Abstract:
Hydrocarbon-containing feedstocks are processed to produce useful intermediates or products, such as fuels. For example, systems are described that can process a petroleum-containing feedstock, such as oil sands, oil shale, tar sands, and other naturally-occurring and synthetic materials that include both hydrocarbon components and solid matter, to obtain a useful intermediate or product.

Description:
RELATED APPLICATIONS 
     This application is a continuation of PCT/US2010/035331, filed May 18, 2010, which claimed priority to U.S. Provisional Application Ser. No. 61/179,995, filed May 20, 2009, U.S. Provisional Application Ser. No. 61/218,832, filed Jun. 19, 2009, and U.S. Provisional Application Ser. No. 61/226,877, filed Jul. 20, 2009. The complete disclosure of each of these applications is hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     Processing hydrocarbon-containing materials can permit useful intermediates or products to be extracted from the materials. Natural hydrocarbon-containing materials can include a variety of other substances in addition to hydrocarbons. 
     SUMMARY 
     Systems and methods are disclosed herein for processing a wide variety of different hydrocarbon-containing materials, such as light and heavy crude oils, natural gas, bitumen, coal, and such materials intermixed with and/or adsorbed onto a solid support, such as an inorganic support. In particular, the systems and methods disclosed herein can be used to process (e.g., crack, convert, isomerize, reform, separate) hydrocarbon-containing materials that are generally thought to be less easily processed, including oil sands, oil shale, tar sands, and other naturally-occurring and synthetic materials that include both hydrocarbon components and solid matter (e.g., solid organic and/or inorganic matter). 
     Such materials can be especially difficult to mix with liquids, e.g., with water or a solvent system during processing. For example, if the materials are low density, the materials tend to float to the surface of the liquid, or if the materials are high density they tend to sink to the bottom of the mixing vessel, rather than being dispersed. In some cases, the materials can be hydrophobic, highly crystalline, or otherwise difficult to wet. At the same time, it is desirable to process the feedstock in a relatively high solids level dispersion, for efficiency and in order to obtain a high final concentration of the desired product after processing. 
     The inventors have found that dispersion of a feedstock in a liquid mixture can be enhanced, and as a result in some cases the solids level of the mixture can be increased, by the use of certain mixing techniques and equipment. The mixing techniques and equipment disclosed herein also enhance mass transfer. In particular, jet mixing techniques, including for example jet aeration and jet flow agitation, have been found to provide good wetting, dispersion and mechanical disruption. By increasing the solids level of the mixture, the process can proceed more rapidly, more efficiently and more cost-effectively, and the resulting concentration of the intermediate or product can be increased. 
     In some implementations, the process further includes treating the feedstock to facilitate recovery of the hydrocarbon. For example, exposure of the materials to particle beams (e.g., beams that include ions and/or electrons and/or neutral particles) or high energy photons (e.g., x-rays or gamma rays) can be used to process the materials. Particle beam exposure can be combined with other techniques such as sonication, mechanical processing, e.g., comminution (for example size reduction), temperature reduction and/or cycling, pyrolysis, chemical processing (e.g., oxidation and/or reduction), and other techniques to further break down, isomerize, or otherwise change the molecular structure of the hydrocarbon components, to separate the components, and to extract useful materials from the components (e.g., directly from the components and/or via one or more additional steps in which the components are converted to other materials). Radiation may be applied from a device that is in a vault. Methods of treating hydrocarbon-containing materials are described in detail in U.S. patent application Ser. Nos. 12/417,786 and 12/417,699, both of which were filed on Apr. 3, 2009, the complete disclosures of which are incorporated herein by reference. 
     The systems and methods disclosed herein also provide for the combination of any hydrocarbon-containing materials described herein with additional materials including, for example, solid supporting materials. Solid supporting materials can increase the effectiveness of various material processing techniques. Further, the solid supporting materials can themselves act as catalysts and/or as hosts for catalyst materials such as noble metal particles, e.g., rhodium particles, platinum particles, and/or iridium particles. The catalyst materials can increase still further the rates and selectivity with which particular intermediates or products are obtained from processing the hydrocarbon-containing materials. Such additional materials and their use in processing are described in the above-incorporated U.S. patent application Ser. No. 12/417,786. 
     Many of the intermediates or products obtained by the methods disclosed herein, such as petroleum products, can be utilized directly as a fuel or as a blend with other components for powering cars, trucks, tractors, ships or trains. The hydrocarbon products can be further processed via conventional hydrocarbon processing methods. Where hydrocarbons were previously associated with solid components in materials such as oil sands, tar sands, and oil shale, the liberated hydrocarbons are flowable and are therefore amenable to processing in refineries. 
