Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present non-provisional application claims the benefit of U.S. Provisional Patent Application No. 60/896,131, entitled TWO-STAGE THERMAL OXIDATION OF DRYER OFFGAS, filed Mar. 21, 2007, which is specifically incorporated herein by reference thereto. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a method and equipment for reducing contaminants such as volatile organic compounds (VOC&#39;s) and carbon monoxide (CO) normally present in dryer offgas that is discharged into the atmosphere from a moist organic product drying process. The equipment includes a product pre-dryer/waste heat evaporator, a primary product dryer, thermal oxidizing apparatus, a furnace, which serves to deliver hot products of combustion to the thermal oxidizing apparatus, and a gas-to-gas heat exchanger of the indirect type having a hot gas side and a cool gas side, hereinafter referred to as the primary heat exchanger, for bringing the hot gaseous output from the thermal oxidizing apparatus that is ultimately discharged into the atmosphere into indirect heat exchange relationship with recycle dryer offgas to increase the temperature of the recycle dryer offgas prior to its reentry into the dryer. 
         [0004]    Efficient thermal oxidation of VOC&#39;s and CO requires correlation of four factors occurring simultaneously: 
         [0005]    1) Adequate temperature; 
         [0006]    2) Adequate oxygen concentration; 
         [0007]    3) Adequate residence time; and 
         [0008]    4) Adequate turbulence. 
         [0009]    In the present process, a rotary waste heat evaporator is utilized to remove moisture from a portion of the dryer offgas thereby allowing the thermal oxidizing apparatus to achieve a much higher temperature while maintaining an adequate oxygen concentration than compared to conventional processes. The amount of moisture removed from the dryer off gas by the rotary waste heat evaporator and the input of fuel to the furnace are controlled and adjusted to provide a hot gaseous output from the thermal oxidizing apparatus that is at a temperature of at least about 1600° F. with an optimum 5% oxygen content by volume, which are sufficiently high to substantially oxidize VOC&#39;s and CO in dryer offgas that is discharged into the atmosphere. 
         [0010]    2. Description of the Prior Art 
         [0011]    Dryers have been used for many years to lower the moisture content of a variety of organic products, such as grain, including distiller&#39;s grain and the like, which nominally may have a water content as high as 60-75%. The recent emergence of ethanol plants producing substantial quantities of moist distiller&#39;s grain as output residue requiring drying for further commercial use, has rekindled interest in more efficient drying processes while, at the same time, necessitating that dryer offgas discharged into the atmosphere contain reduced amounts of VOC&#39;s and CO. 
         [0012]    Commercial drying equipment has been previously designed and constructed to dry organic products to a predetermined acceptable level, which is normally about 10% moisture by weight, wet basis. It has been known for some time to incorporate thermal oxidizing apparatus in processes and equipment for drying moist organic products in order to lower the VOC and CO content of the product output from the dryer. In order to reduce the VOC and CO content of dryer offgas introduced into the atmosphere employing a thermal oxidizer, the hot gaseous output from the oxidizer should be at least about 1600° F. and the oxygen concentration should be at least about 5% by volume. Heretofore, the temperature of the output from the thermal oxidizer has been limited to temperatures in the order of 1400° F. when the oxygen concentration is increased to 5% by volume; hence, VOC and CO reduction has not been optimum. 
         [0013]    Even though residence time of the offgas being oxidized was not restricted and gas turbulence not a significant factor, it was not heretofore feasible to adequately control both the temperature of the thermal oxidizer and its oxygen concentration, in order to significantly lower the VOC and CO content of the offgas introduced into the atmosphere. The temperature and the oxygen concentration could be controlled individually, but not simultaneously for most efficient operation of the thermal oxidizing apparatus. 
       SUMMARY OF THE INVENTION 
       [0014]    In one embodiment of the present invention there is provided a process of reducing the VOC and CO emissions in dryer offgas that is discharged into the atmosphere from a moist organic product dryer. The process generally comprises separating the dryer offgas into first and second portions. The first portion is directed to a hot gas flow side of a rotary waste heat evaporator. In the rotary waste heat evaporator, moisture is removed from the first portion of the dryer offgas thereby forming a reduced moisture dryer offgas portion. Fuel and combustion air are combusted in a combination furnace and mixing chamber. The reduced moisture dryer offgas portion from the rotary waste heat evaporator is directed into the mixing chamber for mixing with the combustion products. The combined gaseous mixture is then introduced into a thermal oxidizer to form a hot gaseous output, the temperature of which is sufficient to decrease the VOC and CO content of the mixture entering the thermal oxidizer. 
         [0015]    The second portion of dryer offgas is brought into indirect heat exchange relationship with the hot gaseous output from the thermal oxidizer within a primary heat exchanger to preheat the second portion of dryer offgas. The preheated second portion of dryer offgas then is recycled back to the dryer, and the hot gaseous output from the thermal oxidizer is discharged to the atmosphere after indirect heat exchange with the second portion of the dryer offgas. 
         [0016]    In another embodiment of the present invention there is provided a process of drying moist organic material and reducing the VOC and CO emissions from dryer offgas generated in the process that is discharged into the atmosphere. The process generally comprises introducing a moist organic material and pre-dryer air into a product flow side of a rotary waste heat evaporator for removal of moisture from the moist organic material and producing a primary dryer product feed and pre-dryer discharge air. The pre-dryer discharge air is separated from the primary dryer product feed. The primary dryer product feed then is directed to a primary dryer where moisture is removed therefrom by contacting the primary dryer product feed with hot dryer gas thereby producing a dried organic product and dryer offgas. 
