Patent Publication Number: US-2023149981-A1

Title: Method for manufacturing water absorbent resin

Description:
TECHNICAL FIELD 
     The present invention relates to a method for manufacturing a water-absorbent resin. 
     BACKGROUND ART 
     A water-absorbent resin can be manufactured by polymerizing monomers serving as a raw material to form an aqueous gel-like polymer and then drying the polymer. During the polymerization reaction, heat of polymerization is generated. Thus, the polymerization tank for use in polymerization is often connected to a heat exchanger to remove the heat of polymerization. This type of heat exchanger is typically configured to cool and condense a gas delivered from the polymerization tank, and then return the condensed matter to the polymerization tank (see Patent Literature 1). 
     The gas delivered to the heat exchanger usually contains a polymer. As pointed out in Patent Literature 1, the polymer may clog the pipes for heat transfer in the heat exchanger. In order to deal with this problem, Patent Literature 1 proposes suppressing the clogging of pipes with a polymer by obliquely attaching a plate-like member with a plurality of through holes in a chamber connected to the upstream side of a plurality of pipes for heat transfer, and collecting the polymer at a low place on the top surface of the member. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2013-204860A 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, even with the method described in Patent Literature 1, the clogging of the pipes cannot be sufficiently prevented, and the pipes still often become clogged. Accordingly, it is necessary to regularly stop the operation of the heat exchanger, disassemble the heat exchanger, and clean the inside thereof, which is very burdensome to users. Similar problems may occur in other heat exchangers connected to a vessel holding an aqueous gel-like polymer such as a concentrator for use in evaporating a liquid component after the end of a polymerization reaction. 
     An object of the present invention is to provide a method for manufacturing a water-absorbent resin that is capable of effectively preventing the clogging of pipes of a heat exchanger. 
     Solution to Problem 
     A method for manufacturing a water-absorbent resin according to a first aspect includes a cleaning step of cleaning an inside of a heat exchanger that is connected to a vessel holding an aqueous gel-like polymer as a precursor of the water-absorbent resin and is configured to cool a gas containing the polymer produced in the vessel. The heat exchanger includes: a plurality of pipes that have elongated paths through which the gas passes, and in which the gas is cooled due to heat dissipation during passage through the paths; a chamber that is arranged above the plurality of pipes and is in communication with openings at upper ends of the plurality of pipes; and a cleaner that sprays a cleaning liquid in the chamber. The cleaning step includes at least cleaning an inside of the chamber by spraying the cleaning liquid from the cleaner in the chamber. 
     A method for manufacturing a water-absorbent resin according to a second aspect is the manufacturing method according to the first aspect in which the cleaner is configured to spray the cleaning liquid toward at least an inner wall surface of the chamber. 
     A method for manufacturing a water-absorbent resin according to a third aspect is the manufacturing method according to the second aspect in which the cleaner is configured to spray the cleaning liquid toward at least a top surface included in the inner wall surface of the chamber. 
     A method for manufacturing a water-absorbent resin according to a fourth aspect is the manufacturing method according to any one of the first to third aspects in which the cleaner has a spray nozzle configured to rotate around a first shaft extending in a first direction while rotating around a second shaft extending in a second direction that intersects the first direction, and configured to spray the cleaning liquid while rotating around the first shaft and the second shaft. 
     A method for manufacturing a water-absorbent resin according to a fifth aspect is the manufacturing method according to any one of the first to fourth aspects in which the vessel is a polymerization tank in which monomers serving as a raw material for the water-absorbent resin are polymerized. 
     A method for manufacturing a water-absorbent resin according to a sixth aspect is the manufacturing method according to any one of the first to fifth aspects in which the cleaning step includes removing a lump of the polymer adhering to at least one of the inner wall surface of the chamber and inner wall surfaces of the pipes by spraying the cleaning liquid from the cleaner in the chamber. 
     A method for manufacturing a water-absorbent resin according to a seventh aspect is the manufacturing method according to the sixth aspect in which the cleaning step includes breaking up and knocking off the lump of the polymer adhering to the inner wall surface of the chamber by spraying the cleaning liquid toward the inner wall surface of the chamber. 
     Advantageous Effects of Invention 
     According to the foregoing aspects, the cleaning liquid is sprayed from the cleaner in the chamber that is arranged above the plurality of pipes in the heat exchanger and is in communication with the openings at the upper ends of the plurality of pipes. This makes it possible to remove the aqueous gel-like polymer that is the precursor of the water-absorbent resin adhering to the inside of the chamber, thereby effectively prevent the clogging of the pipes. As a result, the water-absorbent resin can be manufactured efficiently. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a general configuration diagram of an apparatus for manufacturing a water-absorbent resin according to an embodiment of the present invention; 
         FIG.  2    is a cross-sectional view of a heat exchanger according to the embodiment of the present invention; 
         FIG.  3    is a graph indicating cleaning effects according to a first example; and 
         FIG.  4    is a graph indicating cleaning effects according to a second example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A method for manufacturing a water-absorbent resin according to an embodiment of the present invention will be described below with reference to the drawings. 
