U.S. patent application number 16/789608 was filed with the patent office on 2020-06-11 for plate-type membrane distillation module with hydrophobic membrane.
This patent application is currently assigned to KING FAHD UNIVERSITY OF PETROLEUM AND MINERALS. The applicant listed for this patent is KING FAHD UNIVERSITY OF PETROLEUM AND MINERALS. Invention is credited to Atia Esmaeil KHALIFA.
Application Number | 20200179876 16/789608 |
Document ID | / |
Family ID | 68056560 |
Filed Date | 2020-06-11 |
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United States Patent
Application |
20200179876 |
Kind Code |
A1 |
KHALIFA; Atia Esmaeil |
June 11, 2020 |
PLATE-TYPE MEMBRANE DISTILLATION MODULE WITH HYDROPHOBIC
MEMBRANE
Abstract
A membrane distillation module with a circulating line to
circulate a portion of distilled water, which is formed and
accumulated in a distillate zone, to enhance a permeate flux of
water vapor through a hydrophobic membrane of the membrane
distillation module. Various combinations of embodiments of the
membrane distillation module are provided.
Inventors: |
KHALIFA; Atia Esmaeil;
(Dhahran, SA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KING FAHD UNIVERSITY OF PETROLEUM AND MINERALS |
Dhahran |
|
SA |
|
|
Assignee: |
KING FAHD UNIVERSITY OF PETROLEUM
AND MINERALS
Dhahran
SA
|
Family ID: |
68056560 |
Appl. No.: |
16/789608 |
Filed: |
February 13, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15936547 |
Mar 27, 2018 |
10596521 |
|
|
16789608 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 61/366 20130101;
B01D 69/06 20130101; B01D 61/58 20130101; B01D 61/364 20130101;
B01D 61/368 20130101 |
International
Class: |
B01D 61/36 20060101
B01D061/36; B01D 61/58 20060101 B01D061/58; B01D 69/06 20060101
B01D069/06 |
Claims
1. A membrane distillation module, comprising: a vessel with an
internal cavity; a hydrophobic membrane that divides the internal
cavity into a feed zone and a distillate zone; a thermally
conductive divider disposed in the distillate zone to separate a
coolant zone from the distillate zone such that the distillate zone
is sandwiched between the feed zone and the coolant zone; wherein
the hydrophobic membrane is a flat membrane sandwiched in a frame
with the conductive divider and a perforated supporting plate; a
feed inlet fluidly connected to the feed zone for delivering a feed
stream to the feed zone, and a feed outlet fluidly connected to the
feed zone for discharging the feed stream from the feed zone; a
coolant inlet fluidly connected to the coolant zone for delivering
a coolant stream to the coolant zone, and a coolant outlet fluidly
connected to the coolant zone for discharging the coolant stream
from the coolant zone; at least one distillate inlet and at least
one distillate outlet fluidly connected to the distillate zone; a
circulating line disposed outside of the distillate zone that
fluidly connects the at least one distillate outlet to the at least
one distillate inlet; and a pump fluidly connected to the
circulating line, wherein the feed stream comprises water and water
vapor, and a portion of the water vapor permeates through the
hydrophobic membrane and condenses in the distillate zone in the
form of distilled water, wherein the pump circulates at least a
portion of the distilled water in the circulating line, and wherein
at least a portion of the circulating line is disposed in the
coolant zone with a coil shape such that the distilled water of the
circulating line heat exchanges with the coolant stream.
2. The membrane distillation module of claim 1, wherein the
hydrophobic membrane is micro-porous with a mean pore diameter of
0.1 to 1 .mu.m.
3. (canceled)
4. The membrane distillation module of claim 1, further comprising:
at least one nozzle disposed on the at least one distillate inlet
to inject the distilled water into the distillate zone.
5-6. (canceled)
7. The membrane distillation module of claim 1, wherein the
circulating line is made of a metal or a metal alloy having a
thermal conductivity of at least 30 W/(m*K) at a temperature of
20-30.degree. C.
8. The membrane distillation module of claim 1, wherein the
thermally conductive divider is made of a metal or a metal alloy
having a thermal conductivity of at least W/(m*K) at a temperature
of 20-30.degree. C.
9. The membrane distillation module of claim 1, wherein the vessel
has a longitudinal axis, which is vertically oriented, and wherein
the feed inlet and the at least one distillate inlet are arranged
proximal to a top of the vessel, and the feed outlet and the at
least one distillate outlet are arranged proximal to a bottom of
the vessel.
10. The membrane distillation module of claim 1, wherein the vessel
has a longitudinal axis, which is vertically oriented, and wherein
the feed inlet and the coolant outlet are arranged proximal to a
top of the vessel, and the feed outlet and the coolant inlet are
arranged proximal to a bottom of the vessel.
11. The membrane distillation module of claim 1, wherein the vessel
has a longitudinal axis, which is horizontally oriented, a first
end, and a second end, and wherein the feed inlet and the coolant
outlet are arranged on the first end of the vessel, and the feed
outlet and the coolant inlet are located on the second end of the
vessel.
12-20. (canceled)
Description
BACKGROUND OF THE INVENTION
Technical Field
[0001] The present invention relates to a water-gap membrane
distillation module with a circulating line for circulating a
portion of distilled water inside the gap, which is produced by the
membrane distillation module.
Description of the Related Art
[0002] The "background" description provided herein is for the
purpose of generally presenting the context of the disclosure. Work
of the presently named inventors, to the extent it is described in
this background section, as well as aspects of the description
which may not otherwise qualify as prior art at the time of filing,
are neither expressly or impliedly admitted as prior art against
the present invention.
[0003] Membrane distillation (MD) is a thermally-driven membrane
separation technology for separating water vapor from feed water.
In MD processes, saline water with elevated temperature is passed
over a micro-porous hydrophobic membrane. The temperature
difference across the membrane causes a vapor pressure difference,
which leads to a flow of water vapor from the feed side of the
membrane to the permeate side through the membrane pores. The
permeated vapor further condenses either on the cold side of the
membrane, or on a cooling surface arranged therein. The existing
surface tension provided by the hydrophobic membrane prevents
liquid water to permeate through the pores.
[0004] The application of thermally-driven membrane distillation
technology, particularly for water desalination purposes, is
attracting more attention as a potential technology for
small-to-medium scale water treatment plants. The membrane
distillation modules generally operate at low temperatures
(50-90.degree. C.) and low pressures, and provide a desalinized
with nearly 100% salt rejection. Four types of membrane
distillation modules are commonly used for water desalination
including i) Direct Contact Membrane Distillation (DCMD), ii) Air
Gap Membrane Distillation (AGMD), iii) Sweeping Gas Membrane
Distillation (SGMD), iv) Vacuum Membrane Distillation (VIVID). In
all the above mentioned MD modules, the feed is in direct contact
with the hydrophobic membrane, however, the design of the permeate
side is different.
[0005] Water gap membrane distillation (WGMD) (also known as
permeate gap or liquid gap) is another type of membrane
distillation modules that are developed and used for water
desalination purposes [A. E. Khalifa, "Water and air gap membrane
distillation for water desalination--An experimental comparative
study," Sep. Purif. Technol., vol. 141, pp. 276-284, 2015,
incorporated herein by reference in its entirety]. In WGMDs, the
gap between the membrane and the cooling surface is filled with
distilled water. FIG. 1 illustrates a water-gap membrane
distillation module. The mechanism of water evaporation and
permeation in the feed side of a WGMD is similar to other types of
MD modules; however, in the permeate side, the permeated water
vapor condenses immediately at the interface of the membrane
surface (i.e. the permeate side of the membrane) and the distilled
water in the gap.
[0006] Under the same operating and design conditions, the
production capacity of a WGMD module is higher than the production
capacity of an AGMD module, due to more efficient condensation as a
result of insignificant vapor transfer resistance inside the gap
[A. E. Khalifa, "Water and air gap membrane distillation for water
desalination--An experimental comparative study," Sep. Purif.
Technol., vol. 141, pp. 276-284, 2015, which is incorporated herein
by reference in its entirety; M. Essalhi and M. Khayet,
"Application of a porous composite hydrophobic/hydrophilic membrane
in desalination by air gap and liquid gap membrane distillation: A
comparative study," Sep. Purif. Technol., vol. 133, pp. 176-186,
2014; V. Ugrozov and L. Kataeva, "Mathematical modeling of membrane
distiller with liquid gap," Desalination, vol. 168, no. 1-3, pp.
