U.S. patent application number 11/586150 was filed with the patent office on 2007-05-17 for polymeric hollow fiber heat exchange systems.
Invention is credited to Alexander P. Korikov, Praveen B. Kosaraju, Kamalesh K. Sirkar, Dimitrios Zarkadas.
Application Number | 20070107884 11/586150 |
Document ID | / |
Family ID | 38039550 |
Filed Date | 2007-05-17 |
United States Patent
Application |
20070107884 |
Kind Code |
A1 |
Sirkar; Kamalesh K. ; et
al. |
May 17, 2007 |
Polymeric hollow fiber heat exchange systems
Abstract
Heat exchange systems are provided that include one or more
polymeric hollow fibers. Exemplary hollow fibers are asymmetric and
include a microporous wall and a dense skin formed thereon, thereby
preventing liquid transmission and/or contamination through the
wall of the hollow fiber while simultaneously enhancing heat
transfer based on the presence of liquid molecules within the
porous substructure of the hollow fiber. The hollow fibers may be
employed in a variety of heat transfer-related
commercial/industrial applications, including solvent-aqueous
systems, organic-aqueous systems, organic-organic systems,
desalination applications, solar heating applications, applications
in the chemical industry, applications in the biomedical industry,
and applications in the biotechnology or pharmaceutical industry,
e.g., extracorporeal blood oxygenation systems. Heat transfer
systems wherein steam is advantageously condensed on a first side
of a polymeric, hollow fiber-based heat exchanger are also
provided. The condensed steam provides energy that may be used to
heat water and/or other liquids that flow on a second side of the
polymeric, hollow fibers.
Inventors: |
Sirkar; Kamalesh K.;
(Bridgewater, NJ) ; Korikov; Alexander P.;
(Uniondale, NY) ; Kosaraju; Praveen B.; (Harrison,
NJ) ; Zarkadas; Dimitrios; (Fanwood, NJ) |
Correspondence
Address: |
McCARTER & ENGLISH, LLP;Attn.: Basam E. Nabulsi
Financial Centre, Suite 304A
695 East Main Street
Stamford
CT
06901-2103
US
|
Family ID: |
38039550 |
Appl. No.: |
11/586150 |
Filed: |
October 25, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60730954 |
Oct 27, 2005 |
|
|
|
Current U.S.
Class: |
165/133 ;
165/158 |
Current CPC
Class: |
F28F 21/062 20130101;
F28D 7/1669 20130101 |
Class at
Publication: |
165/133 ;
165/158 |
International
Class: |
F28F 13/18 20060101
F28F013/18 |
Claims
1. A heat exchange system comprising: a heat exchange module
configured and dimensioned to receive a plurality of polymeric
hollow fibers; and a plurality of polymeric hollow fibers
positioned in said module, wherein said plurality of hollow fibers
are asymmetric, porous hollow fiber with a dense skin formed on a
surface thereof.
2. A heat exchange system according to claim 1, wherein said heat
exchange module is configured for cross-flow heat exchange.
3. A heat exchange system according to claim 2, wherein the heat
exchange module includes at least one baffle.
4. A heat exchange system according to claim 1, wherein said heat
exchange module is configured for parallel flow heat exchange.
5. A heat exchange system according to claim 1, wherein said heat
exchange module is adapted for use in at least one of the following
applications: a desalination application, a solar heating
application, a chemical application, a biotechnology application, a
biomedical application, a blood oxygenation application, or a
pharmaceutical application.
6. A heat exchange system according to claim 1, wherein a dense
skin is formed on a microporous surface of each of said plurality
of hollow fibers.
7. A heat exchange system according to claim 1, wherein said dense
skin is effective to substantially prevent liquid transmission
through the wall of the hollow fiber.
8. A heat exchange system according to claim 1, further comprising
liquid molecules within the porous substructure of the polymeric
hollow fibers.
9. A heat exchange system according to claim 8, wherein the liquid
molecules facilitate heat transfer through the hollow fiber, but
are substantially prevented from passing through the wall of the
hollow fiber.
10. A heat exchange system according to claim 1, wherein the heat
exchange module is configured to receive process fluids and wherein
the plurality of polymeric hollow fibers are substantially inert to
said processing fluids.
11. A heat exchange system according to claim 1, wherein at least
one of the plurality of polymeric hollow fibers is fabricated from
a polymeric material selected from the group consisting of
polypropylene, polyethersulfone (PES), polyamide,
polyphenylenesulfide, polyimide, polyetheretherketone (PEEK),
polysulfone (PS), and poly-4-methyl-1-pentene (PMP).
12. A heat exchange system according to claim 1, wherein each of
the plurality of polymeric hollow fibers has a wall that is between
about 20-200 .mu.m in thickness.
13. A heat exchange system according to claim 1, wherein the dense
skin is formed by an interfacial polycondensation reaction.
14. A heat exchange system according to claim 1, wherein the dense
skin is formed by the following process steps: fiber pores are
wetted with an aqueous monomer solution, and an organic solution
having a second monomer is passed through the lumen side of the
fibers to form an interfacially polymerized polyamide film on the
inner diameter of the fibers.
15. A heat exchange system according to claim 1, wherein the
internal surface of the asymmetric polymeric hollow fibers is
provided with a bilayer coating.
16. A heat exchange system according to claim 15, wherein the first
layer of the bilayer coating is formed from a reaction product of
first and second reaction products.
17. A heat exchange system according to claim 15, wherein the
second layer of the bilayer coating is applied over the first
layer.
18. A heat exchange system according to claim 15, wherein the
second layer of the bilayer coating is a cross-linked
polydimethlysiloxane (PDMS).
19. A heat exchange system according to claim 18, wherein the PDMS
coating is applied by introducing a silicone solution and a curing
agent to the asymmetric polymeric hollow fiber lumen.
20. A heat exchange system comprising: a heat transfer module that
includes a plurality of polymeric hollow fibers, each of said
plurality of polymeric hollow fibers including a wall that defines
a first heat transfer side and a second heat transfer side; a steam
source that is adapted to supply steam to said heat transfer
module; and a liquid source that is adapted to supply a liquid flow
to said heat transfer module; wherein said steam is condensed on
the first heat transfer side of the polymeric hollow fibers,
thereby providing heat transfer energy to the liquid source on the
second heat transfer side of the polymeric hollow fibers.
21. A heat exchange system according to claim 20, wherein an
overall heat transfer coefficient on the order of or greater than a
liquid-liquid heat exchanger is achieved.
22. A heat exchange system according to claim 20, wherein the
polymeric hollow fibers define a hydrophobic polymeric surface.
23. A heat exchange system according to claim 20, wherein the steam
is condensed in a drop-wise manner.
24. A heat exchange system comprising: a heat transfer module that
includes a plurality of polymeric hollow fibers, each of said
plurality of polymeric hollow fibers including a wall that defines
a first heat transfer side and a second heat transfer side; a brine
source that is adapted to supply brine to said heat transfer
module; and a liquid source that is adapted to supply a liquid flow
to said heat transfer module; wherein heat transfer to the brine is
effected through heat transfer within the heat transfer module in
connection with desalination processing of said brine.
25. A heat exchange system according to claim 24, wherein the
liquid source is selected from the group consisting of brine, water
and steam.
26. A heat exchange system according to claim 24, wherein the
plurality of hollow fibers are asymmetric, porous hollow
fibers.
27. A heat exchange system according to claim 26, wherein the
asymmetric, porous hollow fibers include a dense skin on a surface
thereof.
28. A heat exchange system according to claim 24, wherein the heat
exchange module is adapted for at least one of cross-flow heat
exchange and parallel flow heat exchange.
29. A heat exchange system according to claim 28, wherein the heat
exchange module includes at least one baffle.
30. A heat exchange system according to claim 24, further
comprising liquid molecules within a porous structure of the
polymeric hollow fibers.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] The present application claims the benefit of a co-pending
provisional patent application entitled "Polymeric Hollow Fiber
Heat Exchange Systems," which was filed on Oct. 27, 2005 and
assigned Ser. No. 60/730,954. The entire contents of the foregoing
provisional patent application are incorporated herein by
reference.
