GB2190854A - Method and apparatus for separating oil and water - Google Patents

Abstract

Oil-contaminated water, such as marine bilge water, is passed through a gravity separation chamber (3) in which relatively large drops of oil are removed. The water then passes through a fibrous coalescing bed (9) in which more oil is coalesced to facilitate its removal. The water finally passes through a porous membrane (13) on whose surface substantially all the remaining oil droplets are coalesced into larger drops. The gravity separation chamber (3), the oil coalescing bed (9) and the porous membrane (13) are arranged one above another in a single housing (1a,1b) with an upper feed water inlet (5) communicating with the gravity separation chamber (3) and a lower pure water outlet (12). Oil sensor (21) controls valve (22) through which most of the oil is discharged. Membrane (13) is of polyethylene terephthalate, stated to repel oil and water. <IMAGE>

Description

SPECIFICATION Method and apparatus for separating oil and water The present invention relates to a method and an apparatus for separating oil from oil-contaminated fresh water or seawater such as marine bilge water (referred to herein as 'feed water').

The separation of oil from water is generally carried out by three different methods: firstlythe gravity separation method which utilizes the difference in specific gravity of oil and water; secondly the oil coalescing method in which small droplets of oil are coalesced into larger drops and then removed, and; thirdly the adsorption method in which oil is trapped on an adsorbent bed of activated carbon, fibres orthe like. It is also known to use two or more of these methods in various combinations.

Japanese Patent Publications Nos. 58-41887and No.55-16687 propose the combination of three methods in which a fibrous bed for coalescing the oil droplets is used for coarse separation and a gravity separation chamber and a fibrous bed for the adsorptive capture of oil are used forthefine separation.

However, the known method described above includes introducing a feed water into a gravity separation chamber and an oil-coalescing fibrous bed to separate a portion of the contaminating oil and trapping the residual fine oil droplets by adsorption on the matrix of an adsorptive fibrous bed and hasthefollowing disadvantages. Oil progressively accumulates in the adsorptive fibrous bed which thus becomes progressively blocked thereby producing a sharp increase in the flow resistance to the feed water.When the amount of oil deposited in the fibrous matrix exceeds a certain level, fine oil droplets entering the matrix tend to pass through the fibrous bed without being trapped with the resultthatthe equipment cannot be operated continuously on a long-term basis with a high degree of efficiency To avoid this problem, afrequent regeneration procedure including washing off the trapped oil from the absorptive fibrous bed matrix is thus required.

This problem is particularly serious when thefeed water is heavily contamined with oil, particularlya high-viscosity oil, such as heavy fuel oil.

It is an object of the present invention to provide a method and an apparatus for the separation of oil and water which permits continuous long-term operation with a high separation efficiency.

According to the present invention there is provided a method of separating oil and water in which oil-contaminated water is passed through a gravity separation chamber in which relatively large drops of oil are removed, then through a fibrous oil coalescing means in which a further portion ofthe oil is removed and then through a porous membrane ofwater-and oil-repellent material in which substantially the remainder of the oil is removed. The preferred embodiment includes sensing the presence of oil accumulated at the top of the gravity separation chamber and periodically withdrawing the said accumulated oil in response thereto.

The invention also embraces apparatus including three separation stages for carrying out the method and the preferred embodiment of such apparatus includes a housing which contains the three separation stages with thefirststage above the second stage which is in turn abovethethird stage and which has an inletfor oil-contaminated water communicating with the gravity separation chamber and a pure water outlet at its lower end.

Thus in the method and apparatus ofthe present invention relatively large drops of oil are separated from the feed water in the gravity separation chamber whilst smaller drops are separated by the fibrous oil coalescing means. The feed waterthen passes through the porous membrane and the remaining droplets of oil coalesce into larger drops on the inner and outer surfaces ofthe membrane dueto its oil-repellent properties. The relatively large drops thus formed on the surfaces ofthe membrane then float away or can be readily removed which results in there being no substantial increase in flow resistance across the membrane.

Moreover, the membrane does not tend to swell due to the penetration of water into the membrane material itself by virtue of its water-repellent properties.

