CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a national stage of PCT/JP08/056,245 filed Mar. 28, 2008 and published in Japanese, which has a priority of Japanese no. 2007-095674 filed Mar. 30, 2007, hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a molding machine for molding a gas hydrate by compression molding, and more specifically to a molding apparatus suitable for making pellets by compression molding of a gas hydrate in a high-pressure gas atmosphere. 
     2. Description of Related Art Including Information Disclosed Under 37 CFR §§1.97 and 1.98 
     Generally molding pellets by compressing powder adopts a molding apparatus having a pair of forming rolls having molding concavities on the surface thereof. As shown in  FIG. 6 , for example, the molding apparatus has a casing  4  which contains two forming rolls  6   a  and  6   b  being arranged closely to each other, a feed opening  2  for the powder a at the upper part of the casing  4 , and a discharge opening  3  for the pellets, which were formed by compression molding, at the lower part of the casing  4 . 
     In the molding apparatus  1  having the above structure, the powder a is supplied to the forming rolls  6   a  and  6   b  by a screw transfer unit  16 . The powder a is pressed and compacted in the molding concavities formed on the surface of the respective rolls  6   a  and  6   b  to become pellets P, which pellets P are then discharged from the discharge opening  3 . 
     The applicant of the present invention has already proposed a method for increasing the filling rate of gas hydrate, for improving the stability and safety during transportation and storage of the gas hydrate, or for improving the handling of the gas hydrate loading/unloading through the procedure that a raw material gas, containing methane as a principal component, such as natural gas, is brought into gas-liquid contact with water, under a specified pressure and temperature condition (for example, 5.4 MPa and 4° C.), to thereby form the gas hydrate, which gas hydrate is then molded into pellets through the compression molding by a pair of rotary rolls in said pressure atmosphere (for example, Patent Document 1).
     Patent Document 1: Japanese Patent Application Kokai Publication No. 2002-220353   

     BRIEF SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     When the above-described conventional molding apparatus is used for pelletizing a powder in a high-pressure atmosphere, since the raw material gas is a flammable gas or an ignitable gas like methane, there are requirements for providing each of bearings with a mechanical seal so as not to allow the leak of the high-pressure gas through said bearing parts, which results in an expensive molding apparatus. 
     Means to Solve the Problems 
     The present invention has been perfected to solve the above conventional problems and has features of (1) a gas hydrate compression molding apparatus comprising: a hopper which holds gas hydrate formed by the reaction of a raw material gas with a raw material water under a high pressure; a pair of forming rolls positioned at the upper portion of an opening part of the hopper; and a screw transfer unit which supplies gas hydrate powder in said hopper to said forming rolls, wherein said forming rolls are positioned in a high-pressure vessel which can pressurize thereof to said gas hydrate-forming pressure; (2) the gas hydrate compression molding apparatus in which said screw transfer unit is positioned in said high-pressure vessel; (3) the gas hydrate compression molding apparatus in which a driving unit for driving said forming rolls is positioned in said high-pressure vessel; and (4) the gas hydrate compression molding apparatus in which a driving machine forming the driving unit for driving said forming rolls and/or a driving machine for driving said screw transfer unit is a hydraulic motor. 
     Effect of the Invention 
     According to the invention, since the forming rolls are positioned in the high-pressure vessel allowing pressurizing thereof to a gas hydrate-forming pressure, there is no need of applying expensive mechanical seal, and thus the utility cost can be reduced. 
     Furthermore, only a simple modification that the conventionally used molding machine is positioned in the high-pressure vessel allows the conventional molding machine to be used under a high-pressure under which the gas hydrate forms. 
     According to one aspect of the invention, the screw transfer unit is positioned in the high-pressure vessel so that the screw shaft of the screw transfer unit does not penetrate through the high-pressure vessel. That is, there is no need of mounting a seal mechanism such as mechanical seal at a part where the screw shaft penetrates through the high-pressure vessel, which further suppresses the fabrication cost, and further prevents gas leak through the penetration part of the screw shaft. 
     According to another aspect of the invention, since the driving unit for the forming rolls is positioned in the high-pressure vessel, the drive shaft of the driving unit does not penetrate through the high-pressure vessel. Accordingly, at the part where the drive shaft penetrates through the high-pressure vessel, seal mechanism such as mechanical seal need not be mounted, thus the cost increase is prevented, and the gas leak from said penetrating part is not generated. 
     According to still another aspect of the invention, the driving machine of the driving unit adopts a hydraulic motor so that a spark which is observed in an electric motor is not generated, and thus there is no danger of ignition and explosion in the high-pressure vessel even in a high-pressure and flammable gas atmosphere. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the system of production apparatus for natural gas hydrate provided with the molding machine according to the present invention. 
         FIG. 2  shows a rough plan view of an embodiment of the molding machine according to the present invention. 
         FIG. 3  shows a side view of an embodiment of the molding machine according to the present invention. 
         FIG. 4  shows a side view of another embodiment of the molding machine according to the present invention. 
         FIG. 5  shows a side view of further embodiment of the molding machine according to the present invention. 
         FIG. 6  shows a rough side view of the molding machine having the conventional forming rolls. 
     
