Abstract:
A reactor apparatus including: an internal mixing chamber including a first chamber section having a cross-sectional area expanding from a biomass inlet to the internal mixing chamber to the a second chamber section; the second chamber section having a substantially uniform internal cross-sectional area from the opposite end of the first chamber section to a discharge end of the mixing chamber; the biomass inlet is coupled to a source of pre-treated biomass external to the reactor vessel, and a rotating mixing device in the internal mixing chamber and coaxial with an axis of the first chamber section.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Utility Application 61/393,740 filed on Oct. 15, 2010, the entirety of which is incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to the field of enzymatic conversion of biomass to monomeric sugars and particularly to mixing the biomass with enzymes to promote hydrolysis. 
         [0003]    Biomass feedstock may be solely lignocellulosic material or a mixture of lignocellulosic and other materials. Polysaccharide biomass is typically a mixture of starch and lignocellulosic materials. The starch may be contained in grains or a refined starch added as feedstock to form the biomass. The biomass feedstock may also include polymers and other materials. 
         [0004]    Enzymes, such as cellulose, are mixed with the biomass to promote hydrolysis. Mixing ensures that the enzymes continually and repeatedly move into contact with chemical reaction sites in the biomass. In addition or in place of enzymes other cellulose degrading organisms and biocatalysts, such as thermophilic bacterium or yeast, may be added to the biomass to promote hydrolysis or other degradation of the biomass. 
         [0005]    The different feedstock materials and enzymes (or other degrading materials) are mixed together to form the biomass mixture. The biomass mixture may have characteristics similar to a high matter content powder. Liquid may also be added to the biomass mixture to form a high liquid slurry. Liquid is added to liquefy biomass solids and generate a uniform biomass emulsion formed of feedstock and liquids which have significant differences in their characteristics. 
         [0006]    Mixers, constant stir reactors and other such mixing or agitation devices may be used to mix and liquefy the feedstock and enzymes to form the biomass mixture. These devices conventionally are cylindrical vessels arranged vertically and having mechanical mixing devices, such as stirrers having radial arms and blades. These mixing devices generally rotate about a vertical shaft and move through the biomass. The period of mixing needed for the biomass mixture depends on the feedstocks used to form the biomass. 
         [0007]    Enzymatic liquefaction of lignocellulosic biomass may require several hours of mixing. This mixing process reduces the viscosity of the biomass as the biomass converts from a generally solids composition to a liquefied slurry. Biomass pretreated for enzymatic conversion to monomeric sugars typically starts the mixing process having a fibrous or mud-like consistency. The enzymes added to the biomass typically have a relatively low concentration with respect to the biomass. The biomass and enzyme mixture tends to be highly viscous as it enters a mixing and pretreatment reactor system, which include one or more hydrolysis reactor vessels. 
         [0008]    Due to the high viscosity of the biomass entering the hydrolysis reactor vessel, a large force (torque) is needed to turn the mixing devices and properly mix the enzymes with the biomass. The mixing speed of the mixing arms and other mixer components in the mixing chamber is typically below 300 revolutions per minute (rpm). The required mixing force traditionally limits the size of the mixing vessels. The conventional mixing devices tend to be small diameter vessels because the torque needed to rotate the mixing arms increases exponentially with the radial length of the arms. Due to the high viscosity of the biomass, the radial length of the arms is traditionally been short so that the can be moved arms through the biomass. Similarly, the motors that turn the mixing arms have maximum power limitations that constrain the maximum length of the mixing arms. Due to the constraints of the motor and the mechanical strength of the mixing components, the vessels for mixing the highly viscous pre-treated biomass have conventionally been small and narrow. 
         [0009]    Further, the mixing vessels for enzymatic liquefaction of lignocellulosic biomass have traditionally been operated in a batch mode rather than a continuous mode. Batch mode is often better suited to situations were several smaller mixing vessels feed a larger downstream vessel, such as a digester or other reactor vessel. 
         [0010]    Recirculation of liquefied material to dilute the incoming pretreated biomass has been proposed to decrease the viscosity, and improve the mixing. Recirculation has a disadvantage in that additional mixing volume is required to achieve the desired retention time in the vessel. Batch processing adds volume to the system, as time has to be provided to fill and empty the vessel. 