         [0017]    The dryer offgas is separated into a first portion and a second portion, with the first portion of dryer offgas being directed into a hot gas flow side of the rotary waste heat evaporator. Moisture is removed from the first portion of the dryer offgas within the rotary waste heat evaporator thereby forming a reduced moisture dryer offgas portion. Fuel and combustion air are combusted in a combination furnace and mixing chamber. The reduced moisture dryer offgas portion is directed into the mixing chamber and mixed with the combustion products from the furnace. The mixture then is delivered to a thermal oxidizer which produces a hot gaseous output from the thermal oxidizer. The temperature of the hot gaseous output from the thermal oxidizer is raised to a sufficient level so as to decrease the VOC and CO content of the mixture input to the thermal oxidizer. 
         [0018]    The second portion of dryer offgas is brought into indirect heat exchange relationship with the hot gaseous output from the thermal oxidizer within a primary heat exchanger to preheat the second portion of the dryer offgas thereby forming the hot dryer gas which is recycled back to the primary dryer. The hot gaseous output from the thermal oxidizer then is discharged to the atmosphere after indirect heat exchange with the second portion of the dryer offgas. 
         [0019]    In yet another embodiment of the present invention, there is provided equipment for reducing the VOC and CO content of dryer offgas that is discharged into the atmosphere from a moist organic product drying process. The equipment generally comprises a rotary waste heat evaporator including a product flow side and a hot gas flow side. The product flow side presents a moist product and pre-dryer air inlet and a pre-dried product and air outlet. The hot gas flow side presents a hot gas inlet and a cool gas outlet. A first separator is operably connected with the pre-dried product and air outlet for separating the pre-dryer air and pre-dried product exiting the product flow side of the rotary waste heat evaporator. The first separator includes a discharge air outlet and a pre-dried product outlet. A primary dryer is also provided presenting a product inlet, a dryer air inlet, and a dryer outlet through which the dried organic product and dryer offgas exit. A conveyor leads from the first separator pre-dried product outlet to the primary dryer inlet for delivering pre-dried product from the first separator to the primary drier. A second separator is provided for separating the dried organic product from the dryer offgas, the second separator presenting a dryer offgas outlet and a dried product outlet. A duct leads from the dryer offgas outlet to the hot gas inlet of the rotary waste heat evaporator for delivery of the dryer offgas from the primary dryer to the rotary waste heat evaporator. The dryer offgas serves as the primary heat source to the rotary waste heat evaporator. 
         [0020]    The equipment further comprises thermal oxidizing apparatus including a thermal oxidizer having an input and an output, a combination furnace, and mixing chamber operably connected to the input of the thermal oxidizer, and a tempering chamber that communicates with the thermal oxidizer. A duct connects the rotary waste heat evaporator cool gas outlet with the combination furnace and mixing chamber for delivering cooled gas from the rotary waste heat evaporator to the combination furnace and mixing chamber. The cooled gas is mixed with the combustion products of the combination furnace and mixing chamber within the combination furnace and mixing chamber. 
         [0021]    An indirect primary heat exchanger is provided presenting a cool gas side and a hot gas side. The cool gas side includes a cool gas inlet and a hot gas outlet, and the hot gas side presents a hot gas inlet and a cooled gas outlet. A duct extends between the duct leading to the hot gas inlet of the rotary waste heat evaporator and the primary heat exchanger cool gas inlet for diverting a portion of the dryer off gas to the cool gas side of the primary heat exchanger. A duct leads from the primary heat exchanger hot gas outlet to the dryer air inlet of the primary dryer for the recycle of heated dryer off gas to the primary dryer. A duct leads from the tempering chamber to the hot gas inlet of the primary heat exchanger for supplying hot gas to the hot gas side of the primary heat exchanger. A duct leads from the cooled gas outlet of the primary heat exchanger to the atmosphere for discharging offgas having reduced VOC and CO content to the atmosphere. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a flow diagram of a process for treating dryer offgas according to the present invention; 
           [0023]      FIG. 2  is a cross-sectional view of an exemplary four-pass rotary waste heat evaporator for use with the present invention; 
           [0024]      FIG. 3  is a fragmentary, longitudinal section of the four-pass rotary waste heat evaporator of  FIG. 2  illustrating the dryer off gas flow path therethrough; 
           [0025]      FIG. 4  is a fragmentary, longitudinal section of the four-pass rotary waste heat evaporator of  FIG. 2  illustrating the product flow path therethrough; 
           [0026]      FIG. 5  is a fragmentary view of a section of the four-pass rotary waste heat evaporator illustrating the perforated flights located in a gas flow passage of the evaporator; and 
           [0027]      FIG. 6  is a perspective view of a rotary waste heat evaporator system that may be used with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]      FIG. 1  illustrates an exemplary process  22  according to the present invention for reducing the VOC and CO content of dryer offgas that is discharged into the atmosphere from a moist organic product drying process. The process employs a predryer/waste heat recovery unit  24  (shown schematically in  FIG. 6 ) in order to remove moisture from the moist product. Unit  24  also removes moisture from the dryer offgas so that higher thermal oxidizer temperatures may be achieved while maintaining a sufficient level of oxygen in the output from the thermal oxidizer. As explained hereunder, unit  24  is integrated with a primary product dryer  26  and thermal oxidizing apparatus  28  in order to achieve thermal oxidation of pollutants present in the dryer offgas prior to discharge to the atmosphere. 
