GB2276561A - Continuous separation of immiscible liquids - Google Patents

Abstract

Sludge on the floor of an oil storage tank 1 is broken up by jets of hot water delivered by a tracked robot 4, and the resulting mixture of oil, solids and water is drawn into a vessel 6 and then fed to a separation tank 10. The mixture enters chamber 14 of tank 10 and separates under gravity. Oil flows over a vertically adjustable weir 26 into chamber 20. Water flows through lower outlet 32 to chamber 16 and then over weir 30 to chamber 18. Settled solids are removed from chamber 14 by a chain scraper to outlet 34 and reduced to a cake in centrifuge or decanter 38. The separated oil and water are also purified by centrifuges, with the water being recycled to robot 4. Various stages are steam-heated. Sludge break up may be assisted with chemicals from tank 42. <IMAGE>

Description

CONTINUOUS SEPARATION OF A MIXTURE OF IMMISCIBLE LIQUIDS The present invention relates to apparatus for, and to a method of, continuous separation of a mixture of immiscible liquids, and to a system for the reclamation of sludge from a tank and for separating it into its constituent components. The present invention has particular though certainly not exclusive application in the separation of oil sludge from a crude oil storage tank into its constituent components.

A problem has existed in the oil refining industry in that if a crude oil storage tank ruptures or requires maintenance, it is necessary to pump the crude oil to another storage tank and to remove oil sludge lying on the bottom of the tank before work can be carried out on the tank. The oil sludge on the bottom of the tank is very viscous and, usually, must be removed from the tank manually. This operation is quite arduous and requires personnel to work for extended periods inside the tank.

Further, the working environment inside the tank is extremely hazardous due to the nature of the volatile gases in the tank. Consequently, it is necessary for personnel removing the oil sludge to wear protective clothing and associated breathing apparatus, as well as to take frequent rest periods. These factors result in the time taken to clean a typical crude oil storage tank being about two to four months.

Legislation currently exists in some Australian states requiring crude oil storage tanks to be inspected on a regular basis and similar legislation is expected to be enacted soon in the other remaining states. Each tank inspection will necessitate the emptying and cleaning of each tank, and the oil sludge on the bottom to be removed. It is anticipated that this will have to be done at least once every ten years. Consequently, there is a need for a means of removing oil sludge from crude oil tanks which is both safe and efficient.

The volume of oil sludge in each tank is typically greater than 20,000 litres and consists of a mixture of water, oil and waste residue. The waste residue comprises such matter as sand and rust. The oil sludge has a high hydrocarbon content and, consequently, disposal charges levied by local authorities make the disposal of oil sludge to waste extremely expensive. If, in the alternative, some of the hydrocarbons are extracted from the oil sludge to a more acceptable level in order to reduce disposal costs, and to make the resultant product more compatible with the environment, the additional refining or separation process that is required is both expensive and inefficient.

Furthermore, conventional separation processes usually require sophisticated control devices, such as electric level detecting switches and associated control gear, which can be unreliable in operation and which, by virtue of an electric current being required for them to operate, can present a fire and/or explosion risk, particularly if the oil sludge is mixed with solvents, which is common practice in an effort to dissolve the sludge in those solvents.

Process equipment for continuous collection of oil from a liquid has been previously proposed in US patent 4,980,070 to Lieberman. The equipment disclosed in this patent is designed specifically for the removal of very small amounts of oil from a liquid such as coolants and other treatment liquids used in various metalworking and metalforming processes. The equipment includes a tank separated into two compartments by a partition which has an aperture at its bottom. The mixture of oil and coolant can separate into layers under the effect of gravity in one compartment so that a thin layer of oil lies on top of the coolant. This oil layer is then, in effect, skimmed off by allowing it to flow over a number of weirs which are height adjustable. A fixed height weir is provided in the second compartment for maintaining a fixed level of separated coolant in the two compartments. In use, the height of the adjustable weirs is set so that they are just above the height of the second weir to allow the very thin layer of separated oil to flow over them. This equipment is not suitable for the separation of mixtures comprising a more viscous oil as a more viscous oil requires a longer settling time to enable it to more effectively separate under the effect of gravity. In that regard, if the depth of the oil layer is increased, the separation time will increase and the separated oil will be more pure. This patent does not contemplate controlling the depths of either of the liquids in the first compartment. In addition, such equipment has a very slow separation rate and must, if a cost-effective separation rate is to be obtained, be very large in size.

