| GB1145568A | 1969-03-19 | |||
| US3614874A | 1971-10-26 |
1. A process for obtaining water from saline water, the process including at least the steps of:- - feeding saline water from a source of saline water to a heat exchanger to reduce the temperature of the saline water; feeding chilled saline water exiting the heat exchanger to a refrigeration vessel in which the temperature of the saline water is reduced by a refrigerant to a pre-selected temperature; - feeding the saline water to a freezing vessel at the pre-selected temperature, in which freezing vessel the temperature of the saline water is reduced by a refrigerant to form a two-phase mixture including ice formed from water contained in the saline water and a saline-rich solution; and - separating the ice from the saline-rich solution and melting the ice to obtain a chilled water stream; wherein the chilled water stream and the saline-rich solution from the freezing process are used to cool the refrigerants of the refrigeration and freezing vessels and the saline water fed to the heat exchanger.
2. A process as claimed in claim 1 , wherein spent refrigerant from the freezing vessel is compressed to form a superheated vapour after exiting the freezing vessel and wherein the superheated vapour is used to melt the ice from the freezing vessel in a melting vessel, thereby at least partially condensing the vapour.
3. A process as claimed in claim 2, wherein the at least partially condensed refrigerant vapour is further condensed in a condenser wherein cold saline-rich solution from the freezing vessel is used to condense the remaining vapour.
4. A process as claimed in any one of claims 1 to 3, wherein spent refrigerant from the refrigeration vessel is compressed to form a superheated vapour after exiting the refrigeration vessel and is condensed using the chilled water stream and saline-rich solution exiting the heat exchanger.
5. A process as claimed in claim 3, wherein additional refrigerant from a source other than that supplying the freezing vessel is used to augment the cooling of the ice, molten water and saline-rich solution used to condense the compressed spent vapour from the freezing vessel, thereby reducing the condensation temperature of the vapour.
6. A process as claimed in claim 5, wherein additional refrigerant from the refrigeration vessel refrigeration supply system is used to augment the cooling of the ice, chilled water stream and saline-rich solution used to condense the compressed spent vapour from the freezing vessel.
7. A process as claimed in any one of the preceding claims, wherein a source of waste heat is used to power an absorption refrigeration system which supplies refrigerant to additional cooling coils located in the refrigeration vessel thereby reducing the energy requirements of the refrigeration vessel refrigeration-supply system.
8. A process as claimed in claim 7, wherein the spent refrigerant from the additional cooling coils is fed to the absorber where it is absorbed by a concentrated absorbent solution fed to the absorber from an evaporator to form a refrigerant-absorbent solution.
9. A process as claimed in claim 8, wherein the absorber is cooled by a stream of chilled water and saline-rich solution originating from the freezing vessel.
10. A process as claimed in claim 9, wherein the refrigerant is driven off from the refrigerant-absorbent solution in the evaporator and is condensed in a condenser cooled by a stream of chilled water and saline-rich solution originating from the freezing vessel before being fed to the additional cooling coils.
11. A process as claimed in any one of claim 7 to 10, wherein a fraction of the refrigerant from the absorption refrigeration system is used to augment the cooling of ice, molten water and saline-rich solution used to condense the compressed spent vapour from the freezing vessel.
12. A process as claimed in any one of the preceding claims wherein the stream of chilled water exiting the process can be used as water.
13. A process as claimed in any one of the preceding claims, wherein the source of saline water is the sea.
14. A process according to the invention for obtaining water from saline water substantially as hereinbefore described or exemplified.
15. A process of obtaining water from saline water including any new and inventive integer or combination of integers, substantially as herein described. |
A PROCESS FOR OBTAINING WATER FROM SALINE WATER
Field of the Invention
This invention relates to a process for obtaining water from saline water wherein process streams are utilised to reduce the total energy requirements of the process.
Background to the Invention
Various methods and processes for separating water from saline water such as sea water are know and are being implemented in areas where water is scarce and sea water is readily available. These processes include distillation and reverse osmosis.
