Abstract:
A material drying apparatus for substantially eliminating the liquid content of a sludge, the apparatus including a first stage operating at ambient pressure for heating the sludge; a second stage operating with partial vacuum for receiving heated sludge from the first stage for further heating and for drying the sludge by evaporation of substantially all liquid therefrom leaving a dry particulate material; and an evacuated cyclonic stage for applying sufficient centrifugal force to the dry particulate material to render this material odorless.

Description:
FIELD OF THE INVENTION 
   This invention relates to an apparatus and method for drying sludge to produce a substantially dry benign end product e.g. for use as a fertilizer or an innocuous and biologically benign material which can be readily disposed of. 
   DEFINITION 
   Sludge as used herein shall be construed as including semi-solids, sludges and slurries, e.g. sewage, fecal material, containing evaporable liquids, particularly, though not exclusively, water. 
   BACKGROUND OF THE INVENTION 
   There are many methods of reducing the moisture content of sludges and slurries containing organic material including filtering using filter presses, heating or chemical treatment or applying partial vacuums. A major concern when using heat and a partial vacuum to dehydrate materials is the high temperature to which the material is heated while exposing the material to an optimum vacuum level. In addition the high temperatures involved tend to reduce or destroy the fertile quality of the end product. In addition, prior art proposals are often suited only to dehydration of batches of sludge as opposed to a continuous process suitable for the mass production of a benign particulate end product. 
   OBJECTS OF THE INVENTION 
   It is an object of the invention to provide apparatus for continuously drying sludges to provide an end product having fertile qualities. 
   It is an object of the invention to provide a continuous process to produce an environmentally benign particulate dry final product from sludge, e.g. sewage or fecal material. 
   It is yet a further object of the present invention to use a cyclonic action to produce substantially dry material which is substantially odorless. 
   STATEMENT OF THE INVENTION 
   According to the invention there is provided a material drying apparatus for continuous operation to substantially evaporate the liquid content of a sludge, the apparatus comprising: 
   a first stage operating at ambient pressure for heating the sludge; 
   a second stage operating with partial vacuum for receiving heated sludge from the first stage for further heating and for drying the sludge by evaporation of substantially all liquid therefrom to leave a dry particulate material; and 
   a cyclonic stage operating with a partial vacuum for applying sufficient centrifugal force to the dry particulate material to render this material substantially odorless. 
   Preferably the first stage comprises a cylindrical housing defining a longitudinal axis, a hollow shaft coaxial with the housing and extending entirely through the housing, and conveyor paddles mounted on the hollow shaft and helically inclined relative to the axis to convey, upon rotation of the hollow shaft, sludge from an input end of the cylindrical housing to an outlet end thereof while being heated, wherein the paddles terminate at their radially outer ends sufficiently proximate the cylindrical housing to scrape sludge from the housing during rotation of the paddles about the axis by the hollow shaft and are mounted on the hollow shaft by tubes the interiors of which are open to the interior of the hollow shaft whereby steam introduced into the hollow shaft will enter the tubes to provide a heat source for the sludge. 
   A rotary transfer lock may be disposed between an outlet for the heated sludge from the first stage and the inlet for the heated sludge into the second stage, the transfer lock comprising a cylindrical chamber closed at its ends and housing a finned rotor having a plurality of fins cooperating with the cylindrical chamber to substantially prevent loss of the partial vacuum from the second stage while, upon rotation of the finned rotor, the heated sludge from the first stage is metered through the transfer lock for onward transmission to the second stage; and the finned rotor may be hollow and is provided with openings to allow air introduced into the rotor to pass into the heated sludge being metered through the transfer lock in order to prevent clogging of the finned rotor. The air maybe hot to one of further heat and maintain heat of the sludge. 
   A shredder disposed between the rotary transfer lock and the inlet for the heated sludge into the second stage is desirable, the shredder serving to break the heated sludge into small pieces and particles. 
   Also preferably the second stage comprises a cylindrical housing defining a longitudinal axis, a hollow shaft coaxial with said housing and a plurality of tined projections mounted on the shaft for rotation with the shaft to agitate particles and break up small pieces of the sludge entering the second stage, the tined projections terminating closely adjacent the housing with each projection having a plurality of tines and with the housing supporting blocks which cooperate with the tines upon rotation thereof to remove material from between the tines, and the tined projections may be mounted on the hollow shaft by tubes the interiors of which are open to the interior of the hollow shaft to receive steam passed through the hollow shaft to provide additional heating of the sludge. 
   Again preferably dried particulate material from the second stage is drawn into a cyclone by a vacuum pump connected to the cyclone to provide the partial vacuum in the cyclone and second stage, the material being drawn into the cyclone by way of a venturi to accelerate the particles for entry tangentially into the cyclone whereby centrifugal force generated by passage of the dried particles in the cyclone renders the dried particles substantially odorless, a rotary transfer lock being located to provide an outlet for particles from the cyclone and to substantially prevent loss of the partial vacuum from the second stage and the cyclone. 