According to a first aspect of the present invention there is provided apparatus for continuous separation of a mixture of immiscible liquids, the apparatus comprising first holding means for receiving the mixture and in which the liquids can settle into layers according to their respective specific gravities and second holding means interconnected with the first holding means by interconnection means so that the first and second holding means are in fluid communication, the arrangement being such that only the higher specific gravity liquid will flow into the second holding means from the first holding means under the effect of gravity, the first holding means having first outlet means vertically displaced from second outlet means of the second holding means, the second outlet means being arranged such that it is below the first outlet means but above tne interconnection means, the vertical displacement between the first outlet means and the second outlet means being adjustable to thereby control the depths of the liquids in the first holding means, and first and second collection means associated with the first and second outlet means respectively, the arrangement being such that in use, as the mixture is added to the first holding means, the lower specific gravity liquid flows from the first outlet means into the first collection means and the higher specific gravity liquid flows from the second outlet means into the second collection means.

The first and second holding means may consist of tanks having a common wall and the apparatus may be sized so that it can be loaded into a standard freight container and the first outlet means may be height adjustable so that, as will be described in detail below, the depths of the separated layers of the liquids in the first holding means can be varied to ensure that a sufficiently deep layer of the higher gravity liquid is maintained in the first holding tank In a preferred form of the invention, the interface between the layers of liquids in the first holding means is substantially below the second outlet means. This results in the layer of lower specific gravity liquid having a depth which is sufficient to increase the overall separation time of the mixture into layers. This optimises the purity of the liquid collected by the first collection means.

In addition, the first holding means may be heated to assist in melting of the mixture to thereby facilitate the separation process.

The mixture of immiscible liquids may comprise sludge from a tank and may comprise a oil and water.

According to a second aspect of the present invention there is provided a system for the reclamation of sludge from a tank and for separating it into its constituent components, the system comprising means for spraying water onto the sludge to break it up, means for removing the water and broken up sludge mixture from the tank to an apparatus for continuous collection of oil from the mixture, the apparatus comprising first holding means for receiving the mixture and in which the liquids can settle into layers according to their respective specific gravities and second holding means interconnected with the first holding means by interconnection means so that the first and second holding means are in fluid communication, the arrangement being such that only the higher specific gravity liquid will flow into the second holding means from the first holding means under the effect of gravity, the first holding means having first outlet means vertically displaced from second outlet means of the second holding means, the second outlet means being arranged such that it is below the first outlet means but above the interconnection means, the vertical displacement between the first outlet means and the second outlet means being adjustable to thereby control the depths of the liquids in the first holding means, and first and second collection means associated with the first and second outlet means respectively, the arrangement being such that in use, as the mixture is added to the first holding means, the lower specific gravity liquid flows from the first outlet means into the first collection means and the higher specific gravity liquid flows from the second outlet means into the second collection means.

In a preferred form of the second aspect of the invention, the means for spraying water onto the sludge is a remote controlled robot which is moved about the floor of the tank spraying hot water onto the sludge through a nozzle carried by the robot. The use of a robot eliminates the need for personnel to work inside the tank for extended periods and the use of hot water assists in melting the sludge. If the particular sludge to be reclaimed has a high melting point, the temperature of the water may be raised to a temperature of up to about 80 Celsius, though some oils can be successfully broken up using water which is not heated. A further advantage of the use of hot water is that water vapour results. This water vapour displaces volatile gases inside the tank, thereby reducing the likelihood of an explosion.

