Patent Publication Number: US-3879856-A

Title: Method and apparatus for drying particulate materials

Description:
United States Patent [191 Barr [451 Apr. 29, 1975 METHOD AND APPARATUS FOR DRYING PARTICULATE MATERIALS [75] Inventor: Peter Joachim Barr, London,  
 England [73] Assignee: Barr &amp; Murphy Limited, London,  
 England [22] Filed: Jan. 30,1974  
 [21] Appl. No.: 438,017  
 [30] Foreign Application Priority Data Mar. 23, 1973 United Kingdom 9140/73 [52] US. Cl 34/10; 34/57 E; 432/14;  
  432/58 [51] Int. Cl. F26b 3/08 [58] Field of Search 432/14, 15, 58; 34/10, 34/57 R, 57 E [56] References Cited UNITED STATES PATENTS Shimizu et a1. 34/10 Ritzmann 432/58 Koller 34/10 Primary E.\&#39;aminer.1ohn J. Camby Attorney, Agent, or FirmWenderoth. Lind &amp; Ponack [57] ABSTRACT Material can be dried using gas of temperature higher than a critical temperature of the material, such as softening point, by circulating the material in gas through a circuit. At a point in the circuit is a zone to an inner region of which the hot air is introduced, while circulated gas and material is introduced into an outer region. Drying occurs as the gases in the regions intermingle without the material reaching the the critical temperature.  
 In apparatus of the invention the intermixing occurs in a manifold with inlets such that the gases assume vortical motion.  
 11 Claims, 1 Drawing Figure METHOD AND APPARATUS FOR DRYING PARTICULATE MATERIALS The present invention relates to a method of, and apparatus for, drying particulate materials, such as powdered, granular, pasty materials and the like.  
  It is known to dry materials of this type by subjecting them to the action of heated drying air, the material to be dried and the drying air being introduced into and conveyed continuously through an endless circuit, and means being provided for continuously renewing the drying air. However, the temperature of the drying air used is normally limited by a critical temperature such as the melting or softening point of the material to be dried, or a critical temperature with respect to some other heat sensitive factor of the material since it is undesirable, for instance, that melting or softening of the material should occur during the drying operation. This consideration is of particular importance when drying organic materials such as plastics, stearates, and other such products. 1  
 It is an object of the present invention to provide a 7 method and apparatus for drying solid materials in which drying gas at a temperature higher than a critical p temperature such as the melting or softening temperature of the material to be dried may be utilised.  
  According to one aspect of the present invention there is provided a method of drying particulate material, said method including the steps of circulating a drying gas and material around acircuit, feeding moist material to be dried into the gas in the circuit at a feed point where the gas is at a temperature lower than a critical temperature of the material, introducing hot drying gas of a temperature higher than the critical temperature of the material into a zone in the circuit in such manner that the hot gas is introduced into an inner region of the zone, the cooler circulated gas being introduced into an outer region of the zone surrounding the inner region, and continuously withdrawing spent gas carrying dried material from a withdrawal point in the circuit.  
  The gas in each said region preferably flows as a vortex, the vortices being concentric. As the gas in the two regions interpenetrate and intermix, the freshly introduced hot gas is cooled both by mixing with the cooler gas and more particularly, by evaporation of moisture from the moist material carried therein, with the result that the material carried in the gas is itself dried without ever reaching a temperature near the critical temperature. The vortices may be established by feeding the cooler gas tangentially to the zone, the hot gas assumin g vortical motion due to frictional effects with the cooler gas. An initial vortical motion may if desired be mechanically imparted to the hot gas on or prior to entry into the zone.  
  The invention is particularly envisaged for use where the critical temperature is a softening or melting temperature, although other applications are clearly available.  