BACKGROUND
[0002] 1. Technical Field
[0003] The present disclosure is directed to advantageous heat
exchange systems that include one or more polymeric solid hollow
fibers and, more particularly, asymmetric porous hollow fiber-based
heat exchange systems that provide enhanced heat transfer in a
variety of applications, e.g., desalination applications, solar
heating applications, applications in the chemical industry, and/or
applications in the biotechnology, biomedical or pharmaceutical
industry. Exemplary embodiments of the disclosed heat exchange
systems are characterized by hollow fibers that include a
microporous wall and a dense skin formed thereon, thereby
preventing liquid transmission and/or contamination through the
wall of the hollow fiber while simultaneously enhancing heat
transfer based on the presence of liquid molecules within the
porous substructure of the hollow fiber. The disclosed hollow fiber
systems may be advantageously employed with a variety of heat
exchange applications, including aqueous/aqueous heat exchange
systems, aqueous/steam heat exchange systems, steam/organic solvent
heat exchange systems, and aqueous/organic solvent heat exchange
systems.
[0004] 2. Background Art
[0005] It is known that polymeric materials offer numerous
advantages over metals in the construction of heat exchange
systems, e.g., reduced cost, ease of fabrication and lower weight.
In addition, from an energy requirements standpoint, the
fabrication of polymeric materials involves energy input that is
reduced by a factor of two relative to common metals. Moreover, the
surface of plastics is generally smooth, which translates to: (1)
less fouling than metal tubes, and (2) potentially smaller friction
forces and pressure drops. As a general matter, plastics possess
excellent chemical resistance to acids, oxidizing agents, and many
solvents.
[0006] Despite the advantages associated with polymeric materials
for the fabrication of heat exchange systems, disadvantages remain.
In particular, polymeric materials exhibit low thermal conductivity
relative to metals, thereby significantly reducing their utility in
heat transfer applications. For example, polymeric materials
generally exhibit thermal conductivity in the range of 0.1 to 0.4
W/m.degree. K., which is 100-300 times lower than that of metals.
Thus, the commercial and/or industrial utility of polymeric heat
exchange systems has been limited.
[0007] Conventional use of large-scale metallic tubes in condensers
and heat exchangers employed in multi-stage flash, multiple effect
and vapor compression distillation-based desalination processes has
led to huge capital investments, excessive corrosion/erosion of the
tubes in the presence of hot brine, heavy metal contamination of
waste brine, relatively large footprints, and excessive weight.
Thus, the elimination/replacement of conventional metallic tube
condensers and heat exchangers with polymeric hollow fiber heat
exchange systems that provide effective heat transfer performance
would be highly desirable.
[0008] The patent literature includes teachings with respect to
polymer-based hollow fiber membranes. Exemplary teachings include:
[0009] Knickel, U.S. Pat. No. 4,036,748, which discloses asymmetric
semi-permeable membranes for desalination that can be produced from
solution in the form of hollow fibers. [0010] Leonard, U.S. Pat.
No. 3,724,672, which describes a "skin-core structure." [0011]
Semmens, U.S. Pat. No. 4,960,520, which discloses hollow fiber
membranes of microporous polypropylene with a very thin outside
coating of plasma polymerized disiloxane. The fibers are potted in
a module which resembles a shell and tube heat exchanger. Strippant
is pumped through the shell side of the module and over the outside
of the fibers. The volatile and semi-volatile contaminants in the
water diffuse across the membrane and dissolve into the oil. The
process results in clean water and a smaller volume of more highly
contaminated oil.
[0012] Beyond the foregoing U.S. patents, two Japanese patent
abstracts are noted that discuss the use of polymeric hollow fibers
for heat transfer. Matsuro Suzuki, Japanese Patent No 60200097,
discusses hollow fibers molded of heterogeneous film (polyphenylene
sulfide) to increase thermal efficiency of a heat exchanger. Eiichi
Hamade, Japanese Patent No 61083898, discusses heat transfer of
water, organic liquid or gaseous bodies by hollow fibers.
[0013] The prior art also includes teachings with respect to
systems that include polymeric hollow fibers for heat transfer in
aqueous systems. For example, U.S. Pat. No. 5,863,654 to Frey
discloses a biocompatible porous hollow fiber made of a polyolefin
material that includes a coating of biocompatible carbon material.
Frey discloses that the foregoing hollow fibers may be used in heat
exchange devices, e.g., within oxygenators. U.S. Pat. No. 5,876,667
to Gremel discloses a heat exchanger of polymeric hollow fibers
with a wetting agent coating.
[0014] Zaheed et al. discuss the performance characteristics and
uses of a cross-corrugated, polymer film, compact heat-exchanger
(PFCHE) made of polyetheretherketone (PEEK) in aqueous-aqueous
systems. Zaheed, L., Jachuck, R. J. L., Performance of a Square,
Cross-Corrugated, Polymer Film, Compact, Heat-Exchanger with
Potential Application in Fuel Cells, Journal of Power Sources, v.
140 (2005), pp 304-310. Konagaya discusses hollow fiber
configurations for use in reverse osmosis applications consisting
of a top skin, dense layer, and a microporous layer. Konagaya, S.,
New Chlorine-Resistant Polyamide Reverse Osmosis Membrane with
Hollow Fiber Configuration, Journal of Applied Polymer Science, v.
79 (2001) 517 et seq.
[0015] Despite efforts to date, a need remains for polymeric heat
exchange systems that offer enhanced heat transfer performance. In
addition, processing designs and systems utilizing polymeric heat
exchange elements for effecting heat exchange with steam, e.g., low
temperature steam, are needed. These and other needs are satisfied
by the heat exchange systems disclosed herein.
SUMMARY OF THE DISCLOSURE
[0016] The present disclosure provides advantageous heat exchange
systems that include one or more asymmetric polymeric solid hollow
fibers. Exemplary asymmetric polymeric solid hollow fibers
according to the present disclosure are characterized by hollow
fibers that include a microporous wall and a dense skin formed
thereon, thereby preventing liquid transmission and/or
contamination through the wall of the hollow fiber while
simultaneously enhancing heat transfer based on the presence of
liquid molecules within the porous substructure of the hollow
fiber. The disclosed asymmetric hollow fibers may be employed in a
variety of heat transfer-related commercial/industrial
applications, including desalination applications, solar heating
applications, applications in the chemical industry, and
applications in the biotechnology or pharmaceutical industry. For
example, the disclosed asymmetric polymeric solid hollow fibers may
find advantageous application in extracorporeal blood oxygenation
systems with the heat exchange fluid on the porous wall side.
[0017] In addition, the present disclosure provides heat transfer
systems wherein steam is advantageously condensed on a first side
of a polymeric, hollow fiber-based heat exchanger. The condensed
steam provides energy that may be used to heat water and/or other
liquids that flow on a second side of the polymeric, hollow fibers.
Indeed, by using a steam feed that is adapted to condense on one
side of the hollow fibers (rather than a gas) and a liquid on the
other side, overall heat transfer coefficients on the order of a
liquid-liquid heat exchanger can be achieved with the disclosed
heat transfer system (if not higher). According to exemplary
embodiments of the disclosed steam condensation heat transfer
systems, a hydrophobic polymeric surface (e.g., polypropylene) may
effect drop-wise condensation of the steam, thereby enhancing the
overall heat transfer performance of the disclosed systems.
Condensation of the steam is generally effected to form relatively
small droplet sizes, thereby minimizing the likelihood of
condensation-related flow restriction on the steam side of the
hollow fiber system.