     In one aspect, the invention features a method that includes processing a hydrocarbon-containing feedstock by mixing the feedstock with a liquid medium in a vessel, using a jet mixer. 
     Some embodiments include one or more of the following features. The jet mixer may include, for example, a jet-flow agitator, a jet aeration type mixer, or a suction chamber jet mixer. If a jet aeration type mixer is used, it may be used without injection of air through the mixer. For example, if the jet aeration type mixer includes a nozzle having a first inlet line and a second inlet line, in some cases both inlet lines are supplied with a liquid. In some cases, mixing comprises adding the feedstock to the liquid medium in increments and mixing between additions. The mixing vessel may be, for example, a tank, rail car or tanker truck. The method may further include adding an emulsifier or surfactant to the mixture in the vessel. 
     In some instances, the vessel is or includes a conduit or other structure or carrier for the feedstock. For example, a jet mixer may be disposed in a conduit, e.g., between processing areas. In this case, the jet mixer can serve the dual purpose of mixing and conveying the mixture from one area to another. Additional jet mixers can be disposed in other areas, e.g., in one or more processing tanks, if desired. In some cases, the vessel can be a continuous loop of pipe, tubing, or other structure that defines a bore or lumen, and jet mixing can take place within this loop. 
     In another aspect, the invention features processing a hydrocarbon-containing feedstock by mixing the feedstock with a liquid medium in a vessel, using a mixer that produces generally toroidal flow within the vessel. 
     In some embodiments, the mixer is configured to limit any increase in the overall temperature of the liquid medium to less than 5° C. over the course of mixing. This aspect may also include, in some embodiments, any of the features discussed above. 
     In another aspect, the invention features an apparatus that includes a tank, a jet mixer having a nozzle disposed within the tank, and a delivery device configured to deliver a hydrocarbon-containing feedstock to the tank. 
     Some embodiments include one or more of the following features. The jet mixer can further include a motor, and the apparatus can further include a device configured to monitor the torque on the motor during mixing. The apparatus can also include a controller that adjusts the operation of the feedstock delivery device based on input from the torque-monitoring device. 
     All publications, patent applications, patents, and other references mentioned herein or attached hereto are incorporated by reference in their entirety for all that they contain. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a sequence of steps for processing hydrocarbon-containing materials. 
         FIGS. 2 and 2A  are diagrams illustrating jet flow exiting a nozzle. 
         FIG. 3  is a diagrammatic perspective view of a jet-flow agitator according to one embodiment.  FIG. 3A  is an enlarged perspective view of the impeller and jet tube of the jet-flow agitator of  FIG. 3 .  FIG. 3B  is an enlarged perspective view of an alternate impeller. 
         FIG. 4  is a diagram of a suction chamber jet mixing nozzle according to one embodiment.  FIG. 4A  is a perspective view of a suction chamber jet mixing system according to another embodiment. 
         FIG. 5  is a diagrammatic perspective view of a jet mixing nozzle for a suction chamber jet mixing system according to another alternate embodiment. 
         FIG. 6  is a diagrammatic perspective view of a tank and a jet aeration type mixing system positioned in the tank, with the tank being shown as transparent to allow the jet mixer and associated piping to be seen.  FIG. 6A  is a perspective view of the jet mixer used in the jet aeration system of  FIG. 6 .  FIG. 6B  is a diagrammatic perspective view of a similar system in which an air intake is provided. 
         FIG. 7  is a cross-sectional view of a jet aeration type mixer according to one embodiment. 
         FIG. 8  is a cross-sectional view of a jet aeration type mixer according to an alternate embodiment. 
         FIGS. 9-11  are diagrams illustrating alternative flow patterns in tanks containing different configurations of jet mixers. 
         FIG. 12  is a diagram illustrating the flow pattern that occurs in a tank during backflushing according to one embodiment. 
         FIG. 13  is a side view of a jet aeration type system according to another embodiment, showing a multi-level arrangement of nozzles in a tank. 
         FIGS. 14 and 14A  are a diagrammatic top view and a perspective view, respectively, of a device that minimizes hold up along the walls of a tank during mixing. 
         FIGS. 15 and 16  are views of water jet devices that provide mixing while also minimizing hold up along the tank walls. 