     1. Apparatus for Manufacturing Water-Absorbent Resin 
       FIG.  1    is a general configuration diagram of a water-absorbent resin manufacturing apparatus  100  for use in performing a method for manufacturing a water-absorbent resin according to the present embodiment. Water-absorbent resin is widely used in various applications such as hygiene products for paper diapers and sanitary items, daily necessities such as pet sheets, water-absorbent paper towel for food, water-stop materials for cables, industrial materials such as condensation-preventive material, and water retention agents and soil improvement agents for greening, agriculture, and gardening. Water-absorbent resin is manufactured by polymerizing monomers serving as a raw material to form a polymer. 
     As illustrated in  FIG.  1   , a water-absorbent resin manufacturing apparatus  100  includes a polymerizer  1  and a concentrator  2 . The polymerizer  1  polymerizes monomers serving as a raw material for the water-absorbent resin to form a slurry containing an aqueous gel-like polymer (a liquid containing an aqueous gel-like polymer). The concentrator  2  evaporates a liquid component of the slurry delivered from the polymerizer  1  to concentrate the slurry and produce a concentrated polymer liquid. The manufacturing apparatus  100  further includes a dryer  3 . The dryer  3  dries the concentrated polymer liquid delivered from the concentrator  2  (that is, volatilizes the liquid component). In the case of producing an aqueous gel-like polymer by a reverse-phase suspension polymerization method, the liquid component mainly contains a hydrocarbon dispersion medium and water. In the case of producing an aqueous gel-like polymer by an aqueous polymerization method, the liquid component mainly contains water. 
     As illustrated in  FIG.  1   , the manufacturing apparatus  100  includes two heat exchangers  4 A and  4 B. During the polymerization reaction in the polymerizer  1 , heat of polymerization is generated. During the concentration process in the concentrator  2 , the slurry is heated. The heat exchanger  4 A is connected to the polymerizer  1  to remove the heat of polymerization. The heat exchanger  4 B is connected to the concentrator  2  to remove heat produced during the concentration process. 
     As illustrated in  FIG.  1   , the manufacturing apparatus  100  further includes a control device  5  that controls the operations of the polymerizer  1 , the concentrator  2 , the dryer  3 , and the heat exchangers  4 A and  4 B to control the water-absorbent resin manufacturing process. The control device  5  is typically implemented as a computer controlled by programs. 
     The polymerizer  1 , the concentrator  2 , the dryer  3 , and the heat exchangers  4 A and  4 B will be described in detail below, while touching on various devices connected to these devices  1 ,  2 ,  3 ,  4 A, and  4 B as appropriate. 
     2. Configurations of Components 
     2-1. Polymerizer 
     The polymerizer  1  has a polymerization tank  10 . The polymerization tank  10  is a vessel that holds monomers serving as a raw material for the water-absorbent resin, for example, water-soluble ethylene unsaturated monomers, and a liquid component, such that a gas-phase component is formed above these materials. In the polymerization tank  10 , the monomers and the liquid component are appropriately stirred by a stirrer (not illustrated) and are heated by a heater (not illustrated), so that the polymerization reaction of the monomers proceeds to produce an aqueous gel-like polymer. The stirrer and the heater are connected to the control device  5 , and operations thereof are controlled by the control device  5 . Accordingly, a slurry containing the aqueous gel-like polymer as a precursor of the water-absorbent resin is held in the polymerization tank  10 . 
     The polymerization tank  10  has openings  11  and  12  at the upper part. One end of a pipe L 1  is connected to the upper opening  11 , and the other end of the pipe L 1  is connected to an opening  41  in the heat exchanger  4 A described later. A gas heated by the heat of polymerization resulting from the polymerization reaction is discharged from the opening  11 , passes through the pipe L 1 , and flows into the heat exchanger  4 A via the opening  41 . A valve V 1  is attached to the pipe L 1 . The opening and closing of the valve V 1  is controlled by the control device  5  in order to control the flow of the gas via the pipe L 1 . 
