Patent Publication Number: US-2017348644-A1

Title: Method for producing hollow fiber membrane and hollow fiber membrane-spinning nozzle

Description:
TECHNICAL FIELD 
     The present invention relates to a hollow fiber membrane-spinning nozzle for producing a hollow fiber membrane by applying a fiber-forming dope capable of forming a porous membrane layer on the peripheral surface of a long hollow porous substrate. The present invention also relates to a method for producing a hollow fiber membrane using the hollow fiber membrane-spinning nozzle. 
     BACKGROUND ART 
     Due to tightened regulations and growing concerns over environmental pollution in recent years, people have shown interest in water treatment that uses filtration membranes characterized by compact size and complete separation capability. For such water treatment purposes, filtration membranes are required to exhibit excellent separation capability and permeability along with even higher mechanical properties than before. 
     As for a pressure-resistant porous filtration membrane, a hollow fiber membrane is known to have a porous membrane layer formed on the peripheral surface of a hollow porous substrate. 
     For example, Patent Literature 1 proposes a method for producing a hollow fiber membrane as follows: after a round cord is passed through a liquid immersion bath for a defoaming process, the round cord and a film-forming dope containing a phase-separable film-forming resin are fed out from a double-ring hollow fiber membrane-spinning nozzle and are spun by a wet or a dry-wet spinning process. 
     Furthermore, Patent Literature 2 proposes a hollow fiber membrane-spinning nozzle, capable of suppressing a gas from being entrapped in a composite consisting of a long hollow porous substrate and a film-forming dope so as to prevent abnormal outer diameter portions caused by entrapped gases and defective film portions caused by locally thinned film. The hollow fiber membrane-spinning nozzle is structured to have a substrate feed port for feeding a long hollow porous substrate to be inserted into a membrane, and a circular ring-shaped dope discharge port positioned to surround the substrate feed port on its outer side so as to discharge a film-forming dope in the direction in which the hollow porous substrate is fed out. Moreover, the hollow fiber membrane-spinning nozzle is structured to have a flow channel for exhausting the gas existing in a space of the region from the nozzle tip to the point where the film-forming dope is adhered (laminated) to the peripheral surface of the hollow porous substrate outside the nozzle. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP H05-7746A 
     Patent Literature 2: JP2007-126783A 
     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     When a hollow fiber membrane is produced using the above hollow fiber membrane-spinning nozzle, the discharged film-forming dope may be disturbed because of equipment vibrations, variations in the feed rate of a hollow porous substrate, bubbles entrapped in the film-forming dope, uneven spinnability of the film-forming dope, and the like. When methods for producing hollow fiber membranes in Patent Literatures 1 and 2 are employed under such conditions, the film-forming dope may be temporarily detached from the hollow porous substrate in the region outside the nozzle where the film-forming dope is designed to be adhered to the peripheral surface of the hollow porous substrate. 
     As described above, when a film-forming dope is detached from a hollow porous substrate during the spinning process at the point outside the nozzle where the film-forming dope is designed to be adhered to the peripheral surface of the hollow porous substrate, the film-forming dope fails to follow the hollow porous substrate that continues coming out from the nozzle at a constant speed. As a result, the film-forming dope grows into a large drip-shaped mass near the dope discharge port outside the nozzle. Hereinafter, the film-forming dope that has grown into a mass is referred to as an abnormally discharged portion. 
     When the film-forming dope continues to be discharged from the dope discharge port under the above conditions, the abnormally discharged portion grows into an even greater mass and is reattached to the peripheral surface of the hollow porous substrate, thus resuming the process for applying the film-forming dope to the hollow porous substrate. However, the peripheral surface of the hollow porous substrate will have a bare portion since no film-forming dope is applied thereon between the time of dope detachment and the time of dope reattachment. In addition, the thickness of the film-forming dope is locally made greater where the abnormally discharged portion is reattached to the peripheral surface of the hollow porous substrate. Such variations in the application of the film-forming dope result in a defective hollow fiber membrane. 
     Especially, when a greater abnormally discharged portion exists on a hollow porous substrate during the production process of a hollow fiber membrane, an extra process is required to remove the defective portion at the time of product inspection. Moreover, during rinsing, drying, winding steps and the like subsequent to the spinning step, process failure may arise such as the hollow fiber membrane becoming clogged at the abnormally discharged portion when it passes through the narrow aperture of the equipment, the abnormally discharged portion becoming entangled with another spindle of hollow fiber membrane and causing damage, or the like. Therefore, it is important to prevent the formation of abnormally discharged portions, and if it happens, to reduce their sizes. 
     The present invention has been carried out to solve the above problems. Its objective is to provide a hollow fiber membrane production method capable of suppressing the detachment of a film-forming dope from a hollow porous substrate at the point outside the nozzle where the film-forming dope is designed to adhere to the peripheral surface of the hollow porous substrate, and also capable of promptly reattaching the film-forming dope even if it is detached from the hollow porous substrate, so that the formation of defects caused by abnormally discharged portions of the film-forming dope is prevented while also preventing process failure during steps subsequent to the spinning step. Moreover, its objective is to provide a hollow fiber membrane-spinning nozzle to be used in such a production method. 
     Solutions to the Problems 
     The present invention is characterized by the following aspects.
     [1] A method for producing a hollow fiber membrane using a hollow fiber membrane-spinning nozzle structured as below, the method including a spinning step for applying and coagulating a film-forming dope on the peripheral surface of a hollow porous substrate to form a porous membrane layer; in the method, the linear velocity (V A ) of discharging the film-forming dope from the dope discharge port relative to the feed rate (V B ) of feeding out the hollow porous substrate from the substrate feed port is set to have a draft ratio (V B /V A ) of 1 to 6.   

     (Hollow Fiber Membrane-Spinning Nozzle) 
     A hollow fiber membrane-spinning nozzle, structured to have a substrate insertion hole for the hollow porous substrate to be inserted and a dope flow channel for the film-forming dope to be distributed, in which a ring-shaped dope discharge port for discharging the film-forming dope distributed through the dope flow channel is formed on the outer side of a substrate feed port with a tube-shaped wall disposed between them so as to surround the substrate feed port for feeding out the hollow porous substrate coming through the substrate insertion hole.
     [2] The method for producing a hollow fiber membrane according to [1], in which the aperture area of the dope discharge port is no greater than 3 times the cross-sectional area of the hollow porous substrate cut perpendicular to its longitudinal direction.   [3] The method for producing a hollow fiber membrane according to [1] or [2], in which the aperture area of the dope discharge port is no greater than 15 mm 2 .   [4] The method for producing a hollow fiber membrane according to [3], which uses a film-forming dope with a viscosity at 40° C. set to be 30,000 mPa·s or higher.   [5] A hollow fiber membrane-spinning nozzle for applying a film-forming dope to form a porous membrane layer on the peripheral surface of a hollow porous substrate, structured to have a substrate insertion hole into which the hollow porous substrate is to be inserted and a dope flow channel through which the film-forming dope is to be distributed, where a ring-shaped dope discharge port for discharging the film-forming dope distributed through the dope flow channel is formed on the outer side of a substrate feed port with a tube-shaped wall disposed between them so as to surround the substrate feed port for feeding out the hollow porous substrate coming through the substrate insertion hole, and the thickness of the tip end of the tube-shaped wall is 0.1 mm to 0.75 mm.   [6] The hollow fiber membrane-spinning nozzle according to [5], in which the aperture area of the dope discharge port is no greater than 3 times the cross-sectional area, cut to be perpendicular to a longitudinal direction, of the hollow porous substrate to be inserted in the substrate insertion hole.   [7] The hollow fiber membrane-spinning nozzle according to [5] or [6], in which the aperture area of the dope discharge port is no greater than 15 mm 2 .   [8] The hollow fiber membrane-spinning nozzle according to any of [5] to [7], in which the diameter of the substrate feed port is 1.01 times to 1.20 times the diameter of the hollow porous substrate inserted in the substrate insertion port.   [9] The hollow fiber membrane-spinning nozzle according to any of [5] to [8], in which a straight portion having the same diameter as that of the dope discharge port and a length of at least 1 mm is formed in the dope flow channel extending from near the dope discharge port to the dope discharge port.   [10] The hollow fiber membrane-spinning nozzle according to any of [5] to [9], in which a diameter tapering portion is arranged in the dope flow channel near the dope discharge port to have a diameter decreasing toward the dope discharge port.   [11] The hollow fiber membrane-spinning nozzle according to any of [5] to [10], in which a branching-merging mechanism is formed in the dope flow channel and the film-forming dope passes through the flow channel while branching and merging repeatedly.   [12] The hollow fiber membrane-spinning nozzle according to any of [5] to [11], in which at least two dope flow channels are formed, and a dope lamination portion is formed for the dope flow channels to merge near the dope discharge port so that film-forming dopes coming from the dope flow channels are formed into a composite laminate inside the nozzle.   [13] The hollow fiber membrane-spinning nozzle according to any of [5] to [12], in which at least two dope flow channels are formed, each dope flow channel is structured to have a dope distribution hole for distributing a film-forming dope and a ring-shaped dope reservoir for the film-forming dope coming through the dope distribution hole to be stored on the outer side of the substrate insertion hole, and the dope reservoir for storing a film-forming dope to be laminated on the outer side is shifted in an axis direction of the substrate insertion hole so as to be located on the downstream side of the dope reservoir for storing a film-forming dope to be laminated on the inner side.   [14] The hollow fiber membrane-spinning nozzle according to [13], in which the dope distribution holes are formed to have an interval at least 60 degrees apart from each other around the central axis of the substrate insertion hole.   [15] The hollow fiber membrane-spinning nozzle according to any of [11] to [14], in which the branching-merging mechanism is a porous element and the film-forming dope passes through the element while repeatedly branching and merging.   [16] The hollow fiber membrane-spinning nozzle according to any of [11] to [14], in which the dope flow channel is structured to have a dope reservoir where the film-forming dope is stored in a ring shape on the outer side of the substrate insertion hole, and the branching-merging mechanism is a filler layer with particles filled inside the dope reservoir.   [17] The hollow fiber membrane-spinning nozzle according to any of [11] to [14], in which the dope flow channel is structured to have a dope reservoir where the film-forming dope is stored in a ring shape on the outer side of the substrate insertion hole, and the dope reservoir is vertically divided into two or more storage cells.   [18] The hollow fiber membrane-spinning nozzle according to [13], in which a delay mechanism is formed in the dope flow channel to delay the passage of a film-forming dope, and the delay mechanism is set to be a meandering portion that causes the film-forming dope to meander vertically between the dope reservoir and the dope shaping portion for forming the film-forming dope into a tube shape.   

