Patent Publication Number: US-2010126694-A1

Title: Silo for storing bulk products, in particular dried sludge from water treatment plants

Description:
The invention relates to a silo for the storage of products containing organic matter, in which products self-heating occurs, the silo being of the kind of those which comprise a cylindrical or prismatic outer shell of vertical axis, having a top and a bottom, this shell being made of a thermally conductive material, especially a metallic material, and comprising at least one inlet and one outlet for the products, a roof closing the silo at the top and a discharge device closing the silo at the bottom, the silo having at least one hollow internal shaft made of a thermally conductive material, in particular a metallic material, which communicates with the atmosphere at the top and at the bottom, the internal volume of the shaft being free to allow air circulation and heat dissipation. 
     A silo of this kind is shown in documents BE 521 214 A and FR1 444 136 A. 
     The invention relates more particularly, but not exclusively, to such a silo for the storage of dried sludge from wastewater or similar treatment plants. 
     The storage capacity of current silos is limited by the self-heating of the stored product that occurs. This is because the cross section of the silos, in particular the outside diameter (excluding the wall thickness) in the case of a circular cross section, is limited owing to the self-heating of the stored product. For a given type of product, it is possible to define a critical radius R c  that corresponds to the maximum admissible radial thickness of product in order for the heat generated by self-heating to be sufficiently discharged into the atmosphere through the mass of product and the outer shell, so as to prevent the onset of pyrolysis when the temperature rises. The higher the risk of a product self-heating, by dint of its composition and its particle size, the smaller the critical radius in order to facilitate heat dissipation. 
     In the case of a vertical cylindrical silo of circular cross section, for a given material and given wall thickness of the shell, the distance between the geometric axis of the silo and the outer surface of the shell (i.e. the radius of the shell) must always be less than or equal to the critical radius R c  of the product to be stored. A product of critical radius R c  must be stored in a silo having a radius less than or equal to R c . 
     By way of indication, it is frequently found that silos for dried sludge consist of cylindrical shells some ten meters in height with an inside diameter of 2 meters or less. 
     The object of the invention is most particularly to increase the storage capacity of a silo of the kind defined above, without in any way reducing safety as regards self-heating of the stored products. 
     According to the invention, a silo for the storage of dried bulk products containing organic matter, in particular for the storage of dried sludge from wastewater treatment plants, in which products self-heating occurs, the silo being of the kind defined above, is characterized in that the ratio d/D of the outside diameter d of the shaft to the outside diameter D of the shell is greater than 0.12. 
     According to another aspect of the invention, a silo for the storage of dried bulk products containing organic matter, in particular for the storage of dried sludge from wastewater treatment plants, in which products self-heating occurs, the silo being of the kind defined above, is characterized in that the outside diameter d of the shaft is greater than (D−4R c ), D being the outside diameter of the shell and R c  being the critical radius of the products to be stored in the silo. 
     According to yet another aspect of the invention, a silo for the storage of dried bulk products containing organic matter, in particular for the storage of dried sludge from wastewater treatment plants, in which products self-heating occurs, the silo being of the kind defined above, is characterized in that the shaft is in an eccentric position relative to the shell, the eccentricity of the shaft being equal to one half of its outside diameter, the critical radius R c  being equal to or greater than D/4 (i.e. R c ≧D/4), D being the outside diameter of the shell. 
     Advantageously, the shell includes, on the inside at the bottom, a support means for supporting the internal shaft and suitable for this internal shaft to communicate with the atmosphere, while preventing any ingress of product into said internal shaft. 
     The means for supporting the internal shaft may be designed to allow rainwater to drain away and air to circulate. 
     The support means may comprise tubes welded to the shaft and running into this shaft, each tube passing through that wall of the shell to which it is welded and running to the outside. 
     The support means may comprise three tubes forming a tripod. 
     According to one advantageous embodiment, the diameter of the shell is at least three meters, the internal shaft having a diameter of at least one meter. 
     The internal shaft may be coaxial with the external shell. The silo may comprise several internal shafts of parallel axes. 
    
    
     
       The invention consists, apart from the arrangements explained above, of a number of other arrangements which will be explained more fully below with regard to exemplary embodiments, which are described with reference to the appended drawings but are in no way limiting. In these drawings: 
         FIG. 1  is a schematic view, in perspective and with parts cut away, of a silo according to the invention; and 
         FIG. 2  shows, in a similar manner to  FIG. 1 , an alternative embodiment. 
     