     One end of another pipe L 2  is connected to the other upper opening  12 , and the other end of the pipe L 2  is connected to an opening  42  in the heat exchanger  4 A described later. The gas delivered from the polymerization tank  10  to the heat exchanger  4 A is cooled and condensed to a cooled fluid containing a liquid and a gas in the heat exchanger  4 A. The cooled fluid is discharged from the opening  42 , passes through the pipe L 2 , and flows into the polymerization tank  10  via the opening  12 . Accordingly, the heat of polymerization is removed from the polymerization tank  10 . A valve V 2  is attached to the pipe L 2 . The opening and closing of the valve V 2  is controlled by the control device  5  in order to control the flow of the cooled fluid via the pipe L 2 . 
     The polymerization tank  10  has an opening  13  at the lower part. One end of a pipe L 3  is connected to the lower opening  13 , and the other end of the pipe L 3  is connected to an opening  21  in the concentrator  2  described later. A slurry generated by the polymerization reaction in the polymerization tank  10  is discharged from the opening  13 , passes through the pipe L 3 , and flows into the concentrator  2  via the opening  21 . A valve V 3  is attached to the pipe L 3 . The opening and closing of the valve V 3  is controlled by the control device  5  in order to control the flow of the slurry via the pipe L 3 . 
     2-2. Concentrator 
     The concentrator  2  has a concentration tank  20 . The concentration tank  20  has openings  21 ,  22 , and  23  at the upper part. The upper opening  21  is connected to the opening  13  in the polymerization tank  10  via the pipe L 3  as described above. The concentration tank  20  is a vessel that holds the slurry delivered from the polymerization tank  10  via the pipe L 3  such that a gas-phase component is formed above the slurry. In the concentration tank  20 , the slurry is stirred as appropriate by a stirrer (not illustrated) and is heated by a heater (not illustrated), so that the liquid component in the slurry evaporates and the slurry becomes concentrated. The stirrer and the heater are connected to the control device  5 , and operations thereof are controlled by the control device  5 . Accordingly, the concentration tank  20  holds a concentrated liquid containing an aqueous gel-like polymer as the precursor of a water-absorbent resin. 
     One end of another pipe L 4  is connected to another upper opening  22 , and the other end of the pipe L 4  is connected to the opening  41  in the heat exchanger  4 B described later. A gas heated by the concentration process is discharged from the opening  22 , passes through the pipe L 4 , and flows into the heat exchanger  4 B via the opening  41 . A valve V 4  is attached to the pipe L 4 . The opening and closing of the valve V 4  is controlled by the control device  5  in order to control the flow of the gas via the pipe L 4 . 
     One end of another pipe L 5  is connected to the other opening  23  at the upper part, and the other end of the pipe L 5  is connected to the opening  42  in the heat exchanger  4 B described later. The gas delivered from the concentration tank  20  to the heat exchanger  4 B is cooled and condensed in the heat exchanger  4 B to form a cooled fluid containing a liquid and a gas. The cooled fluid is discharged from the opening  42 , passes through the pipe L 5 , and an oil component therein flows into the concentration tank  20 . Accordingly, heat is removed from the concentration tank  20 . A valve V 5  is attached to the pipe L 5 . The opening and closing of the valve V 5  is controlled by the control device  5  in order to control the flow of the cooled fluid via the pipe L 5 . 
     In the present embodiment, an oil separation device D 1  is attached to the pipe L 5 . The oil separation device D 1  is used to manufacture a polymer by a reverse-phase suspension polymerization method, and is not required to be used in the case of employing a water-solution polymerization method. The oil separation device D 1  is a device that separates an oil component (hydrocarbon dispersion medium) and a water component from a liquid component, and is connected to a collector D 2  via a pipe L 7  branched from the pipe L 5 . The water component separated from the cooled fluid by the oil separation device D 1  is discharged from the system and collected in the collector D 2 , rather than being returned to the concentration tank  20 . This promotes dehydration of the slurry in the concentration tank  20 . On the other hand, the oil component separated from the cooled fluid by the oil separation device D 1  is returned to the concentration tank  20  via the opening  23 . 
     Note that the opening  23 , the valve V 5 , the oil separation device D 1  and the like may be omitted, and all of the cooled fluid discharged from the opening  42  in the heat exchanger  4 B may be discharged from the system and collected in the collector D 2 . In this case, an oil component is preferably added to the concentration tank  20  as appropriate. 
     The concentration tank  20  has an opening  24  at the lower part. One end of a pipe L 6  is connected to the lower opening  24 , and the other end of the pipe L 6  is connected to an opening  31  in the dryer  3  described later. The concentrated liquid generated by the concentration process in the concentration tank  20  is discharged from the opening  24 , passes through the pipe L 6 , and flows into the dryer  3  via the opening  31 . A valve V 6  is attached to the pipe L 6 . The opening and closing of the valve V 6  is controlled by the control device  5  in order to control the flow of the concentrated liquid via the pipe L 6 . 
