U.S. patent application number 11/345724 was filed with the patent office on 2007-08-02 for system for recovering water from flue gas.
This patent application is currently assigned to Siemens Power Generation, Inc.. Invention is credited to Michael S. Briesch, Philip G. Deen, Fred W. Shoemaker, Terrence B. Sullivan.
Application Number | 20070175333 11/345724 |
Document ID | / |
Family ID | 38320718 |
Filed Date | 2007-08-02 |
United States Patent
Application |
20070175333 |
Kind Code |
A1 |
Shoemaker; Fred W. ; et
al. |
August 2, 2007 |
System for recovering water from flue gas
Abstract
A power plant may include a combustion apparatus (11) producing
an exhaust gas (12), an absorber (20) receiving the exhaust gas
(12), the absorber (20) including a desiccant and producing a first
stream of desiccant solution containing water and a first
concentration of desiccant, and an apparatus (29, 70, 94) for
dehydrating the first stream of desiccant solution while
maintaining the water in a liquid phase. The apparatus (29, 70, 94)
may include one or more reverse osmosis apparatus (30, 40) that
receive the first stream of desiccant solution and produce a second
stream of desiccant solution containing a second concentration of
desiccant greater than the first concentration of desiccant. The
apparatus (29, 70, 94) may include a heat exchanger (71, 110), a
crystallizing heat exchanger (74, 96), a separator (78, 98) and a
flash tank (112) for dehydrating the desiccant solution while
maintaining water in a liquid phase and subsequently recovering
water from the solution.
Inventors: |
Shoemaker; Fred W.;
(Longwood, FL) ; Briesch; Michael S.; (Orlando,
FL) ; Deen; Philip G.; (Enterprise, FL) ;
Sullivan; Terrence B.; (Orlando, FL) |
Correspondence
Address: |
Siemens Corporation;Intellectual Property Department
170 Wood Avenue South
Iselin
NJ
08830
US
|
Assignee: |
Siemens Power Generation,
Inc.
|
Family ID: |
38320718 |
Appl. No.: |
11/345724 |
Filed: |
February 2, 2006 |
Current U.S.
Class: |
96/243 |
Current CPC
Class: |
B01D 53/263
20130101 |
Class at
Publication: |
096/243 |
International
Class: |
B01D 53/14 20060101
B01D053/14 |
Claims
1) A power plant comprising: a combustion apparatus producing an
exhaust gas; an absorber receiving the exhaust gas, the absorber
comprising a desiccant and producing a first stream of desiccant
solution containing water and a first concentration of desiccant;
and means for dehydrating the first stream of desiccant solution
while maintaining the water in a liquid phase.
2) The power plant of claim 1, the means for dehydrating comprising
a first reverse osmosis apparatus receiving the first stream of
desiccant solution and producing a second stream of desiccant
solution containing water and a second concentration of desiccant
greater than the first concentration of desiccant.
3) The power plant of claim 2, the means for dehydrating further
comprising a second reverse osmosis apparatus receiving the second
stream of desiccant solution and producing a third stream of
desiccant solution containing water and a third concentration of
desiccant greater than the second concentration of desiccant.
4) The power plant of claim 1, the absorber comprising a media
directly contacting the desiccant with the exhaust gas.
5) The power plant of claim 1 further comprising: a main fluid
connection between an outlet of the absorber and an inlet of the
absorber; a fluid connection between the main fluid connection and
the means for dehydrating; and valving for selectively regulating a
flow of the first stream of desiccant solution between the means
for dehydrating and the inlet of the absorber.
6) The power plant of claim 1, the means for dehydrating comprising
a crystallizing heat exchanger receiving and cooling the first
stream of desiccant solution and producing a second stream of
desiccant solution comprising desiccant crystals and a second
concentration of desiccant greater than the first concentration of
desiccant.
