High efficiency microbubble aeration

14 Pages·2013·1.25 MB·English

USO05674433A ‘ ‘ ‘
United States Patent [19] [11] Patent Number: 5,674,433
Semmens et al. [45] Date of Patent: Oct. 7, 1997
[54] HIGH EFFICIENCY MICROBUBBLE FOREIGN PATENT DOCUMENTS
AERATION
0 138 060 A3 4/1985 European Pat. 01f. .
[75] Inventors: Michael J. Semmens; Charles J. U1" HER PUBLICATIONS
Gantzer; Michael J. Bonnette. all of
“Untersuchung der Blasenbildung und des Stoffaustausches
Minneapolis. Minn.
unter dem Ein?u? oberfl'aichenaktiver Substanzen und
[73] Assignee: Regents of the University of geloster Gase” (English translation of Chapter 5.1 and
Minnesota. Minneapolis. Minn. Figures 89. 90 and 91 as shown in the German text pp. 134.
136. 137 and 138) by Franz Bischof. Feb. 10. 1994.
“High Oxygen Transfer Rate in a New Aeration System
[21] Appl. No.: 519,125
Using Hollow Fiber Membrane”. by Hiroshi Matsuoka et a1.
[22] Filed: Aug. 24, 1995 Biotechnology and Bioengineering, vol. 40. pp. 346-352
(1992).
[5 1] Int. Cl.6 ...................................................... .. B01F 3/04
[52] US. Cl. ......................... .. 261/37; 261/1221; 261/87; Primary Examiner-Tim R. Miles
Attorney, Agent, or Firm-Westman. Champlin & Kelly.
261/93; 261/120; 261/DIG. 75
PA.
[5 3] Field of Search .................................. .. 261/1221. 37.
261/87. 93. 120. DIG. 75 [57] ABSTRACT
[5 6] References Cited An aeration device disperses microbubbles into a liquid and
maintains e?icient transfer of gas to the liquid. The aeration
U.S. PATENT DOCUMENTS
device uses a number of sealed end. hollow ?ber membranes
that are hydrophobic and provided with pores in the walls of
3,228,876 1/1966 Mahon .................................... .. 210/22
3,702,658 11/1972 McNamara etal 210/321 the tubular ?bers that range from about 0.01 to 1.0 microns.
4,181,604 1/1980 Onishi et a1. .. 210/8 so that very small bubbles are formed on the outside surface
4,781,889 11/1988 Fukasawa et al. 422/48 of the hollow ?ber membranes. Gas pressures above the
4,824,444 4/1989 Nomura ........................ .. 55/16 bubble point of the ?ber membranes are used. and a cloud
4,886,601 12/1989 Iwatsuka et al. 210/321.79 of microbubbles is expelled into the liquid as it is forced to
4,950,431 8/1990 Rudjck et a]. 261/1221
?ow past the ?bers. These microbubbles provide a large
5,034,164 7/1991 Semmens . . . . . . . . . . . . . . .. 261/122 surface area for the effective dissolution of gases into the
5,059,374 10/1991 Krueger et a1. ..... .. 204/156
liquid. The length of the hollow ?ber membranes is con
5,143,612 9/1992 Hamanakaetal. 210/3218
5,228,991 7/1993 Strohm et a1. 210/3218 trolled in order to obtain ef?cient small bubble formation.
5,254,253 10/1993 Behmann . . . . . . . . . . . . . . .. 210/607
5,332,498 7/1994 Rogut ................................. .. 210/3218 13 Claims, 6 Drawing Sheets
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US. Patent 0a. 7, 1997 Sheet 5 of 6

5 .674.433
1 2
HIGH EFFICIENCY MICROBUBBLE source. or a remote or distal end can be closed or sealed.)
AERATION The walls of the membrane forming the hollow ?bers are
provided with micropores such that when a gas under
This invention was made with Government support pressure is supplied to the interior of the hollow ?bers.
under NSF/BCS-9l23 175 awarded by the National Science microbubbles will form on the exterior surface of the ?ber
Foundation. The Government has certain rights in this membranes and will be stripped off by liquid moving past
invention. the ?ber membrane exterior surface. The bubble size is
maintained small in order to provide for good gas transfer to
BACKGROUND OF THE INVENTION the liquid. The bubbles are purposefully discharged and
dispersed in a large volumetric ?ow rate so they remain
The present invention relates to a gas transfer device that
separate. and the opportunity for coalescence to form larger
provides highly et?cient transfer of a gas such as oxygen
bubbles is minimized.