     2-3. Dryer 
     The dryer  3  has a drying chamber  30 . The drying chamber  30  has the opening  31  at the upper part. The upper opening  31  is connected to the opening  24  in the concentration tank  20  via the pipe L 6  as described above. The drying chamber  30  receives the concentrated liquid delivered from the concentration tank  20  via the pipe L 6  and heats the concentrated liquid by a heater (not illustrated) to dry the polymer of the water-absorbent resin contained in the concentrated liquid. The heater is connected to the control device  5 , and the operation thereof is controlled by the control device  5 . 
     Note that the concentrator  2  and the heat exchanger  4 B connected thereto may be omitted, and the polymerizer  1  may be connected directly to the dryer  3 . In this case, the dryer  3  concentrates and dries the slurry at the same time. 
     2-4. Heat Exchanger 
     Next, the structure of the heat exchangers  4 A and  4 B will be described with reference to  FIG.  2   . The heat exchanger  4 A and  4 B have the same structure, and hereinafter, they will be collectively called heat exchanger  4  without differentiation therebetween, and the structure thereof will be collectively described. 
     The heat exchanger  4  is a device that cools the gas containing the aqueous gel-like polymer as a precursor of the water-absorbent resin, which is produced in the polymerization tank  10  or the concentration tank  20 . The heat exchanger  4  has a casing  40 . The casing  40  has a cylindrical trunk part  401 , a dome-like upper chamber  402  connected to the upper end of the trunk part  401 , and an inverse dome-like lower chamber  403  connected to the lower end of the trunk part  401 . The inner space of the trunk part  401  and the inner space of the upper chamber  402  are partitioned and separated from each other by a partition wall  404 . Similarly, the inner space of the trunk part  401  and the inner space of the lower chamber  403  are partitioned and separated from each other by a partition wall  405 . 
     The opening  41  is formed in the wall surface part of the upper chamber  402  to let the inside and outside of the upper chamber  402  communicate with each other. The opening  41  is connected to the opening  11  in the polymerization tank  10  via the pipe L 1  or is connected to the opening  22  in the concentration tank  20  via the pipe L 4 , as described above. Therefore, the gas containing the aqueous gel-like polymer is introduced from the polymerization tank  10  or the concentration tank  20  into the upper chamber  402  via the opening  41 . 
     A plurality of pipes  60  that are vertically elongated are arranged inside the trunk part  401 . These pipes  60  extend between a plurality of openings in the upper partition wall  404  and a plurality of openings in the lower partition wall  405 . Therefore, the upper chamber  402  is arranged above the plurality of pipes  60  and is in communication with openings at upper ends of the plurality of pipes  60 . The lower chamber  403  is arranged under the plurality of pipes  60  and is in communication with openings at lower ends of the plurality of pipes  60 . 
     The pipes  60  each define an elongated path. The gas delivered from the polymerization tank  10  or the concentration tank  20  is introduced into the upper chamber  402 , and then is introduced from the upper chamber  402  into these paths. While passing through the paths, the gas dissipates heat and is cooled and condensed into a cooled fluid containing a liquid and a gas. Thereafter, the cooled fluid is introduced into the lower chamber  403 . 
     The lowermost part of the lower chamber  403  has the opening  42  that lets the inside and outside of the lower chamber  403  communicate with each other. The opening  42  is connected to the opening  12  in the polymerization tank  10  via the pipe L 2  or is connected to the opening  23  in the concentration tank  20  via the pipe L 5 . Therefore, the cooled fluid is discharged from the lower chamber  403  into the polymerization tank  10  or the concentration tank  20  via the opening  42 . 
     The wall surface part of the trunk part  401  has the openings  43  and  44  that let the inside and outside of the trunk part  401  communicate with each other. The openings  43  and  44  are separately arranged at the upper part and lower part of the trunk part  401 . A plurality of guide plates  45  are arranged inside the trunk part  401  with predetermined spacings therebetween in the vertical direction. The guide plates  45  have a plurality of openings into which the plurality of corresponding pipes  60  are inserted. 