7) The power plant of claim 6 further comprising a separator
receiving the second stream of desiccant solution and separating a
quantity of the desiccant crystals from the second stream of
desiccant solution, the separator producing a desiccant slurry
containing the second concentration of desiccant.
8) The power plant of claim 7 further comprising a heat exchanger
receiving and heating the desiccant slurry to melt at least a
portion of the desiccant crystals.
9) The power plant of claim 6 further comprising: a heat exchanger
receiving and cooling the first stream of desiccant solution
upstream of the crystallizing heat exchanger.
10) The power plant of claim 1 further comprising a flash tank
receiving a second stream of desiccant solution from the means for
dehydrating.
11) The power plant of claim 1, the means for dehydrating
comprising: a heat exchanger receiving and cooling the first stream
of desiccant solution from the absorber; a desiccant sub-cooler
receiving and further cooling the cooled first stream of desiccant
solution and producing a second stream of desiccant solution
comprising desiccant crystals; and a separator receiving the second
stream of desiccant solution and separating a quantity of the
desiccant crystals from the second stream of desiccant solution,
the separator producing a slurry containing desiccant crystals and
dissolved desiccant in solution, and a flow of desiccant solution
containing a concentration of desiccant lower than a concentration
of desiccant in the slurry.
12) The power plant of claim 11 further comprising a flash tank for
receiving the flow of desiccant solution from the separator, the
flash tank producing water vapor and a return stream of desiccant
solution.
13) The power plant of claim 12 further comprising a return fluid
connection for directing the return stream of desiccant from the
flash tank to the absorber.
14) The power plant of claim 13 further comprising a desiccant
cooler receiving the return stream of desiccant from the flash tank
upstream of the absorber.
15) The power plant of claim 13 further comprising a fluid
connection for directing the slurry to the return fluid connection
upstream of the absorber.
16) A power plant comprising: a combustion apparatus producing an
exhaust gas; an absorber receiving the exhaust gas, the absorber
comprising a desiccant and producing a first stream of desiccant
solution containing water and a first concentration of desiccant;
and a reverse osmosis circuit receiving the first stream of
desiccant solution and producing a stream of recovered water and a
second stream of desiccant solution having a second concentration
of desiccant greater than the first concentration of desiccant.
17) The power plant of claim 16, the reverse osmosis circuit
comprising: a first reverse osmosis apparatus receiving the first
stream of desiccant solution and producing an intermediate stream
of desiccant solution having an intermediate concentration of
desiccant greater than the first concentration of desiccant; and a
second reverse osmosis apparatus downstream of the first reverse
osmosis apparatus and receiving the intermediate stream of
desiccant solution, the second reverse osmosis apparatus producing
the second stream of desiccant solution.
18) The power plant of claim 16 further comprising: a main fluid
connection between an outlet of the absorber and an inlet of the
absorber; a fluid connection between the main fluid connection and
the primary osmosis apparatus; and valving for selectively
regulating a flow of the first stream of desiccant solution between
the primary reverse osmosis apparatus and the inlet of the
absorber.
19) A power plant comprising: a combustion apparatus producing an
exhaust gas; an absorber receiving the exhaust gas, the absorber
comprising a desiccant and producing a first stream of desiccant
solution containing water and a first concentration of desiccant;
and a crystallization circuit receiving the first stream of
desiccant solution and producing a second stream of desiccant
solution having a second concentration of desiccant greater than
the first concentration of desiccant.
20) The power plant of claim 19, the crystallization circuit
comprising: a recuperative heat exchanger receiving the first
stream of desiccant solution; a desiccant crystallizer downstream
of the recuperative heat exchanger receiving the first stream of
desiccant solution from the recuperative heat exchanger, the
desiccant crystallizer producing a stream of desiccant solution
comprising desiccant crystals; and a separator downstream of the
desiccant crystallizer receiving the stream of desiccant solution
comprising desiccant crystals and producing the second stream of
desiccant solution.
