Abstract:
The present invention relates to an improved apparatus and method for conveying fibrous, solid and slurry materials, such as granulated wood, rice hulls, chopped cane and the like, to a pressurized vessel, wherein the material being conveyed is compacted in the feeder in a controlled manner to create a seal at the feeder exit into the pressurized vessel whereby the processing pressure in the vessel is maintained. The invention is particularly useful when used in conjunction with a biomass reactor for the production of gas selectively rich in hydrogen and carbon containing components, such as carbon monoxide, carbon dioxide and methane, which in turn, may be converted into a select end product fuel, such as methanol or ethanol or used as a feed gas for an industrial power plant, such a the biomass reactor for producing gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of Provisional Application for Patent, Ser. No. 60/951,198. filed Jul. 21, 2007, which application is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Funding from Department of Energy under grant DE-FG36-02GO12025 was received in conjunction with the development of this technology. 
     
    
     REFERENCE TO A “MICROFICHE APPENDIX” 
       [0003]    Not applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    The present invention relates to an improved apparatus and method for conveying fibrous, solid and slurry materials, such as granulated wood, rice hulls, chopped cane and the like, to a pressurized vessel. The material being conveyed is compacted in the feeder in a controlled manner to create a seal at the feeder exit to the pressurized vessel which maintains the pressure in the vessel. The invention is particularly useful when used in conjunction with a biomass reactor for the production of gas selectively rich in hydrogen and carbon containing components, such as carbon monoxide, carbon dioxide and methane, which in turn, may be converted into a select end product fuel, such as methanol or ethanol or used as a feed gas for an industrial power plant, such as the biomass reactor for producing gas described in U.S. Pat. No. 6,767,375 owned by the inventor of the present invention. 
         [0005]    Gasification of biomass material such as wood, woodchips, sawdust, wood charcoal, rice, sugar cane hulls and other particulate cellulosic materials has become of increasing interest and importance because of the volatility of petroleum prices, dwindling of fossil fuels, such as domestic petroleum and natural gas resources and the increased dependence of the United States on international imports of these fuels. Gasification of coal and biomass has been practiced for over one hundred years and there are many varieties and types of gasifiers and methods of gasification. One method of gasification applicable to biomass material is pyrolysis. Pyrolysis is the breakdown of the biomass by heat at elevated temperatures (e.g., about 400 to about 1200 degrees Fahrenheit) to yield an intermediate gas which is ultimately transformed into a market fuel (gas or liquid such as methane or ethanol). Inclusion of a transport gas, such as oxygen or steam, during the pyrolysis, assists in the production of an intermediate gas containing carbon monoxide, carbon dioxide and hydrogen, useful in later conversion into fuel such as ethanol, methanol, ammonia or methane. Similarly, other gas additions, such as air or nitrogen, may be used for synthesis gas having other makeup required for different end products. 
         [0006]    As mentioned, the present invention is particularly useful when used in conjunction with the biomass reactor for producing gas as described in U.S. Pat. No. 6,767,375 (“&#39;375 patent”) owned by the inventor of the present invention. The &#39;375 patent provides an improved method and apparatus for producing a synthesis gas from a biomass feed material. In one aspect, the &#39;375 patent incorporates a reactor vessel heated, at least in part, by a heat source such as an electric or natural gas heating unit. The reactor vessel generally includes a helical coil or conduit of many turns utilized for carrying the biomass feed material and an appropriate transport gas, throughout which the pyrolytic process is performed. Some embodiments of the helical coil may have a cooling system associated with at least a portion of a support system interconnecting the helical coil with the reactor vessel. 