         [0011]    There is a need for large mixing vessels capable of mixing highly viscous biomass with enzymes. These vessels would preferably be continuous flow vessels in which biomass flows continuously in, through and out of the vessel. A large vessel would provide efficient, high flow capacity for mixing biomass and enzymes. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0012]    A novel apparatus and method is disclosed herein for mixing, e.g., liquefaction, of biomass. The apparatus and method may be used for the liquefaction and saccharification of polysaccharide containing biomasses, which may have a dryer matter content of above 10% w/w (weight/weight). The apparatus and method combines enzymatic hydrolysis with a mixing process that relies on physical forces, such as gravity and centrifugal force, to ensure that the biomasses are subjected to mechanical forces, such as shear and tear forces. 
         [0013]    The apparatus and method disclosed herein may be applied in processes of biomasses, such as for fermentation of biomass to bio-alcohols such as ethanol or butanol, forming bio-gas, forming specialty carbohydrates for food and feed, forming carbo-hydrate feedstock and for processing biomass into plastics and chemicals. 
         [0014]    A mixing and reactor vessel is disclosed herein comprising: an internal mixing chamber including a first chamber section having a cross-sectional area expanding from a biomass inlet to the internal mixing chamber to the a second chamber section, the second chamber section having a substantially uniform internal cross-sectional area from the opposite end of the first chamber section to a discharge end of the mixing chamber; the biomass inlet is coupled to a source of pre-treated biomass external to the reactor vessel, and a rotating mixing device in the internal mixing chamber and coaxial with an axis of the reactor vessel. 
         [0015]    A method is disclosed herein to mix biomass and an enzyme in a reactor and mixing vessel comprising the steps of: feeding the biomass and enzyme to an to the vessel, wherein the inlet is aligned with a narrow end of a first internal mixing chamber of the vessel; moving and mixing the biomass and enzyme as they flow from the narrow end to a wide end of the first internal mixing chamber section wherein the first internal mixing chamber expands in cross-section along a movement direction of the biomass and enzyme through the chamber; moving and further mixing the mixture of biomass and enzyme from the first internal mixing chamber to a second internal mixing chamber having a substantially uniform cross-sectional area in the movement direction; discharging from the vessel the biomass and enzyme mixture from a discharge outlet of the second internal mixing section. This mixture of biomass and enzyme may be an enzyme such as cellulose, a thermophilic bacterium or other cellulose degrading organism or biocatalyst. 
         [0016]    The first internal mixing chamber may have multiple zones at different elevations in the vessel. These zones may be separated via optional and possibly adjustable bottoms, e.g., baffles or trays, in the vessel to optimize a step-wise transformation of the biomass solids to a slurry. These intermediate bottoms are preferably horizontal and extend substantially the entire cross-section of the vessel at the elevation where the platform is positioned. The bottoms may also be slightly inclined with respect to horizontal. Adjustable openings in the intermediate bottoms may be provided to vary the flow through the bottoms and from one zone to the next one. Depending on the dry matter feedstock and mixing slurry in question (which could be an enzyme mixture), that may be no intermediate bottoms in the vessel such that the downward movement of the biomass mixture is dependent solely on gravity and plugflow downflow of the induced matter through the reactor vessel. 
         [0017]    The already conditioned (liquefied) slurry flows from lower zones (or the bottom) of the mixing vessel. A portion of the slurry flow may be pumped or circulated to upper zones in the vessel to adjust the slowly changing viscosity of the biomass feedstock at the upper elevations of the vessel. 
         [0018]    The conical top may provide approximately constant torque as the material flows through the mixer. The angle of the cone could change as the diameter increases, as the viscosity decrease is fast in the beginning and then slows down. The vessel top may also consist of several stacked concentric cylinders with increasing diameters from top to bottom. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic diagram showing in cross-section a vertically aligned mixing and hydrolysis reaction vessel for biomass. 
           [0020]      FIG. 2  is a chart showing an expected viscosity of the biomass in the reaction vessel shown in  FIG. 1  as a function of retention time of the biomass in the vessel. 
