Patent Publication Number: US-2006000355-A1

Title: Apparatus for generating freshwater

Description:
BACKGROUND OF THE INVENTION  
      1) Field of the Invention  
      The present invention relates to an apparatus for generating freshwater to serve as industrial water, drinking water, living water, and the like, by using exhaust combustion gas and seawater.  
      2) Description of the Related Art  
      An evaporation method, a reverse osmosis membrane method, an electrodialysis method, a refrigeration method, and the like have been conventionally proposed or put to practical use as methods for generating freshwater from seawater or so-called seawater desalting methods. Among them, the evaporation method and the reverse osmosis membrane method are typical seawater desalting methods. The evaporation method is for evaporating seawater in an evaporator, generating a steam, cooling the generated steam, and collecting the cooled steam as freshwater.  
      Since the evaporation method requires a large amount of energy for evaporating the water, a system for making an effective use of the energy has been considered. A typical system of this type is a multi-flash method. With this multi-flash method, a plurality of evaporators is arranged in series, and evaporation temperatures of the respective evaporators are changed by changing pressure reduction degrees thereof. In addition, a heat of condensation of a steam generated on the high temperature-side evaporators is used as a preheat for the seawater supplied to low temperature-side evaporators, thereby performing a heat collection. With this method, however, a problem of a need of much heat still remains unsolved.  
      The reverse osmosis membrane method is for applying a pressure equal to or higher than an osmotic pressure to a seawater side of a membrane using a semi-permeable membrane that selectively transmits water, and for collecting freshwater from the other side of the membrane. This method has, however, a problem of a high power cost because of the application of the pressure equal to or higher than the osmotic pressure to the processed seawater.  
      To deal with such conventional problems, various methods have been proposed, such as a method for condensing a steam, which is obtained by spray flushing and evaporating warm seawater near a surface of the sea, by cold seawater at a relatively low temperature in the sea (Japanese Patent Application Laid-open No. H2-214585), and a method, similar to the spray flush method, for using drainage water from a condenser provided in an LNG thermal plant as warm seawater and drainage water from an LNG vaporizer provided in the LNG thermal plant as cold seawater (Japanese Patent Application Laid-open No. H9-52082).  
      Furthermore, a power-generating and seawater-desalting combined method for combining a desalting apparatus with a power generating device, thereby obtaining a heat or a power necessary for the seawater desalting apparatus based on the evaporation method or the reverse osmosis membrane method (Japanese Patent Application Laid-open No. H10-47015).  
      With the evaporation method represented by the multi-flash method, it is necessary to evacuate the evaporators to vacuum. With the reverse osmosis membrane method, it is necessary to provide a high-pressure pump for feeding a liquid, and to maintain the membrane. Accordingly, in order to provide a small-scale desalination, a construction cost is disadvantageously pushed up.  
      Furthermore, as common problems to the evaporation methods including the spray flash method, since it is necessary to evaporate the seawater to correspond to a desalination amount, a volume of an evaporator increases, a heat energy necessary for the evaporation increases, and a desalination cost thereby increases.  
      The combined method of the power generating device with the seawater desalting apparatus can advantageously ensure a high energy-efficiency as a whole. However, this method has problems, such as the equipment is complicated and large in scale, and the combined apparatus is required to be operated while adjusting a load balance between the power generating device and the seawater desalting apparatus.  
      In view of the conventional problems, it is an object of the present invention to provide a freshwater generating apparatus capable of supplying freshwater even in a small-scale desalination plant.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to at least solve the problems in the conventional technology.  
      An apparatus according to one aspect of the present invention, which is for generating freshwater using exhaust combustion gas and seawater, includes a water spraying unit that sprays the seawater into the exhaust combustion gas; and a freshwater collecting unit that collects the freshwater from the exhaust combustion gas into which the seawater is sprayed.  
      The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a conceptual view of a freshwater generating apparatus according to a first embodiment of the present invention;  
       FIG. 2  depicts the specific device configuration of the freshwater generating apparatus according to the first embodiment; and  
       FIG. 3  is a conceptual view of a freshwater generating apparatus according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      Exemplary embodiments of an apparatus for generating freshwater according to the present invention will be explained below in detail with reference to the accompanying drawings. It should be noted that that the invention is not limited thereto. Furthermore, constituent elements in the embodiments below include elements that persons skilled in the art can easily assume or that are substantially the same.  
