Liquid purification or separation – With means to add treating material – Directly applied to separator
Reexamination Certificate
1999-01-25
2001-01-16
Lithgow, Thomas M. (Department: 1724)
Liquid purification or separation
With means to add treating material
Directly applied to separator
C210S519000, C210S521000, C210S522000
Reexamination Certificate
active
06174435
ABSTRACT:
TECHNICAL FIELD
The invention relates to a device for clarifying by lamellar flotation. The device according to the invention can be used to accelerate the separation of materials in suspension, referred to hereinbelow by the abbreviation MIS, contained in a liquid, by natural flotation, if their density is less than that of the liquid. The invention can also be used in combination with the technique of flotation with dissolved air. In these two applications, the construction and the operating principle of the device according to the invention are the same.
The common name for clarifiers of this type, using flotation and the lamellar technique, is an inclined-plate float or a lamellar float.
The process and the device described are particularly designed to accelerate the separation of materials in suspension from a liquid effluent.
PRIOR ART
The existing devices of this type use the following techniques:
A/ The Technique of Lamellar Separation.
This technique is used to accelerate the separation of the MIS from a liquid when the density of the MIS is different from that of the liquid. If the density of the MIS is less than that of the liquid, this is referred to as lamellar clarification by flotation. The fundamental theory of lamellar separation and the advantages of this technique will not be described herein, since they are considered as being known.
B/ The Technique of Flotation with Dissolved Air.
This technique is often combined with the technique of lamellar separation. It consists in using the property of microbubbles of air (or of another gas), produced by a suitable device, this property lying in the ability of these microbubbles to adhere to the particles, i.e. to the materials in suspension, present in the liquid, and to entrain them to the surface of the liquid. In this case, this is referred to as a forced flotation.
The principle of the lamellar separation by flotation of the MIS from the liquid will briefly be described below, in support of FIGS.
1
and
2
A-
2
C:
it should be pointed out beforehand that, in the case of the use of the technique of flotation with dissolved air, the device for producing microbubbles of air required for the flotation is not described herein, since it is considered as being known. It is simply assumed that the MIS to be separated from the liquid has a density less than that of the liquid, or else that it is “lightened” by the microbubbles and that it therefore floats.
The liquid charged with floating material is introduced between the inclined plates (
1
) also referred to as lamellae (FIG.
1
). The floating material, represented in the various figures by small circles, travels upwards until it reaches the surface of the top plate of the said lamellae. It then rises by sliding along the plate constituting each of the lamellae, in the direction of the arrow represented by mixed lines (
2
), up to the top end of each of the said lamellae. On arrival at this point, the floating material detaches from the plate and rises to the surface of the liquid. The space occupied by the particles, and thus liberated, is taken by the clarified liquid, which “slides” along the surface of the bottom plate in the direction of the arrow (
3
), i.e. in the opposite direction to the upward travel of the floating material, i.e. from the top downwards. Thus, the clarified liquid and the floating material cross between two plates to become separated: the floating material to the top and the clarified liquid to the bottom.
Depending on the direction of the flow of introduction of the liquid to be clarified relative to the lamellae, three types of lamella clarifiers exist:
1/ Co-current Clarifiers (
FIG. 2A
)
The liquid to be clarified (
4
) is introduced from the bottom upwards. Between the plates (
1
), the floating material and the liquid travel in the same direction.
Theoretically, this solution is very advantageous, since the upward travel of the floating material is not perturbed by the movement of the clarified liquid, since the two entities travel in the same direction. In reality, this solution has hardly ever been applied in practice, given the problem associated with removing the clarified liquid. The reason for this is that the floating material and the clarified liquid are recovered on the same side of the lamellae and they readily become remixed in the zone located above the lamellae.
2/ Cross-current Clarifier (
FIG. 2B
)
The liquid to be clarified is introduced laterally via the side of the device in the direction of the arrow (
4
), such that the liquid to be clarified and the floating material circulate perpendicularly relative to the direction of introduction. This technique is also rarely used in practice on account of the problems associated with the equal distribution of the flows.
3/ Counter-current Clarifier (
FIG. 2C
)
The liquid to be clarified is introduced in the direction of the arrow (
4
) from the top downwards. This technique is by far the one most commonly used in practice, since, in this case, the separation of the floating material and of the clarified liquid is very clear: the floating material is recovered above the lamellae, and the clarified liquid is recovered below the lamellae. Nevertheless, the implementation of this solution comes up against several problems:
in order to increase the total projected surface (TPS) and thus the separation capacity of the float for the same ground surface area, it proves to be necessary to;
reduce the distance between the lamellae; however, the smaller this distance, the greater the friction between the veil of rising floating material and the flow of the descending clarified liquid, which perturbs the upward travel of the floating material;
increase the length of the lamellae in order to be able to increase the rate of passage between the lamellae; in reality, the length of the lamellae depends on the amount of MIS to be removed and on the compactability of the veil of floating material. Lamellae which are too long are easily engorged at the top by an excessively large veil of floating material;
in the space located above the lamellae, the floating material concentrated between the lamellae comes into contact with the liquid to be clarified. Consequently, it is rediluted by the liquid and some of it is once again entrained by the liquid between the lamellae, which decreases the efficacy of the separation.
In practice, these problems are reflected by a decrease in the limit speed of ascent Va=flow rate/TPS applicable on a lamellar float relative to a float with vertical flow. This decrease in the limit speed of ascent depends on several factors:
the flotation speed of the particles: the higher the speed, the more the limit speed of ascent of the lamellar float approaches that of the float with vertical flow;
the concentration of MIS: the higher the value, the more the limit speed of ascent of the lamellar float decreases relative to that of the float with vertical flow;
the compactability of the veil of floating material: the more easily the floating material is compacted, the more the limit speed of ascent of the lamellar float approaches that of the float with vertical flow.
The consequence of these various factors results in the lamellar clarification being used well within its theoretical capacities.
As a guide, in the case of clarification of aqueous effluents by flotation with dissolved air, the maximum theoretical flotation speed is about 18 m/h. In practice, floats with vertical flow are limited to about 8 m/h for the most advanced constructions, while certain lamellar float manufacturers are limited to a limit speed of ascent Va of only 2 m/h, essentially on account of the hydraulic constraints mentioned above.
It emerges from these various observations that co-current clarification makes it possible to rapidly separate large amounts of MIS, but, on the other hand, only achieves a coarse clarification. In contrast, counter-current clarification gives good results for relatively small amounts of MIS, and makes it possible to achieve high-quality clarification. Howeve
Lithgow Thomas M.
Wall Marjama & Bilinski
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