into liquids. such as wastewater. using tubular wall ?ber
membranes having micropores through the wall. Open ends Inlet gas pressure is regulated so that microbubbles are
of the ?ber membranes are provided with a gas such as air formed on the ?ber membrane surface at the micropores
or oxygen under pressure sut?cient to cause gas to bubble 15 through the ?ber membrane. The ?ber membranes are
through the micropores. The micropores are sized so that the arranged in the aerator to ensure that they are adequately
bubbles are vm'y small and are released in clouds as the dispersed in the ?uid ?ow to ensure stripping the bubbles
liquid passes over the exterior of the tubular ?ber mem from the ?ber membrane surface. The water velocity past the
branes. external surface of the hollow ?ber membrane disperses the
Conventional wastewater treatment tank aeration devices 20 generated bubbles into a large water ?ow which also aids in
attempt to e?iciently transfer gas through the water while controlling the size of the bubbles.
providing the required level of mixing to keep the micro Microbubbles are de?ned as bubbles which measure less
organisms uniformly dispersed throughout the u'eatment than 100-200 pm in diameter and as a result they have a very
tank. Normally. when gases bubble into the water. the rising large surface area in relation to the volume of gas forming
bubbles generate the required mixing. Studies have shown the bubbles and this enhances the effective mass transfer of
that conventional ?ne bubble diffusers have an optimum gas to the liquid. Bubbles that are less than 100 pm in size
bubble size of around 2 mm to satisfy gas transfer and are less inclined to coalesce. and this is likely due to charge
mixing functions. Typically. conventional diffusers of this acquisition at the bubble interface. As the bubble size is
type dissolve only about 20-50% of the oxygen in the air reduced. the surface area to volume ratio for the bubble
supplied as the input gas. which means that the majority of 30 increases and the effect of the charged surface becomes more
the oxygen is lost back to the atmosphere when the bubbles important in determining the behavior of the bubble.
burst at the water surface. This makes the use of conven Bubbles carrying like electrostatic charges tend to repel one
tional ?ne bubble diffusers inappropriate for use with pure another and this reduces coalescence. These electrostatic
oxygen since a high wastage rate would render the process interactions dominate the behavior of very small bubbles (10
prohibitively expensive. 35 11111)
A commercially available oxygenator is sold under the The ?ber membranes are mounted in manifolds in various
trademark “Vitox System” . In this system. a high pressure con?gurations that aid in dispersing the bubbles into large
pump delivers a high flow rate of water to a Venturi which water flows and thus aid in transfer of gas into the water or
is equipped with small holes in the throat or reduced section liquid ?owing past the ?ber membranes. Many different
of the Venturi. Pure oxygen is blown through the holes and con?gurations of hollow ?ber membrane mounting mani
as the water passes the holes at high velocity. the water folds can be used.
shears ?ne bubbles from the surface of the wall. A jet of ?ne The microbubble forming hollow ?ber membranes pro
bubbles is then discharged into a deep tank and the energy vide a highly efficient transfer of gases. such as oxygen into
of the jet provides mixing. The e?iciency is depth liquids. Small diameter. hollow ?bers made of membranes
45
dependent. but with discharges in deep tanks. the oxygen that have small pores through the walls are suitable.
transfer rate e?iciency is said by the manufacturer to
approach 95%. BRIEF DESCRIPTION OF THE DRAWINGS
A membrane bioreactor that uses a combination of FIG. 1 is a schematic side elevational view of a typical
microbubbles and macrobubbles is shown in US. Pat. No. 50 installation utilizing hollow ?ber membranes made accord
5.254.253. ing to the present invention;
The use of hollow ?ber membranes for gas transfer has FIG. 2 is an enlarged schematic sectional view of a typical
been recognized as potentially reducing the cost of gas hollow ?ber membrane used for the purposes of the present
transfu by reducing energy requirements and increasing the invention;
e?iciency of the gas transfer. When using hollow ?ber 55 FIG. 2A is a schematic representation of a cross?ow
membranes. the mixing and gas transfer functions are sepa arrangement;
rate and may be engineered separately to meet the needs of
FIG. 3 is a view of a modi?ed manifold used for holding
a particular application. The hollow ?ber membranes are
hollow ?ber membranes;
preferably made speci?cally to produce very ?ne bubbles.
FIG. 4 is a schematic representation of a manifold insert
Mixing must be (for this type of aerator) provided by a
packet holding a plurality of hollow ?ber membranes;
separate high ?ow. low head pump. or mixer.
