Combined water purification and power of generating plant

A combined water purification and power generating plant is disclosed having special features designed to maximize the cycle thermal efficiency and salt recovery, with little or no concentrated brine produced therefrom. Using the plant, a volume of salt water is delivered to a plurality of indirect and direct contact feed heaters. Within the direct contact heaters, the salt water is heated and diluted by condensation therein by super-heated steam delivered thereto. Any alkaline salts having reverse solubility characteristics particulate and are filtered therefrom. From the last direct contact feed heater, the diluted salt water is delivered to a plurality of high pressure, high temperature evaporators arranged in a series which are used to further heat, evaporate and filter the salt water in multiple stages thereby improving the plant's efficiency. A steam heater is used to super-heat a steam which delivered to various areas of the plant to heat and evaporate the salt water. High and low pressure steam turbines are also provide which utilize the steam to generate electrical power. The turbines are also arranged so that the exhaust steam therefrom may be used to heat the salt water in the feed heaters and then condensed into fresh water. An optional expansion tank is also provided for additional evaporation of the concentrated brine from the last evaporator.

BACKGROUND OF THE INVENTION 
1. Field Of the Invention: 
This invention relates generally to a combined sea water desalinization and 
electrical power generation system and, more particularly, such a system 
which operates with greater efficiency and high salts recovery, with 
little or no discharge of concentrated brine material. 
2. Description of the Related Art: 
The need for economical desalination plant has grown as reliable supplies 
of fresh water have diminished in traditionally water-short areas of the 
world and in semi-arid regions experiencing rapid population growth. The 
major desalination processes use in such plants today can be broadly 
classified as either thermal or membrane processes. 
Thermal processes are based on the distillation of salt water wherein salt 
water is boiled, and the steam evolved therefrom is collected and 
condensed into desalinated water. The most widely used thermal process 
uses multistage flash distillation, also known as MSF, which is based on 
the principle that water will boil at lower temperatures when it is 
subjected to lower pressures. Using MSF, heated salt water is fed into a 
flash chamber in which the pressure is gradually lowered which allows the 
salt water to boil at lower temperatures. The vapor produced is condensed 
on tubes that carry fresh, cool salt water into the plant. In the 
heat-exchange process, steam heats the cooler salt water, while the vapor 
condenses into desalinated water. The higher concentrated, unevaporated 
sea water is then delivered to a second chamber maintained at a lower 
pressure, where the process is repeated. For large desalination plants, a 
large number of flash chambers may be used. 
Most membrane plants are based on reverse osmosis processes wherein saline 
water is pumped to a pressure above its osmotic pressure. The compressed 
saline water is ultra-filtered by a semi-permeable membrane which allows 
water molecules to pass through while preventing passage of salt 
molecules. Fresh water is then collected from the other side of the 
membrane. 
One major constraint in all desalination plants is undesirable salt 
formation on various surfaces on the machinery. For example, calcium 
sulphate salts precipitation, which can not be prevented by pH control, 
will limit the maximum boiling temperature of sea water to 120 degrees C. 
This is mainly due to the salt material being deposited on heat transfer 
surfaces located in the evaporators which increases the resistance for 
heat transfer. This, in turn, increases the energy requirement for the 
plant. 
Combined desalination and power generation plants, also known as dual 
purpose plants, are commercially used today. In general, these plants 
utilize the expanded steam from the power plants' turbines to supply 
thermal energy used in the desalination process. Currently, research is 
directed towards improving the temperature limitations of the plants by 
chemical pre-treatment of the salt water. One alternative approach has 
been to use direct contact heat transfer equipment which eliminates the 
need for heat transfer surfaces. With direct contact heat transfer 
processes, a heat transfer medium, such as hydrocarbon oils, is used to 
transfer heat to the salt water. Unfortunately, problems with emulsion 
formation, oil-water separation, and oil degradation have limited the 
development of these processes. 
Another approach to resolving the temperature limitations of these plants 
was described by Blaskowski in U.S. Pat. No. 3,352,107. In Blaskowski, 
superheated steam is used as a heat transfer medium in a combined power 
generating-desalination plant. Unfortunately, the system was not 
commercially developed due to important thermodynamic and design 
considerations. 
Another desirable goal for most desalination plants is the total recovery 
of various salts from the sea water. Unfortunately, due to inherent 
thermal inefficiencies in the plant's designs, recovery of salts from 
current desalination plants have not be commercially developed. 
More efficient dual purpose plants are needed today. In addition, such 
plants which allow for greater salt recovery and that substantially reduce 
or eliminate the concentrated brine therefrom, would be highly desirable. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide a combined desalination and 
power generating plant. 