347-353, 2004; L. Francis, N. Ghaffour, A. A. Alsaadi, and G. L.
Amy, "Material gap membrane distillation: A new design for water
vapor flux enhancement," J. Memb. Sci., vol. 448, pp. 240-247,
2013]. Due to the existence of water in the gap of WGMD modules, a
rate of heat transfer is enhanced. The specific energy consumption
(kWh needed to produce one cubic meter of distilled water) of WGMD
modules is usually lower compared to AGMD modules due to an
enhanced flux of distilled water.
[0007] In view of the forgoing, one objective of the present
disclosure is to provide a membrane distillation module with a
circulating line to circulate a portion of distilled water which is
formed and accumulated in a distillate zone to enhance a permeate
flux of water vapor through a hydrophobic membrane of the membrane
distillation module. Another objective of the present disclosure
relates to a membrane distillation module with a circulating line,
wherein a portion of the circulating line is arranged in a coolant
zone of the membrane distillation module.
BRIEF SUMMARY OF THE INVENTION
[0008] According to a first aspect, the present disclosure relates
to a membrane distillation module including i) a vessel with an
internal cavity, ii) a hydrophobic membrane that divides the
internal cavity into a feed zone and a distillate zone, iii) a
thermally conductive divider disposed in the distillate zone to
separate a coolant zone from the distillate zone such that the
distillate zone is sandwiched between the feed zone and the coolant
zone, iv) a feed inlet fluidly connected to the feed zone for
delivering a feed stream to the feed zone, and a feed outlet
fluidly connected to the feed zone for discharging the feed stream
from the feed zone, v) a coolant inlet fluidly connected to the
coolant zone for delivering a coolant stream to the coolant zone,
and a coolant outlet fluidly connected to the coolant zone for
discharging the coolant stream from the coolant zone, vi) at least
one distillate inlet and at least one distillate outlet fluidly
connected to the distillate zone, vii) a circulating line disposed
outside of the distillate zone that fluidly connects the at least
one distillate outlet to the at least one distillate inlet, viii) a
pump fluidly connected to the circulating line, wherein the feed
stream comprises water and water vapor, and a portion of the water
vapor permeates through the hydrophobic membrane and condenses in
the distillate zone in the form of distilled water, and wherein the
pump circulates at least a portion of the distilled water in the
circulating line.
[0009] In one embodiment, the hydrophobic membrane is micro-porous
with an average (mean) pore diameter of 0.1 to 1 .mu.m.
[0010] In one embodiment, the hydrophobic membrane is a PTFE
membrane with an average pore diameter of 0.1 to 0.6 .mu.m.
[0011] In one embodiment, the membrane distillation module further
includes at least one nozzle disposed on the at least one
distillate inlet to inject the distilled water of the circulating
line into the distillate zone.
[0012] In one embodiment, at least a portion of the circulating
line is disposed in the coolant zone such that the distilled water
of the circulating line heat exchanges with the coolant stream.
[0013] In one embodiment, the portion of the circulating line
disposed in the coolant zone has a coil shape or a zig-zag
shape.
[0014] In one embodiment, the circulating line is made of a metal
or a metal alloy having a thermal conductivity of at least 30
W/(m*K) at a temperature of 20-30.degree. C.
[0015] In one embodiment, the thermally conductive divider is made
of a metal or a metal alloy having a thermal conductivity of at
least 30 W/(m*K) at a temperature of 20-30.degree. C.
[0016] In one embodiment, the vessel has a longitudinal axis, which
is vertically oriented, wherein the feed inlet and the at least one
distillate inlet are arranged proximal to a top of the vessel, and
the feed outlet and the at least one distillate outlet are arranged
proximal to a bottom of the vessel.
[0017] In one embodiment, the vessel has a longitudinal axis, which
is vertically oriented, wherein the feed inlet and the coolant
outlet are arranged proximal to a top of the vessel, and the feed
outlet and the coolant inlet are arranged proximal to a bottom of
the vessel.
[0018] In one embodiment, the vessel has a longitudinal axis, which
is horizontally oriented, a first end, and a second end, wherein
the feed inlet and the coolant outlet are arranged on the first end
of the vessel, and the feed outlet and the coolant inlet are
located on the second end of the vessel.
[0019] In one embodiment, the membrane distillation module has a
configuration selected from the group consisting of a flat sheet
membrane configuration, a hollow fiber membrane configuration, and
a spiral wound configuration.
[0020] According to a second aspect, the present disclosure relates
to a multistage membrane distillation system including a plurality
of the membrane distillation modules arranged in series, wherein
the feed outlet of each membrane distillation module is fluidly
connected to the feed inlet of a subsequent membrane distillation
module and the feed stream in delivered to the feed inlet of a
first membrane distillation module, wherein the coolant inlets of
the plurality of the membrane distillation modules are fluidly
connected to a coolant line and the coolant stream is delivered to
the coolant inlets with the coolant line in a parallel fashion,
wherein the distillate inlets of the plurality of the membrane
distillation modules are fluidly connected to the distillate
outlets of the plurality of the membrane distillation modules with
a branched circulating line, and wherein the distilled water is
circulated inside the branched circulating line with the pump.
[0021] In one embodiment, at least a portion of the branched
circulating line is disposed in at least one coolant zone of the
plurality of the membrane distillation modules such that the
distilled water inside the branched circulating line heat exchanges
with the coolant stream.
[0022] According to a third aspect, the present disclosure relates
to a multistage membrane distillation system including a plurality
of the membrane distillation modules arranged in parallel, wherein
the feed inlets of the plurality of the membrane distillation
modules are fluidly connected to a feed line and the feed stream is
delivered to the feed inlets with the feed line in a parallel
fashion, wherein the coolant inlets of the plurality of the
membrane distillation modules are fluidly connected to a coolant
line and the coolant stream is delivered to the coolant inlets with
the coolant line in a parallel fashion, wherein the distillate
inlets of the plurality of the membrane distillation modules are
fluidly connected to the distillate outlets of the plurality of the
membrane distillation modules with a branched circulating line, and
wherein the distilled water is circulated inside the branched
circulating line with the pump.
[0023] According to a fourth aspect, the present disclosure relates
to a method of desalinating the feed stream with the membrane
distillation module, the method involving i) delivering the feed
stream to the feed inlet and delivering the coolant stream to the
coolant inlet, wherein a portion of the water vapor permeates
through the hydrophobic membrane and condenses in the distillate
zone in the form of distilled water, ii) circulating the distilled
water in the circulating line.
[0024] In one embodiment, a portion of the distillate zone is
filled with distilled water before delivering the feed stream.
[0025] In one embodiment, the feed stream has a temperature of
30-90.degree. C., and the coolant stream has a temperature of
5-20.degree. C.
[0026] In one embodiment, at least a portion of the circulating
line is disposed in the coolant zone, wherein the water vapor
permeates through the hydrophobic membrane at a permeate flux of
30-140 kg/m.sup.2/hr.
[0027] In one embodiment, the circulating involves injecting the
distilled water into the distillate zone, wherein the water vapor
permeates through the hydrophobic membrane at a permeate flux of
30-140 kg/m.sup.2/hr.
[0028] The foregoing paragraphs have been provided by way of
general introduction, and are not intended to limit the scope of
the following claims. The described embodiments, together with
further advantages, will be best understood by reference to the
following detailed description taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] A more complete appreciation of the disclosure and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0030] FIG. 1 schematically illustrates a water-gap membrane
distillation module.
[0031] FIG. 2A schematically illustrates the membrane distillation
(MD) module of the present disclosure, wherein a circulating line
with a coil shape is disposed in a coolant zone of the MD
module.
[0032] FIG. 2B schematically illustrates the membrane distillation
(MD) module, wherein the circulating line is disposed outside of
the coolant zone.
[0033] FIG. 2C schematically illustrates the membrane distillation
(MD) module, wherein the circulating line with a zig-zag shape is
disposed in the coolant zone.
[0034] FIG. 3 schematically illustrates a distillate zone of the MD
module with two distillate inlets and two distillate outlets, and a
circulating line that fluidly connects the distillate inlets to the
distillate outlets.