   The invention also provides a method of drying sludge to produce a benign particulate material comprising the steps of:
         a) heating the sludge at ambient pressure to a temperature of at least 200° F. while being continuously conveyed through a first stage of an apparatus;   b) heating and drying the previously heated sludge under a partial vacuum in a second stage of the apparatus;   c) substantially preventing loss of the partial vacuum from the second stage at a location between the first and second stages;   d) shredding the heated sludge to reduce the heated sludge to small pieces and particles prior to entry into the second stage;   e) agitating the small pieces and particles in the second stage to break up the small pieces therein; and   f) subjecting the particles dried in the second stage to a centrifugal force sufficient to provide a substantially odorless particulate product.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic partially sectioned view of apparatus according to the present invention; 
       FIGS. 2 and 3  are fragmentary diagrammatic views of a helical paddle in a first stage of the apparatus of  FIG. 1 ,  FIG. 3  being a view in the direction of arrow A in  FIG. 2 ; 
       FIG. 4  is variation of the arrangement of  FIGS. 2 and 3  in which shaft scraping paddles are provided; 
       FIG. 5  is a diagrammatic sectional view of a tined paddle in a second stage of the apparatus of  FIG. 1 ; 
       FIG. 6  illustrates a helical arrangement of the paddles of  FIGS. 2 and 3 ; 
       FIG. 7  illustrates a helical arrangement of the tined paddles of  FIG. 5 ; and 
       FIG. 8  is a diagrammatic representation of a transfer lock disposed between the first and second stages of the apparatus of FIG.  1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the sludge drying apparatus comprises: a supply hopper  2  which, by way of a rotary shredder  4 , can continuously supply sludge entering the hopper  2  in the direction of arrow  6 , to a first stage  8  for heating the sludge; a second stage  10  to further heat and to dry the sludge received from the first stage  8  by way of a serially connected rotary transfer lock  12  and a shredder  14 ; and a cyclonic stage  18  for deodorizing the dry particulate product from the second stage  10  received by way of a venturi  16  located at the input of a cyclonic stage  18 . 
   The design of the hopper  2  and its rotary shredder  4  will be well known to those of skill in the appropriate art. 
   The first stage comprises a tubular housing  20 , about 2 feet in diameter and about 30 feet long, concentrically surrounding a hollow shaft  22 , about 6 inches in diameter, extending through the length of the tubular housing  20  and beyond the ends thereof by way of seals and bearings  24  for connection to a motor  26  capable of rotating the shaft  22  within the housing  20  at from about 10 to about 50 r.p.m. The hollow shaft  22  is connected at one end to receive high pressure non-turbulent steam at about 300° F. or more. The steam is exhausted at the other end of the shaft  22  after passage therethrough. The housing  20  is surrounded by two serially disposed insulated heat jackets  28  to insulate and heat the housing substantially throughout its length. To provide a source of heat to the heat jackets  28 , steam inlets and outlets  30  are provided. As with the shaft  22  the steam for the jackets  28  is supplied as a high pressure non-turbulent steam at about 300° F. 
   The shaft  22  supports a plurality of helically inclined, relative to the axis  31  of the shaft  22  and housing  20 , paddles  32  (two only being shown in FIG.  1 ). Sixteen or more such paddles  32  may be evenly spaced along the shaft  22  and these may be dispersed in a straight line parallel to the axis  31  of the shaft  22  or in a helical array as shown in FIG.  6 . 
   Each paddle  32  (see  FIGS. 2 and 3 ) is inclined relative to the axis  31  to convey the sludge, entering the housing  20  from the hopper  2 , toward the outlet end  34  in the direction of arrow  36  as the shaft  22  is rotated in the manner of an auger or screw conveyor. The outer end of each paddle  32  is shaped to be in close proximity or contact with the housing  20  throughout its axial length in order to scrape, from the housing, any sludge adhering to the inside of the housing. Each paddle  32  is supported on the shaft  22  by a tube  38  the interior of which is connected to the interior of the hollow shaft  22  whereby steam passing through the shaft may enter and heat the tube (see arrows  40  in FIG.  2 ). 
   As seen in  FIG. 4 , shaft scraping paddles  42  may be placed between adjacent pairs of the shaft mounted paddles  32  to scrape from the shaft  22  any sludge adhering to the shaft  22 . These shaft scraping paddles  42  are usually parallel to the axis  31  and do not play a part in the conveyance of the sludge through the housing. However, the shaft scraping paddles  42  could be helically inclined so as not to hinder helical motion of the sludge along the housing. In this latter case the shaft scraping paddle inner ends are shaped to closely cooperate with the shaft throughout their axial length. The scraping paddles  42  are supported by tubes or rods  44  attached to the housing  20 . 
   The temperature of the sludge is at least 200° F. at the outlet end  34  as determined by its rate of conveyance through the first stage  8  and the heating provided by the steam heated jackets  28  and shaft  22  during this conveyance. 
   The entire housing  20  and heat jackets  28  are insulated to prevent loss of heat from the housing  20 . It should be noted that the heating of the sludge, in the first stage  8 , takes place substantially at ambient pressure. 
   If required the housing  20  and shaft  22  may be constructed of serially interconnected parts and also, if required, two or more first stages  8  may be serially arranged before the heated sludge exits to the transfer lock  12 . 