         [0029]    Moist product to be dried is initially fed to a rotary waste heat evaporator  30 , also referred to herein as a predryer, by way of a product conveyor  32 . Typically, the moist product supplied to process  22  is a high-moisture product having a water content as high as 60-75% by weight. In one embodiment of the present invention, the moist product comprises distiller&#39;s grain and the like, by-products from fermentation processes used in the production of ethanol. The moist product is generally supplied from the fermentation process as a wet cake and/or syrup. In other embodiments of the invention, the moist product to be dried may comprise animal or fish byproducts, municipal sludge, forage materials, or wood-byproducts. 
         [0030]    In order to improve its handling and processing characteristics, the moist product may be combined with recycled dried product from dryer  26  prior to being supplied to predryer  30 . In such embodiments, sufficient quantities of dried product are recycled so that the moist product fed to predryer  30  presents a moisture content of between about 22 to about 45% by weight, and particularly between about 25 to about 35% by weight. In certain embodiments, from about 75 to about 92% by weight of the total dried product discharged from dryer  26  is recycled and mixed with the wet cake and/or syrup before being fed to predryer  30 . 
         [0031]    Preheated air is also supplied to the product inlet  34  of predryer  30  via duct  36 . As explained below, the air delivered via duct  36  may be preheated using energy recovered from the dryer offgas prior to discharge to the atmosphere. In certain embodiments, the predryer air input presents a temperature of between about 50 to about 250° F., and particularly, between about 100 to about 200° F. At product inlet  34 , the moist product supplied by conveyor  32  is combined with the air delivered via duct  36 . This combined moist product/air input is represented in  FIG. 1  as process stream  38 . In one embodiment of the present invention, predryer  30  comprises a unique four-pass predryer, although, it is within the scope of the present invention for predryer  30  to comprise one, two, three, or more passes for the product and dryer offgas. The particulars of this unique predryer/rotary waste heat evaporator are discussed in further detail below. 
         [0032]    The pre-dried product is discharged from predryer  30  via a conveyor  40 . The predryer air discharged from predryer  30  is first separated from entrained pre-dried product by a cyclone  42  (or other type of separator known to those of skill in the art). In  FIG. 1 , the output of air and product from predryer  30  is schematically represented by stream  43  and the separation of pre-dried product and air is schematically illustrated by separator  45 . The discharged air, which may contain significant amounts of moisture, is removed from unit  24  via duct  44  with fan  46  supplying the motive force. In certain embodiments of the present invention, the air discharged from the product side of predryer  30  may be delivered to a yet-to-be-described tempering chamber  48  where it will be used to decrease the temperature of gas exiting thermal oxidizing apparatus  28 . 
         [0033]    The pre-dried product is then directed toward primary dryer  26  where final drying of the product occurs. In certain embodiments of the present invention, dryer  26  is a cylindrical, single-pass, co-current, three-stage, rotary drum, as illustrated, which is rotated about its longitudinal axis. Alternatively, the dryer may be of the rotary multiple-pass type. Further, the dryer may be of the non-rotating tubular type, or any type that incorporates direct-contact heat exchange between the product to be dried and a hot gaseous heat transfer media. 
         [0034]    Heated gas enters dryer  26  through a duct  50  joined to the outlet of the cool gas side of an elongated, transversely rectangular, indirect, gas-to-gas heat exchanger herein referred to as the primary heat exchanger  52 . The heated gas entering dryer  26  generally presents a temperature of between about 300 to about 800° F., or between about 600 to about 700° F., to effect final drying of the product. The heated gas is commingled with the moist product in the rotary dryer  26  in a direct-contact heat exchange process. During this heat exchange process, the moist product receives heat and rejects moisture in the form of steam, and the heated dryer offgas rejects heat, is cooled, and integrates the moisture rejected by the product into its composition. Upon exiting dryer  26 , the dried product and off gas emitted from dryer  26  are delivered into a dropout chamber (not shown) which separates a large fraction of the dried product from the gaseous content of the dryer output. The offgas emitted from the dropout chamber along with the remaining fraction of entrained dried product moves into a separator, such as a centrifugal separator or cyclone  56  via duct  54 , which separates another fraction of the dried product from the gaseous content. A portion of the dried products captured in the dropout chamber and centrifugal separator are recycled and combined with quantities of moist product to be dried that are fed to predryer  30  via product input  38 . The portion of dried product that is not recycled may be conveyed to a cooling drum (not shown) and then discharged from process  22 . Relatively particle-free offgas exits from cyclone  56  through a duct  58  which leads to the inlet of an induced draft fan  60 . In certain embodiments, the dryer offgas presents a temperature of between about 200 to about 260° F. 
         [0035]    The gas is discharged from fan  60  via duct  62  where it is separated into two portions. The first portion of dryer offgas is delivered to the gas flow side of rotary waste heat evaporator  30  via duct  64 . The second portion of dryer offgas is carried through duct  66  toward the cool gas side inlet of primary heat exchanger  52 . In certain embodiments of the present invention, the second portion of dryer offgas that is delivered to primary heat exchanger  52  comprises the majority of the dryer offgas carried by duct  62 . Particularly, between about 60 to about 80% by weight of the total dryer offgas is contained within the second offgas portion carried by duct  66 . 