[0035] FIG. 4A schematically illustrates a multistage membrane
distillation (MD) system with a plurality of MD modules that are
fluidly connected in series relative to a feed line.
[0036] FIG. 4B schematically illustrates a multistage membrane
distillation (MD) system with a plurality of MD modules that are
fluidly connected in parallel.
[0037] FIG. 5 is an image of a bench scale setup of the MD
module.
[0038] FIG. 6 represents a permeate flux of water vapor in the MD
module over various temperatures of a feed stream, with and without
the circulating line.
[0039] FIG. 7 represents a relative increase of the permeate flux
of water vapor in the MD module, relative to a MD module that does
not include a circulating line.
[0040] FIG. 8 illustrates a disassembled view of an embodiment of
the membrane distillation (MD) module of the present disclosure,
wherein the hydrophobic membrane is sandwiched between two metal
plates and sealed with gaskets.
[0041] FIG. 9 illustrates the membrane distillation (MD) module,
wherein the longitudinal axis is horizontally oriented.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0042] Embodiments of the present disclosure will now be described
more fully hereinafter with reference to the accompanying drawings,
in which some, but not all embodiments of the disclosure are
shown.
[0043] The present disclosure will be better understood with
reference to the following definitions. As used herein, the words
"a" and "an" and the like carry the meaning of "one or more."
Within the description of this disclosure, where a numerical limit
or range is stated, the endpoints are included unless stated
otherwise. Also, all values and subranges within a numerical limit
or range are specifically included as if explicitly written
out.
[0044] Referring to FIGS. 2A-2C, according to a first aspect, the
present disclosure relates to a membrane distillation (MD) module
100. The membrane distillation module of the present disclosure is
a kind of a water-gap membrane distillation module, which is
described by Khalifa et al. [A. E. Khalifa, "Water and air gap
membrane distillation for water desalination--An experimental
comparative study," Sep. Purif. Technol., vol. 141, pp. 276-284,
2015, incorporated herein by reference in its entirety]. The
membrane distillation module may preferably be utilized for
desalinating saline water, e.g. seawater, or producing distilled
water, although it may be utilized in other applications to reduce
dissolved salts of a water stream or any other liquid stream,
deionizing water, etc.
[0045] The MD module 100 includes at least a vessel 102 with an
internal cavity, a hydrophobic membrane 124, a thermally conductive
divider 126, a feed inlet 106 and a feed outlet 108, a coolant
inlet 120 and a coolant outlet 122, at least one distillate inlet
112 and at least one distillate outlet 116, a circulating line 128,
and a pump 130. Each of the abovementioned components of the MD
module is fully described hereinafter.
[0046] The vessel 102 may have various shapes/geometries including,
without limitation, a rectangular shape, a cubical shape, a
rectilinear shape, a cylindrical shape, a spherical shape, an
oblong shape, a conical shape, or a pyramidal shape. For example,
in a preferred embodiment, the vessel 102 has a rectangular shape,
and a volume of the internal cavity ranges from 0.01-10 m.sup.3,
preferably 0.1-5 m.sup.3, preferably 0.2-2 m.sup.3, more preferably
0.3-1 m.sup.3. In view of this embodiment, the vessel 102 may
preferably have a longitudinal axis, which is substantially
perpendicular to the ground, and the vessel 102 is considered
vertically oriented, as shown in FIGS. 2A-2C. In certain
embodiments, such as the example shown in FIG. 9, the vessel 102
may have a longitudinal axis, which is substantially parallel to
the ground, and the vessel is considered horizontally oriented. The
vessel 102 can be made of various types of materials that can
withstand moderate process conditions, e.g. a pressure of no more
than 4 bars, preferably no more than 3 bars, and a temperature of
no more than 120.degree. C., preferably no more than 100.degree. C.
Accordingly, in some embodiments, the vessel 102 is made of glass,
aluminum or an aluminum alloy, iron or an iron alloy, quartz,
stainless steel, nickel steel, chromium steel, aluminum, aluminum
alloy, copper and copper alloys, titanium, high density
polyethylene (HDPE), low density polyethylene (LDPE),
polypropylene, polystyrene, polycarbonate, polymethylmethacrylate,
and the like. Preferably, an internal surface of the vessel may be
coated with an oxidation resistant layer as known to those skilled
in the art to prevent internal surface oxidation. Exemplary
oxidation resistant layer may include, without limitation, a
polymeric coating such as epoxy, vinyl ester, polyurethane, urea
formaldehyde, or a non-polymeric coating such as quartz, alumina,
Pyrex.RTM., and the like. The shape of the vessel and the material
of constructing the vessel are not meant to be limiting and various
shapes and materials may be used in addition or in lieu of the
aforementioned ones.
[0047] The membrane distillation module further includes a
hydrophobic membrane 124 arranged inside the vessel 102 such that
it divides the internal cavity into a feed zone 104 and a
distillate zone 110. The hydrophobic membrane 124 may be arranged
parallel or perpendicular relative to the longitudinal axis of the
vessel 102. In some preferred embodiments, the vessel 102 is
vertically oriented and the hydrophobic membrane 124 is arranged
parallel to the longitudinal axis of the vessel, as shown in FIGS.
2A-2C. The hydrophobic membrane 124 may preferably include at least
one compound selected from polytetrafluoroethylene (PTFE),
polyvinylidene fluoride (PVDF), polypropylene (PP), polyethylene
(PE), polyethersulfone (PES), polysulfone (PS), polyacrylonitrile
(PAN), polyamide (PA), or any other suitable hydrophobic material
as known to those skilled in the art. In some preferred
embodiments, the hydrophobic membrane 124 may be sandwiched between
some protective layers to protect the hydrophobic layer against
damage due to abrasion, for example, due to small particles being
present in a liquid stream that is contacted therewith. Said
protective layers may preferably also provide mechanical strength
to the hydrophobic membrane. For example, in some embodiments, the
hydrophobic membrane 124 is sandwiched between two stainless steel
mesh structures that substantially cover a surface of the
hydrophobic membrane. In one embodiment, the hydrophobic membrane
124 is supported by a perforated supporting structure from one
side, as shown in FIG. 8. In one embodiment, the hydrophobic
membrane 124 is sandwiched between two channel plates each with a
thickness of 0.1-10 mm, preferably 0.5-8 mm, preferably 1.0-5.0 mm,
as shown in FIG. 8, and tightly secured and sealed in place by
sealing means as known to those skilled in the art, e.g. gaskets or
O-rings.
[0048] Hydrophobicity is a property of a material's surface, i.e. a
measure of wetting properties of a material's surface.
Hydrophobicity may be quantified with contact angle of a water
droplet on a horizontal surface upon which the drop of water is
placed. In a number of technology fields and industrial
applications, materials with one or more hydrophobic surfaces are
advantageous due to difficulty to wet and self-cleaning properties.
In terms of the present disclosure, a hydrophobic surface is a
surface that provides a water contact angle of greater than
90.degree.. Accordingly, the hydrophobicity surface of the
hydrophobic membrane 124 has a water contact angle in the range of
90.degree. to about 150.degree., preferably from about 100.degree.
to about 145.degree.. The hydrophobicity surface of the hydrophobic
membrane 124 may also provide a reduced biofouling effect.
[0049] The hydrophobic membrane 124 is arranged inside the vessel
102 with a sealing element as known to those skilled in the art to
prevent any leakage from the feed zone 104 to the distillate zone
110. The sealing element may be a soft material so that it can be
compressed to provide a seal. For example, the sealing element may
be made from a natural rubber, a synthetic rubber or any other
suitable soft material that can provide a seal. Examples of the
synthetic rubber include thermoplastic vulcanizates (TPV) and
ethylene propylene diene monomers (EPDM). Both TPVs and EPDM may be
inject-molded. In some embodiments, a layer of glue may be used to
provide a seal. Suitable glues that may be used include
polyurethane, silicone, epoxy and polyacrylate glues. In certain
embodiments, a seal may be provided with a gasket or O-rings, for
example, a rubber gasket made of the synthetic rubber as mentioned
previously.
[0050] In certain embodiments, the hydrophobic membrane 124
includes a hydrophobic layer disposed on a polymeric substrate.
