U.S. patent application number 10/508059 was filed with the patent office on 2005-10-20 for hollow fiber membrane contact apparatus and process.
Invention is credited to Cheng, Kwon-Shun, Parekh, Bipin S., Patel, Rajnikant B..
Application Number | 20050230856 10/508059 |
Document ID | / |
Family ID | 28457143 |
Filed Date | 2005-10-20 |
United States Patent
Application |
20050230856 |
Kind Code |
A1 |
Parekh, Bipin S. ; et
al. |
October 20, 2005 |
Hollow fiber membrane contact apparatus and process
Abstract
The present invention provides a perfluorinated thermoplastic
hollow fiber (14) membrane gas-liquid shell side contactor (10) and
a process for manufacturing the contactor (10) is described. The
present invention also provides a device including a gas-liquid
contactor (10) for producing ozonated water.
Inventors: |
Parekh, Bipin S.;
(Chelmsford, MA) ; Patel, Rajnikant B.;
(Tewksbury, MA) ; Cheng, Kwon-Shun; (Nashua,
NH) |
Correspondence
Address: |
MYKROLIS CORPORATION
129 CONCORD ROAD
BILLERICA
MA
01821-4600
US
|
Family ID: |
28457143 |
Appl. No.: |
10/508059 |
Filed: |
September 17, 2004 |
PCT Filed: |
March 6, 2003 |
PCT NO: |
PCT/US03/06928 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60366857 |
Mar 19, 2002 |
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60397462 |
Jul 19, 2002 |
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Current U.S.
Class: |
261/122.1 |
Current CPC
Class: |
B01D 19/0031 20130101;
B01D 63/02 20130101; B01F 3/04269 20130101; B01D 67/003 20130101;
B01F 3/0446 20130101; B01D 63/021 20130101; B01D 71/36 20130101;
B01D 2325/022 20130101; B01D 67/0018 20130101; B01D 2323/08
20130101; B01F 5/0453 20130101; B01F 2003/04404 20130101; B01D
53/22 20130101; B01D 63/023 20130101; C01B 13/0255 20130101; C10J
1/08 20130101; B01D 2313/14 20130101; B01D 71/32 20130101; B01D
61/00 20130101; C01B 13/10 20130101; B01D 69/08 20130101; B01F
2003/04886 20130101; B01F 5/0456 20130101; B01D 2325/20 20130101;
B01D 71/76 20130101; B01F 5/0465 20130101; B01F 2003/0439
20130101 |
Class at
Publication: |
261/122.1 |
International
Class: |
C10J 001/08 |
Claims
1-21. (canceled)
22. A liquid-gas phase contactor comprising: a plurality of
perfluorinated thermoplastic hollow fiber membranes comprising a
polymer selected from the group consisting of poly
(tetrafluoroethylene-co-perfluoro (alkylvinylether)),
tetrafluoroethylene-co-hexafluoropropylene, and blends of these,
the hollow fibers having a first end and a second end, an outer
surface and an inner surface; the hollow fiber membranes selected
from the group consisting of hollow fiber membranes having a porous
skinned inner surface, a porous outer surface, and a porous support
structure between; hollow fiber membranes having a porous skinned
outer surface, a porous inner surface, and a porous support
structure between; and hollow fiber membranes having a porous outer
surface, a porous inner surface, and a porous support structure
between, a perfluorinated thermoplastic housing wherein each end of
the hollow fibers potted with a perfluorinated thermoplastic seal
forming a unitary end structure with the housing where the fiber
ends are open to fluid flow, the housing having an inner wall and
an outer wall wherein the inner wall of the housing and the outer
surface of the hollow fiber membranes define a fluid flow volume,
the housing having a gas inlet to supply a gas to the first end of
the hollow fiber lumen and a gas outlet for removal of gas from the
second end of the hollow fibers, the housing having a liquid inlet
to supply a liquid to be contacted with the outer surface of the
hollow fiber membranes and a liquid outlet to remove the liquid
contacted with the outer surface of the fibers in the housing.
23. The contactor of claim 22 having hollow fiber membrane with an
outer diameter in the range of from about 300 microns to about 1500
microns.
24. The contactor of claim 22 wherein the porous skinned surface
pores are in the range of 0.001 micron to about 0.05 micron.
25. The contactor of claim 22 wherein the fibers have a packing
density of at least 0.32 in the housing.
26. A method of using the contactor of claim 22 to dissolve a gas
in a liquid comprising: flowing an ozone containing gas through the
hollow fiber lumen and contacting an aqueous liquid with the outer
surface of the fibers.
27. A liquid-gas phase contactor made from a perfluorinated
thermoplastic polymer for contacting a liquid with a gas
comprising: a plurality of porous perfluorinated thermoplastic
hollow fibers having a first end and a second end, the hollow
fibers formed of a polymer selected from the group consisting of
poly (tetrafluoroethylene-co-perfluoro (alkylvinylether)),
tetrafluoroethylene-co-hexafluoropropylene, and blends of these
polymer, the hollow fibers having an outer surface and an inner
surface, a perfluorinated thermoplastic housing wherein each end of
the hollow fiber are potted to form a liquid tight seal forming a
unitary end structure with the surrounding housing wherein the
fiber ends are open; the housing having an inner wall and an outer
wall wherein the inner wall of the housing and the hollow fiber
outer surface defines a liquid flow volume, the housing having a
gas inlet to supply a gas to the first end of the hollow fibers and
a gas outlet to remove gas from the second end of the hollow
fibers; the housing having a spacer to promotes the ingress of
liquid into the flow volume of the housing; the housing having a
liquid inlet to supply a liquid to be contacted with the outer
surface of the hollow fibers and a liquid outlet to remove the
contacted liquid from the housing.
28. The contactor of claims 27 wherein the porous hollow fiber
membranes are unskinned.
29. A method of using the contactor of claim 27 comprising: flowing
in the hollow fibers a gas that contains ozone and contacting the
outer fiber surface with an aqueous liquid.
30. The method of claim 29 wherein the hollow fiber membranes are
unskinned.
31. A contactor that transfers mass between a liquid and a gas
comprising: a housing having a liquid inlet and a liquid outlet,
the liquid inlet and liquid outlet being configured to contact a
liquid with the outer surface of a plurality of perfluorinated
hollow porous fiber membranes positioned within the housing, the
housing having a gas inlet and a gas outlet configured to flow a
gas through the hollow fibers positioned in the housing whereby
mass transfer between the gas in the lumens and the liquid in
contact with the outer surface of the hollow fiber membranes
occurs; the housing and hollow fibers are configured to dissolve in
deionized water at 25.degree. C. at least about 0.34 ppm ozone
gas/liter of deionized water/liter of interior volume of housing
when the ozone gas is at a pressure of 22 psig, the concentration
of ozone gas of 250 gNm.sup.3 the flow ozone gas is 5 slpm, and the
flow of deionized water is 22 lpm.
32. The contactor of claim 31, wherein the hollow fibers are
configured in the housing to dissolve in deionized water at
25.degree. C. at least about 0.4 ppm ozone gas/liter of deionized
water/liter of interior volume of housing.
33. A contactor that transfers mass between a liquid and a gas
comprising: a housing having a liquid inlet and a liquid outlet to
contact a flow of liquid with the outer surface of a plurality of
perfluorinated porous hollow fiber membranes positioned within the
housing, the hollow fiber membranes fluidly sealed to the housing
and open for gas flow through the hollow fibers, the hollow fiber
membranes having an inlet and an outlet, the housing having a gas
inlet and a gas outlet for passing the gas through the porous
hollow fiber membranes, wherein the packing density of hollow
porous fiber membranes is at least 0.34 m.sup.2 of external
membrane area/liter of interior housing volume.
