U.S. patent application number 14/955892 was filed with the patent office on 2017-06-01 for combination multi-effect distillation and multi-stage flash evaporation system.
The applicant listed for this patent is KUWAIT INSTITUTE FOR SCIENTIFIC RESEARCH. Invention is credited to ESSAM EL-DIN FARAG EL-SAYED.
Application Number | 20170151507 14/955892 |
Document ID | / |
Family ID | 57705792 |
Filed Date | 2017-06-01 |
United States Patent
Application |
20170151507 |
Kind Code |
A1 |
EL-SAYED; ESSAM EL-DIN
FARAG |
June 1, 2017 |
COMBINATION MULTI-EFFECT DISTILLATION AND MULTI-STAGE FLASH
EVAPORATION SYSTEM
Abstract
The combination multi-effect distillation and multi-stage flash
evaporation system integrates a multi-stage flash (MSF) evaporation
system with a multi-effect distillation (MED) system such that the
flashing temperature range of the MSF process is shifted upward on
the temperature scale, while the MED distillation process operates
in the lower temperature range. The multi-stage flash evaporation
system includes a plurality of flash evaporation/condensation
stages, such that the multi-stage flash evaporation system receives
a volume of seawater or brine from an external source and produces
distilled water. The multi-effect distillation system includes a
plurality of condensation/evaporation effects, such that the
multi-effect distillation system receives concentrated brine from
the multi-stage flash desalination system and produces distilled
water.
Inventors: |
EL-SAYED; ESSAM EL-DIN FARAG;
(ONTARIO, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KUWAIT INSTITUTE FOR SCIENTIFIC RESEARCH |
SAFAT |
|
KW |
|
|
Family ID: |
57705792 |
Appl. No.: |
14/955892 |
Filed: |
December 1, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 61/027 20130101;
C02F 1/041 20130101; Y02A 20/128 20180101; C02F 1/048 20130101;
C02F 2103/08 20130101; B01D 61/145 20130101; C02F 2301/08 20130101;
B01D 1/26 20130101; C02F 1/442 20130101; B01D 61/147 20130101; C02F
1/06 20130101; C02F 9/00 20130101; B01D 1/2884 20130101; B01D
2311/268 20130101; Y02A 20/131 20180101; B01D 2311/2669 20130101;
C02F 2303/22 20130101; C02F 1/444 20130101; B01D 3/145 20130101;
B01D 1/2887 20130101; C02F 1/04 20130101; B01D 2311/2673 20130101;
B01D 3/065 20130101 |
International
Class: |
B01D 3/06 20060101
B01D003/06; B01D 1/28 20060101 B01D001/28; C02F 1/44 20060101
C02F001/44; B01D 61/14 20060101 B01D061/14; C02F 1/06 20060101
C02F001/06; C02F 1/04 20060101 C02F001/04; B01D 1/26 20060101
B01D001/26; B01D 61/02 20060101 B01D061/02 |
Claims
1. A combination multi-effect distillation and multi-stage flash
evaporation system, comprising: a multi-stage flash evaporation
system comprising a plurality of flash evaporation/condensation
stages, said multi-stage flash evaporation system receiving a
volume of saltwater from an external source and producing distilled
water; and a multi-effect distillation system comprising a
plurality of condensation/evaporation effects and a final
condenser, said multi-effect distillation system receiving
concentrated brine from said multi-stage flash evaporation system
for further desalination thereof and producing a desalinated water
distillate.
2. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 1, wherein each of the flash
evaporation/condensation stages comprises a flash chamber and a
condenser, the condenser having at least one conduit having an
inlet and an outlet, the at least one conduit passing through the
plurality of flash chambers.
3. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 2, further comprising means
for extracting the volume of saltwater from the external source,
passing it through the final condenser and feeding the volume of
saltwater under pressure through the at least one conduit, the
means being in fluid communication with the inlet of the at least
one conduit.
4. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 3, further comprising means
for heating the volume of saltwater after the volume of saltwater
has been delivered through the at least one conduit and prior to
injection thereof into the flashing stage.
5. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 4, further comprising means
for extracting the distilled water from a last stage of the
multi-stage flash evaporation system, wherein the heated volume of
saltwater injected into the plurality of flash chambers is flashed
into vapor within the plurality of flash chambers, and the vapor
condenses on an external surface of the at least one conduit to
form the distilled water.
6. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 5, wherein the means for
extracting the volume of saltwater from an external source and
feeding the volume of saltwater under pressure through the at least
one conduit comprises at least one pump in fluid communication with
the inlet of the at least one conduit.
7. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 6, wherein the means for
heating the volume of saltwater comprises: a brine heater in fluid
communication with the outlet of the at least one conduit; and a
boiler for delivering first heating steam into the brine heater
after the volume of saltwater has been pre-heated by the at least
one conduit.
8. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 7, further comprising a
first desuperheater in communication with the heater for
selectively cooling the first heating steam prior to delivery
thereof into the brine heater, wherein a first portion of condensed
steam produced by the heater is recycled for use in the first
desuperheater and a second portion of the condensed steam produced
by the heater is recycled for use in the boiler.
9. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 8, further comprising a
second desuperheater for selectively cooling a second heating steam
produced by the boiler prior to delivery thereof into a first one
of the plurality of condensation/evaporation effects of said
multi-effect distillation system.
10. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 9, further comprising: a
plurality of feed heaters, wherein each said feed heater is in
communication with the at least one conduit and a respective one of
the effects; and a plurality of flash pots, each flash pot being in
communication with a respective effect and configured for pressure
equalization.
11. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 5, further comprising a
pre-treatment system for filtering the volume of saltwater prior to
delivery thereof to said multi-stage flash evaporation system.
12. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 11, wherein the
pre-treatment system comprises a nanofiltration membrane filter for
removal of hardness ions from the saltwater.
13. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 11, wherein the
pre-treatment system comprises a filtration system selected from
the group consisting of a low pressure microfiltration system, an
ultrafiltration membrane filtration system, and a combination
thereof.
14. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 11, wherein the pretreatment
system further comprises one or more valves to regulate the flow of
the saltwater stream passing through the membrane filtration system
and the remainder of the saltwater stream that is bypassing the
membrane filtration system.
15. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 5, further comprising a pump
in fluid communication with the at least one conduit and the inlet
to the first one of the plurality of condensation/evaporation
effects of the multi-effect distillation system, the pump being
configured for extracting the remaining concentrated brine in the
flash chamber of the last stage in the multi-stage flash
evaporation and delivering it under pressure in two portions,
wherein a first portion thereof circulates in the multi-stage flash
evaporation system for further desalination, and a second portion
thereof passes to the first effect of the multi-effect distillation
system for further desalination.
16. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 5, further comprising a
thermal vapor compressor in fluid communication with a final one of
the plurality of condensation/evaporation effects of said
multi-effect distillation system.
17. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 5, wherein the means for
heating the volume of brine comprises: a brine heater in fluid
communication with the outlet of the at least one conduit; and a
mechanical vapor compressor in fluid communication with at least
one of the flash evaporation stages for delivering vapor generated
in the last flash evaporation stage into the brine heater as first
heating steam for heating the volume of brine after the volume of
brine has been heated by the at least one conduit.
18. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 17, further comprising a
desuperheater for selectively cooling the heating steam prior to
delivery thereof into the brine heater.
19. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 18, wherein the
desuperheater is in communication with the brine heater and at
least a portion of condensed steam produced by the brine heater is
recycled for use in the desuperheater.
20. The combination multi-effect distillation and multi-stage flash
evaporation system as recited in claim 19, further comprising a
pre-treatment system for filtering the volume of saltwater prior to
delivery thereof to said multi-stage flash evaporation system.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to desalination, and
particularly to a system for producing desalinated water from
saltwater, such as seawater using both multi-effect distillation
and multi-stage flash evaporation.
[0003] 2. Description of the Related Art
[0004] A falling film evaporator is an industrial device to
concentrate solutions, especially with heat sensitive components.
The evaporator is a special type of heat exchanger. In general, the
evaporation takes place on the outside surfaces of horizontal or
vertical tubes, although it should be noted that there are also
applications where the process fluid evaporates inside vertical
tubes. In all cases, the process fluid to be evaporated flows
downwards by gravity as a continuous film. The fluid creates a film
along the tube walls, progressing downwards, hence the name
"falling film".
[0005] In a falling film evaporator, the fluid distributor must be
designed carefully in order to maintain an even liquid distribution
for all tubes along which the solution falls. In the majority of
applications, the heating medium is placed inside the tubes, thus
high heat transfer coefficients can be achieved. In order to
satisfy this requirement, condensing steam is commonly used as a
heating medium.
[0006] For internally evaporating fluids, separation between the
liquid phase (i.e., the solution) and the gaseous phase takes place
inside the tubes. In order to maintain conservation of mass as this
process proceeds, the downward vapor velocity increases, increasing
the shear force acting on the liquid film and therefore also the
velocity of the solution. The result can be a high film velocity of
a progressively thinner film, resulting in increasingly turbulent
flow. The combination of these effects allows very high heat
transfer coefficients.
[0007] The heat transfer coefficient on the evaporating side of the
tube is mostly determined by the hydrodynamic flow conditions of
the film. For low mass flows or high viscosities, the film flow can
be laminar, in which case heat transfer is controlled purely by
conduction through the film. Therefore, in this condition, the heat
transfer coefficient decreases with increased mass flow. With
increased mass flow, the film becomes wavy laminar and then
turbulent. Under turbulent conditions, the heat transfer
coefficient increases with increased flow. Evaporation takes place
at very low mean temperature differences between heating medium
(i.e., process stream) and film liquid, typically between 3 K and 6
K, thus such devices are ideal for heat recovery in multi effect
processes.
[0008] A further advantage of the falling film evaporator is the
very short residence time of the liquid and the absence of
superheating of the same. The residence time inside the tubes is
typically measured in seconds, making it ideal for heat-sensitive
products such as milk, fruit juice, pharmaceuticals, and many other
mass-produced liquid products. Falling film evaporators are also
characterized by very low pressure drops, thus they are often used
in deep vacuum applications as well.
[0009] However, due to the intimate contact of the film liquid with
the heating surface, such evaporators are susceptible to fouling
from precipitating solids; liquid velocity, typically low at the
top rows of a bank of horizontal tubes, is usually not sufficient
to perform an effective self-cleaning of the tubes. Falling film
evaporators are therefore typically used only with clean,
non-precipitating liquids.
