Abstract:
A system for removing from a particle mass a liquid permeating the particle mass includes a vaporizing stage having a space wherein the pressure is less than the pressure of the particle mass. As the particle mass enters the vaporizing stage, the lower pressure causes much of the liquid to vaporize. Once vaporized, the vapor can be removed with a pump. A first stripping stage receives the particle mass from the first stage at a first particle inlet port and discharges the particle mass at a first particle outlet port. A first stripping gas inlet near the first particle outlet port of the first stripping stage injects an inert stripping gas into the particle mass. The inert gas mixes with remaining elements of the liquid and any entrained gas formed by the liquid, and the mixture is discharged at a gas outlet near the first particle outlet port. A second stripping stage having construction and operation different from the first stripping stage may receive the particle mass. In a preferred embodiment, at least one of the stripping stages transports the particle mass through gravitational force.

Description:
[0001]    This is a regular application filed under 35 U.S.C. § 1 l(a) claiming priority under 35 U.S.C. §119(e)(1), of provisional application Serial No. 60/341,440, having a filing date of Oct. 30, 2001, which was filed pursuant to 35 U.S.C. §111(b). 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Many oilseed grain products such as corn, sunflowers, and soybeans, and other types of vegetable products such as cocoa (referred to hereafter generally as products), have a substantial vegetable oil component. Often, this oil is extracted at some point while processing the raw products. The oil itself is often 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. The process to be described was developed to form a part of a process for extracting cocoa oil from raw cocoa, but may be used in other vegetable oil extraction processes as well.  
           [0003]    Early steps in the processing grind or otherwise change the form of the raw product to flakes, powder, 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.  
           [0004]    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 raw product has been converted to particles, the particles are immersed in a hydrocarbon liquid solvent such as hexane, heptane, isohexane, butane, or any similar petroleum-based solvent that dissolves the oil.  
           [0005]    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, by for example, pressing or even 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.  
           [0006]    When extracting oil from certain kinds of products, such as flaked or ground cocoa, one process uses butane in a pressurized chamber to dissolve the oil. When pressurized at room temperature to perhaps 3.5 bars (50 psi.), butane is a liquid. At one atmosphere and room temperature, butane is a gas, well known as the fuel for backyard grills around the country. It is convenient for this process that the solvent (butane, e.g.) be a liquid at moderate pressure and room temperature, and a gas at room pressure, but the process can be used, less conveniently, with solvents other than butane that liquefy at different pressures or temperatures. Whatever solvent one chooses should not liquefy at a pressure or temperature that may change the properties of the product particles in an undesirable way. The solvent will be usually referred to hereafter as butane, but the processes should be understood to operate with a number of solvents that dissolve the product oil and have a liquid-gas phase change compatible with room temperature and pressure.  
           [0007]    The pressurized butane solvent liquid forms a solution with the oil in the product, which can be drained from the flaked or ground product. Several stages of pressurized solvent extraction may be used to remove nearly all of the oil from the product particles. Depressurizing the butane-oil solution obtained in each stage boils off the butane which can then be reclaimed. The remaining oil can be used as a food constituent or for other purposes.  
           [0008]    After the oil-solvent solution has been drained from the cocoa particles in the last stage, there is usually a significant amount of solvent still permeating the cocoa particles, perhaps 30% by weight, and a trace amount of oil. 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.  
           [0009]    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.  
           [0010]    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.  
           [0011]    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  
         [0012]    A system for removing from a particle stream, a liquid such as a solvent that permeating the particle stream has at least two stages. The system includes a first vaporizing stage having a chamber where the pressure is maintained lower than the pressure of the entering particle mass. As the particles enter the chamber of the vaporizing stage, the lower pressure causes much of the liquid to vaporize. A pump removes the vapor, thereby maintaining the lower pressure in the vaporizing stage chamber.  
