Abstract:
A gas hydrate slurry dewatering apparatus adapted to feed a raw as into a cylindrical main body of dewatering column so gas to attain pressuriation and so suction any gas from the interior of a drainage chamber disposed around the cylindrical main body so as to attain depressurization. An internal tube ( 8 ) as a constituent of a dewatering apparatus ( 6 ) in which the gas hydrate slurry (S) is introduced is provided with a separating section ( 7 ). A drainage chamber ( 10 ) is formed by the internal tube ( 8 ) and, disposed with a given spacing therefrom, an external tube ( 9 ). An exhaust blower (B 2 ) and a drainage pump (P 2 ) are connected to the drainage chamber ( 10 ). A gas feed blower (B 3 ) for a raw gas (G 1 ) is connected to the internal tube ( 8 ). A differential pressure detector (x 1 ) is provided for detecting any pressure difference between the interior of the internal tube ( 8 ) and the interior of the drainage chamber ( 10 ). Control of the exhaust blower (B 2 ) and/or the gas feed blower (B 3 ) is performed by the signal from the differential pressure detector (x 1 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a national stage of PCT/JP08/055,487 filed Mar. 25, 2008 and published in Japanese, which has a priority of Japanese no. 2007-093991 filed Mar. 30, 2007, hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a dewatering apparatus for a gas hydrate slurry, and more specifically, to a dewatering apparatus in a production plant of gas hydrate in which a gas hydrate slurry is generated by being subjected to a hydration reaction of raw material gas such as methane or the like, and raw material water. 
     2. Description of Related Art 
     In recent years, natural gas which contains methane or the like as a major component has captured much of the spotlight as a clean energy source. Then, for purpose of transportation and storage, a practice of transforming such a natural gas into a liquified natural gas (hereinafter, referred to as LNG) is being conducted. Since, however, the transportation and storage of a gas in the form of a LNG requires maintaining it in a cryogenic state, not only a generation system but also a transportation system and a storage system have become quite expensive. As a consequence, they are limited to only large-scale gas fields, and were economically unfeasible for smaller-scale gas fields. 
     Under these circumstance, studies on manufacturing natural gas hydrate (hereinafter, simply referred to as gas hydrate) by causing natural gas to react with water, and transporting or storing it through the gas hydrate are being carried out. With regard to this gas hydrate, it is well known that the raw material gas and the raw material water are introduced into a reactor in which a predetermined temperature and pressure selected from among, for example, temperatures of 1 to 10° C. and atmospheric pressures of 30 to 100 atmosphere are retained, to generate a slurry which contains a crystalline-like gas hydrate. Then, this slurry is introduced into a dewatering apparatus to separate and remove unreacted water, and is subsequently again brought into contact with the raw material gas to manufacture a powdery gas hydrate having low water content. 
     In a production plant for such a gas hydrate, a horizontal screw press-type dewatering apparatus and a vertical gravity-type dewatering apparatus are proposed as a dewatering apparatus (e.g., Patent Document 1). 
     A horizontal screw press-type dewatering apparatus as described in such a Patent Document 1 is made of a double construction combined with a mesh-processed inner wall, and a cylindrical body constituting an outer shell situated at the outside of the inner wall, and it is configured such that a gas hydrate is drained from meshes processed on the inner wall by advancing the gas hydrate while forcedly squeezing it by a screw shaft mounted inside the inner wall. 
     In such a dewatering apparatus, the gas hydrate was consolidated and was adhered to the surface of a screw, during said process of dewatering said gas hydrate. As a result a load of the screw shaft was increased, and thus such a dewatering apparatus was required to be driven at a high torque. 
     Thus, in order to solve the problem with said dewatering apparatus, the present inventors have studied a dewatering apparatus in which the gas hydrate slurry is supplied into the cylindrical body by a slurry pump, and water is drained naturally from a porous portion of the cylindrical body while causing it to move up in succession, through the use of a vertical-type dewatering apparatus having a separating section formed to be porous at an intermediate section of a cylindrical body (e.g., Patent Documents 2, 3). 
     The vertical-type dewatering apparatus as described in Patent Document 2, the present inventors previously proposed, includes a cylindrical main body with drain holes formed at substantially intermediate section, and a dewatering collecting section (drainage chamber) provided around said drain holes. Then, the gas hydrate slurry supplied to the dewatering apparatus is designed to be dewatered resulting from unreacted water being drained from said drain holes. 
