Abstract:
This invention relates to a coil comprising a first helical tube segment ( 511 ) and a second helical tube segment ( 512, 513 ) extending, parallel to the first helical segment ( 511 ), between a distributor ( 53 ) and a manifold ( 54 ), said first and second helical segments ( 511, 512, 513 ) being centred on a common geometrical axis (X 5 ), with substantially the same bending radius (R 1 ) and nested, such that they form together a globally cylindrical bundle ( 51 ). The method comprises a step which consists in interleaving the helical tube segments ( 511 - 513 ) so as to form a globally cylindrical bundle. The invention also concerns a reactor equipped with such a coil which can be used for treating a viscous medium or for carrying out chemical reactions, such as 6-6 polyamide polymerization.

Description:
This application is an application under 35 U.S.C. Section 371 of International Application Number PCT/FR03/00177 filed on Jan. 20, 2003. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a coil for circulation of coolant fluid and to a method for manufacturing such a coil. The invention also concerns a reactor for the treatment of a viscous medium or for carrying out chemical reactions in a viscous medium, such as a polymerization, such a reactor comprising such a coil. Finally, the invention concerns the use of such a reactor. 
   It is known to effect polymerization of polyamide continuously or discontinuously. In the so-called discontinuous methods, one proceeds by batches with reactors of autoclave type. In that case, it is known to evaporate the water of an aqueous solution of two monomers and the water produced by their polymerization, thanks to an outside heat supply. 
   Such heat supply must be sufficient for the reaction of polymerization to take place in a period of time compatible with the criteria of productivity in force in the industrial world. The heat supply must not be too great in order to avoid, as far as possible, entraining one of the monomers with the vapour phase. If a quantity of monomers is entrained with the vapour phase, this quantity must be constant in order that the characteristics of the polyamide obtained be reproducible. Furthermore, the supply of heat makes it possible to monitor the reaction of polymerization insofar as it allows the evaporation of the water to be controlled. 
   To supply heat to the reaction medium, coils in which a coolant fluid circulates have been used in low-capacity autoclaves, i.e. ones with a volume less than 3 m 3 . 
   For reactors of greater capacity, in particular of the order of 5 to 6 m 3 , it has been possible to envisage using a coil and an agitator, the latter aiming at improving the homogeneity of the reaction medium and at increasing the coefficient of heat transfer. 
   However, this solution cannot be applied to high-capacity reactors, in particular ones with a capacity greater than 8 m 3 , as it is impossible to sufficiently increase the exchange surfaces constituted by the walls of the coil. In effect, if the overall diameter of the coil is increased, it is in that case no longer possible to house an efficient agitator in the vessel of the reactor. If the diameter of the tubes constituting the coil is decreased, the pressure drops associated with the circulation of coolant fluid in these tubes increase significantly. If a coil of very elaborate shape is created, the axial re-circulation of the reaction medium is hindered and the so-called “pumping” effect at the centre of the agitator is cancelled. Finally, a coil of elaborate shape with tubes of small diameter would not satisfy the criteria of mechanical strength allowing it to withstand prolonged use and/or incidents in manufacture. 
   It is a more particular object of the present invention to overcome these drawbacks by proposing a novel coil which allows an efficient heat supply to a reaction medium of large volume, while being compatible with the dimensions of a reactor vessel and with an agitator. 
   BRIEF SUMMARY OF THE INVENTION 
   To that end, the invention relates to a coil for circulation of a coolant fluid, this coil comprising at least one segment of tube wound along a helical generatrix, characterized in that it comprises at least a second segment of tube wound along a helical generatrix extending in parallel to the first segment between a distributor and a manifold, these first and second segments being centred on the same geometrical axis, with substantially the same bending radius and nested so that they together form a substantially cylindrical bundle. 