     Either the opening  43  or the opening  44  constitutes an introduction port through which a heat-exchange fluid is introduced into the trunk part  401 , and the other one constitutes a discharge port through which the heat-exchange fluid is discharged from the trunk part  401 . Although there is no particular limitation this, the heat-exchange fluid is water or air, for example. In the example of  FIG.  2   , the lower opening  44  is the introduction port, the upper opening  43  is the discharge port, and the flow of the heat-exchange fluid is indicated with arrows A 1 . The heat-exchange fluid introduced into the trunk part  401  via the introduction port that is either one of the openings  43  and  44  is guided by the plurality of guide plates  45  in the trunk part  401 , passes through a winding course to the discharge port that is the other one of the openings  43  and  44 , and is discharged from the trunk part  401  via the discharge port. Since the heat-exchange fluid comes into contact with the plurality of pipes  60  while flowing through the trunk part  401  from the introduction port to the discharge port, efficient heat exchange is achieved between the heat-exchange fluid and the gas flowing in the pipes  60  to facilitate the condensation of the gas. 
     As described above, the gas delivered into the heat exchanger  4  contains a polymer. If the polymer excessively enters and adheres to the inside of the pipes  60  in the heat exchanger  4  and clog the pipes  60 , it is necessary to perform burdensome work of stopping the operation of the heat exchanger  4 , and disassembling and cleaning the heat exchanger  4  on a regular basis. In the present embodiment, in order to effectively prevent the clogging of the pipes  60 , a cleaner  7  is arranged in the upper chamber  402 . The cleaner  7  sprays a cleaning liquid in the upper chamber  402  to clean the inside of the heat exchanger  4  (mainly the inside of the upper chamber  402 ). 
     The present inventors have found that the clogging of the pipes  60  is more likely to occur due to the polymer adhering to the inner wall surfaces (in particular, the top surface) of the upper chamber  402  than due to the polymer adhering to the inner wall surfaces of the pipes  60 . That is, if the polymer adheres to the inner wall surfaces, in particular the top surface of the upper chamber  402  and grows to large lumps, the lumps may then drop off to clog the pipes  60 . The same thing can be said for the case where the polymer adheres to the upper surface of the upper partition wall  404 . 
     Based on the above findings, the inventors have discovered that the clogging of the pipes  60  can be effectively prevented by cleaning the inside of the upper chamber  402 , more preferably, by spraying a cleaning liquid onto at least the inner wall surfaces of the upper chamber  402  (cleaning the inner wall surfaces). The inside of the upper chamber  402  includes surfaces defining the inner space of the upper chamber  402 . In the present embodiment, the inside of the upper chamber  402  includes the inner wall surfaces of the upper chamber  402  and the upper surface of the upper partition wall  404 . 
     The inventors have also found that the inner wall surfaces of the upper chamber  402  that are to be cleaned preferably include at least the top surface, and that the clogging of the pipes  60  can be further effectively prevented by not only cleaning the inside of the  5  upper chamber  402  (preferably, the inner wall surfaces of the upper chamber  402 , more preferably, the top surface included in the inner wall surfaces of the upper chamber  402 ) but also cleaning the inside of the pipes  60 . In this respect, the cleaner  7  in the present embodiment can effectively clean not only the inner wall surfaces including the top surface of the upper chamber  402  and the upper surface of the upper partition wall  404  but also the inside of the pipes  60 . 
     The cleaner  7  includes a nozzle  70 , a first shaft  71  that extends vertically, a second shaft  72  that extends horizontally, a first motor  73  that drives the first shaft  71  to rotate, and a second motor  74  that drives the second shaft  72  to rotate. The first motor  73  is fixed to the center of the top surface of the upper chamber  402  in a plan view, rotatably supports the upper part of the first shaft  71 , and drives the first shaft  71  to rotate around an axis parallel to the vertical direction. The second motor  74  is fixed to the lower part of the first shaft  71 , rotatably supports one end of the second shaft  72 , and drives the second shaft  72  to rotate around an axis parallel to the horizontal direction. The nozzle  70  is fixed to the other end of the second shaft  72 . 
     In the present embodiment, the nozzle  70  is arranged in the upper chamber  402  and has two jetting ports  701  and  702 . The jetting ports  701  and  702  are both open in directions intersecting the second shaft  72  (in the present embodiment, directions orthogonal to the second shaft  72 ). The jetting ports  701  and  702  are also arranged at positions separated from each other by 180° around the second shaft  72 , and are open in opposite directions. The first motor  73  and the second motor  74  are connected to the control device  5 . The driving of the first motor  73  and the second motor  74 , that is to say the rotational operations of the first shaft  71  and the second shaft  72 , are controlled by the control device  5 . 
     When executing the cleaning step of cleaning the inside of the heat exchanger  4 , the control device  5  drives the first motor  73  and the second motor  74  at the same time. Accordingly, the jetting ports  701  and  702  included in the nozzle  70  rotate around the first shaft  71  while rotating around the second shaft  72 . Hereinafter, the rotation around the first shaft  71  will also be called “revolution”, and the rotation around the second shaft  72  will also be called “spinning”. 