         [0007]    The many turns of this helical coil may be disposed in the vessel in a number of appropriate locations, but are preferably disposed adjacent a sidewall of the reactor vessel. This preferred arrangement of the coil relative to the reactor vessel may be said to provide an air gap between the coil and the vessel sufficient to produce convective heating. The coil generally receives a feed of the biomass material, preferably in ground or granulated form, which is mixed and transported through the reactor coil utilizing the transport gas. In some embodiments, the transport gas may provide heat and/or chemical support to the pyrolysis process in addition to the externally supplied heat that is utilized to transform the biomass material into a target synthesis gas in the reactor coil. The rate of and control over the pyrolysis process in the reactor coil are preferably effected by the inclusion of separated radiant and convective heat zones in the reactor vessel. These heat zones, at least in one embodiment, may generally be determined by the location of a heat shield disposed in the vessel. This heat shield may exhibit any of a number of appropriate designs. For instance, in one preferred embodiment, the heat shield includes an at least generally cylindrical section. Moreover, this heat shield may be disposed in any effective location relative to the coil. It is, however, preferred that the heat shield be located at least generally, concentrically of the coil. Further, it is also generally preferred that this heat shield be located in an upper region of the vessel above the heat source. The heat shield preferably includes a truncated conical section disposed toward a bottom of the heat shield (closed at an end nearest the heat source) to better establish transition between the radiant and convective heat zones and to facilitate convective heating in the respective zone. 
         [0008]    Preferably, the reactor vessel includes a pressurized mixing vessel in which the biomass feed material is collected, mixed and supplied to the reactor coil. This pressurized vessel may include a number of appropriate mechanisms capable of maintaining a seal against a loss of operating pressure within the mixing chamber while promoting the biomass feed material and/or the transport gas to pass therethrough. 
         [0009]    It is desirable to mix the transport gas and biomass feed together in a pressurized vessel before they enter the reactor. A transport gas utilized to mix and transport the biomass feed and carry it to the reactor is input into the pressurized vessel. The biomass feed materials are generally added to the pressurized vessel at atmospheric pressure, or at some lower pressure than that of the mixer; thus, there would be backflow of the transport gas through the feeding mechanism and into the vessel containing the feed material unless there is an adequate seal between the vessels. 
         [0010]    In the &#39;375 patent, the material is introduced into the pressurized vessel from a hopper through a veined rotary valve. The hopper contains bulk raw material which is supplied to the rotary valve by means of a conventional metering rotary valve feeding the amount of biomass feed material to the pressurized vessel preferably in a manner and/or at a rate sufficient for a particular gas output. In order to ensure that no significant amount of build up of biomass feed material occurs in the hopper at the rotary valve, the rotary valve is preferably operated at a higher RPM than the metering valve. 
         [0011]    As noted, one function of the rotary valve is to at least generally seal the interior of the pressurized vessel to the atmosphere. This generally helps maintain gas pressure within the system as well as promote the pressurized feed of mixed biomass and transport gas traveling from the pressurized vessel to the reactor. The biomass feed is introduced into the pressurized vessel by means of a rotary valve which may be rotated utilizing any appropriate means, such as an electric motor. The rotary valve may be said to facilitate the supply of material in being moved into the lower portion of the drop tube and toward the bottom of the pressurized vessel. Incidentally, the metering valve and the rotary valve are interconnected by an upper portion as the drop tube. The drop tube may be said to contain the biomass feed as a transit to and from the rotary valve. What may be characterized as a mating of veins with side walls of the rotary valve is such that a seal against back pressure is at least generally provided thereby between the lower and upper portions respectively of the drop tube. This seal of sorts may be said to assist in maintaining the pressure of the incoming transport gas to prevent over heating of the rotary valve. 
         [0012]    Due to the nature of the feed materials, they may lump together and flow non-uniformly into the mixer. This non-uniform flow can clog the feeding mechanism and create problems maintaining pressure in the mixer. The &#39;375 patent system utilizes a rotary vane valve as a feeder which has several issues. One issue with that design is pocket plugging. The feed material initially falls into the pockets of the valve. This material is compressed as the pressure from the system enters the pockets. Moisture in the material or from the system transport gas causes the material to stick in the pocket. The material does not exit the pocket at the discharge position, but is carried around to the feed inlet position where less material can enter the pocket. This continues until the pocket is full of material and the feed system is completely plugged. 
         [0013]    Temperature variations also cause problems for the rotary valve feeder. The heat from the transport gas is transferred to the rotary valve feeder which causes expansion and contraction. The lower portions of the rotary valve are exposed to greater heat than the lower portions; therefore, the expansion is not uniform throughout the valve. The seal created by the rotary valve is dependent on a very close tolerance between the rotating vanes and the rotary valve body. Temperature variations in the rotary valve components change these tolerances and cause either loss of pressure and leakage (if the tolerance is increased) or damage to the valve and seizing of the valve (if the tolerance decreases). 