         [0036]    The first portion of dryer offgas that is delivered to the gas flow side of rotary waste heat evaporator  30  is nearly saturated with water. As the dryer offgas passes through rotary waste heat evaporator  30 , a significant portion of the water is condensed and removed from the dryer offgas. As explained below, the removal of water from the dryer offgas contributes to the ability of the present invention to achieve sufficiently high temperatures within the thermal oxidizer, while also achieving sufficiently high oxygen levels to effect thermal oxidation of the VOCs and CO contained within the dryer offgas. In certain embodiments according to the present invention, at least about 25% by weight of the moisture, or between about 30 to about 60% by weight of the moisture, carried by the first portion of dryer offgas is condensed within rotary waste heat evaporator  30 . The condensate is removed from rotary waste heat evaporator  30  via a conduit  68  and the reduced-moisture dryer offgas is removed by flue gas recycle (FOR) fan  70  and directed toward thermal oxidizing apparatus  28  via duct  72 . 
         [0037]    Thermal oxidizing apparatus  28  generally comprises a furnace  74 , a mixing chamber  76 , and a thermal oxidizer  78 . (Note that in certain embodiments, furnace  74  and mixing chamber  76  may present as a combined furnace and mixing chamber as the apparatus is fluidly coupled together.) A fuel supplied by conduit  80  is combusted within furnace  74  with combustion air supplied by duct  82 . In certain embodiments, natural gas is utilized as the fuel that is combusted within furnace  74 . However, it is also within the scope of the present invention for fuels other than natural gas to be used including propane, light and heavy fuel oils, and solid fuels. In yet additional embodiments, the combustion air supplied via duct  82  is preheated as opposed to being supplied at or near ambient temperature. Particularly, the combustion air is preheated to a temperature of between about 100 to about 250° F. 
         [0038]    The hot products of combustion from furnace  74  enter mixing chamber  76  where they are combined with the dryer offgas from duct  72 . The mixture is then directed to thermal oxidizer  78 . If desired, a second thermal oxidizer may be provided in series flow relationship with the thermal oxidizer  78  to provide additional residence time of the thermal oxidizer process. The removal of moisture from the dryer offgas in rotary waste heat evaporator  30  means that less water is being heated within thermal oxidizing apparatus  28 . Consequently, higher thermal oxidation temperatures may be achieved within apparatus  28  while also maintaining sufficient oxygen levels thereby ensuring the thermal oxidation of the VOC and CO pollutants contained within the dryer offgas. The quantity of natural gas, the quantity and temperature of the combustion air introduced into furnace  74 , and the quantity, temperature and moisture content of the dryer offgas introduced into mixing chamber  76  are all controlled such that the hot gaseous output from the thermal oxidizer  78  leading to tempering chamber  48  is at a temperature of at least about 1600° F. and has an oxygen concentration of at least about 5% by voltume. In certain embodiments, the temperature of the output from the thermal oxidizer  78  is at least about 1700° F., and in still other embodiments the temperature of the output from the thermal oxidizer is at least about 1800° F., all while the oxygen concentration is at least about 5% by volume. 
         [0039]    In certain embodiments of the invention, heat exchanger  52  is constructed with conventional materials in order to reduce capital expenses. Thus, it is important that the temperature of the hot gaseous output from the thermal oxidizer  78  be reduced so as to avoid damaging heat exchanger  52 . Otherwise, heat exchanger  52  would need to be constructed from more expensive materials capable of withstanding the extreme temperatures of the hot gaseous output from the thermal oxidizer. The hot gaseous output from the thermal oxidizer  78  is directed to tempering chamber  48  via duct  84 . Within tempering chamber  48 , the temperature of the hot gaseous output from the thermal oxidizer  78  is reduced to less than about 1600° F. before being directed into the hot gas side of the primary heat exchanger  52 . In other embodiments of the present invention, the hot gaseous output from the thermal oxidizer is reduced to a temperature of between about 900 to about 1400° F. This temperature reduction is accomplished at least in part by combining the hot gaseous output from the thermal oxidizer with at least a portion of the air discharged from predryer  30  conducted to tempering chamber  48  by duct  44 . The two gaseous products are well-mixed within tempering chamber  48  and then delivered to the hot gas side of the primary heat exchanger  52  via duct  85 . 
         [0040]    Within primary heat exchanger  52 , heat is indirectly transferred from this hot gaseous mixture to the dryer offgas moving in a counterflow direction on the cool gas side of the primary heat exchanger. During this process the gaseous mixture on the hot gas side of the primary heat exchanger  52  is substantially cooled before exiting the hot gas side of the primary heat exchanger. In certain embodiments of the present invention, the gaseous mixture is cooled to a temperature of between about 300 to about 550° F. within primary heat exchanger  52 . A fan  86  provides the motive force for moving the gaseous products and mixtures through thermal oxidizing apparatus  28 , tempering chamber  48 , and the hot gas side of primary heat exchanger  52 . 
         [0041]    In certain embodiments, the cooled gaseous mixture exiting the primary heat exchanger hot gas side is delivered to the hot gas side inlet of air heater  88  via duct  90 . Air heater  88  may be used to preheat the combustion air supplied to furnace  74  via duct  82  and/or preheat the air input to predryer  30  via input  38 . A stream of cooled gaseous mixture exits the hot gas side of the air heater  88  and exits to the atmosphere through a stack. Again, fan  86  provides the motive force for moving the gaseous mixture through air heater  88  and exhausting to the atmosphere. 
         [0042]    The table below illustrates processing conditions and flow rates that may be encountered in an exemplary embodiment of the process of  FIG. 1 . The label in the first column of the table corresponds with the stream label in  FIG. 1 . 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 1 
                 Product and Predryer Air Input to Rotary 
               
