Process for desalinating water while producing power

A process and apparatus for desalinating seawater or brine and purifying water which contains minerals, salts, and other dissolved solids while simultaneously generating power. The salinous water is heated in a boiler to form steam and a concentrated brine. The concentrated brine is removed from the boiler, the steam produced in the boiler is washed with fresh water to remove trace salts and inorganic materials, and water bearing trace salts and inorganic materials are returned to the boiler. The washed steam is expanded across a turbine to generate electrical or mechanical power which is utilized as a product. The steam exhausted from the turbine is collected and condensed, and one portion of the condensed water is utilized as a fresh water product and another portion of the condensed water is used as the wash water to wash the steam produced in the boiler. Energy efficiency is improved by heat exchanging the hot concentrated brine against the salinous feed water or by flashing the brine to produce steam. Boiler scaling and corrosion may be controlled by feed water pretreatment. By utilizing distillation combined with power generation, demand for fresh water and power can be satisfied simultaneously.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to desalinating processes, and more 
particularly to a process for desalinating seawater or brine or purifying 
fresh containing minerals, salts, and other dissolved solids while 
simultaneously generating power. 
2. Brief Description of the Prior Art 
As world population increases, demand for fresh water and power will also 
increase. Pollutants and drought result in a shortage of fresh water in 
many locations. Therefore, it would be desirable to provide a process 
utilizing desalination and distillation combined with power generation 
whereby demand for fresh water and power can be simultaneously satisfied. 
Most previous methods of desalination have been stand-alone processes. 
Hence, they have focused upon energy efficiency to satisfy economics. 
Several of the commercial methods include reverse osmosis, evaporation, 
and vapor recompression. Dual purpose power plants have also been 
utilized. 
Reverse osmosis is a technology wherein fresh water is extracted from 
saline water by pressure. This is accomplished by circulating saline water 
under high pressure (i.e., 1000-2000 psig) around a loop. One portion of 
the loop is adjacent to a membrane. The membrane selectively allows water 
to pass through it while preventing the passage of most ions. Effectively, 
fresh water is squeezed from the saline water. Excellent energy efficiency 
can be achieved by this method. However, the membranes are prone to 
pluggage and in practice the fresh water produced is not completely free 
of dissolved salts. The present process, on the other hand, produces fresh 
water by a phase change and produces power. 
Evaporation is the boiling of salinous water by the addition of heat 
followed by the condensation of the steam by heat exchange. Evaporators 
may be classified as boiling or flashing. No work is performed by the 
system and a large amount of energy input is required. This method is the 
least energy efficient of the existing methods. The present process, on 
the other hand, performs work and partial condensation of the steam may be 
accomplished by doing the work. 
Vapor recompression is a technology wherein water boils itself. This is 
accomplished by boiling water at low pressure to produce water vapor. The 
water vapor is compressed and heated by doing work upon it. The heated 
water vapor is then condensed by heat exchange against the boiling water. 
The net result is that a phase change is accomplished by doing work. The 
energy efficiency of the system is controlled by the amount of heating of 
the water vapor. Small temperature increases result in high energy 
efficiencies and hence low operating costs for energy. Unfortunately, 
small temperature increases also result in large amounts of heat exchange 
area and hence high capital outlays. The present process, on the other 
hand, requires less heat exchanger area for a given duty and condensation 
may be at least partially achieved by doing work. With the present system, 
work is withdrawn from the system rather than input into the system. 
Dual purpose desalination/power plants currently in use produce fresh water 
by using the exhaust steam as a source of heat for an evaporator. The 
exhaust steam is condensed against the boiler of the evaporator. As the 
boiler duty increases with fresh water production, the temperature of the 
condensing exhaust steam also increases. This reduces the thermodynamic 
efficiency of the power plant providing the steam. The present process 
does not require a second boiler and the efficiency of the power plant is 
not adversely affected by increasing the fresh water production rate. 