         [0014]    When using the rotary valve feeder, after the material is added to the pocket that pocket is pressurized before the material is deposited in the pressurized vessel. If this step is skipped, the pressure in the pressurized vessel may prevent the material from exiting the pocket at the valve outlet. Another issue, is that if the pressurized vessel contains moist gas, that gas seeps into the pocket adding moisture to the feed material and possibly causing clumping to occur. A inert gas such as nitrogen is typically used to pressurize the pocket. Nitrogen dilutes the transport gas and requires larger system components to handle the required amount of transport gas plus the additional inert gas. The present invention eliminates the need to pressurize the feeder. 
         [0015]    After discharging the material, the rotary valve pockets fill with pressurized transport gas. The pressurized transport gas must be evacuated before the valve pocket returns to the feed inlet position. If not, the transport gas will discharge into the feed inlet area and a smaller amount of feed material or none at all will be able to enter the rotary valve pocket. The discharge of transport gas containing moisture at the feed inlet introduces moisture into the feed material and may cause plugging at the feed inlet. 
         [0016]    The present invention is an improvement over the &#39;375 patent feed system and allows for conveying fibrous solid and slurry materials, such as biomass materials, in a uniform matter. It utilizes the material itself to maintain the seal, thus allowing a vessel, such as a mixer, to maintain its pressure while material is being added to it. The present invention solves the challenges listed above by blocking flow of the transport gas from the pressurized vessel into the feeder with the material plug. In the present invention, the material is only exposed to the transport gas beyond the conveying means at the point where it is discharged, so there is no potential for moist feed material to plug the conveying means. Likewise, a close tolerance between rotating and stationary parts is not critical to maintain a seal. Additionally, the moving parts do not rotate past stationary parts which are at different temperatures as with the rotary valve. Thus, the challenges presented by the temperature variations and the rotary valve are eliminated. The conveying mechanism of the present invention does not contact the transport gas thus the exhaust cycle for removing trapped transport gas from the empty rotary valve pocket is eliminated. 
         [0017]    There are numerous commercial examples of auger systems such as those which feed plastic pellets into injection molding machines. In those systems, the plastic pellets melt and flow uniformly into the auger which transports the plastic into the injection molding machine. The present invention differs from those systems because it is able to handle many different compositions of fibrous solid and slurry materials which otherwise tend to flow non-uniformly and provides a uniform flow of material into a pressurized vessel. 
       BRIEF SUMMARY OF THE INVENTION 
       [0018]    The present invention provides an improved apparatus and method for conveying fibrous solid and slurry materials into a pressurized vessel while maintaining the pressure in that vessel by utilizing the material to create a seal. 
         [0019]    One use of the present invention is to feed material into a flowing gas stream at a pressure. Another use of the present invention is to feed material into a vessel, grinder, processing machine or the like which is at a pressure. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a side view of the conveying apparatus including a cross section of the discharge housing. 
           [0021]      FIG. 2  is a side view of the conveying apparatus including a cross section of the discharge housing with a cut away showing the auger. 
           [0022]      FIG. 3  is a side, cross sectional view of the discharge housing showing the conical end being forced out of the material retention chamber. 
           [0023]      FIG. 4  is a side, cross sectional view of the discharge housing showing the material retention chamber open and material exiting the material retention chamber. 
           [0024]      FIG. 5  is a side, cross sectional view of the discharge housing showing the material retention chamber closed off by the conical end. 
           [0025]      FIG. 6  is a perspective view of the conveying apparatus as attached to the pressurized vessel. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Referring to  FIG. 1 , the present invention includes an auger  4  enclosed in a pipe  1  with an inlet  5  and an outlet  10  for transporting biomass material. ( FIG. 2  shows the auger  4 ) The auger  4  is powered by a drive motor  6  (See  FIG. 6 ). The pipe  1  and auger  4  are made of any durable material such as carbon steel or stainless steel. The abrasiveness and hardness of the material should be considered by those skilled in the art when determining the particular materials of construction of the pipe  1  and auger  4 . Because the auger  4  is forcing the biomass material against the interior walls of the pipe  1 , those more highly abrasive materials may cause considerable frictional wear on the interior walls of pipe  1  and auger  4 . Those skilled in the art should recognize the higher the speed of auger  4 , the greater the likely wear on the pipe  1  and auger  4 . In embodiments where the material is highly abrasive, the speed of auger  4  (and thus the material feed speed) should be relatively minimized to reduce wear to a tolerable level. In one embodiment, the material is a fibrous, biomass feed, such as saw dust—a byproduct of processing lumber for use in furniture production or as building material at commercial sawmills, but the material could be any solid or slurry biomass material. 