     Effects of the Invention 
     According to the present invention, a film-forming dope is suppressed from being detached from a hollow porous substrate at a position outside the nozzle where the dope is designed to be attached to the peripheral surface of the substrate, and even if a film-forming dope is detached from a hollow porous substrate, the dope is promptly reattached to the substrate. Therefore, occurrence of defects caused by an abnormally discharged portion of a film-forming dope and process failure in subsequent steps are prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view schematically showing a hollow fiber membrane-spinning nozzle according to an embodiment of the present invention; 
         FIG. 2  is a view showing an end surface corresponding to the arrow-A view of the hollow fiber membrane-spinning nozzle in  FIG. 1  according to an embodiment of the present invention; 
         FIG. 3  is a view showing the end surface corresponding to the arrow-A view of the hollow fiber membrane-spinning nozzle in  FIG. 1  according to an embodiment of the present invention with added cross sections of a hollow porous substrate and a film-forming dope; 
         FIG. 4  is a view showing the B-B cross-section of the hollow fiber membrane-spinning nozzle in  FIG. 1 ; 
         FIG. 5  is a view showing the C-C cross-section of the hollow fiber membrane-spinning nozzle in  FIG. 1 ; 
         FIG. 6  is a view showing the D-D cross-section of the hollow fiber membrane-spinning nozzle in  FIG. 1 ; 
         FIG. 7  is a cross-sectional view schematically showing a hollow fiber membrane-spinning nozzle according to another embodiment of the present invention; 
         FIG. 8  is a cross-sectional view schematically showing a hollow fiber membrane-spinning nozzle according to yet another embodiment of the present invention; 
         FIG. 9  is a plan view schematically showing the second nozzle block of the hollow fiber membrane-spinning nozzle according to the yet another embodiment of the present invention; 
         FIG. 10  is a cross-sectional view schematically showing a hollow fiber membrane-spinning nozzle according to yet another embodiment of the present invention; 
         FIG. 11  is a plan view schematically showing the second nozzle block of the hollow fiber membrane-spinning nozzle according to the yet another embodiment of the present invention; and 
         FIG. 12  is a cross-sectional view schematically showing a hollow fiber membrane-spinning nozzle according to yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     [Hollow Fiber Membrane-spinning Nozzle] 
     The hollow fiber membrane-spinning nozzle related to the present invention is used for producing a hollow fiber membrane structured to have a porous membrane layer on the peripheral surface of a hollow porous substrate (support body). The hollow fiber membrane-spinning nozzle related to the present invention may be used for producing a hollow fiber membrane structured to have a single porous membrane layer or to have two or more porous membrane layers. 
     The following is a description of a hollow fiber membrane-spinning nozzle to be used in a production method of a hollow fiber membrane related to the present invention. 
     A hollow fiber membrane-spinning nozzle  1  according to an embodiment of the present invention (hereinafter referred to as spinning nozzle  1 ) is used for producing a hollow fiber membrane structured to have a double-layered porous membrane consisting of an inner layer and an outer layer formed on the peripheral surface of a hollow porous substrate. As shown in  FIG. 1 , spinning nozzle  1  is structured to allow detachable substrate insertion hole  10 , which is for inserting hollow porous substrate  2 , to be fixed with screws or the like to the downstream-side end surface of vertically installed metallic mounting plate  11 . 
     Mounting plate  11  is formed in a circular shape on a plan view, and substrate insertion hole  10  is formed along its axis. Substrate insertion hole  10  penetrates mounting plate  11  from the upstream-side end surface through the downstream-side end surface. 
     In addition to substrate insertion hole  10 , first intake hole  12  and second intake hole  13  each for introducing a film-forming dope are formed in mounting plate  11 . First and second intake holes  12 ,  13  are located on the outer side of substrate insertion hole  10  to be apart from each other when seen on a plan view of mounting plate  11 , and are formed parallel to substrate insertion hole  10  from the upstream-side end surface through the downstream-side end surface of mounting plate  11 . 
     First film-forming dope  3  for forming an inner porous membrane layer is introduced into first intake hole  12 . Second film-forming dope  4  for forming an outer porous membrane layer is introduced into second intake hole  13 . 
     Spinning nozzle  1  is structured to have triple-layered nozzle main body  5  consisting of first nozzle block  5 A, second nozzle block  5 B and third nozzle block  5 C each formed in a columnar shape and consecutively stacked in that order from the mounting plate  11  side. 
     Although various materials may be used for forming nozzle main body  5 , stainless steel (SUS) is preferred considering heat resistance, corrosion resistance, strength and the like. 
     As shown in  FIGS. 1 and 4 to 6 , first nozzle block  5 A is structured to have columnar block main body  51  and circular tube-shaped protrusion  6 , which is integrated with block main body  51  and protrudes from the center of the end surface opposite mounting plate  11 . 
     Tube-shaped protrusion  6  is structured to have larger-diameter portion  6   a  on the base-end side and smaller-diameter portion  6   b  on the tip-end side. The axes of larger-diameter and smaller-diameter portions  6   a,    6   b  correspond to each other. 
     Substrate insertion hole  7  is formed in tube-shaped protrusion  6  to insert hollow porous substrate  2 . Substrate insertion hole  7  is formed from the tip end of tube-shaped protrusion  6  to the mounting plate  11 -side of block main body  51  so as to be connected to substrate insertion hole  10  formed in mounting plate  11 . As shown in  FIGS. 1 and 2 , substrate feed port  7   a  is formed to feed out hollow porous substrate  2  coming through substrate insertion hole  7 . Hollow porous substrate  2  inserted into substrate insertion hole  10  of mounting plate  11  passes through substrate insertion hole  7  of first nozzle block  5 A and is fed out of substrate feed port  7   a.  It is an option to continuously draw hollow porous substrate  2  from substrate feed port  7   a  by using a winding roller or the like installed on the downstream side of spinning nozzle  1 . 
     The diameter (c) of substrate feed port  7   a  ( FIG. 2 ) is preferred to be 1.01 to 1.20 times, more preferably 1.05 to 1.15 times, the diameter of hollow porous substrate  2  to be inserted in substrate insertion hole  7 . If the diameter of substrate feed port  7   a  is set to be at least the above lower limit, the substrate is suppressed from becoming stuck in the substrate insertion hole so as to improve the feeding stability of the substrate. If the diameter of substrate feed port  7   a  is set to be no greater than the above upper limit, first and second film-forming dopes  3 , 4  discharged from dope discharge port  27   a  are adhered to the peripheral surface of hollow porous substrate  2  with a smaller angle relative to the substrate. 
     As shown in  FIGS. 1, 5 and 6 , second nozzle block  5 B is structured to have columnar block main body  52  and circular tube-shaped protrusion  8 , which is integrated with block main body  52  and protrudes from the center of the end surface opposite mounting plate  11 . 
     Recess  20 , which is shaped to be circular on a plan view, is formed on the block main body  51 -side end surface of block main body  52 . The inner diameter of recess  20  is formed to be greater than the outer diameter of larger-diameter portion  6   a  of tube-shaped protrusion  6 , and the inner wall surface of recess  20  is formed to surround larger-diameter portion  6   a.  The space between recess  20  and larger-diameter portion  6   a  in nozzle main body  5  is set to be circular ring-shaped first dope reservoir  21 . The center of circular ring-shaped first dope reservoir  21  corresponds to the axis of larger-diameter portion  6   a  of tube-shaped protrusion  6 . 
     As shown in  FIGS. 1 and 4 , first dope distribution hole  14  is formed in block main body  51  of first nozzle block  5 A and block main body  52  of second nozzle block  5 B, which are stacked. When seen on a plan view, first dope distribution hole  14  is positioned to be on the outer side of substrate insertion hole  7  in block main body  51 , corresponding to the outer periphery of recess  20  in block main body  52 , and is formed parallel to substrate insertion hole  7  from the upstream-side end surface of block main body  51  to recess  20 . The bottom of first dope distribution hole  14  is made flush with the bottom of recess  20 . 
     First dope distribution hole  14  is connected to first intake hole  12  so that first film-forming dope  3  introduced into first intake hole  12  flows into first dope distribution hole  14 . 
     The cross-sectional shape cut perpendicular in a longitudinal direction of first dope distribution hole  14  is preferred to be circular. However, that is not the only option. In addition, the diameter of first dope distribution hole  14  is not limited to a particular size. 
     First film-forming dope  3  coming through first dope distribution hole  14  flows into first dope reservoir  21 . In first dope reservoir  21 , first film-forming dope  3  distributed through first dope distribution hole  14  is stored in a circular ring shape around larger-diameter portion  6   a  of tube-shaped protrusion  6 . More specifically, first film-forming dope  3  flowed into first dope reservoir  21  from first dope distribution hole  14  is branched into two arc-shaped streams, which merge on the opposite side of first dope distribution hole  14  to ultimately form a circular ring shape in first dope reservoir  21 . 
     In the center of tube-shaped protrusion  8  on a plan view, penetrating hole  22  is connected to recess  20  and extends to the tip-end surface of tube-shaped protrusion  8 . The inner diameter of penetrating hole  22  is set to be greater than the outer diameter of smaller-diameter portion  6   b  of tube-shaped protrusion  6  while the inner-wall surface of penetrating hole  22  is formed to surround smaller-diameter portion  6   b.  The space between penetrating hole  22  and smaller-diameter portion  6   b  is set to be cylindrical first dope shaping portion  23 . The axis of cylindrical first dope shaping portion  23  corresponds to the axis of smaller-diameter portion  6   b  of tube-shaped protrusion  6 . 