Exemplary polymeric substrates may include, without limitation,
polypropylene, polyethylene, polysulfone, polyacrylonitrile,
polyamide, polyethylene terephthalate, polybutylene terephthalate,
polyester, polyacrylate and polymethacrylate, polyvinyl chloride,
polyimide, polyethernitrile, polyethersulfone, polystyrene,
polycarbonate, poly methyl methacrylate, and any combinations
thereof. Exemplary hydrophobic layers may include a fluoropolymer
to provide hydrophobicity, for example, polytetrafluoroethylene
(PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene (TFE),
chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP),
perfluoropropylvinylether (PPVE), perfluoromethylvinylether (PMVE),
and the like.
[0051] In a preferred embodiment, the hydrophobic membrane 124 is
microporous with an average (mean) pore diameter of 0.1-1 .mu.m,
preferably 0.03-5 .mu.m, preferably 0.05-2 .mu.m, preferably 0.08-1
.mu.m, preferably 0.1-0.8 .mu.m, preferably 0.15-0.5 .mu.m,
preferably 0.2-0.3 .mu.m. A microporous membrane may preferably
refer to a membrane having an open spongy morphology of controlled
pore size ranging from 0.03-5 .mu.m, preferably 0.05-2 .mu.m,
preferably 0.08-1 .mu.m, preferably 0.1-0.8 .mu.m, preferably
0.15-0.5 .mu.m, preferably 0.2-0.3 .mu.m. In some preferred
embodiments, the hydrophobic membrane 124 is a PTFE membrane with
an average pore diameter of 0.1-0.6 .mu.m, preferably 0.2-0.5
.mu.m, preferably 0.3-0.45 .mu.m. The hydrophobic membrane 124 may
have a thickness in the range of 0.1 to 5 mm, preferably 0.5 to 4
mm, more preferably 1 to 3 mm. In certain embodiments, the
hydrophobic membrane 124 may have a thickness of 1 to 50 .mu.m,
preferably 2 to 30 .mu.m, more preferably 3 to 20 .mu.m, even more
preferably 5 to 10 .mu.m. In certain embodiments, the hydrophobic
membrane 124 includes the hydrophobic layer disposed on the
polymeric substrate, wherein a thickness of the hydrophobic
membrane 124 may be in the range of 10 to 500 .mu.m, preferably 50
to 300 .mu.m, more preferably 100 to 200 .mu.m, even more
preferably about 105 to 160 .mu.m. In addition, the hydrophobic
membrane 124 may preferably endure a pressure (i.e. a liquid entry,
e.g. the feed stream, pressure) of up to 4 bars, preferably up to 3
bars, preferably in the range of 0.5-2.5 bars. In some embodiments,
an effective surface area of the hydrophobic membrane 124 may be in
the range of 0.05-5 m.sup.2, preferably 0.1-4 m.sup.2, preferably
0.5-3 m.sup.2, preferably 1-2 m.sup.2. In some other embodiments,
the effective surface area of the hydrophobic membrane 124 is less
than 0.05 m.sup.2, preferably 0.001-0.05 m.sup.2, preferably
0.005-0.01 m.sup.2, preferably around 0.007 m.sup.2. The term
"effective surface area" as used here refers to a surface area of
the hydrophobic membrane 124 which is in contact with the feed
stream 104s and permeates water vapor to the distillate zone
110.
[0052] The type of hydrophobic membrane is not meant to be limiting
and various types of hydrophobic membranes as known to those
skilled in the art may alternatively be utilized here.
[0053] The feed zone 104 as used in this disclosure refers to a
space inside the vessel 102, wherein a feed stream 104s is
configured to pass therethrough. Also, the distillate zone 110 as
used in this disclosure refers to a space inside the vessel 102,
wherein the water vapor that is permeated through the hydrophobic
membrane 124 is condensed and accumulated in the form of distilled
water 110s.
[0054] A volume of the feed zone 104 may be 20-50%, preferably
25-45%, preferably 30-40% of the total volume of the vessel 102. A
volume of the distillate zone 110 may be 5-50%, preferably 10-30%,
preferably 15-20% of the total volume of the vessel 102. In one
embodiment, the volume of the distillate zone 110 defines a
distance between the hydrophobic membrane 124 and the thermally
conductive divider 126. Accordingly, in certain embodiments, the
distance between the hydrophobic membrane 124 and the thermally
conductive divider 126 is in the range of 1 mm to 20 cm, preferably
2 mm to 5 cm, preferably 4 to 15 mm, preferably 5 to 12 mm.
[0055] The feed stream 104s as used in this disclosure refers to
any water stream that is delivered to the feed zone 104 and
includes water and water vapor and contains more salinity than
distilled water. Accordingly, in some embodiments, the feed stream
104s is seawater that contains about 3-5% by weight of dissolved
salts. In some embodiments, the feed stream 104s is brackish water
that contains about 0.05-3% by weight of dissolved salts. In some
embodiments, the feed stream 104s is brine, which contains greater
than 5% by weight of dissolved salts (as presented herein % by
weight is relative to the total solution weight). Salts that may be
present in the feed stream 104s may include, without limitation,
cations of sodium, magnesium, calcium, potassium, ammonium, and
iron, and anions of chloride, bicarbonate, carbonate, sulfate,
sulfite, phosphate, iodide, nitrate, acetate, citrate, fluoride,
and nitrite.
[0056] When the feed stream 104s is delivered to the feed zone 104,
a portion of the water vapor permeates through the hydrophobic
membrane 124 and condenses in the distillate zone 110 in the form
of distilled water 110s.
[0057] In terms of the present disclosure, in a preferred
embodiment the distilled water 110s is a kind of water that is
substantially free from dissolved salts, e.g. the abovementioned
salts. The term "substantially free" as used here may refer to a
concentration of less than 100 ppm, preferably less than 50 ppm,
preferably less than 40 ppm, preferably less than 20 ppm,
preferably less than 10 ppm, preferably less than 5 ppm, preferably
less than 2 ppm, preferably less than 1 ppm, preferably less than
0.5 ppm, preferably less than 0.2 ppm, preferably less than 0.1 ppm
of dissolved salts.
[0058] In a preferred embodiment, a weight ratio of dissolved salt
in the distilled water 110s to the dissolved salt in the feed
stream 104s is in the range of 1:10 to 1:1000, preferably 1:50 to
1:900, preferably 1:100 to 1:800. In view of that, the hydrophobic
layer preferably effectively separates dissolved salts present in
the feed stream 104s.
[0059] The distilled water 110s may be collected from the
distillate zone 110 in various ways. For example, in one
embodiment, a distilled water canal 132 is fluidly connected to the
distillate zone 110 to discharge the accumulated distilled water
when it reaches to a certain liquid level in the distillate zone,
as shown in FIGS. 2B and 2C. In some embodiments, a distilled water
line 134 may be fluidly connected to the circulating line 128, and
delivers a portion of the distilled water 110s to a downstream
processing unit, as shown in FIG. 2A. In view of this embodiment, a
control system may preferably be adapted to control a flow rate of
the distilled water 110s in the distilled water line 134 to make
sure the flow rate does not exceed a predetermined flow rate and
that a liquid level of the distilled water 110s in the distillate
zone 110 does not drop below certain liquid levels during operating
the MD module 100.
[0060] The distilled water 110s that is collected from the
distilled water canal 132 and/or the distilled water line 134 may
be delivered to an industrial water treatment plant and/or a
residential water treatment plant, wherein the distilled water may
be further processed to be utilized for drinking, or may be used in
air conditioning or refrigerating systems in residential or
industrial applications. In some embodiments, said distilled water
may be utilized in various chemical reactions and processes where
distilled water is demanded as known to those skilled in the art,
e.g. cement processing, chemical cracking, oil refining processing,
mineral processing, steam generating, methanol processing, LNG
processing, sugar refining, power plants, mining applications,
etc.
[0061] To deliver the feed steam to the feed zone 104, the membrane
distillation module further includes a feed inlet 106 fluidly
connected to the feed zone 104. In addition, the membrane
distillation module includes a feed outlet 108 fluidly connected to
the feed zone 104 for discharging the feed stream 104s from the
feed zone 104.
