Abstract:
An improved desolventizer-toaster (DT) unit is used for removing traces of a hydrocarbon solvent from a mass of vegetable particles of oil. A conventional DT unit has within a housing, a set of solvent removal trays and a main ejector transporting solvent vapor and steam from below the tray set to between a pair of the trays in the set. The improved DT unit has a further scavenger tray between an inlet of the main ejector and the housing floor. A scavenger ejector transports solvent vapor from between the scavenger tray and the housing floor before it exits from the unit, to the space between the tray set and the scavenger tray.

Description:
CROSS-SREFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a regular application filed under 35 U.S.C. §111(a) claiming priority, under 35 U.S.C. §119(e)(1), of provisional application Ser. No. 61/478,799, previously filed Apr. 25, 2011, under 35 U.S.C. §111(b) and provisional application Ser. No. 61/479,096, previously filed Apr. 26, 2011, under 35 U.S.C. §111(b). 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    One very important industrial process is the extraction of vegetable oil from oil-bearing seeds or kernels such as soybeans, cottonseed, canola, and rapeseed. The process generally operates continuously in very large equipment, where a single unit typically extracts many tons per day of the oil. The oil is very valuable, and has many food and non-food uses. The particle mass remaining after the oil removal is also valuable, and may be used as human food or animal feed. 
         [0003]    One type of extraction system first processes the oil-containing portion of the seeds to form a mass of flakes or particles bearing the oil (meal). Then the meal is transported to a container where a solvent such as hexane dissolves the oil in the meal. Much of the solvent-oil solution so formed is then removed from the meal by draining. The process then separates the oil and solvent removed from the meal by distillation for example, allowing the oil to be used as desired, and the solvent reused. 
         [0004]    Hexane and other similar solvents are highly flammable, so the processes used must avoid any possibility of igniting the hexane. Hexane and other solvents also form vapors much heavier than air and water vapor, so solvent vapors tends to settle at the bottom of any vessel containing them. 
         [0005]    The meal after the first solvent-oil removal step still has so much solvent that the meal is unfit for use as animal feed or human food. To correct this situation, a “desolventizer-toaster” (DT) unit may remove a large percentage of the remaining solvent from the meal. This leaves the remaining meal with a small amount of residual solvent. The solvent that the DT unit extracts from the meal can be reused in the process as well, making the process more cost-efficient and environmentally friendly. 
         [0006]    A DT unit passes the meal through a number of heating stages that vaporize nearly all of the solvent remaining in the meal. Each stage comprises a floor or tray that heats the meal and/or allows steam to pass through the meal, in either case vaporizing a portion of the hexane or other solvent in the meal. A stifling element at each stage agitates the meal to assist the vaporization and to eventually shift the meal to an opening in the stage&#39;s floor through which the meal falls under the force of gravity to the next stage. 
         [0007]    Each stage can remove only a percentage of the solvent remaining in the meal. DT units having a reasonable number of stages, say 6-10, do not remove as much of the solvent as desired to provide meal with a suitably small amount of solvent. 
         [0008]    Certain newer DT units now have one or more solvent extraction flash stages at the bottom of the conventional heating stages that use a different process to extract a further percentage of the entrained solvent, meanwhile reusing a portion of the steam. This type of DT unit is explained in both U.S. Pat. No. 6,279,250 (&#39;250) and in an article in Inform, June 2003, pp. 338-339 (Inform). Both &#39;250 and Inform are incorporated by reference into this description. 
         [0009]    It will be helpful for the reader of this description to be familiar with both of these publications. Such a solvent extraction and steam reuse stage form a Vapor Recovery (VR) enhancement of a DT unit. 
         [0010]    This VR stage uses an ejector or other vapor transport device that collects steam and leaked solvent vapor from the rotary valve receiving meal discharged from the final conventional stage, see &#39;250. The ejector recycles this steam and leaked solvent vapor back into an upper conventional stage of the DT unit. The stages at and below the stage receiving the recycled steam can reuse the thermal energy of the recycled steam rather than losing it. The heat in the recycled vapor will heat the meal to extract further solvent while again passing through the conventional stages, thereby reducing solvent lost to the environment and providing more solvent for reuse. 
         [0011]    A DT unit having VR usually has only one VR stage as shown in &#39;250. Some have however, been built with two or more VR stages in order to recover more of the solvent. Experience shows though, that in DT units with multiple VR stages, it is difficult to assure the most efficient venting of vapors from the plurality of VR stages. That is, it is difficult to find the optimum amount of steam and vapor to recycle from above the first VR stage and how much to recycle from above the second VR stage. 
         [0012]    These conventional VR stages do not deal with the problem of pooling or gathering of the heavy solvent vapors due to inadequate agitating of the gasses in the space involved. Hexane for example, has a specific gravity that is more than five times that of water. If the steam and solvent vapors do not thoroughly mix, the heavier solvent vapors settle in the space and eventually exit the DT unit with the meal. 