[0010] Falling film evaporation is the primary principle used in
multi-effect distillation (MED) systems (sometimes also referred to
as "multiple-effect distillation systems"). Multi-effect
distillation is a distillation process often used for sea water
desalination. It consists of multiple stages or "effects". In each
effect, the seawater feed falls as a film over the outside surfaces
of the tubes and is heated by steam inside the tubes. Some of the
falling water film evaporates, and this vapor flows into the tubes
of the next effect, heating and evaporating more water. Each effect
essentially reuses the energy from the previous effect. Although
the tubes can be submerged in the feed water, it is far more common
that the seawater feed is sprayed on the top of a bank of
horizontal tubes, and then drips from tube to tube until it is
collected at the bottom of the effect.
[0011] FIG. 2 illustrates a typical prior art multi-effect
distillation evaporator 100. In the first effect 102, seawater is
fed, via an inlet 108, to one or more sprayers or nozzles 110
positioned within the first effect 102. Heated steam, produced by
an external boiler or the like, is fed through a tube 112. As the
sprayed seawater lands on the external surface of the tube 112, and
forms a thin liquid film thereon, heat transferred from the heating
steam causes the seawater to evaporate, forming water vapor V. The
heat transfer cools the steam, producing condensate in the tube
112, which is then returned back to the boiler for subsequent
re-heating. The seawater which does not evaporate (indicated as S
in FIG. 2), drips from one portion of the tube 112 to another (or
from tube to tube, in the case where multiple such tubes are used),
until it is collected at the bottom 114 of the first effect. A pump
116 then delivers this collected seawater into the second effect
104, where it is sprayed by sprayers or nozzles 120, similar to the
spraying in the first effect 102.
[0012] The water vapor V from the first effect is transferred by a
second tube 118 into the second effect and acts in a similar manner
to the steam passing through tube 112 in the first effect, except
that the condensate in second tube 118, rather than being returned
to the boiler, is drawn out through a product conduit 124, where
distilled water is collected. The seawater S which does not
evaporate into water vapor V in the second effect 104, once again,
falls from tube portion to tube portion (or tube to tube) to be
collected on the bottom 122 of the second effect 104. A pump 126
then delivers this collected seawater into the third effect 106,
where it is sprayed by sprayers or nozzles 128, similar to the
spraying in the first and second effects 102, 104.
[0013] The water vapor V from the second effect 104 is transferred
by a third tube 130 into the third effect 106 and acts in a similar
manner to the steam passing through tube 112 in the first effect
102 and the heated vapor passing through the second tube 118 in
second effect 104. In the third effect 106, the condensate in third
tube 130 is drawn out through the product conduit 124, where it
mixes with the desalinated water from the second effect 104 to be
collected. The seawater S which does not evaporate into water vapor
V in the third effect 106, once again, falls from tube portion to
tube portion (or tube to tube) to be collected on the bottom 132 of
the third effect 106, where it is then pumped, by pump 134, to the
next effect. Although only three effects 102, 104, 106 are shown in
FIG. 2, it should be understood that this is shown for exemplary
and illustrative purposes only. An example of a conventional
multi-effect distillation system is shown in U.S. Pat. No.
3,481,835, which is hereby incorporated by reference in its
entirety.
[0014] Conventional multi-effect distillation systems, such as the
above, which generally rely on falling film evaporation, suffer
from a number of drawbacks, each of which typically limits the
design capacity of the units and the maximum permissible operating
temperatures. On a broad level, many MED designs involve complex
and often circuitous paths for heated seawater and vapor to
minimize usage of pumps, maintain wettability of the tubes to avoid
scaling, and to maximize energy recovery from the flashing brine
and distillate. The farther the pumps, vessels, water routes and
vapor routes are from minimal, optimized paths, the more the design
suffers from excessive losses.
[0015] In addition to multi-effect distillation systems,
multi-stage flash (MSF) evaporation is also relatively commonly
used to produce desalinated water from saltwater sources, such as
seawater. FIG. 3 shows a conventional prior art MSF system or plant
200, where feed seawater or brine enters the system under pressure,
being drawn into the plant 200 via a pump 228 or the like. The
seawater or brine is transported, under pressure, through conduits
or pipes 232 to a brine heater 214, which then delivers heated
brine to flash chambers 216. A steam generator 212, which is a
separate simple steam power plant, external to the MSF system,
supplies the brine heater 214 with the heating steam needed to heat
up the brine. The steam generator 212 is a simple steam power plant
(preferably a Rankine cycle power plant), and consists of a pump
240, a boiler 242, and a steam turbine 244, in addition to the
condenser 214, which also acts as the brine heater. It should be
understood that the steam turbine 244 shown in FIG. 3 is not a
component of a typical MSF process, but is merely shown in this
example as part of an exemplary plant utilizing MSF. The steam
passing to the brine heater 214 may be extracted from a turbine,
such as steam turbine 244, or may be fed directly from boiler 242.
It should be understood that the simplified illustration of FIG. 3
is provided merely to describe a conventional MSF process and
system. Typically, a de-superheater would also be used to condition
the steam, whether extracted from a turbine or passing directly
from a boiler, prior to entering the brine heater 214, thus
ensuring that the steam is saturated and not superheated.
Conventional MSF systems are well-known. U.S. Pat. Nos. 3,966,562
and 8,277,614, both of which are hereby incorporated by reference
in their entirety, show conventional MSF systems.