FIG. 5 is a schematic representation of a side view of a
SUMMARY OF THE INVENTION further modi?ed installation;
The present invention relates to the use of tubular or FIG. 6 is a schematic top view of the installation shown
hollow ?ber membranes that have at least one open inlet end 65 in FIG. 5;
for receiving a gas such as oxygen or air. (Both ends of the FIG. 7 is a schematic front elevational view of the
?ber membranes can be connected to an air or oxygen installation of FIG. 6.
5,674,433
3 4
FIG. 8 is a schematic top plan view of a number of hollow remain separated. Water may be discharged through the
?ber membranes supported on radially extending arms that pores under pressure. Water condensing in the interior of the
rotate with a hub about a central axis; ?bers usually will transfer to end section 52 of the ?ber
FIG. 9 is a schematic side elevational view of the device membranes and be forced out of the pores adjacent this end.
of FIG. 8. The length of the hollow ?ber membrane sections 30 can
FIG. 10 is a schematic plan view of a further modi?ed vary but the length is usually at least 5 cm and ranges up to
10 cm from the surface of the supporting manifold.
form of the present invention used in a circular mixer
arrangement; Referring to FIG. 2. a typical hollow ?ber membrane 26
is illustrated. As shown. the hollow ?ber membrane 26 has
FIG. 11 is a schematic side view of the device in FIG. 10;
10 a ?rst open end 42 potted or supported in a suitable potting
and
material 44. An end opening 46 opens to the manifold 24. so
FIG. 12 is a graphical representation of the e?'ect on the
that oxygen or air in the manifold plenum chamber 47 is
gas pressure on the transfer rate into a liquid with di?‘erent
provided to the lumen 48 of each hollow ?ber membrane 26.
e?‘ective lengths of hollow ?ber membranes made according
The hollow ?ber membranes 26 have walls made of suitable
to the present invention.
hydrophobic material that have micropores indicated sche
matically at 50 therein. The micropores are very small. and
DETAILED DESCRIPTION OF THE
in the range of 0.01 to 10 microns and since the membrane
PREFERRED EMBODIMENTS
material is hydrophobic the pores remain dry and gas ?lled
In FIG. 1. a schematic representation of a high ef?ciency when it is contacted with water. The end sections 30 of the
rnicrobubble aeration device is illustrated generally at 10. 20 hollow ?ber membranes adjacent the manifold 24 are chemi
The aeration device 10 includes a pipe section 12 through cally treated to close the pores in sections 30 so that the
which water will ?ow from a pump 14 or other ?ow source oxygen does not pass through the membrane wall in these
as indicated by the arrow 16. The pipe section 12 can have end sections.
unions at its opposite ends indicated at 18 for connecting The midsection 28 of the hollow ?ber membranes 26 (also
into any type of desired ?ow pipe. 25 called ?bers for convenience) is where the microbubbles are
The liquid ?ow rate through the pipe section 12 is formed by gas transfening through the pores of the mem
adjustable by adjusting ?ow source or pump 14. and the branes.
cross sectional size of the pipe section 12 can be changed as The hollow ?ber membranes 26 will spread out or
desired to provide the ?ow conditions for best gas transfer. separate. as shown in FIG. 1. by the action of water ?owing
As shown schematically. a source of oxygen under pres over the outside of the ?ber membranes. The hollow ?ber
sure 20 is connected through a conduit 22 and a ?tting 23 to membranes tend to “?uidize" and become distributed
a manifold illustrated schematically at 24. The manifold 24 throughout the cross section of pipe section 12.
can be of any desired type that will transfer gases into the The micropores permit trapped liquid to transfer out of the
interior of hollow ?ber membranes. For example. reference interior of the ?ber membranes when su?icient pressure is
35
is made to US. Pat. No. 5.034.164. and to co-pending US. present on the interior of the ?ber membrane.
application Ser. No. 081303.021 ?led Sep. 8. 1994. now The pressure within the lumena 48 of the hollow ?ber
abandoned both owned by the assignee of this application. membranes is adjusted to suit the length of the ?ber mem
for showings of suitable manifolds. branes for increasing e?iciency. Generally speaking. a gas
The manifold 24 provides gas under pressure to the pressure of at least 45 psi. above the water pressure will be
interior of each of a plurality of tubular or hollow ?bm' utilized. and higher pressures also can be used. The pressure
membranes 26 which have ?rst ends open to the interior of can be selected to suit the ?ber membrane being used. the
the manifold 74 and which have sealed or closed remote size of the ?ber membrane. the pore size and the like.