         [0027]    The inlet  5  is connected to a metering device (not shown) which controls the feed rate of the material to be conveyed by the auger  4  to the pressurized vessel  2 . The metering device can be any commercially available metering device such as a metering screw, belt feeder, rotary valve or the like, such as the Fuller-Kovako rotary feeder illustrated in U.S. Pat. No. 6,767,375). The rate of material transfer is not controlled by the auger  4  speed, but by the speed of the metering device; therefore, to avoid accumulation of feed material at the inlet  5 , the auger  4  should be operated at a speed capable of handling more material than is being conveyed by the metering device. The ratio of metering device outlet speed to auger  4  speed is dependent on many parameters such as size, type of material, and type of metering device. For example, in one embodiment, the auger runs between 10% to 33% faster than the metering device. 
         [0028]    The outlet  10  is adjacent to a discharge housing  15  at a first side opening  16  in the discharge housing  15 . The discharge housing  15  also has a second side opening  17  and a bottom opening  18 . A material retention chamber  20  is disposed between the outlet  10  of the pipe  1  and the first side opening  16  of the discharge housing  15 . The material retention chamber  20  extends into the discharge housing  15 . In one embodiment the material retention chamber  20  is created by a portion of the pipe  1  which extends beyond the auger  4 . In another embodiment, the material retention chamber  20  is a separate section of pipe attached to the outlet  10  of pipe  1 . The material retention chamber  20  has a chamber inlet  21  and a chamber outlet  22 . 
         [0029]    A piston  25  is connected to the discharge housing  15  at the second side opening  17 . The piston  25  is comprised of a cylinder  26  and a plunger  27  which is enclosed in the cylinder  26 . The plunger  27  moves horizontally within the cylinder  26  when pressure is applied to either end of the cylinder  26 . The plunger has a conical end  28  which extends into the discharge housing  15  and a rear end  29  which is enclosed in the cylinder  26 . The conical end  28  may be made of plastic or metal and has a smooth surface, however having a surface sufficiently suitable wear resistant to withstand the frictional flow of the compressed biomass material. In one embodiment, pressure is applied to the rear end  29  of the cylinder  26  by an air or hydraulic pressure system through pressure fitting  32 . The pressure setting is controlled as part of the overall control of the conveying system. The air or hydraulic pressure setting can be controlled or correlated to the power input of the auger  4 , for instance by monitoring the drive motor  6 . In another embodiment, the pressure control can be set to adjust the air or hydraulic load to keep the pressure higher. 
         [0030]    Referring now to  FIG. 2 , the pressure on the cylinder is set so that the plunger  27  extends out of the cylinder  26  and into the discharge housing  15  far enough that the conical end  28  is inserted into the chamber outlet  22  of the material retention chamber  20  closing the material retention chamber  20  off from the discharge housing  15 . The amount of pressure set on the rear end  29  of the plunger  27  and the horsepower delivered by the auger  4  varies according to the characteristics of the material, such as particle size, moisture, compressibility, friability, and elasticity to name a few. For example, finer, dry materials require more pressure to compress to a consistency where the gas will not permeate through void spaces in the mixture and the auger  4  must deliver more horsepower. Coarse, wet materials require less pressure to compress because the moisture fills void spaces more readily and prevents any gas from permeating through the plug and the auger  4  can deliver less horsepower. The total horsepower delivered by the auger must overcome both the wall friction and the cone pressure. The cone pressure adds to the wall friction to give the required total compaction power. 