   The transfer lock  12  is a rotary device somewhat akin to a revolving door. The transfer lock  12  comprises a housing  48  defining an inlet  50 , connected to the outlet end  34  of the first stage, and an outlet  52 , to supply the heated sludge to the second stage  10  by way of the shredder  14 , with a cylindrical portion  54  therebetween. The cylindrical portion  54  is closed at its ends. Housed in the portion  54  is a finned rotor  56  coaxial with cylindrical portion  54  comprising at least three, preferably six or eight, fins  58  mounted on a hollow shaft  60  with their radially outer ends contacting or in very close proximity to the cylindrical portion  54  and their axial ends contacting or in very close proximity to the closed ends of the cylindrical portion so that the path from the inlet  50  to the outlet  52  is always closed by at least two of the fins  58  to prevent loss of the partial vacuum from the second stage  10  while passing the heated sludge to the outlet  32 . The hollow shaft is sealed and supported in the ends of the cylindrical housing. The finned rotor  56  (shown in greater detail in  FIG. 8 ) is driven by a motor  46  at a rate of rotation to provide desired metering of the heated sludge from the outlet end  34  of the first stage  8  to the evacuated environment of the second stage  10 . 
   Air is supplied through the hollow shaft  60  to openings  62  for onward passage into the heated sludge to prevent clogging. If the air is hot it will further heat the sludge, or at least maintain its temperature, so that heated sludge enters the second stage  10  at about 200° F. or more. 
   The heated sludge passes from the outlet  52  to a small shredder  14  driven by a shredder motor  66 . The shredder  14  is sealed to prevent loss of vacuum from the second stage  20  and has a multi-bladed rotor driven at between about 200 and about 2000 r.p.m. to break the heated sludge into small pieces and particles. The small pieces and particulate material so formed is fed into the input end of the second stage  10  as shown by arrow  68 . 
   The second stage  10  comprises a cylindrical housing  70  coaxial with a hollow rotatable shaft  72  supported in bearings  74  and sealed by seals  76 , at the ends of the housing  70 , for rotation at about 10 to about 50 r.p.m. by a motor  78 . The housing has a diameter of about two feet, a length of about 15 feet and is surrounded by an insulated heat jacket  80 . The hollow shaft  72  has an outside diameter of from about six inches to about one foot. Mounted on the shaft  72  are a plurality of tined projections  82  (see  FIGS. 1 ,  5  and  7 ) distributed along and around the shaft  72  ( FIG. 7 ) and supported on the shaft by tubular supports  84  the interiors of which are open to the interior of the shaft  72  whereby non-turbulent high pressure steam, at about 300° F., passing through the shaft  72  can enter and heat the tubular supports and tined projections  82 . 
   Each tined projection  82  has three equi-spaced tines  86  terminating at or closely proximate the interior cylindrical surface of the housing  70 . Affixed to the housing are blocks  88  sized to closely pass between the tines  86  as the tined projections  82  are rotated by the shaft  72  thereby to remove any particulate material held between the tines. 
   The heat jacket  80  has connections  90  for the admission and exhaust of high pressure non-turbulent steam at about 300° F. whereby the steam passes through the heat jacket  80  to heat the housing  70  and the particulate material therein. 
   The housing  70  is evacuated to about 20 inches Hg. below ambient pressure by a vacuum pump  106  attached to a cyclone  18  which receives particulate material dried in the second stage  10  by way of a duct  94  and a venturi  16 . The pressure in the housing may be adjusted by controlling (bleeding) air into air inlet  98  at the inlet end of the housing  70 . 
   The venturi  16  may have an air inlet  100  for pressurized air to increase acceleration of the dried particulate material into the cyclone  18  substantially tangentially along the cyclone&#39;s inner surface. 
   The cyclone  18  has a cylindrical upper body connected to an inverted cone which decreases in diameter to an outlet controlled by a rotary particle lock  102  which functions in similar manner to the transfer lock  12  to remove the particulate material from the cyclone  18 . This lock  102  is driven by motor  104 . 
   The accelerated particulate material entering the cyclone is subjected to centrifugal force due to its motion around the interior of the cyclone and to the force of gravity. As a result the particulate material follows a helical path to the particle lock  102 . 
   The second stage  10  and cyclone are evacuated by the vacuum pump  106  connected to an outlet  108  at the top of the cyclone  18 . A baffle  110  ensures that the particulate material is not drawn into the vacuum pump  106 . 
   The forces applied to the particulate material in the cyclone serve to substantially destroy odors and to destroy any pathogens remaining in the particulate material. 
   Although preferably horizontal the first and second stages may be inclined downwardly from their inlets to their outlets in order to assist in conveyance of the sludge/particulate material therethrough. 
   It will be appreciated that while the use of steam for heating has been described above, other sources of heat, e.g. hot air, electrical elements, solar energy, geothermal energy, could be used. The source for heating the various components of the invention may also be combined into a single source with an appropriate distribution system and/or different types of heat source could be used in different components or combinations of components. 
   Similarly, some or all of the motors could be replaced by a single motor with appropriate transmission arrangements to drive associated components. 
   It will also be appreciated that the vacuum pump  106  may have an outlet connected to a condenser to condense water vapor or steam removed from the sludge and removed through the vacuum pump  106 . 
   
