WO1999050181A1

WO1999050181A1 - Air saturation cylinder

Info

Publication number: WO1999050181A1
Application number: PCT/US1999/004753
Authority: WO
Inventors: Richard P. Hebert; William A. Gero
Original assignee: Beloit Technologies Inc
Current assignee: Beloit Technologies Inc
Priority date: 1998-03-31
Filing date: 1999-03-03
Publication date: 1999-10-07
Anticipated expiration: 2000-09-30
Also published as: AU2801999A

Abstract

A device (20) for dissolving air in water at super atmospheric pressure. Air injectors (32) formed of sintered stainless steel pipe form very small bubbles typically having diameters of twenty to forty microns. Water is injected between the outer (44) and the inner pipes (46) adjacent to the head (48) and tangential to the outer pipe. The outer pipe is longer than the inner pipe and is capped by a second head (50). Water flows in a spiral path around the inner pipe towards the second head, enters the inner pipe and flows to an outlet (54) through the common head. The air supplied to the vessel is divided into two equal parts, one part is injected through six air injectors (32) along the outer pipe, and the other half of the air is injected along the axis of the inner pipe through a sintered porous pipe which extends through the second head.

Description

TITLE: AIR SATURATION CYLINDER

FIELD OF THE INVENTION The present invention relates to dissolved air clarifiers in general and to apparatus for dissolving air in water in particular.

BACKGROUND OF THE INVENTION Many industrial processes utilize large quantities of water. The paper industry has long been noted for the large amounts of water needed to produce each ton of paper. At a bare minimum more than a hundred tons of water are mixed with each ton of fiber to make up the stock from which paper is formed. Over time, particularly in the last few decades, the water needed for paper production has been greatly reduced by recycling water within the papermaking process to the point where net water usage and discharge of waste water have been radically reduced or even eliminated.

The science and engineering involved in cleaning and purifying water for recycling involves many techniques. These techniques are divided into those useful for removing dissolved solids, and those techniques useful for removing suspended solids. The techniques used for removing suspended solids are in turn divided into various techniques including filtering, settling, flotation, flocculation followed by settling or flotation, and conglomeration by settling or flotation. The flotation process is in turn divided into those processes where surfactants are used to connect particles of a particular type to large air bubbles, for example ink particles composed of carbon; and dissolved air clarification where air bubbles having a diameter of ten to forty microns in diameter are formed by allowing water containing super saturated dissolved air to form bubbles around small nucleations. Air coming out of solution forms around small particles and thus allows flotation of suspended particles which have substantially the same density as water and therefore are difficult to separate by conventional settling techniques.

Dissolved air clarifiers typically divide the flow of water into two streams of approximately 20 percent and 80 percent. The 20 percent stream is sent through an air dissolving chamber where air is mixed with water at relatively high pressures to form an air water solution which will be supersaturated at atmospheric pressure. A device for forming a supersaturated solution of air and water requires three things: water supplied at elevated pressures, air supplied at elevated pressures, and a means for mixing the air and water. Compressing water involves some expense and compressing air generally a greater expense. Conventional techniques can require recirculating water and undissolved air. Because so much water must be recycled for each unit of paper made, controlling costs associated with clarifying and using water is critical. Any cost increment or saving is multiplied by more than a factor of one-hundred when its final cost effect on the product is determined. Thus, even small improvements in the efficiency of the recycling process are important.

What is needed is an apparatus for dissolving air in waste water with increased efficiency.

