Patent Publication Number: US-2012042784-A1

Title: Aeration apparatus including water-repellent layer and seawater flue gas desulfurization apparatus including the same

Description:
FIELD 
     The present invention relates to wastewater treatment in a flue gas desulphurization apparatus used in a power plant such as a coal, crude oil, or heavy oil combustion power plant. In particular, the invention relates to an aeration apparatus for aeration used for decarboxylation (air-exposure) of wastewater (used seawater) from a flue gas desulphurization apparatus for desulphurization using a seawater method. The invention also relates to a seawater flue gas desulphurization apparatus including the aeration apparatus. 
     BACKGROUND 
     In conventional power plants that use coal, crude oil, and the like as fuel, combustion flue gas (hereinafter referred to as “gas”) discharged from a boiler is emitted to the air after sulfur oxides (SO x ) such as sulfur dioxide (SO 2 ) contained in the flue gas are removed. Known examples of the desulphurization method used in a flue gas desulphurization apparatus for the above desulphurization treatment include a limestone-gypsum method, spray dryer method, and seawater method. 
     In a flue gas desulphurization apparatus that uses the seawater method (hereinafter referred to as a “seawater flue gas desulphurization apparatus”), its desulphurization method uses seawater as an absorbent. In this method, seawater and flue gas from a boiler are supplied to the inside of a desulfurizer (absorber) having a vertical tubular shape such as a vertical substantially cylindrical shape, and the flue gas is brought into gas-liquid contact with the seawater used as the absorbent in a wet process to remove sulfur oxides. The seawater (used seawater) used as the absorbent for desulphurization in the desulfurizer flows through, for example, a long water passage having an open upper section (Seawater Oxidation Treatment System: SOTS) and is then discharged. In the long water passage, the seawater is decarbonated (exposed to air) by aeration that uses fine air bubbles ejected from an aeration apparatus disposed on the bottom surface of the water passage (Patent documents 1 to 3). 
     Citation List 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2006-055779 
     Patent Literature 2: Japanese Patent Application Laid-open No. 2009-028570 
     Patent Literature 3: Japanese Patent Application Laid-open No. 2009-028572 
     SUMMARY 
     Technical Problem 
     Aeration nozzles used in the aeration apparatus each have a large number of small slits formed in a rubber-made diffuser membrane that covers a base. Such aeration nozzles are generally referred to as “diffuser nozzles.” These aeration nozzles can eject many fine air bubbles of substantially equal size from the slits with the aid of the pressure of the air supplied to the nozzles. Conventionally, in the case of a rubber-made diffuser membrane, the length of the slit is about 1 to 3 millimeters. 
     When aeration is continuously performed in seawater using the above aeration nozzles, precipitates such as calcium sulfate in the seawater are deposited on the wall surfaces of the slits of the diffuser membranes and around the openings of the slits, causing the gaps of the slits to be narrowed and the slits to be clogged. This results an increase in pressure loss of the diffuser membranes, and the discharge pressure of discharge unit, such as a blower or compressor, for supplying the air to the diffuser is thereby increased, so that disadvantageously the load on the blower or compressor increases. 
     The occurrence of the precipitates may be due to the following reason. Seawater present outside a diffuser membrane permeates inside the diffuser membrane through its slits and comes into continuous contact with air passing through the slits for a long time. Drying (concentration of the seawater) is thereby facilitated, and the precipitates are deposited. 
     In view of the above problem, it is an object of the present invention to provide an aeration apparatus that can suppress and avoid generation of precipitates in the slits of diffuser membranes, and a seawater flue gas desulfurization apparatus including the aeration apparatus. 
     Solution to Problem 
     According to an aspect of the present invention, an aeration apparatus that is immersed in water to be treated and generates fine air bubbles in the water to be treated, includes: an air supply pipe for supplying air through a discharge unit; and an aeration nozzle including a diffuser membrane having a slit, the air being supplied through the slit to the aeration nozzle. A water-repellent layer is provided at least at one of an opening and vicinity thereof of the slit. 