34. The contactor of claim 33 wherein the packing density is at
least 0.6 m.sup.2 external membrane area/liter of interior housing
volume.
35. The contactor of claim 33 further comprising a spacer.
36. A liquid-gas contactor, the contactor comprising a housing and
a plurality of porous conduits having an inner surface and an outer
surface, the conduits fluidly sealed to the housing and capable of
exchanging corrosive gases with a liquid in contact with the outer
surface of the conduits, the contactor characterized by being
capable of dissolving at least about 0.34 ppm ozone gas/lpm
deionized water/liter of internal cartridge volume without bubbles
at 25.degree. C. when the ozone gas is at a pressure of 22 psig,
the concentration of ozone gas is 250 gNm.sup.3, the flow ozone gas
is 5 slpm, and the flow of deionized water is 22 lpm.
37. The contactor of claim 36, wherein the contactor dissolves
greater than 0.4 ppm ozone gas/lpm deionized water/liter of
internal cartridge volume.
38. The contactor of claim 36, wherein the efficiency is greater
than 0.6 ppm ozone gas/lpm deionized water/liter of internal
cartridge volume.
39. A contacting device that dissolves an ozone containing gas in a
fluid comprising: a contactor including a housing with a liquid
inlet and a liquid outlet to contact a liquid with the outer
surface of a plurality of porous conduits positioned within the
housing, the conduits fluidly sealed to the housing and open to gas
flow through the conduits, the conduits having an inlet and an
outlet, the housing having a gas inlet and a gas outlet for passing
a gas through the conduits, a source of an ozone containing gas
connected to the gas inlet of the contacting device, an aqueous
liquid source connected to the liquid inlet of the contacting
device, the ozone containing gas dissolving into the aqueous fluid
through the gas contacting conduits.
40. The device of claim 39 further comprising a spacer.
41. The device of claim 39, wherein the gas conducting conduits are
perfluorinated porous hollow fibers.
42. The device of claim 41, wherein the hollow fibers are
twisted.
43. A contactor that transfers mass between fluids comprising: a
housing having a first fluid inlet and a first fluid outlet, the
first fluid inlet and first fluid outlet being configured to
contact a first fluid with the outer surface of a perfluorinated
membrane positioned within the housing, the housing having a second
fluid inlet and a second fluid outlet configured to contact a
second fluid with the inner surface of said perfluorinated membrane
whereby mass transfer between the two fluids occurs; the housing
and membrane are configured such that if the first fluid was
deionized water at 25.degree. C. and a flow rate of 22 lpm and the
second fluid was ozone gas at a pressure of 22 psig, a
concentration of 250 gNm.sup.3 and a flow rate of 5 slpm, at least
about 0.34 ppm ozone gas/liter of deionized water/liter of interior
volume of housing would be produced.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a hollow fiber membrane contactor
for phase contact applications. The contactor is made from
perfluorinated alkoxy polymeric materials, has a high packing
density providing high useful contacting area, and the ability to
operate with liquids of low surface tension.
BACKGROUND OF THE INVENTION
[0002] Liquid-gas contactors are used to transfer one or more
soluble substances from one phase to another. Examples of
conventional contactors are packed towers, plate columns and wetted
wall columns. In these systems, gas absorption of one or more
components from a gas stream is accomplished by dispersing the gas
as bubbles in packed towers and plate columns in a countercurrent
flow to a liquid stream. Absorption efficiency is controlled apart
from solubility considerations by the relative rate of the flows
and the effective surface area of the gas flow bubbles. In wetted
wall contactors the gas stream flows past a downward flow of liquid
on the inside wall of a vertical tube. Gas stripping is used to
transfer a gas dissolved in liquid into a gas stream. Similar
contactors are used for gas stripping.
[0003] Conventional contactors have several deficiencies. Primary
among these is the fact that the individual gas and liquid flows
cannot be varied independently over wide ranges. Tray columns are
prone to such problems as weeping at low gas flows and flooding at
high liquid flows. Packed towers can flood at high flow rates. The
use of low liquid flow rates in a packed tower can lead to
channeling and reduced effective surface area Excessive frothing or
foam formation can lead to process inefficiency. Wetted wall
contactors have inherently low mass transfer coefficients, and can
flood at high gas flow rates. The development of membrane
contactors has overcome these deficiencies.
[0004] Membrane contactors are devices through which two fluid
phases flow separated by a membrane permeable to the gas being
transferred. If a microporous membrane is being used, the preferred
method relies on the non-wetting characteristic of the membrane
material and the pore size to prevent liquid from intruding into
the pores and filling them. Gas transfer then occurs through the
gas filled pores to or from the liquid, depending on whether the
process is absorption or stripping. If a non-porous membrane is
used, gas transfer is controlled by the diffusion rate in the
non-porous layer of the membrane. While other membrane geometries
are available for this use, hollow fiber membranes are ideally
suited as contactors.
[0005] A hollow fiber porous membrane is a tubular filament
comprising an outer diameter, an inner diameter, with a porous wall
thickness between them. The inner diameter defines the hollow
portion of the fiber and is used to carry one of the fluids. For
what is termed tube side contacting, the liquid phase flows through
the hollow portion, sometimes called the lumen, and is maintained
separate from the gas phase, which surrounds the fiber. In shell
side contacting, the liquid phase surrounds the outer diameter and
surface of the fibers and the gas phase flows through the
lumen.
[0006] The outer or inner surface of a hollow fiber membrane can be
skinned or unskinned. A skin is a thin dense surface layer integral
with the substructure of the membrane. In skinned membranes, the
major portion of resistance to flow through the membrane resides in
the thin skin. The surface skin may contain pores leading to the
continuous porous structure of the substructure, or may be a
non-porous integral film-like surface. In porous skinned membranes,
permeation occurs primarily by connective flow through the pores.
Asymmetric refers to the uniformity of the pores size across the
thickness of the membrane; for hollow fibers, this is the porous
wall of the fiber. Asymmetric membranes have a structure in which
the pore size is a function of location through the cross-section,
typically, gradually increasing in size in traversing from one
surface to the opposing surface. Another manner of defining
asymmetry, is the ratio of pore sizes on one surface to those on
the opposite surface.
[0007] Manufacturers produce membranes from a variety of materials,
the most general class being synthetic polymers. An important class
of synthetic polymers are thermoplastic polymers, which can be
flowed and molded when heated and recover their original solid
properties when cooled. As the conditions of the application to
which the membrane is being used become more severe, the materials
that can be used becomes limited. For example, the organic
solvent-based solutions used for wafer coating in the
microelectronics industry will dissolve or swell and weaken most
common polymeric membranes. The high temperature stripping baths in
the same industry consist of highly acid and oxidative compounds,
which will destroy membranes made of common polymers.
Perfluorinated thermoplastic polymers such as
poly(tetrafluoroethylene-co- -perfluoro(alkylvinylether))
(poly(PTFE-CO-PFVAE)) or
poly(tetrafluoro-ethylene-co-hexafluoropropylene) (FEP) are not
adversely affected by severe conditions of use, so that membranes
made from these polymers would have a decided advantage over
ultrafiltration membranes made from less chemically and thermally
stable polymers. These thermoplastic polymers have advantages over
poly(tetrafluoroethylene) (PTFE), which is not a thermoplastic, in
that they can be molded or shaped in standard type processes, such
as extrusion. Perfluorinated thermoplastic hollow fiber membranes
can be produced at smaller diameters than possible with PTFE.
Fibers with smaller diameters, for example, in the range of about
350 micron outer diameter to about 1450 micron outer diameter, can
be fabricated into contactors having high membrane surface area to
contactor volume ratios. This attribute is useful for producing
compact equipment, which are useful in applications where space is
at a premium, such as in semiconductor manufacturing plants.