    
    
     DESCRIPTION OF THE REFERENCE SYMBOLS 
     
         
         
           
               1 ,  22  molding apparatus 
               2 ,  30  supply opening 
               3 ,  31  discharge opening 
               4  casing 
               5   a ,  5   b ,  34 , rotary shaft 
               42 ,  45   
               6   a ,  6   b , forming roll 
               33   a ,  33   b    
               7   a ,  7   b ,  7   c , bearing 
               7   d ,  35   a,    
               35   b ,  35   c,    
             
               35 
               d  
             
               8 ,  40  driving unit 
               9 ,  41 ,  42  electric motor 
               38  reduction gear 
               36  synchronous unit 
               13 ,  47  hopper 
               14  screw shaft 
               15 ,  46  screw 
               16 ,  43  screw transfer unit 
               20  slurry-forming apparatus 
               21  dewatering apparatus 
               22  molding apparatus 
               23  cooler 
               24  depressurizing apparatus 
               25  high-pressure vessel 
               26  molding machine 
               27  lid 
               28  bolt 
               29  support leg 
               32  frame 
               37 ,  39  support member 
               44   a ,  44   b  hydraulic motor 
               48  guide 
               50  mechanical seal 
           
         
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments of the molding machine according to the present invention will be described below referring to  FIGS. 1 to 4 . 
     Example 1 
       FIG. 1  shows a system diagram of the gas hydrate production apparatus provided with the molding machine according to the present invention. In  FIG. 1 , the reference symbol  20  signifies the slurry-forming apparatus,  21  signifies the dewatering apparatus,  22  signifies the molding machine,  23  signifies the cooler, and  24  signifies the depressurizing apparatus. 
     Into the slurry-forming apparatus  20  maintained to a specified pressure and temperature (for example, 5.4 MPa and 4° C.), a raw material gas g and a raw material water w are introduced, and an agitation method or a bubbling method is applied to bring the raw material gas g and the raw material water w into contact reaction, and to form a slurry h 1  containing gas hydrate. 
     Then, said slurry is dewatered in the dewatering apparatus  21  to prepare a gas hydrate h 2  in powder form containing large amounts of gas hydrate. 
     Said powdery gas hydrate h 2  is supplied to the molding apparatus  22  to produce pellets P having approximate lengths ranging from 5 to 30 mm. The gas hydrate pellets P are cooled by the cooler  23 , (for example, to about −20° C.), and are then depressurized to atmospheric pressure (0.1 MPa) by the depressurizing apparatus  24 . The depressurized pellets P are stored in a storage tank (not shown) positioned at the downstream side. 
     Next, the description will be given to the molding apparatus  22  of the gas hydrate, being applied to the gas hydrate production apparatus such as the one shown in  FIG. 1 , referring to  FIGS. 2 and 3 . 
     In  FIGS. 2 and 3 , the reference number  25  signifies a high-pressure vessel, which high-pressure vessel  25  is equipped with a lid  27  fixed by bolts  28 . The high-pressure vessel  25  is mounted on a base (not shown) by support legs  29  positioned at the lower portion of the vessel  25 . At the upper portion of said high-pressure vessel  25 , a supply opening  30  for the powdery gas hydrate h 2  is positioned. At the lower portion of said high-pressure vessel  25 , a discharge opening  31  for discharging the pellets P is positioned 
     In such structured high-pressure vessel  25 , a frame  32  is positioned. On the frame  32 , there are mounted a molding machine  26 , a synchronous unit  36 , and a reduction gear  38 . 
     A driving unit  40  which rotates the forming rolls  33   a  and  33   b  forming said molding machine  26  is composed of an electric motor  41 , the reduction gear  38 , and the synchronous unit  36  equipped with a synchronous gear. A rotary shaft  42  of the electric motor  41  penetrates through a side wall  25   a  of said high-pressure vessel  25 , and a mechanical seal  50  is provided at the penetrating part. 
     A screw transfer unit  43  is formed by a hopper  47  which holds the gas hydrate h 2 , and a screw  46  which supplies the gas hydrate h 2  in the hopper  47 . Said screw  46  is attached to a rotary shaft  45  of a hydraulic motor  44   a  located inside the high-pressure vessel  25 . The hopper  47  of said screw transfer unit  43  has a guide  48  which introduces the gas hydrate h 2  supplied from the supply opening  30  into the hopper  47 . 
