EP0954379A2

EP0954379A2 - Apparatus and method for induced air flotation

Info

Publication number: EP0954379A2
Application number: EP98904132A
Authority: EP
Inventors: Marko Zlokarnik
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-01-23
Filing date: 1998-01-23
Publication date: 1999-11-10
Also published as: WO1998032538A2; WO1998032538A3

Abstract

A device (10) for induced air flotation that uses a self aspirating and radially discharging funnel-shaped nozzle for producing extremely fine gas bubbles. The nozzle in addition may be used with a flotation cell (30) that consists of a mixing chamber (32) and an annular channel (34) around it thereby enabling vertical motion of the liquid through from top to bottom of the annular channel (34).

Description

APPARATUS AND METHOD FOR INDUCED AIR FLOTATION

FIELD OF THE INVENTION

The present invention concerns induced air flotation.

BACKGROUND OF THE INVENTION With the exception of mineral processing, where flotation processes have been used for decades for selective classification of ore constituents, flotation has found relevance primarily in waste water treatment, where it is used for thickening of excess biological sludge. For this particular task, extremely fine gas bubbles and an absence of turbulence in liquid flow are essential. Both are achieved by a so-called dissolved air flotation technique, in which approximately 30% of clarified waste water is recycled via a pressurized vessel in which the feed is contacted with air at approximately 70 pounds per square inch (psi) to roughly 70% of the saturation concentration. The liquid throughput is subsequently decompressed in a flotation trough or tub by a decompression valve. As result, tiny gas bubbles are formed with bubble diameters of about 40-60 micrometers

(μm) which attach themselves to the hydrophobic surface of the sludge flocks and thereby float the sludge to the surface of the liquid.

In order to introduce flotation as a mechanical liquid/solid separation process for the chemical industry, such as for the clarification of process waters, a so- called induced air flotation process has been devised. For this purpose, a self-aspirating and radially discharging aeration device in the form of a funnel-shaped nozzle is known, such as was described in European Patent No. 0 035 243, issued to BAYER AG, D - Leverkusen (Inventor: M. Zlokarnik) and dated 7 May 1986. The nozzle consists of a deflecting cone which is arranged in a funnel-shaped housing thus forming an annular channel in the space between the cone and the funnel . This channel provides a Venturi-type confuser/diffuser. Above the top of the cone, a liquid propulsion nozzle line is placed. The liquid jet impinges against the deflecting cone, is fanned out by it, and becomes uniformly distributed over the whole cross- section of the annular channel. Here, the air, which is sucked into the mixing chamber, becomes intimately mixed with the liquid and discharged over the whole circumference of the deflecting cone, forming a free jet of the gas/liquid dispersion, which is afterwards uniformly distributed over the bottom of the cell.

This funnel-shaped nozzle produces relatively tiny gas bubbles of approximately 200 μm and distributes them evenly over the floor of the flotation cell . This technology has been successfully applied in many industrial tasks, see for example, M. Zlokarnik, "New Approaches in Flotation Processing and Waste Water Treatment in the Chemical IndustryX in Ger. Chem. Eng. 5 (1982) 2, 109-115 and Kern. Ind. (Zagreb) 34 (1985) 1, 1-6. An example of a large- scale application of this technology is the recovery of silver salts from washwaters of a large film factory as described in European Patent No. 0 059 227 issued to AGFA Leverkusen, and dated 24 April 1984.