           [0013]    A first stripping stage receives the particles from the first stage at a first particle inlet port and discharges the particles at a first particle outlet port. A first stripping gas inlet near the first particle outlet port injects an inert stripping gas into the particles. The inert gas mixes with remaining elements of the liquid and any entrained gas formed by the liquid, and the mixture is discharged at a gas outlet near the first particle outlet port.  
           [0014]    A second stripping stage may also be present to receive the particle mass from the first stripping stage. The second stripping stage may have a construction different from the first stripping stage. In one embodiment, at least one of the stripping stages, preferably the second, transports the particle mass through gravitational force.  
           [0015]    In one embodiment, the second stripping stage comprises a fluid removal chamber having a cylinder to be mounted in an approximately upright position. The cylinder has an enclosed passage from an upper opening to a lower opening. The cylinder has adjacent to the lower opening, a gas inlet into which an inert gas such as nitrogen can be introduced. We use the term “cylinder” here to mean any sort of hollow chamber having a cross section approximately constant along its axis. The cross section is often circular, but can also be square or other convenient shape. We intend the term “cylinder” to include chambers whose cross section varies somewhat along the axis, say where the chamber cross section tapers to become smaller toward the lower opening.  
           [0016]    A particle outlet port forms a part of the lower opening of the cylinder. The particle outlet port regulates flow of particles from the cylinder at a predetermined flow rate. A source of pressurized inert gas is to be connected to provide pressurized gas to the gas inlet. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 shows a two dimensional diagram of a system built according to the invention, for removing a liquid such as a solvent from a stream of particles.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    [0018]FIG. 1 shows a system for removing a liquid such as a solvent permeating a mass of product particles, typically flowing in a stream. The system employs first, second, and third stages  10 ,  20  and  30  to progressively remove a percentage of a liquid remaining in a mass of product particles such as cocoa particles. Very little of the liquid remains in the particles at the end of the third stage. The description following is for a system intended to remove a liquid such as pressurized butane or other solvent from a stream  48  comprising cocoa particles, but similar systems can be used to remove other types of liquids from other types of particles.  
         [0019]    The first stage  10  uses a vaporizing process to remove a large amount of the solvent initially present in the particle stream  48 . A pump  42  constantly removes this vaporized solvent.  
         [0020]    The second and third stages each use a stripping process to remove remaining solvent carried in the particle stream  48 . Stripping is carried out by forcing an inert or other gas through a chamber substantially filled with a part of the particle stream  48 . The inert gas mixes with and if necessary vaporizes much of the remaining solvent, and sweeps the solvent vapor from the particle stream  48 .  
         [0021]    Structure  
         [0022]    The particle stream  48  enters inlet port  53  of a rotary valve or airlock  65 . Stream  48  flows from earlier process steps operating at a high pressure relative to the pressure in stages  10 ,  20  and  30 . As mentioned in the Background section, this pressure may be in the range of 50 psi. at room temperature. These earlier steps have removed most of the oil in the particles but have left a relatively large amount of liquid solvent permeating the particle stream  48 . One set of earlier process steps provides a particle stream  48  that is 30% liquid solvent by weight, but for desolventizing, this system may operate with varying concentrations of solvent.  
         [0023]    Rotary valve  65  controls the flow rate of particle stream  48  from port  53  to an inlet port  62  of a first stage chamber  15 . Valve  65  is a conventional device having a number of radially extending walls defining pie-shaped chambers between them. A central shaft is attached to the inner edges of the radial walls and in use is rotated as shown by the arrow, by a motor not shown. As the valve  65  rotates, particles in stream  48  fall into the individual chambers of valve  65  and are conveyed to inlet port  62 . The particles passing through valve  65  fall through port  62  to the bottom of chamber  15  and form a particle mass  66 .  
         [0024]    Valve  65  opposes leakage of fluids and particles in stream  48  from inlet port  53  to inlet port  62 , other than fluid and particles carried in the chambers of valve  65 . The speed at which valve  65  rotates and the size of the individual chambers control the rate of particle flow into chamber  15 . One finds that a certain amount of leakage of at least fluids through these rotary valves is usual.  
