An appraisal of mop skimmer technology.
By Peter Ilsley, Founder of OPEC LTD

Skimmer technology offers many alternative types such as mop, disc, drum, brush, belt, tube or weir skimmers.

1.0 Background

		A World patent was awarded to Shell international in 1967, for the use of a continuous belt of material, synthetic or otherwise, which would remove oil from the surface of water, by means of adsorption or absorption. From that time until the formation of the company - Oil Pollution Environmental Control Ltd., (in 1980), I, together with two other former colleagues, developed a "Rope Mop" system.
Fig 1. Example of two OPEC Mop Skimmers in operation on an airfield interceptor in the UK

This, utilized a polypropylene material to form a continuous loop of material, which would float on the surface of water, at the interface of the oil and water, to adsorb oil and not water, to remove surface oil contamination from sea water or fresh water. Other companies came and went during this time with variations, which failed for one reason or another. Others have since started up making versions of this. The technology they employ is still circa early 1970's. We - on the other hand - have continued to develop the systems, constantly improving the design as a result of experience gained in the day to day continuous operation of our Mop skimmer systems, world-wide. Also during this time and subsequent periods, various other skimming systems have evolved as listed above.

2.0 Comparison of OPEC mop skimmers and other mop skimmers

Two important factors need to be noted:

		2.1 The Mop The yarn used by OPEC to produce the mop material has a very high degree of surface area, due to the network structure of the special polypropylene base yarn. This produces a phenomenal surface area, whether it be for use in heavy oil (viscous) or in light oils such as gas oil, diesel, Jet fuel, spindle oil, light to medium lube oil, etc....Other manufacturers use a much coarser material that does not have the surface area that the OPEC material has. Also, the construction of the mop itself is special and we incorporate a centre tape within the structure (not on the outside) which gives a much better performance and strength support. We manufacture two basic types of mop, one Black and one White. The white mop is deployed when picking up light oils and the black mop is used for picking up the heavy oils.
Fig 2. OPEC Mop Skimmer vertical deployment

There are subtle differences in the two mop types which make them so efficient in the oil recovery duty they perform. Also, because of the tremendous surface area we have, the adsorption and retention of oil is so much better than any other mop. Also note that we can produce ready looped mops at the factory, which can be threaded into the OPEC mop drive/oil extraction machines, as ready looped mop, so reducing the time spent in setting up on site, by not having to cut, splice & join mops on site. This is unique!!

2.2 The mop drive/oil extraction unit

Our range of equipment offers the unique facility of threading ready looped mop in between the drive rollers of the "E" Series machines - the whole range and the same with the OM range. Again this gives two very important main advantages over competitor’s equipment: -

2.2.1 Health & Safety

The fact that OPEC MOP Skimmer Systems can be set up and deployed on site, without the need for the drive rollers to be rotating, offers the client a major advantage of not presenting a potential DANGER to the operatives. If an operator should accidentally trap his or her fingers in between the rollers during the threading of the mop, it can cause serious injury and we are aware of at least one major claim in Indonesia against one of our competitors. An operator had his hand crushed by a diesel driven unit, before they could shut down the engine when his hand was trapped between the drive rollers.

2.2.2 Ease and speed of equipment deployment

By having ready looped mops ready to thread through the rollers of the OPEC equipment, it obviates the need to cut, splice and join - open ended mops. It makes the job much easier and simpler for the operators, especially on industrial sites, where mop replacement may be a regular occurrence.

3.0 Comparison of OPEC mop skimmers and other skimmer technology

3.1 Belt Skimmers

These are only available as vertical deployment systems. This seriously limits the effectiveness of the skimmer, since it is located down through the oil layer at one very localized location. O.K. for boreholes, but not lakes lagoons or rivers where oil may be spread over a wide area, or on an industrial interceptor, which may have a large surface area. Also the belt skimmers are prone to limitations in performance due to the presence of debris. This blocks up the feed of the oil into the V shaped hopper feed when the oil is scraped off the belt. This causes any oil removed from the belt to spill over and fall back into the area from which it has been picked up. The belt also has a very small surface area compared to a mop - see notes above and there is a very poor retention of oil because of the relatively smooth surface which allows recovered oil to run off - especially the lighter oil range.

Because the belt has to be kept straight, there has to be a bottom roller attached to the bottom of the belt. This then means that the belt passes through the oil on the surface and due to the movement of the belt, will carry some of the surface oil down with it and displace it into the water phase. - Not a good idea. Again note there is a very small contact surface area with the oil.

OPEC Mop skimmers can be deployed vertically - down bore hole or through access manholes in tanks. With the use of floating pulleys (or as in the case of permanent installations in industrial sites, fixed guide rollers) the mop can be set out with different flow patterns on the oil/water surface to create surface oil movement. It will also make a large contact surface area with the floating oil. As regards to floating debris, - such as dead birds, wooden logs, tree branches, leaves, packaging materials rubber gloves etc. If the debris comes into contact with the mop and is small enough to adhere to it, the worst that can happen, will be for the debris to be squeezed between the drive rollers and will drop into the recovered oil container. The larger items of debris will not adhere to the mop, so will not affect the operation.

3.2 Drum Skimmers

Basically the same as 3.1 above. Surface area is small, oil pushed below the surface by drum rotation, problems with handling debris at the scrape off point.

3.3 Tube Skimmers

Small surface area, same problems with debris, small surface area contact. Same as 3.1/3.2

3.4 Disc Skimmers

Relatively small surface area. Localized oil contact and pick as for 3.1-3.3 above. Due to the centrifugal force created by the rotation of the discs, oil will be ejected from the discs (even "T" discs), so putting the oil back into the water phase. Again prone to debris problems, especially being caught up between the discs, so distorting them and causing mechanical damage. Also causes a blockage of the recovered oil to the oil collection in the centre of the disc skimmer, which can even cause the skimmer to sink.

3.5 Weir Skimmers

Due to the inherent design, even auto adjusting weir skimmers, will pick up a lot of free water. Also, very prone to debris blocking the flow of oil to the weir - depending upon the location of the support floats. This is particularly apparent when "sticky" - viscous oil is present, - such as Bunker "C" fuel oil.

3.6 Brush skimmers

Better than most of the units described above, but again prone to problems with debris being caught up in the bristles. Works ok in very viscous oil, but with light oil, not so good, because the oil readily runs off the filaments. Not constructed like the mop, with a fibrous structure.
In all cases noted above 3.1-3.6 inclusive. All of these systems have to be deployed using an oil transfer pump at all times, no matter what the duty, either emergency spill response or as industrial fixtures. In the majority of cases the OPEC mop skimmer systems do not need transfer pumps to be fitted and if a pump is required, this can be provided independently by the contractor on spill clean up.

		This could be the road tanker integral pumping system. Also most of the systems described have to be hydraulically driven, to allow the skimmer drive - say the rotation of the drum, or the brush elements or the discs or the belt.... As well as providing a separate drive for the pump system. Again the OPEC mop skimmer system is supplied with a direct drive, which can be electric, hydraulic, air, or diesel. In the case of the diesel drive unit, we drive both the drive rollers for the mop drive rollers and the pump (where fitted) from the same diesel drive motor. The OPEC mop skimmer system provides so many advantages over its competitors - and to top it all we manufacture in Stainless Steel!!
Fig. 3 OPEC Skimmer ready for shipment

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