     First film-forming dope  3  formed in a circular ring shape in first dope reservoir  21  flows into first dope shaping portion  23  so as to be formed in a cylindrical shape. 
     As shown in  FIGS. 1 and 6 , recess  24  in a circular shape on a plan view is formed on the block main body  52 -side end surface of third nozzle block  5 C. The inner diameter of recess  24  is formed greater than the outer diameter of tube-shaped protrusion  8  and its inner-wall surface is set to surround tube-shaped protrusion  8 . The space between recess  24  and tube-shaped protrusion  8  in nozzle main body  5  is set to be circular ring-shaped second dope reservoir  25 . The center of circular ring-shaped second dope reservoir  25  corresponds to the axis of smaller-diameter portion  6   b  of tube-shaped protrusion  6 . 
     In spinning nozzle  1 , first dope reservoir  21  is formed in block main body  52  of second nozzle block  5 B, and second dope reservoir  25  is formed in third nozzle block  5 C positioned on the downstream side of second nozzle block  5 B. Namely, second dope reservoir  25  for storing second film-forming dope  4  to be laminated on the outer-layer side is shifted in an axis direction of substrate insertion hole  7  so as to be positioned on the downstream side of first dope reservoir  21  for storing first film-forming dope  3  to be laminated on the inner-layer side. As structured in spinning nozzle  1  related to the present invention, a dope reservoir for storing a film-forming dope to be laminated on the outer-layer side is shifted in an axis direction of the substrate insertion hole so as to be positioned on the downstream side of another dope reservoir for storing a film-forming dope to be laminated on the inner-layer side. By so setting, the spinning nozzle is designed compact in a width direction, enhancing the processing efficiency and productivity of the film-forming apparatus. 
     Second dope distribution hole  15  is formed where block main body  51  of first nozzle block  5 A, block main body  52  of second nozzle block  5 B and third nozzle block  5 C are stacked together. When seen on a plan view, second dope distribution hole  15  is formed to be on the outer side of first dope distribution hole  14  in block main bodies  51 ,  52 , corresponding to the outer periphery of recess  24  in third nozzle block  5 C, and is formed parallel to substrate insertion hole  7  from the upstream-side end surface of block main body  51  to recess  24 . The bottom surface of second dope distribution hole  15  is made flush with the bottom surface of recess  24 . 
     Second dope distribution hole  15  is connected with second intake hole  13  so that second film-forming dope  4  introduced into second intake hole  13  flows into second dope distribution hole  15 . 
     The shape of a cross section perpendicular to a longitudinal direction of second dope distribution hole  15  is preferred to be circular. However, that is not the only option. The diameter of second dope distribution hole  15  is not limited to a particular size. 
     In the embodiment above, first dope distribution hole  14  and second dope distribution hole  15  are formed in such a way that substrate insertion hole  7 , first dope distribution hole  14  and second dope distribution hole  15  align linearly on a plan view. 
     In the present invention, it is an option to arrange multiple dope distribution holes to be separated at intervals of 60 degrees or greater around the central axis of the substrate insertion hole when seen on a plan view. It is preferred to arrange multiple dope distribution holes as above, since origination points of cracking that may occur in an axis direction are dispersed among layers, and formation of cracking is thereby suppressed. 
     First film-forming dope  3  coming through second dope distribution hole  15  flows into second dope reservoir  25 . In second dope reservoir  25 , second film-forming dope  4  coming through second dope distribution hole  15  is stored in a circular ring shape around tube-shaped protrusion  8 . More specifically, second film-forming dope  4  coming through second dope distribution hole  15  enters second dope reservoir  25  and is branched into two arc-shaped streams, which merge on the opposite side of second dope distribution hole  15  to ultimately form a circular ring shape in second dope reservoir  25 . 
     In the center of third nozzle block  5 C on a plan view, penetrating hole  26  is connected to recess  24  and extends to the end surface opposite second nozzle block  5 B. The inner diameter of penetrating hole  26  is greater than the outer diameter of smaller-diameter portion  6   b  of tube-shaped protrusion  6 , and the inner-wall surface of penetrating hole  26  is formed to surround smaller-diameter portion  6   b.  In addition, the inner diameter of penetrating hole  26  is set slightly greater than the inner diameter of penetrating hole  22 . 
     The space between penetrating hole  22  and smaller-diameter portion  6   b  in nozzle main body  5  is set to be cylindrical dope lamination portion  27 . The axis of cylindrical dope lamination portion  27  corresponds to the axis of smaller-diameter portion  6   b  of tube-shaped protrusion  6 . 
     Second film-forming dope  4  formed in a circular ring shape in second dope reservoir  25  flows into dope lamination portion  27  and is laminated on the outer side of first film-forming dope  3  to be a composite laminate while being formed in a cylindrical shape. 
     As shown in  FIGS. 1 to 3 , on the end surface of third nozzle block  5 C positioned opposite second nozzle block  5 B, circular ring-shaped dope discharge port  27   a  is formed as an opening end of dope lamination portion  27 . 
     Dope discharge port  27   a  is positioned to surround substrate feed port  7   a  on its outer side, but is separated from substrate feed port  7   a  by tube-shaped wall  6   c,  which is the tip of smaller-diameter portion  6   b  of tube-shaped protrusion  6 . 
     As described above, nozzle main body  5  is structured to have first dope flow channel  28 , which includes first dope distribution hole  14 , first dope reservoir  21  and first dope shaping portion  23 , and to have second dope flow channel  29 , which includes second dope distribution hole  15  and second dope reservoir  25 . First dope flow channel  28  and second dope flow channel  29  merge at dope lamination portion  27  near dope discharge port  27   a  in nozzle main body  5 . 
     First film-forming dope  3  flowing through first dope flow channel  28  and second film-forming dope  4  flowing through second dope flow channel  29  become a composite laminate in dope lamination portion  27  with second film-forming dope  4  being laminated on the outer side of first film-forming dope  3 ; then, the composite laminate is discharged from dope discharge port  27   a  in a cylindrical shape. Cylindrical first and second film-forming dopes  3 , 4  discharged from dope discharge port  27   a  are continuously adhered to the peripheral surface of hollow porous substrate  2  fed out from substrate feed port  7   a.    
     The thickness (a) ( FIG. 2 ) of the tip of tube-shaped wall  6   c  is preferred to be 0.1 mm to 0.75 mm, more preferably 0.25 mm to 0.60 mm. 
     The thickness (a) of tube-shaped wall  6   c  set to be within the above range contributes to forming a smaller angle when first and second film-forming dopes  3 , 4  discharged from dope discharge port  27   a  are adhered to the peripheral surface of hollow porous substrate  2 . Moreover, such a thickness contributes to forming a smaller angle when first and second film-forming dopes  3 , 4  are adhered and stretched diagonally by hollow porous substrate  2 . Accordingly, first and second film-forming dopes  3 , 4  are consistently adhered to the peripheral surface of hollow porous substrate  2 . Moreover, a thickness (a) of tube-shaped wall  6   c  set to be within the above range shortens the distance from when first and second film-forming dopes  3 , 4  are discharged from dope discharge port  27   a  to when the dopes are adhered to the peripheral surface of hollow porous substrate  2 , which in turn shortens a duration of instability for first and second film-forming dopes  3 , 4  after they are discharged from dope discharge port  27   a  until they are adhered to the peripheral surface of hollow porous substrate  2 . 
     Accordingly, it is easier to suppress the detachment of first and second film-forming dopes  3 , 4  from hollow porous substrate  2  at the position outside spinning nozzle  1  where first and second film-forming dopes  3 , 4  are designed to adhere to the peripheral surface of hollow porous substrate  2 . 
     Furthermore, if thickness (a) of tube-shaped wall  6   c  is within the above range, even when first and second film-forming dopes  3 , 4  are detached from hollow porous substrate  2  at the position where the dopes are designed to adhere to the peripheral surface of hollow porous substrate  2 , the distance is short between the resultant abnormally discharged portion and the peripheral surface of hollow porous substrate  2 , thus making it shorter timewise before the abnormally discharged portion is reattached to the peripheral surface of hollow porous substrate  2 . As a result, even when first and second film-forming dopes  3 , 4  are detached from hollow porous substrate  2 , the dope reattachment occurs promptly, thereby resulting in a smaller abnormally discharged portion. 
     In addition, if thickness (a) of tube-shaped wall  6   c  is within the above range, it is easier to secure sufficient pressure-resistant strength at the tip of tube-shaped protrusion  6 . 
     The aperture area (d) of dope discharge port  27   a  ( FIG. 2 ) is preferred to be no greater than three times, more preferably 1 to 2.5 times, the cross-sectional area, cut to be perpendicular to the longitudinal direction, of hollow porous substrate  2  to be inserted in substrate insertion hole  7 . By so setting, it is easier to suppress the detachment of first and second film-forming dopes  3 , 4  from hollow porous substrate  2  at the position outside spinning nozzle  1  where the dopes are designed to adhere to the peripheral surface of hollow porous substrate  2 . 