     As indicated with arrow A 2  in  FIG.  2   , the first shaft  71  and the second shaft  72  internally have a path through which a cleaning liquid flows. In the cleaning step, the cleaning liquid is supplied from a pump (not illustrated) into the path, flows into the nozzle  70 , and is sprayed through the jetting ports  701  and  702 . The pump is connected to the control device  5  so that the control device  5  controls the discharge timing and discharge pressure of the cleaning liquid. The type of the cleaning liquid is not particularly limited, but is preferably water. 
     As described above, in the cleaning step, the jetting ports  701  and  702  spray the cleaning liquid while revolving and spinning. As a result, the cleaning liquid is sprayed three-dimensionally in every direction from the jetting ports  701  and  702  in the upper chamber  402 . The cleaning liquid is sprayed onto all the inner wall surfaces of the upper chamber  402  including the top surface and side surfaces, and collides with all the surfaces to remove the adhering polymer from all the surfaces. The cleaning liquid is also sprayed down from the nozzle  70 , that is to say onto the upper surface of the upper partition wall  404  and the inside of the pipes  60  to remove the adhering polymer from the upper surface of the upper partition wall  404  and the inner wall surfaces of the pipes  60 . 
     The lower limit of the flow rate of the cleaning liquid is preferably 50 L/min, more preferably 60 L/min, and further preferably 70 L/min. The upper limit of flow rate of the cleaning liquid is preferably 250 L/min, more preferably 200 L/min, and further preferably 180 L/min. The lower limit of discharge pressure of the cleaning liquid is preferably 0.2 MPaG, more preferably 3.0 MPaG, and further preferably 5.0 MPaG. The upper limit of discharge pressure of the cleaning liquid is preferably 15 MPaG, more preferably 10 MPaG, and further preferably 8 MPaG. When the flow rate and the discharge pressure of the cleaning liquid are within the foregoing ranges, it is possible to efficiently break up and knock off lumps of the polymer on the inside of the upper chamber  402  while effectively suppressing the usage of the cleaning liquid. 
     3. Method for Manufacturing Water-Absorbent Resin 
     A method for manufacturing a water-absorbent resin using the manufacturing apparatus  100  will be described below, taking a reverse-phase suspension polymerization method as an example. 
     The processes for manufacturing a water-absorbent resin by the manufacturing apparatus  100  are mainly controlled by the control device  5 . First, monomers serving as a raw material for a water-absorbent resin and a liquid component (a hydrocarbon dispersion medium) are put into the polymerization tank  10 . The control device  5  switches the valve V 1  and the valve V 2  to the open state to put the polymerization tank  10  and the heat exchanger  4 A in communication with each other. In this state, the control device  5  drives the stirrer and the heater included in the polymerizer  1 . 
     Accordingly, a polymerization reaction proceeds in the polymerization tank  10 , and a gas containing an aqueous gel-like polymer heated by the heat of polymerization is delivered to the heat exchanger  4 A via the pipe L 1 . The gas passes through the upper chamber  402  in the heat exchanger  4 A and is cooled during passage through the pipes  60  to form a cooled fluid. The cooled fluid is then returned to the polymerization tank  10  via the pipe L 2 . In this manner, a slurry containing the aqueous gel-like polymer is produced in the polymerization tank  10 . 
     Upon completion of the process in the polymerizer  1 , the control device  5  stops the driving of the stirrer and the heater included in the polymerizer  1  and switches the valve V 3  to the open state. Accordingly, the slurry in the polymerization tank  10  is delivered to the concentration tank  20 . After the delivery of all the slurry from the polymerization tank  10  to the concentration tank  20 , the control device  5  returns the valve V 3  to the closed state. The control device  5  then switches the valves V 4  and V 5  to the open state to put the concentration tank  20  and the heat exchanger  4 B in communication with each other. In this state, the control device  5  drives the stirrer and heater included in the concentrator  2 . Accordingly, the process of concentration of the slurry in the concentration tank  20  proceeds, and a gas containing the heated aqueous gel-like polymer is delivered to the heat exchanger  4 B via the pipe L 4 . The gas passes through the upper chamber  402  in the heat exchanger  4 B, and is cooled during passage through the pipes  60  to form a cooled fluid. Then, the oil component contained in the cooled fluid is returned to the concentration tank  20  via the pipe L 5 . In this manner, a concentrated liquid of the slurry is produced in the concentration tank  20 . 