[0016] As shown, the seawater or brine may also be first drawn
through a cooler 230 in order to reduce the temperature of the
feed, thus also the temperature of the last stage. The brine is
then passed through the feed heater conduits 232. The feed heaters
are condenser type heat exchangers where feed is heated by the heat
released from condensing the vapor flashed off in each stage. Feed
brine reaches the first stage at an elevated temperature, however
it is not high enough to start flashing, and therefore, additional
heat must be supplied to the brine. The brine heater 214 receives
steam from the external steam generator 212, and elevates the brine
temperature to the level suitable to start flashing. The brine is
then injected into the flash chambers 216. It should be understood
that the number of flash chambers 216 shown in FIG. 3 is shown for
exemplary purposes only, and is a simplification of the number of
flash stages. Typical MSF plants have between fifteen and forty
stages or flash chambers. The brine delivered by the heater 214
typically has a temperature of between approximately 90.degree. C.
and 120.degree. C., depending upon the chemical treatment or scale
prevention technique used, the quality of heating steam, and the
ejection system maintaining pressure in each stage.
[0017] The operating pressure in the flash chambers 216 is lower
than that in the heater, thus causing the heated brine to rapidly
boil or "flash" into vapor. Typically, only a small percentage of
this water is converted into vapor. Consequently, the remaining
water will be sent through a series of additional stages or flash
chambers 216, as shown, each possessing a lower operating pressure
than the previous chamber. The brine is delivered through each
successive flash chamber 216 or stage through any conventional
method. As vapor is generated, it is condensed in the same stage or
flash chamber on the pipes 232, which run through each chamber. The
condensed water is then collected by collection trays 218 and is
removed by a pump 220 to produce a stream of desalinated water 222.
The pipes 232 and trays 218 form the condensers for each flash
stage. The remaining brine with a high saline concentration may be
drawn out by a separate pump 224, and removed as waste at 226.
[0018] In the MSF process, heat transfer surfaces, which are on the
brine side, are never subject to change of phase and are always
kept wet and relatively free of scale precipitation by effective
scale control techniques, typically involving chemical treatment of
feed water and on-line mechanical cleaning. Flashing of the brine
occurs at a safe distance from heat transfer tubes. This procedure
makes the MSF process fairly protected from scale formation and
precipitation up to the temperatures at which sulfate-based scales
begin to form (i.e., above 121.degree. C.).
[0019] In the MED process, on the other hand, evaporation takes
place directly on the outside surfaces of the heat transfer tubes
as the brine film reaches the liquid superheat temperatures needed
for the change of phase to occur. Such an evaporation mechanism
makes heat transfer surfaces highly vulnerable to scale formation
and precipitation, especially since only chemical treatment can be
used to retard scale formation while on-line mechanical cleaning is
not possible. This situation imposes severe restrictions on the
maximum practical operating temperatures in the MED process, which
must be kept within a safe range (i.e., below 70.degree. C.).
[0020] The conventional MSF process suffers from three primary
sources of thermodynamic loss, namely boiling point elevation loss,
pressure drop loss, and non-equilibrium loss. The boiling point
elevation loss is due to the presence of salts at high
concentrations in the brine, thus it is a loss that must be present
in any process involving boiling or change of phase and its value
depends on the state of the brine solution in terms of its
temperature and concentration. Boiling point elevation loss
increases with temperature as well as with concentration. In the
MSF process, both driving forces of the boiling point elevation act
conversely, since flashing brine temperature decreases while its
concentration increases as the brine flows toward the lower
temperature stages. Consequently, the resulting effect of this
behavior minimizes the variations in the boiling point elevation
across the MSF stages.
[0021] The pressure drop caused by the flow of vapor through the
demisters and through the tube bundle results in vapor expansion,
which is accompanied by a drop in its corresponding saturation
temperature. This is known as pressure drop loss and it is far less
in magnitude as compared with boiling point elevation or
non-equilibrium losses, and it usually increases as the brine flows
toward the lower temperature stages. The non-equilibrium loss,
unlike the previous two losses, is an inherited characteristic of
the MSF process. The amount of this loss is inversely proportional
to the stage thermal level and it is directly proportional to the
flashing brine depth. To illustrate such a characteristic, one can
define the vapor equilibrium temperature in the brine pool at a
given depth below the surface as
T b * = T 0 * + T * P .gamma. h b , ##EQU00001##
where T.sub.0* is the vapor equilibrium temperature at the stage
pressure,
T * P ##EQU00002##
represents the rate of change in vapor saturation temperature vs.
pressure, and .gamma.h.sub.b represents the hydrostatic pressure in
the brine pool at a given depth h.sub.b below the surface.