In the design of activated sludge units as deep shafts or high towers (instead of "ponds") having a liquid height, H, greater than about 30 feet, the subsequent sludge- sedimentation causes serious problems. This is likely a result of dissolved C0₂ which desorbs and therefore hinders the sedimentation process. In this application, flotation should therefore be carried out continuously, which requires a completely new concept for the flotation cell. Preferably the flotation cell is subdivided into two different spaces by inserting an inner vessel concentrically into a larger vessel . The inner vessel is equipped with the funnel-shaped nozzle and serves as the actual flotation cell . Between the inner and outer vessel an annulus is formed, thus enabling vertical liquid flow from top to bottom. In the upper portion of the annulus, flocks accumulate and form a filter which guarantees a complete hold back of the residual flocks . The annulus serves as ^■ a calming zone where the laminar motion of the liquid throughput facilitates the final separation of the finest flocks. See for example M. Zlokarnik, "Neue Flotationstechniken zur Abtrennung und Eindickung von Klaerschlamm bei der biologischer Abwasserreinigung" , in Chem. -Ing. -Tech. 57 (1985) 5, 442- 446. This technology has been successfully introduced to tower biology plants in breweries, see E. Schmidt, "Vollbiologische Betriebsklaeranlage" , Brauwelt 123 (1983) 42, 1830-1841 and G. Kubeck, "Das neue Abwasserreinigungskonzept der Bittburger Brauerei - Verfahren und erste Erfahrungen" , Forum der Brauerei 6 (1984), 126-131. However, it has turned out that in some applications both the aeration device (e.g., the funnel-shaped nozzle) as well as the above-described flotation cell display certain disadvantages which inhibit their universal use.

For some solids dispersions, the funnel-shaped nozzle produces gas bubbles that are too coarse to ensure satisfactory separation. To fulfill this task optimally, the liquid propulsion jet in the annular space of the funnel-shaped nozzle must therefore be tuned to provide the ut ost shear rate. This task cannot be accomplished by an increase in suction alone, since it hardly makes any sense to increase the suction of the funnel-shaped nozzle while at the same time choking the gas throughput as it is done in funnel-shaped nozzles with a constant hydraulic cross- sectional area (see M. Zlokarnik and J. Susa, in "Selbstansaugende und radialstrahlende Trichtesduese" , Chem. -Ing. -Techn. 68 (1996) 12, 1572-1574).

The hitherto concept of a flotation cell in which the actual flotation chamber is surrounded by an annulus was also described in a German patent application No. 33 47 525 Al, issued to BAYER AG, D - Leverkusen (Inventors: M. Zlokarnik) , dated 5 Sept 1985, where the liquid stream is calmed down by laminar motion from top to bottom, and completely depleted from solid particles.

Unfortunately, large-scale applications (with flotation cell diameters, D, >> 10 feet) of this approach encounter two serious disadvantages .

The first disadvantage is the fact that the flotate (froth) is pushed by the skimmer over the whole cross- sectional area of the cell. This perturbs the liquid motion in the annulus and has the disadvantage that parts of the compacted flotate may sediment before they can be removed from the rim of the cell. The second disadvantage concerns the calming zone (the annulus) itself. Its dimensions are typically chosen according to the superficial velocity, V, found in model- scale measurements (normally v = 30 ft/h) , and additional requirements for proper laminar flow (i.e. keeping the Reynolds number Re at about 2000) . Both requirements can be fulfilled only by inserting a calming grid of arbitrary shape in the annulus. In doing so, one has to accept that flocks may cover the grid surface thereby constricting the cross-sectional area.

The present invention aims at purposeful changes of the funnel-shaped nozzle and of the flotation cell to eliminate these disadvantages.

SUMMARY OF THE INVENTION

The present invention is an aeration device for inducing a liquid/gas dispersion which includes a cone- shaped deflecting element arranged inside of and spaced apart from a funnel-shaped outer casing such that an annular channel is formed between the cone and the funnel . A liquid propulsion nozzle is disposed adjacent the apex of the funnel and arranged to cause the liquid exiting therefrom to impinge upon the deflecting cone and to be fanned out thereby.

A first novel aspect of the present invention are directed to disposing one or more trip edges within the annular channel. The trip edge(s) prevent formation of a laminar boundary layer along the deflecting element, thereby promoting turbulence in the fluid flow within the annular channel. The trip edges also promote turbulence within the annular channel itself, improving the suction action of the funnel shaped nozzle and thereby also strongly intensifying the shear rate in the liquid flow which in turn is a prerequisite to producing gas bubbles which are as fine as possible.

The trip edges may be disposed on either the cone- shaped deflecting cone element surface or on the inner surface of the funnel shaped casing. Preferably the annular channels between the deflecting cone and the funnel shaped housing deflecting cone element and funnel have approximately a constant hydraulic diameter.