US patent numbers 3,314,881 and 3,616,612, GB patent number 954 926 and South African patent number 2007/10039 describe processes in which sea water is frozen to obtain ice which can be molten to obtain water. The current process seeks to provide an energy efficient means for obtaining water from saline water and specifically from sea water.
Summary of the Invention
According to the invention, there is provided a process for obtaining water from saline water, the process including at least the steps of:- feeding saline water from a source of saline water to a heat exchanger to reduce the temperature of the saline water; feeding chilled saline water exiting the heat exchanger to a refrigeration vessel in which the temperature of the saline water is reduced by a refrigerant to a pre-selected temperature;
feeding the saline water to a freezing vessel at the pre-selected temperature, in which freezing vessel the temperature of the saline water is reduced by a refrigerant to form a two-phase mixture including ice formed from water contained in the saline water and a saline-rich solution; and separating the ice from the saline-rich solution and melting the ice to obtain a chilled water stream; wherein the chilled water stream and the saline-rich solution from the freezing process are used to cool the refrigerants of the refrigeration and freezing vessels and the saline water fed to the heat exchanger.
Spent refrigerant from the freezing vessel may be compressed to form a superheated vapour after exiting the freezing vessel. Thereafter, the superheated vapour may be used to melt the ice from the freezing vessel in a melting vessel, thereby at least partially condensing the vapour.
The at least partially condensed refrigerant vapour may be further condensed in a condenser wherein cold saline-rich solution from the freezing vessel may be used to condense the remaining vapour.
The chilled water stream and saline-rich solution may be passed through the heat exchanger to reduce the temperature of the saline water being fed to the process.
Spent refrigerant from the refrigeration vessel may be compressed to form a superheated vapour after exiting the refrigeration vessel and may then be condensed using the chilled water stream and the saline-rich solution exiting the heat exchanger.
The saline water may be sea water.
An additional refrigerant from a system independent of that supplying the freezing vessel to supply additional cooling via a separate condenser to the condensing system of the compressed vapour from the freezing vessel may be used to lower the condensing temperature of this vapour. This reduces the compressor work required, thereby reducing the energy requirements of the refrigeration system supplying the freezing vessel.
This also lowers the temperature of the chilled water stream and saline-rich solution leaving the condensing system of the freezing vessel refrigeration system. This lowers the temperature of the saline solution leaving the heat exchanger and reduces the refrigeration load of the refrigeration vessel. The energy required from the compressor which forms part of the refrigeration vessel refrigeration system is thereby reduced.
Part or all of this energy reduction may be used to decrease the freezing temperature used in the freezing vessel, thus increasing the water/ saline water ratio which will reduce the energy requirements of the pumping system supplying saline water to the process.
In this embodiment of the invention, the refrigeration may be supplied from the refrigeration system which feeds the refrigeration vessel, the spent refrigerant being combined with the spent refrigerant from the refrigeration vessel prior to compression. The additional energy required for the refrigeration compressor is far less than the energy saved by the freezer compressor.
In the event that a source of waste heat is available, a further aspect may be included. The waste heat may be used to energise an absorption refrigeration system. The refrigerant generated may be supplied to separate cooling coils located in the refrigeration vessel. This reduces the refrigeration required to be supplied by the system supplying the refrigerant to the refrigeration vessel thereby reducing the energy requirements of the process. The absorber and
condenser of the absorption process may be cooled by the chilled water stream and saline-rich solution exiting the refrigeration vessel refrigerant condensing system.
A fraction of the refrigerant supplied by the absorption refrigeration system may be used to supply additional cooling to the condensing system of the compressed vapour from the freezing vessel.
Detailed Description of the Invention
The invention will now be described by way of the following non-limiting example with reference to the accompanying drawings.
In the drawings:-
Figure 1 shows a schematic representation of an embodiment of a process for obtaining water from saline water in accordance with the present invention; and
Figure 2 shows a schematic representation of a portion of the process as per Example 5 below.