           [0021]      FIG. 3  is a schematic diagram showing in cross-section a conical mixing and hydrolysis vessel connected to a cylindrical mixing and hydrolysis reaction vessel. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 1  shows schematically a reactor and mixing vessel  10  having a conical upper section  12  and a cylindrical lower section  14 . These sections  12 ,  14  define an internal reaction chamber in which the biomass is mixed with the enzyme(s) and is hydrolyzed. The internal reaction chamber may have a volume in a range of 50 cubic meters to 2,500 cubic meters. Narrower ranges of 200 cubic meters to 1,200 cubic meters or 400 cubic meters to 800 cubic meters will also be suitable depending on the specific application of the reaction and mixing process. The reaction chamber may be substantially larger in volume than batch mixing/reactor vessels conventionally used to mix highly viscous biomass. 
         [0023]    The vessel includes a rotatable shaft  16  extending along the vertical axis of the vessel. The shaft is driven (rotated) by a motor and gear box drive assembly  18 , which may be mount to the top or bottom of the vessel. The shaft  16  may be to a vertical axis of the vessel and extend the height of the vessel. The shaft turns a mixing device  28 , e.g., mixing arms and paddles, that moves through and churn the biomass in the vessel. 
         [0024]    A source  20  of biomass and enzymes may be continuously fed to an upper inlet  22  of the vessel  10 . The biomass and enzymes may be fed as a mixture to the vessel or fed separately to the vessel. The source  20  may include a short retention time horizontal mixer, in which the biomass and enzymes are brought into initial contact with each other. If desired, recycled low viscosity hydrolyzed material  21  is introduced into the source  20  or the upper inlet  22  of the vessel. 
         [0025]    The inlet  22  feeds the biomass to a narrow region of the conical upper section  12 . The cross-sectional area of the upper section  12  expands from the upper narrow region to the transition  24  between the upper section  12  and the lower section  14 . The cross-section area of the lower section  14  may be uniform along its entire height. The bottom of the lower section is adjacent a discharge outlet  26  for the hydrolyzed biomass continuously flowing out of the vessel  10  to other process units, such as a digester, fermenter or continuing enzymatic hydrolysis vessels. The bottom of the lower section may be sloped to provide a uniform discharge from the entire cross-sectional area of the bottom of the lower section. 
         [0026]    A mixing device  28  (shown schematically by a tree of rotating arms  30  in  FIG. 1 ) is mounted to the shaft  16  and rotates through the biomass and enzymes moving downward through the upper and lower sections  12 ,  14  of the vessel. The mixing device  28  may include radially extending arms or spokes  30  at various elevations in the vessel. The arms may extend horizontally or may be oblique with respect to horizontal. The arms  30  may be arranged as spokes extending from the shaft. The arms may have mixing paddles, blades or fingers  32  arranged at the radial end of the arms and optionally at various positions along the radial length of each of the arms. 
         [0027]    The arms  30  may be adjusted to be positioned at various elevations and positions in the vessel. Similarly, the paddles, blades or fingers  32  may be adjustably mounted on each of the arms. The adjustment may change, for example, the angle at which the paddles, blades or fingers are oriented with respect to the direction of rotation of the arms. The orientation of the paddles, blades or fingers may be set to provide a slight radially outward flow to the biomass to distribute the biomass evenly through the cross-sectional area of the vessel. The rotation of arms with the paddles, blades or fingers at one or more elevations or radii may be provide may also apply a slight uplift of the biomass to prevent short-circuiting of the biomass flowing down from above through the vessel. 
         [0028]    The arms turn in a circular rotational pattern through the biomass in the vessel. The arms are turned by the rotating shaft  16 . The movement of the arms and mixing paddles, blades or fingers mix the enzyme into the biomass and thereby cause the enzyme to come into contact with reaction sites in the biomass. The reactions between the enzyme and the biomass promotes hydrolysis of the biomass in the vessel. 