       FIG. 1  is a conceptual view of the freshwater generating apparatus according to the first embodiment of the present invention. The freshwater generating apparatus  3  is an apparatus for generating freshwater  13  using an exhaust combustion gas  11  from a burning unit  1  and seawater  12 . The freshwater generating apparatus  3  includes a water spraying unit  15  that sprays the seawater  12  into the exhaust combustion gas  11  and a freshwater collecting unit  17  that cools an exhaust combustion gas  16  into which the seawater is sprayed.  
      The exhaust combustion gas  11  used in the freshwater generating apparatus  3  is branched from a main gas flue  2  according to a necessary desalination amount, and a flow rate of the exhaust combustion gas  11  is adjusted by a gas volume adjustment unit (not shown) such as a damper. The freshwater generating apparatus  3  can be disposed on the main gas flue  2 . In this case, the burning unit  1  needs to be stopped during maintenance of the freshwater generating apparatus  3 . On the other hand, when the freshwater generating apparatus  1  is disposed separately from the main gas flue  2  as shown in  FIG. 1 , it is unnecessary to stop the burning unit  1  even for the maintenance of the freshwater generating apparatus  3 . Therefore, an excellent operativity is ensured. While an exhaust combustion gas  18  after the freshwater generating apparatus  3  collects the freshwater  13  is returned again to the main gas flue  2  and discharged from an exhaust flue  4 . Alternatively, a different gas flue and a different exhaust flue can be provided for the exhaust combustion gas  18 .  
      According to the first embodiment, the seawater  12  is evaporated in the water spraying unit  15  by an exhaust heat of the exhaust combustion gas  11 , so that it is unnecessary to apply a fresh heat. Furthermore, the freshwater  13  can be collected from not only the fluid evaporated from the seawater  12  but also the fluid inherent in the exhaust combustion gas  11 . It is, therefore, possible to generate more freshwater as compared with the simple evaporation method. To collect the freshwater  13  from the fluid in the exhaust combustion gas  11 , a temperature of the exhaust combustion gas  11  needs to be set equal to or lower than a temperature (dew point) at which the fluid in the exhaust gas is condensed. However, by spraying the seawater  12  using the water spraying unit  15 , the fluid is humidified and cooled until the fluid turns into a saturation state. As a result, the temperature of the exhaust combustion gas decreases and the dew point increases, so that a reduction in a size of the cooling unit provided in the freshwater collecting unit  17  can be realized.  
      As a fuel for the burning unit  1 , a clean fuel that hardly causes generation of a sulfur oxide, a dust, and the like after being burned is preferably used. When the sulfur oxide, the dust, and the like exist in the exhaust combustion gas, they can be possibly mixed into the collected freshwater. To eliminate them, an additional device is necessary. Examples of such a clean fuel include hydrocarbon, alcohol, and coal gasified gas from which impurities are eliminated. It is particularly preferable to use one of light hydrocarbons such as natural gas and liquefied petroleum gas as the clean gas. These fuels are higher in hydrogen content, so that a water concentration in the exhaust combustion gas  11  is higher. As a result, a saturation temperature of the exhaust combustion gas  16  at an exit of the water spraying unit is higher, making it possible to collect the freshwater in larger amounts. Furthermore, since impurities other than the hydrocarbon exist only in small amounts, amounts of impurities, for example, salt in the freshwater can be suppressed to be small. If coal, heavy oil or the like is used as the fuel, by contrast, the saturation temperature of the exhaust combustion gas  16  is lower and an additional device for eliminating impurities is required. As a result, the cost of desalination is increased.  