21) The power plant of claim 20 further comprising the separator
producing the second stream of desiccant solution and a stream of
recovered water.
22) The power plant of claim 21 further comprising: a fluid
connection directing the second stream of desiccant solution from
the separator to the recuperative heat exchanger; and a fluid
connection directing the second stream of desiccant solution from
the recuperative heat exchanger to the absorber.
23) The power plant of claim 20 further comprising the separator
producing a third stream of desiccant solution and the second
stream of desiccant solution as a slurry containing desiccant
crystals and dissolved desiccant in solution.
24) The power plant of claim 23 further comprising a flash tank for
receiving the third stream of desiccant solution from the
separator, the flash tank producing water vapor and a return stream
of desiccant solution.
25) The power plant of claim 24 further comprising: a condenser
receiving the water vapor and producing a flow of recovered water;
a fluid connection directing the return stream of desiccant
solution from the flash tank to the absorber; and a fluid
connection directing the slurry containing desiccant crystals and
dissolved desiccant in solution from the separator to the
absorber.
26) A system for use with a fossil fuel burning power plant
producing an exhaust gas, the power plant including a water
absorber receiving the exhaust gas and producing an aqueous
desiccant solution containing water and a first concentration of
desiccant, the system comprising: means for dehydrating the aqueous
desiccant solution while maintaining the water in a liquid phase,
the means for dehydrating producing a second stream of desiccant
solution having a second concentration of desiccant greater than
the first concentration of desiccant.
27) The system of claim 26, the means for dehydrating comprising a
reverse osmosis apparatus receiving the aqueous solution from the
water absorber and producing the second stream of desiccant
solution.
28) The system of claim 26, the means for dehydrating comprising: a
recuperative heat exchanger receiving the aqueous solution; a
desiccant crystallizer downstream of the recuperative heat
exchanger receiving the aqueous desiccant solution from the
recuperative heat exchanger, the desiccant crystallizer producing a
stream of desiccant solution comprising desiccant crystals; and a
separator downstream of the desiccant crystallizer receiving the
stream of desiccant solution comprising desiccant crystals and
producing the second stream of desiccant solution.
29) The system of claim 28 further comprising the separator
producing a third stream of aqueous desiccant solution and the
second stream of desiccant solution as a slurry containing
desiccant crystals and dissolved desiccant in solution.
30) The system of claim 29 further comprising a flash tank for
receiving the third stream of desiccant solution from the
separator, the flash tank producing water vapor and a return stream
of desiccant solution.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to the field of recovering
water from a flue gas and more particularly to recovering water
from a flue gas produced by the combustion of a fossil fuel.
BACKGROUND OF THE INVENTION
[0002] Water is a natural byproduct of the combustion of
hydrocarbon or fossil fuels. Permits for water are becoming
increasingly difficult to obtain for power plants, which consume
relatively large volumes of water during operation. In some cases,
the difficulty with obtaining water permits for wells, or use of
surface water may preclude construction of a needed power plant.
Thus, recovering water from power plants is desirable to obviate
the need of obtaining water permits.
[0003] Fossil fuel exhaust or flue gas, such as that exhausted from
a combustion turbine engine, or downstream of a coal-fired boiler,
can contain varying concentrations of water. Water concentration
may depend on ambient conditions, fuel composition, inlet air
treatment, fuel treatment, flue gas treatment and other factors. If
the flue gas exhaust stream were cooled, a portion of that water
could be recovered. It is known that cooling an exhaust stream in a
condenser to below the precipitation temperature of the moisture in
the exhaust gas will result in the condensation of a portion of
that moisture. The quantity and percentage of recovered moisture
depends on the temperature to which the exhaust can be cooled by
the condenser.
[0004] Ambient air is commonly the ultimate heat sink for
condensers, and the ambient air temperature thus determines the
amount of moisture that can be removed by a condenser. In an arid
desert environment, for example, the effectiveness of water removal
by an ambient air-cooled condenser is limited. Given such high
ambient temperatures and the limits of heat exchange equipment,
direct condensation alone becomes technically untenable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a schematic of an exemplary embodiment of a system
for removing water from a flue gas and recovering water from a
desiccant stream.