[0018] Of note, the disclosed polymeric hollow fibers offer
numerous industrial advantages. For example, the polymeric hollow
fibers disclosed herein are substantially inert to a wide range of
processing fluids/systems, e.g., brine-to-brine, brine-to-water,
and steam-to-brine desalination applications. The polymeric hollow
fibers are not susceptible to corrosion and/or erosion (which can
limit the utility of metal heat exchange tubes), and may be
fabricated in highly compact designs. Highly compact designs are
feasible, at least in part, because the disclosed polymeric hollow
fibers have a very high surface area per unit equipment volume (as
compared to non-polymeric heat exchange systems or even other
plastic heat exchange systems). Thus, an equivalent volume may be
an order of magnitude smaller, thereby drastically reducing the
associated system weight, footprint and cost.
[0019] Indeed, polymeric hollow fibers fabricated from such
exemplary polymeric systems as polypropylene, polyethersulfone
(PES), polyetheretherketone (PEEK), polyimides, polyphenylene
sulfide (PPS), polyethylene (PE), polytetrafluoroethylene (PTFE),
polysulfone (PS), poly-4-methyl-1-pentene (PMP) and the like, are
generally unaffected by hot brine, cold brine, pH values over a
wide range, a host of chemical systems, and a host of solvents. The
disclosed polymeric hollow fibers may be advantageously employed in
a variety of organic-aqueous and organic-organic heat transfer
systems, provided the polymeric material provides suitable levels
of stability with the organic molecules to be processed.
[0020] Additional advantageous features and functions of the
disclosed heat exchange systems and associated heat exchange
processes will be apparent from the detailed description which
follows, particularly when read in conjunction with the appended
figures.
BRIEF DESCRIPTION OF THE FIGURES
[0021] To assist those of skill in the art in making and using the
disclosed heat exchange systems, reference is made to the
accompanying figures, wherein:
[0022] FIG. 1 is a plan view of an exemplary cross/parallel flow
module employed in experimental work according to the present
disclosure;
[0023] FIG. 2 is a plan view of a further exemplary cross/parallel
flow module according to the present disclosure;
[0024] FIG. 3 is a plan view of an additional cross/parallel flow
module according to the present disclosure;
[0025] FIG. 4 is a plan view of a plurality of modules according to
the present disclosure;
[0026] FIG. 5 provides a schematic cross-sectional diagram of a
cross/parallel flow module with flow direction at the shell side
utilized in the experimental work described herein;
[0027] FIG. 6 is a schematic diagram of an experimental system used
in measuring heat exchange performance according to the present
disclosure;
[0028] FIG. 7 is a schematic diagram of an experimental setup for
membrane gas permeation measurements according to the present
disclosure;
[0029] FIG. 8a is a temperature profile plot in solid hollow fiber
heat exchangers for a hot brine-cold water/brine system;
[0030] FIG. 8b is a temperature profile plot in solid hollow fiber
heat exchangers for a steam-cold water/brine system;
[0031] FIG. 9 is a plot of overall heat transfer coefficient (U) of
module HEPP1 relative to feed inlet temperature obtained through
experimental studies according to the present disclosure;
[0032] FIG. 10 is a plot of overall heat transfer coefficient (U)
of module HEPP1 relative to interstitial velocity of hot brine
flowing on the shell side in crossflow, obtained through
experimental studies according to the present disclosure;
[0033] FIG. 11 is a plot of overall heat transfer coefficient (U)
of module HEPP1 relative to linear velocity of cooling water
flowing on the tube side with brine on shell side in crossflow,
obtained through experimental studies according to the present
disclosure;
[0034] FIG. 12 is a plot of overall heat transfer coefficient (U)
of module HEPP1 relative to linear velocity of hot brine flowing on
the tube side with D.I. water flowing in shell side in crossflow,
obtained through experimental studies according to the present
disclosure;
[0035] FIG. 13 is a plot of overall heat transfer coefficient (U)
of module HEPP1 relative to linear velocity of hot brine flowing on
the shell side in parallel flow, obtained through experimental
studies according to the present disclosure;
[0036] FIG. 14 is a plot of overall heat transfer coefficient (U)
of module HEPEEK1 relative to interstitial velocity of hot brine
flowing on the shell side in crossflow, obtained through
experimental studies according to the present disclosure;
[0037] FIG. 15 is a plot of overall heat transfer coefficient (U)
of module HEPES1 relative to linear velocity of hot brine flowing
on the tube side and D.I. water flowing on the shell side in
parallel flow, obtained through experimental studies according to
the present disclosure;
[0038] FIG. 16 is a plot of overall heat transfer coefficient (U)
of module HEPP1 relative to linear velocity of steam flowing on the
tube side and tap water flowing on the shell side in cross flow,
obtained through experimental studies according to the present
disclosure;
[0039] FIG. 17 is a plot of overall heat transfer coefficient (U)
of module HEPP2 relative to linear velocity of steam condensing on
the tube side and tap water flowing on the shell side in cross
flow, obtained through experimental studies according to the
present disclosure;
[0040] FIG. 18 is a plot of overall heat transfer coefficient (U)
of a large module 041939 relative to linear velocity of hot brine
flowing on the shell side in parallel flow with tap water in tube
side flow, obtained through experimental studies according to the
present disclosure;
[0041] FIG. 19 is a table setting forth design details with respect
to modules fabricated at NJIT;
[0042] FIG. 20 is a table setting forth design details with respect
to solid hollow fiber modules obtained from Membrana (Charlotte,
N.C.);
[0043] FIG. 21 is a table setting forth experimental data for
experimental runs using module HEPP1 according to the present
disclosure;
[0044] FIG. 22 is a table setting forth solid hollow fiber heat
exchanger performance data for hollow fiber systems with hot
brine/water according to the present disclosure; and
[0045] FIG. 23 is a plot of heat transfer coefficient relative to
temperature for an exemplary solid hollow fiber exchanger (HEPP4)
according to the present disclosure.
DESCRIPTION OF EXEMPLARY EMBODIMENT(S)
[0046] As noted above, the present disclosure provides advantageous
heat exchange systems that include one or more polymeric solid
hollow fibers and, more particularly, asymmetric porous hollow
fiber heat exchange systems that provide enhanced heat transfer in
a variety of applications, e.g., desalination applications, solar
heating applications, applications in the chemical industry,
applications in the biomedical industry and/or applications in the
biotechnology industry. Exemplary embodiments of the disclosed heat
exchange systems are characterized by hollow fibers that include a
microporous wall and a dense skin formed thereon, thereby
preventing liquid transmission and/or contamination through the
wall of the hollow fiber while simultaneously enhancing heat
transfer based on the presence of liquid molecules within the
porous substructure of the hollow fiber. The disclosed heat
exchange systems advantageously provide improved processing
performance and reduced cost for industrially significant
processing schemes, e.g., thermally-driven desalination processes,
by employing thermally, chemically and mechanically stable
polymeric solid hollow fibers with an ultrathin wall.
[0047] Exemplary heat exchange systems according to the present
disclosure are described in greater detail below, with particular
reference to experimental studies associated therewith. As
described herein, compact and lightweight devices for heat exchange
are provided. The heat transfer characteristics of such devices for
steam-brine and water-brine heat exchange have been studied and
experimental reports reported below. As demonstrated by such
experimental results, advantageous modifications to asymmetric
hollow fibers have been undertaken so as to achieve
improved/enhanced heat exchange performance. The disclosed
polymeric solid, hollow fibers are chemically inert,
environmentally benign and have strong, but ultrathin, walls (e.g.,
about 40-100 .mu.m thickness), which yield advantageous and
industrially acceptable wall heat transfer coefficient values.
Technical Approach to Experimental Studies
[0048] Step 1: Procure solid hollow fibers fabricated from a
variety of polymers.
[0049] Step 2: Fabricate small heat exchanger modules of 20-50 cm
length containing 200-400 solid hollow fibers (see Step 1) and
procure solid hollow fiber modules of larger surface area.
[0050] Step 3: Develop setup(s) to study heat exchange between
steam and brine, hot brine and cold brine, and hot brine and cold
water.