         [0031]    As material is transferred through the pipe  1  by the auger  4 , it collects in the material retention chamber  20  until material retention chamber  20  is filled. Referring now to  FIG. 2 , as the material fills the material retention chamber  20 , the solid particles are compressed together creating a material plug as at  23  in the material retention chamber  20 . The build up of material in material retention chamber  20  exerts pressure on the conical end  28 . When enough material has built up in the material retention chamber  20  that the pressure on the conical end  28  exceeds the pressure setting on the rear end  29 , the material forces the plunger  27  to move horizontally in the cylinder  26 , opening the chamber outlet  22  so that material exits the material retention chamber  20  into the discharge housing  15  as shown in  FIGS. 3 and 4 . The drive motor  6  on the auger  4  should be sized to successfully drive the auger  4  against the pressure created by piston  25  and force the material to compact creating material plug  23 . 
         [0032]    The material plug  23  thus creates a seal between the pipe  1  and the discharge housing  15  preventing the backflow of gas from the pressurized vessel  2  into the pipe  1  when the conical end  28  is not itself inserted into the material retention chamber  20 . Although some material is discharged, the auger  4  feeds enough new material into the material retention chamber  20  to maintain the material plug  23 . The pressure on rear end  29  forces the plunger  27  to move horizontally in the cylinder  26 , extending the plunger  27  far enough into the discharge housing  15  that it is reinserted into the chamber outlet  22 . (See  FIG. 5 ) 
         [0033]    The pressure setting on the rear end  29  of the plunger  27  and the speed of the auger  4  are parameters that are controlled to maintain the material plug  23  and the seal it creates. These two parameters determine the degree of compaction of the material and the force exerted by the material on the walls of the pipe  1 . The speed of the auger  4  should be set such that the material discharge rate exceeds the rate of permeation of the pressurized gas from pressurized vessel  2  through the plug  23 . In one embodiment the auger  4  runs at a constant speed to ensure no accumulation of material at the inlet  5  and a constant rate of discharge of material at chamber outlet  22 . In another embodiment, the auger  4  speed varies, but is dependent on the metering device speed and is maintained at a speed slightly greater than the metering device speed. This would be particularly useful in situations where the operating parameters of the pressurized vessel fluctuate. In yet another embodiment, the auger  4  speed will be set to never drop below a minimum speed in order to maintain the plug  23  and seal and prevent the loss of system pressure. This would also be particularly useful in situations where the operating parameters of the pressurized vessel fluctuate. 
         [0034]    The amount of pressure on the rear end  29  of the plunger  27  is dependent on the characteristics of the feed material and the pressure of the pressurized vessel  2 . In one embodiment, the pressure on the rear end  29  is varied by adjusting the horsepower delivered by the auger or the speed of the auger. In another embodiment, the pressure on the rear end  29  is 30 psig greater than the maximum system pressure in the pressurized vessel  2 . 
         [0035]    The size of the material retention chamber  20  and the distance from the chamber outlet  22  and the conical end  28  of the plunger  27  varies according to the type, grain size, moisture content, and other characteristics of the material. The angle of vertex of a cone is the angle between the axis of the cone and the sloped side of the cone. The larger the diameter of the material retention chamber  20 , the greater the degree of the angle of the vertex of the cone of the conical end  28 . In embodiments where the material is fine and free flowing, the diameter of the base of conical end  28  should be equal to or greater than the diameter of chamber outlet  22  so that the conical end  28  creates a seal. In other embodiments where the material is drier and resists free flow, the diameter of the base of conical end  28  is less than the diameter of chamber outlet  22 . 
         [0036]    The pressure, speed of auger controlled to maintain seal  23  is very dependent on the type of material biomass material which is being fed. In the illustrated examples for sawdust, the pressure applied to the plunger should produce a force on the sawdust plug in the range of 15 to 25 psi. This is applicable to examples 1 and 2, below and assumes an auger speed which is 10 to 50% greater than the metering flow. Changes in the compaction nature of the material, as for materials other than the sawdust preferably used, will cause this requirement to change. As stated above, finer materials (including sawdust) will require higher compaction to ensure that the reactor gases do not permeate backwards through the sealing plug  23 . This requires plug forces which produce more than the 15 to 40 psi which are used for the materials of the examples. Also, as the reactor pressure increases, a higher force should be used. As may be observed, the force, 15 to 40 psi, is in the range of the pressures sealed against in the illustrative examples. At the start, as the reactor pressure is increased, the plunger force applied to the sawdust should equal the reactor pressure. 