[0006] FIG. 2 is a schematic of another exemplary embodiment of the
system of FIG. 1.
[0007] FIG. 3 is a schematic of an exemplary embodiment of a system
for removing water from a flue gas and recovering water from a
desiccant stream.
[0008] FIG. 4 is a schematic of another exemplary embodiment of the
system of FIG. 3.
[0009] FIG. 5 is a schematic of an exemplary embodiment of a system
for removing water from a flue gas and recovering water from a
desiccant stream.
[0010] FIG. 6 is a schematic of another exemplary embodiment of the
system of FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
[0011] FIG. 1 is a schematic of an exemplary embodiment of a water
recovery system 10 for recovering water from a flue or exhaust gas
12 and removing water from a desiccant stream. System 10 may be
used to recover water from a flue gas produced using a fossil fuel
to generate power such as a combustion turbine power plant. One
such power plant is a Model SGT5-5000F sold by Siemens Power
Corporation, the assignee of the present invention. It will be
appreciated that embodiments of system 10 may be used with various
types of plants combusting fossil fuels in a combustion apparatus
or furnace such as coal-fired, oil-fired or biomass-fired plants.
Examples of combustion turbine power plants are disclosed in U.S.
Pat. No. 6,804,964, which is specifically incorporated herein by
reference in its entirety. Embodiments of the present invention
provide lower capital costs and improved water recovery rates
compared to conventional water recovery systems such as those
relying on large quantities of heat for evaporation.
[0012] Before flue gas 12 is released to the ambient atmosphere 13,
it is first treated by system 10. FIG. 1 illustrates that flue gas
12 exiting a combustion apparatus 11 may be directed to a water
stripper or absorber 20. Absorber 20 may define an interior portion
or plenum containing a fill material or media 22. Media 22 may be a
packed media based system using polyethylene, ceramic, metal or
other suitable materials. Media 22 provides surface area contact
between flue gas 12 and a flow of aqueous desiccant solution, for
example, entering absorber 20 through inlet connection 24. Other
desiccant solutions may be used comprising solvents and desiccant
solutes recognized by those skilled in the art. Embodiments of
system 10 may be adapted to use a solid form of desiccant, such as
a desiccant wheel exposing the desiccant to flue gas 12 in absorber
20. Absorber 20 may be one disclosed in pending application having
application Ser. No. 11/183,696 filed Jul. 18, 2005, which is
specifically incorporated herein by reference in its entirety.
[0013] In an exemplary embodiment, flue gas 12 passes into absorber
20 through a flue gas inlet 26. Flue gas 12 may enter absorber 20
at approximately 200.degree. F.-300.degree. F., or hotter and
contain approximately 5%-10% by volume of moisture, or more. It
will be appreciated that the flue gas temperature and moisture
content may vary as a function of ambient conditions, performance
objectives of the fossil fuel combustor and other operating
parameters of a fossil fuel burning plant. The desiccant solution
may flow into absorber 20 through inlet connection 24. Water is
chemically absorbed from flue gas 12 by the desiccant solution. The
desiccant solution may contain various desiccant compounds such as
calcium chloride (CaCl.sub.2), bromide, lithium chloride, various
hydroxides such as lithium hydroxide or sodium hydroxide, or
organic liquids such as polypropylene glycol, or mixtures thereof,
for example.
[0014] Moisture removal from flue gas 12 in absorber 20 is a highly
exothermic process. This process causes the desiccant solution
temperature, such as a CaCl.sub.2 aqueous solution, for example, to
increase and the concentration of CaCl.sub.2 in the solution to
decrease by weight. As the moisture content in the desiccant
solution increases, moisture in flue gas 13 exhausting to
atmosphere decreases. The temperature and concentration of
CaCl.sub.2 in the desiccant solution exiting absorber 20 depend on
the relative quantity and inlet temperature of the CaCl.sub.2
desiccant solution, and the moisture content and temperature of
flue gas 12 entering absorber 20.
