Patent Application
20180154277
Not Applicable.
Not Applicable.
Not Applicable.
A multi-stage flash desalination system is commonly used to produce a distillate or drinking water from seawater. Such systems are typically combined with a power plant, from which typically an extraction steam from a condensing turbine or a discharge steam from a backpressure turbine is supplied and used as heating steam.
The steam supplied from the power plant to the multi-stage flash desalination system contains energy which could be converted into electrical power and has as such a value, so that it is of advantage if the amount of steam to be supplied from the power plant can be reduced.
The steam is supplied from a power plant typically at a pressure of 2.5 to 3.0 bar (36 to 44 psi) while the heating steam pressure required in the desalination system may be typically in the range of 1.0 to 1.8 bar (15 to 26 psi). Therefore typically a steam control valve is installed to reduce the steam supply pressure as required. The required steam pressure may depend to a certain degree on the plant design criteria, but also on the operation mode of the desalination plant which may vary over an annual cycle between summer condition and winter condition and may also vary in daily cycles between day and night conditions. Downstream of the steam control valve typically a de-super heater is installed through which the temperature of the superheated steam discharging from the steam control valve is reduced by condensate injection to saturation temperature or near saturation temperature before it is used as heating steam.
The heating steam is used in a brine heater to heat a coolant, typically seawater or a re-circulating brine, to a top temperature before it enters into a multi-stage flash evaporator as flashing brine. The brine heater is typically a tube and shell heat exchanger, in which the coolant is passing through the tubes while the heating steam is condensing on the outside surface of the tubes. The condensate generated in the brine heater by condensation of the heating steam is returned to the power plant with a temperature equal or close to the saturation temperature of the heating steam, typically in the range of 100 to 115° C. (212 to 240° F.).
While a condensate returned in a power plant from a condensing steam turbine may have typically a temperature in the range of 35 to 45° C. (95 to 115° F.), such condensate is typically heated up by utilizing low grade heat of exhaust gases of a steam generator which is improving the heat rate or thermal efficiency of a power plant. As such, the condensate returned from a desalination plant at a much higher temperature is not desirable as the low grade heat from the exhaust gases can't be utilized. It would be preferred if the energy content of the condensate would be used to a maximum degree in the desalination plant, while it would be also preferred if the required steam mass flow to be supplied to the desalination plant would be reduced.
The main part of a multi-stage flash desalination system is a multi-stage flash evaporator, comprising a plurality of flash stages including a first flash stage which is operating at the highest temperature and a last flash stage which is operating at the lowest temperature. Each flash stage comprises typically a tube bundle. While a heated coolant, typically seawater or re-circulating brine, enters as flashing brine into the first flash stage at its highest temperature, it flashes down in each consecutive flash stage to a lower temperature, releases some vapor which is condensing on the tube bundles and being collected as distillate. The salt concentration of the flashing brine is increasing toward the last flash stage.
In a “once through” configuration of a multi-stage flash evaporator, typically seawater is used as a coolant, entering with its lowest temperature into the tube bundle at the last flash stage and passing through all tube bundles of the flash stages toward the first flash stage in a serial flow communication, while its temperature increases in each flash stage relative to its temperature in the previous flash stage as vapor is condensing on the outside of the tube bundles.
The most common configuration of a multi stage flash evaporator is the “brine re-circulation” concept, in which the evaporator comprises a heat recovery section and a heat rejection section. The heat rejection section comprises a plurality of flash stages, typically 2 to 4, including the last flash stage, in which typically seawater is used as a coolant for the tube bundles. The heat rejection section is designed such, that the coolant is capable to remove together with the discharging distillate and the discharging concentrated flashing brine, the majority of the heat introduced into the system by the steam supplied from the power plant. In the heat recovery section, which comprises typically more than 10 flash stages including the first flash stage, the heat released from the flashing brine is recovered by a re-circulating brine as it is passing through the tube bundles as a coolant while the vapor released from the flashing brine is condensing on the outside of the tube bundles. The re-circulating brine is a mixture of a part of the concentrated flashing brine discharging from the last flash stage of the multi-stage flash evaporator and a part of the seawater discharging from the tube bundles of the heat rejection section. The portion of the seawater used as part of the re-circulating brine, replaces primarily the amount of distillate and concentrated flashing brine discharged from the system. It may be treated by chemical dosing in order to limit scaling of the tube bundles and it may be passed through a deaerator to remove non-condensable gases in order to limit corrosion in the evaporator.
