Abstract:
This method achieves spray drying by injection super heated slurry type material counter currently into a duct delivering preheated drying gas. Drying gas velocity is above flooding velocity, and the feed spray is delivered by a variable-flow nozzle valve which maintains constant kinetic energy per unit mass of spray over its flow range. Spray cone angle is also adjusted over the same flow range. Conventional facilities are used for heating drying gas and for collecting dried solids.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Application Ser. No. 09/690,861 Combined Spray Nozzle and Throttle Valve filed Oct. 8, 2000. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Int. Cl. 
                 B01F 3/4 
               
               
                   
                 U.S. Cl. 
                 21/62; 261/117; 261/dig. 9 
               
               
                   
                   
               
             
          
         
       
     
     Reference cited: U.S. Pat. No. 6,419,210 B1 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     There has been no federally sponsored research or development in this application. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     In many industrial operations a liquid slurry or solution of a product is used during its preparation. Then it is a common practice to remove the liquid, most often water, and ship only dry product to the customer. The customer may then use the product in its dry form, or he may reconstitute it with a suitable liquid. Common products so handled are instant coffee and powdered milk. 
     FIG. 1 shows a typical spray dryer system used industrially to remove liquid from a slurry or solution. 
     Feed stream  1  is a liquid carrying suspended or dissolved solids which is pumped on to a spinning disk  17  driven at high speed by a motor  3 . This action of disk  17  results in atomization of stream  1  into fine droplets  18  within the body of a spray chamber  5 . Although a spinning disk is the most common atomizer, other devices include a spray nozzle, two phase spray nozzles and high frequency devices. 
     A hot-arid drying-gas  2  is introduced through a manifold  4  which directs gas  2  downward into spray chamber  5  where it forms the surrounding atmosphere for droplets  18 . As droplets  18  fall through drying gas  2  their liquid component is evaporated which leaves dry particles of product. Most of the dry product passes out of spray chamber  5  at a lower solids discharge  7 . Fine particles of product are carried with the drying gas and water vapor through a gas outlet  6  to a cyclone separator  9 . 
     Most remaining solids leave cyclone  9  at a cyclone solids outlet  16  where they join the bulk of collected solids from discharge  7  to form a solids product  12  passing to a final separator  10 . Drying gas and water vapor  15  leaving cyclone  9  and carrying very few solids joins vent gas  11  from final separator  10 . Both streams pass through a final scrubber or bag filter  14  before venting to a suitable release point  13 . Solids leave separator  10  at an outlet  8 . 
     Although FIG. 1 represents a general spray dryer design, there are many variations depending upon the material being processed and the desired product. 
     The spray dryer design shown by FIG. 1 is highly adaptable to many materials, and it is used widely in many industrial applications. However much of its cost lies in the large spray chamber which may also require a major investment in building space in addition to the spray chamber itself. Also the spray disk and its drive are expensive precision devices, since they often reach speeds in the range of 14,000 rpm. Electrical energy to power the disk is a continuing operating cost. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention improves spray-drying equipment by simplifying it which results in savings in investment and in electrical operating cost. A contact zone in the duct used to convey hot gas  2  to manifold  4  and duct  2  itself are used to replace Spray chamber  5  in FIG. 1. A reversed jet nozzle valve described by U.S. Pat. No. 6,419,210 B1 and application Ser. No. 09/690,861 is used to replace spray disk  17  and its electric motor drive  3 . 