     Advantageously, in the aeration apparatus, the water-repellent layer is a coating layer made of a hydrophobic material. 
     Advantageously, in the aeration apparatus, the water-repellent layer is any one of a fluorine coating layer, a silicone coating layer, and a wax coating layer. 
     Advantageously, in the aeration apparatus, the water-repellent layer is a fractal structure layer. 
     Advantageously, in the aeration apparatus, the diffuser membrane is made of rubber, metal, or ceramic. 
     According to another aspect of the present invention, an aeration apparatus that is immersed in water to be treated and generates fine air bubbles in the water to be treated, includes: an air supply pipe for supplying air through a discharge unit; and an aeration nozzle including a diffuser membrane having a slit, the air being supplied through the slit to the aeration nozzle. The diffuser membrane is formed by adding a hydrophobic material thereto in an amount from 25 to 95 parts by weight per 100 parts by weight of a rubber material, and a water-repellent layer is provided at least at one of an opening and vicinity thereof of the slit. 
     According to still another aspect of the present invention, an aeration apparatus that is immersed in water to be treated and generates fine air bubbles in the water to be treated, includes: an air supply pipe for supplying air through a discharge unit; an aeration nozzle including a diffuser membrane having a slit, the air being supplied through the slit to the aeration nozzle; and a hydrophobic-material supply unit that adds a hydrophobic material to the air supply pipe. 
     According to still another aspect of the present invention, a seawater flue gas desulphurization apparatus includes: a desulfurizer that uses seawater as an absorbent; a water passage for discharging used seawater discharged from the desulfurizer; and the aeration apparatus according to any one of claims  1  to  7  that is disposed in the water passage, the aeration apparatus generating fine air bubbles in the used seawater to decarbonate the used seawater. 
     Advantageous Effects of Invention 
     According to the present invention, generation of precipitates can be suppressed and avoided in the slits of the diffuser membranes of the aeration apparatus. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of a seawater flue gas desulphurization apparatus according to an embodiment. 
         FIG. 2A  is a plan view of aeration nozzles. 
         FIG. 2B  is a front view of the aeration nozzles. 
         FIG. 3  is a schematic diagram of the inner structure of an aeration nozzle. 
         FIG. 4  is a schematic diagram of an aeration apparatus according to the embodiment. 
         FIG. 5  is a schematic diagram of an opening of a slit formed in a diffuser membrane of the aeration nozzle according to the embodiment. 
         FIG. 6A  depicts the outflow of air (humid air having a low degree of saturation), the inflow of seawater, and a state of concentrated seawater in the slit of the diffuser membrane. 
         FIG. 6B  depicts the outflow of air, the inflow of seawater, and states of concentrated seawater and precipitates in the slit of the diffuser membrane. 
         FIG. 6C  depicts the outflow of air, the inflow of seawater, and states of concentrated seawater and precipitates (when precipitates grow) in the slit of the diffuser membrane. 
         FIG. 7  is a schematic diagram of another aeration apparatus according to the embodiment. 
         FIG. 8  is an example of a pattern diagram of a fractal structure. 
         FIG. 9  is a chart obtained by analyzing precipitates by X-ray diffraction. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to embodiments described below. The components in the following embodiments include those readily apparent to persons skilled in the art and those substantially similar thereto. 
     Embodiments 
     An aeration apparatus and a seawater flue gas desulphurization apparatus according to embodiments of the present invention will be described with reference to the drawings.  FIG. 1  is a schematic diagram of the seawater flue gas desulphurization apparatus according to one embodiment. 
     As shown in  FIG. 1 , a seawater flue gas desulphurization apparatus  100  includes: a flue gas desulphurization absorber  102  in which flue gas  101  and seawater  103  comes in gas-liquid contact to desulphurize SO 2  into sulfurous acid (H 2 SO 3 ); a dilution-mixing basin  105  disposed below the flue gas desulphurization absorber  102  to dilute and mix used seawater  103 A containing sulfur compounds with dilution seawater  103 ; and an oxidation basin  106  disposed on the downstream side of the dilution-mixing basin  105  to subject diluted used seawater  103 B to water quality recovery treatment. 
