Abstract:
A desolventizing system for removing solvent from a quantity of solvent-laden particles such as flakes comprises first and second desolventizer units, each having an inlet port for receiving solvent-laden particles, an outlet port for discharging at least partially desolventized particles, and a solvent vapor port. A solvent trap is connected between the outlet port of the first desolventizer through a first airlock, and to the inlet port of the second desolventizer unit. Particles entering the solvent trap through the first airlock are conveyed to the inlet port of the second desolventizer. The second desolventizer unit has an airlock connected to the outlet port of the second desolventizer unit. The solvent trap has a vent preferably in the upper part of the trap for connection to a vacuum source that maintains a partial vacuum within both the solvent trap and the second desolventizer unit allowing liquid solvent and water permeating particles within the second desolventizer unit to vaporize efficiently at a relatively low temperature. Solvent vaporized in each desolventizer unit can be drawn out through the solvent vapor port of the desolventizer unit for further processing and later reuse.

Description:
This application claims benefit of U.S. Provisional Application 60/329,790 filed Oct. 15, 2001. 

   BACKGROUND OF THE INVENTION 
   Many vegetable grain or seed products such as corn, sunflowers, and soybeans (generally referred to as oilseeds) have a substantial vegetable oil component. Often, this oil is extracted at some point while processing the raw products. The oil itself is a valuable commercial material used in foods, plastics, etc. The solids remaining after extracting the oil are also valuable and can be used for both human and animal foods, as well as for other purposes. 
   Early steps in processing change the form of the raw oilseed product to flakes or other types of particulate material. This particulate material is still permeated with most of the original natural oil. The oil is then extracted from this particulate material. 
   A number of different processes for removing or extracting the oil from this particulate material have been developed. The type of oil removal process of interest here is termed solvent extraction. After the grain has been converted to particles, the particles are immersed in a hydrocarbon fluid solvent such as hexane, heptane, isohexane, or any similar petroleum-based solvent that dissolves the oil. 
   Upon immersing the particles, the solvent forms a liquid solution with the oil in the particles. The oil-solvent solution is then removed from the particles in some manner, usually by simple gravity draining. In gravity draining, a screen supports the particulate material and allows the oil-solvent solution to drain through the screen to a catch basin. The solvent and oil are then separated with a conventional process. Usually, the solvent recovered during this separation step can be used again in the extraction process. 
   After the oil-solvent solution has drained from the particulate material, there is usually still a significant amount of solvent and a smaller amount of oil permeating the particulate material. Where the particulate material will be used as human food or animal feed, it is important for a number of reasons to remove nearly all of the solvent from the particulate material. First, the solvent may be toxic, so removing the solvent from the particulate material prevents harm to whomever or whatever might consume the end product of the process. Secondly, whether the solvent is toxic or not, it may be an air pollutant so it&#39;s important to prevent as much of the solvent as possible from reaching the atmosphere. Third, the solvent is valuable. Extracting it from the particulate material allows its reuse in the oil extraction process. 
   U.S. Pat. No. 5,630,911 (Kratochwill) discloses apparatus and process for removing a substantial amount of the remaining solvent following gravity draining or other type of oil-solvent removal. The Kratochwill apparatus uses, within an enclosed vessel or volume, a number of inclined conveyors that carry the particulate material over heating plates. The particulate material permeated by the solvent still present is heated to vaporize the solvent. This solvent vapor can then be removed from the enclosed space. Some oil remains in the particulate material, but it forms a small percentage of the total mass. Kratochwill is incorporated by reference into this application. 
   One feature of the Kratochwill apparatus is that the process occurs at a temperature high enough to reduce the protein dispersability index (PDI) of particulate material having high protein content. A high PDI is preferred for some processed oilseed materials; for these materials, lower process temperature is an advantage. 
   BRIEF DESCRIPTION OF THE INVENTION 
   I have developed a desolventizing system capable of removing all but a negligible amount of the solvent from a quantity of solvent-laden vegetable flakes or particles. The system includes a first desolventizer unit having an inlet port for receiving solvent-laden particles, an outlet port for discharging at least partially desolventized particles, and a solvent vapor port. The first desolventizer unit heats the particles to vaporize the liquid solvent. A solvent-steam vapor that is almost all solvent results and can be drawn from the solvent vapor port. 
   A first airlock has an inlet port in flow connection to the outlet port of the first desolventizer unit, and an outlet port. The first airlock transports the at least partially desolventized particles from the first airlock&#39;s inlet port to the first airlock&#39;s outlet port while at least partially maintaining any pressure difference existing between the first airlock inlet and outlet ports. 
   A solvent trap has a chamber from which inlet and outlet ports extend upwardly. The trap&#39;s inlet port is connected to receive particles and vapor from the first airlock&#39;s outlet port. The trap&#39;s outlet port is connected to provide particles to the inlet port of the second desolventizer. The trap has a vapor vent preferably located near the trap&#39;s top for connection to a vacuum source. 