     Further a vertical-type dewatering apparatus as described in Patent Document 3, the present inventors previously proposed, is configured such that a dewatering column is made of a double cylindrical construction consisting of two cylindrical bodies of an internal tube and an external tube, and dewatering filtration elements are provided on both side walls of the internal tube and external tube respectively, then the unreacted water is caused to outflow to the outside of the column through both the filtration elements provided on the internal tube and the external tube. 
     Incidentally, since a dewatering apparatus as described in said Patent Document 2 is configured such that water and hydrate are separated by the action of gravity, there was a problem of slow rates at which the unreacted water is drained from said drain holes. In addition, the dewatering column must be high enough to enhance dewatering efficiency, and thus there was a problem with the increase in size of the apparatus. 
     A dewatering column as described in the other Patent Document 3 includes an annular-shaped bottom plate, an annular-shaped shielding plate, a gas hydrate-crushing device, and plural tabular blades provided in radial form at the lower end and so on, to form a complicated construction. Therefore, there was a problem that a period required to manufacture the dewatering column becomes longer, along with a higher cost.
     Patent Document 1: Japanese Patent Application Kokai Publication No. 2003-105362   Patent Document 2: Japanese Patent Application Kokai Publication No. 2006-111769   Patent Document 3: Japanese Patent Application Kokai Publication No. 2006-257359   

     BRIEF SUMMARY OF THE INVENTION 
     Thus, the present inventors, in view of the problems in said Patent Documents 2 and 3, have sought to provide a dewatering column of a simple construction that restricts the height of a cylindrical main body of the dewatering column and improves a drainage capability in the middle part of a gas hydrate layer. 
     The present invention was made to solve the above-described conventional problems, and a dewatering method in a production plant of a gas hydrate according to the present invention is a method for dewatering unreacted water contained in a gas hydrate slurry generated through gas-liquid contact between raw material water and raw material gas, characterized in that an external tube is arranged around an internal tube of said dewatering apparatus to form a drainage section, and a pressure difference between said drainage section and a gas hydrate layer formed at an upper level above a separating section of said internal tube is generated by exhausting a gas of said drainage section and/or introducing a gas from the upper part of said internal tube. 
     Then, the dewatering apparatus in the production plant of the gas hydrate according to the present invention is an apparatus to dewater the unreacted water contained in the gas hydrate slurry purified through gas-liquid contact between the raw material water and the raw material gas, characterized in being configured such that an external tube is arranged around an internal tube of said dewatering apparatus to form a drainage section, and a pressure difference between said drainage section and the gas hydrate layer formed at the upper level above the separating section of said internal tube is generated by exhausting a gas in said drainage section and/or introducing a gas from an upper part of said internal tube. 
     EFFECT OF THE INVENTION 
     With a dewatering method for a gas hydrate according to the invention, a difference between a pressure inside a drainage chamber and a pressure inside an internal tube where the gas hydrate comes up is detected by a differential pressure detector, and the operation of an intake blower and/or a gas feed blower is controlled according to its signal. Therefore, a pressure difference between inside the drainage chamber and inside the internal tube can be retained at a predetermined value and its differential pressure can be increased, and as the unreacted water contained in the gas hydrate is squeezed from the drainage section, dewatering efficiency is improved. 
     With a dewatering apparatus of the gas hydrate according to the invention, a difference between a pressure inside the drainage chamber and a pressure inside an internal tube where the gas hydrate comes up is detected by a differential pressure detector, and the operation of an intake blower and/or gas a feed blower is controlled according to its signal. Therefore, a pressure difference between inside the drainage chamber and inside the internal tube can be retained at a predetermined value, and its differential pressure can be increased, and the unreacted water contained in the gas hydrate is squeezed and drained from the drainage section. As a result, a dewatering apparatus having good performance and in a small size can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of the first exemplary embodiment of a dewatering apparatus in a production plant of a gas hydrate according to the present invention. 
         FIG. 2  is a schematic view of the second exemplary embodiment of a dewatering apparatus in a production plant of a gas hydrate according to the present invention. 
         FIG. 3  is a schematic view of the third exemplary embodiment of a dewatering apparatus in a production plant of a gas hydrate according to the present invention. 