[0022] FIG. 4 shows plots for T.sub.b* at different values of
h.sub.b over typical flashing ranges of the conventional MSF
process. FIG. 4 shows that the effect of the brine depth on vapor
equilibrium temperature in the brine pool is quite insignificant
for high thermal level stages and it becomes rapidly significant
for the lower thermal level stages. In other words, taking
(T.sub.b).sub.in and (T.sub.b).sub.out as the brine bulk
temperatures at the stage inlet and outlet less the boiling point
elevation, then (T.sub.b).sub.in>T.sub.b*; is a condition
necessary for evaporation to occur at any point on the brine
surface and below to a maximum depth of h.sub.b. For high thermal
level stages, this condition is usually furnished even for the
maximum submergence in the brine pool. However, for evaporation to
remain effective as the brine travels through the stage towards its
outlet, the condition (T.sub.b).sub.out>T.sub.b* must be
sustained for a significant depth in the brine pool. As the brine
flows toward lower thermal level stages, the condition
(T.sub.b).sub.in>T.sub.b*>(T.sub.b).sub.out becomes prevalent
even for minimum brine depth, which indicates that evaporation may
take place only near the surface at the stage inlet and it lessens
until it diminishes as the brine approaches the stage outlet, thus
making a significant part of the stage nonproductive.
[0023] FIG. 5 is a plot of the three losses and the resulting total
thermodynamic losses across the stages of a typical prior art MSF
unit. FIG. 5 shows the changes in the relative magnitudes of these
losses as fractions of the total average temperature difference
between flashing and recycling brine along the stages of the MSF
unit. Contrary to MSF, non-equilibrium thermodynamic losses are
non-existent in the MED process. This is because evaporation occurs
in the superheated liquid film rather than by flashing of the
liquid pool. On the other hand, both boiling point elevation and
pressure drop losses exist to an extent similar to that of the MSF
process. However, these losses have far less significance as far as
the thermal performance of the MED process is concerned, mainly
because the evaporation temperature range is already limited to a
narrow low temperature stretch, and also because the overall heat
transfer coefficient at these low thermal levels is almost double
that of the MSF process.
[0024] The combined effect of these losses is illustrated by the
per-stage and the accumulated mass flow rates of product distillate
shown in FIG. 6 for a typical prior art MSF unit. FIG. 6 clearly
shows that stage productivity is directly dependent on stage
thermal level, and that low stage productivity in the stages of the
lower temperature range in the MSF process is an inherent
characteristic.
[0025] Two basic quantities must be first established when an MSF
or an MED plant is under consideration, namely the plant's
production capacity and the available thermal energy in the form of
low-grade steam required to drive any of these plants to produce
the desired output. The guidelines for measuring MSF and MED plant
effectiveness, or the process potential, are usually based on these
two quantities and are known in combination as the gain output
ratio (GOR) or the performance ratio (PR). The GOR is defined as
the mass ratio between the product distillate (in kilograms per
unit time) and the steam supplied to the process (also in kilograms
per unit time). The PR is defined either as the amount of
distillate mass (in kilograms per a predefined quantity of latent
heat due to condensation of the heating steam, measured in
kilojoules) or the amount of heat supplied (in kilojoules) to
produce one kilogram of distillate. These ratios depend on several
parameters, some of which are the top brine temperature (TBT),
number of evaporation stages or effects, available flashing
temperature range, mass ratio of the brine subject to evaporation
and the product distillate, concentration of the brine, and
effectiveness of evaporation stage or effect. There are, however,
certain technical and economic limitations to the upper values of
the GOR or PR that can be achieved for any process. However, one
must be cautious when comparing these quantities (GOR or PR) for
MSF with that of MED, since heating steam conditions, and, hence,
the grades of energy supplied to each process, are usually quite
different. It would be desirable to be able to integrate MSF with
MED such that the flashing temperature range of the MSF process is
shifted upward on the temperature scale for better performance of
the MSF at relatively higher operating temperatures, while the MED
subunit incorporated into the MSF system operates in the lower
temperature range for better performance in this range.
[0026] Thus, a combination multi-effect distillation and
multi-stage flash evaporation system solving the aforementioned
problems is desired.
SUMMARY OF THE INVENTION
[0027] The combination multi-effect distillation and multi-stage
flash evaporation system integrates a multi-stage flash (MSF)
evaporation system with a multi-effect distillation (MED) system
such that the flashing temperature range of the MSF process is
shifted upward on the temperature scale (e.g., 70-120.degree. C.),
while the MED distillation process operates in the lower
temperature range (e.g., below 70.degree. C.). The multi-stage
flash evaporation system includes a plurality of flash
evaporation/condensation stages, such that the multi-stage flash
evaporation system receives a stream of saltwater (e.g., seawater
or brine) after being preheated by feed heaters of the multi-effect
distillation system, and produces pure distilled water. The
multi-effect distillation system includes a plurality of
condensation/evaporation effects, such that the multi-effect
distillation system receives the heated concentrated brine from the
multi-stage flash evaporation system, for further distillation, and
produces pure distilled water.
[0028] A brine heater is in fluid communication with the
multi-stage flash evaporation stages, and a boiler is provided for
delivering first heating steam into the brine heater for further
heating of the brine stream after the brine stream has been
preheated by the feed heaters of the multi-effect distillation
system and evaporation stages of the multistage flash evaporation
system. A first desuperheater may be provided for selectively
cooling and conditioning the first stream of heating steam prior to
infusion thereof into the brine heater. Preferably, a first portion
of condensed steam produced by the brine heater is recycled for use
in the first desuperheater. A second portion of the condensed steam
produced by the brine heater may be recycled for use in the
boiler.