[0008] Being chemically inert, the Poly(PTFE-CO-PFVAE) and FEP
polymers are difficult to form into membranes using typical
solution casting methods as they are difficult to dissolve in the
normal solvents. Tey can be made into membranes using the Thermally
Induced Phase Separation (TIPS) process. In one example of the TIPS
process a polymer and organic liquid are mixed and heated in an
extruder to a temperature at which the polymer dissolves. A
membrane is shaped by extrusion through an extrusion die, and the
extruded membrane is cooled to form a gel. During cooling the
polymer solution temperature is reduced to below the upper critical
solution temperature. This is the temperature at or below which two
phases form from the homogeneous heated solution, one phase
primarily polymer, the other primarily solvent. If done properly,
the solvent rich phase forms a continuous interconnecting porosity.
The solvent rich phase forms a continuous interconnecting porosity.
The solvent rich phase is then extracted and the membrane
dried.
[0009] Hydrophobic microporous membranes are commonly used for
contactor applications with an aqueous solution that does not wet
the membrane. The solution flows on one side of the membrane and a
gas mixture preferably at a lower pressure than the solution flows
on the other. Pressures on each side of the membrane are maintained
so that the liquid pressure does not overcome the critical pressure
of the membrane, and so that the gas does not bubble into the
liquid. Critical pressure, the pressure at which the solution will
intrude into the pores, depends directly on the material used to
make the membrane, inversely on the pore size of the membrane, and
directly on the surface tension of the liquid in contact with the
gas phase. Hollow fiber membranes are primarily used because of the
ability to obtain a very high packing density with such devices.
Packing density relates to the amount of useful membrane surface
per volume of the device. It is related to the number of fibers
that can be potted in a finished contactor. Also, contactors may be
operated with the feed contacting the inside or the outside
surface, depending on which is more advantageous in the particular
application. Typical applications for contacting membrane systems
are to remove dissolved gases from liquids, "degassing"; or to add
a gaseous substance to a liquid. For example, ozone is added to
very pure water to form a solution used to wash semiconductor
wafers. Many processing steps involved in chip manufacturing use
very aggressive chemicals such as hot sulfuric acid, hydrogen
peroxide, phosphoric acid, etc. for etching purpose. Since these
chemicals are toxic and dangerous, transport, storage and proper
disposal of these chemicals pose serious health and safety hazards
to the workers in this industry. New processing technologies have
been developed in the last few years by a number of chip tool
manufacturers. Unlike the conventional process which uses many
aggressive chemicals, the new processing technology utilizes only
two chemicals-ozonated DI water and HF water. It has been
demonstrated that almost all existing processing bath can be
replaced using only these two chemicals.
[0010] While HF water can easily be produced, a good source for
ozonated water has been a challenge. Although ozonated water is
being used in chip plants today. Most are for cleaning operations
where only a couple of ppm of ozone concentration are needed in the
fluid stream. However, to replace aggressive etch baths, much
higher ozone concentration is needed. In general, the concentration
ranges between 10-80 ppm. The water flow rate ranged between 5-40
ppm. Typical requirement is about 15 ppm at 20 lpm water flow
rate.
[0011] Prior to the present invention, a tube side contacting
device has been provided wherein the hollow fibers are formed of a
polyfluorinated alkoxyvinylether (PFA) polymer. This device is
characterized by undesirably limited gas mass transfer through the
hollow fibers. In addition, contact devices have been provided
wherein the hollow filter membranes or spirally pleated membranes
are formed of PTFE.
[0012] Ohmi et al, J. Electrochem. Soc., Vol. 140, No. 3, March
1993, pp. 804-810, describe cleaning organic impurities form
silicon wafers at room temperature with ozone-injected ultrapure
water. U.S. Pat. No. 5,464,480 shows that ozone diffused through a
subambient temperature deionized water will quickly and effectively
remove organic materials such as photoresist from waters without
the uses of other chemicals. It is believed that lowering the
temperature of the solution enables a sufficiently high ozone
concentration in solution to substantially oxidize all of the
organic material on the wafer to insoluble gases. The means for
diffusing a gas can be any means which provides fine bubbles of
ozone or other gases into the tank and uniformly distributes the
gas throughout the tank.
[0013] In U.S. Pat. No. 5,464,480, preferably, the bubbles that are
provided by the diffuser are initially about 25 to about 40 microns
in diameter. The gas diffuser preferably are initially about 25 to
about 40 microns in diameter. The gas diffuser preferably is made
from a mixture of polytetrafluoroethylene (PTFE) and
perfluoroalkoxylvinylether. By varying the temperature and pressure
under which the mixture is prepared by methods known in the art,
both porous and nonporous members are formed. The impermeable and
permeable members are preferably comprised of about 95% PTFE and
about 5% perfluoroalkoxylvinylether. The permeable member and the
impermeable member may be joined by any number of methods as long
as the result is a composite member that will not come apart under
the stresses in the tank. Preferably, the members are heat sealed
together, essentially melting or fusing the members together using
carbon-carbon bonds. Once the permeable member is formed, a trench
is bored out of the PTFE in the top portion of the member. The
resulting diffuser has on the order of about 100,000 pores of a
size of about 25 to about 40 microns in diameter through which gas
may permeate into the treatment tank. The use of the trench in the
diffuser allows the gas to diffuse into the tank as very fine
bubbles. In applications for the semiconductor manufacturing
industry, a device that supplied homogeneous bubble free ozone
dissolved in ultrapure water would provide more efficient oxidation
reactions because the reaction would not be localized at the
bubbles. The more homogeneous solution would provide for a more
uniform cleaning reaction. Furthermore, the high surface area to
volume ratio inherent in hollow fiber devices would give a compact
system, suitable for semiconductor operations.
[0014] Dissolved oxygen in ultrapure water is another problem in
semiconductor device manufacturing. Oxygen removal to less than one
part per billion (ppb) is required to prevent uncontrolled oxide
growth. Potential problems associated with uncontrolled oxide
growth are prevention of low temperature epitaxy growth, reduction
of precise control of gate-oxide films, and increased contact
resistance for VIA holes. This uncontrolled growth can be overcome
by stripping dissolved oxygen to less than 1 ppb from the ultrapure
water used in the manufacturing process. The high packing density
and cleanliness associated with an all perfluorinated thermoplastic
contactor are advantages in such applications.
[0015] U.S. Pat. No. 5,670,094 provides an oxidized water producing
method in which a pressurized ozone gas is generated by an electric
discharge type ozonator is dissolved in water to be treated through
a hollow fiber membrane, characterized in that the water pressure
inside the membrane is maintained higher than the pressure of the
ozone gas supplied to the outside of the hollow fiber membrane to
prevent tiny bubbles and impurities from getting mixed into the
water being treated, and the ozone concentration in the treated
water is controlled on the basis of the concentration of the ozone
gas. This reference discloses only PTFE membranes and does not
contemplate the use of an all perfluorinated thermoplastic
contactor.
[0016] Commercially all available PTFE hollow tube contactors are
referred to as "hollow tubes", probably because they are relatively
large. Patent PJ7213880A discloses the fiber manufacturing process
for maling composite PTFE hollow tubes for ozonizing applications.