     
    
    
     LIST OF REFERENCE LETTERS 
     
         
         
           
               1  reactor 
               2  gas supply line 
               3  water supply line 
               4  coolant 
               5  slurry line 
               6  dewatering apparatus 
               7  separating section 
               8  internal tube 
               9  external tube 
               10  drainage chamber 
               11  exhaust line 
               12  drainage line 
               13  hydrate layer 
               14  storage section 
               15  screw conveyor 
               16  gas supply line 
               17  first external tube 
               18  second external tube 
               19  partition wall 
               20  communicating chamber 
             B 1  raw material gas supply blower 
             B 2  exhaust blower 
             B 3  gas feed blower 
             P 1  slurry pump 
             P 2  drainage pump 
             S slurry 
             G gas 
             W water 
             H gas hydrate 
             x 1  differential pressure detector 
             x 2  level gauge 
           
         
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, exemplary embodiments of a dewatering apparatus in a production plant of a gas hydrate according to the present invention will be described with reference to  FIG. 1  to  FIG. 3 . 
     Example 1 
       FIG. 1  is a schematic view for illustrating the first exemplary embodiment of a dewatering apparatus in a production plant of a gas hydrate according to the present invention. In  FIG. 1 , a reactor  1  is retained at predetermined pressure and temperature. A raw material gas G 1  from a gas supply line  2  to the reactor  1 , and raw material water W 1  from a water supply line  3  are respectively introduced, wherein a gas hydrate slurry S is generated. 
     Then, the slurry S is supplied via a slurry line  5  having a slurry pump P 1  to a dewatering apparatus  6 , where being separated into unreacted water W 2  and a gas hydrate H. To describe it in detail, the dewatering apparatus  6  is configured such that an internal tube  8  having a separating section  7  constituted by, for example, porous elements or the like, and an external tube  9  arranged to have a predetermined spacing from the internal tube  8  form a drainage chamber  10 , one end of an exhaust gas line  11  having an exhaust blower B 2  is connected to the upper part of said drainage chamber  10 , one end of a drainage line  12  having a drainage pump P 2  is connected to the lower part of said drainage chamber  10 , then a differential pressure detector x 1  for detecting a differential pressure between a pressure inside said internal tube  8  and a pressure inside said drainage chamber  10  is provided, and thereby said exhaust blower B 2  is controlled according to the signal from the differential pressure detector x 1 . 
     In addition, there is provided a supply line  16  for raw material gas connected to the upper part of a reactor where a gas hydrate slurry S is generated, as well as being connected to the upper end side of the internal tube  8 , and a gas feed blower B 3  is provided on the supply line  16 , and configured to be controlled according to the signal from said differential pressure detector x 1 . 
     In such a configuration, a pressure in the internal tube  8  is maintained higher by a predetermined value of pressure than a pressure in the drainage chamber  10  by driving either one or both of the exhaust blower B 2  and the gas feed blower B 3  under the action of the differential pressure detector x 1 . 
     Then, when the gas hydrate slurry S generated in said reactor  1  is introduced from the lower part of the internal tube  8  constituting the dewatering apparatus  6 , the slurry S moves up in the internal tube  8  to reach a separating section  7 , where the unreacted water W 2  forming the slurry S is drained into the drainage chamber  10 . 
     A gas hydrate H from which the unreacted water W 2  has been drained moves further up in the internal tube  8 , which forms a gas hydrate layer  13  at the upper side of the internal tube  8 . At this moment, a part of the unreacted water W 2  moves up to the lower part of the gas hydrate layer  13  (near the separating section  7 ) due to capillarity and it is likely to form a gas hydrate layer having a high water content. But, as a raw material gas G 1  is introduced into the internal tube  8  and thus a pressure inside the internal tube  8  becomes higher than a pressure inside the drainage chamber  10 , the unreacted water W 2  is squeezed from the holes of the separating section  7 , thereby to be drained into the drainage chamber  10 . 
     The unreacted water W 2  which has been drained into the drainage chamber  10  is sucked by a drainage pump P 2 , and returned via a drainage line  12  to the reactor  1 . A level gauge x 2  is equipped in said drainage chamber  10 , and the drainage pump P 2  is controlled according to the signal from the level gauge x 2  such that a fluid level of the unreacted water W 2  that has been drained into the drainage chamber  10  is controlled to be maintained at a predetermined position. 
     Then, the gas hydrate H which has been dewatered is supplied to equipment on the downstream side thereof by a screw conveyor  15  as a discharge device. 
     According to the present Example, a pressure inside the drainage chamber can be reduced lower than a pressure inside the internal tube  8  by sucking a gas in the drainage chamber  10  with the use of the exhaust blower B 2 , which enables to suck the unreacted water W 2  contained in the slurry. 