     The aperture area (d) of dope discharge port  27   a  ( FIG. 2 ) is preferred to be no greater than 15 mm 2 , more preferably 1 mm 2  to 15 mm 2 . If aperture area (d) of dope discharge port  27   a  is no greater than 15 mm 2 , it is easier to suppress the detachment of first and second film-forming dopes  3 , 4  from hollow porous substrate  2  at the position outside spinning nozzle  1  where the dopes are designed to adhere to the peripheral surface of hollow porous substrate  2 . 
     The outer diameter (b) of dope discharge port  27   a  ( FIG. 2 ) is preferred to be 1 to 5 mm, more preferably 2 to 5 mm. 
     In the present embodiment, near the dope discharge port of a dope flow channel, a straight portion at least 1 mm long extending to the dope discharge port is preferred to be formed having the same diameter as the dope discharge port. In the present embodiment, dope lamination portion  27  near dope discharge port  27   a  for first and second dope flow channels  28 ,  29  is preferred to have a straight portion extending at least 1 mm from dope discharge port  27   a  to have the same diameter as dope discharge port  27   a.  By so setting, consistent downward discharge of film-forming dopes is achieved. 
     In addition, a diameter tapering portion is preferred to be formed near the dope discharge port of a dope flow channel to extend to the dope discharge port with its diameter decreasing toward the dope discharge port. For example, dope lamination portion  27  near dope discharge port  27   a  for first and second dope flow channels  28 ,  29  is preferred to have a diameter tapering portion, which extends to dope discharge port  27   a  while decreasing its diameter toward dope discharge port  27   a.  Since such a setting makes the distance shorter for film-forming dopes passing through a narrow passage, the pressure required to discharge film-forming dopes is set lower, thus enhancing production stability. 
     In the embodiments of the present invention, a branching-merging mechanism is preferred to be arranged so that a film-forming dope passes through the dope flow channel while repeatedly branching and merging. Such a structure makes it easier to suppress formation of originating points of cracking that may occur along an axis direction in the porous membrane layer of a produced hollow fiber membrane. 
     Examples of a branching-merging mechanism are a porous element through which a film-forming dope passes while repeatedly branching and merging. 
     An example of such a porous element is the one described in WO2012/070629, and it is preferred to be a porous material having a three-dimensional network structure. A three-dimensional porous network structure means three-dimensional passages are formed inside so that the flow of a film-forming dope that passes through the structure is not linear but branches vertically and horizontally. 
     Considering strength, heat conductivity, drug resistance and homogeneous structure, sintered metal fine particles are preferred to be used to form a porous element. However, such a structure is not the only option, and a porous element may be selected from among sintered metal fibers, metal-mesh laminates or sintered laminates, ceramic porous materials, porous-sheet laminates or sintered laminates, metal particle fillers and the like. 
     It is preferred to form a cylindrical porous element so that a film-forming dope passes through it from the outer peripheral surface toward the inner peripheral surface. In the embodiments of the present invention, it is especially preferable to install a cylindrical porous element inside a dope reservoir. 
     An example of a hollow fiber membrane-spinning nozzle having a porous element is hollow fiber membrane-spinning nozzle  100  as shown in  FIG. 7  (hereinafter referred to as spinning nozzle  100 ). 
     Spinning nozzle  100  is structured to have triple-layered nozzle main body  110  consisting of first nozzle block  111 , second nozzle block  112  and third nozzle block  113  which are consecutively stacked from the upper side in that order. First nozzle block  111 , second nozzle block  112  and third nozzle block  113  are formed to be the same as first nozzle block  5 A, second nozzle block  5 B and third nozzle block  5 C in spinning nozzle  1 . 
     In block main body  111   a  and circular tube-shaped protrusion  111   b  in first nozzle block  111 , substrate insertion hole  114  is formed to insert a hollow porous substrate. At the tip of tube-shaped protrusion  111   b,  substrate feed port  114   a  is formed to feed out the hollow porous substrate coming through substrate insertion hole  114 . 
     In nozzle main body  110 , the space between recess  115  formed in block main body  112   a  of second nozzle block  112  and larger-diameter portion  111   c  of tube-shaped protrusion  111   b  is set to be circular ring-shaped first dope reservoir  132 . In block main body  111   a  of first nozzle block  111  and block main body  112   a  of second nozzle block  112 , first dope distribution hole  131  is formed to be connected to first dope reservoir  132 . The space between penetrating hole  116  formed in tube-shaped protrusion  112   b  of second nozzle block  112  and smaller-diameter portion  111   d  of tube-shaped protrusion  111   b  is set to be cylindrical first dope shaping portion  133 . 
     Cylindrical porous element  117  is formed in first dope reservoir  132 . 
     In nozzle main body  110 , the space between recess  118  and tube-shaped protrusion  112   b  formed in third nozzle block  113  is set to be circular ring-shaped second dope reservoir  142 . Second dope distribution hole  141  is formed in block main body  111   a  of first nozzle block  111 , block main body  112   a  of second nozzle block  112  and third nozzle block  113 , and is connected to second dope reservoir  142 . The space between penetrating hole  119  formed in third nozzle block  113  and smaller-diameter portion  111   d  of tube-shaped protrusion  111   b  is set to be cylindrical dope lamination portion  143 . On the lower end surface of third nozzle block  113 , circular ring-shaped dope discharge port  143   a  is formed as an opening end of dope lamination portion  143 . 
     Cylindrical porous element  120  is formed in second dope reservoir  142 . 
     Dope discharge port  143   a  is positioned to surround substrate feed port  114   a  on its outer side, but is separated from substrate feed port  114   a  by tube-shaped wall  111   e,  which is the tip of smaller-diameter portion  111   d  of tube-shaped protrusion  111   b.    
     The thickness of the tip of tube-shaped wall 111e is preferred to be 0.1 mm to 0.75 mm, more preferably 0.25 mm to 0.60 mm. 
     Spinning nozzle  100  is structured to have first dope flow channel  130  which includes first dope distribution hole  131 , first dope reservoir  132  and first dope shaping portion  133 , and to have second dope flow channel  140  which includes second dope distribution hole  141  and second dope reservoir  142 . First dope flow channel  130  and second dope flow channel  140  merge at dope lamination portion  143 . 
     The first film-forming dope coming through first dope distribution hole  131  flows into first dope reservoir  132  and is made into a circular ring shape on the outer side of porous element  117 . The circular ring-shaped first film-forming dope passes through porous element  117  while minutely branching and merging from the outer peripheral surface toward the inner peripheral surface, and flows into first dope shaping portion  133 . 
     In addition, the second film-forming dope coming through second dope distribution hole  141  flows into second dope reservoir  142  and is made into a circular ring shape on the outer side of porous element  120 . The circular ring-shaped second film-forming dope passes through porous element  120  while minutely branching and merging from the outer peripheral surface toward the inner peripheral surface, and flows into first dope lamination portion  143 , where the second film-forming dope is formed into a composite being laminated on the outer side of the first film-forming dope that has flowed from first dope reservoir  133 . The composite laminate is then discharged from dope discharge port  143   a.  First and second film-forming dopes discharged through dope discharge port  143   a  are continuously adhered to the peripheral surface of the hollow porous substrate fed out from substrate feed port  114   a.    
     It is an option to employ a filler layer, for example, where particles are filled inside a dope reservoir as the branching-merging mechanism. 
     Such particles may be shaped in a spherical, rectangular or filler form, an uneven three-dimensional structure or the like. 
     The material of particles is not limited particularly, and metals such as stainless steel and alloys, inorganic materials such as glass and ceramics, and resins such as Teflon® and polyethylene that are insoluble in film-forming dopes may be used. Specific examples of particles may include steel balls. 
     The size and number of particles may be determined appropriately as desired. 
     The height of filler layers may also be determined appropriately as desired. 
     An example of a hollow fiber membrane-spinning nozzle with a filler layer is hollow fiber membrane-spinning nozzle  200  as shown in  FIGS. 8 and 9  (hereinafter referred to as spinning nozzle  200 ). Spinning nozzle  200  is used for producing a hollow fiber membrane where a single porous membrane layer is laminated on the outer side of a hollow porous substrate. 
     Spinning nozzle  200  is structured to have double-layered nozzle main body  210  consisting of first nozzle block  211  and second nozzle block  212  which are stacked from the upper side in that order. 
     Substrate insertion hole  213  is formed to insert a hollow porous substrate in block main body  211   a  and circular tube-shaped protrusion  211   b  protruding downward from block main body  211   a  of first nozzle block  211 . At the tip of tube-shaped protrusion  211   b,  substrate feed port  213   a  is formed to feed out the hollow porous substrate coming through substrate insertion hole  213 . 
     In nozzle main body  210 , the space between recess  214  formed on the upper portion of second nozzle block  212  and tube-shaped protrusion  211   b  is set to be circular ring-shaped dope reservoir  222 . In first and second nozzle blocks  211 ,  212 , dope distribution hole  221  is formed to be connected to dope reservoir  222 . On the lower portion of second nozzle block  212 , penetrating hole  215  is formed to be connected to recess  214 . The space between penetrating hole  215  and tube-shaped protrusion  211   b  in nozzle main body  210  is set to be cylindrical dope shaping portion  223 . 