The first step of this process involves extruding PTFE paste
derived from a mixture of PTFE powder and lubricants. After the
tube is formed, the lubricants are extracted and the powder
sintered into a slightly porous PTFE solid tube. The tube is then
stretched longitudinally to make it porous. This is different than
typical PTFE sheet membranes made by a similar process. To generate
very fine microporous structures, characterized by a node to
fibrils network, most PTFE membranes are made by biaxial
stretching. For hollow fibers, the equivalent process would have
been stretching the fiber radially. Probably because of the
impracticality of such a step, this radial stretching step is
missing from the disclosed process. Consequently, the pores in this
tube are only "half-formed", i.e., it did not attain the "node to
fibril network" of flat sheet membrane. To compensate for this
deficiency, the tube underwent a second step of laminating a
regular microporous flat sheet membrane on top of the external
surface of the porous tube. This step involves lamination of a long
narrow strip of PTFE microporous membrane spirally on the surface
of the tubing. This is a tedious, labor intensive process. Also,
with the membrane laminated to the outside of the hollow tube, the
resistance to mass transfer in tube-side flow could be higher in
cases were the fluid partially intrudes into the support layer.
This arrangement diminishes the potential of housing the membrane
as the barrier for separating the two fluid phases. These
deficiencies are overcome with the hollow fiber membranes of the
present invention.
[0017] An advantage for contacting applications is that the very
low surface tension of these perfluorinated polymers allows use
with low surface tension liquids. For example, highly corrosive
developers used in the semiconductor manufacturing industry may
contain surface tension reducing additives, such as surfactants.
These developers could not be degassed with typical microporous
membranes because the liquid would intrude the pores at the
pressures used and permeate, causing solution loss and excess
evaporation. In addition., liquid filling the pores would greatly
add to the mass transfer resistance of gas transport. U.S. Pat. No.
5,749,941 describes how conventional follow fiber membranes of
polypropylene or polyethylene cannot be used in carbon dioxide or
hydrogen sulfide absorption into aqueous solutions containing an
organic solvent without the use of a solution additive to prevent
leakage. While (TFE) membranes would work in these applications,
presumably because of their lower surface tension, they are
difficult to process into hollow fibers. The membranes of the
present invention are made from polymers having similar surface
tension properties to PTFE and are more readily manufactured into
small diameter hollow fiber membranes.
[0018] Accordingly, it would be desirable to provide a hollow fiber
membrane contactor apparatus for forming a liquid solution from a
gas and a liquid which provides high mass transfer rates of gas
through the hollow fiber membranes. Such an apparatus can be formed
of a suitably small size to permit its use with currently available
apparatus for delivering a reagent to a conventional etching
process for making electronic devices.
SUMMARY OF THE INVENTION
[0019] In a first embodiment of the present invention, a shell side
contact device is provided comprising a shell containing
perfluoroalkoxy resin thermoplastic hollow fibers used as a porous
barrier. The perfluoroalkoxy resin comprises a copolymer of
tetrafluoroethylene and perfluoroalkyl vinyl ether or a
tetrafluoroethyl-co-hexafluropropylene (FEP)copolymer. The
perflu6roalkoxy resin is impermeable to water and is permeable to
gases such as oxygen, nitrogen or ozone. The perfluoroalkoxy resin
hollow fibers can be unskinned, skinned on their inner surface or
skinned on their outer surfaces. It is preferred that the hollow
fibers be unskinned.
[0020] In a second embodiment, a shell side contact device is
provided comprising a shell, perfluoroalkoxy resin hollow fibers
and spacer means for separating the hollow fibers adjacent an inlet
to the shell for liquid to be introduced into the shell. The spacer
means permit liquid flow through the shell at desirably high flow
rates with acceptable pressure drop through the shell.
[0021] This invention provides for contact device including a
thermoplastic perfluoroalkoxy resin hollow fiber membrane contactor
with unitary end structures having a high packing density.
[0022] The contactor is comprised of a bundle of substantially
parallel hollow fiber membranes potted at both ends and having
unitary end structure(s) with the housing containing the fibers.
The lumens of the hollow fibers are exposed at both ends of the
hollow fibers. The perfluorinated thermoplastic hollow fiber
membranes of this invention are made of a polymer of a
tetrafluoroethyl-co-hexafluoropropylene copolymer or poly
(tetrafluoroethylene-co-perfluoro (alkyl-vinylether). Typically,
alkyl can be propyl wherein the polymer is referred to in the art
as PFA or a mixture of methyl and propyl wherein the polymer is
referred to in the art as MFA. PFA is manufactured by DuPont,
Wilmington, Del. MFA is described in U.S. Pat. No. 5,463,006. A
preferred polymer is Hyflon.RTM. POLY (PTFE-CO-PFVAE) 620,
obtainable from Ausimont USA, Inc., Thorofare, N.J.
[0023] The fibers are made by a Thermally Induced Phase Separation
(TIPS) method, in which polymer is dissolved in a halocarbon
solvent at high temperatures and extruded through an annular die
into a cooling bath. The resulting gel fiber is wound as a
continuous coil on a steel frame with the fibers substantially
parallel and not touching. The frame and coil are placed in an
extraction bath to remove the solvent from the gel fiber. After
extraction, the fibers are annealed on the frame for about 24 hours
and then cooled. The fibers are removed from the annealing oven and
cooled. They are then gathered into a cylindrical bundle and potted
and bonded in a single step.
[0024] Potting is a process of forming a tube sheet having liquid
tight seals around each fiber. The tube sheet or pot separates the
interior of the final contactor from the environment. The pot is
thermally bonded to the housing vessel in the present invention to
produce a unitary end structure. The unitary end structure
comprises the portion of the fiber bundle which is encompassed in a
potted end, the pot and the end portion of the perfluorinated
thermoplastic housing, the inner surface of which is congruent with
the pot and bonded to it. By forming a unitary structure, a more
robust contactor is produced, less likely to leak or otherwise fail
at the interface of the pot and the housing. The potting and
bonding process is an adaption of the method described in U.S.
patent application Ser. No. 60/117,853 filed Jan. 29, 1999, the
disclosure of which is incorporated by reference.
[0025] Potting and bonding are done in a single step. An external
heating block is used for potting one end at a time. The
perfluorinated thermoplastic end seals are preferably made of poly
(tetrafluoroethylene-co-perfluoro (alkylvinylether)) having a
melting point of from about 250.degree. C. to about 260.degree. C.
A preferred potting material is Hyflon.RTM. 940 AX resin, from
Ausimont U.S.A., Inc., Thorofare, N.J. Low viscosity poly
(tetrafluoroethylene-co-hexafluoroprop- ylene) with low end-of-melt
temperatures as described in U.S. Pat. No. 5,266,639 is also
suitable. The process involves heating the potting material in a
heating cup at about 275.degree. C. until the melt turns clear and
are free of trapped bubbles. A recess is made in the molten pool of
potting material that remains as a recess for a time sufficient to
position and fix the fiber bundle and housing in place.
Subsequently, the recess will fill with the molten thermoplastic in
a gravity driven flow.
[0026] A unitary end structures, by which is meant that the fibers
and the pot are bonded to the housing to form a single entity
consisting solely of perfluorinated thermoplastic materials is
prepared by first pretreating the surfaces of both ends of the
housing before the potting and bonding step. This is accomplished
by melt-bonding the potting material to the housing. The internal
surfaces on both ends of the housing are heated closer to its
melting point or just at the melting point and immediately immersed
into a cup containing powdered poly
(tetrafluoroethylene-co-perfluoro (alkylvinylether)
(PTFE-CO-PFVAE)) potting resin. Since the surface temperature of
the housing is higher than the melting point of the potting resins,
the potting resin is then fused to the housing resin. The housing
is then removed and polished with a heat gun to fuse any excess
unmelted powder. Without this pretreatment step, the housing
surfaces often detach from the potting surfaces because of absence
of intermixing of the two resins.
[0027] The unitary end structure(s) is cut and the lumen of the
fibers exposed. The potting surfaces are then polished further
using a heat gun to melt away any smeared or rough potted surfaces.
A solder gun can be used to locally remelt and repair any defective
spot, sometimes with the help of a drop of melted resin.
