The rate of oil recovery in drum-skimming operations is dependent on the rate of cycling of the skimmer, the velocity of the recovery vessel on which the cycling drum is installed, and the absorbent material used.
These conclusions were reached by Dr. Mostafa E. Aboul-Fetouh after analyzing results from a series of drum-skimming tests performed at Al-Azhar University in Cairo.
The velocity of the skimming vessel is important because there is a critical velocity, above which an hydraulic jump will form in the oil along an arc ahead of the skimming vessel. This causes oil to escape around the end of the booms, which are used to concentrate the oil as the vessel moves through the water.
The rate of cycling of the drum is significant because, if it is too great, water is picked up by the absorbing element and the oil-removal efficiency is reduced.
EXPERIMENTAL PROCEDURE
Aboul-Fetouh and his associates at the Department of Mining and Petroleum Engineering used a model drum skimmer in their experimental testing program (Fig. 1). Design features and details of the apparatus have been previously published.1
The drum speed was adjusted to 25 rpm and the motor was turned on. At the same time, the squeezing operation was finely adjusted so that each roller just touched the absorbent material.
The drum and the squeezing rollers ran continually while samples of both water and oil-water emulsion were drawn at time intervals of 1, 2, 5, 10, 20, and 30 min.
After settling for 24 hr, the emulsion was separated from the clear water using a separating funnel. The water content of the emulsion was analyzed by the standard IP/74 method. The values determined at the smaller time intervals were important in assessing the initial rate of oil recovery.
Two types of absorbent material were tested by this method: polyurethane foam and felt. Both materials had fixed dimensions of 62 x 80 cm with a thickness of 6 cm.
The procedure was run on five types of oils with viscosities of 10, 50, 500, 1,000, and 1,500 cSt. Similar experiments were performed using drum speeds of 50, 75, and 100 rpm.
Other series of tests using this method were run on each type of oil, using felt as the absorbent material.
The effect of the submergence depth on the percent of oil recovered was also investigated at the previously mentioned drum speeds using felt as the absorbing material. Depths studied were 2, 2.5, and 3 cm.
DRUM SPEED
The following standard conditions were used for comparing oil recoveries:
- Drum speed, 25 rpm
- Oil viscosity, 10 cSt
- Submergence depth, 2.5 cm
- Thickness of absorbent material, 6 cm
- Oil layer thickness, 10 mm.
A 10-mm oil layer was used because the effectiveness of drum-type skimmers is reported to be reduced when the oil-film thickness is less than 0.25 in.2
Statistical analysis of the test results showed that as drum speed increased from 25 to 75 rpm, the percentage of oil recovered decreased, between 1 and 30 min (Table 1). On the other hand, the volume of recovered water-oil emulsion increased with the increase in speed.
These results were expected because, for a constant period of time, the contact between the absorbent material and the emulsion increases with drum speed. The percentage of oil recovered decreased because at high speeds the contact time between the absorbent and the oil layer is relatively short, and therefore insufficient for complete oil absorption.
The researchers found no significant difference in the data obtained at drum speeds of 75 and 100 rpm, keeping all of the other conditions constant.
ABSORBENT MATERIAL
No statistical difference was found between the absorbing power of polyurethane foam and felt at standard conditions. But the analysis did show a difference at drum speeds of 50-100 rpm (Table 1).
In fact, at these speeds, felt withstood the pressure of the squeezing rollers without any apparent tearing or deformation. In other words, felt maintained its homogeneity such that the pressure produced by the rollers is distributed uniformly over the area of contact.
In contrast, many defects occurred over the polyurethane foam. Its tightness around the drum was lost and, consequently, some parts were folded radially over the others, resulting in a nonuniform thickness.
Felt, therefore, is a more efficient absorber for oil recovery than polyurethane foam because no creeping occurs at high drum speeds, according to Aboul-Fetouh.
SUBMERGENCE DEPTH
For the felt-covered drum, the percentage of oil recovered decreased significantly with increasing submergence depth at higher drum speeds (75-100 rpm), at constant conditions.
Aboul-Fetouh says this is because high submergence depths usually result in a greater quantity of water being absorbed. This causes the emulsion formed to have a much higher water content. Such an emulsion invariably results in a lower efficiency of oil recovery.
REFERENCES
- Aboul-Fetouh, M.E., "Oil Removal from Water Surface Using Absorbent Synthetic Materials," Third National Meeting of Chemists, Mar. 12-14, 1989, Dhahran.
- Sittig, M., Oil Spill Prevention and Removal Handbook, first edition, Noyes Data Corp., New Jersey, 1974, p. 465.
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