     In the seawater flue gas desulphurization apparatus  100 , the seawater  103  is supplied through a seawater supply line L 1 , and part of the seawater  103  is used for absorption, i.e., is brought into gas-liquid contact with the flue gas  101  in the flue gas desulphurization absorber  102  to absorb SO 2  contained in the flue gas  101  into the seawater  103 . The used seawater  103 A that has absorbed the sulfur components in the flue gas desulphurization absorber  102  is mixed with the dilution seawater  103  supplied to the dilution-mixing basin  105  disposed below the flue gas desulphurization absorber  102 . The diluted used seawater  103 B diluted and mixed with the dilution seawater  103  is supplied to the oxidation basin  106  disposed on the downstream side of the dilution-mixing basin  105 . Air  122  supplied from an oxidation air blower  121  is supplied to the oxidation basin  106  from aeration nozzles  123  to recover the quality of the seawater, and the resultant water is discharged to the sea as treated water  124 . 
     In  FIG. 1 , reference numeral  102   a  represents spray nozzles for injecting seawater upward as liquid columns;  120  represents an aeration apparatus;  122   a  represents air bubbles; L 1  represents a seawater supply line; L 2  represents a dilution seawater supply line; L 3  represents a desulphurization seawater supply line; L 4  represents a flue gas supply line; and L 5  represents an air supply line. 
     The structure of the aeration nozzles  123  is described with reference to  FIGS. 2A ,  2 B, and  3 . 
       FIG. 2A  is a plan view of the aeration nozzles;  FIG. 2B  is a front view of the aeration nozzles; and  FIG. 3  is a schematic diagram of the inner structure of an aeration nozzle. 
     As shown in  FIGS. 2A and 2B , each aeration nozzle  123  has a large number of small slits  12  formed in a rubber-made diffuser membrane  11  that covers the circumference of a base and is generally referred to as a “diffuser nozzle.” In such an aeration nozzle  123 , when the diffuser membrane  11  is expanded by the pressure of the air  122  supplied from the air supply line L 5 , the slits  12  open to allow a large number of fine air bubbles of substantially equal size to be ejected. 
     As shown in  FIGS. 2A and 2B , the aeration nozzles  123  are attached through flanges  16  to headers  15  provided in a plurality of (eight in the present embodiment) branch pipes (not shown) branched from the air supply line L 5 . In consideration of corrosion resistance, resin-made pipes, for example, are used as the branch pipes and the headers  15  disposed in the diluted used seawater  103 B. 
     For example, as shown in  FIG. 3 , each aeration nozzle  123  is formed as follows. A substantially cylindrical support body  20  that is made of a resin in consideration of corrosion resistance to the diluted used seawater  103 B is used, and a rubber-made diffuser membrane  11  having a large number of slits  12  formed therein is fitted on the support body  20  so as to cover its outer circumference. Then the left and right ends of the diffuser membrane  11  are fastened with fastening members  22  such as wires or bands. 
     The slits  12  described above are closed in a normal state in which no pressure is applied thereto. In the seawater flue gas desulphurization apparatus  100 , because the air  122  is continuously supplied, the slits  12  are constantly in an open state. 
     A first end  20   a  of the support body  20  is attached to a header  15  and allows the introduction of the air  122 , and the support body  20  has an opening at its second end  20   b  that allows the introduction of the seawater  103 . 