   A second desolventizer unit has an inlet port for receiving particles from the solvent trap chamber, an outlet port for discharging finally desolventized particles, and a solvent vapor port, and also heats the particles. The second desolventizer unit is preferably mounted with the inlet port and the solvent vapor port thereof both at an elevation above the solvent trap&#39;s vapor vent to prevent vapor entering the solvent trap from the first airlock, from flowing into the second desolventizer unit. 
   A second airlock has an inlet port connected to the outlet port of the second desolventizer and receives therefrom the finally desolventized particles. The second airlock has an outlet port through which the finally desolventized particles flow. 
   A vacuum source attached to the solvent trap vent will create a vacuum within the second desolventizer unit sufficient to lower the boiling point of any liquid solvent or water remaining in the particles to below the temperature that lowers the PDI excessively. In one version, this vacuum is around −7 psig. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side sketch view of the key elements of the desolventizing system. 
       FIG. 2  is a block diagram of a preferred condenser/pump unit for forming a vacuum in portions of the desolventizing system. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows one version of the invention intended to remove a very high percentage of solvent permeating a quantity of particulate vegetable material of various shapes (hereafter “flakes” or “flaked material” for convenience). The solvent has been added in earlier steps of processing to extract the oil in the flakes. The invention is particularly suited for removing solvent from flakes comprising a substantial amount of protein and whose temperature should be kept below, say 180° F. to allow the protein to retain its high PDI. Solvents having a boiling point below approximately 180° F. at reduced pressure and that in the gaseous state have a higher mass density than air at similar temperature and pressure are suitable for this application. Hexane, isohexane, and heptane are examples of such solvents. 
   Structure 
   The invention uses at least two processing stages, each comprising a desolventizer unit  10  or  11 . Each unit  10  or  11  removes a high percentage of solvent present in flaked material entering the particular unit  10  or  11 . Each of the desolventizer units  10  and  11  in the preferred embodiment is similar in structure to that disclosed in Kratochwill, although different types of desolventizer units may also be used with little reduction in efficiency. Since the description in Kratochwill is completely adequate to explain the structure and operation of desolventizer units  10  and  11 , no detailed discussion of the internal features of units  10  and  11  is deemed necessary. The desolventizer units  10  and  11  each include flake inlet ports  15  or  38  and flake outlet ports  16  or  39 . Solvent-laden flakes of an oilseed such as soybeans fall into the inlet port  15  or  38 . 
   Units  10  and  11  each cause the flakes to traverse a circuitous path from flake inlet port  15  or  38  to flake outlet port  16  or  39 . Units  10  and  11  apply heat to the flakes during this traversal to vaporize most of the solvent and some of the water in the flakes and form a solvent-steam vapor mixture. Drawing off solvent-steam vapor through solvent vents at  13  and  41  removes a substantial percentage of the solvent literally boiling from the flakes as they pass through the respective unit  10  or  11 . At the end of the traversal through each unit  10  or  11 , flakes from which has been removed much of the solvent originally present at the input port  15  or  38  of the respective desolventizer unit  10  or  11  fall through outlet ports  16  or  39 . 
   Airlocks or rotary valves  18 ,  25 , and  28  are devices that are intended to support a partial vacuum within desolventizer unit  2   11 . One form of the airlocks  18 ,  25 , and  28  has a revolving gate much like a revolving door for transporting flakes under the influence of gravity from inlet ports at  18   a ,  25   a ,  28   a  to the corresponding outlet ports  18   b ,  25   b ,  28   b . Airlocks using gates or flaps that provide for transport of the flakes from the inlet port to the outlet port while supporting a pressure difference between inlet and outlet ports are also suitable. This structure allows a pressure differential to exist from the airlock inlet ports at  18   a ,  25   a ,  28   a  to the corresponding airlock outlet ports  18   b ,  25   b ,  28   b.    
   A solvent trap  30  has an inlet port  30   a  that receives both solvent-steam vapor and flakes from desolventizer unit  1   10 . Most of this solvent-steam vapor leaks through or simply accompanies flakes passing through airlock  18 . The solvent-steam vapor is mostly solvent, with a small amount, say 5%, of steam or water vapor. Solvent trap  30  has an outlet port  30   b  through which flakes pass on the way to desolventizer unit  2   11 . A collection chamber  31  at the bottom of solvent trap  30  collects the heavy solvent-steam vapor from desolventizer unit  1   10  where it commingles with the flakes falling from airlock  18 . A solvent trap vent  23  near the top of solvent trap  30  is used to create a partial vacuum within solvent trap  30  and to remove solvent vapors from the system. 