In Figure 1 , reference numeral 10 generally indicates a process for obtaining water from saline water in accordance with the present invention.
A process 10 for obtaining water 26 from saline water 14 includes at least the steps of feeding saline water from a source of saline water via pump 15 to a heat exchanger 16 to reduce the temperature of the saline water and feeding the chilled saline water 14 exiting the heat exchanger 16 to a refrigeration vessel 38 in which the temperature of the saline water 14 is further reduced by refrigerant 21 and then feeding the refrigerated saline water 14 to a freezing vessel 18 in which the temperature of the saline water 14 is further reduced by a refrigerant
20 to form a two-phase mixture including ice 22 formed from water contained in the saline water 14 and a saline-rich solution 24.
The ice 22 is separated from the saline-rich solution 24 and is melted. The chilled water stream 26 and the saline-rich solution 24 from the freezing process 18 are used to cool the refrigerant 20, the saline water 14 fed to the heat exchanger 16 and the spent refrigerant 21 fed to condenser 36.
Spent refrigerant 20 from the freezing vessel 18 is compressed in compressor 28 to form a superheated vapour after exiting the freezing vessel 18. Thereafter, the superheated vapour is used to melt the ice 22 from the freezing vessel 18 in a melting vessel 30, thereby at least partially condensing the refrigerant vapour 20.
The at least partially condensed refrigerant vapour 20 is further condensed in condenser 34 wherein cold saline-rich solution 24 from the freezing vessel 18 or fresh refrigerant 20 is used to condense the remaining vapour. The condensed refrigerant 20 is recycled to the freezing vessel 18.
The condensing temperature of the refrigerant vapour 20 is lowered by the addition of condenser 32 which is cooled by a fraction of the refrigerant 21 , the spent refrigerant 21 having been combined with the spent refrigerant from the refrigeration vessel 38.
Spent refrigerant 21 from the refrigeration vessel 38 and, if included, condenser 32 is compressed in compressor 29 to form a superheated vapour. This superheated vapour is condensed in condenser 36 which is cooled by the chilled water stream 26 and the saline-rich solution 24 fed from the heat exchanger 16.
The condensed refrigerant 21 is fed to the refrigeration vessel 38. A fraction of this refrigerant 21 is fed to condenser 32, if included.
In the event that a source of waste heat is available, a second cooling vessel 40 is located immediately downstream from the heat exchanger 16 to reduce the temperature of the saline water 14 exiting the heat exchanger 16. A refrigerant 42 is used to cool the saline water 14 exiting the heat exchanger 16. The refrigerant 42 originates from an absorption cycle 44.
In the absorption cycle 44, the refrigerant 42 is driven off from a refrigerant- absorbent solution 46 in an evaporator 48 using waste heat in the form of steam 50 available from an unrelated process 52 after which the refrigerant 42 is condensed in a condenser 54 cooled by chilled water stream 26 and saline-rich solution fed from the absorber 56.
Spent refrigerant 42 from the second cooling vessel 40 is fed to an absorber 56 where it is absorbed by a concentrated absorbent solution 58 fed to the absorber 56 from the evaporator 48 to form a refrigerant-absorbent solution 60.
The absorber 56 is cooled by chilled water stream 26 and saline-rich solution fed from the condenser 36.
The refrigerant-absorbent solution 60 is fed to a heat exchanger 62 where it is heated by a concentrated absorbent solution 58 from the evaporator 48 prior to being recycled to the evaporator 48.
The following examples serve to illustrate the invention:
EXAMPLES
Five examples are used to demonstrate the effectiveness of the invention. All five use the same power source, which is a gas turbine driving an alternator. The alternator produces 58207 kW. The gas turbine has an exhaust mass flow of
166, 1 kg/s. The exhaust temperature is 424°C. The exhaust flow is used to
heat a boiler 52 which produces steam at 67, 92°C. This steam is used to provide the heat for the absorption cycle 44. The exhaust gas leaves the boiler 52 at a temperature of 69,2°C, which will give it a vertical acceleration of 0,07g in an atmosphere at a temperature of 45°C.