     In addition, a raw material gas G 1  is circulated by the gas feed blower B 3  from the upper part of the internal tube  8  to the drainage chamber  10 , and thus the raw material gas can be brought into countercurrent contact with the hydrate layer  13  and the unreacted water W 2  can be purged and removed. In this case, it is enough to put the exhaust blower B 2  at a standstill and to allow the raw material gas G 1  to flow into a bypass line (not shown). 
     In the case of the dewatering process, a part of the unreacted water W 2  is subjected to a hydration reaction so as to become hydrated through the contact with the raw material gas G 1 , which thus exerts effectiveness that the water content of the hydrate layer  13  can further be reduced. In addition, it is easy to control a pressure inside the internal tube  8  so as not to be lower than that inside a generator  1 , whereby there is also no risk that the hydrate may be decomposed during the process of dewatering. 
     Further, a gas in the drainage chamber  10  may be sucked by the exhaust blower B 2 , while circulating the raw material gas G 1  by the gas feed blower B 3  from the upper part of the internal tube  8  to the drainage chamber  10 . In that case, since the above-described effectiveness can be obtained at the same time, an excellent dewatering effectiveness can be obtained. 
     Example 2 
       FIG. 2  is a schematic view for illustrating the second exemplary embodiment of a dewatering apparatus of a gas hydrate according to the present invention, the same reference letters as those of  FIG. 1  denote the same names, and their descriptions will be omitted. 
     In the  FIG. 2 , a dewatering apparatus  6  includes an internal tube  8  having a separating section  7 , an external tube  9  arranged to have a predetermined spacing from the internal tube  8 , and a partition wall  19  situated between the external tube  9  and the internal tube  8  and attached to the upper part of said separating section  7 , wherein a communicating chamber  20  that communicates with an interior of the internal tube  8  over the partition wall  19  and a drainage chamber  10  below the communicating chamber  20  are formed. 
     A differential pressure detector x 1  is designed to detect a differential pressure between inside the communicating chamber  20  and inside the drainage chamber  10  and to control the exhaust blower B 2  and/or the gas feed blower B 3 . 
     A level gauge x 2  is provided in said drainage chamber  10 , and the drainage pump P 2  is controlled according to the signal from the level gauge x 2  such that a liquid level of the unreacted water W 2  drained into the drainage chamber  10  is maintained at a predetermined position. 
     In the dewatering apparatus  6  configured in this way, a pressure inside the internal tube  8  is maintained higher by a predetermined value of pressure than a pressure inside the drainage chamber  10  by driving the gas feed blower B 3 , while being under the action of said differential pressure detector x 1 . Then, when a gas hydrate slurry S generated in said reactor  1  is introduced from the lower part of the internal tube  8  constituting the dewatering apparatus  6 , the slurry S moves up in the internal tube  8  to reach the separating section  7 , where the unreacted water W 2  forming the slurry S is drained into the drainage chamber  10 . 
     A gas hydrate H from which the unreacted water W 2  has been drained moves further up in the internal tube  8 , which forms a gas hydrate layer  13  at the upper side of the internal tube  8 . At this moment, a part of the unreacted water W 2  moves up to the lower part of the gas hydrate layer  13  (near the separating section  7 ) due to capillarity and it is likely to form a gas hydrate layer having a high water content. But, as a raw material gas G 1  is introduced into the internal tube  8  and thus a pressure inside the internal tube  8  becomes higher than a pressure inside the drainage chamber  10 , the unreacted water W 2  is squeezed from the holes of the separating section  7 , thereby to be drained into the drainage chamber  10 . 
     The unreacted water W 2  which has been drained into the drainage chamber  10  is sucked by a drainage pump P 2 , and is returned via a drainage line  12  to the reactor  1 . A level gauge x 2  is equipped in said drainage chamber  10 , and the drainage pump P 2  is controlled according to the signal from the level gauge x 2  such that a fluid level of the unreacted water W 2  that has been drained into the drainage chamber  10  is controlled to be maintained at a predetermined position. 
     Then, the gas hydrate H which has been dewatered is supplied to equipment on the downstream side thereof by a screw conveyor  15  as a discharge device. 
     According to the present Example, the dewatering apparatus  6  is made of a double tube construction with the drainage chamber  10  in the outer side and the internal tube  8  in the inner side, which has improved pressure resistance compared with a construction in which the external tube is provided in a part of the internal tube. Therefore, a pressure difference (differential pressure) between inside the drainage chamber  10  and inside the internal tube  8  can take a larger value by the activation of the exhaust blower B 2  and/or the gas feed blower B 3 , and the unreacted water W 2  of the slurry S can be drained more powerfully than the above-described Example. 