Power generation using steam expansion is a common process. Condensate is 
fed to a boiler and heated. Steam is removed from the boiler and typically 
superheated. It then expands across a turbine, thereby doing work. The 
steam is then condensed and recycled to the boiler. A moderate amount of 
liquid is intermittently withdrawn from the boiler to prevent sludge 
accumulation. Treated fresh water is added to the system to compensate for 
material losses. The present process, on the other hand, withdraws the 
condensate as a product. Also, treated salinous water is fed to the boiler 
and liquid is continuously removed from the boiler to reduce scaling and 
prevent supersaturation by salt. In addition, the steam produced is washed 
by a stream of condensate to remove volatized salts and other inorganic 
compounds such as silica. 
There are several patents which disclose various desalinating processes, 
some of which also generate power. 
Ellis et al, U.S. Patent discloses a process which utilizes geothermal 
brine to generate power in a closed system with the exclusion of air to 
minimize corrosion. Steam from geothermal brine contains significant 
quantities of soluble salts including sodium and potassium chloride, 
calcium salts and iron and manganese salts, which have a strong corrosive 
action on turbine blades and related equipment. In this process, hot 
geothermal brine is flashed in a flash zone to form steam and concentrated 
brine and the steam is used to drive a power-generating turbine. The 
exhaust steam from the turbine is condensed and the major portion of the 
condensed steam is combined with the concentrated brine to form a restored 
brine, and the restored brine is returned to the geothermal hot brine 
well. There is no suggestion of a fresh water product. 
Kutchinson et al, U.S. Pat. No. 3,893,299 discloses a geothermal heat 
recovery process wherein hot water from a geothermal well is passed 
through successive flash chambers operating at successively lower 
temperatures and the steam from each flash chamber is passed in heat 
exchange relationship with a working fluid operating in a closed loop 
which is expanded in a power extracting gas expansion device for 
generating power. The hot fluid at the output of each heat exchange is 
either combined with the steam at the output of the next flash chamber or 
applied to the input of the next flash chamber with the hot fluid that is 
not converted to steam. There is no suggestion of a fresh water product. 
Spears, Jr., U.S. Pat. No. 4,078,976 discloses a potable recovery and power 
generating process which utilizes solar power for recovering potable water 
from salinous water. A portion of salinous water and an air stream are 
introduced into a solar radiation heat sink and heated water-containing 
air is withdrawn and condensed into potable drinking water. The heated 
salinous water is withdrawn from the solar radiation heat sink and 
recycled, and a part of the heated salinous water is flashed and the 
resultant vapor is passed through turbines to generate power and the 
exiting turbine vapors are cooled or condensed by contact with a second 
portion of the salinous water to recover addition potable water. 
Pitcher, U.S. Pat. Nos. 4,267,022 and Gress, 4,310,382 disclose processes 
which utilize air as a working fluid for desalination and heat pumps for 
transferring latent heat associated with vaporizing or condensing water 
from one part of the process to another. Both processes require work input 
rather than producing work. 
Mock, U.S. Pat. Nos. 4,276,124 and Elmore, 5,096,543 are essentially 
low-efficiency evaporator systems which utilize air as a working fluid to 
transport water vapor from one part of the system to another. 
The present invention is distinguished over the prior art in general, and 
these patents in particular by a process and apparatus for desalinating 
seawater or brine and purifying water containing minerals, salts, and 
other dissolved solids while simultaneously generating power. The salinous 
water is heated in a boiler to form steam and a concentrated brine. The 
concentrated brine is removed from the boiler, the steam produced in the 
boiler is washed with fresh water to remove trace salts and inorganic 
materials, and water bearing trace salts and inorganic materials 
are-returned to the boiler. The washed steam is expanded across a turbine 
to generate electrical or mechanical power which is utilized as a product. 