In addition, the mixture in the first holding means and/or the sludge in the tank may be heated to thereby facilitate the separation process. This heating may be provided by way of pipework, through which steam can pass, arranged so that heat is transmitted to the sludge or the mixture. If the sludge in the tank is to be heated, the pipework may be arranged on the floor of the tank and if the mixture is to be heated, the pipework may be arranged on the interior of the first holding means.

Further, pushing means may also provided within the first holding means for pushing waste residue to an associated outlet. This waste residue may then be further processed to reduce its moisture content and stabilised by the addition of suitable stabilisers.

The separated liquids may be further refined by a centrifuge and/or a hydrocyclone and water separated by the centrifuge, the hydrocyclone, and the water from the second collection means may be recycled by connection to a water source which provides water to the means for spraying water.

According to a third aspect of the present invention there is provided a method of continuous separation of a mixture of immiscible liquids, the method comprising introducing the mixture into first holding means in which the liquids can settle into layers according to their respective specific gravities and from which the higher specific gravity liquid flows under the effect of gravity into second holding means by way of means interconnecting the second holding means with the first holding means, allowing the lower specific gravity liquid to flow out of first outlet means associated with the first holding means into first collection means and allowing the higher specific gravity liquid to flow out of second outlet means associated with the second holding means into second collection means, the second outlet means being arranged such that it is below the first outlet means but above the means interconnecting the second holding means with the first holding means, and adjusting the height of the first outlet means relative to the second outlet means to control the depths of the layers of liquid in the first holding means.

The depth of the lower specific gravity liquid layer in the first holding means is dependent on the displacement between the first outlet means and the second outlet means and the specific gravity of the liquid. If, for example, the liquid to be separated is oil having a specific gravity of 0.85 and the vertical displacement between the first and second outlets is 50mm, the depth of the oil, Doil, is calculated by the following relationship: Doil x S.G.oil = (Doil - 50) x S.G.water Consequently, if S*Geoil = 0.85, and since S.G.water = 1, D011 = 333mm.

The adjustment of the distance between the first and second outlets controls the depth of the lower specific gravity liquid layer, depending on its specific gravity, within a range sufficient to provide a sufficiently deep layer of separated water having a minimum amount of oil entrained therein in the first holding tank and allow sufficient space for other ancillary equipment, such as a scraper on the floor of the first holding means, yet still enable the separation process to be completed within an apparatus having a particular maximum overall height.

The present invention will now be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic view of a system for reclaiming oil sludge from a crude oil storage tank and for separating the oil sludge into its constituent components; Figure 2 shows a schematic view of an embodiment of an apparatus for separating a mixture of the oil sludge and water: the apparatus being depicted with its outer walls removed for clarity; Figure 3 shows a side view of the apparatus of Figure 2 in the direction of arrow A; and Figure 4 shows a side view of the apparatus of Figure 2 in the direction of arrow B.

Figure 1 shows a schematic view of an embodiment of a system according to the present invention for removing oil sludge from a crude oil tank 1 and for separating the oil sludge into its constituent components, namely oil, water and waste residue, such as sand and rust. A crude oil tank is typically more than 30 meters in diameter and the oil sludge lying on the bottom of the tank may be more than two meters high.

When it is required to remove the oil sludge from the crude oil tank 1, an operator dons protective clothing and breathing apparatus, enters the tank through a man-way 2 in the side of the tank 1 and removes some of the oil sludge using a shovel, for example, to clear a portion of the floor of the tank adjacent the man-way 2. A tracked, remote controlled robot 4 is then inserted through the man-way 2 and is set down on the cleared portion of the tank floor, after which the operator exits the tank. The robot 4 carries a nozzle through which high pressure hot water can be sprayed.