   A flake elevator  32  forms a hermetically sealed conveyor duct from airlock outlet  18   b  and solvent trap input port  30   a  through solvent trap  30  and solvent trap outlet port  30   b  to desolventizer unit  2  inlet port  38 . Flakes fall from airlock  18  to the collection chamber  31  at the bottom of solvent trap  30 . Elevator  32  carries solvent-bearing flakes falling from airlock outlet port  18   b  at the elevator inlet  20  through solvent trap  30  to the elevator outlet  35 , from which flakes fall into inlet port  38  of desolventizer unit  2   11 . Most of the solvent-steam vapor in collection chamber  31  stays in chamber  31  until removed by a condenser/pump unit  21 . 
   Flake elevator  32  may move flakes with a conveyor element comprising a chain  24  (partially shown) having crossbars attached to the chain  24 . The chain  24  and crossbars ascend along the bottom interior surface of elevator  32  carrying the flakes upwards and descend along the top interior surface of elevator  32 . A drive sprocket  29  (driven by a motor not shown) near the elevator outlet  35  moves the chain  24  within elevator  32 . Guide sprockets  26  or channels are located strategically throughout the remainder of elevator  32  to hold the chain in positions that efficiently move flakes from inlet  20  to outlet  35  and prevent descending portions of the chain  24  from catching on ascending portions of chain  24 . Alternatively, an auger may be used in place of chain  24  and the crossbars as the conveyor element. 
   Both inlet port  38  and solvent vents  41  of desolventizer unit  2   11  are preferably located at elevations above the solvent trap vent  23 . This limits migration of the heavy solvent-steam vapor from airlock  18  into desolventizer unit  2   11 . It&#39;s possible however, that by properly arranging the features of solvent trap  30  this elevational relationship may be unnecessary. 
   A condenser/pump unit  21  shown in more detail in  FIG. 2  is attached to vent  23  of solvent trap  30 . A housing  50  contains condenser coils  53  receiving cool water from a pipe  62  and discharging warmed water to pipe  63 . Liquid pump  58 , a steam venturi or other vacuum source connected to vacuum port  56 , and condensing coils  53  all cooperate with airlocks  18  and  25  to form a partial vacuum of around −7.0 psig. within housing  50 . This vacuum is applied through vent  23  to solvent trap  30 , elevator  32 , and desolventizer unit  2   11 . The vacuum maintained within housing  50  draws the gasses within solvent trap  30  across coils  53  where the solvent vapor condenses into liquid solvent flowing in duct  55 . Pump  58  removes the condensed solvent. 
   Operation 
   In operation, a continuous volume of solvent-laden flakes from a previous stage of the solvent extraction process passes through an airlock  12  to the inlet  15  of desolventizer unit  10 . The mixture at this point in one process of interest may be about 32% solvent by weight, with &lt;1% oil and about 13% water. The large amount of solvent may result from a solvent wash to remove as much oil as possible from the flakes. The flake and solvent temperature may be at around 142° F. The solvent and water is substantially in liquid form and completely permeates the flakes. 
   Heat is applied to the volume of flakes as they traverse through desolventizer unit  1   10 , causing the temperature of the solvent-water mixture permeating the flakes to rise from 142° F. to about 160° F. This causes most of the solvent and some of the water to vaporize forming a solvent-steam vapor that fills the volume of desolventizer unit  1   10 . Because the lower boiling point of the solvent, less of the water vaporizes within desolventizer unit  1   10  because the solvent has a lower boiling point than water. This solvent-steam vapor is highly concentrated (about 95%) solvent. 
   As the solvent converts from liquid to vapor within desolventizer unit  1   10  the solvent-steam vapor is drawn by a low vacuum pump (not shown) through the solvent ports  13  as shown by the arrows. The vapor is collected as highly concentrated solvent-steam vapor mixture at duct  17 . The solvent and water can be condensed and separated by equipment not shown and the solvent reused. A lesser amount of the highly concentrated solvent-steam vapor leaks through airlock  18  into solvent trap  30 . 
   At the end of the traverse through desolventizer unit  1   10 , the flakes reach the outlet port  16 . At this point they have a temperature of about 160° F. and contain a liquid comprising about 0.7% solvent and 11% water because most of the original solvent and some of the water has already been vaporized during the traverse through desolventizer unit  1   10  and removed through ports  13 . 
   From outlet port  16 , the flakes fall through the airlock inlet port  18   a , the mechanisms of airlock  18 , the outlet port  18   b  of airlock  18 , and the inlet port  30   a  of trap  30  to reach the partial vacuum of solvent trap  30  and the elevator inlet  20 . Because of unavoidable leakage of solvent-steam vapor from desolventizer unit  1   10  through airlock  18 , some of the highly concentrated solvent-steam vapor mixture reaches trap  30 . The relatively high specific gravity of the highly concentrated solvent-steam vapor mixture causes this vapor mixture to collect in the collection chamber  31  of trap  30  to approximately the level shown as dotted interface line  27 . 
