In some cases, previously described oil removal technologies based on the gravity separation of oil from the cleaning solution are unjustifiably cumbersome and expensive. Fortunately, smaller scale solutions are available for use in such applications.
Oil Skimmers –
Oil skimmers utilize a material which preferentially attracts oil (hydrophobic) to skim floating oil from a surface leaving the majority of any aqueous component behind.
The oil, which coats the surface of the attractant is then removed from the surface and collected for disposal or re-use. The oil-attracting material is usually configured as a belt or disc which is partially submerged in the liquid being skimmed. A squeegee or scraper is used to continually remove oil for disposal or re-use. A typical example is shown below.
Devices like that shown above work well in many relatively non-critical applications to remove oil.
Oil Absorbing “Filters” –
In more critical applications additional means such as oil absorbing filters are required for complete oil removal. Such “filters” are similar in construction to those used to filter out particles and, in fact, in some cases may remove both particles and oil from a recirculated solution. They contain materials which absorb oil and reject aqueous liquids. Limitations include the fact that most of these devices have only a limited capacity to remove oil. Once the absorbant has become saturated, the canister (filter) must be discarded and replaced. This, of course, is not a practical solution if there is a large amount of oil present.
Under specific and controlled conditions, ultrafiltration may be utilized to remove oil from a processing bath even if it is in the form of an emulsion. Basically, the cleaning chemistry must be designed to have a molecular size smaller than that of the oil being removed. Oil molecules are typically rather large compared to those found in many cleaning chemistries. To filter out the oil molecules typically requires the use of a filter made of porous stainless steel (or other material) which is coated with a material which provides the very small passages required to allow water and cleaning chemistry to pass while retaining the oil molecules. Problems with this method of oil removal include the possibility of clogging of the filter media. I will provide a more thorough discussion of ultrafiltration as a means of oil removal in a later blog.
In summary, oil removal is a major consideration in any cleaning process where oil is present as a contaminant. Selection and implementation of the proper oil removal means will determine success or failure in many applications and should be considered early in the design phase of any cleaning equipment.
– FJF –