[0062] In some embodiments, the feed inlet 106 and the feed outlet
108 are substantially the same, wherein each is a cylindrical canal
with a diameter of 1-100 mm, preferably 5-80 mm, preferably 10-50
mm, preferably 12-25 mm. The feed inlet 106 and the feed outlet 108
are configured to operate at various temperatures and pressures,
preferably a pressure of no more than 4 bars, preferably no more
than 3 bars, and a temperature of no more than 120.degree. C.,
preferably no more than 100.degree. C. In some preferred
embodiments, an internal surface of the feed inlet 106 and the feed
outlet 108 may be coated with the oxidation resistant layer, as
described previously, to prevent internal surface oxidation.
[0063] Each of the feed inlet 106 and the feed outlet 108 may be
arranged perpendicular, parallel, or at a certain inclination angle
relative to the longitudinal axis of the vessel. For example, in
one embodiment, the vessel 102 is vertically oriented, wherein the
feed inlet 106 and the feed outlet 108 are arranged parallel to the
longitudinal axis of the vessel 102, as depicted in FIG. 2A. In
some alternative embodiments, the feed inlet 106 and the feed
outlet 108 may be disposed perpendicular to the longitudinal axis
of the vessel 102.
[0064] The membrane distillation module further includes at least
one distillate inlet 112 and at least one distillate outlet 116
fluidly connected to the distillate zone 110. Said distillate
outlet 116 is configured to discharge the distilled water 110s from
the distillate zone 110, and the distillate inlet 112 is configure
to deliver distilled water 110s to the distillate zone 110.
[0065] In some embodiments, the distillate inlet 112 and the
distillate outlet 116 are substantially the same, wherein each is a
cylindrical canal with a diameter of 1-100 mm, preferably 5-80 mm,
preferably 10-50 mm, preferably 12-25 mm. The distillate inlet 112
and the distillate outlet 116 are configured to operate at various
temperatures and pressures, preferably a pressure of no more than 4
bars, preferably no more than 3 bars, and a temperature of no more
than 30.degree. C., preferably no more than 20.degree. C. In some
preferred embodiments, an internal surface of the distillate inlet
112 and the distillate outlet 116 may be coated with the oxidation
resistant layer, as described previously, to prevent internal
surface oxidation.
[0066] Each of the distillate inlet 112 and the distillate outlet
116 may be arranged perpendicular, parallel, or at a certain
inclination angle relative to the longitudinal axis of the vessel.
For example, in one embodiment, the vessel 102 is vertically
oriented, wherein the distillate inlet 112 and the distillate
outlet 116 are arranged parallel to the longitudinal axis of the
vessel 102, as depicted in FIG. 2A. In some alternative
embodiments, the distillate inlet 112 and the distillate outlet 116
may be disposed perpendicular to the longitudinal axis of the
vessel 102. In one embodiment, two distillate inlets are fluidly
connected to a top of the distillate zone, and two distillate
outlets are fluidly connected to a bottom of the distillate
zone.
[0067] In some preferred embodiments, the membrane distillation
module further includes at least one nozzle 114 disposed on the at
least one distillate inlet 112 to inject the distilled water 110s
into the distillate zone 110. The nozzle 114 may be any type as
known to those skilled in the art, e.g. a hydro jet nozzle, a
propelling nozzle, a magnetic nozzle, a spray nozzle such as a
swirler, etc. The type of the nozzle as used here is not meant to
be limiting and various other types of nozzle may be used
alternatively. The nozzle 114 is used to inject/jet the distilled
water 110s into the distillate zone 110 to form turbulence in the
distilled water, which is accumulated in the distillate zone 110.
The turbulence may effectively enhance a permeate flux of water
vapor through the hydrophobic membrane 124. For example, in some
embodiment, the presence of the nozzle 114 in a MD module increases
the permeate flux of water vapor through the hydrophobic membrane
by 30-60%, preferably 35-50%, preferably 40-45%, relative to a MD
module that does not include the nozzle.
[0068] The membrane distillation module further includes a
thermally conductive divider 126 disposed in the distillate zone
110 to separate a coolant zone 118 from the distillate zone 110
such that the distillate zone 110 is sandwiched between the feed
zone 104 and the coolant zone 118. The thermally conductive divider
126 is preferably planar and provides a cool surface to cool the
distilled water accumulated in the distilled zone for better
condensation of water vapor that permeates through the hydrophobic
membrane. In one embodiment, the thermally conductive divider 126
is a duct with a rectangular, a circular, or an elliptical
cross-section. The thermally conductive divider 126 is preferably
impermeable to water molecules and, in some preferred embodiments,
is made of a thermally conductive material selected from copper,
brass, iron, cast iron, steel, carbon steel, aluminum, lead,
bronze, silver, tin, titanium, tungsten, chromium, cobalt, nickel,
iridium, gold, zinc, and alloys thereof. In a preferred embodiment,
the thermally conductive divider 126 is made of a metal or a metal
alloy having a thermal conductivity of at least 30 W/(m*K),
preferably at least 40 W/(m*K), preferably at least 50 W/(m*K),
preferably at least 100 W/(m*K), at a temperature of 20-30.degree.
C., preferably 22-28.degree. C., preferably about 25.degree. C. The
thermally conductive divider 126 may be arranged parallel or
perpendicular relative to the longitudinal axis of the vessel 102.
For example, in some preferred embodiments, the vessel 102 is
vertically oriented and the thermally conductive divider 126 is
planar with a longitudinal axis, which is arranged parallel to the
longitudinal axis of the vessel 102, as shown in FIGS. 2A-2C.
[0069] The thermally conductive divider 126 is arranged inside the
vessel 102 with the sealing element, as described previously, to
prevent any leakage from the coolant zone 118 to the distillate
zone 110. In some embodiments, a layer of glue may be used to
provide a seal. Suitable glues that may be used include
polyurethane, silicone, epoxy and polyacrylate glues. In certain
embodiments, the thermally conductive divider 126 is welded to the
vessel 102 and further coated with the glue and/or the sealing
element.
[0070] The coolant zone 118 as used in this disclosure refers to a
space inside the vessel 102, wherein a coolant stream 118s is
configured to pass therethrough. A volume of the coolant zone 118
may be 20-50%, preferably 25-45%, preferably 30-40% of the total
volume of the vessel 102.
[0071] The coolant stream 118s as used in this disclosure refers to
any fluid stream, preferably a cool-water stream with a temperature
of no more than 25.degree. C., preferably no more than 20.degree.
C., preferably 5-20.degree. C., preferably 10-15.degree. C., which
is configured to cool down the thermally conductive divider 126 to
cool the distilled water 110s in the distillate zone 110.
Alternative coolant streams as known to those skilled in the art
may be used in lieu of the cool-water stream. Examples of the
coolant stream may include, without limitation, a cooled air, a
refrigerant such as ammonia, a cooled nitrogen gas, liquid
nitrogen, a cooled argon gas, etc. The coolant stream may be
delivered from an upstream refrigeration system or an air cooler,
which may be electrically powered, mechanically powered, or
preferably solar powered.
[0072] In some preferred embodiments, the pump 130 is powered with
solar energy. Accordingly, solar radiation rotates an electric
drive motor by means of photovoltaic solar cells, and the drive
motor further rotates the pump.
[0073] The membrane distillation module further includes a coolant
inlet 120 fluidly connected to the coolant zone 118 for delivering
the coolant stream 118s to the coolant zone 118, and a coolant
outlet 122 fluidly connected to the coolant zone 118 for
discharging the coolant stream 118s from the coolant zone 118.
[0074] In some embodiments, the coolant inlet 120 and the coolant
outlet 122 are substantially the same, wherein each is a
cylindrical canal with a diameter of 1-100 mm, preferably 5-80 mm,
preferably 10-50 mm, preferably 12-25 mm. The coolant inlet 120 and
the coolant outlet 122 are configured to operate at various
temperatures and pressures, preferably a pressure of no more than 4
bars, preferably no more than 3 bars, and a temperature of no more
than 30.degree. C., preferably no more than 20.degree. C. In some
preferred embodiments, an internal surface of the coolant inlet 120
and the coolant outlet 122 are coated with the oxidation resistant
layer, as described previously, to prevent internal surface
oxidation.
[0075] Each of the coolant inlet 120 and the coolant outlet 122 may
be arranged perpendicular, parallel, or at a certain inclination
angle relative to the longitudinal axis of the vessel. For example,
in one embodiment, the vessel 102 is vertically oriented, wherein
the coolant inlet 120 and the coolant outlet 122 are arranged
parallel to the longitudinal axis of the vessel 102, as depicted in
FIG. 2A. In some alternative embodiments, the coolant inlet 120 and
the coolant outlet 122 may be disposed perpendicular to the
longitudinal axis of the vessel 102.