         [0029]    Mixing baffles  32  may be installed on the inside vessel wall of the lower section  14  and optionally the upper section  12 . The biomass flowing through the lower section will have a relatively low viscosity, as compared to the viscosity at the vessel inlet. Mixing baffles are most suitable for low viscosity flows through a mixing vessel. Trays or baffles could also be installed between the mixing arms to aid in distribution of the biomass material. 
         [0030]    The shaft and mixing arms may provide indirect cooling or heating to the biomass, such as by cooling or heating passages in the arms. Similarly, the interior walls of the vessel may be jacketed or provided with cooling or heating coils  34 . 
         [0031]    As an example, to hydrolyze 1200 tons of biomass per day, where the biomass has a 25% solids loading, the reactor vessel should be sized to process about 5000 cubic meters of biomass during a twenty-four (24) hour retention period in the vessel. The vessel should be larger if the biomass retention period is longer, such as 72 to 120 hours. A vessel having an internal chamber volume of 15,000 cubic meters to 25,000 cubic meters may be needed to provide long retention periods of a continuous flow of a large amount of biomass, e.g., 1200 tons/day, being hydrolyzed. 
         [0032]    The diameter, height and other dimensions of the vessel depend on the flow of biomass and retention period of the biomass in the vessel. By way of example, a reactor vessel  10  may need an effective internal volume of about 1200 cubic meters to handle 1200 tons of biomass per day at a 25 percent solids loading and a six hour retention period. Assuming that the aspect ratio (diameter to height) of the vessel is six, the diameter of the vessel would be about 5.4 meter and its height would be greater than 33 meters. 
         [0033]    The conical upper section  12  is narrowest at the upper inlet that receives the highly viscous biomass entering the vessel. The viscosity of the biomass is greatest at the top inlet to the vessel. While the high viscosity increases the starting torque needed to turn the mixing device, the torque is lessened because of the short mixing arms at the narrow top. The biomass becomes less viscous as it mixes it the enzyme and moves down through the vessel. The lessening viscosity allows for the mixing arms to be longer without increasing the torque needed to turn shaft. The arms in the lower portions of the upper conical section are longer than most or all the upper arms  30 . Longer arms require more torque to be turned through the biomass, assuming the viscosity of the biomass remains constant. The combined effects of the reduction in viscosity of the biomass and the longer arms results in acceptable torque requirements for the mixing device in the upper conical section. 
         [0034]    The conical geometry of the upper section reduces the starting torque requirement. Less power is required for mixing, the biomass can be more thoroughly mixed, and the biomass is less susceptible to channeling down through the vessel. The conical shape also results relatively frequent and robust mixing near the inlet of the vessel, where mixing may be most beneficial to promote hydrolysis. 
         [0035]    Torque increases with the diameter squared. The torque required to move (mix) a fluid in a circle is a function of the force required to move the fluid times the radius of that force from the center of rotation. The force required to move the fluid is a function of the viscosity of the fluid, the velocity of the motion and the distance that the fluid has to move. 
         [0036]    Assuming a constant fluid viscosity and constant rotation of the mixing device, the torque required to turn the mixing device depends on the square of the radius of the vessel. Due to the squared relationship between torque and vessel diameter, reducing the vessel diameter dramatically reduces the amount of torque or allows the same amount of torque to mixing a highly viscous biomass flow. 
         [0037]    The conical upper section  12  is suited for short mixing arms in the upper region of the vessel where viscosity is high. The shortest mixing arms are at the top of the vessel where the biomass viscosity is greatest and the resistance of the biomass to mechanical mixing is high. As the biomass moves down through the upper section, the viscosity of the biomass lessens, the resistance to mixing decreases and longer mixing arms may be used in view of the increasing diameter of the conical portion of the vessel. 
         [0038]    By knowing the viscosity of the biomass at various elevations in the upper section  12 , the angle of the cone of the upper section may be selected such that the radius of the mixing arms increases at a rate that results in uniform torque on the arms at each elevation. Thus, each mixer arm may require the same torque to mix the material, even through the diameter of the conical section is increasing in a downward direction. 