It is another object of the present invention to provide such a combined 
plant which is more energy efficient than currently available combined 
plants. 
It is a further object of the present invention to provide such a plant 
which provides for salt recovery. 
It is a still another object of the present invention to provide such a 
plant that rejects little or no concentrated brine. 
An energy efficient, high recovery water purification, power generating and 
salt recovery plant is disclosed herein. The plant, which in the preferred 
embodiment uses sea water as a working medium, operates on an energy 
efficient regenerative-reheat, thermodynamic cycle. Although in the 
preferred embodiment, the plant is concerned with desalting sea water, it 
should be understood that the plant may be adapted to recover fresh water 
from other contaminated water sources. With such modifications, of course, 
the material recovered from the plant will vary with the source of the 
contaminated water. 
The plant includes special features designed to maximize cycle thermal 
efficiency. In the plant a volume of sea water is gradually heated by 
condensation of super-heated steam extracted from turbines at different 
conditions. During treatment, the solubility of the salt changes so that 
filters may be used to remove any particulate material therefrom. The salt 
water is diluted over a series of steps and then delivered to multi-stage 
evaporators which are used to evaporate the water. In the preferred 
embodiment, a multi-stage evaporator scheme is used to reduce the 
deficiencies resulting from boiling point elevation, to reduce steam 
pressure losses, to reduce liquid entrainment, to reduce equipment size, 
and to improve salt recovery. Sensible heat and heat of vaporization are 
provided in the evaporators by direct contact with super-heated steam. The 
super-heated steam used in each evaporator is compressed to compensate for 
pressure losses in steam ducts and any other pressure losses result from 
contact with the salt water or brine located therein. The steam and water 
vapor produced from each evaporator is passed through a demister (a 
gas-liquid separation device commonly used in thermal desalination 
processes) to remove any entrained liquid. From the demister, the steam is 
delivered to the steam heater where it is super-heated for use in the 
regenerative-reheat cycle. Part of the super-heated steam from the steam 
heater is also recycled through the evaporators. For initial start up, an 
auxiliary boiler may be used to produce an initial steam flow to the steam 
heater and the evaporators. In addition, the steam from the turbines of 
the power generation cycle is delivered to a condenser which uses the sea 
water pumped into the plant as a coolant to produce fresh water. 
In the evaporators, the salt water is processed to produce concentrated 
brine. In the last evaporator, the brine is at high temperature and 
pressure with a salt concentration over 10 times the sea water entering 
the plant. In one embodiment, the brine is delivered to an expansion tank 
wherein the pressure is reduced which allows up to 60% of the water to be 
removed by evaporation. Conduits are provided for carrying the steam 
produced from the expansion tank to other parts of the plant, such as the 
direct contact feed heaters, where it is used to pre-heat the sea water. 
After treatment in the expansion tank, the brine is then delivered to solar 
ponds from which the remaining water is slowly evaporated. Alternatively, 
the brine may be brought into direct contact with the steam heater's flue 
gas for rapid evaporation. In this manner, little or no concentrated brine 
produced by the plant is rejected into the environment. 
Various conduits are also provided to deliver the super-heat steam between 
the steam heater and other areas in the plant to supply the sensible heat 
and the heat of vaporization to the treated sea water. 
The thermal efficiency of this plant is within 1 to 2% of a typical 
regenerative-reheat cycle which means lower energy requirements for the 
desalination process. By using super-heated steam in direct contact 
equipment, the need for heat transfer surfaces is eliminated, thereby 
reducing the problems associated therewith, such as corrosion and scale 
formation. To handle the high temperature and pressure, the vessels used 
in the plant may be made of enforced, lined concrete can be utilized which 
reduces the current capital cost of the expensive alloys currently used in 
thermal desalination plants. In addition to the energy and capital cost 
savings, salt recovery is greatly improved for commercialization.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Referring now to the drawing, wherein like reference characters designate 
like elements, there is shown in FIG. 1, a combined desalination and power 
generating plant, designated generally 10. 
Sea water 1 is pumped by a typical intake pumping system 32 into a water 
treatment tank 33 where it undergoes chlorine or ultraviolet treatment, 
designated Q. The treated sea water 2 is then transported by a conduit to 
a condenser 34 where it is used to remove the heat of condensation from 
the expanded exhaust steam 27 delivered from the low pressure turbine 47. 
The condenser 34 is an indirect contact heat exchanger wherein the exhaust 
steam 27 is at pressure below atmospheric. 