         [0025]    An auger  21  carried on shaft  30  is mounted near the floor of chamber  15  and is rotated by a motor  68 . The floor of chamber  15  may be shaped to cooperate with auger  21 . As auger  21  rotates, particles forming particle mass  66  are simultaneously agitated and conveyed or transported toward an outlet port  33  of chamber  15 .  
         [0026]    Auger  21  may have a pitch and rotational speed to provide a transport time for particle mass  66  from the inlet port  62  to the outlet port  33  of around two minutes. The optimal transport time will vary depending on a number of factors, such as the type of material comprising particle mass  66 , depth of particle mass  66 , type of solvent, and size and shape of particles in particle mass  66 .  
         [0027]    A pump  42  draws gasses and vapors that form within chamber  15  through evaporation of solvent through a hood  56  and a filter  50  to maintain the pressure within chamber  15  substantially lower than the internal pressure level of the particle stream  48 . The chamber  15  pressure should be held low enough to allow the liquid solvent in the particle stream  48  to vaporize within chamber  15  at a temperature that is easy to maintain. As this liquid solvent entrained in particle mass  66  vaporizes, pressure increases within chamber  15 . Arrows  60  symbolize vapor removal from chamber  15  by pump  42  at a rate allowing much of the liquid solvent permeating particle stream  66  to vaporize during the transport of the particles in particle mass  66 .  
         [0028]    Pump  42  compresses the vaporized solvent and forces the solvent into tank  45  through pipe  44 . Compressing the solvent vapor heats it, and cooling the solvent as it flows through pipe  44  reliquefies it. In most cases, pipe  44  should be a heat exchanger of some type to remove the heat from the solvent as it passes through pipe  44 .  
         [0029]    Particles from mass  66  may disperse into dust suspended in the gasses occupying the space  17  above particle mass  66  due to the impact of the falling particle stream at port  62 , the boiling off of the solvent from the particle mass  66 , and the agitation by auger  21 . Filter  50  is provided to keep these particles from reaching pump  42 .  
         [0030]    A heat source, a part of which is shown generically as a heating element  58 , supplies heat to chamber  17  to replace the heat taken up by the vaporizing solvent. In one version, auger  21  may be heated by the heat source as well as chamber  17 . One way to heat auger  21  is by carrying auger  21  on a hollow shaft  30  as shown through which hot fluid  31  of some type is pumped.  
         [0031]    As auger  21  rotates, particles in mass  66  are propelled toward an outlet port  33  of chamber  15 , where they fall in a cascade  68  onto a second rotary valve  71 . At outlet port  33 , we find that in one design for stage  10  the particle stream  66  comprises perhaps 0.5% solvent by weight, the other 29.5% having boiled off within chamber  15 . However, the remaining 0.5% solvent in the cascade  68  of particles still poses somewhat of a problem for some uses of the particles.  
         [0032]    A first stripping chamber  35  provides a second stage of solvent removal. Particles in cascade  68  are carried through rotary valve  71  to an inlet port  72  of chamber  35 . An auger  83  rotated by motor  74  transports the particle mass  80  to an outlet port  87 , where the particle mass then falls by gravity through an inlet port  87  into a second stripping chamber  91 . An optional inert stripping gas inlet  77  may be provided near the outlet of chamber  35 . A suitable transport time for particles from inlet port  72  of chamber  35  to inlet port  87  of second stripping chamber  91  might be around 30 sec.  
         [0033]    Second stripping chamber  91  serves as a third stage of solvent removal. Chamber  91  comprises an upright cylinder relatively tall compared to its width. During operation, a particle mass  95  formed from stream  48  at least partly fill chamber  91 .  
         [0034]    A sleeve or hopper  98  encloses the bottom opening of chamber  91 , forming an annular opening  96  around the periphery of the lower end of chamber  91 . A plenum  108  seals the interior of sleeve  98  against the outer surface of chamber  91 . The gas inlet  90  near the top of sleeve  98  is to be connected to a pressurized source of a stripping gas. The seal between sleeve  98  and the outer surface of chamber  91  causes the inert gas to flow through the annular opening  96  into the particle mass  95 .  