[0015] The desiccant solution may exit absorber 20 through outlet
connection 28 and be pumped to a means for dehydrating the
desiccant solution while maintaining the water in a liquid phase,
such as reverse osmosis circuit 29. Reverse osmosis circuit 29 may
include a primary or first reverse osmosis apparatus 30 comprising
a membrane porous to water, but not to desiccant to separate at
least a portion of water from desiccant. The flow of desiccant
solution exiting absorber 20 may have no head pressure so
pressurization pump 32 may be provided to increase the pressure to
that required by primary reverse osmosis apparatus 30.
[0016] The heated flow of desiccant solution flowing into primary
reverse osmosis apparatus 30 will have a lower concentration of
CaCl.sub.2 than that of the desiccant solution entering absorber 20
through inlet connection 24. This is due to the absorption of
moisture into the desiccant solution in absorber 20. The
concentration of desiccant within the desiccant solution within
system 10 may be referred to herein in relativistic terms as being
"weak" or "strong" but is not intended to imply specific
concentrations.
[0017] Primary reverse osmosis apparatus 30 may be configured with
internal modules containing membranes that allow water to pass
there through while retaining desiccant materials at the molecular
level. This may be accomplished when pressure is applied to the
desiccant feed solution stream flowing through inlet fluid
connection 34 by a high-pressure pump such as pump 32. In an
exemplary embodiment, a secondary or second reverse osmosis
apparatus 40 may be provided that operates in conjunction with
primary reverse osmosis apparatus 30.
[0018] It will be appreciated that the employment of secondary
reverse osmosis apparatus 40 may be predicated on the efficiency of
absorber 20 and/or primary reverse osmosis apparatus 30. In this
respect, the type of desiccant used, the rate of desiccant solution
recirculation through absorber 20, contaminant level in the
desiccant feed solution stream and/or the design specifications of
reverse osmosis apparatus 30, 40 may influence the desirability of
using secondary reverse osmosis apparatus 40.
[0019] In an embodiment, first reverse osmosis system 30 may
produce an intermediate stream of desiccant solution that flows to
the second reverse osmosis apparatus 40. The intermediate stream
may have an intermediate concentration of desiccant that is greater
than the concentration of desiccant in the desiccant solution
entering the first reverse osmosis apparatus 30. The intermediate
stream may flow into second reverse osmosis apparatus 40 through
fluid connection 42. The desiccant solution flowing out of second
reverse osmosis apparatus 40 may have a concentration of desiccant
that is greater than the concentration of desiccant in the
desiccant solution flowing into the first reverse osmosis apparatus
30 from absorber 20.
[0020] Additional reverse osmosis apparatus may be used as desired
to perform the separation of desiccant from liquid water in
sequential or parallel stages. For example, a third reverse osmosis
apparatus 41 and respective fluid connections 43 are shown in
phantom in FIGS. 1 & 2. Reverse osmosis systems or apparatus
30, 40, 41 may be commercially available ones such as suitably
adapted FlowMAX reverse osmosis systems available from
USFilter.
[0021] After entering primary reverse osmosis apparatus 30, none,
all or a portion of the desiccant solution stream may be directed
by control valve 31 to flow into secondary reverse osmosis
apparatus 40 through connection 42. Control valve 31 may regulate
the amount of flow between secondary reverse osmosis apparatus 40
and connection 48. The amount of desiccant solution flow from
apparatus 30 to apparatus 40 may depend on the amount of desiccant
concentration reduction within apparatus 30. Apparatus 30, 40 may
produce a flow of recovered water 44 that may be used for various
purposes such as within other systems of a power plant. A control
valve 33 may regulate the amount of flow between apparatus 40 and
apparatus 41.