         [0037]    Angle of the vertex of cone as compared to the size of chamber varies depending on material type and it is important to ensure that adequate force is applied to the sawdust (and any processed biomass material) to deliver the required compaction to seal against pressure leakage at the plug  23 . The cone also serves to break up the plug (i.e., change the direction of the biomass material flow) so that it can enter the steam entrainment area. As the size of the chamber increases (relative to the size of the particles) the cone angle must also increase. It is appropriate angles for materials other than the sawdust of the examples disclosed must be optimized to maintain the flow and plug however it is expected that that the angles will be between 45 and 80 degrees. For very large chamber diameters, it is anticipated that cone angles of slightly less than 90 degrees will be appropriate, albeit appearing as almost be a flat plate. The ultimate cone angle limit is 90 degrees. 
         [0038]    Other features of the invention will become apparent in the course of the following examples which are given for illustrations of the invention and are not intended to be limiting thereof. 
       EXAMPLE 1 
       [0039]    The following is an example of the apparatus of the present invention. The material retention chamber  20  has a three inch diameter, the angle of the vertex of the cone of the conical end  28  is approximately 45 degrees, the utilized motor horsepower is 10 hp, the maximum speed of the auger is 50 rpm, and the auger diameter is 3 inches. The sawdust wood rate of feed is about 100 pounds per hour and the reactor pressure is 10 psig, the sawdust has an average moisture of about 10% and grain top size of about ⅛ inch. 
       EXAMPLE 2 
       [0040]    The following is an example of a second embodiment of apparatus of the present invention. The material is sawdust and the material retention chamber  20  has an eight inch diameter, the angle of the vertex of the cone of the conical end  28  is between 70 and 80 degrees. The auger speed is 25 rpm, producing a feed rate of about 800 pounds of sawdust per hour with a reactor pressure of about 40 psig. The useful horsepower of the motor is 15 hp. The auger size is 8 inches in diameter. When the pressurized vessel  2  is run at 30 psig, the pressure on the rear end  29  is 60 psig. It is noted that the 50% increase of horsepower utilized in example 2 represents the ratio of frictional area over the volume for the diameter (3 vs. 8 inches) of the feeders. It is observed that for materials other than sawdust (at the given % of moisture) a starting point is proposed in the application of the data above as adjusted for the coefficient of friction between the “new” material and the material of construction used for the retention chamber. If the “new” coefficient is higher than the coefficient for the sawdust, more horsepower for the drive motor will be required. The increase will be roughly the ratio of coefficients of friction. 
         [0041]    It is preferred that the ratio of the metering device flow to auger speed be an auger speed which will deliver 10% more flow than the metering device. Running at less of a ratio usually induces problems with buildup in the auger inlet. It is preferable to generally operate at an auger speed which will deliver 25 to 50% more flow than the metering device. It is noted that running at auger speeds of 200 to 300% of the metering device can be maintained, there is little useful effect. It is observed that to avoid excessive wear in the feeder stream, the 25 to 50% range appears to be appropriate for most applications. 
         [0042]    The material exits the discharge housing  15  through the bottom opening  18  which is attached to a pressurized vessel  2  at a feed inlet  31 . (See  FIG. 2 ) In some embodiments, a grinder  32  may be attached to the bottom opening  18  to break up the material as it enters the pressurized vessel  2  through the feed inlet  31  (See  FIG. 2 ). A grinder  32  may be included depending on the material and its moisture content. For material that is nominally dry and friable enough to break up on its own, like sawdust, a grinder  32  is not required. However, a grinder  32  is useful for materials which have a higher moisture content and exhibit a paste-like consistency when compacted, like chicken litter. The grinder  32  may be any device which helps break up clumped material and maintain a uniform flow into the pressurized vessel  2 . Some examples of acceptable configurations of grinders  32  are a rotating drum against rotating drum or breaker plate, but any commercially available grinder that is contained within the system piping and does not allow accumulation of material above the grinder  32  is acceptable. 
         [0043]    Upon start up, material is fed through the auger  4  until material plug  23  is formed. When the pressure exerted by the material plug  23  on the conical end  28  exceeds the pressure on rear end  29 , the feed valve  33  connecting the feeder to the feed inlet  31  of the pressurized vessel  2  is opened to start the material feed. (See  FIG. 6 ) 
         [0044]    Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all alterations and modifications that fall within the true spirit and scope of the invention.