         [0013]    Experience also shows that conventional VR systems leave a small percentage of the solvent remaining in the meal. While this remaining solvent is not considered to affect the quality of the meal, it is still wasted. It would be advantageous to extract a further portion of this remaining solvent, for reuse if for no other reason. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0014]    A desolventizer-toaster (DT) unit removes solvent from a mass of vegetable meal in the form of particles or flakes holding the solvent in liquid form in the meal. The DT unit is of the type having a floor and above the floor, a series of permeable vapor recovery (VR) trays including a bottom tray. Each VR tray receives steam for heating the meal and permeating the meal. The meal cascades downward through ports in the VR trays to a bottom tray. A mixture of the steam and vaporized solvent flows upwards through the permeable trays. The bottom VR tray and a floor of the unit each having a transport device for passing meal to the space below but resisting passage of vapors. 
         [0015]    The DT unit further comprises a main ejector with an inlet in the space below the bottom tray and an outlet between adjacent VR trays above the bottom tray 
         [0016]    The invention is an improvement that allows removing a further fraction of the solvent remaining in the meal when it reaches the floor of the DT unit. This improvement includes a scavenger tray between the main ejector inlet and the floor. A first scavenger ejector has an inlet in the space between the scavenger tray and the floor and an outlet in the space between the scavenger tray and the bottom tray. 
         [0017]    The scavenger ejector transports vapors in the space between the scavenger tray and the floor to the space between the bottom VR tray and the scavenger tray. Some of the vapors that pass through the scavenger ejector are then transported by the main ejector to 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0018]      FIG. 1  is an internal side elevation view of a multi-stage desolventizer-toaster (DT) unit with a final solvent scavenger stage. 
           [0019]      FIG. 2  is a top plan view at a section of the final solvent scavenger stage. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]      FIGS. 1 and 2  show a DT unit  10  having many of the components shown in the &#39;250 patent mentioned earlier. Unit  10  as shown in  FIGS. 1 and 2  include a housing or enclosure  16  with a generally circularly cylindrical cross section within which occurs removal of solvent from vegetable meal. Masses or layers  40  of meal are shown throughout housing  16  in various stages of solvent removal. 
         [0021]    The stages of solvent removal comprise various vapor recovery (VR) trays  37   a  and  37   b ,  49   a  -  49   e , and tray  59 , collectively VR tray sets  37 ,  49 , and  59  respectively. The tray sets  37 ,  49 , and  59  all support layers of meal masses  40  as the meal passes through the unit  10 . 
         [0022]    Tray sets  37 ,  49 , and  59  and floor  90  are all hollow. Steam flows into and through them to heat the meal masses  40  they hold to vaporize the solvent in the meal masses  40  on them. Both top and bottom plates forming trays  49   a  -  49   d  are porous to allow steam to percolate through the meal masses  40  held thereon, and to allow vapors from lower stages to also flow through. The upper plate of tray  49   e  is porous to allow steam to flow upwards through the mass  40  thereon, but the lower plate of tray  49   e  is non-porous to prevent flow of vapor from space  55 . 
         [0023]    Each tray and floor  90  has a stirrer  88 , shown only for tray  49   c  and floor  90  in the FIGS. Stirrers  88  may each comprise for example, an arm and paddle blade driven by a shaft  93  causing stirrers  88  to rotate circularly around the upper surface of the trays in each tray set  37 ,  49 , and  59 . Stirrers  88  mix and agitate the individual meal masses  40  to maintain constant temperature therein, to release trapped solvent vapors, and to assist vaporizing the solvent in the masses  40 . 
         [0024]    Solvent-containing meal enters enclosure  16  through a port  52  at the top of enclosure  16  in a zone  45   a  within enclosure  16 . The entering meal initially falls under the force of gravity onto to the upper surfaces of tray  37   a . From tray  37   a , the stirrers sweep meal masses  40  through the trays&#39; respective openings  43   a , etc. to cascade through enclosure  16  from each of the trays to the tray directly below prevents most vapor leakage upwards through tray  49   e.    
         [0025]    Stirrers  88  continuously sweep across individual trays of tray sets trays  37 ,  49 , and  59  causing agitation of meal masses  40  held on trays  37 ,  49 , and  59 . Stirrers  88  also shift meal masses  40  to openings  43   a ,  43   b , and  53   a - 53   d , through which the meal falls to the tray surface below. Block arrows  53   a - 53   d  represent this falling meal. 
         [0026]    A transport device such as rotary valve  56  moves meal from tray  49   e  to tray  59 . Such a transport device prevents most vapor leakage upwards through tray  49   e.    
         [0027]    The heated tray sets  37 ,  49 , and  59  vaporize much of the solvent in the meal masses  40 . The steam injected into trays of tray set  49  vaporizes much of the solvent in the masses  40  thereon to form a solvent-steam vapor. Much but not all of this solvent-steam vapor exits through vent  13 . Equipment receiving the gasses from vent  13  maintain a pressure lower than that within housing  16 , as for example by condensing the solvent-steam vapor in the course of separating the oil and solvent. 