In another preferred embodiment, the annular channel may be segmented into at least four sections, with each of the sections having trip edges disposed therein. The segments channel may be shaped to provide convergent, divergent, or a Venturi shaped exit path to provide the desired fluid flow characteristics.

Yet another aspect of the invention concerns a flotation cell provided with an inner cylindrical chamber and an outer annular space, in which the annular space serves as the flocculator or "mixing" chamber and the inner chamber serves as the tranquilizer or "calming" chamber. Yet another aspect of the invention concerns a flotation cell provided with an inner cylindrical chamber and an outer annular space, in which the annular space serves as the flocculator or "mixing" chamber and the inner chamber serves as the tranquilizer or "calming" chamber. The calming zone exhibits laminar vertical motion from top to bottom thereby facilitating the separation of the residual flocks.

Within the flotation cell, liquid throughput forming the liquid propulsion jet is provided by a recycling centrifugal pump. By reversing the traditional functions of the inner chamber and annular space in the flotation cell, a number of advantages are achieved.

In addition, the majority of the flocks float in immediate vicinity of the outer cell wall and, therefore, may be removed by a skimmer without perturbing the liquid surface of the tranquilizer. Indeed, the accumulated flocks along the upper part of the tranquillizing zone are now left practically undisturbed by the skimmer and the laminar motion within it is maintained. In the lower portion of the inner cylindrical vessel, a simple cross shaped grid may be inserted. This may, therefore, be implemented far more easily in the cylindrical vessel than in the annular space, thereby permitting the dimensions of the cross sectional area of the inner cylinder to be relatively large at the expense of the area of the outer annular space.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views.

Fig. 1 is a cross sectional view of a self-aspirating and radially discharging aeration device according to the invention.

Fig. 2 is a plot of the suction characteristics of the aeration device.

Fig. 3A is a cross-sectional view of an alternative embodiment of the aeration device .

Fig. 3B is a top end view of the aeration device of Fig. 3A. Fig. 4 is a cross sectional view of a flotation cell in which the aeration device may be disposed to provide an induced air flotation for separating activated sludge.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Turning attention now to the drawings more particularly, Fig. 1 depicts a cross sectional elevational view of a self-aspirating and radially discharging funnel shaped aeration device 10 according to the invention. The aeration device 10 consists of a deflecting section 12 shaped preferably as a cone which is arranged within a funnel-shaped housing 14. The cone 12 and funnel 14 are spaced apart to thereby form an annular channel 16 between them. The channel 16 therefore provides a Venturi-type confuser or diffuser depending upon the exact relative angular orientation of the cone 12 and funnel 14. The spacing between the cone 12 and funnel 14 is maintained by spacers 18 disposed between them. Above the top of the cone 12 is disposed a liquid propulsion jet nozzle 20 typically within the funnel. The liquid jet nozzle 20 causes liquid exiting therefrom to impinge against the apex of the cone of the deflecting cone 12. Liquid is fanned out by the cone 12 becoming uniformly distributed over the cross sectional area of the annular channel 16. Also, within the annular channel, any air being sucked into the device through the top of the funnel 14 is ultimately mixed with liquid exiting from the jet nozzle 20 being discharged over the circumference of the deflecting cone 12. The resulting free jet of gas-liquid dispersion is therefor afterwards uniformly distributed over the floor of the flotation cell.

The inventive improvement of the funnel-shaped nozzle consists of inserting trip-edges 22 to either wall of the annular channel 16. Their purpose is to rip off the' laminar boundary layer from the deflecting cone 12 and from the casing 14, thus promoting the utmost turbulence in the annular channel 16. The use of the trip-edges 22 improves the suction action of the funnel-shaped nozzle 10. However, of possibly even greater importance is the fact that the trip edges 22 also promote the utmost shear rate in liquid flow which is a prerequisite to producing gas bubbles which are as fine as possible.