[0022] A respective flow of strong desiccant solution may exit
apparatus or systems 30, 40 into connection 48 that are pumped by a
supply pump 50 to a heat exchanger or desiccant cooler 52. Cooler
52 may be connected to an outside heat sink or cooling source (not
shown) for cooling the flow of strong desiccant solution to a
desired temperature for optimizing absorption within absorber
20.
[0023] FIG. 2 illustrates an exemplary embodiment of system 10 with
like components to those of FIG. 1 having like reference numerals.
FIG. 2 illustrates that a weak desiccant solution flows out of
absorber 20 through outlet connection 28 to control valve 56, which
may be automatically or manually controlled to regulate the volume
of desiccant solution flowing through system 10. For example, it
may be desirable to allow all or a portion of the weak desiccant
solution to bypass reverse osmosis circuit 29 and flow directly to
a mixer or connector 60 through fluid connection 62. In this
respect, fluid connections 28, 62, 48, 24 may constitute a main
fluid connection between an outlet of absorber 20 and an inlet of
the absorber 20.
[0024] Fluid connection 64 allows for a flow of desiccant solution
to pass through pressurization pump 32 into reverse osmosis circuit
29. In this respect, it may be desirable to allow a portion of weak
desiccant solution exiting absorber 20 to flow directly to mixer 60
so the weak desiccant solution mixes with a flow of strong
desiccant solution flowing from reverse osmosis circuit 29 through
fluid connection 66 to mixer 60. This allows for optimizing the
desiccant concentration flowing through desiccant cooler 52 back to
absorber 20 as a function of various operating parameters of a
fossil fuel burning plant, system 10 and desiccant chemistry, for
example.
[0025] FIG. 3 illustrates an exemplary embodiment of system 10 with
like components to those of FIG. 1 having like reference numerals.
FIG. 3 illustrates that weak desiccant solution exiting absorber 20
may be pumped by a pressurization pump 32 through fluid connection
68 to a means for dehydrating the desiccant solution while
maintaining the water in a liquid phase, such as a crystallization
circuit 70. Crystallization circuit 70 may include a heat exchanger
71 that provides a first stage of cooling the flow of desiccant
solution, which may then flow through fluid connection 72 to a
desiccant crystallizer 74.
[0026] The desiccant solution entering crystallizer 74 may be
supersaturated whereby the dissolved desiccant ions are susceptible
to separation from water and forming crystals. Crystallizer 74 may
be a heat exchanger having an outside heat sink or cooling source
(not shown) sufficient to cause the concentration of desiccant in
the desiccant solution to crystallize. In the embodiments of FIGS.
3 and 4, it is anticipated that crystallizer 74 will approach or
achieve 100% crystallization of desiccant within the desiccant
solution.
[0027] The crystallized desiccant solution may flow through fluid
connection 76 to a liquid/solid separator 78. Separator 78 may
comprise appropriate filters or be a centrifuge for separating the
crystals from water as recognized by those skilled in the art.
Separator 78 may include continuously or intermittently backwashed
filters or centrifuges ("spinners") such as those employed in water
treatment or salt production. Separator 78 produces a flow of
recovered water 44 that may be pumped to other power plant systems
as desired. Separator 78 may also produce a flow of crystallized
desiccant solution or slurry containing a mixture of desiccant
crystals and solution.
[0028] The flow of crystallized desiccant solution may flow from
separator 78 through fluid connection 80 to heat exchanger 71
wherein the solution is reheated. Heat exchanger 71 may
sufficiently reheat the crystallized desiccant solution to melt the
crystals so that a strong desiccant solution flows from heat
exchanger 71 through fluid connection 82. The strong desiccant
solution may be directed to fluid connection 48 by valve or mixer
60 and pumped by pump 50 back to absorber 20. After dilution of the
strong desiccant solution due to absorption of water in absorber
20, the solution exiting absorber 20 may be cooled as desired,
upstream of crystallization circuit 70.