[0028] The shell side contactor of the invention provides
substantial advantages over the tube side contactor of the prior
art. In one aspect of this invention, the shell side contactor is
highly efficient in that the ozonated product comprises at least
0.34 ppm ozone/liter aqueous liquid/liter of interior shell
volume.
[0029] In another aspect of this invention, the shell side
contactor is characterized by a high pacling density of at least
0.60 m.sup.2 membrane area/liter of interior shell (housing)
volume.
[0030] In another aspect of this invention, the shell volume
containing the hollow fibers is free of apparatus which promotes
turbulent liquid flow within the shell.
DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is an illustration of a shell side hollow fiber
membrane contactor of this invention.
[0032] FIG. 2 is an illustration of a shell side hollow fiber
contactor of this invention including parallel plate spacers.
[0033] FIG. 3 is an illustration of a shell side contactor of this
invention including a thick shell wall spacer.
[0034] FIG. 4 is an illustration of a shell side hollow fiber
contactor of this invention including a tubular spacer extending
through the liquid inlet to a shell.
[0035] FIG. 5 compares the shell side flows vs. tube side flow on
ozone concentration for the contactor of Example 1.
[0036] FIG. 6 compares the shell side flows vs. tube side flow on
ozone concentration for the contactor of Example 2.
[0037] FIG. 7 is an illustration of a prior art, tube side
contactor.
[0038] FIG. 8 is an illustration of an ozone test system.
[0039] FIG. 9 compares the shell side flows vs. tube side flow on
ozone concentration for the contactor of Example 3.
[0040] FIG. 10 illustrates the efficiency of a shell side hollow
fiber contactor of this invention.
[0041] FIG. 11 illustrates the performance of a shell side hollow
fiber contactor of this invention.
[0042] FIG. 12 illustrates the relationship between ozonation and
temperature.
[0043] FIG. 13 illustrates the effect of water flow rate on
ozonation.
[0044] FIG. 14 illustrates the effect of water flow rate vs.
pressure drop.
[0045] FIG. 15 is a schematic of an ozonation recirculating
loop.
DETAILED DESCRIPTION OF THE INVENTION
[0046] The asymmetric skinned hollow fiber membrane is produced by
the process described in concurrent U.S. patent application Ser.
No. 60/117,854 filed Jan. 29, 1999, the disclosure of which is
incorporated by reference. That process is based on the Thermally
Induced Phase Separation (TIPS) method of making porous structures
and membranes. A mixture of perfluorinated thermoplastic polymer
pellets, usually ground to a size smaller than supplied by the
manufacturer, from about 100 to about 1000 microns, preferably
about 300 microns, more preferably supplied or ground to a powder,
and an solvent, such as chlorotrifluoroethylene oligimer, is first
mixed to a past or past-like consistency. The polymer comprises
between approximately 12% to 75%, preferably 30% to 60%, by weight
of the mixture. The polymers are perfluorinated thermoplastic
polymers, more specifically poly (tetrafluoroethylene-co-perfluoro
(alkylvinylether) such as PFA or MFA or
tetrafluoroethylene-co-hexafluoropropylene) (FEP), or blends of
these polymers, which are dissolved in a solvent to give a solution
having an upper critical solution temperature, and which when the
solution is cooled, separates into two phases by liquid-liquid
phase separation.
[0047] The solvent is chosen so the membrane formation occurs by
liquid-liquid phase separation, rather than solid-liquid phase
separation when the solution is extruded and cooled. Preferred
solvents are saturated low molecular weight polymers of
chlorotrifluoroethylene. A preferred solvent is HaloVac.RTM. 60
from Halocarbon Products Corporation, River Edge, N.J. The choice
of the solvent is dictated by the ability of the solvent to
dissolve the polymer when heated to form an upper critical solution
temperature solution, but not to excessively boil at that
temperature. Fiber extrusion is referred to as spinning and the
extruded fiber length from the die exit to the take-up station is
referred to as the spin line. The paste is metered into a heated
extruder barrel where the temperature raised to above the upper
critical solution temperature so that dissolution occurs. For
inside skinned hollow fiber membranes, the homogeneous solution is
then extruded through an annular die directly into a liquid cooling
bath with no air gap. The lumen diameter is maintained with a
constant pressure of gas. The liquid cooling bath is maintained at
a temperature below the upper critical solution temperature of the
polymer solution. The preferred bath liquid is not a solvent for
the thermoplastic polymer, even at the extrusion temperature. Upon
cooling, the heated and shaped solution undergoes phase separation
and a gel fiber results. The die tip is slightly submerged for
vertical spinning, i.e.; the spin line falls downward, in the
direction of a freely falling body. For horizontal spinning, where
the spin line exists directly in the horizontal attitude, and is
maintained more or less in that plane until at least the first
guide roll, a specially design die is used. The die is firmly
positioned against an insulated wall with the die tip penetrating
through an opening having a liquid-tight seal in the insulator
wall. A trough for cooling liquid flow is placed in a recess in the
opposite side of the insulating wall, in a manner that will
maintain the die nose outlet in a submerged condition. Cooling
liquid flows in the trough and overflows in a region of the trough
of lesser depth, keeping the die nose outlet submerged with a flow
of cooling liquid. In both the vertical and horizontal methods, a
booster heater and temperature control means is used to briefly
raise the solution temperature at the die tip to prevent premature
cooling. In the subsequent step, the dissolution solvent is removed
by extraction and the resultant hollow fiber membrane is dried
under restraint to prevent membrane shrinkage and collapse.
Optionally, the dried fiber may be heat set at 200.degree. C. to
300.degree. C. Preferably the fiber will be heat set or annealed
under restraint at a temperature near the melting temperature of
the fiber, which for the preferred polymer of this invention is
within a range of from about 270.degree. C. to about 290.degree. C,
preferably from about 275.degree. C. to about 285.degree. C., with
the most preferred range from about 278.degree. C. to about
282.degree. C. In order to minimize shrinkage during potting, a
second, unrestrained, annealing step at similar temperatures is a
preferred step. Annealing times for these steps is from about 6 to
about 48 hours, more preferably, from about 18 to about 30
hours.
[0048] In the invention described in U.S. Ser. No. 60/117,854,
controlled evaporation of solvent from at least one surface of the
hollow fiber as it exists the die tip is combined with higher
polymer solids solutions and the submerged extrusion process to
produce inner diameter skinned asymmetrical hollow fiber porous
membranes from perfluorinated thermoplastic polymers. For this
embodiment, the lumen is maintained with a constant pressure of a
gas continuously fed into the inner diameter of the lumen. In this
embodiment the superheated solvent evaporates inside the lumen as
soon as it emerges from the die. The loss of solvent causes a
superficial increase in solids concentration on the inner lumen
surface. As the melt is quenched, a very thin skin is formed on the
lumen surface, while the rest of the membrane forms a microporous
structure due to its being submerged in a cooling or quenching bath
which prevents the porogen from flashing off the outer surface and
prevents the formation of a skin on the outer surface.
[0049] To produce an asymmetric skinned perfluorinated
thermoplastic hollow fiber membrane with the skin on the outer
surface, the process described above is adapted so the lumen is
filled with a liquid to prevent evaporation at the inner surface
and the outer surface is exposed to the atmosphere in a very short
air gap before entering the cooling bath. The lumen-filling liquid
can be a liquid that does not boil or excessively vaporize during
the extrusion process. Preferred liquids are mineral oil, silicone
oil, and dioctylpthlate, with a most preferred liquid being a low
molecular weight saturated chlorotrifluorohydrocarbon polymer.
[0050] To produce unskinned perfluorinated thermoplastic
microporous hollow fiber membrane, the teachings of U.S. patent
application Ser. Nos. 60/117,852 and 60/117,853 filed Jan. 29,
1999, are used. This application provides for high flux, skin-free
hollow fiber porous membranes, more specifically, microporous
membranes, from perfluorinated thermoplastic polymers, more
specifically PFA or MFA or blends of these polymers.