     Heating equipment to supply hot-arid air to the dryer including its blower is essentially the same as for a conventional spray dryer. Also, the collection system is similar to that used by conventional spray dryers of the type that includes fluidized bed drying/stripping of residual moisture from the solids. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a schematic drawing of a typical conventional spray dryer. FIG. 1 is after a typical spray dryer system shown on page 4 of “SPRAY DRYING HANDBOOK”, fifth edition by K. Masters. 
     FIG. 2 is a schematic drawing showing the proposed spray dryer design using a reversed-jet nozzle valve for atomization and without a spray chamber. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 2 a liquid feed stream  22  containing dissolved or suspended solids is pumped under pressure in the range of 50 psig. This pressure may be higher or lower depending upon stream  22 &#39;s viscosity, abrasiveness, and the amount of heat or superheat needed at a contact zone  38 . Stream  22  passes through an optional pre-heater  23  where the stream&#39;s temperature may be raised to include superheat above the atmospheric flash point of liquid stream  22 . This superheat is particularly preferred when the solid product is difficult to dry, and it should be preferably used unless the high temperature degrades the final product. Feed stream  22  enters a nozzle valve  19  at a valve inlet  39 . Nozzle valve  19  is attached to a flange  21  at an elbow in a duct  37 . Nozzle valve  19  is axially aligned along the centrally located longitudinal axis of duct  37 , and its spray  41  is countercurrent to a drying gas  20  flowing in duct  37 . Feed stream  22  becomes a spray  41  directed into duct  37  at a rate determined by the degree of opening of nozzle valve  19  and the pressure (potential energy) of feed stream  22 . Spray  41  is a narrow solid-cone type usually having an included angle of 20 degrees or less. It is a characteristic of nozzle valve  19  that its spray  41  has constant kinetic energy per unit weight of spray  41  over its useful turndown range of approximately six to one. Another characteristic of valve  19  is that the included angle of spray  41  will decrease slightly with decreasing flow of stream  22  so as to approximately maintain the penetration of spray  41  into gas  20  even though drop size in spray  41  decreases with decreasing flow. Nozzle valve  19  is described in detail by U.S. Pat. No. 6,419,210B1 and in application Ser. No. 09/690,861. 
     Previously heated drying gas  20  reaches the system through duct  37 . Gas  20  should have a symmetrical velocity profile about a central longitudinal axis of duct  37  before gas  20  reaches contact zone  38 . This symmetrical velocity profile can usually be achieved by twenty diameters of straight upstream duct length. However, the necessary length can vary depending upon the configuration of preceding duct work. Gas  20  is preheated to a temperature sufficient to vaporize liquid in stream  22  and in sufficient quantity to match the rate of stream  22 . The velocity of gas  20  should be at least enough to cause a complete reversal of a counter current spray  41  in contact zone  38  where gas  20  meets spray  41  of stream  22 . This reversal velocity for gas  20  will be in the range of 80 feet/sec. at temperatures and pressures normally encountered in a spray dryer where water is being evaporated with hot air. However, the required velocity of gas  20  can vary depending upon its density and the density of spray  41 . Downstream of contact zone  38  a combined gas stream  15  should have a velocity in the range of 50 ft/sec. for the same spray dryer. This minimum velocity will also vary depending upon combined gas and water vapor density and the density of solids being conveyed. 
     Contact zone  38  results from the high-velocity countercurrent collision of spray  41  at typically 70 to 80 feet per second with gas  20  also moving at a similar velocity in the opposite direction. At contact zone  38  the counter current collision causes droplets from spray  41  to shatter into smaller drops with an increase in surface area. This increased surface area leads to a rapid transfer of heat from gas  20  to droplets in spray  41 . When stream  22  has been superheated above its atmospheric flash point, by for example ten degrees by pre heater  23 , solids in feed stream  22  are above their final dry temperature before reaching contact zone  38 . Also by this preheating, the task of transferring heat from drying gas  20  to a drying solid particle becomes less difficult. By superheating stream  22 , spray  41  partially flashes as it leaves nozzle-valve  19 . This flashing further reduces drop size with increased surface area for even more rapid evaporation of liquid in spray  41 . The bulk of drying takes place in contact zone  38 . However, removal of residual moisture continues in downstream solids recovery equipment. 