[0029] FIG. 4 illustrates an exemplary embodiment of system 10 with
like components to those of FIG. 3 having like reference numerals.
FIG. 4 illustrates that a weak desiccant solution flows out of
absorber 20 through outlet connection 28 to control valve 56, which
may be automatically or manually controlled to regulate the volume
of desiccant solution flowing through system 10. For example, it
may be desirable to allow all or a portion of the weak desiccant
solution to bypass crystallization circuit 70 and flow directly to
a mixer or connector 60 through fluid connection 62.
[0030] Fluid connection 64 allows for a flow of desiccant solution
to pass through pressurization pump 32 into crystallization circuit
70. In this respect, it may be desirable to allow a portion of weak
desiccant solution exiting absorber 20 to flow directly to mixer 60
so the weak desiccant solution mixes with a flow of strong
desiccant solution flowing from crystallization circuit 70 through
fluid connection 82 to mixer 60. This allows for optimizing the
desiccant concentration flowing back to absorber 20.
[0031] It will be appreciated that the embodiments of FIGS. 3 and 4
illustrate a recuperative heat exchanger 71 within which the
hotter, weak desiccant solution flowing into heat exchanger 71 from
absorber 20 exchanges heat with the cooler, strong desiccant
solution flowing into heat exchanger 71 from liquid/solid separator
78. Alternate embodiments allow for separate heat exchanging
systems to be used to perform the cooling and heating of respective
streams of weak and strong desiccant solution, respectively.
Additional crystallizers 74 and separators 78 may be used to
perform additional stages of cooling the desiccant solution and
separating crystallized desiccant from water.
[0032] FIG. 5 illustrates an exemplary embodiment of system 10 with
like components to those of FIG. 1 having like reference numerals.
FIG. 5 illustrates that weak desiccant solution exiting absorber 20
may be pumped by a forwarding pump 90 through fluid connection 92
to a means for dehydrating the desiccant solution while maintaining
the water in a liquid phase, such as a crystallization circuit 94.
Crystallization circuit 94 performs a first stage of separation. In
this respect, weak desiccant solution exiting absorber 20 passes
through a heat exchanger 96, which may be a recuperative heat
exchanger, and is cooled.
[0033] The desiccant solution then passes through a desiccant
sub-cooler or heat exchanger 98 having an outside heat sink or
cooling source (not shown), such as forced air cooling, local water
sources or condensate from a power plant. Heat exchanger or
desiccant sub-cooler 98 cools the desiccant solution sufficiently
to cause at least a portion of the desiccant in the desiccant
solution to crystallize. The temperature to which the desiccant
solution is cooled by heat exchanger 98 may depend on the amount of
water in the aqueous desiccant solution. It may be desirable to
cool the desiccant solution as far as possible in heat exchanger 98
under ambient conditions to promote crystallization of the
desiccant.
[0034] In the embodiments of FIGS. 5 and 6, it is anticipated that
less than 100% of the desiccant within the desiccant solution will
crystallize. In this respect, the desiccant solution may flow from
heat exchanger 98 to a separator 100, which may be a cyclone
separator for separating the fraction of crystallized desiccant
from the desiccant solution. It will be appreciated that heat
exchanger 98 and separator 100 may be one device that performs
cooling and crystal separation. For example, a properly configured
commercially available device referred to in the industry as a
Spiractor.RTM. may be used depending on the kinetics of the
desiccant crystal formation.
[0035] The affects of heat exchanger 98 and separator 100 are to
produce separate streams of solution having different
concentrations of desiccant. In this aspect, a first stream may be
produced containing a relatively weaker concentration of desiccant
and hence a higher vapor pressure. The first stream may exit
separator 100 and flow through fluid connection 102 back to
recuperative heat exchanger 96.