ends. The hollow ?ber membranes 26 are in the interior Pressurized gas such as oxygen is pumped into the lumena
passage way 12A of the pipe section 12. and the ?ow of 45 48 of very thin hollow ?ber membranes 26. which have an
water past the hollow ?ber membranes 26 will tend to outer diameter approximately in the range of 100 to 1.000
?uidize the outer or remote ends of the ?ber membranes so microns. The micropores. as stated. are very small. so that
that the hollow ?ber membranes 26 remain spaced apart. A small bubbles are formed at the exterior surfaces of the
central length portion of each hollow ?ber membrane 26. hollow ?ber membranes.
generally represented by the position of double arrow 28 is By pressurizing the lumena of the hollow ?ber mem
50'
where transfer of gas from the interior of the hollow ?ber branes with oxygen at a pressure above the bubble point of
membranes to the exterior will take place. The gas-to-liquid the membrane. oxygen will ?ow through the micropores 50
transfer is through very small pores which cause and form bubbles 54 on the outside surfaces of the ?ber
microbubbles to form at the exterior surface of the hollow membranes 26. The size of the bubbles is determined not
?ber membranes. 55 only by the dimension of the micropores. but also by the
The inner end section of the hollow ?ber membranes character of the external ?ber membrane surface. which
indicated by the double arrow 30 usually will be treated determines how quickly the bubbles shear off. the pressure
chemically to close the pores in the hollow ?ber membranes of the oxygen inside the ?ber membrane. the quality of the
to prevent bubble formation adjacent to the manifold High water being oxygenated and the water (liquid) velocity past
gas pressure present at the manifold 24 could result in excess 60 the external surface of the hollow ?ber membranes.
formation of bubbles on the end sections 30 of the hollow When pressurized gas is pumped into the lumena 48 of the
?ber membranes. Because the hollow ?ber membranes 26 very thin hollow ?ber membranes to form microbubbles.
are close together adjacent to the manifold. if bubbles there is a signi?cant pressure drop along the lumena 48 of
formed along section 30 they would tend to coalesce quickly each ?ber membrane extending from the manifold 24. As a
to form larger bubbles. In the central regions or sections 28. result. the micropores near the mounting or proximal end of
the hollow ?ber membranes 26 become separated. so that the ?ber membranes release bubbles under the in?uence of
rnicrobubble formation is enhanced and the bubbles tend to a high pressure inside the ?ber membranes while further
5 674.433
5 6
down the ?ber membranes. (at distal ends) the bubbles are ?uidize. keeping them apart in the sections where bubble gas
formed at signi?cantly lower internal pressure because of transfer takes place. Each ?ber membrane 26 is preferably
the pressure drop. It has been found that under ?ber lengths unaffected by interference from other ?bers. to minimize
longer than a length in the range of 30 cm to 50 cm cause bubble coalescence in the sections 28 of the ?ber mem
the pressure inside the ?ber membrane to drop to a level too branes 26.
low to form rnicrobubbles through the micropores and the Water velocity past the ?ber membranes increase the
distal end of the ?ber is not used e?ectively for gas transfer. shear forces that tend to pull bubbles from the ?ber mem
The length of the ?ber membranes 26 affects the lateral brane surfaces. and increased velocity therefore tends to
?ber membrane movement in a ?ow stream. Longer ?bers result in the formation of smaller bubbles. The boundary
provide little lateral movement of sections of the ?bers near layer or the thickness of the stagnant liquid layer on the ?ber
the manifold. where bubble formation is enhanced since the membrane surface is very small. and the shear velocities
?bers tend to remain in one position due to the drag on the need to be very large to exert in?uence at the ?ber membrane
downstream ?ber length. By comparison. short ?bers are surface. Membrane material types and the design of the
much more free to move independently in a turbulent flow membrane module also affect the shearing of the bubbles.
?eld. and as a result of the increased movement of the ?bers The preferable ?ber membranes used are made by Mitsub
and the enhanced liquid shear action at the ?ber surfaces. ishi Rayon Corporation and have performed well.
bubble dispersion is likely to be more uniform. A known hydrophilic material. such as polyvinyl alcohol
The effectiveness or e?iciency of a hollow ?ber mem or other suitable material. may be used to coat the external
brane microbubble aerator diminishes with increasing surface of the ?ber membranes. This procedure can be used
lengths of the ?ber membranes. The length of the ?bers is 20 to modify the surface of the membrane to encourage the
selected to provide a high e?iciency. formation of microbubbles. The coating tends to raise the
Another factor in e?icient gas transfer is the density of gas pressure required to form microbubbles at the outer
?ber membranes. that is. the spacing of the ?ber membranes surface. In addition. the rnicrobubbles form over a slightly
at the manifold 24 and in the pipe passageway 12A. As the larger pressure range if the ?ber membranes are coated.