[0029] Saltwater is delivered from an external source and a
pre-treatment system is further provided for filtering the
saltwater prior to delivery thereof to said feed heaters of the
multi-effect distillation system. The pre-treatment system may
selectively include, for example, a low pressure microfiltration or
ultrafiltration membrane system and a nanofiltration membrane
system. Additionally, a second desuperheater may be provided for
selectively cooling and conditioning second heating steam prior to
infusion thereof into a first one of the plurality of distillation
effects of the multi-effect distillation system. The second heating
steam may be produced by the boiler. Additionally, a brine
circulation pump is preferably provided between the multistage
flash evaporation system and the multi-effect distillation system
such that a first portion of unevaporated concentrated brine
delivered by the pump is circulated back into the multistage flash
evaporation system after mixing with pre-treated, pre-heated
saltwater, thus forming a continuous brine stream for further
heating, flashing and condensation in the multistage flash
evaporation system. A second portion of the unevaporated
concentrated brine is delivered to a first one of the plurality of
distillation effects of the multi-effect distillation system for
further distillation. Additionally, a pure water distillate stream
is drawn from the last stage of the multistage flash evaporation
system and is delivered to a first one of a plurality of
receptacles in the multi-effect distillation system for further
flashing and recovery of available latent heat of the pure water
distillate.
[0030] In an alternative embodiment, a thermal vapor compressor is
in fluid communication with a final one of the plurality of
evaporation effects of the multi-effect distillation system, such
that the thermal vapor compressor produces the second heating
steam. The thermal vapor compressor is operated by relatively
medium/low pressure motive steam provided by the boiler. In a
further alternative embodiment, a mechanical vapor compressor is in
fluid communication with at least one of the last flash evaporation
stages for infusing heating steam into the brine heater, rather
than using heating steam from the boiler.
[0031] These and other features of the present invention will
become readily apparent upon further review of the following
specification and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 diagrammatically shows a combination multi-effect
distillation and multi-stage flash evaporation system according to
the present invention.
[0033] FIG. 2 diagrammatically shows a conventional prior art
multi-effect distillation system.
[0034] FIG. 3 diagrammatically shows a conventional prior art
multi-stage flash evaporation system.
[0035] FIG. 4 is a graph showing plots for vapor equilibrium
temperature in a brine pool at a given depth below the surface for
varying depths, taken over typical flashing ranges for a
conventional prior art multi-stage flash evaporation process.
[0036] FIG. 5 is a graph showing plots of boiling point elevation
loss, pressure drop loss and non-equilibrium loss, along with a
resultant total thermodynamic loss, across the stages of a
conventional prior art multi-stage flash evaporation system.
[0037] FIG. 6 is a graph showing the combined effect of the losses
of FIG. 5, shown as per-stage and accumulated mass flow rates of
product distillate, for a conventional prior art multi-stage flash
evaporation system.
[0038] FIG. 7 diagrammatically shows an alternative embodiment of
the combination multi-effect distillation and multi-stage flash
evaporation system.
[0039] FIG. 8 diagrammatically shows a further alternative
embodiment of the combination multi-effect distillation and
multi-stage flash evaporation system.
[0040] Similar reference characters denote corresponding features
consistently throughout the attached drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] The combination multi-effect distillation and multi-stage
flash evaporation system 10, as shown in FIG. 1, combines a
multi-effect distillation (MED) system, similar to the MED system
100 of FIG. 2, with a multi-stage flash (MSF) evaporation system,
similar to the MSF evaporation system 200 of FIG. 3. The
multi-stage flash evaporation portion of system 10, shown in FIG.
1, begins with a mixture of seawater feed and recycled concentrated
brine entering the system under pressure, being drawn into conduits
or pipes 32 via a mixer 68 or the like. The seawater feed is drawn
from an outside source by a pump 28 and passes through the final
condenser 24 of the multi-effect distillation portion of system 10,
as will be described in detail below. Prior to injection into the
MSF process, the total volume (or, alternatively, only a first
portion) of seawater feed is preferably pre-treated by passage
through a filtering system 40 using a nanofiltration (NF) membrane
or the like. Selective flow control of the first portion of
seawater feed passing to the filtering system 40 may be provided by
any suitable type of valve 46. A second portion of seawater feed
bypasses the filtering system 40 through any suitable type of valve
42, as shown in FIG. 1. Filtering system 40 may be coupled
selectively with a secondary, low-pressure microfiltration (MF) or
ultrafiltration (UF) membrane filtration system 44. Untreated
seawater or rejected brine from the filtering system 40 may be
expelled from the system via outlet 48. The pre-treated seawater is
then passed to the MSF portion of system 10 through the feed
heaters 20 of the multi-effect distillation portion of system 10,
as will be described in detail below. It should be understood that
while seawater is discussed herein, other kinds of saltwater, e.g.,
brine, can be treated by the multi-effect distillation and
multi-stage flash evaporation system.
[0042] The pre-treated, pre-heated seawater stream joins the first
portion of the recycled concentrated brine stream from conduit or
pipe 64, and the two streams are then mixed together in mixer 68 or
the like. The mixture of seawater and brine is transported, under
pressure, through conduits or pipes 32 to a brine heater 14, which
then delivers heated brine to flash chambers 16. A boiler 12, which
combusts fuel to heat recycled condensate along with additional
makeup water, acts as the steam generator, supplying the brine
heater 14 with the heating steam needed to heat up the brine.
Following heat transfer to the brine, the steam condenses, and this
condensate follows conduit or pipe 52 back into boiler 12 for
recycling as steam. The condensate is pressurized by a pump 50.