SUMMARY OF THE INVENTION The air saturation cylinder of this invention is a device for dissolving air in water at super atmospheric pressure so the water leaving the cylinder becomes supersaturated with dissolved air. The air saturation cylinder utilizes air injectors which form very small bubbles typically having diameters of twenty to forty microns (millionths of a meter). The air injectors are composed of sintered stainless steel pipe formed by sintering fine grains of stainless steel metal in such a way as to form a porous pipe. A mixing vessel is formed of an outer pipe and a coaxial inner pipe joined to a single head or flange. Water is injected between the outer and the inner pipe adjacent to the head and tangential to the outer pipe circumference. The outer pipe is longer than the inner pipe and is capped by a second head, opposite the common head. Water from the inlet flows in a spiral path around the inner pipe toward the second head where it enters the inner pipe and flows to an outlet from the inner pipe through the common head. The air supplied to the vessel is divided into two equal parts, one part is injected through six air injectors arrayed along the outer pipe, and the second half of the air is injected along the axis of the inner pipe through a sintered porous pipe which extends from the second head. The entire air saturation cylinder will typically be formed of stainless steel and constructed so that the air injectors and the entire device may be readily disassembled for cleaning and maintenance.

It is a feature of the present invention to provide an apparatus for dissolving air in water at super atmospheric pressures which has increased efficiencies.

It is another feature of the present invention to provide an apparatus for dissolving air in water which is more easily maintained.

It is yet another feature of the present invention to provide a method for forming a supersaturated solution of air and water. It is a further feature of the present invention to provide a method of clarifying waste water produced in the papermaking process.

Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric cross-sectional view of the air saturation cylinder of this invention.

FIG. 2 is a cross-sectional view partly cutaway of the air saturation cylinder of FIG.1 taken along section line 2-2 wherein the location of the air injection ports have been rotated for clarity.

FIG. 3 is a schematic view of a clarifier system employing the air saturation cylinder of FIG.1

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to FIGS. 1 -3 wherein like numbers refer to similar parts, an air saturation cylinder 20 is shown in FIG. 3. As indicated by arrow 21 , a stream of water containing particular matter flows from a source of waste water toward a clarifier 24. A portion of the stream 22 comprised of approximately twenty percent of the total volume of the flow is diverted through a pump 26 which pumps the diverted twenty percent into the air saturation cylinder 20. An air compressor 28 supplies air to the air saturation cylinder 20 where air is mixed and dissolved in the compressed water. Water containing dissolved air is mixed with the remaining eighty percent of the waste water and injected into the clarifier 20. The mixing is accompanied by directing water from the air saturation cylinder back to the pipe leading into the clarifier 24 as shown in FIG. 3. FIG. 3.

The clarifier 24 consists of a large open tank where small particles form nucleation points for air as it comes out of solution. The nucleation points float the particles to the surface where a skimmer 30 removes them. An exemplary clarifier is shown in U.S. Application Number 08/971 , 1 73, filed November 1 4, 1 997, which is incorporated herein by reference.

The air saturation cylinder 20 receives water containing suspended particles at approximately seventy psi from the pump 26. Air is supplied to each of the air injectors 32 at approximately eighty psi. The air injectors 32 form very small bubbles typically having diameters of twenty to forty microns (millionths of a meter). The air injectors 32 are formed of sintered stainless steel pipe formed by sintering fine grains of stainless steel metal in such a way as to form a porous pipe 34. The fine grains in the sintered pipe 34 produce pores having a diameter of about twenty microns. The porous pipe 34 is welded to a plug 36 and a flange 38.

Air hoses 40, shown in FIG. 3, are connected to the air injectors 32 and each air injector is bolted to the flange 42 of an air port 43 which extends radially outwardly from the outer cylindrical tube 44 of the air saturation cylinder 20. The air hoses 40 are connected to conventional adapters (not shown) which screw into threads 41 in the flanges 38.

A mixing vessel is formed of a housing having an outer pipe 44 closed by heads 48, 50, and a coaxial inner pipe 46 joined to a common flange or head 48. The flow of particulates and water is injected into an annular volume 47 defined between the outer pipe 44 and the inner pipe 46. The injected flow enters the housing adjacent to the head 48 and tangential to the outer pipe 44 as shown in FIG. 2. To avoid unnecessary interference with the fluid flow through the annular volume 47, the air injectors 32, which extend generally perpendicular to the inner pipe 46, do not extend into the annular volume.

The outer pipe 44 is longer than the inner pipe 46 and is capped by the second head 50, opposite the common head 48. Water is injected through an inlet pipe 52 and flows in a spiral path around the inner pipe 46 towards the second head 50 where it enters the inner pipe and flows to an outlet 54 in the common head 48.