     In the support body  20 , the side close to the first end  20   a  is in communication with the inside of the header  15  through an air inlet port  20   c  that passes through the header  15  and the flange  16 . The inside of the support body  20  is partitioned by a partition plate  20   d  disposed at some axial position in the support body  20 , and the flow of air is blocked by the partition plate  20   d.  Air outlet holes  20   e  and  20   f  are formed in the side surface of the support body  20  and disposed on the header  15  side of the partition plate  20   d.  The air outlet holes  20   e  and  20   f  allow the air  122  to flow between the inner circumferential surface of the diffuser membrane  11  and the outer circumferential surface of the support body, i.e., into a pressurization space  11   a  for pressurizing and expanding the diffuser membrane  11 . Therefore, the air  122  flowing from the header  15  into the aeration nozzle  123  flows through the air inlet port  20   c  into the support body  20  and then flows through the air outlet holes  20   e  and  20   f  formed in the side surface into the pressurization space  11   a,  as shown by arrows in  FIG. 3 . 
     The fastening members  22  fasten the diffuser membrane  11  to the support body  20  and prevent the air flowing through the air outlet holes  20   e  and  20   f  from leaking from the opposite ends. 
     In the aeration nozzle  123  configured as above, the air  122  flowing from the header  15  through the air inlet port  20   c  flows through the air outlet holes  20   e  and  20   f  into the pressurization space  11   a.  Since the slits  12  are closed in the initial state, the air  122  is accumulated in the pressurization space  11   a  to increase the inner pressure. The increase in the inner pressure of the pressurization space  11   a  causes the diffuser membrane  11  to expand, and the slits  12  formed in the diffuser membrane  11  are thereby opened, so that fine bubbles of the air  122  are injected into the diluted used seawater  103 B. Such fine air bubbles are generated in all the aeration nozzles  123  to which air is supplied through branch pipes L 5A  to L 5H  and the headers  15  (see  FIGS. 6 and 7 ). 
       FIG. 4  is a schematic diagram of the aeration apparatus according to the present embodiment. As shown in  FIG. 4 , an aeration apparatus  120  according to the present embodiment is immersed in diluted used seawater (not shown), which is water to be treated, and generates fine air bubbles in the diluted used seawater. This aeration apparatus  120  includes: an air supply line L 5  that supplies the air  122  from blowers  121 A to  121 D serving as discharge units; and aeration nozzles  123  each including the diffuser membrane  11  having slits for supplying air. 
     Two cooling units  131 A and  131 B and two filters  132 A and  132 B are respectively provided in the air supply line L 5 . Accordingly, air compressed by the blowers  121 A to  121 D is cooled and then filtered. The cooled and filtered air is supplied by all the aeration nozzles  123  that receive air supply through branch pipes L 5A  to L 5H  and the headers  15 , thereby generating fine air bubbles. 
     There are four blowers, but normally, three blowers are used for operation, and one of them is a reserve blower. Since the aeration apparatus must be continuously operated, only one of the two cooling units  131 A and  131 B and only one of the two filters  132 A and  132 B are normally used, and the others are used for maintenance. 
     The aeration apparatus according to the present embodiment is explained below. In the present invention, water-repellent treatment is applied to at least one of the opening and the vicinity thereof of the slit to be formed in the diffuser membrane  11  to prevent the inflow of seawater into the slit, and precipitation of calcium sulfate and the like in the slits  12  can be suppressed and avoided. 
       FIG. 5  is a schematic diagram of an opening of the slit  12  formed in the diffuser membrane  11  of the aeration nozzle  123  according to the present embodiment. 
     As shown in  FIG. 5 , the slit  12  according to the present embodiment is provided with a water-repellent layer  150  formed on a slit wall surfaces  12   a  and an edge  12   b  of the opening. In this manner, by applying the water-repellent treatment to the opening and the vicinity thereof, precipitation of precipitates can be suppressed and avoided. 
     The salt concentration in seawater is 3.4%, and 3.4% of salts are dissolved in 96.6% of water. The salt includes 77.9% of sodium chloride, 9.6% of magnesium chloride, 6.1% of magnesium sulfate, 4.0% of calcium sulfate, 2.1% of potassium chloride, and 0.2% of other salts. 
     Of these salts, calcium sulfate is deposited first as seawater is concentrated (dried), and the precipitation threshold value of the salt concentration in seawater is about 14%. 