     Gas  20  combined with vaporized water and precipitated solids from stream  22  leaves contact zone  38  as a combined stream  15  in duct  37  before entering a cyclone  26 . A temperature controller  24  senses gas temperature at a thermometer  25  through a signal line  31 , and it positions nozzle valve  19  using a control line  59  and a valve operator  36 . Controller  24  is set so as to reduce the flow of stream  22  if temperature at  25  drops to a pre determined low value which signals the danger of allowing wet solid material. 
     The following description of a solids recovery system is not unique to a nozzle valve spray dryer. It is included as an example of the general type of solids recovery area which includes fluidized bed stripping/drying that is preferred for this nozzle valve spray dryer. And it allows a visual comparison with an alternate recovery system as shown by FIG.  1 . 
     Cyclone  26  separates gas and water vapor from stream  15  which then exit with very fine solids as a gas stream  16  through a duct  40 . Most solids, separated from stream  15 , leave cyclone  26  through a bottom down leg  42  and a rotary valve  33  in route to a solids discharge  30 . As a preferred method of operation, a hot dry gas  28  should be introduced at the lower end of down leg  42  to form a fluidized bed dryer/stripper between rotary valve  33  and cyclone  26 . This fluidized bed further reduces moisture in recovered solids and reduces the humidity of gas entrained with precipitated solids leaving rotary valve  33 . A differential pressure transmitter  49  is used to measure bed level in down leg  42 . A low pressure  48  is sensed at the top of cyclone  26 . A high-pressure sensor line  55  joins dry gas connection  28  to sense pressure at the bottom of down leg  42 . This differential pressure measurement is a direct function of fluid bed level in down leg  42 , and this information is sent from transmitter  49  to a level controller  50  by a signal line  47 . A controller  50 , acting through a control line  57 , regulates the speed of rotary valve  33  to maintain a desired fluid bed level in down leg  42 . 
     Air, water vapor and residual fine solids make up gas stream  16  which enters a bag filter  27  through duct  40 . Air and water vapor exit filter  27  at a gas outlet  35 . Heat in outlet  35  may be recovered at some point such as a heat inlet line  44  for pre-heater  23  with a final vent  45  for example where feed stream  22  can be heated but not superheated. Solids removed from gas stream  16  by filter  27  collect in a down leg  43 . Down leg  43  preferably should also employ a fluidized bed as a final drying/stripping step for difficult-to-dry solids. Similar to cyclone  26 , a differential pressure transmitter  53  measures a low pressure  52  in filter  27  and a high pressure  56  at a hot dry-gas inlet  29  used to sustain a fluidized bed in down leg  43 . Differential pressure, as a function of fluid bed level in down leg  43 , is sent by a signal line  51  to a level controller  54 . Controller  54 , through a control line  58  adjusts the speed of a rotary valve  34  to maintain a desired fluidized bed level in down leg  43 . Solids leaving valve  34  may exit to join duct  30  as product, or they may exit at a duct  32  for recycle. 
     All recovery system surfaces  46  should be insulated and heated if necessary to maintain these surfaces safely above the dew point of stream  15 . Residual drying of solid particles continues in this downstream equipment. 
     Additional equipment may be added at inlet  39  to facilitate operation of nozzle-valve  19 . These additions are convenient for operation, but they are not essential for a nozzle-valve type spray dryer. These additions are a block-and-bleed valve system to assure valve  19 &#39;s isolation when needed, a recycle line for returning slurry to its make-up tank, and liquid and gas purge lines for clean out use. 