[0051] Step 4: Determine the overall heat transfer coefficients
achieved in the smaller heat exchange modules (see Step 2) for
steam-cold brine as well as hot brine-cold water heat exchange.
[0052] Step 5: Measure the moisture permeance of selected polymeric
hollow fibers.
[0053] Step 6: Make a preliminary heat transfer coefficient
measurement in the hollow fiber module having a larger surface
area.
Experimental Details
[0054] 1. Initial Experimental Modules
[0055] Three cross flow modules of solid polypropylene (PP) hollow
fibers, two cross flow modules of solid polyetheretherketone (PEEK)
hollow fibers, and two parallel flow modules of asymmetric
polyethersulfone (UltraPES) hollow fibers were fabricated in a
laboratory at New Jersey Institute of Technology (Newark, N.J.);
three large parallel flow modules of solid PP hollow fibers were
obtained from Membrana GmbH (Charlotte, N.C.). Dense PP hollow
fibers having a solid wall thickness of 75 .mu.m and outer diameter
(OD) of 575 .mu.m used in the NJIT modules and Membrana modules
(described below) were obtained from Membrana; the dense PEEK
hollow fibers were obtained from Upchurch Scientific, Inc. (Oak
Harbor, Wash.) and the porous UltraPES hollow fibers were also
obtained from Membrana. The porous UltraPES hollow fibers were
coated with a nonporous polyamide film on the internal diameter of
the fibers by interfacial polymerization performed in the NJIT
laboratories. The coating system and procedure are described
below.
[0056] The interfacial polycondensation reaction of ##STR1## was
carried out using the following system: [0057] Aqueous solution: 1%
1,6-Hexanediamine [0058] Organic solution: 1% Sebacoyl chloride in
Xylene [0059] Alkaline agent to remove HCl formed: Na.sub.2CO.sub.3
[0060] Reaction time: 2 minutes at room temperature [0061] Number
of coatings: 3
[0062] The exemplary coating procedure performed according to the
present disclosure involved the following steps: Fiber pores were
wetted with the aqueous monomer (hexanediamine) solution and the
organic solution having monomer (sebacoyl chloride) was passed
through the lumen side of the fibers to form an interfacially
polymerized polyamide film on the inner diameter (ID) of the
fibers.
[0063] According to further exemplary embodiments of the present
disclosure, the internal surfaces of the disclosed fibers have been
provided with an advantageous bilayer coating. In an exemplary
bilayer coating process with respect to asymmetric PES
(polyethersulfone) hollow fibers (e.g., UltraPES fibers; Membrana),
a first coating layer is applied to the lumen side of the PES
fibers by reacting an aqueous solution of a first reactant, e.g.,
0.5% poly(ethyleneimine), and an organic solution of a second
reactant, e.g., 0.5% iso-phthaloyl dichloride in xylene. Exemplary
reaction conditions involve a reaction at room temperature for
about ten (10) minutes, followed by heat treatment at an elevated
temperature (e.g., about 100.degree. C.) for about twenty (20)
minutes. Thereafter, a second coating layer, e.g., a cross-linked
polydimethylsiloxane (PDMS), is applied to the lumen side of the
PES fibers. The PDMS coating may be applied to the disclosed PES
fibers that already possess a first coating layer by introducing a
silicone solution (e.g., 1% solution of silicone in hexane) and a
curing agent at an appropriate concentration (e.g., 0.1%) to the
fiber lumena and curing such second coating. Exemplary conditions
include a first curing step in an oven at an elevated temperature,
e.g., 110.degree. C. for about forty (40) minutes, and a second
cure at room temperature for an extended period, e.g., about two
(2) days. The disclosed bilayer coating and comparable coating
treatments may advantageously prevent and/or minimize any potential
leakage through the fiber wall during heat exchange operations. The
characteristics of the foregoing hollow fibers and the modules are
listed in FIGS. 19 and 20.
[0064] FIGS. 1-3 schematically depict exemplary modules fabricated
at NJIT and utilized in the experimental studies described herein.
Each of the test modules was fabricated to accommodate
cross/parallel flow. With further reference to FIGS. 1-3, exemplary
modules were fabricated to include: (i) 79 polypropylene (PP)
hollow fibers; (ii) 400 PP hollow fibers, (iii) 200 hollow fibers;
(iv) 79 polyetheretherketone (PEEK) hollow fibers; and (v) 6
asymmetric ultra-polyethersulfone (UltraPES) fibers. Turning to
FIG. 4, a schematic depiction of three modules obtained from
Membrana and used in the experimental work described herein is
provided. The respective modules included: (i) 950 PP solid hollow
fibers, and (ii) 2750 PP solid hollow fibers.
[0065] In order to maximize the brine-side boundary layer heat
transfer coefficient on the shell side and investigate the effect
of module flow configuration (parallel flow or cross flow) on the
overall heat transfer coefficient, three PP-based and two
PEEK-based cross/parallel flow modules were fabricated. A schematic
diagram of the cross/parallel flow module is shown in FIG. 5. The
liquid entrance on the shell side of hollow fibers included a
diverging section. The central feeder tube, with a wide size
distribution of open holes, was fabricated using a PP tube. The
hole distribution was implemented such that the holes near the
entrance were smaller and those further out were larger. This hole
distribution was adopted to ensure substantially uniform flow of
the liquid (hot brine) in cross flow outside of and perpendicular
to the hollow fibers, thereby providing effective heat transfer in
the module. When liquid was introduced through the side entrance of
module shell, parallel flow was obtained. The module shell
consisted of a transparent PP tube having an appropriate thickness
and heat transfer resistance.
[0066] 2. Experimental Apparatus and Procedures
[0067] The experimental apparatus employed in the experimental work
described herein was developed such that the heat exchange process
could be studied easily for small as well as large modules. A
schematic diagram of the experimental system is set forth in FIG.
6. The heat exchange module was assembled in heat transfer setups
of differing size/scale--a smaller heat transfer setup and a
corresponding experimental setup for larger modules.
[0068] In the smaller scale experimental setup, the system piping
and storage tanks were thoroughly insulated to minimize heat loss
to the environment. The liquids employed in the experiments were
deionized (D.I.) water or tap water as cooling liquid and 4% (wt)
solution of NaCl as feed solution. In the experimental procedure,
the feed solution was introduced to the shell side/tube side from a
hot brine reservoir by a digital Masterflex peristaltic pump at a
constant flow rate. Two temperature controllers maintained the hot
brine bath temperature at a given value and thus ensured the
required temperature for the hot feed. Outside the module, the feed
was circulated back to the feed reservoir and was rewarmed. Changes
in shell side flow configuration (i.e., parallel flow versus cross
flow) were implemented by switching to the corresponding liquid
entrance as described above.
[0069] Deionized water or tap water was introduced as the cooling
liquid on the fiber lumen side/shell side from a cooling bath by a
second digital Masterflex peristaltic pump at a constant flow rate.
The temperature of cooling liquid was maintained by a Cole-Parmer
Polystat refrigerated bath (Model No. 12111-20) at a given low
temperature before the water entered the module.
[0070] In the steam-water system, steam was generated by a steam
generator having a heating capacity of 3 kW (Sussman Electric
Boilers, Long Island City, N.Y.). The steam flow rate was
controlled with a ball valve and measured by cooling
down/condensing all steam flow into liquid. Steam was conducted
from steam generator into the hollow fiber module using a thick
wall polypropylene tubing. Before steam entered the module, the
condensed water was trapped by a glass well. Normally solid wall PP
cannot bear such high temperatures (around 120.degree. C.). In the
disclosed system, steam was passed through the tubing ID, while the
outside surface of the tubing was exposed to air. The average
temperature of the tubing was therefore maintained at a
considerably lower level. The PP tubing worked well over the course
of many experiments. The same conclusions proved valid for the
stability of the dense PP hollow fibers in the test modules, since
cold water was running on the other side (relative to steam within
the fibers).