      Examples of a type of the burning unit  1  include a boiler, a turbine, and an engine. Among them, the boiler is particularly preferably used as the burning unit  1  for the following reason. When the boiler is used, an excess air ratio during burning can be suppressed to be low. Therefore, the water concentration in the exhaust combustion gas  11  is increased and the freshwater collection amount can be increased. As the exhaust combustion gas  11 , a clean exhaust gas that hardly contains the sulfur oxide, the dust and the like, and that has a high water concentration is preferably used. The exhaust gas obtained by burning the light hydrocarbon such as the natural gas or the liquefied petroleum oil in the boiler is more preferable in those respects.  
      With reference to  FIG. 2 , a specific example of the freshwater generating apparatus will be explained.  
      As shown in  FIG. 2 , the freshwater generating apparatus includes the water spraying unit  15  that sprays the seawater  12  into the exhaust combustion gas  11  and the freshwater collecting unit  17  that cools the exhaust combustion gas  16  humidified and cooled by the seawater in the water spraying unit  15  and that collects the freshwater  13 .  
      The exhaust combustion gas  11  introduced into the water spraying unit  15  is preferably an exhaust combustion gas having a temperature equal to or higher than 130° C. after the heat is collected for the burning unit. The reason is as follows. When the temperature of the exhaust combustion gas  11  is low, the temperature of the exhaust combustion gas  16  cooled and humidified after spraying the seawater. As a result, the water amount retained in the exhaust combustion gas  16  is reduced and a collectable amount of the freshwater is reduced. Accordingly, the higher the temperature of the exhaust combustion gas  11  is, the greater the freshwater collection amount becomes. However, since the heat collection amount is reduced on the burning unit side, the exhaust combustion gas at a temperature equal to or lower than 200° C. is practically used.  
      The water spraying unit  15  includes a gas-liquid contact unit  30  that causes a gas-liquid contact between the seawater  12  and the exhaust combustion gas, a seawater supply pump  31 , a seawater supply nozzle  32 , a seawater unevaporated-water receiving unit  33 , and a demisting unit  34 . The seawater  12  is pumped up by the seawater supply pump  31  to the seawater unevaporated-water receiving unit  33 , and sprayed into the exhaust combustion gas  11  by the seawater supply nozzle  32 . A part of the seawater  12  sprayed by the seawater supply nozzle  32  is evaporated by the heat of the exhaust combustion gas  11 , whereas a remainder thereof enters the seawater unevaporated-water receiving unit  33 .  
      A salt concentration in the seawater in the seawater unevaporated-water receiving unit  33  is slightly increased by evaporation. Therefore, a part of the seawater therein is returned to the sea via a pipe  35  and fresh seawater  12   a  is supplied from a pipe  36 . Preferably, solid components of the seawater  12   a  newly supplied from the pipe  36  are eliminated by an operation such as filtration before the seawater  12   a  is supplied.  
      A type of the gas-liquid contact unit in the water spraying unit  15  is not limited to a specific form and any normally used gas-liquid contact unit can be used. In the example of the water spraying unit shown in  FIG. 2 , a spray tower method for simply spraying the seawater into the exhaust combustion gas  11  from the seawater supply nozzle  32  is shown. Alternatively, a liquid column tower method for arranging a spray header in a lower portion and blowing up a liquid or a packed tower method for providing a packed object for the gas-liquid contact can be used. In addition, a direction in which the exhaust combustion gas  11  flows can be either a horizontal direction shown in  FIG. 2  or an upward flow direction or a downward flow direction as a vertical direction.  
      The demisting unit  34  is disposed on an exhaust combustion gas exit of the water spraying unit  15 . This is intended to prevent a part of the seawater  12  sprayed into the exhaust combustion gas  11  from entering the freshwater collecting unit  17  to accompany the exhaust combustion gas as a mist, being mixed with the collected freshwater, and increasing the salt content in the freshwater. Therefore, a performance of this demisting unit  34  is determined, so that the salt concentration of the generated freshwater is equal to or lower than a required specified value.  
      On the other hand, the salt concentration in the seawater in the seawater unevaporated-water receiving unit  33  of the water spraying unit  15  increases by the evaporation of the seawater by the water spraying unit. When the salt concentration increases, a boiling point rises. As a result, the timing of the evaporation becomes late, and problems such as scaling, corrosion of materials, and an increase in the salt concentration in the collected freshwater occur. To prevent these problems, a part of the seawater is discharged as purge seawater  12   b  from the pipe  35 , and the fresh seawater  12   a  is supplied from the pipe  36  instead. This seawater supply can be performed by either using the circulation pipes for spray as shown in  FIG. 2  or supplying the seawater to the seawater unevaporated-water receiving unit  33 .  