               
                   
                 Waste Heat Evaporator 
               
               
                   
                 192.0° F. (product) 
               
               
                   
                 200.0° F. (air) 
               
               
                   
                 320,865.3 lb/hr (product) 1   
               
               
                   
                 120,000.0 lb/hr (air) 
               
               
                   
                 36.55% - Moisture (product) 
               
               
                   
                 117,259.2 lb/hr - H 2 O 
               
               
                 2 
                 Predryer Air Output to Tempering Chamber 
               
               
                   
                 155.0° F. 
               
               
                   
                 150,000.0 lb/hr - Total 
               
               
                   
                 30,000.0 lb/hr - H 2 O 
               
               
                 3 
                 Dryer Feed 
               
               
                   
                 192.0° F. 
               
               
                   
                 290,865.3 lb/hr - Total 
               
               
                   
                 30.0% - Moisture 
               
               
                   
                 203,605.71 lb/hr - Solids 
               
               
                   
                 87,259.59 lb/hr - H 2 O 
               
               
                 4 
                 Dryer Gas Input 
               
               
                   
                 740.0° F. 
               
               
                   
                 278,523 lb/hr - Total 
               
               
                   
                 28,080 lb/hr - N 2   
               
               
                   
                 8,483 lb/hr - O 2   
               
               
                   
                 18 lb/hr - CO 2   
               
               
                   
                 241,941 lb/hr - H 2 O 
               
               
                   
                 222,550 acfm 
               
               
                 5 
                 Dryer Air Leaks 
               
               
                   
                 50° F. 
               
               
                   
                 −12.6° F. - Dewpoint 
               
               
                   
                 5.3% - Relative Humidity 
               
               
                   
                 9,000 lb/hr - Total 
               
               
                   
                 6,906 lb/hr - N 2   
               
               
                   
                 2,086 lb/hr - O 2   
               
               
                   
                 4 lb/hr - CO 2   
               
               
                   
                 4 lb/hr - H 2 O 
               
               
                   
                 2,000 acfm 
               
               
                 6 
                 Dryer and Piping Radiation and Convection 
               
               
                   
                 Losses 
               
               
                   
                 100,000 Btu/hr 
               
               
                 7 
                 Dryer Product Output 
               
               
                   
                 210° F. 
               
               
                   
                 231,370.13 lb/hr - Total 
               
               
                   
                 12.0% Moisture 
               
               
                   
                 203,605.71 lb/hr - Solids 
               
               
                   
                 27.764.42 lb/hr - H 2 O 
               
               
                 8 
                 Dryer Offgas 
               
               
                   
                 235.7° F. 
               
               
                   
                 347,018 lb/hr - Total 
               
               
                   
                 34,986 lb/hr - N 2   
               
               
                   
                 10,570 lb/hr - O 2   
               
               
                   
                 23 lb/hr - CO 2   
               
               
                   
                 301,440 lb/hr - H 2 O 
               
               
                   
                 205.7° F. - Dewpoint 
               
               
                   
                 160,710 acfm 
               
               
                 9 
                 Dryer Offgas to Primary Heat Exchanger 
               
               
                   
                 235.7° F. 
               
               
                   
                 278,523 lb/hr - Total 
               
               
                   
                 28,080 lb/hr - N 2   
               
               
                   
                 8,483 lb/hr - O 2   
               
               
                   
                 18 lb/hr - CO 2   
               
               
                   
                 241,941 lb/hr - H 2 O 
               
               
                   
                 205.7° F. - Dewpoint 
               
               
                   
                 128,989 acfm 
               
               
                 10 
                 Dryer Offgas to Rotary Waste Heat Evaporator 
               
               
                   
                 235.7° F. 
               