The steam exhausted from the turbine is collected and condensed, and one 
portion of the condensed water is utilized as a fresh water product and 
another portion of the condensed water is used as the wash water to wash 
the steam produced in the boiler. Energy efficiency is improved by heat 
exchanging the hot concentrated brine against the salinous feed water or 
by flashing the brine to produce steam. Boiler scaling and corrosion may 
be controlled by feed water pretreatment. By utilizing distillation 
combined with power generation, demand for fresh water and power can be 
satisfied simultaneously. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a process for 
desalinating seawater or brine and/or purifying fresh water which contains 
minerals, salts, and other dissolved solids while simultaneously 
generating power. 
It is another object of this invention to provide a process for 
desalination and distillation combined with power generation whereby 
demand for fresh water and power can be simultaneously satisfied. 
Another object of this invention is to provide a process for desalinating 
seawater or brine and/or purifying water containing minerals, salts, and 
other dissolved solids, which overcomes the obstacles of corrosion, 
scaling, and steam contamination normally associated with the production 
of power from steam. 
Another object of this invention is to provide a process for desalinating 
seawater or brine and/or purifying water containing minerals, salts, and 
other dissolved solids, which is energy efficient and performs work and 
wherein partial condensation of the steam is accomplished by doing the 
work. 
Another object of this invention is to provide a process for desalinating 
seawater or brine and/or purifying water containing minerals, salts, and 
other dissolved solids, which allows reduction of the heat exchanger area 
for a given duty and at least partially achieves condensation by the 
system doing work. 
Another object of this invention is to provide a process for desalinating 
seawater or brine and/or purifying water containing minerals, salts, and 
other dissolved solids, wherein work is withdrawn from the system rather 
than input into the system. 
A further object of this invention is to provide a process for desalinating 
seawater or brine and/or purifying water containing minerals, salts, and 
other dissolved solids which eliminates the need for a second boiler 
commonly used in conventional dual purpose desalination/power plants and 
the efficiency of the power plant is not adversely affected by increasing 
the fresh water production rate. 
A still further object of this invention is to provide a process for 
desalinating seawater or brine and/or purifying water containing minerals, 
salts, and other dissolved solids, wherein the condensate produced is 
withdrawn as a saleable product, treated salinous water is fed to the 
boiler and liquid is continuously removed from the boiler to reduce 
scaling and prevent supersaturation by salt, and steam produced is washed 
by a stream of condensate to remove volatized salts. 
Other objects of the invention will become apparent from time to time 
throughout the specification and claims as hereinafter related. 
The above noted objects and other objects of the invention are accomplished 
by a process and apparatus for desalinating seawater or brine and 
purifying water containing minerals, salts, and other dissolved solids 
while simultaneously generating power. The salinous water is heated in a 
boiler to form steam and a concentrated brine. The concentrated brine is 
removed from the boiler, the steam produced in the boiler is washed with 
fresh water to remove trace salts and inorganic materials, and water 
bearing trace salts and inorganic materials is returned to the boiler. The 
washed steam is expanded across a turbine to generate electrical or 
mechanical power which is utilized as a product. The steam exhausted from 
the turbine is collected and condensed, and one portion of the condensed 
water is utilized as a fresh water product and another portion of the 
condensed water is used as the wash water to wash the steam produced in 
the boiler. Energy efficiency is improved by heat exchanging the hot 
concentrated brine against the salinous feed water or by flashing the 
brine to produce steam. Boiler scaling and corrosion may be controlled by 
feed water pretreatment. By utilizing distillation combined with power 
generation, demand for fresh water and power can be satisfied 
simultaneously.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing by numerals of reference, there is shown 
schematically, a preferred process for desalinating seawater or brine and 
purifying fresh water which contains minerals, salts, and other dissolved 
solids while simultaneously generating power. In the present process, 
condensed steam is removed as a product and saline water is used as boiler 
feed water. The process and apparatus described hereinafter overcomes the 
obstacles of corrosion,, scaling, and steam contamination normally 
associated with the production of power from steam derived from salinous 
water. 
As shown in the drawing, seawater is introduced via line 10 to a boiler 11. 