?ber density increases. the opportunity for ?ber to ?ber 25 Membrane modules may be designed in two con?gura
contact increases. bubble interactions increase and bubble tions: 1) parallel ?ow as shown in FIG. 1. where the ?uid
coalescence also can increase. with a resulting decrease in ?ow roughly parallel to the axis of the hollow ?bers; or 2)
gas transfer e?iciency. If the ?ber membranes are less cross?ow as shown schematically in FIG. 2A. where the
densely packed. they can spread out more. ?ber to ?ber direction of ?uid ?ow is substantially perpendicular to the
contact is reduced and formed bubbles have an opportunity 30 axis of the hollow ?bers. The preferred direction of ?ow for
to move away from the ?ber bundle before the bubbles wastewater treatment applications is parallel to the ?ber
coalesce with other small bubbles. membranes when the wastewater contains solids. However.
Bubble size is affected by several factors. such as gas when waters are substantially free of solids the cross?ow
pressure. pore size in the membrane wall and velocity and design is effective. The advantage of the cross?ow con?gu
direction of the ?uid relative to the ?ber membrane surface. 35 ration is that a thinner boundary layer is formed around the
At low gas pressures. no bubbles are formed. and gas hollow ?ber membranes when the ?ber lengths are generally
transfer will occur by direct dissolution at the ?ber mem transvmse to the ?ow. and this appears to result in a more
brane surface. As the gas pressure rises into the pressure rapid detachment of the bubbles from the ?ber membrane
range of 30-50 psi. a transition occurs and transfer is surface. As shown in FIG. 2A. a manifold 24A similar to
dominated by the formation of a very large number of manifold 24 can support open ends of ?bers 26A. The
microbubbles. that is. bubbles less than 100 microns in opposite ends of the ?bers 26A can be held in potting
diameter. that stream from the micropores 50 of the hollow compound in a support 29 on the opposite side of a ?ow
?ber membranes 26. At pressures in excess of 50-60 psi. the housing 31. The support 29 can be a manifold such as 24A
size of the bubbles increases. and bubbles as large as 2 mm and provided with oxygen also. if desired
45
may be formed. In reality. designs of membrane contacting devices allow
These numbers are for clean ?ber membranes of a par for water to ?ow in both parallel and cross?ow modes. For
ticular type. If the ?ber membranes have been exposed to example. if there is turbulent ?ow in the device shown in
wastewaters for a period of time the pressure ranges for FIG. 1 there will be a cross?ow component even though the
diiferent size bubble formation may increase. bulk ?ow is parallel to the ?ber membranes. In addition it is
50
The ?ber membranes used for the tests conducted and the bene?cial to develop a device having elements of both
data provided were Mitsubishi Rayon America polyethylene parallel and cross?ow. The parallel ?ow component keeps
membranes number EHF39OC. the ?bers clean if solids are present and yet the cross?ow
Different ?bers manufactured by other companies will component provides for higher gas transfer rates and smaller
have different pore size distributions. inside diameters. wall bubble formation.
55
thicknesses and surface chemistry. These di?°erences will External liquid ?ows which are parallel to the ?bers will
lead to different operating conditions for optimum bubble tend to keep the ?bers separated. and permit solids and ?ocs
formation. Some membranes may not be able to form to pass between the ?ber membranes. since the ?ber mem
microbubbles because their surface chemistry and physical branes separate and the solids can move between the ?bers
morphology are inappropriate. without impediment.
For example. if a membrane has a more open structure Additional con?gurations of microbubble aerators that
and larger pore sizes it is to be expected that it will generate can be used are illustrated in FIGS. 3 and 4. where packets
bubbles at lower gas pressures. As the average pore size of of ?ber membrane indicated at 60 are formed by potting a
the membrane decreases the operating pressure for optimum selected number of ?ber membranes into a base material to
bubble formation will increase. fonn a packet of ?bers as shown in FIG. 4 and then a number
The water velocity past the ?ber membranes influences of these packets are glued into a manifold 62 leading from
the shearing of the bubbles. and it also causes the ?bers to a source of oxygen 64. The packets shown in FIG. 4 include