[0071] In the larger scale experimental setup for heat exchange
operation, feed was introduced to the shell side of the larger
module from a reservoir by a centrifugal pump (model: TE-4-MD-HC,
Little Giant Co., Oklahoma City, Okla.) at a constant flow rate
controlled by a ball valve. The flow rate of the liquid system
could be varied between 5-40 liters per minute (LPM). Feed solution
in a 200-liter stainless steel tank was heated by a heating system
having two heaters (OMEGA, EMT-312E2/240 three-phase moisture
resistant heater (12 kW); and EMT-309E2/240 three-phase moisture
resistant heater (9 kW); total 21 kW heating capacity), two OMEGA
rugged transition joint probes, two OMEGA three phase DIN rail
mount solid state relays and two OMEGA CN77333 controllers. For
safety, a liquid level switch was installed in the feed tank.
Outside the module, the exiting feed was circulated back to the
feed reservoir and was re-warmed. The steam generator used in this
larger system had a heating capacity of 20 kW (Model # MBA2033
Sussman Electric Boilers, Long Island City, N.Y.).
[0072] The cooling system was mainly composed of a Remcor chiller
having a cooling capacity of 12 kW (model: CH3002A, voltage (full
load Amps): 230/60/3, IMI Cornelius Inc., Anoka, Minn.) with a
recirculation pump and a 10 gallon tank. Deionized water was
generally introduced as the cooling liquid on the fiber lumen side
of the module from the reservoir at a constant flow rate. The
exiting hot liquid stream from the module was cooled to a given
temperature by the chiller before entering the module again. Tap
water having a constant temperature (around 20.degree. C.) was also
used as an alternative source of cooling water on the tube
side.
[0073] Each liquid solution (including the feed solution and the
cooling water in the small scale system and the larger scale
system) was filtered by passing through a post-filter cartridge (1
.mu.m) (US Filter-Plymouth Products, Sheboygan, Wis.) before
entering the module. The inlet and outlet temperatures of the hot
feed solution/steam and the cold water were measured by four
thermocouples with 0.1.degree. C. accuracy. The pressure drops of
the feed and the cold water through the module were also monitored.
Conductivity meters (Model No. 115, Orion Research, Beverly, Mass.)
were used to monitor potential leaks of the hollow fibers and/or
the module. When the readings of the flow rates and temperatures
reached constant values, it was assumed that a steady state had
been reached. The steady inlet and outlet temperatures, pressures
and the flow rates of the brine solution/steam and cold water were
recorded for the calculation of heat exchange under the given
experimental conditions.
[0074] Overall heat transfer coefficient, U, was calculated from
the following relation: U = F .times. .rho. .times. .DELTA. .times.
.times. t .times. c p s .times. .DELTA. .times. .times. T .times.
.times. m ( 1 .times. a ) ##EQU1## where F: liquid stream flow
rate; .rho.: its density; .DELTA.t: liquid stream's inlet and
outlet temperature difference; c.sub..rho.: liquid stream heat
capacity; .DELTA.T.sub.m: logarithmic mean bulk temperature
difference between shell side and tube side (see equation 1b); s:
hollow fiber surface area for heat transfer based on the inside
area: s=n.pi.d.sub.1L; .DELTA. .times. .times. T .times. .times. m
= ( T s .times. .times. 1 - T f .times. .times. 2 ) - ( T s .times.
.times. 2 - T f .times. .times. 1 ) n .function. [ T s .times.
.times. 1 - T f .times. .times. 2 T s .times. .times. 2 - T f
.times. .times. 1 ] ( 1 .times. b ) ##EQU2## Here T.sub.s1 and
T.sub.s2 are the hot stream inlet and outlet temperatures and
T.sub.f1 and T.sub.f2 are the cold stream inlet and outlet
temperatures.
[0075] Leak testing: All modules listed in the tables of FIGS. 19
and 20 were tested for leakage before heat exchange measurements.
Both exits of the tube side of module were connected to a N.sub.2
cylinder; shell side openings were connected with an end of a thin
tubing. The module was immersed into water. Gas pressure was slowly
increased to 15 psi. Then the system was stabilized for 1 hr. If
bubbles came from the shell side installed tubing, it indicated
that the module was leaking; if there were no air bubbles, there
was no leakage. The leakage was also measured by passing 4% NaCl
solution through shell side at 8.degree. C. or higher and 5 psi of
inlet pressure; deionized water flowed through the tube side at
room temperature. The conductivity of the deionized cold water was
monitored with increasing brine pressure. If the conductivity of
the distillate water rose with operating time, the test module was
leaking. Otherwise, the module was leak free. The leak testing
indicated that all modules listed in the tables of FIGS. 19 and 20
appeared to be in a good operational state under the test
conditions.
[0076] Gas permeation: A system was also established for the
measurement of gas permeance of the coated hollow fiber membranes
using a gas permeation apparatus (FIG. 7). The N.sub.2 gas from the
cylinder permeated through the membrane from the tube side to the
shell side. The upstream and downstream pressures were measured by
Ashcroft Test Gauge (PT. No. 63-5631). The downstream flow rate of
the gas was measured using a soap bubble flow meter. During the
permeation measurements, the upstream pressure was maintained at a
constant pressure, between 0.1-0.6 psig (0.5-3.1 cm Hg gauge). The
permeation measurements were made at room temperature. As noted
above, the permeant gas was N.sub.2.
[0077] The N.sub.2 permeance of the hollow fiber membranes is
related to the measured steady-state permeation rate of nitrogen
through the membrane by Eq. (2): Q N 2 .delta. M .times. (
permeance ) = P 1 .times. V 1 .times. T 0 P 0 .times. T 1 s .DELTA.
.times. .times. P N 2 ( 2 ) ##EQU3## In Eq. (2), T.sub.0=273.15 K,
P.sub.0=760 Torr, .DELTA.P.sub.N.sub.2 corrected to STP is pressure
difference across the membrane, s is the inside membrane area,
P.sub.1 is the atmospheric pressure, T.sub.1 is the room
temperature, V.sub.1 is the volume flow rate of gas through the
membrane during measurement at room temperature, Q.sub.N.sub.2 is
the permeability coefficient of N.sub.2 permeation through the
membrane of effective thickness .delta..sub.M.
[0078] Calculation of Reynolds numbers: Reynolds number is normally
defined in the following way: Re = D .times. V .times. .rho. .mu. (
3 ) ##EQU4## Where: Re: Reynolds number; D: characteristic
dimension; V: velocity; .rho.: density; .mu.: dynamic viscosity
(absolute viscosity).
[0079] The Reynolds numbers of any liquid flowing through the shell
or the tube side were defined as diameter-based Reynolds number
(Re.sub.d) In the calculation of Re.sub.d based on Eq. (3), fiber
I.D. (d.sub.i) and linear velocity are used for tube side parallel
flow, and fiber O.D. and interstitial velocity/linear velocity for
shell side cross flow/parallel flow. Interstitial velocity=brine or
liquid flow rate/open area for flow through the shell side (4)
[0080] The open area for flow through the shell side has been
defined at the bottom of the table of FIG. 29. Linear velocity=flow
rate/open area for flow through the (tube side/shell side) (5)
[0081] In the literature, boundary layer heat transfer coefficients
are almost always estimated from empirical correlations. For
laminar flow in a circular tube (i.e., fiber lumen), the
Sieder-Tate equation is popularly employed (Gryta et al., 1997;
Hobler, 1986):
Nu.sub.c=1.86(d.sub.iRe.sub.dPr/L).sup.0.33(.mu./.mu..sub.w).sup.0.14
(6) where Nusselt number, Nu.sub.c=h.sub.cd.sub.i/k,
Re.sub.d=(linear velocity)d.sub.i.rho./.mu. and the Prandt1 number,
Pr=c.sub..rho..mu./k. Further h.sub.c is the tube side boundary
layer heat transfer coefficient, d.sub.i is the tube/fiber I.D., k
is the liquid thermal conductivity, .mu..sub.w is the liquid
viscosity evaluated at the tube-wall temperature, and L is the tube
length. The viscosity correction factor (.mu./.mu..sub.w).sup.0.14
normally is negligible (Lawson and Lloyd, 1996b). Equation (6) is
suitable for laminar tubular flow conditions
(Re.sub.d<2100).