[0076] The membrane distillation module 100 further includes a
circulating line 128 disposed outside of the distillate zone 110
and fluidly connects the distillate outlet 116 to the distillate
inlet 112. The circulating line 128 is configured to circulate a
portion of the distilled water 110s, which is accumulated in the
distillate zone 110. Circulation of the distilled water 110s may
form a laminar or a turbulent flow of the distilled water 110s in
the distillate zone 110, depending on the circulation flow rate and
pressure, and may further enhance the permeate flux of water vapor
through the hydrophobic membrane 124. In some embodiments, the
presence of the circulating line 128 in the MD module 100 for
circulating the distilled water 110s enhances the permeate flux of
water vapor through the hydrophobic membrane by 20-40%, preferably
25-35%, preferably about 30%, relative to a MD module that does not
include the circulating line. In some preferred embodiments, the MD
module 100 includes the circulating line 128 and the nozzle 114,
wherein the permeate flux of water vapor through the hydrophobic
membrane 124 is 30-70%, preferably 45-65%, preferably 50-60% higher
than the permeate flux of a MD module that does not include the
circulating line and the nozzle. In some other preferred
embodiments, the feed stream has a temperature of 50-90.degree. C.,
preferably 60-90.degree. C., preferably 70-90.degree. C. and the MD
module 100 includes the circulating line 128 and the nozzle 114,
wherein the permeate flux of water vapor through the hydrophobic
membrane 124 is 30-140 kg/m.sup.2/h, preferably 50-135
kg/m.sup.2/h, preferably 70-130 kg/m.sup.2/h.
[0077] The presence of distilled water 110s in the distillate zone
110 may reduce an efficiency of the MD module 100 by dissipating
thermal energy and reducing a cooling effect in the coolant zone
118, which is provided by the coolant stream 118s and the thermally
conductive divider 126. In view of that, in some preferred
embodiments, at least a portion of the circulating line 128 is
disposed in the coolant zone 118, as shown in FIGS. 2A and 2C.
Accordingly, the distilled water 110s of the circulating line 128
heat exchanges with the coolant stream 118s in a continuous fashion
to continuously keep the temperature of the distilled water 110s
close to the temperature of the coolant stream 118s (i.e. a
temperature of no more than 25.degree. C., preferably no more than
20.degree. C., preferably 5-20.degree. C., preferably 10-15.degree.
C.). In some preferred embodiments, an efficiency of the MD module
100, which includes the circulating line 128 disposed in the
coolant zone 118, is at least 10%, preferably 20-60%, preferably
30-50% higher than the efficiency of a MD module that does not
include a circulating line. In one embodiment, an efficiency of the
MD module 100, which includes the circulating line 128 disposed in
the coolant zone 118, is at least at 10%, preferably 15-40%,
preferably 20-30% higher than the efficiency of a MD module that
has a circulating line, but the circulating line is not disposed in
the coolant zone. The term "efficiency" as used herein refers to
specific energy consumption of the MD module 100, which is total
power consumption per one cubic meter of the distilled water
formed. The total power consumption is a sum of the power
consumption for operating the pump 130, the power consumption for
heating the feed stream 104s, and the power consumption for
reducing the temperature of the coolant stream 118s.
[0078] The portion of the circulating line 128, which is disposed
in the coolant zone 118, may have various shapes to provide an
extended contact surface area. For example, in some preferred
embodiments, said portion of the circulating line has a coil shape
as depicted in FIG. 2A, a zig-zag shape as depicted in FIG. 2C, a
helix shape, a serpentine shape, etc.
[0079] In one embodiment, the circulating line 128 is a tube with a
rectangular, a circular, or an elliptical cross-section. The
circulating line 128 is preferably impermeable to water molecules
and, in some preferred embodiments, is made of a thermally
conductive material selected from copper, brass, iron, cast iron,
steel, carbon steel, aluminum, lead, bronze, silver, tin, titanium,
tungsten, chromium, cobalt, nickel, iridium, gold, zinc, and alloys
thereof. In a preferred embodiment, the circulating line 128 is
made of a metal or a metal alloy having a thermal conductivity of
at least 30 W/(m*K), preferably at least 40 W/(m*K), preferably at
least 50 W/(m*K), preferably at least 100 W/(m*K), at a temperature
of 20-30.degree. C., preferably 22-28.degree. C., preferably about
25.degree. C. In some preferred embodiments, an internal surface of
the circulating line 128 is coated with an oxidation resistant, as
described previously, to prevent internal surface oxidation.
[0080] In one embodiment, an internal diameter of the circulating
line 128 ranges from about 0.1-20 cm, preferably 0.5-10 cm,
preferably 1-5 cm. In one embodiment, a thickness of the
circulating line 128 ranges from about 1-50 mm, preferably 2-30 mm,
preferably 3-20 mm, preferably 4-10 mm. In some embodiments, the
circulating line 128 is configured to bear a pressure of up to 1
bar, preferably up to 2 bars, preferably up to 3 bars, preferably
up to 5 bars, and endure a temperature of up to 20.degree. C.,
preferably up to 30.degree. C., more preferably up to 50.degree.
C.
[0081] In some embodiments, the MD module 100 includes more than
one distillate inlet 112 and more than one distillate outlet 116,
wherein said distillate inlets and outlets are fluidly connected to
the circulating line 128 with multiple-way tube connectors 302, as
shown in FIG. 3.
[0082] The membrane distillation module further includes a pump 130
disposed on the circulating line 128 to circulate the distilled
water 110s in the circulating line 128. The phrase "circulating the
distilled water 110s in the circulating line 128" as used in this
disclosure involves sucking the distilled water 110s and delivering
it to the distillate inlet 112, wherein the distilled water 110s is
preferred injected into the distillate zone 110 with the nozzle
114.
[0083] The type of the pump that is used in this disclosure is not
meant to be limiting and various types of pumps as known to those
skilled in the art may be utilized here. For example, in some
embodiments, the pump 130 is a positive displacement pump, an
impulse pump, a centrifuge pump, a rotary lobe pump, a progressive
cavity pump, a rotary gear pump, a piston pump, a diaphragm pump, a
screw pump, a gear pump, a hydraulic pump, a rotary vane pump, a
peristaltic pump, a rope pump, and the like. The pump 130 may be
electrically powered or mechanically powered, e.g., with a shaft of
a turbine. The power consumption of the pump 130 may preferably not
exceeding 100 kWh, preferably in the range of 0.5-20 kWh,
preferably 1.0 to 15 kWh, preferably 2.0 to 10 kWh.
[0084] In some preferred embodiments, the pump 130 is powered with
solar energy. Accordingly, solar radiation rotates an electric
drive motor by means of photovoltaic solar cells, and the drive
motor further rotates the pump 130. Alternatively, solar radiation
may produce steam in heat recovery steam generators, or any other
types of steam generators. The steam further rotates steam turbines
and generates a shaft work, and the shaft work operates the pump
130. Using solar energy to power the pump may be advantageous
particularly for remote areas, e.g. islands, deserts, remote
villages, etc. where solar radiation is abundant and any form of
energy is not accessible. In addition, the use of solar energy may
provide manufacturing compact/portable MD modules as described in
the present disclosure in different sizes, e.g. in a pilot scale or
a bench scale, particularly for remote areas where a power plant is
not accessible.
[0085] Said inlets and outlets may be disposed at various locations
in the MD module 100, preferably with respect to the efficiency of
the MD module.
[0086] For example, in a preferred embodiment, the vessel 102 is
vertically oriented, wherein the feed inlet 106 and the distillate
inlet 112 are arranged proximal to a top of the vessel 102, and the
feed outlet 108 and the distillate outlet 116 are arranged proximal
to a bottom of the vessel 102. Accordingly, a direction of the feed
stream 104s is co-current relative to a direction of the distilled
water 110s when circulated. In another preferred embodiment, the
vessel 102 is vertically oriented, wherein the feed inlet 106 and
the coolant outlet 122 are arranged proximal to the top of the
vessel 102, and the feed outlet 108 and the coolant inlet 120 are
arranged proximal to the bottom of the vessel 102. The distillate
inlet 112 may be arranged proximal to the bottom, preferably
proximal to the top of the vessel 102, and the distillate outlet
116 is arranged at the opposite end of the vessel 102. The
arrangement in view of this embodiment is shown in FIG. 2A. In view
of that, a direction of the feed stream 104s in the feed zone 104
is counter-current relative to a direction of the coolant stream
118s in the coolant zone 118. In one embodiment, the vessel 102 is
horizontally oriented with a first end and a second end, wherein
the feed inlet 106 and the coolant outlet 122 are arranged on the
first end of the vessel 102, and the feed outlet 108 and the
coolant inlet 120 are located on the second end of the vessel 102.