    
    
       FIG. 1  of the drawings shows a silo S for the storage of bulk products P containing organic matter. More particularly, the products P consist of dried sludge from wastewater treatment plants. Such products undergo self-heating and may, as the temperature rises, result in the onset of pyrolysis if the heat generated is not sufficiently dissipated into the atmosphere. 
     The silo S comprises an external shell  1 , generally made of steel sheet. The shell  1  may have a cylindrical shape, with a circular cross section, of vertical axis, having a top  1   a  and a bottom  1   b . Means for supporting this shell  1  in a vertical position are provided, but not shown in the drawings. Other shapes of shells  1  are possible, for example prismatic shells. As a variant, the axis of the shell could be inclined relative to the vertical at an angle that does not affect the fall of the bulk product under gravity. 
     The silo is closed at the top by a roof  2 , especially one formed by a sheet metal disk, closing off the upper portion of the cylinder. An inlet E for the bulk products is provided at the top, for example in the cylindrical wall of the shell  1 . An outlet A for the products is provided at the bottom. 
     The silo S has at the bottom a product delivery device G formed by a concentrating cone  3 , the small base of which is downwardly directed. This small base is open and forms the outlet A. The cone  3  allows the bulk product to collect at the outlet. Advantageously, the cone  3  is equipped with a vibrating bottom  4  in order to facilitate outflow of the bulk products. 
     According to the invention, the silo S includes at least one hollow internal shaft  5  made of thermally conductive material, especially a metal shaft, communicating with the atmosphere at the top  5   a  and at the bottom  5   b . The shaft  5  is generally made of steel sheet. 
     According to the embodiment shown in  FIG. 1 , the shaft  5  is cylindrical, of circular cross section and coaxial with the silo S. The shaft  5  could have a different shape, especially a prismatic shape with a polygonal cross section. The shaft  5  passes through a hole provided in the roof  2 . The gap between the outline of the shaft  5  and the perimeter of the hole provided in the roof  2  may be sealed by a weld bead, which forms a mechanical link between the two components. The upper end  5   a  of the shaft is completely open to the atmosphere, or it may be covered with a protected grille or grating, allowing air to flow freely through it. 
     The lower portion  5   b  of the shaft communicates with the atmosphere via tubes  6  welded to the lower portion of the shaft, these running into the shaft  5 . The tubes  6  are downwardly inclined from the shaft, in planes passing through the axis of the shaft, and pass in a sealed manner through the wall of the cone  3 , opening to the outside. Each tube  6  is fastened, in particular by welding, to the wall of the cone  3  at the orifice via which it passes through this wall, the set of tubes  6  constituting a means B for supporting the shaft  5 . Preferably, three tubes  6  are provided, angularly spaced apart by 120°, in order to form a tripod for supporting the shaft  5 . Any other means for supporting the shaft  5  in the silo S may be envisaged. 
     The central shaft  5  is thus open at the bottom via the tubes  6 . The openings of the tubes  6  allow a natural draft to occur and rainwater to be removed. The combined cross-sectional area of the tubes  6  is designed so as not to attenuate the natural draft of the shaft  5 . The lower axial end of the shaft  5  is closed, for example by a disk. 
     Where necessary, in order to increase the natural ventilation flow, a fan  7  may be provided in the shaft  5 , preferably at the top. 
     The height of the cylindrical part of the silo is denoted by H, the outside diameter of the shell  1  is denoted by D and the outside diameter of the shaft  5  is denoted by d. The d/D ratio is preferably equal to or greater than 0.12 (i.e. d/D≧0.12). By way of nonlimiting example, the diameter d may be of the order of 1 meter, whereas the diameter D may be between 3 and 8 meters. 
     In the case of a vertical cylindrical silo, with no shaft of inside diameter D, the products that can be stored safely must correspond to a critical radius R c  equal to or greater than D/2 (R c ≧D/2). Products having a critical radius less than this value cannot be stored safely in the silo. 
     In the case of a vertical cylindrical silo according to the invention of outside diameter D, with a coaxial shaft having an outside diameter d (excluding wall thicknesses), the maximum distance between a point in the stored product and a wall (shell wall or shaft wall) in contact with the atmosphere is equal to (D−d)/4. The products to be safely stored may have a critical radius R c ≧(D−d)/4, which is much smaller than that for a conventional silo of the same outside diameter. 
     The outside diameter d of the shaft is calculated in such a way that the following safety requirement defined by the radius R c  is met: 
         D≦ 4 R   c   +d,    
       that is: 
         d≧D− 4 R   c . 
     The shaft  5  thus created may be equipped with access means, such as a safety ladder or a spiral staircase, for installing instrumentation or equipment in the shaft. 
     The concentric arrangement of the shaft in  FIG. 1  is optimal from the mechanical strength standpoint, thanks to the concentricity of the forces on the central shaft  5 , and from the self-heating standpoint. 
     However, it is possible, as illustrated in the alternative embodiment shown in  FIG. 2 , to place the shaft  5 ′ in an eccentric position relative to the shell  1 . The top and bottom of this shaft are denoted by  5 ′ a  and  5 ′ b . According to the example illustrated, the eccentricity of the shaft  5 ′ is equal to one half of its outside diameter, i.e. d/2. In this case, the maximum distance between a point in the stored product and a wall (shell wall or shaft wall) in contact with the atmosphere is equal to D/4, i.e. one half of the radius of the external shell, since the wall of the shaft  5 ′ contains the geometric axis of the shell  1 . The critical radius R c  is then equal to or greater than D/4 (i.e. R c ≧D/4). 
     The eccentricity may take different values, but the coaxial solution remains the preferred one. 
     It is also possible to provide several shafts inside the shell  1 , with parallel axes, distributed so as to ensure good heat dissipation. For example, the axes of the shafts could be uniformly spaced over a circumference concentric with the shell  1 . 
     The invention makes it possible to increase the outside diameter of the silo, while still complying with the critical radius for self-heating. The heat generated by the self-heating may be dissipated just as well by the internal shaft  5 ,  5 ′ as by the external shell  1 . 
     The tables given below show that the storage capacity per linear meter of shell  1  of the silo is increased by a factor of greater than 4, for the same critical radius. 
     For the same quantity to be stored, it is possible to reduce the number of silos needed and to reduce correspondingly the safety and process equipment necessary for each silo, such as those in the following non-exhaustive list:
         CO detector;   explosion vent (with fracture detection);   filling unit/draining unit;   level measurement;   temperature probes;   very-high level detection;   safety valve.       