         [0028]    &#39;250 explains how (referring to  FIG. 1  of this description) main ejector  20  introduces high-temperature steam into space  45   b  to increase the amount of solvent in mass  40  on bed  49   a  that is vaporized. Main ejector  20  is a “medium pressure ejector” that pulls in vapor at approximately atmospheric pressure and has sufficient pressure rise, perhaps between 6″ water column (0.22 psi.) positive pressure and 70″ we (2.5 psi) positive pressure. The precise pressure rise depends on the size of the plant, total number of trays, etc. 
         [0029]    The term “ejector” here should be taken to include not only those gas transport devices that use momentum transfer between a steam jet and the solvent vapor, but also other types of pumps and fans that accomplish similar transport of the gasses at the ejector inlet to the ejector outlet. Because of the flammability of oil solvents such as hexane, it is likely that steam-based ejectors are preferable, since they mostly avoid the possibility of a spark within the ejector itself. 
         [0030]    Port  61  in tray  59  allows meal with entrained liquid solvent and any of the heavier solvent vapors to fall onto floor  90  as the stirrer for tray  59  shifts and mixes the meal lying on tray  59 . The solvent in the space above tray  59 , being substantially denser than steam, also tends to flow through port  61 . 
         [0031]    A transport device such as rotary valve  64  prevents most vapor leakage upwards through floor  90  from outside chamber  16 . Valve  64  is at the bottom of housing  16  in the Fig. and shown in &#39;250 as valve  58 , removes masses of meal from space  87  while allowing only a small amount of air to enter space  87 . Because of the high specific gravity of solvent vapor (in the case of hexane, nearly 5 times as heavy as steam), space  87  between floor  90  and tray  59  tends to accumulate solvent vapor. Then, as meal moves through rotary valve  64 , solvent vapor can escape with the meal. 
         [0032]    The invention includes an additional solvent vapor transfer and mixing device comprising tray  59 , a first scavenger ejector  30 , and a second optional scavenger ejector  30 ′ preferably diametrically located from ejector  30  on housing  16 . Ejectors  30  and  30 ′ carry vapors from space  87  into space  55 , and also enhance circulation of the vapors in space  87 . Ejector  30  is preferably one with relatively low pressure rise. Ejector  30 ′ has a similar structure and operation. 
         [0033]    Ejectors  30  and  30 ′ may have bell-shaped inlet openings  91  and  91 ′ that are substantially larger than the duct leading into the respective ejector  30  or  30 ′. Such inlets  91  and  91 ′ should be directed in a generally tangential direction, with reference to the nearby wall of housing  16 , should face toward or upstream relative to the movement of vapors circulating as a result of stirrer  88  movement. The added areas of the inlet openings  91  and  91 ′ may pull more solvent vapor into the ejectors  30  and  30 ′. 
         [0034]    Steam flows into ejector  30  through pipe  81  and into ejector  30 ′ through pipe  81 ′. The steam flow supplies momentum to any solvent vapor molecules to carry them into space  55 . Ejector  30  outlet  75  is preferably located close to the inlet  72  of main ejector  20 . 
         [0035]      FIG. 2  shows the stirrer  88  in space  87  in the form of a stirring arm rotating circularly around the upper surface of scavenger tray with counterclockwise rotation. The inlet openings  91  and  91 ′ preferably face opposite the direction of stirring arm rotation. 
         [0036]    Movement of stirrers  88  generates a slow counterclockwise rotational movement of the vapors in space  87 . The openings  91  and  91 ′ preferably face against this rotational movement to gather added amounts of solvent vapors for transfer to space  55 . 
         [0037]    The pressure in space  55  in the vicinity of the exit for duct  24  is substantially the same as the pressure within space  87 . Ejectors  30  and  30 ′ transfer some of the solvent vapor in space  87  to space  55 . Main ejector  20  in recirculating vapor from space  55  to space  45   b , also then transfers some of the solvent vapor that previously was in space  87 . 
         [0038]    By placing at least one outlet  75  near inlet  72 , this transfer is enhanced. In any case, some of this vapor that was within space  87  then will flow upwards and exit housing  16  through vent  13 . 
         [0039]    It may be possible to provide more than two of these scavenger ejectors to eliminate any stagnant pockets in space  87  in which heavy hexane settles. But even one of these ejectors  30  and  30 ′ enhances circulation of vapors, allowing their transport by electors  30  and  30 ′ into space  55 , where they may be further transported into the intermediate tray set  49 . 
         [0040]    The motive steam for both ejector  30  and main ejector  20  is almost completely collected by main ejector  20  and forced back to an early stage of set  49  to recycle through unit  10 . Its energy is thus almost completely re-used to vaporize solvent in the meal masses  40 . There is thus little energy cost associated with operating a unit  10  with one or more scavenger ejectors  30  and  30 ′. In fact, considerable energy is saved by the recovery of the steam and solvent in space  87  which would otherwise be lost to the discharge conveyors. 
         [0041]    The net result of these features is to reduce the solvent lost to the environment, to require less fresh solvent to be purchased at processing plants, and to do this with robust, simple, reliable, easy to control, low cost, low energy-consuming apparatus.