Additionally, a desuperheater 54 may be provided, as shown. The
desuperheater 54 is used to inject controlled amounts of cooling
water (i.e., condensate selectively provided by pump 50 through
pipe or conduit 56) into the superheated steam flow to reduce or
control steam temperature.
[0043] The flash chambers 16 act in a manner similar to those of
the conventional MSF system 200 of FIG. 3, yielding desalinated
distilled water, which is drawn out of the MSF portion by pipe or
conduit 58. Recycle pump 60 passes first portion of the
concentrated brine through recycle pipe or conduit 64 to mix at 68
with the pre-treated, pre-heated seawater, under control of a valve
62. The remainder of the concentrated brine enters the feed water
inlet 36 of the MED portion of system 10, under control of valve
66.
[0044] The MED portion includes multiple effects 18, which operate
in a manner similar to the conventional multi-effect distillation
evaporator 100 of FIG. 2. As shown in FIG. 1, a portion of the
steam generated by boiler 12 may be diverted along pipe or conduit
70, supplying heated steam to the steam chest of the first effect.
A desuperheater 72, similar to desuperheater 54, may be provided in
pipe or conduit 70, with a portion of the condensate being
delivered thereto via pipe or conduit 74 and being pressurized by
pump 76.
[0045] Heating vapor for each further effect of the MED portion is
provided from vapor generated in the previous effect, after passing
through the feed heaters 20, to heat up a portion of the brine
entering from the feed water inlet 36 and converting it to vapor.
The condensed vapor from each effect 18, which is desalinated
water, is collected in the respective receptacle 22 via a pipe or
conduit 26, with the first receptacle receiving the distillate from
the MSF portion of system 10 via pipe or conduit 58. The vapor from
the final effect passes through the final condenser 24. After
condensing in the final condenser, the condensate is mixed with the
distillate stream from last receptacle 22 via pipe or conduit 34,
forming the final distillate product (i.e., desalinated water),
which is removed by distillate pump 80. Reject brine is removed
from final effect 18 by pump 78. Receptacles 22 are preferably
provided with flashing pots, such that the distillate from the MSF
portion, along with distillate from each subsequent effect of the
MED portion, is fed to each flashing pot associated with a
particular effect, where the pressure in the flashing pot is
maintained at a specific vacuum, thus causing flashing of the
distillate to occur at a desired rate. For example, pressure in the
first one of the plurality of flash pots is equalized with the
pressure in the first effect via pipe or conduit 30, and the first
flash pot receives the distillate from the MSF portion of system 10
via pipe or conduit 58. Pressure in the second flash pot is
equalized with pressure in the second effect via pipe or conduit
30, and the second flash pot receives distillate from the first
effect via pipe or conduit 26, along with remaining unevaporated
distillate from the first flash pot. This process continues until
the pressure in the last flash pot is equalized with the pressure
in the final condenser 24 via pipe or conduit 30. The last flash
pot receives distillate from the last effect via pipe or conduit 26
along with the remaining unevaporated distillate from the previous
flash pot.
[0046] System 10 shifts the flashing temperature range of the MSF
process upward on the temperature scale while incorporating an MED
subunit into the MSF system in the lower temperature range. Typical
MSF plants operate under normal conditions with a flashing
temperature range between about 40.degree. C. and 90.degree. C. In
system 10, though, maintaining a similar 50.degree. C. flashing
span, the same MSF portion of system 10 can be operated for a
flashing temperature range between 60.degree. C. and 110.degree.
C., leaving the low temperature range between 40.degree. C. and
60.degree. C. for the added MED portion. Effectively, this is an
expansion of the flashing temperature range similar to that in the
high temperature operation of some MSF plants, but with a far
better utilization of the flashing temperature range, especially
the lower temperature range.
[0047] In FIG. 1, the MED portion operates on the lower temperature
side of system 10. As described above, a portion of the
concentrated brine from the MSF and the entire MSF product
distillate continue to produce vapor by boiling and flashing
through the MED portion, while the remaining portion of the
concentrated brine is recycled back in the MSF portion after mixing
with the makeup seawater feed. It should be noted that apart from
the necessary changes in the process temperature and pressure
gradients, the MSF process in system 10 remains relatively
unchanged from a conventional MSF system, with the exception of the
heat rejection section in the conventional system, which is no
longer required in system 10. Instead, the heat rejection stages
are added to the heat recovery section.
[0048] In the alternative embodiment of FIG. 7, heat is recycled
within the MED portion. In FIG. 7, vapor in the lowest temperature
effect 18 and vapor flashed in the last receptacle 22 are recycled
back by the thermal vapor compressor (TVC) 84 to be used as the
heating steam driving the first effect. In the further alternative
embodiment of FIG. 8, heat is recycled in both the MED and MSF
portions. In FIG. 8, the system operates in a manner similar to
that of the system of FIG. 7, however a further appropriate amount
of vapor produced in the last few stages of the MSF is used to
replace the heating steam supplied to the brine heater 14. In FIG.
8, this is shown being performed by a mechanical vapor compressor
(MVC) 82, however it should be understood that any suitable type of
compressor, including a TVC, may be used in this recycling
process.