               
                   
                 68,495 lb/hr - Total 
               
               
                   
                 6,906 lb/hr - N 2   
               
               
                   
                 2,086 lb/hr - O 2   
               
               
                   
                 4 lb/hr - CO 2   
               
               
                   
                 59,499 lb/hr - H 2 O 
               
               
                   
                 205.7° F. - Dewpoint 
               
               
                   
                 31,721 acfm 
               
               
                   
                 6.61 lb H 2 O/lb dry gas 
               
               
                 11 
                 Condensate from Rotary Waste Heat 
               
               
                   
                 Evaporator 
               
               
                   
                 201.5° F. 
               
               
                   
                 30,000 lb/hr - H 2 O 
               
               
                   
                 30,309,746 btu/hr 
               
               
                 12 
                 Dryer Offgas Output from Rotary Waste Heat 
               
               
                   
                 Evaporator 
               
               
                   
                 201.5° F. 
               
               
                   
                 100.0% Relative Humidity 
               
               
                   
                 38,495 lb/hr - Total 
               
               
                   
                 6,906 lb/hr - N 2   
               
               
                   
                 2,086 lb/hr - O 2   
               
               
                   
                 4 lb/hr - CO 2   
               
               
                   
                 29,499 lb/hr - H 2 O 
               
               
                   
                 16,263 acfm 
               
               
                 13 
                 Natural Gas Fuel 
               
               
                   
                 3,549.5457 lb/hr 
               
               
                   
                 77,685,356 Btu/hr (HHV) 
               
               
                   
                 1,306 Btu/lb Water Evaporated 
               
               
                 14 
                 Combustion Air 
               
               
                   
                 50° F., preheated to 200° F. 
               
               
                   
                 45.00% - Excess Air 
               
               
                   
                 81,189 lb/hr - Total 
               
               
                   
                 62,295 lb/hr - N 2   
               
               
                   
                 18,820 lb/hr - O 2   
               
               
                   
                 41 lb/hr - CO 2   
               
               
                   
                 34 lb/hr - H 2 O 
               
               
                   
                 23,350 acfm 
               
               
                 15 
                 Furnace/Mixing Chamber/Thermal Oxidizer 
               
               
                   
                 Radiation and Convection Losses 
               
               
                   
                 200,000 Btu/hr 
               
               
                 16 
                 Gaseous Output from Thermal Oxidizer 
               
               
                   
                 1853.6° F. 
               
               
                   
                 123,234 lb/hr - Total 
               
               
                   
                 69,477 lb/hr - N 2   
               
               
                   
                 7,927 lb/hr - O 2   
               
               
                   
                 9,074 lb/hr - CO 2   
               
               
                   
                 36,755 lb/hr - H 2 O 
               
               
                   
                 0 lb/hr - SO 2   
               
               
                   
                 144,917 acfm 
               
               
                   
                 4.9927% v/v O 2   
               
               
                 17 
                 Output from Tempering Chamber 
               
               
                   
                 985.0° F. 
               
               
                   
                 273,234 lb/hr - Total 
               
               
                   
                 66755 lb/hr - H 2 O 
               
               
                 18 
                 Output from Primary Heat Exchanger 
               
               
                   
                 341.0° F. 
               
               
                   
                 273,234 lb/hr - Total 
               
               
                 19 
                 Heat Exchanger Radiation and Convection 
               
               
                   
                 Losses 
               
               
                   
                 200,000 Btu/hr 
               
               
                 20 
                 Atmospheric Exhaust 
               
               
                   
                 296° F. 
               
               
                   
                 273,234 lb/hr - Total 
               
               
                   
               
               
                   1 Includes 145,446 lb/hr wet cake, 66.04% moisture and 175,136.3 lb/hr recycle from dryer, 12.0% moisture. 
               
             
          
         
       