   The flakes falling onto elevator chain  24  are transported through the outlet port  30   b  of solvent trap  30  to elevator outlet  35 , from where they fall through inlet port  38  into desolventizer unit  2   11 . Upon entering unit desolventizer  2   11 , the flakes undergo another stage of desolventizing at the partial vacuum of −7 psig. within unit  2   11 . This partial vacuum lowers the boiling point of both the liquid solvent and the water within desolventizer unit  2   11 . The solvent and water within desolventizer unit  2   11  thus vaporizes at a lower temperature than when at atmospheric pressure. Accordingly, the temperature within desolventizer unit  2   11  can be kept lower, thereby avoiding reduction of the flake PDI. 
   During traverse of the flakes through desolventizer unit  2   11 , the flake temperature is maintained at about 160° F. At −7 psig., the boiling point of water is around 180° F. and the boiling point of the hexane solvent involved here is around 135° F. At the flake temperature of 160° F., most of the remaining solvent vaporizes, and much of the water evaporates as well. The atmosphere within desolventizer unit  2   11  reaches a state of approximately 30% hexane solvent, 60% steam, and 10% air. I find that a certain amount of air enters through leaks in airlocks  25  and  28 . 
   The solvent-steam vapor forming in desolventizer unit  2   11  is continuously drawn off through solvent vapor ports  41  and supplied to solvent trap  30  through duct  47 , although less efficiently, this vapor can also be handled in other ways. The vapor from unit  2   11  has a much lower concentration of solvent than does the vapor from unit  1   10  within trap  30  because of the efficiency of desolventizer unit  1   10 . Hence the unit  2   11  vapor has a much lower specific gravity than does the heavier solvent-steam vapor from desolventizer unit  1   10 , which tends to collect at the bottom of trap  30 . A representative interface line between these two vapor mixtures is shown at  27 . 
   The continuous removal of vapor from trap  30  by condenser/pump unit  21  keeps the vapors in trap  30  from accreting to the point where unit  1   10  vapor can reach unit  2   11  either through elevator  32  or by backflow through duct  47  and vapor ports  41 . 
   One possible advantageous modification to desolventizer unit  2   11  is injecting a sparging gas. A sparging gas inlet port  42  may be connected to a source of sparging gas such as steam (preferably) or nitrogen. A small amount of injected sparging gas may increase the amount of solvent vapor swept from the flakes. The sparging gas along with solvent vapor flows into solvent vapor ports  41 , and from there enters solvent trap  30 . I prefer to locate the sparging gas inlet port  42  on the side of desolventizer unit  2   11  opposite the side at which solvent vapor ports  41  are located. 
   At the end of the traverse of flakes through unit  2   11 , the flakes fall in order, through airlock  25 , vacuum chamber  37  and airlock  28 , to a flake-cooling step in the processing. It may be desirable to hold vacuum chamber  37  with a vacuum source not shown, at a vacuum approximating that within desolventizer unit  2   11 , thereby reducing the amount of air leakage into desolventizer unit  2   11 . 
   At the flake exit point at the outlet port  28   b , the PDI level is still quite high due to the low processing temperature within desolventizer unit  2   11 . Due to the efficiency at which desolventizer unit  2   11  operates, the solvent level remaining in the flakes has been reduced to approximately 300 ppm at the airlock outlet  28   b.    
   The condenser/pump unit  21  is shown in greater detail in  FIG. 2 . Vapors removed from trap vent  23  enter condenser housing  50  through condenser inlet port  51  and flow over cooling coils  53 . Cooling water enters coils through port  62  and exits, warmed somewhat, at port  63 . The solvent vapor entering at port  51  condenses on coils  53  and drips into outlet port  52 . 
   The condensed solvent and water are drawn from condenser housing  50  through duct  55  by pump  58 . Pump  58  returns the low-pressure liquid (mainly solvent) in duct  55  to atmospheric pressure. This liquid is now more or less at room temperature. Water vapor and air along with trace amounts of solvent vapor within housing  50  are drawn through port  56  by the vacuum source. If desired, further processing can remove more of the water from the condensed solvent in duct  55 . At any rate, the solvent condensate can be reused in the process, thereby at least partially avoiding the need to dispose of the used solvent and improving the environmental friendliness of the process. Similarly, the vapor exiting vacuum port  56  can either be subjected to further processing to remove any solvent still present, or if the solvent level is negligible, can be discharged safely and legally to the atmosphere. 
   In one version of this process, the vacuum source comprises a steam venturi unit operating according to well-known principles. Such a vacuum source is advantageous because no moving parts are required to generate the vacuum, adding reliability to the process. A mechanical pump or fan may also be used as the vacuum source.