     On the lower end surface of second nozzle block  212 , circular ring-shaped dope discharge port  223   a  is formed as an opening end of dope shaping portion  223 . 
     Filler layer  217  filled with particles  216  is formed in dope reservoir  222 . 
     Dope discharge port  223   a  is positioned to surround substrate feed port  213   a  on its outer side but is separated from substrate feed port  213   a  by tube-shaped wall  211   c,  which is the tip of tube-shaped protrusion  211   b.    
     The thickness of the tip of tube-shaped wall  211   c  is preferred to be 0.1 mm to 0.75 mm, more preferably 0.25 mm to 0.60 mm. 
     As described above, spinning nozzle  200  is structured to have dope flow channel  220 , which includes dope distribution hole  221 , dope reservoir  222  and dope shaping portion  223 . 
     The film-forming dope coming through dope distribution hole  221  flows into dope reservoir  222 , passes downward through the reservoir while minutely branching and merging through the filler layer  217 , flows into dope shaping portion  223 , and is finally discharged through dope discharge port  223   a.  The film-forming dope discharged from dope discharge port  223   a  is continuously adhered to the peripheral surface of a hollow porous substrate fed out of substrate feed port  213   a.    
     When a dope flow channel is structured to have a dope reservoir in a hollow fiber membrane-spinning nozzle related to the present invention, the dope reservoir may be vertically divided into two or more dope storage cells. Such a structure suppresses formation of originating points of cracking that may occur in an axis direction in the porous membrane layer of a produced hollow fiber membrane. 
     For example, a hollow fiber membrane-spinning nozzle related to the present invention may be a hollow fiber membrane-spinning nozzle  300  as shown in  FIGS. 10 and 11  (hereinafter referred to as spinning nozzle  300 ). 
     Spinning nozzle  300  is structured to have triple-layered nozzle main body  310 , consisting of first nozzle block  311 , second nozzle block  312 , and third nozzle block  313  consecutively stacked from the upper portion in that order. 
     Substrate insertion hole  314  is formed to insert a hollow porous substrate in block main body  311   a  and circular tube-shaped protrusion  311   b  protruding downward from block main body  311   a  in first nozzle block  311 . At the tip of tube-shaped protrusion  311   b,  substrate feed port  314   a  is formed to feed out the hollow porous substrate coming through substrate insertion hole  314 . 
     In nozzle main body  310 , the space between recess  315  formed on the upper portion of block main body  312   a  of second nozzle block  312  and larger-diameter portion  311   c  of tube-shaped protrusion  311   b  is set to be first storage cell  322   a.  In addition, the space between recess  317  formed on the upper portion of third nozzle block  313  and tube-shaped protrusion  312   b  of second nozzle block  312  is set to be second storage cell  322   b.    
     In block main body  311   a  of first nozzle block  311 , dope distribution hole  321  is formed to be connected to first storage cell  322   a.  Moreover, in block main body  312   a  of second nozzle block  312 , eight supply routes  323  connecting first storage cell  322   a  and second storage cell  322   b  are formed along the inner wall surface of block main body  312   a.  As described, spinning nozzle  300  is structured to have dope reservoir  322  which is vertically divided into double-stage first and second storage cells  322   a,    322   b.    
     First storage cell  322   a  is structured to have circular ring-shaped portion  325   a  with a circular ring-shaped cross section on a plan view, and eight peripheral portions  325   b  formed when portions of the inner wall surface of block main body  312   a  are indented outward from circular ring-shaped portion  325   a.  Eight supply routes  323  are respectively formed in eight peripheral portions  325   b  in first storage cell  322   a.  When seen on a plan view, one of peripheral portions  325   b  in first storage cell  322   a  corresponds to dope distribution hole  321 . 
     In the above embodiment, eight peripheral portions  325   b  are formed in such a way that the bottom surfaces of peripheral portions  325   b  are lowered in stages from the dope distribution hole  321  side toward the opposite side. Forming height differences among peripheral portions  325   b  contributes to supplying a film-forming dope more evenly from each of supply routes  323  to second storage cell  322   b.    
     Second storage cell  322   b  is structured to have circular ring-shaped portion  325   c  with a circular ring-shaped cross section on a plan view, and eight peripheral portions  325   d  are formed in the upper portion of circular ring-shaped portion  325   c  where portions of the inner wall surface of the third nozzle block are indented outward. The film-forming dope is supplied from eight supply routes  323  respectively to eight peripheral portions  325   d.    
     In nozzle main body  310 , a space is formed between penetrating hole  316 , which is formed in tube-shaped protrusion  312   b  of second nozzle block  312 , and smaller-diameter portion  311   d  of tube-shaped protrusion  311   b;  and another space is formed between penetrating hole  318 , which is formed on the lower portion of third nozzle block  313  to be connected to recess  317 , and smaller-diameter portion  311   d  of tube-shaped protrusion  311   b.  Those two spaces are set to be cylindrical dope shaping portion  324 . On the lower end surface of third nozzle block  313 , circular ring-shaped dope discharge port  324   a  is formed as an opening end of dope shaping portion  324 . 
     Dope discharge port  324   a  is positioned to surround substrate feed port  314   a  on its outer side, but is separated from substrate feed port  314   a  by tube-shaped wall  311   e,  which is the tip of smaller-diameter portion  311   d  of tube-shaped protrusion  311   b.    
     The thickness of the tip of tube-shaped wall  311   e  is preferred to be 0.1 mm to 0.75 mm, more preferably 0.25 mm to 0.60 mm. 
     As described above, spinning nozzle  300  is structured to have dope flow channel  320 , which includes dope distribution hole  321 , dope reservoir  322  and dope shaping portion  324 . 
     The film-forming dope coming through dope distribution hole  321  flows into first storage cell  322   a  of dope reservoir  322 , part of which is formed in a circular ring shape and flows into dope shaping portion  324  while the rest is supplied to second storage cell  322   b  from supply routes  323 . The film-forming dope supplied to second storage cell  322   b  is formed in a circular ring shape and flows into dope shaping portion  324 . Then, the film-forming dope coming through dope shaping portion  324  is discharged from dope discharge port  324   a  and adhered continuously to the peripheral surface of a hollow porous substrate fed out from substrate feed port  314   a.    
     Moreover, a dope flow channel related to the present invention is preferred to have a delay mechanism capable of delaying the passage of film-forming dope through the nozzle. As for the delay mechanism, a meandering portion is preferred so as to cause the film-forming dope to vertically meander between the dope reservoir and dope shaping portion. 
     An example of a hollow fiber membrane-spinning nozzle may be hollow fiber membrane-spinning nozzle  400  as shown in  FIG. 12  (hereinafter referred to as spinning nozzle  400 ). 
     Spinning nozzle  400  is structured to have triple-layered nozzle main body  410  consisting of first nozzle block  411 , second nozzle block  412  and third nozzle block  413  which are consecutively stacked from the upper side in that order. 
     Substrate insertion hole  414  is formed to insert a hollow porous substrate in block main body  411   a  and circular tube-shaped protrusion  411   b  protruding downward from block main body  411   a  in first nozzle block  411 . At the tip of tube-shaped protrusion  411   b,  substrate feed port  414   a  is formed to feed out the hollow porous substrate coming through substrate insertion hole  414 . 
     On the lower end side of block main body  411   a  in first nozzle block  411 , circular ring-shaped recess  415  is formed to surround tube-shaped protrusion  411   b.  The space in nozzle main body  410  between recess  415  and second nozzle block  412  is set to be dope reservoir  432 . In block main body  411   a,  first dope distribution hole  431  is formed to be connected to first dope reservoir  432 . 
     On the upper portion of second nozzle block  412 , recess  416  is formed to be circular on a plan view. Recess  416  is formed in such a way that the inner wall surface of second nozzle block  412  surrounds tube-shaped protrusion  411   b.  In the center of recess  416 , penetrating hole  417  is formed to extend to the lower end surface of second nozzle block  412 . 
     From the lower end surface of first nozzle block  411 , cylindrical first gate  418  is formed to surround tube-shaped protrusion  411   b  while hanging down into recess  415 . The tip of first gate  418  is separated from the bottom of recess  416 . In addition, around penetrating hole  417  at the bottom of recess  416  in second nozzle block  412 , second gate  419  is formed rising up to be inside first gate  418 . The tip of second gate  419  is separated from the lower end surface of first nozzle block  411 . Because of such a structure, meandering portion  433  is formed in recess  416  so that the film-forming dope coming from first dope reservoir  432  flows vertically meandering through the structure toward the center while maintaining its circular ring shape. 
     The space between penetrating hole  417  and tube-shaped protrusion  411   b  in nozzle main body  410  is set to be cylindrical first dope shaping portion  434 . 
     On the lower-end side of second nozzle block  412  and on the outer side of recess  416 , circular ring-shaped recess  420  is formed to surround tube-shaped protrusion  411   b.  The space between recess  420  and third nozzle block  413  in nozzle main body  410  is set to be second dope reservoir  442 . In block main body  411   a  of first nozzle block  411  and second nozzle block  412 , second dope distribution hole  441  is formed to be connected to second dope reservoir  442 . 