The air supplied to the vessel is divided into two equal parts, one part is injected through six air injectors 32 arrayed along the outer pipe, and the second half of the air is injected along the axis of the inner pipe 46 through a sintered porous pipe 56 which forms an air injector 57 which is similar to the air inlet 32 except that the pipe 56 is substantially longer and thus extends from the second head 50 into the inner pipe 46. The entire air saturation cylinder will typically be formed of stainless steel and constructed so that the air injectors 32, and the entire device may be really disassembled for cleaning and maintenance.

Easy removal of the air injectors 32, 57 is important because the water which is processed through the air saturation cylinder 20 contains particles which can clog the small air passages formed in the sintered pipe making up the air injectors. Normally air is flowing through the sintered pipe preventing water from moving particles into the air passages. However if the air compressor is shut down before the pump or if there is a power failure small particles of paper fiber are driven into the air passages formed between the grains of stainless steel which make up the sintered pipe. The easily removed air injectors 32, 57 can be cleaned by soaking them in a caustic solution which is a well known solvent for cellulose.

By maintaining a set of spare air injectors, rapid replacement is possible minimizing down time. Because many of the papermaking processes are capital intensive, equipment is often run 24 hours a day. Holding tanks can be used to store waste water for a short period of time. However if cleaning of the waste water equipment cannot be effected sufficiently rapidly, equipment which depends on the clarified water will of necessity have to be shut down resulting in considerable expense.

By breaking up the injected air into very small bubbles the dissolution of the air in the water is facilitated. The properties of air bubbles are governed by surface tension in accordance with the following formula: P = 4T/r, where P is pressure in units of force per unit area and T is surface tension recorded in force per unit length and r is the radius of the bubble in units of length. By examining this equation it is clear that as the size of an air bubble decreases the internal pressure increases. At the same time as an air bubble is decreased in size the surface area to volume ratio increases linearly as radius is decreased because volume decreases as the third power, and surface area only decreases as the second power of the decrease in radius. These effect of increased pressure and increased surface area is to increase the rate of dissolution of the air in the water. The dissolution is also aided by a turbulent environment which assures mixing of the air and water.

A number of instrument ports 60 are positioned on the air saturation cylinder 20 or on the inlet 52 or outlet 54 so that pressure drops across various portions of the air saturation cylinder can be measured. Such measurements can be used to monitor proper function and operation of the air saturation cylinder 20. The test port on the outlet can also be used to bleed air from a system during startup or at other times as necessary.

It should be understood that wherein pipe form of sintered stainless steel is describe other sintered materials including glass and ceramic or natural air stone, or other mounted granular materials, which are durable and which our resistance to caustic solutions could be used.

It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.

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Claims

WHAT IS CLAIMED IS:

1 . An apparatus for dissolving air in water comprising: a cylindrical housing extending a first length along an axis from a first end to a second end; a first head mounted to the housing first end; an inner pipe which extends a second length along the axis within the housing from a first end which is mounted to the first head to a second end, wherein the inner pipe is coaxial with the housing and the second length is less than the first length, the inner pipe defining an interior, and wherein an annular space is defined between the housing and the inner pipe; a second head mounted to the housing second end; a water injection port near the first head, the injection port having portions which direct water injected therethrough into the annular space tangential to the inner pipe; a plurality of air injection ports spaced about the housing, wherein each air injection port has a sintered metal pipe having a multiplicity of air openings sized so as to create bubbles having diameters of about 1 0 to about 40 microns; and portions of the first head defining an outlet from the housing which opens to the interior of the inner pipe.

2. The apparatus of Claim 1 wherein each sintered metal pipe further comprises a means for removably mounting the sintered metal pipe to the vessel.

3. The apparatus of Claim 1 further comprising an axial air injection port comprising a sintered metal tube which extends along the axis a substantial portion of the first length.