     
         [0043]    As noted above, certain embodiments of the present invention employ a unique four-pass rotary waste heat evaporator  30 . As shown in  FIG. 2 , rotary waste heat evaporator  30  comprises a plurality of concentric tube sections, with each tube section defining either a product flow pass or an air flow pass. The innermost tube section comprises a generally cylindrical outer wall  92  and a generally cylindrical inner wall  94 . The outer surface of inner wall  94  is provided with a plurality of longitudinally extending flights  96 . Flights  96  comprise a radially projecting portion  98  and a transversely extending outer toe portion  100 . The inner surface of outer wall  92  also presents a plurality of longitudinally extending product flow flights  102 . Each flight also comprises an inwardly extending portion  104  and an obliquely extending distal segment  106 . It is noted that distal segment  106  presents a varied geometry between the central portion of the flight and the outer ends of the flight. The central portion of the flight presents a distal segment  106   a  that extends away from the inwardly extending portion  104  at an angle of approximately 30°. The outer ends of the flight present distal segments  106   b  that extend away from the inwardly extending portion  104  at a greater angle, approximately 60°. This altered geometry is required because of the presence of frustoconical section  108  formed by inner wall  94  (see,  FIG. 4 ). Thus, the more angled distal segments  106   b  extend along the inner surface of outer wall  92  at least in the region of frustoconical section  108 . In certain embodiments of the present invention, the distal segments  106   b  extend along approximately the outer  24  inches of the flight. The outer surface of outer wall  92  presents a plurality of radially projecting gas flow flights  109 . The gas flow flights of rotary waste heat evaporator  30  present a unique configuration that is discussed in greater detail below. 
         [0044]    A second tube section is disposed about the outside of the innermost tube section and comprises an inner wall  110  and an outer wall  112 . The inwardly facing surface of inner wall  110  presents a plurality of longitudinally extending, inwardly projecting gas flow flights  114 . The outer facing surface of inner wall  110  presents a plurality of radially projecting flights  116  which are very similar in configuration to flights  96 . The inwardly facing surface of outer wall  112  presents a plurality inwardly projecting product flow flights  118  which are very similar in configuration to product flow flights  102 , except that the geometry of the flights is substantially uniform and the distal segments extend away from the inwardly extending portions at an angle of approximately 30°. The outer surface of outer wall  112  presents a plurality of gas flow flights  120  that are very similar in configuration to flights  114 . 
         [0045]    A third tube section is disposed around the outside of the second tube section and comprises an inner wall  122  and an outer wall  124 . The inwardly facing surface of inner wall  122  presents a plurality of inwardly projecting gas flow flights  126  that are similar in configuration to gas flow flights  114  and  120 . The outer surface of inner wall  122  presents a plurality of flights  128  that are similar in configuration to flights  96  and  116 . The inwardly facing surface of outer wall  124  presents a plurality of inwardly projecting product flow flights  130  that are similar in configuration to flights  118 . The outer surface of outer wall  124  presents a plurality of radially projecting gas flow flights  132  similar in configuration to gas flow flights  120 . 
         [0046]    A fourth tube section is disposed around the outside of the third tube section and comprises an inner wall  134  and an outer wall  136 . The inwardly facing surface of inner wall  134  presents a plurality of inwardly projecting gas flow flights  138  that are similar in configuration to gas flow flights  126 . The outer surface of inner wall  134  presents a plurality of flights  140  that are similar in configuration to flights  128 . The inwardly facing surface of outer wall  136  presents a plurality of inwardly projecting product flow flights  142  that are similar in configuration to flights  130 . The outer surface of outer wall  136  presents a plurality of radially extending gas flow flights  144  similar in configuration to gas flow flights  132 . 
         [0047]    Surrounding the fourth tube section is an outer drum  146 . The inwardly facing surface of drum  146  presents a plurality of inwardly projecting gas flow flights  148  that are similar in configuration to gas flow flights  138 . 
         [0048]    In operation, rotary waste heat evaporator  30  is rotated in the direction indicated by arrow  150  (clockwise in  FIG. 2  which is looking toward the hot gas inlet) by a motor  152 . Rotary waste heat evaporator  30  is provided with track or tire sections  154  which contact trunnion wheels  156  during rotation thereof. 
         [0049]    Turning now to  FIG. 5 , the unique configuration of gas flow flights  109 ,  114 ,  120 ,  126 ,  132 ,  138 ,  144 , and  148  will be explained.  FIG. 5  is a fragmentary, perspective view of a portion of outer wall  112  of the second tube section and a portion of inner wall  122  of the third tube section. Gas flow flights  120  and  126  present as longitudinally extending, perforate plates normally projecting from the respective wall surface. In the embodiment illustrated, flights  120  and  126  comprise two rows of orifices  158 . The precise dimensions of the gas flow flights, including the quantity and arrangement of orifices therein, is dependent upon the distance between adjacent wall surfaces. For example, gas flow flights  109  and  114  reside in the gas flow passage between outer wall  92  of the innermost tube section and inner wall  10  of the second tube section. As the radii of these inner tube sections is less than, for example, the radii of the outer two tube sections, the distance between walls  92  and  110  is greater than the distance between walls  124  and  134  in order to accommodate the volume of gas flowing through the rotary waste heat evaporator  30 . 