     At the end of the process in the concentrator  2 , the control device  5  stops the driving of the stirrer and heater included in the concentrator  2  and switches the valve V 6  to the open state. Accordingly, the concentrated liquid in the concentration tank  20  is delivered to the drying chamber  30 . After the delivery of all the concentrated liquid from the concentration tank  20  to the drying chamber  30 , the control device  5  returns the valve V 6  to the closed state. The control device  5  then drives the heater included in the dryer  3 . Accordingly, the drying of the polymer contained in the concentrated liquid proceeds in the drying chamber  30 . In this manner, a dried water-absorbent resin is manufactured in the drying chamber  30 . 
     After the end of the process in the polymerizer  1 , at the same time as the subsequent concentration process or the drying process or after these processes, the step of cleaning the inside of the heat exchanger  4 A is executed. The control device  5  drives the first motor  73 , the second motor  74 , and the pump included in the cleaner  7  at the same time, and causes the nozzle  70  to spray a cleaning liquid from the jetting ports  701  and  702  while revolving and spinning in the upper chamber  402 , thereby cleaning the inside of the upper chamber  402  and the inside of the pipes  60 . At this time, along with the revolution and spinning of the nozzle  70 , the cleaning liquid sprayed from the jetting ports  701  and  702  is distributed with great force directly to all the wall surfaces of the upper chamber  402  including the top surface and side surfaces and the upper surface of the upper partition wall  404 . Due to the pressure, the cleaning liquid removes the adhering polymer from the inner wall surfaces of the upper chamber  402  and the upper surface of the upper partition wall  404 . Accordingly, the lumps of polymer adhering to the inside of the upper chamber  402  are broken up and knocked off from the inside of the upper chamber  402 . 
     Similarly, along with the revolution and spinning of the nozzle  70 , the cleaning liquid sprayed from the jetting ports  701  and  702  is also distributed with great force directly to the inside of the pipes  60 . Due to the pressure, the cleaning liquid removes the adhering polymer from the inner wall surfaces of the pipes  60 . Accordingly, the lumps of polymer adhering to the inner wall surfaces of the pipes  60  are also broken up and knocked off from the inner wall surfaces of the pipes  60 . As a result, the polymer is removed from the inside of the upper chamber  402  and the inside of the pipes  60 . The removed polymer passes through the pipes  60  together with the cleaning liquid and is collected in the lower chamber  403 . The cleaning liquid containing the polymer collected in the lower chamber  403  is appropriately discharged from the system via the opening  42 . 
     After the end of the process in the concentrator  2 , at the same as or after the subsequent drying process, the step of cleaning the inside of the heat exchanger  4 B is executed. The cleaning step is executed similarly to the step of cleaning the inside of the heat exchanger  4 A described above, and thus detailed description thereof will be omitted. 
     In the cleaning step described above, it is possible to effectively remove the polymer adhering to the inside of the pipes  60  and the inside of the upper chamber  402 . As a result, it is possible to effectively prevent the clogging of the pipes  60 . Consequently, the water-absorbent resin can be manufactured efficiently. 
     4. Modification Examples 
     One embodiment of the present invention has been described above. However, the present invention is not limited to the foregoing embodiment, and can be modified in various manners without departing from the gist of the present invention. For example, the following modification examples are possible. 
     4-1 
     The first shaft  71  does not need to extend vertically, and the second shaft  72  does not need to extend horizontally. The first shaft  71  and the second shaft  72  do not need to be orthogonal to each other, although they preferably intersect each other from the viewpoint of spraying the cleaning liquid in various directions. 
     4-2 
     The driving system of the nozzle  70  in the cleaner  7  is an electromotive driving system in the foregoing embodiment. Alternatively, the driving system may be a water-flow driving system. 
     4-3 
     In the foregoing embodiment, the cleaning liquid sprayed from the nozzle  70  is distributed directly to both the inside of the pipes  60  and the inside of the upper chamber  402  (the inner wall surfaces of the upper chamber  402  and the upper surface of the upper partition wall  404 ) in order to remove the adhering polymer from both the parts. Alternatively, the cleaning liquid sprayed from the nozzle  70  may be distributed directly to only the inside of the upper chamber  402  to remove the adhering polymer from the inside of the upper chamber  402 . The portions inside the upper chamber  402  to be cleaned do not need to include the top surface of the upper chamber  402 . That is, the portions inside the upper chamber  402  to be cleaned may be only the side surfaces of the upper chamber  402 , only the upper surface of the upper partition wall  404 , or only the side surfaces of the upper chamber  402  and the upper surface of the upper partition wall  404 . 