   Thanks to the use of two nested helical tube segments, it is possible that these segments each have a relatively short length, with the result that the pressure drop that they generate is relatively slight, even if the cross-section of the tube used may also be small. In addition, the fact that the helical tube segments have a relatively short length induces a relatively considerable slope thereof, i.e. greater than in the case of a single circular segment extending over the whole height of the coil. In this way, in the case of the coolant fluid being supplied in vapour phase able to condense in the tubes, the flow of the condensed fluid in these segments is more rapid, hence less risk of accumulation of condensates and less space requirement for liquid. The fact that these segments form a cylindrical bundle avoids their significantly disturbing the flow or re-circulation of the reaction medium in the central part of a reactor. 
   According to a first advantageous and non-obligatory aspect of the invention, the coil comprises a second bundle, formed by at least one segment of tube wound along a helical generatrix, extending between the distributor and the manifold and centred on the axis of the first helical segments, this second bundle being of substantially cylindrical shape, with a radius smaller than the radius of the first bundle. In that case, the second bundle is advantageously formed by at least two nested helical segments of tube, extending in parallel between the distributor and the manifold. 
   According to other advantageous but non-obligatory aspects of the invention, the coil incorporates one or more of the following characteristics:
         the first bundle is formed by three nested helical segments of tube.   the helical segments have substantially the same length and/or induce substantially the same pressure drop on the flow of the coolant fluid between the distributor and the manifold.   a tube is provided which extends, in a direction substantially parallel to the axis of the first bundle, between the first and second bundles, this tube being connected either to the distributor or to the manifold.   the distributor and/or the manifold are toric in shape, centred on the axis of the first bundle. In that case, the distributor and/or the manifold may be provided to be curved with a radius substantially equal to the radius of the first bundle or possibly of the second bundle, with the result that they are substantially in line with this first bundle or possibly with this second bundle.       

   The invention also relates to a method for manufacturing a coil as described hereinabove and, more specifically, to a method which comprises a step consisting in interleaving two segments of tube wound along helical generatrices and with substantially the same bending radius, so as to form a substantially cylindrical bundle. 
   The segments of tube are advantageously interleaved by a movement of “screwing” about a geometrical axis common to these segments. 
   The invention also relates to a reactor for the treatment of a viscous medium or for carrying out chemical reactions in a viscous medium, such as a polymerization, this reactor comprising, inter alia, a coil as described hereinbefore. 
   According to a first advantageous aspect of the invention, this reactor may comprise an agitator arranged around or inside the coil. The agitator may be suspended from the ceiling of the reactor and form a cage surrounding the coil, the supply and evacuation of the coolant fluid towards or from the coil being effected through the bottom of the reactor. According to another form of embodiment of the invention, the agitator may be formed by an endless screw centred on the geometrical axis of an inner bundle or of the single bundle of the coil. 
   According to another aspect of the invention, the inner bundle or the single bundle of the coil forms a central well of radius included between 20 and 70% of the radius of the vessel, which allows a good re-circulation of the reaction medium in the vessel. In the case of a coil with two bundles, the central well formed by the inner bundle preferably has a radius included between 20 and 40% of the radius of the vessel. 
   Finally, the invention relates to the use of a reactor as described hereinabove having a volume greater than about 8 m 3 , for the treatment of a viscous medium or the preparation of polymers such as polyamides, in particular 6-6 polyamide, or polyesters. This use may be discontinuous, for example for manufacturing batches of polymers of large volume, or continuous. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood and other advantages thereof will appear more clearly on reading the following description of a form of embodiment of a coil and a reactor according to the invention, of their respective manufacture and use, given solely by way of example and made with reference to the accompanying drawings, in which: 
       FIG. 1  is a longitudinal section through a reactor according to the invention equipped with a coil according to the invention. 
       FIG. 2  is a half-section of the coil shown in  FIG. 1 . 
       FIG. 3  schematically shows a step of manufacture of the coil of  FIG. 2 . 
       FIG. 4  is a view similar to  FIG. 3  for another step of manufacture of the coil. 
       FIG. 5  is a plan view of the coil of  FIG. 2 . 
       FIG. 6  is a view from underneath of the coil of  FIG. 2 , and 
       FIG. 7  is a view in perspective of the coil of  FIGS. 2 to 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The reactor  1  shown in  FIG. 1  is used for the polymerization of polyamide. It has a volume V of the order of 11 m 3 . This reactor  1  comprises a vessel  2  of substantially cylindrical shape with a substantially truncated bottom  21 . A cover (not shown) is provided to be mounted on the vessel  2  in order to constitute the ceiling thereof and to ensure seal of the interior volume V of the reactor  1  with respect to the ambient medium. 