Individual types of evaporators may be further differentiated by the tube bundle configuration such as ‘long tube’ evaporators and ‘cross tube’ evaporators.
Concepts of multi-stage flash desalination systems of prior art, in which thermal vapor compressors driven by a motive steam are used, are designed to extract a vapor from a flash stage of the multi-stage flash evaporator operating at low temperature and discharging the mixture of compressed extracted vapor and the motive steam into a flash stage operating at higher temperature or into the brine heater.
As steam supplied from a power plant typically contains chemicals harmful for human consumption, this process is not suitable for drinking water production if the condensate of such steam would be mixed in the flash stages with the distillate produced from vapor releases from the flashing brine.
As the heating steam supplied from the power plant and the condensate generated in the brine heater by condensation of the heating steam have a high purity as required in the power plant for the steam generation, it is not desirable to use a configuration as per prior art where a vapor extracted with a thermal vapor compressor from a flash stage of a multi-stage flash evaporator would become a part of the heating steam in the brine heater, because a vapor extracted from a flash stage contains a relatively high quantity of salt which would contaminate the condensate in the brine heater and, when returned to the power plant it would need to be purified before it could be re-used as boiler feed water.
The multi-stage flash desalination system of the present invention comprises a multi-stage flash desalination system of prior art and a thermal vapor compressor and a condensate flash tank, configured to receive the condensate generated in the brine heater and to flash down the condensate and to release a vapor which is then compressed in the thermal vapor compressor, so that it can be used as a part of the heating steam in the brine heater, which reduces the required amount of steam to be supplied from the power plant, while the temperature of the condensate discharged from the condensate flash tank and returned to the power plant is reduced relative to the temperature of the condensate discharging from the brine heater, which allows in the power plant to use a low grade heat from the exhaust gases of a steam generator to warm up the returned condensate, which increases the thermal efficiency of the power plant.
In the multi-stage flash desalination system of the present invention, the thermal vapor compressor is configured to receive a motive steam and to generate a steam pressure on a discharge connection substantially equal to the heating steam pressure required in the brine heater and a suction pressure on a suction connection which is lower than the steam pressure on the discharge connection.
The thermal vapor compressor may be installed in parallel to a steam control valve in which case the thermal vapor compressor would receive a first part of a steam supplied form a power plant as motive steam, while the remaining second part of the steam supplied from the power plant would pass as a bypass steam through the steam control valve. The steam discharging from the thermal vapor compressor and the bypass steam discharging from the steam control valve would be mixed and the temperature of this mixture would be reduced in a de-super heater to a temperature as needed for the heating steam.
Alternatively, the thermal vapor compressor may be installed downstream of the steam control valve in which case substantially all the steam supplied from the power plant would pass through the steam control valve and would then enter as motive steam into the thermal vapor compressor. In this case, the steam discharging from the thermal vapor compressor would pass through the de-super heater where the temperature of the steam would be reduced as needed for the heating steam.
The condensate flash tank is configured such, that it receives the condensate generated in the brine heater by condensation of the heating steam at a temperature substantially equal to the saturation temperature corresponding with the pressure of the heating steam entering into the brine heater, while the suction side of the thermal vapor compressor is connected to the condensate flash tank, where it is generating a pressure which is lower than the pressure of the heating steam in the brine heater, which results in a flash down of the condensate and a vapor release.
The thermal vapor compressor is configured to receive the vapor released in the condensate flash tank through a suction connection and to compress this vapor substantially to the pressure as required for the heating steam. The thermal vapor compressor is further configured to mix the compressed vapor with the motive steam entering into the thermal vapor compressor, so that the steam mass flow discharging from the thermal vapor compressor is equal to the mass flow of the motive steam plus the mass flow of the vapor released from the condensate in the condensate flash tank.