[0051] The process to produce these membranes is based on the
Thermally Induced Phase Separation (TIPS) method of making porous
structures and membranes. A mixture of polymer pellets, usually
ground to a size smaller than supplied by the manufacturer, from
about 100 to about 1000 microns, preferably about 300 microns, more
preferably supplied or ground to a powder and an solvent, such as
chlorotrifluoroethylene oligimer, is first mixed to a paste or
paste-like consistency. The polymer comprises between approximately
12% to 35% by weight of the mixture. The solvent is chosen so the
membrane formation occurs by liquid-liquid, rather than
solid-liquid phase separation when the solution is extruded and
cooled. Preferred solvents are saturated low molecular weight
polymers of chlorotrifluoroethylene. A preferred solvent is
HaloVac.RTM. 60 from Halocarbon Products Corporation, River Edge,
N.J. Choice of the solvent is dictated by the ability of the
solvent to dissolve the polymer when heated to form an upper
critical solution temperature solution, but not to excessively boil
at that temperature. Fiber extrusion is referred to as spinning and
the extruded fiber length from the die exit to the take-up station
is referred to as the spin line. The paste is metered into a heated
extruder barrel where the temperature raised to above the upper
critical solution temperature so that dissolution occurs. The
homogeneous solution is then extruded through an annular die
directly into a liquid cooling bath with no air gap. The liquid
cooling bath is maintained at a temperature below the upper
critical solution temperature of the polymer solution. The
preferred bath liquid is not a solvent for the thermoplastic
polymer, even at the extrusion temperature. Upon cooling, the
heated and shaped solution undergoes phase separation and a gel
fiber results. The die tip is slightly submerged for vertical
spinning, i.e.; the spin line falls downward, in the direction of a
freely falling body. For horizontal spinning, where the spin line
exits directly in the horizontal attitude, and is maintained more
or less in that plane until at least the first guide roll, a
specially design die is used. The die is firmly positioned against
an insulated wall with the die tip penetrating through an opening
having a liquid-tight seal in the insulator wall. A trough for
cooling liquid flow is placed in a recess in the opposite side of
the insulating wall, in a manner that will maintain the die nose
outlet in a submerged condition. Cooling liquid flows in the trough
and overflows in a region of the trough of lesser depth, keeping
the die nose outlet submerged with a flow of cooling liquid. In
both the vertical and horizontal methods, a booster heater and
temperature control means is used to briefly raise the solution
temperature at the die tip to prevent premature cooling. In a
subsequent step, the dissolution solvent is removed by extraction
and the resultant hollow fiber membrane is dried under restraint to
prevent membrane shrinkage and collapse. Optionally the dried fiber
may be heat set at 200.degree. C. to 300.degree. C.
[0052] The potting method is described in concurrent U.S. patent
application Ser. No. 60/117,853, filed Jan. 29, 1999, incorporated
by reference. This application describes a simplified method for
manufacturing a filter element of perfluorinated thermoplastic
hollow fiber membranes potted with a perfluorinated thermoplastic
polymer. The method comprises vertically placing a portion of a
bundle of hollow fiber membrane lengths with at least one closed,
by the closed end, into a temporary recess made in a pool of molten
thermoplastic polymer held in a container, holding the fiber
lengths in a defined vertical position, maintaining the
thermoplastic polymer in a molten state so that it flows into the
temporary recess, around the fibers and vertically up the fibers,
completely filling the interstitial spaces between fibers with the
thermoplastic polymer. A temporary recess is a recess that remains
as a recess in the molten potting material for a time sufficient to
position and fix the fiber bundle in place and then will be filled
by the molten thermoplastic. The temporary nature of the recess can
be controlled by the temperature at which the potting material is
held, the temperature at which the potting material is held during
fiber bundle placement, and the physical properties of the potting
material. A temporary recess can also be recess in a solid
thermoplastic which will fill when the thermoplastic is heated to a
temperature sufficiently above its softening or melting temperature
to flow, and held at that temperature for the time necessary to
fill the recess. The end of the fiber can be closed by sealing,
plugging, or in a preferred embodiment, by being formed in a
loop.
[0053] Referring to FIG. 1, the shell side contactor 10 of this
invention includes a shell 12 formed of a copolymer of
tetrafluoroethylene and perfluoroalkyl vinyl ether such as PFA or
MFA or the like. Positioned within the shell are a plurality of
hollow fibers 14 formed of a polymer composition described above.
The fibers 14 are potted at each end of the shell 12 with a potting
composition 16 as set forth above. In use, a liquid enters shell 12
through inlet 18 and is removed from shell 12 through outlet 20.
Gas enters the lumens of hollow fiber 14 through gas inlets 22
utilizing a conventional manifold (not shown) and is removed from
the lumens through gas outlets 24. Gas such as ozone, passes
through the hollow fiber walls and is dissolved in the liquid such
as water to form an aqueous ozone solution that can be utilized as
an etchant. Ozone is produced by subjecting an oxygen containing
gas such as 99% oxygen and 1% nitrogen to an electrical discharge
in a marmer well known in the art.
[0054] Referring to FIG. 2, the shell side contactor 26 includes
the potting composition 16 and hollow fibers 14 as well as parallel
positioned spacer plates 28 that spread the hollow fiber 14 thereby
to promote free flow of liquid within contactor 26 to reduce
pressure drop within shell 12.
[0055] Referring to FIG. 3, contactor 30 includes shell wall
portion 30 which is thicker adjacent the liquid inlet 18 that the
remainder of the shell will 32 positioned remote from the inlet 18.
This configuration increases the volume of open space adjacent
inlet 18 thereby to reduce pressure drop within the shell 12.
[0056] Referring to FIG. 4, the contactor 36 includes an extended
tube 38 which separates hollow fiber 14. The tube 38 includes holes
40 through which liquid can pass into the shell 12. The tube 38
promotes ingress of liquid into the shell 12 and to reduce pressure
drop through the shell 12.
[0057] The present invention provides a shell side contactor which
is highly efficient for forming relatively high concentration of
ozonated water which contains at least about 0.34 ppm ozone/liter
aqueous liquid/liter of interior housing volume, preferably, at
least 0.43 ppm ozone/liter aqueous liquid/liter of interior housing
volume.
[0058] This efficiency is at least about 70% more efficient in
forming ozonated water over the most efficient prior art ozonator,
an Infuzor.TM. cartridge that produced an performance of 0.167
ppm/liter acqueous liquid/liter of interior shell volume. The
Infuzor.TM. cartridge is made by Pall Corporation of East Hills,
N.Y.
[0059] All performance claims in this patent application pertain to
ozone contacting use an ozone gas condition of 250 g/Nm.sup.3, 5
slpm and 22 psig of gas pressure.
[0060] The efficiency produced by the present invention permits
utilizing a desirably lower volume shell at relatively high flow
rates of ozone and water through the shell side contactor of this
invention. In addition, the efficiency of the present invention
permits operating the shell side contactor on a one-pass basis of
liquid through the contactor. This operation eliminates the need
for a liquid flow path and accompanying pumping capacity to effect
passage of the liquid through the contactor a plurality of times in
order to produce an ozonated water product containing a desired
minimum ozone concentration. Thus, the shell side contactor of this
invention provides substantial advantages over prior art contactors
which require a multiple pass liquid flow path.