4. An apparatus for dissolving air in water comprising: a cylindrical housing extending a first length along an axis from a first end to a second end; a first head mounted to the housing first end; an inner pipe which extends a second length along the axis within the housing from a first end which is mounted to the first head to a second end, wherein the inner pipe is coaxial with the housing and the second length is less than the first length, the inner pipe defining an interior, and wherein an annular space is defined between the housing and the inner pipe; a second head mounted to the housing second end; a plurality of air injection ports spaced about the housing, wherein each air injection port includes a mean for creating bubbles having diameters of about 1 0 to about 40 microns; a water injection port positioned axially between the first head and an air injection port, the water injection port having portions which direct water injected therethrough into the annular space tangential to the inner pipe; and portions of the first head defining an outlet from the housing which opens to the interior of the inner pipe.

5. The apparatus of Claim 4 wherein each means for creating bubbles is removably mounting to the vessel.

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6. The apparatus of Claim 4 further comprising an axial air injection port comprising a sintered metal tube which extends along the axis a substantial portion of the axial length of the vessel.

7. An apparatus for dissolving air in water comprising: a cylindrical housing having portions defining an outlet at a first end; an inner pipe which extends coaxially within the housing and which is fixed to the housing at the first end such that fluid entering the inner pipe at a second end of the housing exits the housing at the outlet; a plurality of air injection ports spaced axially along the housing between the first end and the second end; a plurality of sintered metal air injection pipes, wherein each pipe is mounted to the housing at an air injection port, each air injection pipe having portions defining a multiplicity of air openings such that air introduced through the pipes is divided into multiple small bubbles; and a water injection port positioned axially between the housing first end and the air injection ports, the water injection port being disposed to direct a flow of water with entrained particles into the housing to travel in a generally spiral path between the housing and the inner pipe, and wherein the inner pipe has portions defining an opening spaced axially from the second end of the housing into which the injected air and flow of water with entrained particles passes and thence through the outlet.

1 1

8. The apparatus for dissolving air in water further comprising: portions of the housing at the first end which define an axial air injection port; and a sintered metal pipe which extends through he axial air injection portion through the housing second end and into the housing within the inner pipe to introduce air therein.

9. The apparatus for dissolving air in water wherein the air injection pipes extend substantially perpendicularly to the cylindrical housing, and wherein an annular volume is defined between the housing and the inner pipe, and wherein the air injection pipes do not extend into the annular volume.

1 0. A method of removing particulates from water comprising the steps of: dividing a flow of water containing particulates into a first stream and a second stream; pumping the first stream into a vessel at a pressure of about

70 psi gauge; causing the first stream to follow a path within the vessel which promotes mixing within the vessel; injecting air in the form of bubbles having a diameter of about

20 to about 40 microns into the vessel at a plurality of injection ports leading into the vessel and spaced along the path within the vessel; allowing the first stream to exit the vessel; mixing the first stream after it exits from the vessel with the

12 second stream and injecting the mixed streams into a clarifier tank; and clarifying the flow of water by removing particulates which float to the top of the clarifier tank.

1 1 . The method of Claim 10 wherein the first stream comprises about 20 percent of the flow of water.

1 2. The method of Claim 10 wherein the vessel is comprised of an outer cylindrical tube and an inner cylindrical tube and wherein the method further comprises injecting the first stream tangential to the outer cylindrical tube, between the inner cylindrical tube and the outer cylindrical tube, and causing the first stream to pass between the outer cylindrical tube and the inner cylindrical tube, and to then pass through the inner cylindrical tube and exit the vessel.

1 3. The method of Claim 1 2 wherein the step of injecting air comprises taking a flow of air and dividing it into a first air stream and a second air stream which are substantially equal, the process further comprising the steps of: injecting the first air stream through a plurality of injection means spaced along the outer cylindrical tube; and injecting the second air stream through a second injection means extending along an axis defined by the inner cylindrical tube.

1 4. The method of Claim 1 3 wherein the process of clarifying water includes the step of removing the first injection means, and the second injection means and placing the first and second means in a caustic

13 solution to removal cellulosic contaminants from the first and second injection means.

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