     Recess  421  shaped circular on a plan view is formed on the upper portion of third nozzle block  413 . Recess  421  is formed in such a way that the inner wall surface of third nozzle block  413  surrounds tube-shaped protrusion  411   b.  In the center of recess  421 , penetrating hole  422  is formed to extend to the lower-end surface of third nozzle block  413 . 
     In recess  421 , two each of cylindrical first and second gates  423 ,  424  are positioned alternately toward the center on a plan view to surround tube-shaped protrusion  411   b  by protruding respectively from the lower-end surface of second nozzle block  412  and the bottom surface of recess  421 . The tip of each first gate  423  is separated from the bottom surface of recess  421 . The tip of each second gate  424  is separated from the lower-end surface of second nozzle block  412 . Because of such a structure, meandering portion  443  is formed in recess  421  so that the film-forming dope coming from second dope reservoir  442  flows vertically meandering through the structure toward the center while maintaining its circular ring shape. 
     The space between penetrating hole  422  and tube-shaped protrusion  411   b  in nozzle main body  410  is set to be cylindrical dope lamination portion  444 . On the lower-end surface of third nozzle block  413 , circular ring-shaped dope discharge port  444   a  is formed as the opening end of dope lamination portion  444 . 
     Dope discharge port  444   a  is positioned to surround substrate feed port  414   a  on its outer side, but is separated from substrate feed port  414   a  by tube-shaped wall  411   c,  which is the tip of tube-shaped protrusion  411   b.    
     The thickness of the tip of tube-shaped wall  411   c  is preferred to be 0.1 mm to 0.75 mm, more preferably 0.25 mm to 0.60 mm. 
     Spinning nozzle  400  is structured to have first dope flow channel  430  which includes first dope distribution hole  431 , first dope reservoir  432 , meandering portion  433  and first dope shaping portion  434  as well as second dope flow channel  440  which includes second dope distribution hole  441 , second dope reservoir  442  and meandering portion  443 . First dope flow channel  430  and second dope flow channel  440  merge at dope lamination portion  444 . 
     The first film-forming dope coming through first dope distribution hole  431  flows into first dope reservoir  432  and is formed in a circular ring shape. Then, the first film-forming dope flows vertically meandering through meandering portion  433 , and flows into first dope shaping portion  434 . 
     The second film-forming dope coming through second dope distribution hole  441  flows into second dope reservoir  442  and is formed in a circular ring shape. Then, the second film-forming dope flows vertically meandering through meandering portion  443 , and flows into dope lamination portion  444 . In dope lamination portion  444 , the second film-forming dope is laminated on the outer side of first film-forming dope coming from first dope shaping portion  434  to form a composite laminate. The first and second film-forming dopes are discharged from dope discharge port  444   a  and are continuously adhered to the peripheral surface of the hollow porous substrate fed out of substrate feed port  414   a.    
     [Method for Producing Hollow Fiber Membrane] 
     The hollow fiber membrane produced by the production method related to the present invention is structured to have a porous membrane layer formed on the peripheral surface of a hollow porous substrate (support body). The method for producing a hollow fiber membrane related to the present invention may be used for producing a hollow fiber membrane structured to have a single porous membrane layer or to have two or more porous membrane layers. 
     According to the present invention, the method for producing a hollow fiber membrane includes a spinning step. In such a method, a hollow fiber membrane-spinning nozzle is used to apply a film-forming dope for a porous layer on the peripheral surface of a hollow porous substrate, and the film-forming dope is coagulated by using a coagulation liquid. In the production method of a hollow fiber membrane according to the present invention, steps subsequent to the spinning process are conducted by a known method. 
     The following is a description of a method for producing a hollow fiber membrane related to the present invention. A method for producing a hollow fiber membrane related to the present invention may include spinning, rinsing, removing, drying and winding steps as described below, for example. 
     Spinning step: using a hollow fiber membrane-spinning nozzle, a film-forming dope for a porous layer is applied on the peripheral surface of a hollow porous substrate, and the film-forming dope is coagulated by using a coagulation liquid to obtain a hollow fiber membrane precursor; 
     Coagulating step: the film-forming dope applied on the peripheral surface of a hollow porous substrate is coagulated by a coagulation liquid to obtain a hollow fiber membrane precursor. 
     Rinsing step: the solvent remaining in the hollow fiber membrane precursor is rinsed off; 
     Removing step: the opening agent remaining in the rinsed hollow fiber membrane precursor is removed to form a hollow fiber membrane; 
     Drying step: the hollow fiber membrane is dried after the removal step; and 
     Winding step: the dried hollow fiber membrane is wound. 
     (Spinning Step) 
     In the spinning step, the linear velocity (V A ) at which a film-forming dope discharged from the dope discharge port of a hollow fiber membrane-spinning nozzle and the feed rate (V B ) at which a hollow porous substrate is fed out from the substrate feed port are set to have a draft ratio (V B /V A ) of 1 to 6 when the film-forming dope is applied on the peripheral surface of the hollow porous substrate. 
     When the draft ratio (V B /V A ) is 1 to 6, the linear velocity (V A ) is sufficiently close to the feed rate (V B ). Thus, at the position where the film-forming dope is designed to be adhered to the peripheral surface of the hollow porous substrate, the film-forming dope is unlikely to be detached from the hollow porous substrate. Moreover, even when the film-forming dope is detached from the hollow porous substrate at the designated position, the dope is promptly reattached to the substrate, thus reducing the size of an abnormally discharged portion. 
     The draft ratio (V B /V A ) is set to be 1 to 6, preferably 2 to 5.5. 
     The feed rate (V B ) of a hollow porous substrate to be fed out from a substrate feed port is preferred to be 10 to 50 m/min., more preferably 15 to 45 m/min. 
     In the embodiments of the present invention, the linear velocity (V A ) of a film-forming dope to be discharged from a dope discharge port is obtained when the amount of a film-forming dope supplied to the hollow fiber membrane-spinning nozzle using a gear pump or the like is divided by the aperture area of the dope discharge port. Also, the feed rate of a hollow porous substrate to be fed out of a substrate feed port is obtained from the rotation speed of a drive roller such as a winding roller positioned on the downstream side of a hollow fiber membrane-spinning nozzle so as to draw out the hollow porous substrate. 
     For example, when the aforementioned spinning nozzle  1  is used, hollow porous substrate  2  is introduced to substrate insertion hole  10  of mounting plate  11 , first film-forming dope  3  is introduced into first intake hole  12 , and second film-forming dope  4  is introduced into second intake hole  13 . Hollow porous substrate  2  introduced into substrate insertion hole  10  is inserted into substrate insertion hole  7  of spinning nozzle  1 , and is fed out from substrate feed port  7   a.  First and second film-forming dopes  3 , 4  introduced into first and second intake holes  12 ,  13  flow respectively through first and second dope flow channels  28 ,  29  of spinning nozzle  1 . Next, the dopes are formed in cylindrical shapes at lamination portion  27 , while forming a composite by laminating second film-forming dope  4  on the outer side of first film-forming dope  3 . The composite laminate is then discharged from dope discharge port  27   a.  Outside the nozzle, first and second film-forming dopes  3 , 4  formed into a composite laminate and discharged from dope discharge port  27   a  are adhered to the peripheral surface of hollow porous substrate  2  fed out of substrate feed port  7   a.  Accordingly, first and second film-forming dopes  3 , 4  are applied on the peripheral surface of hollow porous substrate  2 . 
     In the above step, the linear velocity (V A ) at which first and second film-forming dopes  3 , 4  are discharged from dope discharge port  27   a  and the feed rate (V B ) at which hollow porous substrate  2  is fed out from substrate feed port  7   a  are adjusted respectively so that the draft ratio (V B /V A ) will be 1 to 6. 
     Examples of a hollow porous substrate are those known to be used for forming hollow fiber membranes. Specific examples are hollow braided or knitted cords made of various fibers such as polyester or polypropylene fibers. A hollow porous substrate may be one made of a single fiber material or in combination of multiple fiber materials. 
     Examples of fibers used for forming hollow braided or knitted cords are synthetic or semi-synthetic fibers, recycled fibers, natural fibers and the like. Fibers may have any form, for example, monofilament, multifilament, spun yarns or the like. 
     Moreover, a hollow porous substrate may be a porous hollow fiber membrane obtained by a melt-drawing technique. It is yet another option to use those obtained by immersing the aforementioned hollow porous substrate into a film-forming auxiliary liquid, or by applying a film-forming auxiliary liquid to the peripheral surface of the aforementioned hollow porous substrate. 
     Considering the productivity of a substrate and result of adhering a porous membrane layer to the substrate, a hollow porous substrate is preferred to be a braided cord made of a single multifilament. 
     A hollow porous substrate may take any form as long as it has at least one hollow portion in a cross section perpendicular to a longitudinal direction of the substrate extending in a longitudinal direction so that a liquid is transferred from the peripheral surface to the hollow portion and further transferred in the longitudinal direction. 