[0061] In another aspect of this invention, the shell (housing)
side contactor of this invention has a high packing density of at
least 0.34 m.sup.2 membrane area/liter of interior shell volume,
preferably at least 0.60 m.sup.2 membrane area/liter of interior
shell volume, so that the efficiencies set forth above are
obtained. These high packing densities are obtained when utilizing
hollow fibers having an exterior diameter of between about 300.mu.
and 1500.mu. and preferably between about 600.mu. and 1000.mu. and
an interior diameter between about 250.mu. and 1100.mu.. A packing
density up to about 1.2 m.sup.2 membrane area/liter of interior
shell volume can be obtained with the contactor of this
invention.
[0062] In addition, the present invention provides a shell side
contactor which is free of apparatus which effects turbulent liquid
flow within the shell. The exclusion of such apparatus is
advantageous since particle formation from turbulent flow activator
apparatus is eliminated. The lack of such particles is essential
when processing ozonated aqueous composition utilized in the
electronics industry. In addition, such an apparatus provides a
simple construction which reduces manufacturing costs. The
apparatus of this invention contrasts, for example with the
Liqui-Cel.TM. contactor provided by Hoechest Celanese Corporation
which utilizes hollow fibers positioned on a flexible substrate,
such as a woven substrate which includes a turbulent liquid
inducing baffle within the shell.
[0063] In operating the shell-side contactor of this invention, the
inlet gas pressure at the hollow fiber inlets typically is between
about 1 and about 45 psig, preferably between about 10 and about 45
psig. Typical outlet gas pressure drop at the hollow fiber outlets
is between about 0.1 and about 5 psig, preferably between about 0.1
and about 1 psig. Typical liquid pressure drop at the shell inlet
typically is between about 5 and about 45 psig, preferably between
about 2 and about 15 psig. When operating under these conditions of
pressure, the efficiency and high packing densities set forth above
are obtained. In addition, when operating under these conditions,
gas bubbles within said liquid are prevented while permitting
dissolution of the gas such as ozone in the liquid such as
water.
Characterization Methods
[0064] The shell side contactor of this invention provides
substantial advantages over a tube side-contactor. The mass
transfer equation of tube-side flow is characterized by the
following equation.
Sh=K*d/D.sub.ab=1.64*Re.sup.0.33Sc.sup.0.33*(D/L).sup.0.33
[0065] Where Sh=Sherwood Number,
[0066] K=mass transfer coefficient, cm2/s,
[0067] L=length of fiber
[0068] d=ID of fiber, cm.
[0069] D.sub.ab=diffusion coefficient of ozone
[0070] Re=Reynold's Number, pvd/.mu.
[0071] Sc=Schmidt's Number, .mu./.rho.D.sub.ab
[0072] v=velocity, cm/sec
[0073] .rho.=density, cm3/sec.
[0074] It can be deduced from the above equation that as the device
gets longer (larger L), the mass transfer coefficient per membrane
area will drop accordingly. Keeping the device short and increasing
the number of fibers also doesn't provide satisfactory results. The
reason is that with more fibers, the flow per fiber (v in the above
equation) will drop, again resulting in reduced mass transfer/area.
Therefore, in tube-side flow, although adding membrane area always
results in higher performance, the mass transfer performance always
increases much less than the increase in membrane area. The reason
for reduced mass transfer is that as the fiber length increases, so
is the thickness of diffusion the boundary layer. Thicker boundary
layer means lower mass transfer.
[0075] Shell-side mass transfer is much more efficient, the mass
transfer is characterized by the following equation:
Sh=K*d/D.sub.ab=0.36*Re.sup.0.55Sc.sup.0.33
[0076] Notably missing from the above equation, when compared with
the tube-side equation, is the dependency of d, diameter of the
fiber and L, the length of the fiber. The benefit of shell-side
transfer can be estimated by dividing the shell-side equation by
the tube-side equation. It can be found that shell-side is between
5.times. to 10.times. better than tube-side. An additional benefit
of shell-side transfer surface is the external membrane area. Since
external membrane area is always larger than the internal area
(tube-side transfer), shall-side transfer has the benefit of a
larger contact area.
[0077] Although shell-side mass transfer has many advantages, in
general, design and construction of the device is more complicated.
Usually it involves a center tube distribution for the liquid. The
fibers may have to be woven onto a supporting mat. All these
additional construction elements, which are absent in the tube-side
configuration, pose significant challenges to the cartridge
manufacturing process. In addition, supporting materials such as
threads and mats are potential particle generators which may
significantly contribute to microcontamination.
[0078] The advantage of this invention is that the ability to
obtain substantial benefits from shell-side mass transfer with
relatively simple construction method. While this method doesn't
provide the full benefit of shell-side mass transfer, the
manufacturing cost for such a module is also less.
[0079] A tube-side module is usually constructed with two large
fittings at both ends for water to flow through the lumen of the
hollow fibers. Two small gas fittings would be located on the shell
near the exit and the entrance of the module. In this invention,
the fittings and the flow of the liquid and gas are reversed. In
other words, two small fittings, e.g. 1/4" are located at either
end of the module for gas flow, while large fittings are bonded on
the shell at cross direction near the exit and entrance of the
module. The large fittings, e.g. 1/2" to 1" are needed for carrying
water flow rate up to 10 gpm.
[0080] A shell-side module cannot be made just by reversing the
flow and the fittings because the fiber bundle would produce
enormous pressure drops (>30 psig@5 gpm). The packing density
has to be reduced and the fiber bundle arrangement near the
fittings has to be modified to minimize the pressure drop but at
the same time avoiding massive channeling of water flow. We have
found that pressure drop can be substantially reduced by decreasing
the packing density from 58% to 48% and also offsetting the fiber
bundle at the potted area. In addition, it also has been found that
the mass transfer efficiency can be enhanced by jetting water into
the fiber bundle and introducing gaps into the bundle.
[0081] FIG. 7 shows a typical tube side flow pattern of the Prior
Art hollow fiber contactor. In the tube side configuration the
liquid flows inside the lumen of the fiber and the ozone gas flows
(on the outer surface of the fiber) across shell side. The porous
structure of hollow fibers, without dense skin, allows only gas to
diffuse through the membrane and dissolve in the water flowing in
the lumen. The liquid and gas flows are switched for the shell side
configuration--the liquid flows through the shell side and the
ozone gas is routed through the fiber lumens. For optimum
performance the gas and liquid flows should be countercurrent. The
contactor can be mounted either horizontally or vertically. As we
describe below the two flow configurations offer widely different
ozone transfer efficiency. Performance depends on the gas side flow
rate, pressure, and concentration; and the liquid side flow rate,
pressure, temperature, and pH. All experiments were performed on
the system shown in FIG. 8. It is a recirculation type system; the
all PFA degassers are installed to provide a constant DI feed
stream free of ozone.
EXAMPLE 1
[0082] A contactor with dimensions of 2.25" ID and 12" in length
was made using porous skinless PFA hollow fibers. The fiber outer
diameter (OD) was about 800 microns and the inner diameter (ID)
about 500 microns. The number of the fibers was about 2100 and the
packing density was around 0.46 m.sup.2 of external membrane
area/liter of internal cartridge volume. The fiber bundle was
potted with an offset of 1/4" gas fitting at both end of the
module. Two 1" fittings for water flow were bonded on the shell at
cross-direction near the exit and entrance. The inside of the
fitting was reduced to 1/2" using an insert to create jetting
action of water into the bundle. The water fittings are located
perpendicular to the offset of the potting such that a cavity is
formed right under the fitting.
[0083] The contactor was tested for ozonation efficiency. Ozone gas
at 22 psig., 250 gNm3 and 5 slpm was fed into the gas port of the
contactor. Deionized (DI) water was pumped into the contactor using
the shell-side water fitting at a rate of 5 gpm at 25C. The water
pressure drop across the module was about 5 psig. The concentration
of ozone in the outlet water was measured using an IN-USA ozone
sensor. After a couple of minutes, the ozone concentration in the
water reached 23 ppm. Under the same operating conditions, the same
contactor would produce less than 15 ppm using tube-side mode mass
transfer. Therefore, the ozonation efficiency improvement was about
50%. The ozonation efficiency for this module was 0.4 ppm/lpm/liter
of cartridge volume. A comparison of the effect on ozone
concentration over a shell side contactor of this invention and a
tube side contactor is shown in FIG. 5.