[0082] For the calculation of the boundary layer heat transfer
coefficient of liquid flowing on the shell side of cross flow
hollow fiber modules, Zukauskas equation is often used for cross
flow over tube bundles in heat exchangers when
10<Re.sub.d<5.times.10.sup.2 (Incropera and Dewitt, 2002;
Kreith and Bohn, 2001):
Nu.sub.h=1.04Re.sub.d.sup.0.4Pr.sup.0.36(Pr/Pr.sub.w).sup.0.25F.sub.c
(7) where Nusselt number, Nu.sub.h=h.sub.hd.sub.o/k . Further
h.sub.h is the shell side boundary layer heat transfer coefficient,
d.sub.o represents the tube/fiber O.D., Pr.sub.w is the Prandt1
number evaluated at the tube-wall temperature, F.sub.c is the
tube-row correction factor. All properties except Pr.sub.w are
evaluated at arithmetic mean of the fluid inlet and outlet
temperatures. These equations are provided herein at least in part
to provide a basis for using Re.sub.d in reporting experimental
data even though there are potential problems due to fibers
potentially moving and/or flow irregularities from the entrance
section. Further the velocity used in Re.sub.d is the interstitial
velocity which takes into account the fiber packing density.
[0083] Definitions of heat transfer coefficients: At steady state,
the effective heat flux at the two liquid fiber wall interfaces
(see FIGS. 8a and 8b) may be described by:
Q=h.sub.hA.sub.rh(T.sub.h-T.sub.hm)=h.sub.hA.sub.rh.DELTA.T.sub.h=h.sub.c-
A.sub.rc(T.sub.cm-T.sub.c)=h.sub.cA.sub.rc.DELTA.T.sub.c, (8) where
Q is the effective heat flux through the wall, .DELTA.T.sub.h is
the temperature difference between brine bulk temperature, T.sub.h
and the temperature of the brine-wall interface on the hot side,
T.sub.hm, .DELTA.T.sub.c is the temperature difference between the
temperature of the wall-cold water/brine interface, T.sub.cm, and
the cold water/brine bulk temperature on the cold side, T.sub.c. In
the PP hollow fiber module, the fiber wall thickness is six (6)
times smaller than the inside diameter of the fiber. This
significant diameter differential results in considerable
difference between the outside and inside area of the hollow
fibers. Accordingly, a change of the surface area for heat transfer
should be taken into account. A.sub.r represents the area ratio for
the heat transferred through the fiber wall. Since the internal
diameter-based surface area is being utilized as the base point,
A.sub.rh for the interfacial area between the hot brine and the OD
is (d.sub.o/d.sub.i); the corresponding A.sub.rc for the cold water
and the ID is (d.sub.i/d.sub.i)=1.
[0084] Heat is conducted through the nonporous solid polymeric wall
at a rate: Q=h.sub.mA.sub.r ln(T.sub.hm-T.sub.cm)=h.sub.mA.sub.r ln
.DELTA.T.sub.m (9) where h.sub.m is the tube wall heat transfer
coefficient, .DELTA.T.sub.m is the trans-wall temperature
difference (T.sub.fm-T.sub.cm). The surface area ratio (A.sub.r ln)
is defined as (d.sub.r ln/d.sub.i) where d.sub.r ln is the
logarithmic mean diameter,
((d.sub.o-d.sub.i)/ln(d.sub.o/d.sub.i)).
[0085] The heat transfer mechanism in a heat exchanger can be
described as heat being transferred through a series of
resistances; the overall heat transfer coefficient of the heat
exchange process, U, is conventionally obtained as a series of
resistances defined here with respect to A.sub.rc: hot brine film
resistance (1/h.sub.h), effective tube wall/membrane resistance
(1/h.sub.m) and cold brine/distillate film resistance (1/h.sub.c):
UA rc = [ 1 A rh .times. h h + 1 A r .times. .times. ln .times. h m
+ 1 A rc .times. h c ] - 1 ( 10 ) ##EQU5## Therefore, the local
heat flux Q can be expressed as Q = [ 1 A rh .times. h h + 1 A r
.times. .times. ln .times. h m + 1 A rc .times. h c ] - 1 .times.
.DELTA. .times. .times. T = UA rc .times. .DELTA. .times. .times. T
( 11 ) ##EQU6## where .DELTA.T is the local bulk temperature
difference, T.sub.h-T.sub.i; the value of A.sub.r for U depends on
the basis of calculation, it can be A.sub.rh or A.sub.rc, or
A.sub.r ln. Here A.sub.rc is taken as the basis.
[0086] Normally heat transfer efficiency across a given wall with a
given h.sub.m is decided by boundary heat transfer coefficients
h.sub.h and h.sub.c. Heat transfer efficiency can be described via
the temperature polarization coefficient (TPC): TPC = T hm - T cm T
h - T c = .DELTA. .times. .times. T m .DELTA. .times. .times. T (
12 ) ##EQU7## TPC is the fraction of external applied thermal
driving force that contributes to the heat transfer.
[0087] If an overall boundary layer heat transfer coefficient h is
defined via 1 hA r .times. .times. ln = 1 A rh .times. h h + 1 A rc
.times. h c ( 13 ) ##EQU8## then TPC can be defined by TPC =
.DELTA. .times. .times. T m .DELTA. .times. .times. T = 1 - UA rc
hA r .times. .times. ln ( 14 ) ##EQU9##
[0088] Temperature polarization has a negative influence on the
heat exchange process as a consequence of the decrease in the
temperature of the hot brine on the fiber surface and its increase
on the fiber surface of cold side. Ideally, TPC should be equal to
one (1), but usually it is lower. In order to maximize the shell
side heat transfer coefficient, crossflow modules were fabricated
at NJIT for testing herein.
Experimental Results and Discussion
[0089] Initially, the results of hot brine-cold water heat transfer
in solid wall PP hollow fiber-based modules according to the
present disclosure is described. The heat transfer performance of
asymmetric UltraPES hollow fibers having an impervious
interfacially polymerized coating is considered next. The use of
solid PEEK-fiber based module in heat transfer studies is also
described herein, as are the performances of solid wall PP hollow
fiber-based devices for steam-cold tap water system. In addition,
the performances of the larger module initially obtained from
Membrana will be discussed. Of note, none of the modules of
different fibers (but primarily of PP) described in the tables of
FIGS. 19 and 20 had any measurable N.sub.2 or H.sub.2O vapor
permeance. Thus, these hollow fiber modules were in appropriate
condition for use in the heat transfer studies described
herein.
[0090] FIG. 9 illustrates the performance of the PP module HEPP1
with regard to the variation of the overall heat transfer
coefficient, U, as the hot brine (4% NaCl) inlet temperature was
varied at a given interstitial velocity of hot brine on the shell
side and linear velocity of the cold D.I. water on the tube side.
The increase of the overall heat transfer coefficient, U, with hot
brine inlet temperature is due to the decrease in viscosity of the
fluids, and the corresponding increase in the Reynolds number. FIG.
10 illustrates for the same system the variation of the overall
heat transfer coefficient, U, with the magnitude of the
interstitial velocity of the hot brine on the shell side.
[0091] From the test results reported herein, it is clear that the
magnitude of the interstitial velocity strongly influences the
value of the overall heat transfer coefficient, U, until a plateau
is reached around 1600-1800 W/m.sup.2-.degree. K. This plateau is
ultimately due to the combined resistance of the wall resistance
and the tube-side resistance. More likely, it is due to the
resistance of the solid PP fiber wall whose wall heat transfer
coefficient value is in this range, which conclusion is supported
by the results shown in FIG. 11. For the same system of FIGS. 9 and
10, FIG. 11 shows how the overall heat transfer coefficient, U,
varies with the linear velocity of the cooling water on the tube
side for the highest shell-side brine interstitial velocity of FIG.