     Moreover, the cross-sectional shape of the hollow of a hollow porous substrate and the peripheral shape of the substrate cross section are not limited to a particular shape, and may be circular or irregular. In addition, the cross-sectional shape of the hollow of a hollow porous substrate and the peripheral shape of the substrate cross section may be the same as or different from each other. Considering pressure resistance, shape formation and the like, the peripheral shape of a substrate cross section is preferred to be circular. 
     The outer diameter of a hollow porous substrate is preferred to be 0.3 mm to 5 mm. Since variations in the outer diameter of a hollow porous substrate affect the quality such as spinning stability and film thickness, it is preferred to select a hollow porous substrate capable of maintaining a consistent outer diameter. 
     For example, when using a hollow porous substrate having a circular cross section perpendicular to a longitudinal direction and an outer diameter of 0.3 mm to 5 mm, the variation rate of the outer diameter of the hollow porous substrate is preferred to be no greater than ±0.3 mm. 
     In the embodiments of the present invention, heat treatment is preferred to be conducted on a hollow porous substrate before it is inserted into the substrate insertion hole of a spinning nozzle. Such a treatment reduces expansion/contraction rates of the hollow porous substrate, thereby stabilizing the outer diameter size. 
     As for film-forming dopes, any known types used for forming a porous layer of a hollow fiber membrane may be used. A film-forming dope is a solution obtained when a film-forming resin and an opening agent to control phase separation are dissolved in an organic solvent that is a good solvent for such components. 
     The film-forming resin is selected from generally used resins for forming the porous membrane layer of a hollow fiber membrane; specific examples are resins such as polysulfone, polyether sulfone, sulfonated polysulfone, polyvinylidene fluoride, polyacrylonitrile, polyimide, polyamide-imide, polyesterimide and the like. 
     They may be selected appropriately as desired. Among them, polyvinylidene fluoride is especially preferable because of its excellent drug resistance properties. 
     Examples of an opening agent are hydrophilic polymers such as monool, diol and triol represented by polyethylene glycols, polyvinylpyrrolidone and the like. They may be selected appropriately as desired. Among them, polyvinylpyrrolidone is preferred because of its excellent thickening effects. 
     Examples of an organic solvent are those capable of dissolving the film-forming resin and additives, and dimethyl sulfoxide, dimethylacetamide, dimethylformamide or the like may be used. 
     Any optional additive may be used unless it blocks the phase separation of the film-forming dope. 
     In the embodiments of the present invention, a film-forming dope is preferred to have a viscosity at 40° C. of 30,000 mPa·s or higher, more preferably 60,000 mPa·s or higher, even more preferably 150,000 mPa·s or higher. A higher viscosity of a film-forming dope makes it easier to make smaller pores in a porous membrane layer, thereby forming smaller voids and enhancing the quality of a hollow fiber membrane. Also, a higher draft ratio contributes to controlling the stability of film-forming dope discharged from a hollow fiber membrane-spinning nozzle. 
     As shown in the example using spinning nozzle  1 , when two or more film-forming dopes are used to form two or more layers on the peripheral surface of a hollow porous substrate, the viscosity at 40° C. of at least one of the film-forming dopes is preferred to be 30,000 mPa·s or higher, more preferably 60,000 mPa·s or higher, even more preferably 150,000 mPa·s or higher. 
     The upper limit of viscosity at 40° C. of a film-forming dope is preferred to be 500,000 mPa·s, more preferably 300,000 mPa·s. 
     Furthermore, when two or more film-forming dopes are used to form two or more layers on the peripheral surface of a hollow porous substrate, the molecular weight distribution of a film-forming resin contained in at least one film-forming dope is preferred to be 3 or less, and the molecular weight distribution of the film-forming resin contained in the film-forming dope to be applied on the outer side is preferred to be wider than that of the film-forming resin contained in the film-forming dope to be applied on the inner side By so setting, it is easier to form a dense structure while maintaining water permeability. 
     The film-forming dope applied on the peripheral surface of a hollow porous substrate is coagulated by a coagulation liquid to form a hollow fiber membrane precursor. The film-forming dope on the hollow porous substrate is coagulated by the coagulation liquid as it is phase-separated. 
     In the embodiments of the present invention, from the viewpoint of enhancing water permeability, it is preferred to employ a dry-wet spinning method having a blank section where the hollow porous substrate with applied film-forming dope runs in air for a certain distance between the spinning nozzle and the coagulation liquid. However, it is an option to employ a wet spinning method so that a film-forming dope is discharged from the spinning nozzle directly into a coagulation liquid. 
     The coagulation liquid needs to be a solvent that does not dissolve the film-forming resin while it is a good solvent for an opening agent. Examples of a coagulation liquid are water, ethanol, methanol and the like, including a mixture thereof. Among them, a mixture of water and the solvent to be used for the film-forming dope is preferred considering the working environment and operational management. 
     (Rinsing Step) 
     The obtained hollow fiber membrane precursor is rinsed off using a rinsing liquid so that the solvent remaining in the precursor is removed. 
     The rinsing liquid is preferred to be water considering its high rinsing effect. Examples of water are tap water, industrial water, river water, well water and the like. It is also an option to mix water with alcohols, inorganic salts, oxidants, surfactants and the like. 
     (Removing Step) 
     The opening agent remaining in the rinsed hollow fiber membrane precursor is removed by using an oxidant to obtain a hollow fiber membrane. More specifically, the rinsed hollow fiber membrane precursor is immersed in a chemical solution containing an oxidant such as hypochlorite, and is heated in a gas phase so that the opening agent is oxidized and decomposed. Then, the precursor is rinsed in a rinsing solution to remove the opening agent. 
     When the opening agent remaining in the layer formed by coagulating the film-forming dope is removed from the hollow fiber membrane precursor, portions where the opening agent was present become pores to form a porous membrane layer. Accordingly, a hollow fiber membrane is obtained. 
     (Drying Step) 
     The method for drying the obtained hollow fiber membrane is not limited specifically, and a dryer such as a hot air dryer may be used. 
     (Winding Step) 
     The dried hollow fiber membrane is wound using a bobbin or the like. 
     When a film-forming dope is applied on the peripheral surface of a hollow porous substrate, the greater the draft ratio (V B /V A ) is, the more stretched is the film-forming dope on the peripheral surface of the hollow porous substrate where the dope is adhered. Accordingly, the discharging process of a film-forming dope tends to be affected if the process is disturbed by causes such as equipment vibrations, change in feed rates of the hollow porous substrate, bubbles entrapped in the dope, variations in spinnability of the dope and the like. As a result, the film-forming dope tends to be detached from the hollow porous substrate at the position outside the nozzle where the dope is designed to be adhered to the peripheral surface of the substrate. 
     By contrast, using the production method of a hollow fiber membrane according to the present invention, a draft ratio (V B /V A ) controlled to be 1 to 6 during the spinning process reduces the degree of stretching the film-forming dope adhered to the outer periphery of the hollow porous substrate. Thus, even if the discharging process of a film-forming dope is disturbed, adhesion of the film-forming dope is less likely to be affected. As a result, the film-forming dope is suppressed from being detached from the hollow porous substrate at the position outside the nozzle where the dope is designed to be adhered to the peripheral surface of the substrate. Moreover, even if the film-forming dope is detached, it will be promptly reattached to the hollow porous substrate, thus preventing occurrence of defective portions caused by the abnormally discharged portion of the film-forming dope and process failure in the subsequent steps. 
     The present invention is not limited to the aforementioned embodiments, and various design modifications are possible within a range that does not deviate from the gist of the present invention. 
     The method for producing a hollow fiber membrane according to the present invention may be a method for forming a hollow fiber membrane structured to have a single porous membrane layer, or to have two or more porous membrane layers. 
     In the following the present invention is described in further detail by referring to examples. However, the present invention is not limited to those examples. 
     EXAMPLE 1 
     A hollow fiber membrane was prepared using spinning nozzle  1  shown in  FIGS. 1 to 6 . 
     As for hollow porous substrate  2 , five polyester fibers (fineness: 84 dtex, number of filaments: 36) were combined and knitted using a round knitting machine to form a round hollow knitted cord. A continuous heat drawing treatment was conducted on hollow porous substrate  2  using a 200° C. heating die (aperture diameter of 2.5 mm) at the upstream of spinning nozzle  1  so as to provide the substrate with low expansion contraction properties and outer diameter stability. The outer diameter of hollow porous substrate  2  was 2.5 mm and its inner diameter was 1.5 mm. 
     Film-forming dope (R 1 ) having the composition makeup specified in Table 1 was used as first film-forming dope  3 , and film-forming dope (R 2 ) having the composition makeup specified in Table 1 was used as second film-forming dope  4 . Raw materials shown below were used: 
     polyvinylidene fluoride A: product name Kynar 301F, made by Arkema Co., Ltd. 
     polyvinylidene fluoride B: product name Kynar 9000HD, made by Arkema polyvinylpyrrolidone: product name PVP-K79, made by Nippon Shokubai Co., Ltd. N,N-dimethylacetamide 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Film-forming 
                 Film-forming 
               
