you can remove the excess water and decant the water back to the API separator. The decanting is usually done by gravity.

The biggest problem that you will encounter in the oil separation process is the flow variation. Oil separators work best when operating at a constant flow rate. Unfortunately, a constant flow scenario is not usually the case. You have to try to anticipate the event where a high flow rate may potentially wash oil out of the oil separator.