   An agitator  4  is provided in the volume V. It is driven by a shaft  41  aligned on a central axis X-X′ of the reactor and passing through the cover. The agitator  4  comprises substantially helical blades mounted on a cage which surrounds a coil  5 . In order to render the drawing clearer, the agitator  4  is silhouetted in dashed and dotted lines solely in  FIG. 1 . 
   Other forms of agitators may be envisaged, as long as they are compatible with the space available in the volume V. 
   The vessel  2  is of double envelope type in order to allow the circulation of a coolant fluid, this allowing the volume V to be heated. 
   A coil  5  is installed inside the vessel  2  and is supplied with coolant fluid from a vaporizer  6  which may be of any known type. Two partition passages  59  connect the coil  5  to tubes  61  and  62  respectively allowing supply of the coil  5  with hot coolant fluid and evacuation of comparatively less hot coolant fluid in the direction of the vaporizer  6 . 
   In the present case, the coolant fluid is an oil in vapour phase at a temperature included between 300 and 350° C. Oil in vapour phase behaves like a pure body and works in latent heat, with the result that it conserves its temperature, so that the reaction medium is subjected to a substantially homogeneous heat supply over the length of coil. 
   Arrows E 1  and E 2  represent the flow of the oil in the passages  59 . 
   As is more particularly visible in  FIGS. 2 to 7 , the coil  5  is formed by two bundles of tubes. More specifically, a first bundle  51  is formed by tubes with a substantially constant bending radius R 1 . A second bundle of tubes  52  is formed by tubes with a bending radius R 2  less than radius R 1 . The bundles  51  and  52  are substantially cylindrical and centred on a central axis X 5  of the coil  5 , this axis merging with axis X-X′ when the coil is installed in the reactor  1 . 
   The bundle  51  is formed by three segments of tube  511 ,  512  and  513  each wound along a helical generatrix and nested in one another, i.e. together forming the bundle  51 . 
   In the same way, the bundle  52  is formed by two segments of tube  521  and  522  each wound along a helical generatrix and nested in each other. 
   As is visible in  FIG. 3 , the bundle  51  is formed by “screwing” the segments  511 ,  512  and  513  about axis X 5  which is their common central axis. The arrow F 1  represents the interleaving of the segment  512  in the segment  511 , this interleaving being translated by an advance of the segment  512  parallel to axis X 5 , as represented by arrow F 2 . In the same way, the segment  513  may be interleaved between the segments  511  and  512 . 
   As is visible in  FIG. 4 , the bundle  52  is formed by the segment  522  interleaved in the segment  521  thanks to a movement of screwing represented by arrows F 1  and F 2 . 
   When the two bundles  51  and  52  are each formed with a substantially cylindrical shape and with a predefined radius R 1  or R 2 , it is possible to connect the segments of tube  511  to  513 ,  521  and  522  to a supply tank  53  forming distributor and to an outlet manifold  54 , said tank and manifold each being of substantially toric shape and centred on axis X 5 . 
   Elements  53  and  54  have a diameter greater than that of the segments of tube  511  to  513 ,  521  and  522 , with the result that they make it possible to efficiently supply these segments with coolant fluid and to efficiently collect the fluid coming from these segments, as is represented by arrows of flow E in  FIGS. 5 and 6 . 
   The radius R 3  of the torus formed by the tank  53  is chosen to be equal to radius R 2 , in the same way as radius R 4  of the outlet manifold  54 . In this way, elements  53  and  54  are substantially aligned with the bundle  52 , with the result that they do not disturb a flow in the central part of the coil  5 , such flow being represented by arrow E′ in  FIG. 1 . 
   The tank  53  is provided with two lifting ears  531  and  532  for supporting the coil  5  when it is placed in position in the vessel  2  or when it is extracted therefrom. Other lifting means may be envisaged, on the tank  53  or on other parts of the coil  5 . 