10. These test results demonstrate that, as the tube-side velocity
is increased, the overall heat transfer coefficient, U, reaches a
plateau of around 1800 W/m.sup.2-.degree. K. Therefore, the overall
heat transfer coefficient, U, appears to be limited by the wall
heat transfer coefficient at high values of the shell-side and
tube-side velocities.
[0092] In the tests reported thus far, the hot brine was flowing
always on the shell side of the HEPP1 module in cross flow. If the
hot brine flows on the tube side, the values of the overall heat
transfer coefficient, U, are smaller since cross flow is generally
more efficient in heat transfer at low Reynolds numbers, especially
with the lower viscosity fluid (hotter fluid). This relationship is
illustrated in FIG. 12, where the cold water is in cross flow on
the shell side. However, if hot brine flows on the shell side in
parallel flow at a high Reynolds number, a high value of overall
heat transfer coefficient, U, can be obtained, as shown in FIG. 13.
However, the limiting value of the overall heat transfer
coefficient, U, is still around 1800 W/m.sup.2-.degree. K.
[0093] The limiting value for the overall heat transfer
coefficient, U, observed for solid PP hollow fibers at high values
of the convective film coefficients is most likely due to the
limiting wall resistance. As previously noted, the wall heat
transfer coefficient of PP solid hollow fiber of wall thickness is
around h wall = k wall .delta. m = 0.17 75 .times. 10 - 6 .times.
.times. W m 2 - .degree. .times. .times. K = 2125 .times. .times. W
m 2 - .degree. .times. .times. K , ##EQU10## where the thermal
conductivity of PP was assumed to be around 0.17 W/m.sup.2-.degree.
K. A small increase in this value could change h.sub.wall and U
considerably. For example, a 20% increase in k.sub.wall will
increase h.sub.w to 2500 W/m.sup.2-.degree. K. As is apparent, this
value is the maximum overall heat transfer coefficient, U, that can
be achieved in a solid PP hollow fiber-based exchanger where
.delta..sub.m is 75 .mu.m.
[0094] FIG. 14 illustrates the performance of a solid hollow fiber
module made out of solid hollow fibers of the polymer PEEK. The
overall heat transfer coefficient values, U, obtained are somewhat
smaller relative to the results reported for the HEEP1 module for
two reasons. First, the ID of the fibers in the HEPEEK1 module are
smaller, so a higher pressure drop is needed for a comparable
velocity/Reynolds number. In addition, the wall thicknesses of the
fibers were significantly higher at 105 .mu.m, even though their
thermal conductivity were higher than that of PP. Thus, a lower
tube side Reynolds number and a higher wall thickness leads to a
marginally lower overall heat transfer coefficient, U. However,
PEEK fibers have much higher temperature capabilities than PP
fibers.
[0095] According to the present disclosure, advantageous heat
transfer performance may be achieved through the use of asymmetric
microporous hollow fibers that are characterized by micropores in
the skin that are closed off. According to the present disclosure,
less resistance would be experienced due to the wall being
substantially porous.
[0096] FIG. 15 illustrates the heat transfer performances of a
module built with eight (8) porous asymmetric UltraPES hollow
fibers whose ID was coated with a NYLON 6-10 film by interfacial
polymerization. The test results set forth in FIG. 15 were
generated with a parallel flow module. At high values of the
shell-side velocity of cold D.I. water and hot brine on the tube
side, the overall heat transfer coefficient value, U, was around
3100 Watts/m.sup.2-.degree. K . This heat transfer coefficient is
significantly better than results previously reported, i.e., almost
1.7 times greater than the value achieved with solid wall PP
fibers. Of note, there was a small amount of salt leakage observed
in this test module. The bilayer coating system described herein
above is an exemplary coating system that may be employed to
further reduce the potential for leakage across the fiber wall. The
larger squares in FIG. 15 correspond to initial experimental runs
where there was no salt leakage.
[0097] Turning next to experimental results generated in
experimental procedures with steam on one side and cold tap water
flowing on the other side, FIG. 16 reflects the variation of
overall heat transfer coefficient, U, relative to the linear
velocity of steam flowing on the tube side with tap water flowing
on the shell side in cross flow. The experimental results reflected
in FIG. 16 were generated with steam condensing on the tube side of
module HEPP1, which included solid PP hollow fibers. At higher
velocities of the cold tap water flow in the shell side, a plateau
value of heat transfer coefficient, U, was reached at around
1600-1700 W/m.sup.2-.degree. K. In FIG. 17, test results are
provided for experimental runs with the HEPP2 module where steam
was condensing on the tube side and cold tap water was in cross
flow on the shell side. As the linear velocity of the steam was
increased, the value of the overall heat transfer coefficient, U,
climbed to as much as about 1500 W/m.sup.2-.degree. K. These
overall heat transfer coefficient values were determined via a
special definition since there are two regions on the steam side:
steam and condensate. These steam transfer coefficient results--as
reported herein--are a composite of the heat transfer coefficients
in the two regions.
[0098] Of note, condensation of steam in the disclosed solid hollow
fiber heat exchange systems generates relatively small condensation
water droplets. Small water droplets are advantageous in that they
do not cause flow blockage through the hollow fiber. Moreover,
condensation of the relatively small water droplets is effective in
supplying heat/energy for transfer through the fiber wall, e.g., to
the tap water flowing on the opposite side of the hollow fibers.
According to exemplary embodiments of the present disclosure,
prefiltration may be undertaken to ensure that impurities do not
cause undesirable blockage and/or increases in pressure drop across
the disclosed hollow fiber system.
[0099] Reference is made to a publication entitled "Polymeric
Hollow Fiber Heat Exchangers: An Alternative for Lower Temperature
Applications," authored by Dimitrios M. Zarkadas and Kamalesh K.
Sirkar, which appeared in Ind. Eng. Chem. Res. 2004, 43, 8093-8106,
incorporated herein by reference. The foregoing publication
describes advantageous heat exchange applications wherein exemplary
solvent/aqueous systems are employed, e.g., ethanol/water and
water/aqueous solution of ethylene glycol (33% by volume).
Alternative organic solvents may be employed in the disclosed heat
exchange systems, as will be readily apparent to persons skilled in
the art. Selection of appropriate organic solvent/polymeric solid
hollow fiber systems may be influenced, at least in part, by the
stability of the polymeric fibers in the presence of the proposed
organic solvent. Accordingly, heat exchange applications that
include organic-aqueous or organic-organic heat transfer systems
may be employed according to the present disclosure.
[0100] As noted above, relatively large (1 m.sup.2+) heat transfer
modules can be obtained from Membrana based, at least in part, on
preliminary experiments indicating that these heat exchanger
modules were extremely efficient heat transfer devices. However, it
was determined that the experimental facility would not be able to
supply adequate steam or hot water for efficient heat transfer.
Therefore, somewhat smaller modules with surface areas varying
between 0.15 and 0.44 m.sup.2 were obtained. FIG. 18 describes the
behavior of the overall heat transfer coefficient, U, relative to
the linear velocity of hot brine flowing on the shell side in
parallel flow. Based on the performance results reported herein, it
has been concluded that the linear velocities used here were not
large enough. Overall heat transfer coefficient values of around
750 W/m.sup.2-.degree. K. were observed. The heat transfer surface
area packing density of these modules was high, i.e., around 3000
m.sup.2/m.sup.3, as compared to the values of the modules
fabricated at NJIT (e.g., 300 to 1500 m.sup.2/m.sup.3). Therefore,
the product of the heat transfer coefficient, U, times the surface
area/volume in this module generates a value of 2.3.times.10.sup.6
W/m.sup.3-.degree. K., a value which is almost an order of
magnitude larger than those in a conventional metallic heat
exchangers (1.3-1.5.times.10.sup.5 W/m.sup.3-.degree. K.).