EXAMPLE 2
[0084] A contactor with dimensions of 2" ID and 15" in length was
made using porous skinless PFA hollow fibers. The fiber OD was
about 800 micron and the ID about 500 micron. The number of the
fibers was about 1700 and the packing density was around 0.42
m.sup.2 membrane/liter of internal cartridge volume. The contactor
was fitted with 1/4" gas fitting at both end of the module. Two
1/2" fitting for water flow were bonded on the shell at cross
direction near the exit and entrance.
[0085] The contactor was tested for ozonation efficiency. Ozone gas
at 22 psig. 250 g/Nm3 and 5 slpm was fed into the gas port of the
contactor. DI water was pumped into the contactor using the
shell-side water fitting at a rate of 5 gpm at 25C. The pressure
drop was less than 5 psig. The concentration of ozone in the outlet
water was measured using an IN-USA ozone sensor. After a couple of
minutes, the ozone concentration in the water reached 26 ppm. Under
the same operating conditions, the same contactor would produce
less than 12 ppm using tube-side mode mass transfer. Therefore, the
ozonation efficiency improvement was more than 100%. The ozonation
efficiency of this module was 0.45 ppm/lpm/liter of internal
cartridge volume. A comparison of the effect on ozone concentration
over a shell side contactor of this invention and a tube side
contactor is shown in FIG. 6.
EXAMPLE 3
[0086] A contactor with dimensions of 2.25" ID and 12" in length
was made using porous skinless PFA hollow fibers. The fiber outer
diameter (OD) was about 700 microns and the inner diameter (ID)
about 400 microns. The number of the fibers was about 4000 and the
packing density was around 0.86 m.sup.2 of external membrane
area/liter of internal cartridge volume. The fiber bundle was
potted with an offset of 1/4" gas fitting at both end of the
module. Two 1" fittings for water flow were bonded on the shell at
cross-direction near the exit and entrance. The inside of the
fitting was reduced to 1/2" using an insert to create jetting
action of water into the bundle. The water fittings are located
perpendicular to the offset of the potting such that a cavity is
formed right under the fitting.
[0087] The contactor was tested for ozonation efficiency. Ozone gas
at 22 psig., 250 gNm3 and 5 slpm was fed into the gas port of the
contactor. Deionized (DI) water was pumped into the contactor using
the shell-side water fitting at a rate of 5 gpm at 25C. The water
pressure drop across the module was about 8 psig. The concentration
of ozone in the outlet water was measured using an IN-USA ozone
sensor. After a couple of minutes, the ozone concentration in the
water reached at least 35 ppm. Under the same operating conditions,
the same contactor would produce less than 16 ppm using tube-side
mode mass transfer. Therefore, the ozonation efficiency improvement
was about 100%. The ozonation efficiency for this module was 0.61
ppm/lpm/liter of cartridge volume.
[0088] A comparison of the effect on ozone concentration over a
shell side contactor of this invention and a tube side contactor is
shown in FIG. 9. The shell side module output increases from 25 ppm
ozone at 3 slpm gas flow to 35 ppm at 7 slpm (at 20 lpm DI flow
rate, 250 g/Nm.sup.3 gas concentration and 22 psi gas pressure).
Such a high level of performance results from a high conversion
efficiency (the amount of ozone transferred from gas side to water
side) of over 60% at 3 slpm (FIG. 10). The higher recovery is
achieved as shell-side turbulence lowers the boundary layer
impedance and results in higher mass transfer per unit membrane
area. The high recovery helps lower the cost of ownership of the
process tool. As seen in FIG. 11, increasing the ozone gas side
concentration increases the water ozone output.
EXAMPLE 4
[0089] The effect of water side conditions on performance were
investigated. The effect investigated in this example was the
effect of flow rate and temperature.
[0090] At a given temperature, the dissolved ozone level output in
DI water depends on the water flow rate. At the gas-water
interface, at the fiber wall, the ozone concentration is the
equilibrium value given by the Henry's law, Equation 1, which
states that the ozone concentration in liquid, X (mol ozone/mol
solvent), is proportional to ozone pressure in the gas phase, P
(atm).
P=HX (1)
[0091] The proportionality constant (H) is called Henry's
coefficient, which varies with temperature (T) and pH, Equation 2
[John A. Roth, "Solubility of Ozone in Water", Ind. Eng. Chem.
Fundam. 1981, 20, 137-140].
H=3.8.times.10.sup.7[OH].sup.0.035exp(2428/T) (2)
[0092] The values of Henry's constant are available in literature
[Handbook of Chemical Engineering, Year, page #2-125; and B.
Parekh, "Ozone in Wet Cleans (Part I: Technology), Applications
Note MAL 126, Mykrolis corporation, Bedford, Mass. USA]. From
Equations 1 and 2 one can calculate the equilibrium solubility of
ozone as a function of temperature for a given ozone generator
condition. As an example, FIG. 12 shows the plot of equilibrium
ozone solubility (ppm) in water as a function of temperature at
ozone gas side pressure of 0.11 atm. This is the maximum
concentration achievable at a given temperature. Decreasing the
solution pH increases ozone solubility; however below pH 2, HCl
lowers ozone solubility as Cl.sup.- ion reacts with ozone.
EXAMPLE 5
[0093] The effect of a water side condition on performance
investigated in this example was the effect of flow rate on device
output.
[0094] The device output will approach the equilibrium value
(predicted by FIG. 12) at very low water flow rates and decrease
with an increase in water flow rate. This is because the contact
time for ozone transfer into DI water is greater at low DI flow
rates. Plots in FIG. 13 show ozone output as a function of water
flow rate for shell side and tube side contactors.
EXAMPLE 6
[0095] The effect of a water side condition on performance
investigated in this example was DI water flow rate versus pressure
drop. The pressure drop versus water flow rate data are plotted in
FIG. 14.
EXAMPLE 7
[0096] The shell side ozonator of the present invention should
improve the cost of ownership for an ozone based wet cleans tool
because of (1) its high productivity (ozone output per device
volume), (2) smaller footprint, rapid start-up (faster mass
transfer rate), and (3) ease of installation (only four tube
connections). Because of its simple compact design and efficient
performance, the module is appropriate for both a once-through
(single pass) mode and in recirculation mode operations. It is
easily adaptable in the cleaning processes using immersion baths,
single wafer spin processors and in batch spray processors. The
hollow fiber ozonator may produce ozone-DI water for various
applications including room temperature wafer cleanings (5 ppm to
50 ppm ozone) and low temperature photoresist stripping (5.degree.
C., 100 ppm ozone).
[0097] In some applications the ozonated water is produced at a
central location and then recirculated for delivery to individual
tool at point-of-use; in some designs make up amount ozone is added
at the point-of-use. FIG. 15 shows schematics of an ozone-DI
recirculating loop.
[0098] The present invention allows for improvements in the start
up time to reach the "desired" ozone concentration in cleaning
tools and maintain the stability of the ozone concentration in a
recirculating control loop over time for a bubble free DI-Ozone
application. The smaller size/footprint, shorter time to reach the
desired ozone concentration, and the ease of controlling the device
of the present invention in a narrow range of the ozone
concentration allows the user better process control. An added
advantage is the ability of the module to withstand high water
pressure of about 2-2.5 bar, with an intermittent pressure spikes
of 3-4 bars.
* * * * *