From the condenser 34, the warm sea water 3 is delivered to an indirect 
contact heat exchanger 35 which heats it using the condensed steam 28 from 
the indirect contact feed heater 36 to produce heated sea water 5. The 
excessive treated sea water 4 used for cooling is delivered back to the 
sea 99. The heated salt water 5 is then delivered to the indirect contact 
feed heater 36 where it is further heated by bleeded steam 31(a) from the 
low pressure turbine 47 described further below. The indirect contact feed 
heater 36 is an indirect contact heater rather than a direct contact 
heater because the bleeded steam 31(a) from the low pressure turbine 47 is 
below atmospheric pressure. 
From the indirect contact feed heater 36, the heated salt water 5 is 
delivered to a plurality of multi-stage direct contact feed heaters 
37(a)-(f) which are used to gradually increase the temperature of the 
heated sea water 5 to a temperature found in the first evaporator 40(a). 
In the preferred embodiment, there are six direct contact feed heaters 
37(a)-(f). Connected to each direct contact feed heater 37(a)-(f) is a 
pump 38(a)-(f), respectively, which is used to pump the salt water 5 to 
the pressure of the subsequent direct contact feed heater. Once delivered 
to the first direct contact feed heater 37(a), the salt water 5 is heated 
by condensation from the expanded steam 15 produced from the expansion 
tank 44 and the bleeded steam 31(b) from the low pressure turbine 47. 
Inside the first contact feed heater 37(a), the combined steams 15 and 
31(b) are condensed directly on the salt water 5 so that no heat transfer 
surface is needed. Similar methods are used to heat the salt water 5 in 
the other direct contact feed heaters 37(b)-(f). Either a spray feed 
heater such as the employed in power plant, or bubbling steam into heated 
salt water 5 or any direct contact apparatus can be used as the direct 
contact feed heaters 37(a)-(f). 
Connected between each direct contact feed heater 37(a)-(f) is a filter 
38(a)-(f), respectively, which is used to filter the exiting stream of 
salt water 5 from each direct contact feed heater 37 to recover any 
insoluble salts 48 therefrom. While being heated in the direct contact 
feed heaters 37(a)-(f), the salt water 5 is gradually diluted. As a 
result, alkaline salts having inverse solubilities, such as Magnesium 
Hydroxide, Magnesium Chloride, and Calcium Carbonate, may be recovered 
using the filters 38(a)-(f). Because the salt water 5 is being diluted in 
the process, other salts, such as Calcium Sulfate, are not expected to 
precipitate. Since various magnesium salts are found in sea water 1, the 
plant 10 may be used for magnesium production. Enhancing the precipitation 
of Mg(OH).sub.2 salt over other alkaline salts from the heated salt water 
5 can be achieved by adjusting the pH and/or by sludge recirculation 
through the direct contact feed heaters 37(a)-(f). 
After being filtered by the last filter 38(f), the heated, dilute salt 
water 5, now referred to as salt water 6, is then pumped using pump 39(g) 
to five multi-stages evaporators 40(a)-(e). The evaporators 40(a)-(e) are 
used to evaporate the salt water 6 to produce concentrated brine 8. The 
salt water 6 is delivered to the first evaporator 40(a) having a 
temperature and pressure near the saturation conditions. For the 
embodiment disclosed herein, the concentrated salt water 6 exiting from 
the last evaporator 40(e) is at a pressure of 165 bar and a temperature of 
372 C. and has a saturated NaCl concentration of about 417,000 ppm. 
Although the number of evaporators may be varied for special plants, with 
plant 10 over 45% of the salt water 6 is evaporated in the first 
evaporator 40(a), about 67% of the salt water 6 delivered from the first 
evaporator 40(a) is evaporated in the second evaporator 40(b), about 40% 
of the salt water 6 delivered from the second evaporator 40(b) is 
evaporated in the third evaporator 40(c), about 28% of the salt water 6 
delivered from the third evaporator 40(c) is evaporated in the fourth 
evaporator 40(d), and about 30% of the salt water 6 delivered from the 
fourth evaporator is evaporated in the fifth evaporator 40(e). The heat of 
vaporization and the sensible heat needed in each evaporator 40(a)-(e) is 
supplied by the super-heated steam 24 from the steam heater 45. The 
super-heated steam 24 is compressed by fans or compressors 42(a)-(e) 
located at the entrance of each evaporator 40(a)-(e), respectively, to 
compensate for pressure losses in steam ducts and other losses resulting 
from contact with the salt water 6. 