Furtherfeatures and details of the present invention will be apparent from the following description oftwo specific embodiments which is given by way of example with reference to the accompanying drawings, in which Figure 1 is a vertical front sectional view of separation apparatus in accordance with the present invention; Figure2 is a sectional view on the line Il-Il in Figure 1; Figure3 is a scrap sectional view on the line Ill-Ill in Figure 1; Figure 4 is a viewsimilarto Figure 3 of an alternative type of oil coalescing element.

The apparatus includes a closed housing 1 comprising an upper compartment la and a lower compartment 1 b detachably connected together by abutting flanges 1c. The top ofthe upper compartment la communicates with an oil outlet pipe 2 through an upwardly directed, conical open end thereof.

The upper compartment 1a and a cylindrical partition 4within ittogether define a gravity separation chamber3 with which a feed water supply pipe 5 communicates.

Provided in the partition 4 is an orifice 6 adjacent to divider which extends tangentiallyfrom the partition 4 to the outerwall ofthe compartment ia. In use, feed water flows in through the pipe 5,circumferentially around the gravity separation chamber 3 and then enters the space within the cylindrical partition 4 via the orifice 6.

Further provided within the gravity separation chamber 3 at a position generally diametrically opposite the supply pipe 5 is a dam plate 8 over which the feed water in the cha mber 3 flows.

Afibrous oil coalescing bed 9 is disposed in the upper compartment 1 a below the partition 4. This bed, which is30to 100 mm thick, consists of metallic fibres with a diameter of 20to 70 microns packed with a packing density of 250 to 300 kg/m3. The bed 9 may alternatively be made of synthetic resin fibers and may also be of composite construction consisting of a bottom layer of fibres with a comparatively large diameter and atop layer of fibres with a comparatively small diameter.

Disposed in the lower compartment 1 b is an oil coalescing element 11 removably attached to a partition 10 whose outer edge is removably clamped between the flanges 10 ofthe upper and lower compartments 1 a and 1 b. Awateroutletpipe 12 communicates with the bottom ofthe lower compartment 1 b.

The oil coalescing element 11 comprises a porous membrane 13 made of water- and oil-repellent material, such as polyethyleneterephthalate, interposed between a porous inner cylinder 14,which mayforexample be a wire-mesh screen, and a porous outer cylinder 15 which may also be a wire-mesh screen. The inner and outer cylinders 14 and 15 are removably mounted by stay bolts 19 on a header 18 which is removably attached to the partition 10 by bolts 20. The header 18 is provided with oil outlet ports 16 at its top and water inlet ports 17 in its side.

In orderto increase the effective surface area ofthe porous membrane 13, a plurality of oil coalescing units 11 may be provided, each of which is a cylindrical cartridge of the type described above. Alternatively, the porous membrane 13 may be pleated orofzig-zag configuration as illustrated in Figure4.

Disposed in the upper portion of the upper compartment 1 a is an oil sensor 21 which is connected to a solenoid valve 22 in the oil outlet pipe 2 and is arranged to sense the presence of oil and to open the valve 22 when it does so. An oil outlet 23 disposed below the oil coalescing bed 9 is connected to the outlet pipe 2 via a conduit26 provided with a valve 24and a check valve 25. At the upper portion of the lower compartment 1 bis an oil outlet pipe 28 provided with a valve 27 and at the lower portion ofthe gravity separation chamber 3 is a sludge outlet pipe 30 provided with a valve 29.

In use, feed water, such as oil-contaminated bilge water, is admitted into the gravity separation chamber3 from the feed pipe Sand flows circumferentially. The sludge in this feed water settles on the bottom ofthe gravity separation chamber3 and is periodically withdrawn from the apparatus via the sludge outlet 30.

Large oil drops in the feed waterfloat to the top ofthe upper compartment 1 a.

Thewaterthusfreed ofsludge and large oil drops flows into the space within the cylindrical partition 4 through the orifice 6 and as it flows dwn within the partition 4, oil drops in it float up towards the top ofthe uppercompartment 1a. The oil floating uptothetop ofthe upper compartment 1a progressively accumulates and when its presence is detected by the sensor21 the valve 22 is opened and it is discharged through the outlet pipe 2.