      The supplied sweater  12   a  at a higher temperature is advantageous because, when the temperature of the supplied seawater  12   a  is higher, the evaporation in the water spraying unit  15  is more accelerated and the saturation temperature of the exhaust combustion gas  16  at the exit is higher. Accordingly, when the seawater used by the freshwater collecting unit  17  (described later) for cooling or the high-temperature seawater used by the plant side such as the boiler for cooling is used, a higher desalination efficiency can be attained.  
      The seawater can be sprayed by a one-path flow instead of circulating the seawater from the seawater unevaporated-water receiving unit  33  as shown in  FIG. 2 . In this case, the seawater  12  is fed to the seawater supply nozzle  32  by the seawater supply pump  31  or a pump that replaces the pump  31 , and sprayed. Furthermore, the unevaporated seawater is temporarily collected in the unevaporated-water receiving unit  33  and then discharged from the pipe  35  without being used in a circulating manner. In this case, the temperature of the sprayed seawater decreases, so that the saturation temperature of the exhaust combustion gas  16  decreases and a desalination amount is slightly reduced. Nevertheless, this can advantageously simplify the apparatus.  
      The exhaust combustion gas  16  output from the water spraying unit  15  then enters the freshwater collecting unit  17 . This freshwater collecting unit  17  includes a gas-liquid contact unit  40  that causes a gas-liquid contact between the humidified exhaust combustion gas  16  and the freshwater, a freshwater supply pump  41 , a freshwater supply nozzle  42 , a freshwater collection tank  43 , and a freshwater cooling unit  44 .  
      The exhaust combustion gas  16  humidified and cooled by the water spraying unit  15  is in direct contact with the freshwater supplied from the freshwater supply pump  41  as the gas-liquid contact, thereby cooling the exhaust combustion gas  16 . As a result, the fluid in the exhaust combustion gas  16  is condensed and the resultant exhaust combustion gas  16  enters, together with the freshwater from the freshwater supply nozzle  42 , the freshwater collection tank  43 . The freshwater  13  collected in the freshwater collection tank  43  is partially discharged from a pipe  45  whereas a remainder thereof is cooled by the freshwater cooling unit  44  and used in a circulating manner for cooling the exhaust combustion gas  16 .  
      A type of the gas-liquid contact unit of the freshwater collecting unit  17  is not limited to a specific form similarly to the water spraying unit  15 , and any normally used gas-liquid contacting unit is can be used. While in  FIG. 2 , an example of providing a packed bed  46  for the gas-liquid contact to accelerate the cooling of the exhaust combustion gas  16  is shown, the spray tower or the liquid column tower can be employed without providing the packed bed.  
      A standard cooling temperature for cooling the exhaust combustion gas  16  is preferably 35° C. to 50° C. for the following reason. If the cooling temperature is too high, a saturated water concentration in the cooled exhaust combustion gas increases and the amount of the collected freshwater (desalination amount), therefore, decreases. This is because, when the cooling temperature decreases, the collected freshwater amount increases, and the freshwater cooling unit  44  is made larger in scale, whereby there is no merit in increasing the collected freshwater amount.  
      The freshwater cooling unit  44  is not limited to a specific form as long as the device can indirectly cool the freshwater  13  using a low-temperature fluid  47 . For example, a plate heat exchanger can be used. In the freshwater cooling unit  44  shown in  FIG. 2 , an example of providing the indirect heat exchanger that cools the freshwater  13  using the low-temperature fluid  47  on a cooling freshwater circulation line is shown. Alternatively, the indirect heat exchanger that cools the freshwater  13  using the low-temperature fluid  47  can be provided in the freshwater collection tank  43 .  