The steam will provide 63 057,54 kJ/s of latent heat to the evaporator 48 of the absorption cycle 44 which uses a lithium bromide - water solution as its operating fluid.
Details of the absorption cycle are:
Poinl : Temperature Pressure X Enthalpy Mass Mass x
Enthalpy
("C) (mm Hg) (kJ/kg) (kg) (kJ)
1 29,44 5,817 0,55 -171 ,5774 1 -171 ,5774
1-3 57,22 31 ,766 0,55 -116,4189 1 -116,4189
2 66,67 31 ,766 0 2623,9583 0,0517 135,6586
3 66,67 31 ,766 0,58 -104,6700 0,9483 -99,2586
3-1 37,22 31 ,766 0,58 -162,8330 0,9483 -154,4125
4 30,56 31 ,766 0 125,6040 0,0517 6,4937
5 3,33 5,817 0 2556,7392 0,0517 132,1834
C 3/1 = 0,8936 E 3/1 = 0,8861
Where,
Point 1 describes conditions of the refrigerant at exit from the absorber 56.
Point 1-3 describes conditions of the refrigerant at exit from the heat exchanger
62 and entry into the evaporator 48.
Point 2 describes conditions of the refrigerant at exit from the evaporator 48.
Point 3 describes conditions of the absorbent at exit from the evaporator 48.
Point 3-1 describes conditions of the absorbent at exit from the heat exchanger
62.
Point 4 describes the conditions of the condensed water at exit from condenser
54.
Point 5 describes conditions of the water vapour after expansion through cooling vessel 40 and condenser 32.
C3/1 is the heat capacity ratio of the absorbent to the refrigerant, measured over the same temperature range.
E3/1 is the heat exchanger effectiveness of 62 and is a function of C3/1.
This cycle will yield the following results:
Heat to evaporator 48 is 63 057,54 kJ/s.
Heat from absorber 56 is 61 625,4691 kJ/s.
Heat from condenser 54 is 53 295,3109 kJ/s.
Heat to cooling vessel 40 and condenser 32 is 51 863,24 kJ/s.
These results yield a COP of 0,8225 which is good by absorption standard but very poor by vapour compression standards and illustrates the fact that, wherever possible, the waste heat should be used to generate shaft power first
(more than 11 MW can be generated by a steam turbine with these exhaust gas conditions) and then the exhaust heat can be used for the absorption cycle.
Neither this option nor the use of sensible heat are used for these illustrative examples.
The seawater has a temperature of 27°C. Its salinity is 34,325 (19 parts of chlorine per 1 000 parts of solution by mass). Seawater with this salinity will begin to freeze at - 1 ,8649°C.
The seawater must be pumped into the system through a total head of 50m. The pump efficiency is 80%. This will require an energy input of 0,6131 kJ/kg seawater.
The examples illustrate the different water yields (N kg/s) achieved with or without the use of this invention with a given shaft power input.
EXAMPLE 1
Freezing temperature in 18 is -3.5°C. The seawater to water ratio is then 2,1453. The condensing temperature is 1 ,11 0 C.
The use of both cooling vessel 40 and condenser 32 is illustrated in this example.
The results of the calculations in Tables of Results, Table 1.
EXAMPLE 2
Neither cooling vessel 40 nor condenser 32 are used. Freezing temperature is -3.5°C. The seawater to water ratio remains 2,1453. Condensing temperature is now 1 ,5°C.
The results of the calculations are given in Table 1.
EXAMPLE 3 The freezing temperature in 18 is -3.0 0 C. The seawater to water ratio is 2,6498.
Condensing temperature is 1 ,11 0 C.