As the vapor released from the condensate becomes part of the heating steam, the mass flow of the steam supplied from the power plant can be reduced.
After the flash down of the condensate and vapor release, the condensate returns to the power plant at a temperature significantly lower than the condensate temperature at the discharge of the brine heater, which allows to utilize low grade heat of exhaust gases to heat up the condensate in the power plant, while otherwise such low grade heat would be wasted.
In this configuration of the present invention the vapor received by the thermal vapor compressor from the condensate flash tank has substantially the same high purity as the steam supplied from the power plant, so that the condensate generated in the brine heater by condensation of the heating steam can be returned to the power plant substantially at the same high purity as the steam supplied form the power plant.
This brief summary has been provided, so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings.
The present invention will be better understood from the following detailed description of an exemplary embodiment of the present invention, taken in conjunction with the accompanying drawings in which like reference numerals refer to like parts and in which:
For a better understanding of the present invention, the flow of liquids and vapor are shown in individual Figures in form of arrows indicating in individual positions the main flow direction.
A multi-stage flash desalination system of the present invention comprises a multi-stage flash desalination system of prior art like the systems shown in
Multi-stage flash desalination systems of prior art as shown in
The multi-stage flash desalination system of prior art comprises further a brine heater 6, typically designed as a tube and shell heat exchanger, and a multi-stage flash evaporator 7, which comprises a plurality of flash stages 8a, 8b . . . to 8n, each comprising a tube bundle 9a, 9b . . . to 9n.
The brine heater 6 is configured to receive a coolant discharging from the tube bundle 9a located in the first flash stage 8a and to convey this coolant through the tubes of the brine heater. Further the brine heater is configured to receive the heating steam 102a and to condense the heating steam 102a on the tubes of the brine heater to increase the temperature of the coolant passing through the tubes of the brine heater to a top temperature. Further, the first flash stage 8a of the multi-stage flash evaporator 7 is configured to receive the coolant discharging from the brine heater 6 at the top temperature as flashing brine 108.
Further, the flash stages 8a, 8b . . . 8n of the multi-stage flash evaporator are configured to allow the flashing brine 108 to flash down in each consecutive flash stage to a lower temperature and to release a vapor 109 and to condense this vapor on the tube bundles located in the individual flash stages and to collect the condensed vapor as a distillate 104. The distillate system 4 is configured to convey the distillate 104 from the last flash stage 8n out of the multi-stage flash desalination system.
Further, the steam supply system 2 is configured to control the flow rate and to reduce the steam pressure of the steam 102 supplied from the power plant through the steam control valve 2a as required for the heating steam 102a and to reduce the steam temperature of the steam discharging from the steam control valve 2a to a temperature as required for the heating steam 102a by injection of a condensate 103b in the de-super heater 2b, wherein the condensate system 3 is configured to branch off the condensate 103b from a condensate 103a generated by condensation of the heating steam 102a in the brine heater 6, wherein the condensate system 3 is further configured to convey the condensate 103 remaining after the condensate 103b is branched off from the condensate 103a, at a temperature substantially equal to the saturation temperature of the heating steam 102a, back to the power plant.
In case of a “once through” configuration as shown in
In the case of a “brine recirculation” configuration as shown in
The heat rejection section 7b comprises a plurality of typically two to four flash stages including the last flash stage 8n with the tube bundle 9n. The tube bundle 9n located in the last flash stage 8n of the multi-stage flash evaporator 7 is configured to receive the seawater 101 conveyed through the seawater supply system 1 as a coolant while all tube bundles located in the heat rejection section 7b are configured to convey this coolant through all tube bundles of the heat rejection section 7b in a serial flow communication.