The exiting stream of steam and water vapor 8(a)-8(g) from the evaporators 
40(a)-(e), respectively, are combined to form a combined stream of steam, 
designated 8. Each exiting stream 8(a)-(g) is passed through a demister 
43(a)-(g), respectively, to remove any entrained liquid or solid 
therefrom. The combined stream of steam 8 is then delivered to the fuel 
steam heater 45 where it is super-heated. 
When the combined stream of steam 8 is super-heated to produce a stream of 
super-heated steam 22, it is then divided into two streams of super-heated 
steam, 23 and 24. One stream 23 is delivered to a high pressure turbine 46 
while the second stream 24 is delivered back to the evaporators 40(a)-(e) 
where it is used to heat and evaporate the salt water 6 contained therein. 
After being delivered to the high pressure turbine 46, the bleeding 
streams of steam 25(c)-(f) therefrom are used to heat the salt water 5 in 
the third, fourth, fifth, and sixth feed heaters 37(c)-(f), respectively, 
at the corresponding thermodynamic state (allowing 1 degree C. driving 
force). The expanded exhaust steam 26 from the high pressure turbine 46 is 
delivered back to the steam heater 45 for reheating at constant pressure. 
The stream of reheated steam 29 from the steam heater 45 is then delivered 
to the low pressure turbine 47. As mentioned above, the salt water 5 is 
heated in the indirect contact feed heater, 36, and the first and second 
direct contact feed heaters 37(a),(b) by bleeding steam 31(a), (b), and 
(c), respectively, at a corresponding thermodynamic state (allowing 1 
degree C. driving force). The power produced from the streams of steams 
23, 29, in the turbines 46, 47, respectively can be converted into 
electrical power by generators (not shown). Other combination of turbines 
can be adopted. 
For initial start-up of the plant, an auxiliary boiler (not shown) is used 
to provide the super-heated steam 24 needed for the evaporators 
40(a)-(e)and for preheating the salt water 1. In all of the evaporators 
40(a)-(e), the super-heated steam 24 and concentrated salt water 6 
contained therein are in direct contact without any heat transfer surface. 
Spray columns or super heated steam bubbling through the salt water 6 or 
any direct contact method can be employed. To account for pressure losses 
in the evaporators 40(a)-(e), compressors or circulation fans 42(a)-(e) 
may be used at the entrance of the super-heated steam 24 to each 
evaporator 40(a)-(e). To account for salt water pressure losses, a pump 
29(a)-(d) is used between each evaporator 40(a)-(d), respectively. 
As evaporation takes place in the evaporators 40(a)-(e), Calcium Sulphate 
salts 50 are expected to precipitate due to their increased concentration 
in the salt water 6. These salts 50 can be collected by a medium filter 
41(a)-(e) after each evaporator 40(a)-(e), respectively. The concentrated 
salt water, now referred to as brine 8, exits from the last evaporator 
40(e) is delivered to an expansion tank 44 which has a pressure equal to 
the pressure of the first direct contact feed heater 37(a). As a result of 
this lower pressure, over 60% of the brine 8 is further evaporated thereby 
allowing the NaCl 13 to precipitate and be collected therefrom. The 
remaining fluid in the brine 8 may be delivered to a salt recovery tank 49 
where it is brought in direct contact with the flue gas 51 from the steam 
heater 45 for total recovery of NaCl 13. Thus, concentrated brine 8 is 
prevented from being rejected into the environment. 
The expanded exhaust steam 27 from the low pressure turbine 47 is condensed 
in the condenser 34. In the preferred embodiment, exhaust steam 27 is 
combined with the condensed steam 39 from the heat exchanger 35. From the 
combined streams 27, 39 fresh water may be produced from the plant 10. By 
adding the steam 15 produced from the expansion tank 44, an overall water 
recovery above 90% can be realized. 
In the plant 10, a water conduit system 67 and a steam conduit system 69 
are used to transport the contaminated water and the steam and 
super-heated steam, respectively, between the various components discussed 
above. 
In summary, a combined desalination and power generating plant 10 is 
provided which is more energy efficient than currently available combined 
plants. Plant 10 also provides for high salt recovery with little or no 
concentrated brine 8 rejected into the environment. 
In compliance with the statute, the invention has been described in 
language more or less specific as to structural features. It should be 
understood, however, that the invention is not limited to the specific 
features shown since the means and construction shown comprises the 
preferred forms of putting the invention into effect. The invention is, 
therefore, claimed in any of its forms or modifications within the 
legitimate and valid scope of the amended claims, appropriately 
interpreted in accordance with the doctrine of equivalents.