      As the low-temperature fluid  47  used in the freshwater cooling unit  44 , the seawater is normally used. However, the low-temperature fluid  47  is not limited to the seawater as long as the fluid  47  can cool the freshwater down to a temperature equal to or lower than the cooling temperature for cooling the exhaust combustion gas. When a liquefied natural gas is used as the fuel, a cold heat of the liquefied natural gas can be used. When the freshwater can be easily cooled using the cold heat or the like of the liquefied natural gas, it is preferable to set the cooling temperature as low as possible to increase the collected freshwater amount.  
      To cool the exhaust combustion gas  16 , not the gas-liquid contact method but a method for generating the freshwater by assembling the indirect heat exchanger into the packed bed  46  and cooling the exhaust combustion gas  16  can be considered. However, the method according to the present invention shown in  FIG. 2  is advantageous over the latter method since the cooled collected freshwater is brought into a direct contact with the gas, and the method according to the present invention can thereby more effectively cool the exhaust combustion gas.  
       FIG. 3  is a schematic diagram of an example of a freshwater generating apparatus according to a second embodiment of the present invention, when the natural gas is used as a fuel and a boiler exhaust combustion gas is used as the exhaust gas. Since the freshwater generating apparatus according to a second embodiment of the present invention is substantially equal to that according to the first embodiment, like components are designated with like reference signs, and redundant explanations thereof will be omitted.  
      As shown in  FIG. 3 , the exhaust combustion gas from a boiler  1   a  is branched from the main gas flue  2  and fed to the freshwater generating apparatus  3 . By branching the exhaust combustion gas from the main gas flue and feeding the exhaust combustion gas to the freshwater generating apparatus, the following advantages can be attained. It is unnecessary to stop the boiler for a checking and the like of the freshwater generating apparatus. In addition, the apparatus can be employed according to a necessary desalination amount. For these advantages, a damper  5  is provided in a branch portion from the main gas flue. This damper  5  can be provided with a gas volume adjustment device so as to be able to branch the exhaust combustion gas according to the desalination amount when it is necessary.  
      The exhaust combustion gas  11  branched from the main gas flue  2  is fed first to the water spraying unit  15 .  
      The water spraying unit  15  includes the seawater supply pump  31 , the seawater supply nozzle  32 , the seawater unevaporated-water receiving unit  33 , and the demisting unit  34 . The exhaust combustion gas  11  fed to the water spraying unit  15  comes into contact with the seawater sprayed by the seawater supply nozzle  32 , and humidified and cooled down to near the saturation temperature.  
      The exhaust combustion gas  11  entering the water spraying unit  15  normally has a water content of less than 16% although the water content differs according to a composition of the natural gas serving as the fuel or boiler burning conditions. In addition, the temperature of the exhaust combustion gas  11  is near 200° C. although the temperature differs according to boiler conditions. For this reason, the water concentration in the exhaust combustion gas  11  is far apart from the saturation state. By humidifying and cooling the exhaust combustion gas  11  down to the saturation temperature, more fluid in the seawater can be evaporated.  
      Furthermore, a seawater spray amount is determined according to the gas-liquid contacting unit method. When the spray tower or the liquid column tower is used, the standard seawater spray amount is normally about 0.1 to 4 (I/Nm 3 ) relative to the exhaust combustion gas amount.  
      When the temperature of the exhaust combustion gas at an inlet of the water spraying unit is about 180° C., this saturation temperature is about 60° C. although the saturation temperature differs according to the water concentration in the exhaust combustion gas  11 , the temperature of the exhaust combustion gas  11 , the temperature of the seawater supplied from the seawater supply nozzle or the like. The water content in the exhaust combustion gas at the exit of the water spraying unit increases by about 6% to near 22%. After the mist component of this humidified and cooled exhaust combustion gas is eliminated by the demisting unit, the resultant exhaust combustion gas is fed to the freshwater collecting unit  17 . The mist is eliminated by the demisting unit so that the salt concentration in the freshwater collected by the freshwater collecting unit  17  is equal to or lower than a required level (for example, 250 ppm that is the WTO standard).  