Only cooling vessel 40 is used in this example as the ice/water and brine provide enough condensation cooling for the refrigerant for 18. Calculation results are given in Table 1.
EXAMPLE 4
Conditions are the same as in Example 3 but cooling vessel 40 and condenser
32 omitted. Calculation results are given in Table 1.
EXAMPLE 5
Conditions are the same as for Example 1 but the refrigeration for 32 is supplied by 54. Figure 2 illustrates this example.
The only changes in the tables of results below occur in the following entries:
Example 1 Example 5
Refrigeration required 43,1626 + 2.0987 43,1626
Available 40 51863.24 51863.24 - 2.0987
N N
The refrigeration required from 38:
Example 1 = 43,1626 + 2,0987 - 51863.24 = 45.2613 - 51863.24
N N
Example 5 = 43,1626 - (51863,24 - 2,0987) = 45,2613 - 51863,24
N N It is to be appreciated, that the invention is not limited to any particular embodiment or configuration as hereinbefore generally described or illustrated.
TABLES OF RESULTS
Table 1
Example 1 Example 2 Example 3 Example 4
Freezing
Temperatures (t)
Freezing -3,5 -3,5 -3,0 -3,0
Condensing 1 ,1 1 1 ,5 1,11 1,1 1
Seawater/water mass ratio 2,1453 2,1453 2,6498 2,6498
Refrigeration required (kJ/kg water) 353,4694 353,4694 : 557,3159 357,3159
Refrigerant (kJ/kg refrig.) 1 1255,8074 1254,0164 1255,8074 1 i 255,8074
Work (kJ/kg refrig.) 25,4304 27,5724 22,6299 22,6299
COP 49,3821 45,5026 55,4933 55,4933
Cooling required (kJ/kg water) 360,6272 361,2375 363,7548 363,7548
Cooling available (5-11) 337,9632 339,5897 337,9580 337,9580
(5.12) 2,0987 0 0 0
(5.13) 20,5653 22,2998 26,6833 26,6833
Shaft Energy (kJ/kg water) 7,1578 7,7681 6,4389 6,4389
Heat Exchanger(kWh/k/ water) 1,9883 2,1578 1,7886 1,7886
Heat Exchanger (16)
Temperatures (t)
Water and brine: In 1,11 1,5 1,1 1 1 ,1 1
Out 23,1 167 23,1 75 23,1167 23,1 167
Seawater: In 27 27 27 27
Out 4,9944 5,325 4,9944 4,9944
Refrigeration.
Temperatures ( 3 C)
Refrigerator (40) 3,33 - 3,33 -
Refrigerator (38) 0 0 0 0
Condensing (36) 27,611 29,222 27,611 28,833
Refrigeration (kJ/kg water)
Required 43,1626 +2.0987 46,0193 53,3029 53,3029
Available (40) 51863.24 0 51863,24 0
N N
Refrigerant (38) (kJ/kg refrig.) 1134,0646 1126,1992 1 134,0646 1128,1565
Work (kJ/kg refrig.) 145,4462 153,7205 145,4462 151 ,6906
COP 7,7971 7,3263 7,7971 7,4372
Shaft Energy (kJ/kg water) 4,3406 6,2814 5,3084 7,1670
(kWh/ki water) 1,2057 1,7448 1,4746 1,9908
Water produced N (kg/s) 4542,5554 3788,3384 4352,9341 3821,7284
(M//hr) 16,3532 13,6380 15,6706 13,7582
(M//day) 392,4768 327,3124 376,0935 330,1973
Table 2 : Shaft Energy Utilisation
Example 1 Example 2 Example 3 Example 4
Freezing (28) (kWh/k/) 1,9883 2,1578 1,7886 1,7886
Refrigerator (29) (kWh/k/) 1,2057 1 ,7448 1,4746 1,9908
Pump (15) (kWh/k/)| 0,3654 0,3654 0,4513 0,4513
Total (kWh/k/) 3,5594 4,2680 3,7144 4,2307