     A result of analysis of precipitates adhered to a slit is shown in  FIG. 9 .  FIG. 9  is a chart obtained by analyzing a precipitate by X-ray diffraction. As shown in  FIG. 9 , it was found that most peaks are derived from calcium sulfate. 
     A mechanism in which precipitates are deposited in the slits  12  is explained with reference to  FIGS. 6A to 6C . 
       FIG. 6A  depicts the outflow of air (humid air having a low degree of saturation), the inflow of seawater, and a state of concentrated seawater in the slit of the diffuser membrane.  FIG. 6B  depicts the outflow of air, the inflow of seawater, and states of concentrated seawater and precipitates in the slit of the diffuser membrane.  FIG. 6C  depicts the outflow of air, the inflow of seawater, and states of concentrated seawater and precipitates (when precipitates grow) in the slit of the diffuser membrane. 
     In the present invention, the slits  12  are cuts formed in the diffuser membrane  11 , and the gap of each slit  12  serves as a discharge passage of air. 
     The seawater  103  is in contact with slit wall surfaces  12   a  that form the passage. The introduction of the air  122  causes the seawater  103  to be dried and concentrated to form concentrated seawater  103   a.  A precipitate  103   b  is then deposited on the slit wall surfaces  12   a  and clogs the passage in the slits  12 . 
       FIG. 6A  depicts a state in which salt content in seawater is gradually concentrated to form the concentrated seawater  103   a  due to low relative humidity of the air  122  (low degree of saturation). However, even if the concentration of the seawater is initiated, deposition of calcium sulfate and the like does not occur when the salt concentration in the seawater is about 14% or less. 
     In the state shown in  FIG. 6B , the precipitate  103   b  is generated in portions of the concentrated seawater  103   a  in which the salt concentration in the seawater locally exceeds 14%. In this state, the amount of the precipitate  103   b  is very small. Therefore, although the pressure loss when the air  122  passes through the slits  12  increases slightly, the air  122  can pass through the slits  12 . 
     On the other hand, in the state shown in  FIG. 6C , because the concentration of the concentrated seawater  103   a  has proceeded further, a clogged (plugged) state due to the precipitate  103   b  is formed, and the pressure loss becomes high. Even in this state, the passage of the air  122  remains even in this state; however, a large burden is imposed on a discharge unit. 
     Therefore, to avoid such a problem, the water-repellent layer  150  is provided at least at one of an opening and the vicinity thereof of the slit  12  to prevent the inflow of seawater into the slit, and suppress and avoid generation of the precipitate  103   b  in the slit, thereby enabling a stable operation for a long time. 
     Various water-repellent materials can be mentioned as a material for forming the water-repellent layer. For example, a coating layer formed of a hydrophobic material using talc or silica powder, a fluorine coating layer coated with a fluorine resin, a silicone coating layer coated with a silicone resin, and a wax coating layer coated with wax can be mentioned. 
     At the time of coating the hydrophobic material, it is desired to use a fixing agent or the like so that the hydrophobic material does not exfoliate immediately. The water-repellent layer can be formed at the time of mold release of the diffuser membrane or thereafter. 
     As a result of chemically applying the water-repellent treatment by using a water-repellent material in this manner, the surface of the slit has a hydrophobic property to repel water. 
     Accordingly, the inflow of seawater into the slit can be suppressed and avoided, the salt concentration of seawater is not increased, and precipitation of precipitates is prevented. 
       FIG. 8  is a pattern diagram of a fractal structure. The surface of the slit can be formed as a fractal structure layer in which an infinite number of physical concave-convex surfaces are formed, thereby improving its water repellency. The fractal structure has a structure in which concave and convex structures are nested such that small concavity and convexity are present in large small concavity and convexity, such as the Koch curve, and smaller concavity and convexity are present in the small concavity and convexity, thereby increasing its wettability. 
     At the time of forming the slit, for example, the opening is formed by plasma processing to form an infinite number of concave-convex surfaces in the opening portion. At this time, it is desired that the opening is formed in an inert atmosphere. This is for preventing generation of oxygen functional groups. 