      On the other hand, the unevaporated component of the seawater sprayed from the seawater supply nozzle is collected into the seawater unevaporated-water receiving unit  33  in the lower portion of the water spraying unit  15 . Thereafter, a part of the unevaporated component of the seawater is discharged as the purge seawater  12   b  by the pipe  35  for adjustment of the salt concentration whereas a remainder thereof is used again for humidifying and cooling the exhaust combustion gas by the seawater supply pump  31 .  
      The freshwater collecting unit  17  includes the gas-liquid contacting unit that has the freshwater collection tank  43  provided in a lower portion and the packed bed  46  for the gas-liquid contact provided in an upper portion. The freshwater supply nozzle  42  for spraying the freshwater cooled by the freshwater cooling unit  44  is provided above the gas-liquid contacting unit. The humidified and cooled exhaust combustion gas  16  comes into contact with the cooled freshwater sprayed from the freshwater supply nozzle, and the temperature of the exhaust combustion gas  16  decreases. The fluid in the exhaust combustion gas  16  in a supersaturation state is condensed and enters the freshwater collection tank  43 . When the saturated water concentration shown above is, for example, 22% and the exhaust combustion gas having such a saturated water concentration is cooled down to near 45° C., the saturated water concentration in the burned gas decreases to near 9%. In addition, the water of slightly less than 13% is condensed and the condensed water enters the freshwater collection tank  43 . On the other hand, the exhaust combustion gas  18  cooled by the freshwater is fed to the main gas flue  2  and discharged from the exhaust flue  4 .  
      The freshwater collected into the freshwater collection tank  43  and sprayed for cooling and the condensed water from the exhaust combustion gas  16  are discharged from the freshwater supply pump  41 . They are collected as the freshwater  13  by as much as an amount corresponding to the condensed water from the exhaust combustion gas  16 , whereas a remainder thereof is fed to the freshwater cooling unit  44 .  
      The freshwater fed to the freshwater cooling unit  44  is indirectly cooled by the seawater  12  fed by a cooling seawater pump  48 , fed to the freshwater supply nozzle  42  provided above the gas-liquid contact unit of the freshwater collecting unit  17 , and used again for cooling the exhaust combustion gas. On the other hand, a part of the seawater the temperature of which is raised by a heat exchange with the freshwater in the freshwater cooling unit  44  is supplied to the seawater unevaporated-water receiving unit of the water spraying unit  15  through the pipe  36  as the supplemental seawater  12   a  for the water spraying unit  15 . The remainder thereof is discharged into the sea. When the flow rate of this supplemental seawater  12   a  is higher, an increase in the salt concentration in the seawater supplied in a circulating manner by the water spraying unit is more suppressed. However, the temperature of the supplied seawater  12   a  is normally lower than the temperature of the circulating seawater. Due to this, the temperature of the exhaust combustion gas  16  at the exit of the water spraying unit decreases and the desalination amount is reduced. Conversely, if the flow rate of the supplemental seawater  12   a  is too low, then the salt component in the seawater  12   a  is condensed and the problems such as the boiling point rise and the scaling unfavorably occur.  
      According to the above embodiments, the freshwater of a volume little over about 100 m 3  can be generated from the exhaust combustion gas of a volume of 1 million Nm 3 . In addition, even if the fuel is changed from the natural gas to the liquefied petroleum gas, the seawater can be similarly generated except that the collectable desalination amount is slightly reduced by the reduction in the water concentration in the exhaust combustion gas  11  (under the same conditions as those according to the embodiments).  
      As describe above, according to the present invention, the freshwater can be effectively collected even from the fluid inherently present in the exhaust combustion gas. An evaporation amount of the seawater necessary for desalination can be, therefore, reduced. In addition, the heat used for evaporating the seawater is an exhaust heat, so that it is unnecessary to apply a fresh heat. At the same time, the exhaust combustion gas for collecting the fluid in the exhaust combustion gas is cooled, and a cooling cost can be, therefore, reduced.  
      Furthermore, according to the present invention, it is unnecessary to additionally provide a special device such as a pressure reducing device normally required in the evaporation method or the pressurizing device for the reverse osmosis membrane method. It is, therefore, possible to generate freshwater at an economical cost even in a small-scale desalination plant.  
      Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.