         [0035]    In FIG. 1, the stripping gas is shown as nitrogen (N 2 ), but many other inert or even chemically active gasses may also be used, depending on the particular fluid permeating particle mass  95 . Nitrogen is simply cheaper than other gasses, which is why we prefer it. The overlap between the bottom of chamber  91  and sleeve  98  prevents particle mass  95  from covering or sealing gas inlet  90 . Pressurized stripping gas then flows as arrows  100  indicate through annular opening  96  into and through particle mass  95 .  
         [0036]    Sleeve  98  is shown with a cross section area reducing or tapered toward the bottom end. In one embodiment, this taper is sufficient to limit the flow of particle mass  95  through hopper  98 . By restricting particle flow rate through sleeve  98 , the height of particle mass  95  within chamber  91  stays at a substantial percentage of the total chamber height.  
         [0037]    A particle flow control device of some kind, for example the rotary valve  101  shown, can be used to first of all, impound sufficient particles to form the particle mass  95  column within chamber  91 , and then to control flow rate of particle mass  95  from sleeve  98 . By varying the speed of rotation, valve  101  the particle volumetric flow rate can be adjusted. The volume flow rate for the particle stream  106  should, once steady state is reached, nearly equal the volume flow rate of particles at inlet port  87 . Some simple flow control mechanism may be required to maintain a suitable height and flow rate for particle mass  95 .  
         [0038]    A transport time through third stage  30  on the order of a minute will strip a high percentage of the remaining solvent from mass  95 . Using this criterion, to process about 130 tons/day (180 lb./min.) of a particle mass whose density is 30 lb./ft. 3 , we suggest the following parameters for chamber  91  operation:  
                                                           Flow rate of particle mass 95 downward   0.1   ft./sec.           Cross section area of chamber 91   1.0   ft. 2             Upward flow velocity of stripping gas   0.3   ft./sec.           Height of particle mass column in chamber 91   6.0   ft.                      
 
         [0039]    Parameters for first and second stages  10 ,  20  can be easily derived to match these given for chamber  91 .  
         [0040]    Cocoa particles have a density of around 25-30 lb./ft. 3 . Soybean meal density may be somewhat higher, perhaps 35 lb./ft. 3 .  
         [0041]    Explanation  
         [0042]    Particle stream  48  having a relatively high pressure atmosphere passes through valve  65  to enter the relatively low pressure within first chamber  15 . The lower pressure causes the entrained solvent to vaporize. Pump  42  removes the solvent vapor at a rate that maintains the pressure within chamber  15  at a level allowing continuous vaporization of the solvent. For a butane solvent entering inlet port  53  at 50 psi., pressure within chamber  15  may be held at approximately 15 psi. At the same time, the heating source  58  and the hot fluid  31  flowing through auger shaft  30  keep the particle mass  66  warm enough to support the boiling or vaporazation of solvent entrained in particle mass  66 . Since the vaporization occurs at a relatively low temperature, the characteristics of mass  66  are not changed.  
         [0043]    Motor  68  rotates auger  21  and shaft  30 , transporting elements of particle mass  66  toward outlet port  30 , and at the same time stirring particle mass  66  so as to aid vaporizing of solvent entrained in particle mass  66 . By the time each element of particle mass  66  reaches the outlet port  33 , much of the solvent initially entrained in that element has boiled off or vaporized. Speed of motor  68  may be such that the total transit time for most elements of mass  66  to cascade  68  from inlet port  62  is approximately two minutes. The depth of the mass  66  should not extend much above auger  21  as shown to assure thorough agitation and stirring of the mass  66  while moving toward outlet port  33 .  
         [0044]    For cocoa particles initially 30% solvent at inlet port  62 , the solvent concentration may be reduced to about 0.5% at outlet port  33 . However, this concentration may still be higher than desired for some particles composed of some types of materials.  