In use, the robot 4 is driven about the floor of the tank 1 and a jet of the hot water is directed at the oil sludge to break it up. The use of hot water assists in melting of the sludge. If the particular sludge to be reclaimed has a high melting point, the water is raised to a temperature of up to about 80 Celsius, though some oils have a low melting point and can be successfully broken up using water which is not heated. A further advantage of the use of hot water is that water vapour results which displaces volatile gases inside the tank, thereby reducing the likelihood of an explosion, as can occur with conventional cleaning methods in which a spark, for example, can ignite the gas in the tank The mixture of oil sludge and water on the floor of the tank is then extracted from the tank by way of a flexible pipe S having an inlet end lying in the tank, its other, or outlet, end being connected to a vacuum tank 6 which is connected to a liquid ring vacuum pump 8. In use, the vacuum pump 8 creates a vacuum inside the vacuum tank 6 which draws the mixture from the crude oil tank 1 into the vacuum tank 6. The mixture then flows continuously from the vacuum tank 6 into a separating apparatus 10 which, as will be described below, continuously separates the mixture into its constituent components. The vacuum tank 6 also has steam heated coils (not shown) wound about its inner and outer surfaces in order to provide additional heat to the mixture to assist in melting the sludge to facilitate the subsequent separation process which will be described below.

The separating apparatus 10 comprises a rectangular outer tank 12 which is sized so that it can be loaded into a standard freight container for ease of transport to a location near a crude oil tank to be cleaned. Referring to the Figures, the outer tank 12 is partitioned to provide a separation tank 14, an intermediate water holding tank 16, a water tank 18 and an oil tank 20. In that regard, a partition 22 extends laterally across the outer tank 12, dividing the separation tank 14 from the intermediate water holding tank 16, water tank 18 and oil tank 20. Partition 22 consists of a fixed portion 24 and an oil weir 26 which is height adjustable; the fixed portion 24 standing between the separation tank 14 and the intermediate water holding tank 16 and the oil weir 26 standing between the separation tank 14 and the oil tank 20. The oil weir 26 is mounted on the outer tank byway of a screw threaded arrangement, for example, so that by rotating an associated hand screw, for example, the oil weir 26 may be raised or lowered to adjust the depth of the oil layer lying in the separation tank 14. Further, as most clearly shown in Figures 2 and 3, the fixed portion 24 of partition 22 has an aperture 32 at its bottom.

The intermediate water holding tank 16 and the water tank 18 are separated from the oil tank 20 by another partition 28 which extends in the longitudinal direction, and the intermediate water holding tank 16 and the water tank 18 are separated by a water weir 30 having a fixed height. The water weir 30 is arranged so that its height is less than that of the oil weir 26.

Prior to commencing the separation process, water is pumped into the separation tank 14 so that it then flows into the intermediate water holding tank 16 by way of the aperture 32 until it overflows the water weir 30 whence the level of water in both the separation tank 14 and the intermediate water holding tank 16 is, of course, the same. The oil sludge and water mixture is then permitted to flow from the vacuum tank 6 into the separation tank 14 so that the water and oil separate into layers according to their respective specific gravities. As the mixture separates, and as the mixture flows continuously into the separation tank 14, only the water flows continuously into the intermediate holding tank through the aperture 32 since it has the higher specific gravity and the heavier particles in the mixture, such as sand and rust, settle to the bottom of the separation tank 14. The flow of water through the aperture 32 is indicated by arrow X in Figure 3.

The oil and water in the separation tank 14 and the water in the intermediate holding tank 16 then attain respective new equilibrium levels such that the upper surface of the oil layer is higher than the level of water in the intermediate holding tank 16. This is due to the layer of oil lying on the water having a specific gravity of less than that of the water which has a specific gravity of 1.00. A typical specific gravity for oil is about 0.85.