     Heat and mass calculations for a nozzle valve dryer are similar to those for a conventional dryer. However, a velocity calculation for spray  41  and a pressure drop calculation for contact zone  38  are unique to a nozzle valve dryer. A sample design problem is given below for the area around spray  41  and contact zone  38  in FIG.  2 . 
     Assume: 
     Feed stream  22 =Water based, 3 gpm, 35% solids and 1.1 s.g. Dry solids having low porosity and specific heat=0.2. 
     Valve inlet  39 =50 psig,  138  C. (280 deg. F.) 
     Gas velocity at combined stream  15 =greater than 50 feet/sec. 
     Estimated partial pressure of water vapor at combined stream  15 =3 psia. 
     Hot gas  20 =350 C. (662 F.) Combined gas at  15 =95 C. (203 F) 
     Assume dryer pressure at one atmosphere. 
     Ignore: Inlet water vapor, outlet solids volume and dryer heat loss. 
     Velocity of spray  41 : 
     V=sq.rt. 2 gh where V=ft./sec., g=32.2 ft./sec./sec, h=ft. head at inlet  39   
     V sq.rt. (2×32.2×50 psi×2.3 ft.water/psi×1.0 sg/1.1 s.g.) 
     V=82. ft./sec. 
     Content of feed stream  22  at valve inlet  39 : 
     Total feed=3 gpm×8.337 pounds/gal.×1.1 s.g.×1 minute/60 sec.=0.4585 pounds/sec. 
     Solids feed=0.35×0.4585=0.1605 pounds/sec. 
     Water feed=0.65×0.4583=0.298 pounds/sec. 
     Water feed=0.298 pounds/sec./18 pounds water/pound mole=0.0166 moles/sec. 
     Heat needed from drying gas  20 : 
     Enthalpy of water at 280 F. and 50 psig.=249 B.T.U./pound 
     Enthalpy of water vapor at 203 F. and 3 psia.=1150 B.T.U./pound 
     Heat lost from solid in cooling from 280 F. to 203 F.=(280 F.-203 F.)×0.2 specific heat=15.4 B.T.U./pound 
     Heat needed from gas  22 =0.298 pounds water/sec×(1150-249)B.T.U./pound-0.16 pounds solid/sec.×15.4 B.T.U./pound=266 B.T.U./sec=958,000 B.T.U./hr. 
     Pounds/hr. of dry gas  20  cooled from 662 F. to 203 F. to supply 958,000 B.T.U./hr.: 
     B.T.U./hr=Pounds of gas 20/hr×specific heat gas  20 ×change in F. 
     Pounds of  20 /hr=(958,000 B.T.U./hr)/0.24 specific heat×(662 F.-203 F.) 
     Pounds of gas 20/hr.=8,690. 
     Pound moles of gas 20/hr=8,690/29 pounds air/pound mole=300 
     Pound moles of gas 20/sec.=300 moles/hr×1/3600 sec./hr.=0.08327 
     Volume of gas  20  at 350 C. (662 F.) Assume 358.9 cuft/pound mole 
     Cuft/hr=358.9 cuft/mole×(273 C.+350 C.)/273 C.×300 moles/hr 
     Cuft/hr=245,500=68. cuft/sec. 
     Volume of combined gas stream  15  at 95 C. (203 F.):                Total                 volume     =            volume                 of                 gas                 20                 plus                 volume                 of                 water                          vapor                 from                 spray                 41                 =              .08327                 moles                 gas                 20     +     .01605                 moles                 water                   =            .09932                 moles        /          sec   .                   =            .09932                 moles        /        sec   ×   358.9                 cuft        /        mole   ×                              (     95                   C   .     +   273                       C   .       )     /   273                     C   .                   =            48.                 cuft        /          sec   .                                    
     Diameter of Duct  37  at stream  15 : 
     Assume  12  inside diameter=0.7854 sq.ft. 
     Velocity of combined stream  15 : 
     Velocity of stream  15 =48 cuft/sec./0.7854 sq.ft=61 ft/sec. 
     Velocity of gas  20 : 
     Velocity of gas  20 =68. cuft/sec/0.7854 sq.ft.=86.5 ft/sec. 
     Pressure drop across contact zone  38 : 
     Pressure drop across zone  38  is equal to the force needed to decelerate spray  41 . 
     F=W/g×A Where F=pounds force, W=pounds/sec., A=ft/sec 
     F=0.298 pounds/sec water/32.2 ft/sec/sec×(82+61) ft/sec+0.1605 pounds solid/sec 32.2 ft/sec/sec×(82+30)ft/sec.=1.88 pounds force 
     Pressure drop across zone  38 =1.88 pounds/0.7854 sqft.=2.4 pounds/sqft. 
     Inches of water=2.4 pounds/sqft/62.4 pounds/sqft×12 inches water/ft.=0.46 inches of water 
     The velocity of gas  20  has nil effect on pressure drop across contact zone  38  as long as its velocity is sufficient to cause complete reversal of spray  41 .