     While a rubber-made diffuser membrane is desired, the present invention is not limited thereto, and a stainless-steel or resin diffuser membrane can be used, for example. 
     As a fluorine resin, for example, polytetrafluoro-ethylene (a tetrafluorinated resin, abbreviated as PTFE), polychloro-trifluoroethylene (a trifluorinated resin, abbreviated as PCTFE or CTFE), polyvinylidene fluoride (abbreviated as PVDF), polyvinyl fluoride (abbreviated as PVF), perfluoroalkoxy fluororesin (abbreviated as PFA), tetrafluoroethylene/hexafluoropropylene copolymer (abbreviated as FEP), ethylene/tetrafluoroethylene copolymer (abbreviated as ETFE), ethylene/chlorotrifluoroethylene copolymer (abbreviated as ECTFE) can be exemplified. 
     This water-repellent treatment is applied after formation of slits. 
     A hydrophobic material can be added and kneaded to the diffuser membrane  11  itself. 
     For example, the hydrophobic material can be added in an amount from 25 to 95 parts by weight per 100 parts by weight of a rubber material to form the diffuser membrane. As a result, the diffuser membrane can have a water-repellent layer provided at least at one of an opening and the vicinity thereof of the slit  12 . If the added amount of the hydrophobic material is out of the above range, a water-repellent effect cannot be developed, which is not preferable. 
     For example, the hydrophobic material can include talc and silica power; however, the present invention is not limited thereto. 
     Further, it is preferable to use ethylene-propylene-diene monomer rubber (EPDM rubber) as the rubber material. 
       FIG. 7  is a schematic diagram of another aeration apparatus according to the present embodiment. 
     As shown in  FIG. 7 , an aeration apparatus  120 A according to the present embodiment further includes a hydrophobic-material supply unit  161  that adds a hydrophobic material  160  in the aeration apparatus  120  shown in  FIG. 4 , to supply the hydrophobic material  160  into the air supply line L 5  through a hydrophobic material line L 6 . 
     For example, as the hydrophobic material  160  to be added, it is desired that at least one of talc and silica powder is used. 
     As the supply of the hydrophobic material  160 , at the time of supplying the air  122  to supply fine air from the aeration nozzles  123 , it is desired to remove the precipitate from the slit  12  after pressure fluctuation, and then to apply water-repellent treatment. 
     As the removal of precipitates, an air purge operation or an air suspending operation is performed so as to give fluctuation to the slit  12  of the diffuser membrane  11 , thereby removing the precipitates adhered to the slit  12 . 
     By applying the water-repellent treatment, the slit  12  has water repellency and becomes stain-resistant. 
     In the present embodiment, while seawater has been exemplified as the water to be treated, the present invention is not limited thereto. For example, plugging caused by deposition of contamination components such as sludge on diffuser slits (membrane slits) can be prevented in the aeration apparatus for aeration of contaminated water in decontamination processing, and thus the aeration apparatus can be stably operated for a long time. 
     In the present embodiment, while tube-type aeration nozzles have been exemplified for explaining the aeration apparatus, the present invention is not limited thereto. For example, the invention is applicable to disk-type and flat-type aeration apparatuses and to diffusers made of ceramic or metal (ex. stainless). 
     INDUSTRIAL APPLICABILITY 
     As described above, in the aeration apparatus according to the present invention, generation of precipitates can be suppressed and avoided in the slits of the diffuser membranes of the aeration apparatus. For example, when applied to a seawater flue gas desulphurization apparatus, the aeration apparatus can be continuously operated in a stable manner for a long time. 
     REFERENCE SIGNS LIST 
       11  diffuser membrane 
       12  slit 
       100  seawater flue gas desulphurization apparatus 
       102  flue gas desulphurization absorber 
       103  seawater 
       103 A used seawater 
       103 B diluted used seawater 
       105  dilution-mixing basin 
       106  oxidation basin 
       120 ,  120 A aeration apparatus 
       123  aeration nozzle 
       150  water-repellent layer 
       160  hydrophobic material