         [0050]    In the embodiment of the present invention illustrated in  FIG. 2 , three different gas flow flight configurations are employed. The largest gas flow flights (in dimension and number of orifices) are gas flow flights  109  and  114  positioned between walls  92  and  110 . These gas flow flights generally present three rows of orifices. The next largest gas flow flights are flights  120  and  126  positioned between walls  112  and  122 , which are shown in  FIG. 5 . The distance between walls  112  and  122  is less than the distance between walls  92  and  110 . Flights  120  and  126  present two rows of orifices. The outer two gas flow regions, the space between walls  124  and  134  and the space between wall  136  and drum  146 , comprises the smallest gas flow flights. Flights  132 ,  138 ,  144 , and  148  present a single row of orifices. 
         [0051]    In certain exemplary embodiments, the gas flow flights present widths of approximately 1.5, 2.5, and 3.5 inches, respectively. The orifices formed in the flights are approximately 0.5 inch in diameter and spaced approximately one inch apart. However, the flights need not necessarily present these dimensions or orifice arrangements. Thus, the sizing and configuration of the perforate gas flow flights may depend upon a number of factors such as the size of the rotary waste heat evaporator and the throughput for which it is designed. 
         [0052]    The orifices present in the gas flow flights allow for more effective contact between the gas and condensate flowing within the gas flow regions. This enhanced contact leads to increased condensation of moisture from the dryer offgas and greater transfer of heat between the gas flow side and product flow side of the rotary waste heat evaporator  30 . 
         [0053]    Turning now to  FIGS. 3 and 4 , both the product and gas flow paths through rotary waste heat evaporator  30  are illustrated. All flighting has been removed from these figures for clarity purposes. The flow of dryer offgas through rotary waste heat evaporator  30  is shown in  FIG. 3 . Dryer offgas is delivered to a hot gas inlet  160  via duct  66 . Within inlet  160 , the first pass of dryer offgas is split into two portions. The first portion of dryer offgas continues through frustoconical section  108  into a central passage  162  defined by wall  94 . The second portion of dryer offgas is diverted into gas flow passage  164  that is defined as the space between walls  92  and  110 . Passages  1   62  and  164  combined represent the first pass of dryer offgas through rotary waste heat evaporator  30 . Proximate the end of rotary waste heat evaporator  30  opposite the hot gas inlet  160 , the two portions of dryer offgas from the first pass are recombined and directed outwardly into gas flow passage  166  that is defined as the space between walls  112  and  122 . At this point, the dryer offgas flow will also contain water that has been condensed during the first pass. Thus, the flow of material through the gas flow side of rotary waste heat evaporator  30  is generally two phase, gaseous and liquid. 
         [0054]    The dryer offgas (and any condensed water) flows through passage  166  in a countercurrent manner with respect to the offgas flowing through passages  162  and  164 . Once the dryer offgas reaches the end of passage  166  proximate hot gas inlet  160 , the offgas is directed outwardly into gas flow passage  168  that is defined as the space between walls  124  and  134 . The dryer offgas flows through passage  168  in a co-current manner with respect to the flow through passages  162  and  164  and in a countercurrent manner with respect to the flow through passage  166 . 
         [0055]    Upon reaching the end of passage  166  that is opposite inlet  160 , the dryer offgas (and condensed water) are directed outwardly into yet another gas flow passage  170  that is defined by wall  136  and drum  146 . The dryer offgas flows through passage  170  in a co-current manner with respect to the flow through passage  166  and countercurrent with respect to the flow through passages  162 ,  164 , and  168 . The dryer offgas (and condensed water) exit passage  168  via a gas outlet  172 . The condensate is collected by plenum  173  and is removed from rotary waste heat evaporator  30  by conduit  68  (see,  FIG. 6 ). The cooled, reduced-moisture dryer offgas is removed from rotary waste heat evaporator  30  by means of fan  70  and directed to thermal oxidizing apparatus  28  via duct  72 . Thus, the dryer offgas makes a total of four passes through rotary waste heat evaporator  30  from inlet  160  to outlet  172 . Further, the input and discharge of dryer offgas occurs at the same end of the rotary waste heat evaporator  30 . 
         [0056]      FIG. 4  illustrates the flow path of moist product through rotary waste heat evaporator  30 . Moist product and predryer air enter rotary waste heat evaporator  30  through a product inlet  174 . Upon entering, the product is directed outwardly around the innermost tube section inner wall  94  into a first product flow passage  176  that is defined by walls  92  and  94 . Product flows through passage  176  in a countercurrent manner with respect to the dryer offgas flow through central gas flow passage  162 . Generally, throughout rotary waste heat evaporator  30 , each consecutive pass of product is countercurrent to the respective pass of dryer offgas. Near the end of passage  176  opposite product inlet  174 , passage  176  becomes constricted or narrowed because of frustoconical section  108 . 
         [0057]    The product is then directed outwardly into a second product flow passage  178  that is defined by walls  110  and  112 . Product flows through passage  178  in a countercurrent manner with respect to the flow of product through passage  176 . Upon nearing the end of passage  178  that is proximate product inlet  174 , the product is directed outwardly into a third product passage  180 . Product passage  180  is defined by walls  122  and  124 . Product flows through passage  180  in a countercurrent manner with respect to the flow of product through passage  178  and in a co-current manner with respect to the flow of product through passage  176 . Upon nearing the end of passage  180  that is opposite product inlet  174 , the product is directed outwardly into a fourth product passage  182 . Product passage  182  is defined by walls  134  and  136 . Product flows through passage  182  in a co-current manner with respect to the flow of product through passage  178  and in a countercurrent manner with respect to the flow of product through passages  176  and  180 . Product flows through passage  182  until it reaches product outlet  184  where it falls into conveyor  40  (see,  FIG. 6 ) and is directed toward primary dryer  26 . 
         [0058]    It is to be understood that the foregoing description of various embodiments of the present invention is provided by way of illustration and nothing therein should be taken as a limitation upon the overall scope of the invention.

Technology Category: f