The heat recovery section 7a comprises typically more than 10 flash stages including the first flash stage 8a with the tube bundle 9a. The tube bundles of the heat recover section are configured to convey a re-circulating brine 110 as coolant through all tube bundles located in the heat recovery section 7a in a serial flow communication and to discharge this coolant from the tube bundle 9a located in the first flash stage 8a. A brine re-circulation system 10 comprising at least one brine re-circulation pump 10a, is configured to receive and mix a first part of seawater 101a branched off from the seawater 101 discharging from the tube bundles of the heat rejection section 7b and a first part of the flashing brine 105a discharging from the last flash stage 8n, and to convey this mixture as re-circulating brine 110 through the tube bundles of the heat recovery section 7a and the brine heater 6.
A deaerator 1a is configured to receive the first part of seawater 101a and to remove non-condensable gases from the seawater before it is used as part of the re-circulating brine 110.
The brine discharge system 5 is configured to discharge a concentrated brine 105 out of the multi-stage flash desalination system, wherein this concentrated brine 105 is the remaining part of the flashing brine discharging from the last flash stage 8n, after the first part of flashing brine 105a has been conveyed to the brine re-circulation system 10.
The seawater system 1 is configured to convey a remaining part of seawater 101b out of the multi-stage flash desalination system, wherein this remaining part of seawater 101b is the part remaining from the seawater 101 discharging from the tube bundles of the heat rejection section 7b after a first part of seawater 101a has been conveyed to the brine re-circulating system 10.
In a multi-stage flash desalination system of the present invention, the thermal vapor compressor 2c is configured to receive a motive steam 102d through a motive steam connection 2d and to discharge at a discharge connection 2f a steam 102f at a pressure substantially equal to the pressure of the heating steam 102a. Further, the thermal vapor compressor 2c is configured to generate a suction pressure at a suction connection 2e, which is lower than the pressure of the heating steam 102a. Further, the thermal vapor compressor 2c is configured to receive a vapor 102e through the suction connection 2e.
In a multi-stage flash desalination plant of the present invention as shown in
Alternatively, in a multi-stage flash desalination system of the present invention as shown in
In both configurations of the thermal vapor compressor 2c as shown in
Further, the condensate flash tank 6a may comprise a mist eliminator 6e, which is configured to separate mist or droplets contained in the vapor 102e released from the condensate, before it is conveyed to the thermal vapor compressor 2c.
Further, the thermal vapor compressor 2c is configured to compress the vapor 102e received from the condensate flash thank 6a, to a pressure substantially equal to the pressure of the steam 102f discharging from the thermal vapor compressor and to mix the compressed vapor 102e with the motive steam 102d and to discharge both combined.
Further, the de-super heater 2b is configured to reduce the temperature of the steam passing through the de-super heater to a temperature as required for the heating steam 102a by injection of a condensate 103b.
Further, the condensate system 3 is configured to convey the condensate 103c remaining in the condensate flash tank 6a after the release of the vapor 102e, out of the condensate flash tank 6a through a condensate discharge connection 6c, and to branch off the condensate 103b and to convey it to the de-super heater 2b and to return the remaining condensate 103 to the power plant.
As the vapor 102e released from the condensate 103a becomes part of the heating steam 102a, the mass flow of the steam 102 supplied to the multi-stage flash desalination system can be reduced compared to the steam mass flow 102 required in a multi-stage flash desalination system of prior art.
As the condensate 103 is returned to the power plant at a lower temperature compared to the condensate temperature in multi-stage flash desalination systems of prior art, it allows to utilize low grade heat of exhaust gases in the power plant which otherwise would be wasted. The condensate 103 can be warm-up in the power plant by the exhaust gases of a steam generator without using additional fuel energy, while the fuel energy allocated to the steam supplied to the multi-stage flash desalination system of the present invention is reduced proportional to the reduced steam mass flow.
Depending on limitations of size and capacity of a thermal vapor compressor 2c and a total capacity required, in both configurations as shown in
The multi-stage flash desalination systems described and shown in
Details like the configuration of a condensate flash tank, a thermal vapor compressor, a multi-stage flash evaporator, etc are not shown, since those are commonly known details to those skilled in the field.
Although an exemplary embodiment of the invention has been described above by way of example only, it will be understood by those skilled in the field that modifications may be made to the disclosed embodiment without departing from the scope of the invention, which is defined by the appended claims.