[0049] In the embodiments of FIGS. 7 and 8, the MSF portion is of
recycle type, but the heat rejection stages are joined with the
heat recovery stages. The MED portion still operates in a
conventional manner, where feed water is heated in the MED feed
heaters 20 and then mixed with recycled brine for further heating
in the MSF stages 16 until it reaches the brine heater 14. The
portion of the concentrated brine withdrawn from the MSF portion is
fed to the first MED effect for further boiling, flashing, and
evaporation.
[0050] The distillate from the MSF portion is fed to first flashing
pots 22, where the pressure therein is maintained at the vacuum of
the first effect, thus causing flashing of the distillate to occur
at the desired rate. The vapor released by flashing of the
distillate is passed on to join the vapor heating the feed in the
respective feed heater 20. The brine reject from each effect 18,
operating on the higher temperature side of the system, is passed
on to the subsequent effect to allow further boiling and flashing
and generation of vapors. Similarly, the product distillate of each
effect 18 is passed on to the next lower temperature flash pot 22
to allow recovery of its excess heat by partial flashing. The vapor
released in the last effect 18 can either be passed on to the final
condenser 24, where it condenses at the lowest process temperature,
and, thus, the lowest pressure, or alternatively be compressed by a
thermal vapor compressor (TVC) 84 or the like for reuse, as in the
alternative embodiments of FIGS. 7 and 8.
[0051] In order to show the effectiveness of the combination
multi-effect distillation and multi-stage flash evaporation system
10, Tables 1A and 1B below show sample performance characteristics
of conventional MED and MSF systems compared against the
combination MED-MSF desalination system operating as a heat-driven
system (i.e., the embodiment of FIG. 1); a TVC-driven MED portion
and heat-driven MSF portion of the combination MED-MSF desalination
system (i.e., the embodiment of FIG. 7); and a TVC-driven MED
portion and a MVC-driven MSF portion of the combination MED-MSF
desalination system (i.e., the embodiment of FIG. 8). Process
performance indicators include equivalent gain output ratio (GOR)
in unit mass of distillate per unit mass of equivalent amount of
heating steam (e.g., kg distillate/kg equivalent heating steam),
total energy input (thermal and electrical) in kWh per ton of
distillate, total exergy input (including the sum of all actual
useful energy based on the Second Law of thermodynamics) in kWh per
ton of distillate, and product water recovery ratios (distillate
mass/seawater makeup feed, or distillate mass/total seawater feed
including cooling). As can be seen in Tables 1A and 1B, the GOR is
significantly higher for the combination MED-MSF desalination
system embodiments compared against the conventional MED process
and the conventional MSF process. Further, the energy input and
exergy input are significantly lower for the combination MED-MSF
desalination system compared against the conventional MED process
and the conventional MSF process. The most significant of these
indicators is the exergy input, which shows that the heat-driven
MED-MSF combination is the most efficient of all three embodiments.
Furthermore, in terms of product water recovery ratio, the MED-MSF
combination is clearly superior compared to the conventional MED
process and the conventional MSF process. The product water
recovery ratio for the heat-driven MED-MSF combination system is
the highest in all three embodiments of the MED-MSF combination
system. With such superior performances and higher product water
recovery ratios, the combination MED-MSF desalination system
clearly outperforms the other systems and techniques in terms of
operating and overall product water unit costs.
TABLE-US-00001 TABLE 1A Comparison of the Combination MED-MSF
Desalination System Against Conventional MSF Number TVC of TBT
.degree. C./ Motive Stages or Last Stage or Effect Heating Steam
Effects Temp .degree. C. Steam .degree. C. .degree. C. System
Description MSF MED MSF MED MSF MED MED Conventional MED N/A 9 N/A
75/40 N/A 80 N/A System (FIG. 2) Conventional MSF 19 N/A 90.6/40.6
N/A 100 N/A N/A System (FIG. 3) Conventional MSF 23 N/A 110/40.6
N/A 120 N/A N/A System (FIG. 3) Combination MED-MSF 19 9 110/76.3
76.3/40 120 82.2 N/A System (FIG. 1) Combination MED-MSF 19 9
110/76.4 76.4/40.5 120 N/A 141.68 System (FIG. 7) Combination
MED-MSF 19 9 110/82.2 82.2/46 N/A N/A 141.68 System (FIG. 8)
TABLE-US-00002 TABLE 1B Comparison of the Combination MED-MSF
Desalination System Against Conventional MSF Equivalent Performance
Exergy Input Product Water GOR Ratio (Thermal + Recovery Ratio (Kg
distillate/ (kWh/ton Pumps, kWh/ton (ton distillate/ System
Description kg steam) distillate) distillate) ton seawater)
Conventional MED 7.71 82.87 11.56 0.398/0.0675 System (FIG. 2)
Conventional MSF 7.14 316.97 61.37 0.3745/0.0914 System (FIG. 3)
Conventional MSF 8.9 248.22 58.61 0.3745/0.1149 System (FIG. 3)
Combination MED-MSF 10.57 57.79 3.69 0.495/0.107 System (FIG. 1)
Combination MED-MSF 9.48 52.8 8.0 0.388/(N/A) System (FIG. 7)
Combination MED-MSF 15.4 58.6 14.73 0.4762/(N/A) System (FIG.
8)
[0052] It is to be understood that the present invention is not
limited to the embodiments described above, but encompasses any and
all embodiments within the scope of the following claims.
* * * * *