  According to another aspect of the invention there is provided apparatus for drying material including an endless circuit through which drying gas and material may circulate, a manifold defining a zone in the circuit, the manifold having a first inlet on its axis for introduction of hot gas to an inner region of the zone, a second inlet which is offset from the axis of the manifold and at the same end thereof as the first inlet for introducing previously circulated gas containing material to be dried into an outer region of the zone, and an outlet, and the circuit further including a gas circulator, a feed point for introduction of moist material to the circuit and an exhaust means for removing spent gas and dried material from the circuit, the exhaust means being connected between the outlet from the manifold and the feed point.  
  The manifold is preferably curved and more preferably circular in cross section and may be conical or cylindrical. Preferably the feed point is external to the manifold and between the outlet of the gas circulator and inlet to the manifold so that in operation moist material passes directly into the manifold without having previously passed through the gas circulator. It is however possible for the feed point to be positioned between the outlet from the manifold and the inlet to the gas circulator. The inlet for hot gas in the manifold may have means such as fixed vanes which impart a spiral path to hot gas entering the manifold.  
 In a preferred apparatus the manifold is a cylindrical or conical drum, having an axial warm air inlet at one end, a tangential cool air and feed inlet adjacent said end and a tangential spent gas outlet at the opposite end of the manifold, means associated with the spent gas outlet for passing a part of the spent gas containing a major part of the feed solids to a solids collector. and a return conduit for circulating the remainder of the cool spent gas to the air inlet of the manifold, said conduit including a feed inlet for wet solids feed material.  
  The drying gas to be used in accordance with the invention is preferably air but other suitable gaseous media, for instance an inert gas such as nitrogen or combustion gases from a fuel gas, may also be used.  
  A preferred embodiment according to the invention will now be described by way of example only, with reference to the accompanying diagramatic drawing.  
  As shown in the drawing a closed circuit for air, or any other suitable gaseous drying medium, is defined by a cylindrical manifold l and a circulating fan 2 by way of a duct 3, which connects a tangential outlet 4 of the manifold with the fan, and a duct 5, which connects the fan with a tangential inlet 6 of the manifold. A feeder 7 for moist material opens into duct 5.  
  The outlet 4 of the manifold also communicates by means of a duct 8 and a product collector 9 with an exhaust fan 10 and a valve 11. The product collector 9 contains a conventional bag filter. A baffle shown in dotted lines at 40 is so arranged in the duct 3 adjacent outlet 4 that in operation the outer (lower as shown) layer of the stream of material passing through the outlet 4 is deflected into the duct 3 whereas the inner (upper) layer is passed into the duct 8.  
  A heater 12 is situated at the entrance to a duct 13 which opens into the manifold 1 through an axial inlet 15. The inlet 15 is provided with swirling vanes shown in dotted lines at 14 to promote rotary flow of the incoming gas.  
  In operation, moist material is introduced into duct 5 from feeder 7 and conveyed by gas already in the circuit into manifold l at the inlet 6. Fresh air is drawn through heater l2 and introduced axially into manifold 1 through duct 13, swirling vanes 14 and inlet 1(5, the arrangement therefore being such that in mani&#39;fold 1 there is provided in an inner region of the ma ,ifold a central vortex of hot air within a vortex of cot pler air carrying the material to be dried which is in ari outer region of the manifold. As the two vortices interpenetrate each other the hot air is cooled and the material dried.  
  Air and particles of material emerge from the mani fold at the outlet 4. The ce&#39;htrifugal force produced in operation results in the outer (lower) layer of the emergent stream in outlet 4 being composed of a heavier fraction of undried and partially dried material suspended in air whereas the inner (upper) layer of the stream is composed of the lighter and drier fraction of material suspended in air. The baffie 40 in or adjacent outlet 4 causes the heavier fraction to pass into duct 3 for recirculation and the lighter fraction to pass into duct 8. The lighter fraction, composed of dried material and spent air is conveyed to the product collector 9, the dried material being removed therefrom through valve 11 at the bottom of a hopper and spent air drawn off through exhaust fan 10. Movement of the air through ducts 8 and 13 and product collector 9 is caused by the exhaust fan 10.  