[0101] The table of FIG. 21 sets forth the raw data for one set of
experimental runs described herein. Specifically, the data
illustrated in FIG. 11 for module HEPP1 was obtained from the raw
data that is reported in the table of FIG. 21. By comparing the raw
data in the table of FIG. 21 with the plot of FIG. 11, it will be
readily apparent how the plotted heat transfer coefficient values,
U, were obtained according to the present disclosure.
[0102] The table of FIG. 22 sets forth summary performance data for
solid hollow fiber heat exchanges according to the present
disclosure. As is readily apparent from the data set forth in FIG.
22, the disclosed solid hollow fiber systems are highly effective
in providing advantageous heat transfer levels.
[0103] A polymeric hollow fiber heat exchanger module (HEPP4) built
by Membrana, Inc. on a much larger scale than those shown in FIGS.
4 and 20 has also been tested. The structure of this device is
somewhat similar to that of FIG. 5 except that the crossflow on the
shell side was generated by radial flow outward from the central
feed tube which was blocked at the middle. A baffle was positioned
on the shell side so that the shell-side crossflow in the first
half of the module flows around the baffle and comes down across
the fibers in the other half of the module; the flow stream is
collected and withdrawn through the other half of the central
feeder tube. The total number of polypropylene (PP) solid hollow
fibers was 12,100, each solid hollow fiber having an inner diameter
of 430 .mu.m and an outer diameter of 570 .mu.m. The effective
length of each hollow fiber was 24 cm, with the total length of
each fiber being 33 cm. The central feed distribution tube had an
outer diameter of 3.2 cm. The total available heat transfer surface
area was 4 m.sup.2 based on a fiber inner diameter for a shell side
housing inner diameter of 9.7 cm, which translates to heat transfer
surface area per unit volume of 22.5 cm.sup.-1.
[0104] With reference to FIG. 23, the results of the foregoing
exemplary heat exchange apparatus for one heat transfer run are
shown. In this heat transfer operation, cold city water was used on
the shell side at a flow rate of 11.3 liters/min and a temperature
of about 21.3-22.3.degree. C. The plot of FIG. 23 shows heat
transfer coefficients at varying brine feed temperatures on the
tube side at a brine flow rate of 20 liters/min. Because of the
enormous heat transfer surface area in the small exemplary heat
transfer device and the higher brine flow rate (about two times
that of the city water), at higher brine temperatures it is noted
that the heat transfer surface area is not properly utilized. As a
result, we get the highest heat transfer coefficient of around 630
W/m.sup.2-.degree. K. at the lowest brine temperature of
.about.42.degree. C. However, the value of the overall conductance
per unit volume is quite high and advantageous: 630 .times. .times.
W m 2 .times. - .degree. .times. .times. K . .times. 2250 .times.
.times. m 2 m 3 = 1.4175 .times. 10 6 .times. .times. W m 3 .times.
- .degree. .times. K . . ##EQU11##
[0105] The heat exchanger modules described herein are sufficiently
new that cost information is not readily available. However, the
cost of such modules is likely to be comparable to or less than
those of hollow fiber membrane contactors that are currently
commercially available from industry manufacturers, e.g., Membrana,
Inc. Further, the overall cost necessarily depends on volume of
sales, the number of modules being sold to one user, and related
commercial factors. Obviously, the larger the volume, the smaller
the cost, varying by as much as a factor of 2-3.
[0106] The results described herein are novel and exciting,
indicating the possibilities of polymeric hollow fiber modules made
out of asymmetric polymeric solid hollow fibers for use in heat
transfer. These lightweight, highly compact heat exchangers are
potential candidates for heat exchange in a wide range of
applications, e.g., thermal desalination processes and blood
oxygenation applications. Although the heat exchanger systems of
the present disclosure have been described with reference to
exemplary embodiments and experimental implementations thereof, the
present disclosure is not limited to such exemplary embodiments
and/or experimental implementations. Rather, the heat exchanger
systems of the present disclosure are susceptible to a variety of
modifications, variations and/or enhancements without departing
from the spirit or scope hereof. Accordingly, it is to be
understood that the present disclosure extends to and encompasses
such modifications, variations and/or enhancements.
Notation
[0107] A.sub.r membrane area ratio for heat transfer through a
membrane surface [0108] A.sub.rc value of A.sub.r for cold
water-tube wall interface, d.sub.i/d.sub.i, which is equal to 1
[0109] A.sub.rh value of A.sub.r for hot brine-tube wall interface,
d.sub.o/d.sub.i [0110] A.sub.r ln value of A.sub.r for logarithmic
mean membrane area, d.sub.r ln/d.sub.i, where logarithmic mean
diameter, d.sub.r ln=(d.sub.o-d.sub.i)/ln(d.sub.o/d.sub.1) [0111]
cm centimeter [0112] c.sub.p liquid heat capacity [0113] d.sub.i
fiber inside diameter (I.D.) [0114] d.sub.o fiber outside diameter
(O.D.) [0115] d.sub.r ln logarithmic mean diameter, d.sub.r
ln=(d.sub.o-d.sub.i)/ln(d.sub.o/d.sub.i) [0116] D characteristic
dimension [0117] D.I. deionized water [0118] F liquid stream flow
rate [0119] F.sub.c tube-row correction factor [0120] h overall
boundary layer heat transfer coefficient [0121] h.sub.c tube side
boundary layer heat transfer coefficient [0122] h.sub.h shell side
boundary layer heat transfer coefficient [0123] h.sub.m tube wall
heat transfer coefficient [0124] I.D. internal diameter [0125] k
liquid thermal conductivity [0126] L fiber length [0127] min
minute(s) [0128] n number of fibers in a membrane module [0129]
Nu.sub.c tube side Nusselt number [0130] Nu.sub.h shell side
Nusselt number [0131] O.D. outside diameter [0132] P.sub.1
atmospheric pressure [0133] PEEK polyetheretherketone [0134] PMP
poly(4-methyl-1-pentene) [0135] PP polypropylene [0136] Pr Prandt1
number [0137] Pr.sub.w Prandt1 number evaluated at the tube-wall
temperature [0138] Q effective heat flux through the membrane
[0139] Q.sub.n.sub.2 permeability coefficient of N.sub.2 permeation
through the membrane of effective thickness .delta..sub.M [0140]
Q.sub.N.sub.2/.delta..sub.M N.sub.2 permeance [0141] Re Reynolds
Number [0142] Re.sub.d diameter-based Reynolds number [0143] s
inside membrane area (=n.pi.d.sub.iL) [0144] STP T.sub.0=273.15 K,
P.sub.0=760 Torr [0145] T.sub.1 room temperature [0146] T.sub.c
cold water/brine bulk temperature on the cold side [0147] T.sub.cm
temperature of the tube wall-cold water/brine interface [0148]
T.sub.h brine bulk temperature [0149] T.sub.hm temperature of the
brine-tube wall interface on the hot side [0150] TPC temperature
polarization coefficient [0151] U overall heat transfer coefficient
[0152] UltraPES Ultra-polyethersulfone [0153] V velocity [0154]
.rho. density [0155] .mu. dynamic viscosity (absolute viscosity)
[0156] .mu..sub.w liquid viscosity evaluated at the tube-wall
temperature [0157] .delta..sub.M effective thickness of
membrane/tube wall [0158] .DELTA.P.sub.N.sub.2 N.sub.2 pressure
difference across the membrane [0159] .DELTA.t liquid inlet and
outlet temperature difference [0160] .DELTA.T bulk temperature
difference between shell side and tube side [0161] .DELTA.T.sub.h
temperature difference between brine bulk temperature and the
temperature of brine-tube wall interface on the hot side [0162]
.DELTA.T.sub.m trans-membrane temperature difference [0163]
.DELTA.T.sub.c temperature difference between temperature of tube
wall-cold water/brine interface and cold water/brine bulk
temperature on the cold side
* * * * *