               
                   
                 dope R1 
                 dope R2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Polyvinylidene fluoride A 
                 12 
                 mass % 
                 20 
                 mass % 
               
               
                 Polyvinylidene fluoride B 
                 12 
                 mass % 
                 0 
                 mass % 
               
               
                 Polyvinylpyrrolidone 
                 12 
                 mass % 
                 10 
                 mass % 
               
               
                 N,N-dimethylacetamide 
                 64 
                 mas % 
                 70 
                 mass % 
               
               
                 Temperature of film-forming dope 
                 60° 
                 C. 
                 60° 
                 C. 
               
               
                 Polyvinylidene fluoride concentration 
                 24 
                 mass % 
                 20 
                 mass % 
               
               
                 in film-forming dope 
               
               
                   
               
            
           
         
       
     
     Spinning nozzle  1  was structured as follows: thickness (a) of tube-shaped  6   c:  0 . 4   mm, outer diameter (b) of dope discharge port  27   a:  4.52 mm, its inner diameter: 3.5 mm, aperture area: 6.42 mm 2 . Moreover, when first and second film-forming dopes  3 ,  4  were applied on the peripheral surface of hollow porous substrate  2 , linear velocity (V A ) of first and second film-forming dopes  3 ,  4  discharged from dope discharge port  27   a  was set at 5.63 m/min., feed rate (V B ) of hollow porous substrate  2  fed out from substrate feed port  7   a  was set at 15 m/min., and the draft ratio (V B /V A ) was 2.7. 
     Next, first and second film-forming dopes  3 , 4  applied on the peripheral surface of hollow porous substrate  2  were immersed in a coagulation liquid in a coagulation bath to coagulate first and second film-forming dopes  3 ,  4 , which were then pulled out of the bath, wound on a winding roller rotating at a constant speed of 15 m/min., and rinsed in 80 to 100° C. hot water. Accordingly, a hollow fiber membrane was obtained. 
     EXAMPLES 2 TO 4, COMPARATIVE EXAMPLES 1, 2 
     Hollow fiber membranes were each prepared the same as in Example 1 except that the following were changed as specified in Table 2: thickness (a) of tube-shaped wall  6   c,  outer diameter (b) of dope discharge port  27   a,  its inner diameter and aperture area, linear velocity (V A ) of first and second film-forming dopes  3 ,  4 , feed rate (V B ) of hollow porous substrate  2 , and the draft ratio (V B /V A ). 
     [Evaluation Methods] 
     (Size of Abnormally Discharged Portion) 
     When a hollow porous fiber membrane was obtained, the maximum outer diameter Tmax of an abnormally discharged portion on the peripheral surface of the hollow porous substrate  2  was measured to be set as the size of the abnormally discharged portion. If the maximum outer diameter Tmax is 4.5 mm or smaller, process failure is less likely to occur in the steps subsequent to the spinning step. Thus, hollow fiber membranes having a maximum outer diameter Tmax of 4.5 mm or smaller were evaluated as passing, and those having a maximum outer diameter of 4.5 mm or greater were evaluated as failing. 
     (Number of Abnormally Discharged Portions) 
     The number of abnormally discharged portions per 1000 km length of a hollow fiber membrane. 
     When the number of abnormally discharged portions is less than one per 1000 km length of a hollow fiber membrane, it is allowable considering the frequency of process failure caused by the abnormally discharged portions and an increase in production cost incurred accordingly. Therefore, when the number of abnormally discharged portions was less than one per 1000 km length of hollow fiber membrane, the hollow fiber membrane was evaluated as passing, and if the number was 1 or more, the hollow fiber membrane was evaluated as failing. 
     Spinning conditions and evaluation results in Examples 1 to 4 and Comparative Examples 1, 2 are shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 Comp. 
                 Comp. 
               
               
                   
                 Unit 
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 1 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Thickness (a) of cylindrical wall 6c 
                 mm 
                 0.4 
                 0.4 
                 0.1 
                 0.6 
                 0.8 
                 0.4 
               
               
                 Outer dia. (b) of dope discharge port 27a 
                 mm 
                 4.52 
                 5.33 
                 4.5 
                 4.5 
                 6.3 
                 6.3 
               
               
                 Inner dia. (c + 2a) of dope discharge port 27a 
                 mm 
                 3.5 
                 3.5 
                 2.9 
                 3.9 
                 4.1 
                 3.5 
               
               
                 Aperture area of dope discharge port 27a 
                 mm 2   
                 6.42 
                 12.69 
                 9.3 
                 3.96 
                 17.97 
                 21.55 
               
               
                 Linear velocity (V A ) of film-forming dope 
                 m/min 
                 5.63 
                 2.85 
                 3.89 
                 9.13 
                 2.01 
                 1.68 
               
               
                 Feed rate (V B ) of hollow porous substrate 
                 m/min 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
               
               
                 Draft ratio (V B /V A ) 
                 — 
                 2.7 
                 5.3 
                 3.9 
                 1.6 
                 7.5 
                 8.9 
               
               
                 Size of abnormally discharged portion 
                 mm 
                 4.2 
                 4.1 
                 3.3 
                 4.5 
                 6.8 
                 4.3 
               
               
                 (maximum outer dia. Tmax) 
               
               
                 Number of abnormally discharged portion 
                 number 
                 0.61 
                 0.82 
                 0.54 
                 0.96 
                 3.34 
                 3.65 
               
               
                   
                 per 1000 km 
               
               
                   
               
            
           
         
       
     
     In each of Examples 1 to 4, where the draft ratio (V B /V A ) was set to be 1 to 6, the number of abnormally discharged portions per 1000 km length of hollow fiber membrane was less than one, and the maximum outer diameter Tmax of each abnormally discharged portion was 4.5 mm or smaller. 
     By contrast, in Comparative Example 1, where the draft ratio (V B /V A ) was set to be above 6, the number of abnormally discharged portions per 1000 km length of hollow fiber membrane was 3.34, and the diameter of each abnormally discharged portion was 6.8 mm, which were beyond allowable ranges. 
     In Comparative Example 2, where the draft ratio (V B /V A ) was also set to be above 6, the number of abnormally discharged portions per 1000 km length of hollow fiber membrane was 3.65, which was beyond the allowable range. 
     DESCRIPTION OF NUMERICAL REFERENCES 
       1  hollow fiber membrane-spinning nozzle
 
 2  hollow porous substrate
 
 3  first film-forming dope
 
 4  second film-forming dope
 
 6   c  tube-shaped wall
 
 7  substrate insertion hole
 
 7   a  substrate feed port
 
 14  first dope distribution hole
 
 15  second dope distribution hole
 
 21  first dope reservoir
 
 23  first dope shaping portion
 
 25  second dope reservoir
 
 27  dope lamination portion
 
 27   a  dope discharge port
 
 28  first dope flow channel
 
 29  second dope flow channel