Heat from a heat exchanger (not shown) is added to the boiler 11 via line 
12. The salinous water is heated in the boiler 11 to form steam and a 
concentrated brine. The steam is removed from the boiler 11 and fed to a 
wash column 13 via line 14 and the concentrated brine is removed from 
boiler via line 15. The concentration of dissolved solids within the 
boiler liquid is regulated by controlling the rate of blowdown removal. 
Boiler operation should be maintained at conditions well below the critical 
point of water to ensure good phase separation. Brine concentration may be 
regulated by using a ratio cascaded flow control loop that controls the 
flow rate of exiting brine and that is actuated by the flow rate of boiler 
feed water. Sodium ion concentration within the boiler brine should be 
maintained at a value exceeding 100,000 ppm. 
Fresh wash water is fed to the wash column 13 from a reservoir/accumulation 
tank 16 via line 17. Within the wash column 13, the fresh wash water 
introduced via line 17 and the steam introduced via line 14 are directly 
contacted. Therefore, any salts and inorganic materials contained within 
the steam are transferred into the water. Washed steam is removed from 
wash column 13 via line 18 and fed to a power-generating turbine 19. Water 
that contains trace salts and inorganic materials is removed from the wash 
column 13 via line 20 and recycled to the boiler 11. 
To prevent corrosion resulting from chloride attack within the boiler 11, 
the boiler, the wash column 13 and the associated feed and removal lines 
are formed of corrosion resistant material, such as titanium, hastelloy, 
inconel, incoloy, or monel. Monel would be a preferred material due to its 
cost. However, if monel is used, dissolved ammonia must be excluded from 
the process fluids. 
After entering the turbine 19 via line 18, the washed steam expands against 
the turbine thereby doing work or generating power which is removed via 
shaft 21. Steam that may be partially condensed exits turbine 19 via line 
22 and enters the reservoir/accumulation tank 16. Steam is withdrawn from 
the reservoir/accumulation tank 16 via line 23 and fed to a condenser 24 
where condensation takes place. Heat is removed from the condenser 24 via 
heat exchanger 25 and fresh water (condensate) is removed from the 
condenser via line 26 and recycled to the reservoir/accumulation tank 16. 
The fresh water (condensate) is removed from tank 16 via line 27 and is 
divided into product water and fresh wash water. The fresh wash water is 
recycled to the wash column 13 via line 17 and the fresh product water is 
withdrawn via line 28. 
By washing the steam with condensate and thereby causing the contaminants 
to transfer into the liquid phase from the vapor phase, steam 
contamination as a result of slight volatizing of components such as 
chlorides, sulfates, and silicates, is effectively reduced. 
As shown in dotted line, scaling as a result of precipitation of calcium 
carbonate within the boiler and associated equipment may be effectively 
prevented by adding acid to the salinous feed water followed by deareation 
to remove the carbonate and dissolved gases, and by ion exchange using 
conventional water softening devices 29 to remove the calcium and 
magnesium ions. By maintaining sodium ion concentration within the boiler 
brine at a value exceeding 100,000 ppm, favorable ion selectivity is 
ensured in the water softener. 
The boiler 11 may also serve as the source of the concentrated sodium 
chloride solution used for resin regeneration by the water softener. Other 
water treatment programs may also be utilized to prevent scaling but may 
be more expensive. A biocide may also be added to the feed water. 
Additional water treatment such as settling, filtration, addition of 
foaming inhibitors, addition of scaling inhibitors, and the addition of 
corrosion inhibitors may also be utilized in the present process. 
A superheater 30 may be installed between the wash column 13 and the 
turbine 19 to superheat the washed steam and prolong turbine life. 
The boiler 11, the wash column 13, power-generating turbine 19, the 
reservoir/accumulation tank 16, the condenser 24, and the associated 
conduit, lines, and fittings may also be insulated such that the system 
operates adiabatically and thus further conserves energy. 
While this invention has been described fully and completely with special 
emphasis upon a preferred embodiment, it should be understood that within 
the scope of the appended claims the invention may be practiced otherwise 
than as specifically described herein.