         [0039]    Intermediate bottoms, trays or baffles  38  may be installed and adjusted to separate the upper section  12  into multiple zones to optimize a step-wise transformation of the biomass mixture to a slurry with a higher liquid content than the original biomass. The zones may be generally vertically aligned in the vessel. These intermediate and adjustable bottoms may be horizontal in the vessel and may also be slightly inclined with respect to horizontal. Further, adjustable openings in the intermediate bottoms may be used to vary the flow between the zones defined by the bottoms. Similarly, intermediate bottoms, trays and baffles  39  may be arranged in the lower section  14  into multiple zones. 
         [0040]      FIG. 2  includes a chart  40  of viscosity of the biomass in the vessel  10  as a function of time. The chart is for illustrative purposes. The chart shows the viscosity of a biomass which is steam exploded corn stover mixed at a temperature of 50 degrees Celsius and in a vessel having mixing devices rotating at 20 rpm. The chart shows a range of viscosity values in milliPascal-second (mPas) for the biomass undergoing saccharification. The range results from two different starting mixing patterns used for the biomass. 
         [0041]    As shown in the chart  40 , the viscosity of the biomass may reduce quickly such that the viscosity has been reduced by one-half or more after six hours of reaction time in the vessel. It is known that only about six (6) hours of reaction time (or somewhat more reaction time) is needed in the vessel to convert the viscous biomass flow to a flowing, syrupy consistency. During this initial reaction period (e.g., 15 minutes to 8 hours, preferably 1 hour to 6 hours, most preferably 2 hours to 4 hours), the apparent viscosity of the biomass decreases quickly as enzymes break down the polymeric sugars of the biomass to smaller molecule chains. 
         [0042]    The downward flow rate of the biomass through the vessel can be calculated or estimated by conventional means. As illustrated in  FIG. 2 , the reaction time of biomass in a continuous flow vessel  10  correlates with the movement of the biomass down through the vessel. The vessel may have the mixing device, heating coils and intermediate bottoms as shown in  FIG. 1 . The continuous biomass flow through the vessel is represented by diagonal dashes shown in the illustration of the vessel. 
         [0043]    Using the rate of flow through the vessel and the reaction time to reduce the biomass viscosity to a certain level, such as a 50% or less viscosity reduction, the vertical distance down through the vessel can be calculated to determine at which elevation/reaction time  42  the biomass will have a viscosity of one-half the viscosity of the biomass entering the vessel. The conical upper section  12  may be designed such that the transition  24  to the lower cylindrical section  14  occurs at the same elevation where the viscosity of the biomass is reduced by half. 
         [0044]      FIG. 3  is a schematic diagram showing in cross-section a conical mixing and hydrolysis vessel  50  connected to a cylindrical mixing and hydrolysis reaction vessel  52 . The biomass flowing through these vessels is indicated by diagonal dashes. The conical mixing and hydrolysis vessel  50  is similar in many respects to the conical portion of the vessel  10  shown in  FIG. 1 , as is indicated by the common reference numerals in  FIGS. 1 and 3 . 
         [0045]    Biomass and enzymes are fed from a source  20  to the upper inlet  22  of the narrow end of the conical mixer and reaction chamber  50 . A mixing device  28  has arms  36  that increase in length as the conical mixer increases in diameter. Intermediate bottoms, e.g., baffles, trays or other plates  38 , may be arranged in the conical vessel to regulate the downward flow of biomass through the vessel. The viscosity of the biomass falls as the biomass is mixed and reacts in the vessel  50 . The viscosity may be reduced by half as the biomass is discharged from the vessel at port  54 , as compared to the viscosity of the biomass  20  entering the vessel. A tapered or sloped bottom  56  may direct the biomass into the port  54 . 
         [0046]    A transport conduit, e.g., pipe,  58  and a pump  60  may be used to transport the liquefied biomass to an upper inlet port  62  of the cylindrical vessel  52 . The cylindrical vessel includes a mixing device  64  and optionally baffles  32 . The mixing device is connected to a shaft  66  driven by a drive and gear assembly  68 . The hydrolyzed biomass is discharged at port  70  from the cylindrical vessel. 
         [0047]    The invention has now been described in detail for purposes of clarity and understanding. However, it will be appreciated that certain changes and modifications to the disclosed embodiment of the invention may be practiced within the scope of the appended claims.