     
       
             
           
             
             
           
         
             
                 
             
             
               Reference numbers 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               2 
               supply hopper 
             
             
               4 
               rotary shredder 
             
             
               6 
               arrow 
             
             
               8 
               first stage 
             
             
               10 
               second stage 
             
             
               12 
               transfer lock 
             
             
               14 
               shredder 
             
             
               16 
               venturi 
             
             
               18 
               cyclone 
             
             
               20 
               cylindrical housing 
             
             
               22 
               hollow shaft 
             
             
               24 
               seals and bearings 
             
             
               26 
               motor 
             
             
               28 
               heat jackets 
             
             
               30 
               steam inlets and outlets 
             
             
               31 
               axis 
             
             
               32 
               paddles 
             
             
               34 
               outlet end 
             
             
               36 
               arrow 
             
             
               38 
               tube 
             
             
               40 
               arrows 
             
             
               42 
               shaft scraping paddles 
             
             
               44 
               tubes or rods 
             
             
               46 
               transfer lock motor 
             
             
               48 
               lock housing 
             
             
               50 
               inlet 
             
             
               52 
               outlet 
             
             
               54 
               cylindrical portion 
             
             
               56 
               finned rotor 
             
             
               58 
               fins 
             
             
               60 
               hollow shaft lock 
             
             
               62 
               openings 
             
             
               66 
               shredder motor 
             
             
               68 
               arrow 
             
             
               70 
               housing 
             
             
               72 
               shaft 
             
             
               74 
               bearings 
             
             
               76 
               seals 
             
             
               78 
               motor 
             
             
               80 
               heat jacket 
             
             
               82 
               tined projection 
             
             
               84 
               tubular supports 
             
             
               86 
               tines 
             
             
               88 
               blocks 
             
             
               90 
               connections 
             
             
               94 
               duct 
             
             
               98 
               air inlet 
             
             
               100 
               air inlet 
             
             
               102 
               particle lock 
             
             
               104 
               motor 
             
             
               106 
               vacuum pump 
             
             
               108 
               outlet 
             
             
               110 
               baffle