     Further, since a dewatering column is made of a double tube construction, the separating section  7  can be provided from the lower side to the upper side of the internal tube, and thus a dewatering performance of the slurry is improved. Therefore, the size of the dewatering apparatus can be made significantly smaller than that of the conventional vertical gravity-type dewatering apparatus. 
     In the present Example also, a gas contained in the drainage chamber  10  is sucked via an exhaust gas line  11 , and the raw material gas G 1  can be introduced into the internal tube  8  via the supply line  16 . In addition, by sucking a gas contained in the drainage chamber  10  through the use of the exhaust blower B 2 , a pressure inside the drainage chamber  10  can be reduced lower than a pressure inside the internal tube  8 , and the unreacted water W 2  contained in the slurry can be also sucked. 
     Example 3 
       FIG. 3  is a schematic view for illustrating the third exemplary embodiment of a dewatering apparatus of a gas hydrate according to the present invention. In the  FIG. 3 , the same reference letters as those in  FIG. 1  and  FIG. 2  denote the same names and their descriptions will be omitted. 
     In the  FIG. 3 , a first external tube  17  is a skirt-shaped partition wall in which the upper part is a periphery of an internal tube  8  and is attached to the upper part of a separating section  7 , and the lower part is opened. The first external tube  17  and the internal tube  8  form a drainage chamber  10  and a communicating chamber  20  whose lower parts are opened. Difference between a pressure inside the communicating chamber  20  and a pressure inside the drainage chamber  10  is detected by a differential pressure detector x 1 , and an exhaust blower B 2  and/or a gas feed blower B 3  are controlled according to its signal. 
     In addition, an operation of a suction pump  14  is controlled by a level gauge  18  such that the lower end of the first external tube  17  may become lower than a fluid level of unreacted water W 2  which has been drained from a slurry S. The inside of the first external tube  17  (drainage chamber  10 ) and that of the communicating chamber  20  are sealed by the unreacted water W 2 . 
     In the dewatering apparatus  6  configured in this way, a pressure inside a second external tube  18  is kept higher by a predetermined value of pressure than s pressure inside a first external tube  17  by driving the gas feed blower B 3 , while being under the action of said differential pressure detector x 1 . Then, when a gas hydrate slurry S generated in the reactor  1  is introduced from the lower part of the internal tube  8 , the slurry S moves up in the internal tube  8  to reach the separating section  7 , where the unreacted water W 2  forming the slurry S is drained into the first external tube  17 . 
     A gas hydrate H from which the unreacted water W 2  has been drained moves further up in the internal tube  8 , which forms a gas hydrate layer  13  at the upper side of the internal tube  8 . At this moment, a part of the unreacted water W 2  moves up to the lower part of the gas hydrate layer  13  (near the separating section  7 ) due to capillarity and it is likely to form a gas hydrate layer having high water content. But, as a raw material gas G 1  is introduced into the internal tube  8  and thus a pressure inside the internal tube  8  becomes higher than a pressure inside a first external tube  17 , the unreacted water W 2  is squeezed from the holes of the separating section  7 , thereby to be drained into the first external tube  17 . 
     The unreacted water W 2  drained into the first external tube  17  is sucked by a drainage pump P 2  and returned via a drainage line  12  to a reactor  1 . A level gauge x 2  is provided on said first external tube  17 , and the drainage pump P 2  is controlled according to the signal from the level gauge x 2  such that a fluid level of the unreacted water W 2  that has been drained into the first external tube  17  is controlled to be maintained at a predetermined position. 
     Then, the gas hydrate H which has been dewatered is supplied to equipment on the downstream side thereof by a screw conveyor  15  as a discharge device. 
     In the exemplary embodiment, since it is designed to detect a difference between a pressure inside the communicating chamber  20  and a pressure inside the drainage chamber  10 , a drainage pump P 2  will be activated so as to attain a predetermined differential pressure that has been preset in a level gauge x 2 , for example, even if a pressure inside the internal tube  8  is changed by changing operation status. As a consequence, the apparatus can continue to operate without deterioration of a dewatering ratio or a dewatering speed or the like. In addition, if said differential pressure is changed, a fluid level of the unreacted water W 2  that seals the interior of the drainage chamber  10  and that of the communicating chamber  20  is designed to be changed in water level depending on a magnitude of its differential pressure. Consequently, possible damages to the dewatering apparatus when sporadic pressure changes occur will be prevented.