     For example, the inside of the pipes  60  and the upper surface of the upper partition wall  404  can be cleaned by attaching a shower nozzle to the top surface of the upper chamber  402  and spraying the cleaning liquid downward from the shower nozzle. In addition, the side surfaces included in the inner wall surfaces of the upper chamber  402  can also be cleaned by adjusting the flow rate, discharge pressure, and spraying angle of the cleaning liquid sprayed from the shower nozzle. The cleaner  7  in the foregoing embodiment and the shower nozzle can be used in combination. 
     EXAMPLES 
     First and second examples of the present invention will be described below. However, the present invention is not limited to the first and second examples. 
     First Example 
     A manufacturing apparatus similar to the manufacturing apparatus  100  according to the foregoing embodiment was prepared and used to manufacture a water-absorbent resin by the manufacturing method according to the foregoing embodiment. That is, the series of processes of polymerizing monomers in a polymerizer and then discharging a slurry to a concentrator was executed about 350 times. In addition, each time a slurry was discharged to the concentrator, the cleaning of a heat exchanger connected to the polymerizer was performed. However, the cleaning step was suspended during the 168th to 243rd times out of the 350 times. In the cleaning step, the flow rate of the cleaning liquid was set to 73 L/min, and the discharge pressure of the cleaning liquid was set to 6.3 MPaG. A water-flow driving cleaner (automatic rotation nozzle unit “JRN-080PG” manufactured by Sugino Machine Limited) was used as the cleaner. 
     The internal pressure in the polymerization tank and the heat exchanger connected thereto during the polymerization reaction was measured by a pressure gauge attached to the inside of the polymerization tank.  FIG.  3    illustrates the pressure measurement results (maximum values during the polymerization reaction). The horizontal axis in  FIG.  3    indicates the number of times when the series of processes was performed, including the polymerization process to the discharge process of the slurry and the step of cleaning the heat exchanger.  FIG.  3    also illustrates the periods during which the cleaning step was suspended. 
     If the pipes in the heat exchanger become clogged, the pressure in the polymerization tank and the heat exchanger connected thereto increases. Therefore, the pressures described in  FIG.  3    can be said to indicate the cleaning state of the heat exchanger. As seen from the measurement results, the pressure did not increase so much even after the execution of the polymerization process 350 times. After suspending the cleaning step, the pressure became higher. Thereafter, when the cleaning step was restarted, the pressure decreased again. This has confirmed that the cleaning step according to the first example had a high cleaning effect. 
     Second Example 
     In the manufacturing apparatus according to the first example, the cleaner was detached from the heat exchanger connected to the polymerizer, and instead, as described in modification example 4-3, a shower nozzle for spraying downward a cleaning liquid (the nozzle “JJXP150” manufactured by H. IKEUCHI &amp; CO., LTD.) was attached to the top surface of the upper chamber. Then, the manufacturing apparatus was used to perform the series of processes of polymerizing monomers in the polymerizer and then discharging a slurry to the concentrator was performed 350 times as in the first example. Each time the slurry was discharged to the concentrator, the cleaning of the heat exchanger connected to the polymerizer was performed. In the cleaning step, the flow rate of the cleaning liquid was set to 167 L/min, and the discharge pressure of the cleaning liquid was set to 0.3 MPaG. 
       FIG.  4    illustrates the inner pressure (maximum values during the polymerization reaction) in the polymerization tank and the heat exchanger connected thereto during the polymerization reaction measured by a pressure gauge attached to inside of the polymerization tank. The horizontal axis in  FIG.  4    also indicates the number of times when the series of processes was performed, including the polymerization process to the discharge process of the slurry and the step of cleaning the heat exchanger. 
     As can be seen from the measurement results, the pressure did not increase so much even after the polymerization process was executed 200 or more times. In general, if the pressure becomes about 30 kPaG to 40 kPaG, the heat exchanger may need disassembly and cleaning. However, the measurement results were equal to or lower than the reference value even after the polymerization process was executed 300 or more times. This has confirmed that the cleaning step according to the second example, which does not include cleaning the top surface of the upper chamber, had a high cleaning effect, although falling short of the cleaning effect of the cleaning step according to the first example that includes cleaning of the top surface of the upper chamber. 
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               100  Apparatus for manufacturing water-absorbent resin 
               1  Polymerizer 
               10  Polymerization tank 
               2  Concentrator 
               20  Concentration tank 
               3  Dryer 
               30  Drying chamber 
               4  ( 4 A,  4 B) Heat exchanger 
               40  Casing 
               401  Trunk part 
               402  Upper chamber 
               403  Lower chamber 
               60  Pipe 
               7  Cleaner 
               70  Nozzle 
               701 ,  702  Jetting port 
               71  First shaft 
               72  Second shaft 
               73  First motor 
               74  Second motor