The water flowing down within said cylindrical partition 4 passes through the fibrous oil coalescing bed 9 disposed beneath the gravity separation chamber 3. Comparatively large oil drops which were not removed in the gravity separation chamber3 and the cylindrical partition 4 coalesce in the bed 9 into larger oil drops which float up towards the top of the upper compartment 1 a and are then removed.

The relatively uncontaminated water which has passed through the oil coalescing bed 9 flows into the oil coalescing element 11 through the inlet ports 17 in the header 18, and passes through the porous membrane 13.

As the porous membrane 13 is made of an oil-repellent material, fine oil droplets which have not previously been separated coalesce into larger drops on the inner and outer surfaces of the porous membrane 13. The substantiallyoil4ree water which passes through the membrane 13 is discharged through the outlet pipe 12 and the oil drops which have formed on the inner surface of the porous membrane 13float up through the oil outlet ports 16 in the header 18. Any oil drops which form on the outer surface of the membrane 13 float up towards the underside of the partition 10 and are withdrawn periodicallythrough the oil outlet pipe 28.

Oil which accumulates atthe underside of the oil coalescing bed 9 is discharged, by opening the valve 24at timed intervals,throughthe conduit 26 into the oil outlet pipe 2. The checkvalve 25 in the conduit 26 is provided to prevent the backflow of oil to the underside ofthe oil coalescing bed 9, since otherwise when the valve 24 is open while the solenoid valve 22 in the oil outlet pipe 2 is closed the oil collecting at the top ofthe upper compartment 1a might flow to the underside ofthe oil coalescing bed 9.

Awater-oil separation test was conducted with the above equipment using water contaminated by heavy fuel oil and water contaminated by light oil. The oil concentrations (ppm) at a first point A, which is immediately upstream of the fibrous oil coalescing bed 9, at a second point B,which is downstream ofthe fibrous oil coalescing bed 9 and at a third point C, which is downstream of the porous membrane 13 in the oil coalescing element 11,were measured with differing feed waters and the results are set forth in thefollowing table:: oil oil oil concent- concent- concent ration at ration at ration at pointA point point C (ppm) (ppm) (ppm) Water contaminated with 0.5% of heavy fuel oil 1250 90 3.2 Water contaminated with 13%ofheavy fuel oil 5500 105 3.5 Water contaminated with 25% of heavy fuel oil 7000 102 3.6 Water contaminated with 0.5% of light fuel oil 2200 220 3.6 Water contaminated with 13%oflight fuel oil 7500 240 3.8 Water contaminated with 25% of light fuel oil 9300 235 4.0 The method and apparatus ofthe present invention thus include a three stage separation of oil and water and substantially avoid a sudden buildup offlow resistance across the final stage, i.e. the porous membrane.

The use of an oil and water repellent porous membrane obviates the swelling of the porous membrane due to penetration of water and oil and thus ensures a long service life and contributes to an increased separation efficiency. It also permits long term continuous operation with a high separation efficiency and the invention is particularly effective when the feed water is contaminated with a high-viscosity oil such as heavy fuel oil or is contaminated with a large amount of oil.

Claims (5)

1. A method of separating oil and water in which oil-contaminated water is passed through a gravity separation chamber in which relatively large drops of oil are removed, then through a fibrous oil coalescing means in which a further proportion of the oil is removed and then through a porous membrane of water-and oil-repellent material in which substantially the remainder of the oil is removed.

2. Apparatus for separating oil and water comprising three separation stages connected in series, the first stage including a gravity separation chamber, the second stage including fibrous coalescing means through which the oil-contaminated waterflows and the third stage including a porous membrane of water-and oil-repellent material through which the oil-contaminated waterflows.

3. Apparatus as claimed in claim 2 including a housing containing the three separation stages, thefirst stage being above the second stage which in turn is above the third stage, the housing having an inletfor oil-contaminated water communicating with the gravity separation chamber and a pure water outlet at its lower end.

4. A method of separating oil and water substantially as specifically herein described with reference to Figures 1 to 3, optionally as modified by Figure 4, of the accompanying drawings.

5. Apparatus for separating oil and water substantially as specifically herein described with reference to Figures 1 to 3, optionally as modified by Figure 4, ofthe accompanying drawings.