Accordingly, the direction of the feed stream 104s is
counter-current relative to the direction of the coolant stream
118s. In view of this embodiment, the distillate inlet 112 may be
arranged on the first end or the second end of the vessel, and the
distillate outlet 116 is arranged at the opposite end. As used
here, when an inlet/outlet is arranged proximal to a bottom of a
vertically oriented vessel, the inlet/outlet is considered being in
a region that is located at a height of less than 20%, preferably
less than 15%, preferably less than 10% of the total height of the
vessel, when measured from the bottom of the vessel, with 0% being
the bottom and 100% being the top of the vessel. Similarly, when an
inlet/outlet is arranged proximal to a top of a vertically oriented
vessel, the inlet/outlet is considered being in a region that is
located at a height of more than 80%, preferably more than 85%,
preferably more than 90% of the total height of the vessel, when
measured from the bottom of the vessel, with 0% being the bottom
and 100% being the top of the vessel.
[0087] Referring to FIG. 8, in some embodiments, the MD module
includes a feed channel plate, which is arranged substantially
parallel to a coolant channel plate, wherein the hydrophobic
membrane is tightly sandwiched between the feed channel plate and
the coolant channel plate, and further secured in place with metal
frames and sealed with a plurality of gaskets and supporting
plates. The feed channel plate and the coolant channel plate may
preferably be made of a material with low thermal conductivity,
e.g., PVC, HDPE, etc. to minimize a heat loss from the MD module to
the surrounding.
[0088] Referring now to FIG. 4A, according to a second aspect, the
present disclosure relates to a multistage membrane distillation
system 200 including a plurality of the MD modules arranged in
series relative to a feed line 204, wherein the feed outlet of each
MD module is fluidly connected to the feed inlet of a subsequent MD
module. Accordingly, the feed stream 104s is delivered to the feed
inlet of a first membrane distillation module 202 with the feed
line 204. Also, the coolant inlets of the plurality of the MD
modules are fluidly connected to a coolant line 208, and therefore
the coolant stream 118s is delivered to the coolant inlets in a
parallel fashion, as shown in FIG. 4A, that depicts a series
feed/parallel coolant multistage arrangement. The distillate inlets
of the plurality of the membrane distillation modules are fluidly
connected to the distillate outlets of the plurality of the
membrane distillation modules with a branched circulating line 206,
wherein the distilled water 110s of the plurality of the MD modules
is combined (e.g. using a plurality of multiple-way tube connectors
and lines), pumped with the pump 130, and further distributed among
the distillate inlets of the plurality of the MD modules, as shown
in FIG. 4A.
[0089] The branched circulating line 206 is preferably made of
substantially the same material as the circulating line 128, with
substantially the same diameter and thickness. In one embodiment,
the branched circulating line 206 includes a plurality of
multiple-way tube connectors and lines.
[0090] In some preferred embodiments, at least a portion of the
branched circulating line 206 is disposed in at least one coolant
zone of the plurality of the membrane distillation modules such
that the distilled water 110s heat exchanges with the coolant
stream 118s of each of the plurality of the MD modules, as shown in
FIG. 4A.
[0091] Referring now to FIG. 4B, according to a third aspect, the
present disclosure relates to a multistage membrane distillation
system 200 including a plurality of the MD modules arranged in
parallel relative to the feed line, wherein the feed inlets of the
plurality of the membrane distillation modules are fluidly
connected to a feed line 204. Accordingly, the feed stream 104s is
delivered to the feed inlets in a parallel fashion. Similarly, the
coolant inlets of the plurality of the MD modules are fluidly
connected to a coolant line 208 and the coolant stream 118s is
delivered to the coolant inlets in a parallel fashion. FIG. 4B
represents a parallel feed/parallel coolant multistage arrangement.
The arrangement of the distillate inlets, distillate outlets, and
the branched circulating line 206 is substantially the same as the
arrangement as in the multistage membrane distillation system of
the second aspect.
[0092] The multistage membrane distillation system 200 may have
alternative configurations, for example, a series feed/series
coolant or a parallel feed/series coolant arrangement. In any of
the configurations of the multistage membrane distillation system
200 a direction of the coolant stream may be co-current or
counter-current relative to a direction of the feed stream.
[0093] In certain embodiments, the multistage membrane distillation
system 200 may include more than 2, preferably 3-10, preferably 4-8
MD modules. The number of modules in the multistage membrane
distillation system 200 may depend on a temperature difference
between the coolant stream and the feed stream, wherein the
temperature difference may preferably be maintained at
10-20.degree. C., preferably about 10.degree. C. For example, in
one embodiment, the multistage membrane distillation system 200
includes three stages with a series feed/series coolant
arrangement. In some embodiments, the feed stream may be re-heated
and circulated back the multistage membrane distillation system
200, and/or the coolant stream may be re-cooled and circulated back
to the multistage membrane distillation system 200.
[0094] The MD modules of the multistage membrane distillation
system may be combined and inserted into a single module, wherein
all lines and fluid connections are disposed in said module.
[0095] The efficiency of the MD module 100 or the multistage MD
system 200 depends on the size of the MD module or the multistage
MD system. For example, in one embodiment, the MD module 100 is a
bench scale module that operates with an efficiency of 0.1-20 Wh,
preferably 1-10 Wh per one liter of the distilled water produced.
In some preferred embodiments, the efficiency of the multistage MD
system 200 in accordance with the second or the third aspect ranges
from 1-50 kWh, preferably 2-20 kWh, preferably 3-10 kWh, per one
cubic meter of the distilled water produced.
[0096] A production capacity of the MD module 100 or the multistage
MD system 200 depends on the size of the MD module or the
multistage MD system. For example, in one embodiment, the MD module
100 is a bench scale module that produces distilled water at a
production capacity of less than 150 L/h, preferably 30-120 L/h,
preferably 60-100 L/h, per unit area (e.g. one square meter) of the
hydrophobic membrane. In some preferred embodiments, the production
capacity of a pilot-scale multistage MD system in accordance with
the second or the third aspect ranges from 0.1 to 50 m.sup.3/d
(cubic meters per day), preferably 0.5 to 40 m.sup.3/d, and
preferably 1.0 to 30 m.sup.3/d. In some preferred embodiments, the
production capacity of an industrial-scale multistage MD system 200
in accordance with the second or the third aspect ranges from 50 to
10000 m.sup.3/d (cubic meters per day), preferably 100 to 5000
m.sup.3/d, preferably 200 to 4000 m.sup.3/d.
[0097] According to a fourth aspect, the present disclosure relates
to a method of desalinating the feed stream with the MD module.
[0098] The method involves delivering the feed stream to the feed
inlet. In some embodiments, the feed stream is delivered at a flow
rate of 0.1-100 L/min, preferably 0.5-50 L/min, preferably 1.0-20
L/min, preferably 4-10 L/min, preferably about 6.0 L/min. The feed
stream may have various temperatures. FIG. 6 represents the
permeate flux of water vapor through a PTFE hydrophobic membrane at
various temperatures of the feed stream.
[0099] Accordingly, in some embodiments, the feed stream has a
temperature of 30-90.degree. C., preferably 40-90.degree. C.,
preferably 60-90.degree. C. The feed stream may preferably have an
atmospheric pressure (i.e. a pressure of 0.95-1.05 atm), although
in some embodiments, the feed stream is pressurized to a pressure
of no more than 4 bars, preferably no more than 3 bars, preferably
1.5-2 bars before delivering to the feed inlet, to avoid membrane
wetting.
[0100] In the embodiments where a multistage MD system in
accordance with the second aspect of the present disclosure is
used, the feed stream is delivered to the first membrane
distillation module, as shown in FIG. 4A. In the embodiments where
a multistage MD system in accordance with the third aspect of the
present disclosure is used, the feed stream is delivered to the
feed inlets in a parallel fashion, as shown in FIG. 4B.