  It has been found that the process and apparatus of this invention can be used, for example, for drying a polypropylene which has a melting point of 150C. using air having a temperature of 225C. at the inlet 15 of manifold 1, the temperature of spent air emerging from exhaust fan 10 being 75C. As compared with conventional methods, in which air of less than 150C. would have to be used as drying medium and in which a temperature drop of 75C would be available for supplying the heat necessary for drying, the temperature drop in an apparatus according to this invention would be 225 75 (i.e. 150C). Thus only approximately half the quantity of air required in conventional pneumatic dryers of similar evaporative capacity is used by a dryer of this invention. with the result that the drying efficiency is substantially increased.  
 1 claim:  
  1. A method of drying particulate material comprising the steps of:  
 passing a main stream of gas around a closed path which includes a circular cross-sectioned drying zone; introducing moist particulate material into the gas stream at a point outside said drying zone;  
 introducing said main stream of gas including said moist particulate material into said drying zone at an inlet position close to one end thereof in a direction substantially tangential to the axis of said zone whereby a substantially helical stream of relatively cool gas in the region of the periphery of said zone is established; introducing a stream of gas at a higher temperature than said main stream of gas into said zone substantially along the axis of said zone at the same end of said zone as the inlet of the main stream of gas so that contact is established between the hot axial gas stream and the relatively cool helical main gas stream carrying moist particulate matter;  
 withdrawing all of the gas from the drying zone through an outlet at the opposite end of said drying zone from the inlet in a direction substantially tangential to the axis of said drying zone;  
 dividing the gas withdrawn from the drying zone through the outlet into a main moist particle carrying stream and a dry particle carrying stream; recirculating the gas in said moist particle carrying stream to the inlet of said drying zone; and separating and discharging the dry particles from said dry particle carrying stream.  
  2. A method as claimed in claim 1 further comprising the step of imparting an initial vortical motion to the hot gas prior to its entry into said circular section drying zone.  
  3. A method as claimed in claim 1 in which the main gas stream is introduced into the drying zone at a temperature below the melting point of the particulate material; and the hot gas stream is introduced into the drying zone at a temperature above the melting point of the particular material, the hot gas stream in proportion to the main gas stream so that the temperature of the gas stream leaving the drying zone is below the melting point of the particulate material.  
  4. An apparatus for drying particulate material suspended in a stream of gas comprising:  
 a circular-cross-section manifold having a first gas inlet at one end thereof entering said manifold along the axis of said manifold. a second gas inlet through the side of said manifold at the end of said manifold near said first inlet offset from the axis of said manifold and positioned to direct a stream of gas around the inside periphery of said manifold, and an outlet through the side of said manifold and at the end of said manifold opposite said second inlet also offset from the axis of said manifold;  
 hot gas supply means connected to said first gas inlet for supplying hot gas to said manifold;  
 connecting means connecting said outlet to said second inlet to the manifold;  
 gas circulating means for maintaining the flow of gas throughout the apparatus located in said connecting means;  
 introduction means for introducing the moist particulate matter to be dried into the gas flowing through said connecting means, said introduction means located in said connecting means between said gas circulating means and said second inlet to the manifold; and  
 exhaust means for removing the spent gas and dried material, said exhaust means located in said connecting means between said manifold outlet and said gas circulating means.  
 5. An apparatus as claimed in claim 4 wherein said second inlet to said manifold is tangential thereto.  
  6. An apparatus as claimed in claim 4 wherein said manifold is conical.  
  7. An apparatus as claimed in claim 4 wherein said manifold is cylindrical.  
  8. An apparatus as claimed in claim 4 further comprising motion imparting means located in said first inlet for imparting vortical motion to said hot gas entering said manifold.  
  9. An apparatus as claimed in claim 8 wherein said motion imparting means is comprised of fixed vanes.  
  10. An apparatus as claimed in claim 4 wherein said outlet from said manifold is tangential thereto.  
  1 1. An apparatus as claimed in claim 10 further comprising baffle means in said outlet parallel to the axis of said manifold and dividing said outlet into first and second parts for respectively receiving dried material and spent gas in one part and material to be recirculated in the other part.