[0036] The first stream may flow through recuperative heat
exchanger 96 where it may be further heated then pass through fluid
connection 110 to a flash tank 112. Flash tank 112 performs a
second stage of separation of water from desiccant. The attributes
of the first stream, i.e., heated with a diluted concentration of
desiccant, improve the ability of the water within the desiccant
solution to flash within flash tank 112. A cooler-condenser 114 may
be provided that is in fluid connection with flash tank 112 for
condensing vapor from flash tank 112.
[0037] Under certain ambient conditions, such as during relatively
low ambient temperatures, cooler-condenser 114 may be used
advantageously to pull a low vacuum through fluid connection 116,
which allows for pulling a high volume of steam off flash tank 112.
This may increase the amount of recovered water 44 available for
use in other parts of a power plant. It will be appreciated that
flash tank 112 and cooler-condenser 114 may be operated at various
pressures including sub-atmospheric and super-atmospheric.
[0038] A stream of strong desiccant solution may flow from flash
tank 112 through fluid connection 120 and a mixer or connector 60
to a supply pump 122, which pumps the stream to a heat exchanger or
desiccant cooler 124. Desiccant cooler 124 may have an outside heat
sink or cooling source (not shown), such as forced air cooling,
local water sources or condensate from a power plant. Desiccant
cooler 124 cools the stream of strong desiccant solution
sufficiently for mixing with the second stream of desiccant
solution entering fluid connection 126 via connector 106 from
separator 100.
[0039] The second stream of desiccant solution may be produced by
separator 100 and contain a relatively higher concentration of
desiccant, some of which may be crystallized. The second stream may
be a two-phase flow, i.e., a desiccant rich slurry or concentrated
brine containing crystallized desiccant and dissolved desiccant in
solution. The mixed desiccant solutions then flow through fluid
connection 24 into absorber 20. Crystallized desiccant within the
mixed desiccant solution may be dissolved by heat within absorber
20 or by absorption of water within absorber 20.
[0040] FIG. 6 illustrates an exemplary embodiment of system 10 with
like components to those of FIG. 5 having like reference numerals.
FIG. 6 illustrates that a weak desiccant solution flows out of
absorber 20 through outlet connection 28. The weak desiccant
solution may be pumped by pump 90 to control valve 56, which may be
controlled automatically or manually to regulate the volume of
desiccant solution flowing through system 10. For example, it may
be desirable to allow all or a portion of the weak desiccant
solution to bypass crystallization circuit 94 and flow directly to
a mixer or connector 60 through fluid connection 62.
[0041] Fluid connection 64 allows for a flow of desiccant solution
to pass through into crystallization circuit 94. In this respect,
it may be desirable to allow a portion of weak desiccant solution
exiting absorber 20 to flow directly to mixer 60 so the weak
desiccant solution mixes with strong desiccant solution exiting
crystallization circuit 94 through fluid connection 120 to mixer
60. This allows for optimizing the desiccant concentration flowing
to absorber 20, performance of crystallization circuit 94 and
flashing within flash tank 112. It will be appreciated that the
crystallization circuit 94 of FIG. 6 may operate the same as that
of FIG. 5.
[0042] Alternate embodiments allow for independent heat exchangers
to be used in lieu of a recuperative heat exchanger 96 and
additional desiccant sub-coolers 98 and separators 100 may be used
to perform additional stages of crystallizing the desiccant
solution and separating crystallized desiccant from the desiccant
solution. It will also be appreciated that embodiments of the
invention may combine one or more reverse osmosis apparatus 30, 40
with the crystallization circuits 70, 94 to optimize the
dehydration of desiccant solution and water recovery under
different operating conditions of system 10.
[0043] While the preferred embodiments of the present invention
have been shown and described herein, it will be obvious that such
embodiments are provided by way of example only. Numerous
variations, changes and substitutions will occur to those of skill
in the art without departing from the invention herein.
Accordingly, it is intended that the invention be limited only by
the spirit and scope of the appended claims.
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