   A tube  56 , substantially parallel to axis X 5 , is housed between the bundles  51  and  52 , this tube making it possible to supply the tank  53  from the passage  59  connected to the tube  61  of the vaporizer  6 . This tube  56  has an internal cross-section substantially equal to that of the tank  53 . 
   As is more particularly visible in  FIG. 5 , the three segments of tube  511 ,  512  and  513  are connected to the tank  53  by connections  511   a ,  512   a  and  513   a  extending in an essentially radial direction with respect to the tank  53 . Furthermore, the tubes  521  and  522  are connected to this tank  53  by connections  521   a  and  522   a  extending beneath the tank  53 , i.e. in a direction substantially parallel to axis X 5 , while the connections  511   a ,  512   a  and  513   a  are substantially perpendicular to this axis. 
   In the same way and as is visible in  FIG. 6 , the tubes  511  to  513  are connected by essentially radial connections  511   b ,  512   b  and  513   b  to the manifold  54 , while tubes  521  and  522  are connected to the manifold  54  by essentially axial connections  521   b  and  522   b.    
   The radii R 1  and R 2 , the height h 5  of the coil  5  and the positioning of the elements  53  and  54  may be chosen so that the segments of tube  511  to  513 ,  521  and  522  present substantially the same length. These segments have the same internal cross-section. In that case they induce substantially the same pressure drop on the flow of coolant fluid. 
   With the foregoing in mind, it will be readily understood that the segments  511 ,  512 ,  513 ,  521  and  522  are mounted in parallel with respect to one another between the elements  53  and  54 , which makes it possible to obtain relatively slight pressure drops, taking into account, in particular, the fact that the overall pressure drop of the three tubes  511 ,  512  and  513  is substantially less than that which would be generated by a single tube in helical configuration forming, alone, a bundle as dense as bundle  51 . 
   In addition, the unitary slope of each of the tubes  511  to  513 , which may be defined, as illustrated in  FIGS. 1 and 3 , by angle α 1  between a tube and a line Y 5  normal to axis  5 , is substantially greater than the slope that a single tube, in helical configuration, would have in order to constitute the bundle  51  alone. This considerably reduces the risks of accumulation of condensates inside the segments  511  to  513  and minimizes the liquid retention in the lower part of these tubes. 
   The foregoing observations are also applicable to the tubes  521  and  522  of the second bundle  52 . 
   As the tube  56  extends in a direction substantially parallel to axis X-X′ of the vessel  2 , it does not disturb the flow E′ of the reaction medium to a substantial degree. 
   In practice, the radius R 2  of the inner bundle  52  is chosen to have a value included between 20% and 40% of the radius R of the vessel  2 . Under these conditions, the central well P formed by the bundle  52  in the volume V of the reactor  1  is sufficiently wide for the re-circulation of reaction medium generated by the agitator  4  to be efficient. 
   It will also be noted that the construction of the coil  5  allows its geometry to be adapted to that of the bottom  21  of the vessel  2 , with the result that the dead volume of the reactor  1 , i.e. that part where little re-circulation is created, is limited to a maximum. 
   The invention has been shown with a coil  5  comprising an outer bundle  51  and an inner bundle  52 . However, it is applicable with a coil comprising a single bundle composed of at least two nested segments of tube with a helical configuration. 
   In the case of a coil comprising one sole bundle, the radius of this bundle may be chosen with a value included between 20 and 70% of the radius of the reactor vessel. 
   The invention has been shown with the outer bundle  51  comprising three segments of tube  511 ,  512  and  513 . However, it is applicable with a bundle comprising two segments or, on the contrary, more than three segments. 
   The invention has been shown with an agitator  4  arranged around the coil  5 . However, it is also applicable with an agitator which might penetrate in the central well of the coil  5 . In that case, the radii R 1  and R 2  of the coil  5  might be increased and the agitator might take the form of an endless screw. 
   The invention is independent of the exact type of vaporizer  6  and of the nature of the coolant fluid employed. 
   In  FIGS. 3 ,  4  and  7 , different textures have been used solely in order to differentiate the different parts of the coil  5  visually.