         [0045]    Particles flow through second rotary valve  71  and enter second chamber  35  through inlet port  72 . A second auger  83  transports particles toward inlet port  87  of third chamber  91  where they fall to become part of particle mass  95 . Elements of particle mass  95  continuously flow through the bottom end of chamber  91  as stream  106 .  
         [0046]    During this time, pressurized stripping gas is introduced through inlet  90 . This pressurized stripping gas flows or percolates first downwards through the annular opening  96  and then upwards through the particle mass  95 , all as indicated by the dashed arrows  100 . This flow of stripping gas sweeps almost all of the remaining solvent from particle mass  95 . The solvent remaining in the particle mass  95  is displaced by the inert gas.  
         [0047]    The flow velocity of the inert gas should not be so great as to cause the particle mass  95  column to fluidize, where particles are actually lifted from the top of particle mass  95 . For typical vegetable-type particles, this means that the inert gas flow rate within the mass  95  should be less than about 2 ft./sec. At the same time, the flow velocity of the inert gas must be greater than the velocity downwards of the particle mass  95 , so that the inert gas is continuously exiting from inlet port  87 . The previously suggested flow velocities for the inert gas and particle mass  95  satisfy these requirements.  
         [0048]    The pressure difference between particle inlet port  87  and gas inlet  90  affects the speed of gas flow rate in mass  95 . The gas pressure at particle inlet port  87  is controlled by the pressure drop through second stage  20 , the inert gas flow rate, and the pressure maintained at the gas outlet  38 . Pressure drops through second and third stages  20 ,  30  are typically a few tenths of one psi. If outlet  38  flows directly to the atmosphere, the pressure at inlet  90  can be in the range of 0.5-1.0 psi.  
         [0049]    The inert gas exits chamber  95  through particle inlet port  87  and continues to flow backwards through chamber  35  toward the inlet port  72  of chamber  35 . Clearance between auger  83  and the inner wall of chamber  35  should be sufficient to allow this flow. Auger  83  constantly agitates and shifts particles within chamber  35 , exposing individual particles in particle mass  80  to the flow of inert gas. The inert gas at inlet  87  has very little solvent gas mixed with it, since most of the solvent gas entrained in the particle mass at inlet  72  has already been swept from the particles during their transport through chamber  35 . Therefore, the inert gas counterflowing through chamber  35  can still remove a large percentage of the solvent present in the particles within chamber  35  without adding further levels of stripping gas.  
         [0050]    Solvent gas outlet  38  near the particle inlet port  72  allows the inert gas sweeping through chamber  35  to leave chamber  35 . Since the amount of solvent in the particles within chamber  35  is actually quite low, the gasses exiting from outlet  38  it is usually quite safe to allowed their flow into the atmosphere. If not safe, it is easy to impound these gasses as done for stage  10  and remove any solvent vapor still in them. Using the same inert gas flow to strip solvent from both chamber  35  and chamber  91  reduces the amount of inert gas needed.  
         [0051]    In some circumstances, the amount of gas provided at inlet  90  is not adequate to properly strip the solvent from the particles under transport in chamber  35 . We show an optional inert gas inlet  77  for second stage  20 . The gas at inlet  77  should be at a pressure somewhat less than the pressure at inlet  90  so as to assure that a constant reverse flow of inert gas through chamber  91  is present.  
         [0052]    We find that for a particle mass comprising cocoa and a solvent such as butane, this three stage process can take particles having an initial 30% solvent concentration, and reduce the amount of solvent to perhaps one part in 100,000. Such a level is very likely to satisfy the most stringent requirements for solvent removal in human food.  
         [0053]    The use of an auger  83  to transport particle mass  80  in an angled second stage  20  allows both first and third stages  10  and  30  to be located conveniently close to the ground. However, some variation in the selection of stripping stages  20  and  30  is possible. An auger can be used in third stage  30  rather than a columnar type of cylinder  91 . It might even be possible to use two successive columnar cylinders as second and third stages  20 ,  30 , but this would require locating stage  10  inconveniently far above the ground, or stage  30  below the ground.