Fig. 2 is a chart depicting the dependency of the suction characteristics of the funnel-shaped nozzle 10 upon certain characteristics. In this plot, the diameter of the cone base, D_b, was 160 millimeters (mm) ; the diameter of the propulsion jet nozzle, d, was 20 mm; and the liquid head H' above the cone base was H' 800mm. The plot illustrates that spreading of the liquid jet over the whole cross-section of the channel 16 such as encouraged by the trip edges 22 significantly improves the suction action. The process number governing here is the extended Froude number Fr' =q_L ²/d⁴gH' , where q_G and q_L are the air and water throughputs, respectively, and g is the acceleration due to gravity. In funnel-shaped nozzles 10 having diameters of the cone base, D_b, greater than about 500 mm, and a minimum slot width from about 10 - 15 mm, cross-sectional areas of the annular channel may result which yield liquid throughputs of approximately 100 m³/h (26.400 gal/h) with free jet velocities of greater than 1.5 m/s (5ft/s) . To prevent essentially lower free jet velocities when applying the design to larger funnel-shaped nozzles, one may be forced to reduce the circumference of the slot by leaving parts of it blank, such as by implementing segments 26 into the annulus. The segmentation of nozzles has already been already described, in U.S. Patent No. 4,162,971 issued to BAYER Aktiengesellschft, Leverkusen, FRG (Inventors: M. Zlokarnik and Klaus Elgeti) , dated 31 July 1979, but for other reasons and applications.

One preferred design of such a funnel-shaped nozzle with segments 26 is depicted in Figs. 3A and 3B. In the present case, segmentation 26 of the annular channel 16 represents an additional freely adjustable geometric parameter which results in a threefold advantage: a) In connection with a minimum slot width of the annular channel, it assures a high velocity of the free jet of the gas-liquid dispersion. b) The segments 26 may have different geometrical shapes or positions resulting in a confusor-, diffusor- or Venturi-shaped channels 16. Thus it is possible to optimally satisfy each requirement with regard to gas suction and shear rate. For high suction, a diffusor- shaped channel 16 with a slight enlargement of approximately 7° will be favorable. As a general rule, four to eight segments 26 will suffice. c) The segments make it possible to connect the funnel-shaped cone 12 solidly with the casing 14 thus allowing a centric, light, yet stiff connection between both parts .

The present inventive improvement also consists of reversing the functions of an inner chamber and an annular space of a floatation cell within which the jet nozzle may be deployed.

This inventive design is depicted in Fig. 4. In this configuration, the annular space serves as the actual flotation or flocculation space and the inner chamber represents the calming or tranquilizing zone.

As depicted in Fig. 4, the flotation cell 30 consists of an inner preferably cylindrical chamber 32 and outer annular chamber 34. The annular space 34 serves as the actual flotation space in which mixing and aeration occurs whereby the inner space 32 serves as the calming or tranquilizing zone. The aeration unit 10 is disposed beneath the bottom of the cell typically within the center of the outer annular space. A feed inlet 36 and air inlet 38 are coupled to the aeration unit 10. A feed outlet 40 provides an exit point for clarified liquid to exit from the tranquilizing zone 32. A flotate or froth outlet 44 is provided at the top of the cell 30. The cell is activated by connecting a centrifugal pump 42 to drive the aeration unit 10. A motor 46 provides for driving a skimmer 46 to remove floate from the liquid surface of the cell.

The reversing of the functions of the inner chamber and annular space provides a variety of advantages . a) The free jet of the funnel-shaped nozzle 10 distributes the gas-liquid-dispersion evenly across twice cross-sectional area. This results in an essential suppression of bubble coalescence. b) Most of the flocks float in the immediate vicinity of the outer cell wall and therefore are removed by the skimmer 46 without perturbing the liquid surface. c) The filter formed of accumulated flocks in the upper part of the calming zone is now left essentially undisturbed by the skimmer 48, the laminar motion within it also being safeguarded. d) In the lower part (e.g., within the lower A to 2/3 of the height) of the inner cylindrical vessel 32, a cross- or ring-shaped grid 48 maybe inserted. This can be implemented far easier in a cylindrical chamber 32 than in an annular space as in the prior art. This also permits the cross-sectional area of the cylinder 32 to be extended as far as possible at the expense of the annular space 34. The new inventive conception of the functional reversion of both spaces allows a problem-free control of large cell diameters and includes an additional, fifth advantage, which can be exploited when flotation cells are to be piled up above one another.