     In the molding apparatus  22  for the gas hydrate, having such structure, the internal pressure of the pressure vessel  25  is the same as that of said dewatering apparatus  21 . Thus, the gas hydrate h 2  supplied from the dewatering apparatus  21  can be smoothly supplied without ejection caused by a pressure difference. 
     The powdery gas hydrate h 2  supplied from said dewatering apparatus  21  is supplied to the hopper  47  via the supply opening  30  and the guide  48 . The gas hydrate h 2  in the hopper  47  is fed under pressure in between the forming rolls  33   a  and  33   b  of the molding machine  26  by the screw transfer unit  43 . The fed gas hydrate h 2  under pressure is molded by compression molding in the molding concavities (not shown) to become the gas hydrate pellets P, which are then discharged from the discharge opening  31 , and are stored in a storage tank or the like (not shown). 
     According to the example, since the rotary shaft penetrating through the high-pressure vessel  25  is located at only a single site of the electric motor  41  of the molding machine  26 , the molding apparatus  22  can be fabricated at a low cost. 
     Furthermore, pellets can be produced without decomposing the gas hydrate in the high-pressure vessel  25 , and without the leak of the gas hydrate therefrom. 
     Example 2 
       FIG. 4  shows a side view of further another embodiment of the molding apparatus according to the present invention. The same reference symbol as that of  FIGS. 2 and 3  has the same name. 
     In the molding apparatus  22  in the embodiment replaces, the electric motor is replaced by a hydraulic motor  44   b , and further the hydraulic motor  44   b  is positioned in the high-pressure vessel  25 . 
     According to Example 2, since the rotary shaft penetrating through the high-pressure vessel  25  is only a single site of the rotary shaft  45  of the electric motor  42  of the screw transfer unit  43 , the seal part is located at a single site. Consequently, the molding apparatus  22  can be fabricated at a low cost. 
     Furthermore, by switching the electric motor as the driving machine to the hydraulic motor, a possibility of explosion in the high-pressure vessel  25  is eliminated even if the internal atmosphere is flammable gas such as natural gas containing methane as a principal component. 
     Example 3 
       FIG. 5  shows a side view of further another embodiment of the molding apparatus according to the present invention. The same reference symbol as that of  FIGS. 2 to 4  has the same name. 
     In the molding apparatus  22  in the embodiment, the driving unit  40  for the forming rolls  33   a  and  33   b  and the screw transfer unit  43 , are positioned in the high-pressure vessel  25 , and further positions the driving unit for driving the screw  46  of said screw transfer unit  43  are also positioned in the high-pressure vessel  25 . Said driving unit is the hydraulic motor  44   a.    
     According to Example 3, since there is no rotary shaft penetrating through the high-pressure vessel  25 , and there is no need of the seal part as in the above examples, the molding apparatus  22  can be fabricated at a low cost. 
     The embodiments described in said Examples 1 to 3 are only examples, and the present invention is not limited to these examples. A core of the present invention is the technological concept to reduce the mechanical seal parts by locating at least the driving motor in the high-pressure vessel, and it is clear that modifications can be possible within the range not to depart from such concept. 
     That is, the mechanical seal part can be eliminated by positioning the driving unit  40  of the forming rolls  33   a  and  33   b , the screw transfer unit  43 , and the driving unit  44   a  of the screw transfer unit  43  in the high-pressure vessel  25 , and thus the molding apparatus  22  can come to be fabricated at a low cost.
A molding machine for production of gas hydrate pellets under a high pressure in gas hydrate forming conditions, which is inexpensive through minimizing of the use of expensive mechanical seal. The molding machine comprises two forming rolls each fitted to a rotary shaft whose both ends are supported by bearings; a drive unit for rotating the forming rolls; a screw transfer unit for supplying powder to the forming rolls; and a high-pressure vessel, wherein the bearings, the rotary shaft and the forming rolls are all disposed in the high-pressure vessel.