[0101] The method further involves delivering the coolant stream to
the coolant inlet. In some embodiments, the coolant stream is
delivered at a flow rate of 0.1-100 L/min, preferably 0.5-50 L/min,
preferably 1.0-20 L/min, preferably 4-10 L/min, preferably about
6.5 L/min. The flow rate of the coolant stream may preferably be
substantially the same as the flow rate of the feed stream. The
coolant stream may preferably have a temperature of no more than
25.degree. C., preferably no more than 20.degree. C., preferably
5-20.degree. C., preferably 10-15.degree. C. The coolant stream may
preferably have an atmospheric pressure (i.e. a pressure of
0.95-1.05 atm), although in some embodiments, the coolant stream
may be pressurized to a pressure of no more than 4 bars, preferably
no more than 3 bars, preferably 1.5-2 bars before delivering to the
coolant inlet.
[0102] In the embodiments where a multistage MD system in
accordance with the second or the third aspect of the present
disclosure is used, the coolant stream is delivered to the coolant
inlets in a parallel fashion, as shown in FIGS. 4A and 4B.
[0103] Once the feed stream in delivered to the feed zone (or the
feed zones of a multistage MD system), a portion of the water vapor
of the feed stream permeates through the hydrophobic membrane (or
hydrophobic membranes of a multistage MD system) and further
condenses in the distillate zone (or distillate zones of a
multistage MD system) in the form of distilled water.
[0104] The permeate flux of the water vapor depends on the type of
the hydrophobic membrane and the temperature of the feed stream. In
a preferred embodiment, the feed stream has a temperature of
30-90.degree. C., preferably 50-90.degree. C., preferably
60-90.degree. C., wherein the permeate flux of the water vapor
ranges from 30-140 kg/m.sup.2/hr, preferably 40-130 kg/m.sup.2/hr,
preferably 45-130 kg/m.sup.2/hr.
[0105] The method further involves circulating the distilled water
in the circulating line and further injecting the distilled water
into the distillate zone. The MD module of the present disclosure
is a water-gap membrane distillation, and thus the distillate zone
is filled with distilled water before delivering the feed stream to
the feed zone. In some embodiments, the distillate zone is empty,
and circulating the distilled water begins after delivering the
feed stream to the feed zone, preferably when the distilled water
reaches to a predetermined water level. The distilled water is
preferably circulated in the circulating line at a flow rate of
0.1-5 L/min, preferably 0.2-4 L/min, preferably 0.5-3 L/min,
preferably 1.0-2.0 L/min, preferably about 1.4 L/min, depending on
the module dimensions. The distilled water may preferably be
injected into the distillate zone at the abovementioned flow rates.
In one embodiment, the distillate zone may preferably have an
atmospheric pressure (i.e. a pressure of 0.95-1.05 atm) during
circulating and injecting.
[0106] The examples below are intended to further illustrate
protocols for the membrane distillation module, and are not
intended to limit the scope of the claims.
Example 1
[0107] The flowing examples provide a design and a process to
enhance the output permeate flux of the water gap membrane
distillation (WGMD) module through injection and circulation of the
distilled water inside the gap. Circulation and injection of the
distilled water improves the heat and mass transfer through the
membrane and thus resulting in a higher permeate flux. The flux was
further increased by using the coolant channel as heat exchanger to
cool the circulation stream.
[0108] The water gap, which separates the membrane and the cooling
surface, is filled with stagnant distilled water. Distilled water
inside the gap is recirculated from the module bottom side and
injected, in a proper way based upon the module design, into the
gap from the top side of the module using a circulating pump. The
injections are in the form of small jets parallel to the membrane
cold surface. Since the distilled water inside the gap is
circulated and injected in a closed loop, no external water supply
is encountered. Injecting and circulating water into the gap
enhances the heat and mass transfer characteristics in the WGMD
module and increase the flux due to the movement of water inside
the gap on the cold side of the membrane. The proposed design
increases the heat transfer coefficient inside the module and
reduces the resistance to vapor mass transfer across the membrane.
In addition, the design enhances the cooling and condensation
processes inside the WGMD module. The power for pumping and
circulating the distilled water is small since the pressure
required for injection jets and recirculation is small. FIG. 2B
shows the Water Gap MD process with gap water circulation and
injection. According to the experimental results, 50% increase in
flux (on average) was obtained compared to the normal WGMD system,
when the gap water was injected and recirculated, as shown in FIG.
6.
Example 2
[0109] In another embodiment, to provide better cooling for the
water inside the gap, the cooling channel of the module is used as
a heat exchanger to cool the circulated water into the gap (see
FIG. 2C). The circulated water exiting from the bottom of the
module passes in a coil through the coolant chamber (channel)
before being injected into the module gap from the top side. The
coolant channel is fitted with a coil of high thermal conductivity
material (e.g. copper) to be connected to the circulation cycle.
This design helps in carrying out the excess heat transfer to the
water gap through the membrane, and hence increases the temperature
difference across the membrane and augments the condensation of
permeated vapor in the water gap, and eventually increases the
output flux of the system. This cooling process is a simple
modification in the module design which may result in a small or
negligible increase in energy consumption. However, the enhancement
in the output flux is evident.
[0110] FIG. 3 shows the injection and circulation cycle for the
distilled gap water using a small pump. Injection nozzles (two or
more nozzles) are aligned to create water jets inside the gap on
the cold side (distilled water side) of the membrane. These jets
create turbulent flow inside the gap and enhance the heat and mass
transfer across the membrane, and thus increase the permeate flux.
Two jets are used to cover the effective area of the membrane,
although more than two jets may be used alternatively. Injection
pressure and flow rate are varied to obtain a maximum vapor mass
transfer and a maximum permeate flux.
[0111] The distilled water is collected from a tube located at the
top of the gap and its volume (or mass) is measured over the
sampling time to calculate the system permeate flux.
Example 3
[0112] Experiments were conducted to investigate the effects of
circulating and injecting of the distilled gap water on the system
flux. A water gap membrane distillation module was designed and
manufactured as shown in FIG. 5. A hollow plastic frame of 6 mm
thickness was used to create the gap space between the membrane
cold side and the cooling plate. The frame was equipped with two
injection nozzles from the top side of the gap and another two
suction holes at the bottom side of the gap. A small circulating
pump (6-Watt power) was used to circulate and inject the gap water.
The module was operated at the following conditions: PTFE membrane
with mean pore size of 0.22 .mu.m, an effective gap width of 7.5
mm, a feed concentration of 3000 mg/L, a feed flow rate of 6 L/min,
a coolant flow rate of 6.4 L/min, and a coolant temperature of
20.degree. C. The pressure of distilled water during circulation
and injection was set to be atmospheric, and the injection flow
rate was set at 1.4 L/min.
[0113] FIG. 6 represents a comparison between the permeate flux of
the normal water gap MD system (without injection/circulation) and
the proposed design of water gap MD system with gap injection and
circulation, at different feed temperatures from 50 to 90.degree.
C. The permeate flux obtained from the water gap MD system of the
present disclosure, i.e. having gap water circulation and
injection, was found to be higher than the corresponding water gap
MD system with stagnant water inside the gap. A flux of 128
kg/m.sup.2/h was achieved at a feed temperature of 90.degree. C.
when the distilled water was circulated. FIG. 7 shows the
percentage increase in flux when the gap water was circulated
compared to the stagnant gap water, at different feed temperatures.
On average, 50% increase in flux was reported with maximum of 61%
increase at feed temperature of 60.degree. C. and 41% increase at
feed temperature of 80.degree. C. The TDS of the collected permeate
was measured as 6 mg/L (6 ppm). Increasing the circulation flow
rate and pressure increases the system flux.
[0114] Thus, the foregoing discussion discloses and describes
merely exemplary embodiments of the present invention. As will be
understood by those skilled in the art, the present invention may
be embodied in other specific forms without departing from the
spirit or essential characteristics thereof. Accordingly, the
disclosure of the present invention is intended to be illustrative,
but not limiting of the scope of the invention, as well as other
claims. The disclosure, including any readily discernible variants
of the teachings herein, defines, in part, the scope of the
foregoing claim terminology such that no inventive subject matter
is dedicated to the public.
* * * * *