As shown in Figures 1 to 4, the upper edge of the oil weir 26 is higher than the upper edge of the water weir 30 and the aperture 32 is arranged below them so that the liquids can maintain equnit)flum. As more or tne mixture is aaaea to me separation tank 14, the oil overflows the height adjustable oil weir 26 into the oil tank 20 while the separated water concurrently overflows the water weir 30 into the water tank 18. The separated water is then either drained to waste from the water tank 18, or is further processed, for example, through a hydrocyclone 34, and the separated oil is either pumped to storage in another crude oil storage tank, or to a further refining process. The flow of water over the water weir 30 is indicated by the arrow Y in Figure 3 and the flow of oil over the oil weir 26 is indicated by the arrow Z in Figure 4.

The depth of the oil layer in the separation tank 14, as mentioned above, is dependent on the vertical displacement between the upper edge of the oil weir 26 and the upper edge of the water weir 30. If, for example, the liquid has a specific gravity of 0.85 and the vertical displacement between the respective upper edges is 50inn, the depth of the oil, Doil, is calculated by the following relationship: Doil X S.G*oil = (Doil - 50) x S.G.water Consequently, if S.G.oil = 0.85, and since S.G.water = 1, Doil = 333mm.

Accordingly, the height of the oil weir 26 is adjustable to enable the depth of the oil layer, depending on the specific gravity of the particular oil, to be controlled within a range sufficient to provide a sufficiently deep layer of separated water having a minimum amount of entrained oil in the separation tank 14 and allow sufficient space for other ancillary equipment, such as a scraper on the floor of the tank, yet still enable the separation process to be completed within an apparatus having a particular minimum overall height. If the apparatus is sized so that it can be contained inside a standard freight container, the maximum height is 1.7 metres.

In practice, the height of the oil weir can be varied in the range of O - 75mm.

In order to facilitate transport of the system inside a standard freight container, the vacuum tank 6 is attached to the separating apparatus 10 by hinges so that the vacuum tank 6 can be rotated about the hinges for temporary storage in the interior of the separating apparatus. In addition, the vacuum tank 6 is heated by pipework similar to that used for heating the mixture in the separation tank 14.

This separation apparatus is simple, inexpensive to operate and significantly safer in operation than the various separation apparatus found in the prior art as it relies only on gravity, rather than on more sophisticated control devices, such as electric level detecting switches and associated control gear, which can be unreliable in operation and which, by virtue of an electric current being required for them to operate, can present a fire and/or explosion risk The separation tank 14 is also provided with a chain-driven scraper (not shown) which moves across the floor of the separation tank to push waste residue lying on the bottom to an outlet 34 at one end of the separation tank 14. The waste residue is then loaded, via a residue holding tank 36, into a horizontal decanterlcentrifuge 38 from which it exits as a cake. This cake is then mixed in a dual screw mixer (not shown) with a stabilising agent, such as lime, cement or sawdust, and loaded onto a truck for transport to a dumping area for use as land fill.

The stabilised cake has a low hydrocarbon content, being of the order of less than 5%, and does not incur the substantial disposal fees incurred by untreated oil sludge waste.

The separated oil may be further processed such as, for example, straining through strainers 36, heating in a plate heat exchanger 38 and then be further separated in a rotary centrifuge 40.

The separated oil from the hydrocyclone 34 recycles back to the separating apparatus 10, the separated oil from the centrifuge 40 may be pumped to another storage tank and the clean water from the hydrocyclone 34 and/or the water from the water tank 18 is recycled by connection to a pump which supplies water to the nozzle of the robot 4, rather than disposing it to waste.

If the mixture to be separated comprises more than two immiscible liquids having different specific gravities, a number of the above described apparatus may be placed in series in order to separate each liquid sequentially.

Chemicals from a chemical tank 42 may be also added to the water sprayed by the robot 4 to assist in breaking up the oil sludge in the crude oil tank 1 and/or the same or other chemicals, may be added to the separation tank 14 from a chemical tank 44 to promote faster separation of the liquids of the mixture into layers.

The invention has been described by way of example only and modifications are possible within the scope of the invention.

