Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1997-06-18
1999-05-04
Lithgow, Thomas M.
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210708, 210712, 210738, 210199, 210205, 2102212, 210521, 210802, C02F 124, C02F 152, C02F 140, B01D 2108
Patent
active
059001542
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a method and an equipment for the purification of a liquid, wherein the liquid may be polluted by other liquids or solid material, e. g. removing oil from water, comprising a flotation device.
Extracting oil and gas offshore results in large amounts of sea water containing oil. The major part of the oil may be separated from the sea water in separating containers or tanks, but this purification is not sufficient for rendering the sea water back to the sea. Today, authoritative regulations say that spill water from offshore oil- and gas installations shall not contain more than 40 ppm oil.
When removing oil and/or other pollutants from water there are commonly a plurality of methods using centrifuges, hydrocyclones and flotation devices.
Centrifuges are advanced machines giving a high degree of purification, but their specific capacity is low. The use of a purification installation of this type when extracting oil, comprises a plurality of machines connected in parallel. Such installations are very expensive and it is required to keep a large stock of spare parts. The maintenance routines are also quite comprehensive.
Another disadvantage is that installations of this type are rather energy consuming. A typical energy consumption is 1 kwh/m.sup.3 of purified water.
The hydrocyclone may be considered as "common art" because they are preferred in new platform installations. These cyclones have quite good performances at favourable working conditions. Meanwhile, they have some limitations and disadvantages. In the presence of small oil droplets, a chemical flocculation device has to be installed upstream the hydrocyclone installation. The shear forces that may occur in the hydrocyclones are large, and the flakes have to be very robust. Flakes with the required strength may be difficult to produce.
Further, the pressure drop in the hydrocyclone is large, commonly more than 40 m W.P.
Downstream from the hydrocyclone it is required to have a flotation chamber with a rather large capacity when relieving the pressure. According to certain working conditions a hydrocyclone plant may be quite comprehensive to install. The capacities of the hydrocyclones are small, and therefore they have to be connected in groups to serve as large batteries when higher capacity is demanded.
The third type of separating devices, flotation plants, are characterised in that they have low specific capacities and low purification effects. The space needed for this type of installation is large, and they are often difficult to operate. Today, flotation plants are regarded as "technology of the past", and are of little relevance in offshore use because of the disadvantages these plants represent.
SUMMARY OF THE INVENTION
The present invention may be regarded as a sort of flotation plant, but it represents a substantial improved solution among these. Thus, the plant according to the invention needs less space and volume, is substantially lighter and has better purification effects than hydrocyclone installations.
The invention is characterised in that the purification process comprises adding one or more chemicals to the polluted liquid contained in a flocculation device. The flocculation device consists of one or more pipe loop(s) with built-in agitator(s) that effect turbulence and plug-type flow through the loop, followed by separating the purified liquid and the pollutants in the flotation device or in a sedimentation device.
The equipment according to the present invention is characterised in that it comprises a pipe loop with built-in agitator in conjunction with the flotation device.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following, the invention is described in detail with reference to drawings that illustrate embodiments thereof:
FIG. 1 is a schematic illustration of a liquid purification plant according to the invention.
FIG. 2 is a schematic illustration of an alternative embodiment of the plant in FIG. 1.
FIG. 3 is on an enlarged scale an agitator included in the inve
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patent: 4750994 (1988-06-01), Schneider
patent: 5120435 (1992-06-01), Fink
patent: 5156745 (1992-10-01), Cairo
patent: 5167798 (1992-12-01), Yoon et al.
patent: 5356533 (1994-10-01), Nakagawa
Lithgow Thomas M.
Mastrans AS
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