It also happens that the shape of the flotation cell resembles by far more a pan than a deep vessel . For example, for an average flotation rate constant of k=l min^-1 under the afore mentioned boundary conditions (Re = 2.000; v = lOm/h) , the liquid height for a solids depletion of ct/co = 10^"3 will be H =1.15 (3.5 ft) and for ct/co = 10-4 it will amount to H=1.53 m (4.6 ft) .

Consequently, multiple flotation cells 30 may be easily operated in parallel and be stacked up above one another, with 2 to 3 cells above one another representing an optimum utilization of the plant ground expense.

Another advantage of the new flotation cell according to the invention also becomes apparent, due to the favorable circumstance that the flotate (froth) rises in the annular space around the outer cell. The flotate can therefore be easily be sucked out by a vacuum froth cleaner, rotating around the circumference of the cell.

EQUIVALENTS While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the claims .

Claims

CLAIMSWhat is claimed is:

1. An apparatus for separation of solids from solid-liquid suspensions by induced air flotation, the apparatus including a nozzle element including a cone-shaped deflecting element arranged inside of and spaced apart from a funnel-shaped outer casing such that an annular channel is thereby formed between the cone and funnel, with a liquid propulsion jet disposed adjacent an apex of deflecting cone and arranged to cause the liquid exiting therefrom to impinge upon the deflecting cone and to be fanned out thereby, the device characterized in that: one or more trip edges are disposed within the annular channel to perturb a laminar boundary layer formed within the annular channel, thereby promoting turbulence in fluid flow within the annular channel .

2. An apparatus as in claim 1 wherein at least one trip edge is disposed on a surface of the cone, facing the annular channel .

3. An apparatus as in claim 1 wherein at least one trip edge is disposed on a surface of the casing, facing the annular channel .

3. An apparatus as in claim 1 wherein the annular channel has an approximately constant hydraulic diameter.

4. An apparatus as in claim 1 wherein the nozzle element is positioned within a flotation cell. /32538

-16 -

5. An apparatus as in claim 1 wherein the annulus between the deflecting cone and the funnel-shaped casing is formed by multiple individual segments.

6. An apparatus as in claim 1 wherein the individual segments are shaped to provide one of a convergent, divergent or Venturi-shaped channel between them.

7. An apparatus for separation of solids in fluid suspension by induced air floatation, the apparatus including a flotation cell having an inner cylindrical chamber and outer annular space characterized in that the annular space serves as the flotation space and the inner chamber serves as a tranquillizing space.

8. An apparatus as in claim 7 wherein the flotation cell is pan-shaped and of a relatively low liquid height.

9. An apparatus as in claim 8 wherein multiple pan-shaped floatation cells are stacked upon one another.

10. An apparatus as in claim 7 wherein vertical liquid flow occurs within the tranquilizing zone.

11. An apparatus as in claim 7 additionally comprising a froth vacuum for removing froth from around an upper rim of the flotation cell.

12. A method for separation of solids in fluid suspension by induced air floatation, the method comprising the steps of: passing a portion of the clarified liquid through a nozzle including a cone-shaped deflecting element arranged inside of and spaced apart from a funnel-shaped outer casing such that the purified liquid travels through a liquid propulsion jet disposed adjacent an apex portion of the deflecting cone and such that fluid exiting therefrom impinges upon the deflecting cone and is fanned out thereby to pass through an annular channel formed between the cone and funnel; and perturbing the fluid suspension as it passes within the annular channel, thereby promoting turbulence in fluid flow within the annular channel .

13. A method for separation of solids in fluid suspension by induced air flotation, the method comprising the steps of: passing the fluid suspension through a nozzle such that fluid exiting therefrom is fanned out thereby; and causing fluid exiting from the nozzle to enter a flotation separation cell having an inner cylindrical chamber and outer annular space, such the annular space serves as the floatation space and the inner chamber serves as a fluid tranquillizing space.