Claims (11)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. Apparatus for continuous separation of a mixture of immiscible liquids, the apparatus comprising first holding means for receiving the mixture and in which the liquids can settle into layers according to their respective specific gravities and second holding means interconnected with the first holding means by interconnection means so that the first and second holding means are in fluid communication, the arrangement being such that only the higher specific gravity liquid will flow into the second holding means from the first holding means under the effect of gravity, the first holding means having first outlet means vertically displaced from second outlet means of the second holding means, the second outlet means being arranged such that it is below the first outlet means but above the interconnection means, the vertical displacement between the first outlet means and the second outlet means being adjustable to thereby control the depths of the liquids in the first holding means, and first and second collection means associated with the first and second outlet means respectively, the arrangement being such that in use, as the mixture is added to the first holding means, the lower specific gravity liquid flows from the first outlet means into the first collection means and the higher specific gravity liquid flows from the second outlet means into the second collection means.

2. Apparatus according to claim 1, wherein the first and second holding means comprise tanks having a common wall.

3. Apparatus according to claim 1 or 2, wherein said first outlet means is height adjustable so that the depths of the separated layers of the liquids in the first holding means can be varied to ensure that a sufficiently deep layer of the higher gravity liquid is maintained in the first holding tank

4. Apparatus according to claim 1, 2 or 3, wherein the interface between the layers of liquids in the first holding means is substantially below the second outlet means, whereby the layer of lower specific gravity liquid has a depth which is sufficient to increase the overall separation time of the mixture into layers.

5. Apparatus according to any preceding claim, wherein said first holding means is heated to assist in melting of the mixture to thereby facilitate the separation process.

6. A system for the reclamation of sludge from a tank and for separating it into its constituent components, the system comprising means for spraying water onto the sludge to break it up, means for removing the water and broken up sludge mixture from the tank to an apparatus for continuous collection of oil from the mixture, the apparatus comprising first holding means for receiving the mixture and in which the liquids can settle into layers according to their respective specific gravities and second holding means interconnected with the first holding means by interconnection means so that the first and second holding means are in fluid communication, the arrangement being such that only the higher specific gravity liquid will flow into the second holding means from the first holding means under the effect of gravity, the first holding means having first outlet means vertically displaced from second outlet means of the second holding means, the second outlet means being arranged such that it is below the first outlet means but above the interconnection means, the vertical displacement between the first outlet means and the second outlet means being adjustable to thereby control the depths of the liquids in the first holding means, and first and second collection means associated with the first and second outlet means respectively, the arrangement being such that in use, as the mixture is added to the first holding means, the lower specific gravity liquid flows from the first outlet means into the first collection means and the higher specific gravity liquid flows from the second outlet means into the second collection means.

7. Apparatus according to claim 6, wherein said means for spraying water onto the sludge is a remote controlled robot which in use is moved about the floor of the tank, spraying hot water onto the sludge through a nozzle carried by the robot.

8. Apparatus according to claim 6 or 7, wherein the mixture in the first holding means and/or the sludge in the tank is heated to thereby facilitate the separation process.

9. Apparatus according to claim 8, wherein said heating is provided by way of pipework, through which steam can pass, arranged so that heat is transmitted to the sludge or the mixture.

10. Apparatus according to claim 7, 8 or 9, further including pushing means provided within the first holding means for pushing waste residue to an associated outlet.

11. A method of continuous separation of a mixture of immiscible liquids, the method comprising introducing the mixture into first holding means in which the liquids can settle into layers according to their respective specific gravities and from which the higher specific gravity liquid flows under the effect of gravity into second holding means by way of means interconnecting the second holding means with the first holding means, allowing the lower specific gravity liquid to flow out of first outlet means associated with the first holding means into first collection means and allowing the higher specific gravity liquid to flow out of second outlet means associated with the second holding means into second collection means, the second outlet means being arranged such that it is below the first outlet means but above the means interconnecting the second holding means with the first holding means, and adjusting the height of the first outlet means relative to the second outlet means to control the depths of the layers of liquid in the first holding means.