     Table Showing the Variation in Volume and Critical Radius as a Function of the Outside Diameter of a Cylindrical Silo 
     Values calculated for H=10 m and d=1 m; shaft  5  coaxial with the silo ( FIG. 1 ). 
     
       
         
           
               
               
               
            
               
                   
               
               
                 CONVENTIONAL SILO 
                 SILO WITH COAXIAL 
                   
               
               
                 WITH NO SHAFT 
                 INTERNAL SHAFT 
                 COMPARISON 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Silo 
                 Silo volume 
                 Critical 
                 Silo volume 
                 Critical 
                 Volume 
                 Critical 
               
               
                 diameter D 
                 πHD 2 /4 
                 radius 
                 πH(D 2  − d 2 )/4 
                 radius 
                 with shaft 
                 radius 
               
               
                 (m) 
                 (m 3 ) 
                 D/2 (m) 
                 (m 3 ) 
                 (D − d)/4 (m) 
                 (%) 
                 reduction (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 3 
                 70.7 
                 1.5 
                 62.8 
                 0.5 
                 89% 
                 33% 
               
               
                 4 
                 125 
                 2 
                 117 
                 0.75 
                 93% 
                 37% 
               
               
                 5 
                 196 
                 2.5 
                 188 
                 1 
                 96% 
                 40% 
               
               
                 6 
                 282 
                 3 
                 274 
                 1.25 
                 97% 
                 41% 
               
               
                 7 
                 384 
                 3.5 
                 377 
                 1.5 
                 98% 
                 42% 
               
               
                 8 
                 502 
                 4 
                 494 
                 1.75 
                 98% 
                 43% 
               
               
                   
               
            
           
         
       
     
     Thus, for a given critical radius of 1.5 m, a silo according to the invention will have a volume of 377 m 3 , as opposed to only 70.7 m 3  in the case of a conventional silo with no coaxial shaft. 
     Table Showing the Variation in Volume and Critical Radius as a Function of the Diameter of the Silo 
     Values calculated for H=10 m and d=1 m; eccentric shaft  5 ′ of d/2 ( FIG. 2 ). 
     
       
         
           
               
               
               
            
               
                   
               
               
                   
                 SILO WITH 
                   
               
               
                 CONVENTIONAL SILO 
                 ECCENTRIC 
               
               
                 WITH NO SHAFT 
                 INTERNAL SHAFT 
                 COMPARISON 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Silo 
                 Critical 
                 Silo volume 
                 Critical 
                 Volume 
                 Critical 
               
               
                 Silo di- 
                 volume 
                 radius 
                 πH(D 2  − 
                 radius 
                 with 
                 radius 
               
               
                 ameter 
                 πHD 2 /4 
                 D/2 
                 d 2 )/4 
                 D/4 
                 shaft 
                 reduction 
               
               
                 (m) 
                 (m 3 ) 
                 (m) 
                 (m 3 ) 
                 (m) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 3 
                 70.7 
                 1.5 
                 62.8 
                 0.75 
                 89% 
                 50% 
               
               
                 4 
                 125 
                 2 
                 117 
                 1 
                 93% 
                 50% 
               
               
                 5 
                 196 
                 2.5 
                 188 
                 1.25 
                 96% 
                 50% 
               
               
                 6 
                 282 
                 3 
                 274 
                 1.50 
                 97% 
                 50% 
               
               
                 7 
                 384 
                 3.5 
                 377 
                 1.75 
                 98% 
                 50% 
               
               
                 8 
                 502 
                 4 
                 494 
                 2 
                 98% 
                 50% 
               
               
                   
               
            
           
         
       
     
     For a given critical radius of 1.5 m, a silo with an eccentric shaft will have a volume of 274 m 3 , as opposed to only 70.7 m 3  for a conventional silo with no shaft.