         FIG. 17  is a cross-sectional view of a tank having a domed bottom and two jet mixers extending into the tank from above. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic diagram of a technique  100  for processing hydrocarbon-containing materials such as oil sands, oil shale, tar sands, and other materials that include hydrocarbons intermixed with solid components such as rock, sand, clay, silt, and/or solid organic material. These materials may be in their native form, or may have been previously treated, for example treated in situ with radiation as described below. In a first step of the sequence shown in  FIG. 1 , the hydrocarbon-containing material  110  can be subjected to one or more optional mechanical processing steps  120 . The mechanical processing steps can include, for example, grinding, crushing, agitation, centrifugation, rotary cutting and/or chopping, shot-blasting, and various other mechanical processes that can reduce an average size of particles of material  110 , and initiate separation of the hydrocarbons from the remaining solid matter therein. In some embodiments, more than one mechanical processing step can be used. For example, multiple stages of grinding can be used to process material  110 . Alternatively, or in addition, a crushing process followed by a grinding process can be used to treat material  110 . Additional steps such as agitation and/or further crushing and/or grinding can also be used to further reduce the average size of particles of material  110 . 
     In a second step  130  of the sequence shown in  FIG. 1 , the hydrocarbon-containing material  110  can be subjected to one or more optional cooling and/or temperature-cycling steps. In some embodiments, for example, material  110  can be cooled to a temperature at and/or below a boiling temperature of liquid nitrogen. More generally, the cooling and/or temperature-cycling in step  130  can include, for example, cooling to temperatures well below room temperature (e.g., cooling to 10° C. or less, 0° C. or less, −10° C. or less, −20° C. or less, −30° C. or less, −40° C. or less, −50° C. or less, −100° C. or less, −150° C. or less, −200 ° C. or less, or even lower temperatures). Multiple cooling stages can be performed, with varying intervals between each cooling stage to allow the temperature of material  110  to increase. The effect of cooling and/or temperature-cycling material  110  is to disrupt the physical and/or chemical structure of the material, promoting at least partial dissociation of the hydrocarbon components from the non-hydrocarbon components (e.g., solid non-hydrocarbon materials) in material  110 . Suitable methods and systems for cooling and/or temperature-cycling of material  110  are disclosed, for example, in U.S. Provisional Patent Application Ser. No. 61/081,709, filed on Jul. 17, 2008, and U.S. Ser. No. 12/502,629, filed Jul. 14, 2009, the entire contents of which are incorporated herein by reference. 
     In a third step  140  of the sequence of  FIG. 1 , the hydrocarbon-containing material  110  can be exposed to charged particles or photons, such as photons having a wavelength between about 0.01 nm and 280 nm. In some embodiments, the photons can have a wavelength between, e.g., 100 nm to 280 nm or between 0.01 nm to 10 nm, or in some cases less than 0.01 nm. The charged particles interact with material  110 , causing further disassociation of the hydrocarbons therein from the non-hydrocarbon materials, and also causing various hydrocarbon chemical processes, including chain scission, bond-formation, and isomerization. These chemical processes convert long-chain hydrocarbons into shorter-chain hydrocarbons, many of which can eventually be extracted from material  110  as products and used directly for various applications. The chemical processes can also lead to conversion of various products into other products, some of which may be more desirable than others. For example, through bond-forming reactions, some short-chain hydrocarbons may be converted to medium-chain-length hydrocarbons, which can be more valuable products. As another example, isomerization can lead to the formation of straight-chain hydrocarbons from cyclic hydrocarbons. Such straight-chain hydrocarbons may be more valuable products than their cyclized counterparts. 
     By adjusting an average energy of the charged particles and/or an average current of the charged particles, the total amount of energy delivered or transferred to material  110  by the charged particles can be controlled. In some embodiments, for example, material  110  can be exposed to charged particles so that the energy transferred to material  110  (e.g., the energy dose applied to material  110 ) is 0.3 Mrad or more (e.g., 0.5 Mrad or more, 0.7 Mrad or more, 1.0 Mrad or more, 2.0 Mrad or more, 3.0 Mrad or more, 5.0 Mrad or more, 7.0 Mrad or more, 10.0 Mrad or more, 15.0 Mrad or more, 20.0 Mrad or more, 30.0 Mrad or more, 40.0 Mrad or more, 50.0 Mrad or more, 75.0 Mrad or more, 100.0 Mrad or more, 150.0 Mrad or more, 200.0 Mrad or more, 250.0 Mrad or more, or even 300.0 Mrad or more). 
     In general, electrons, ions, photons, and combinations of these can be used as the charged particles in step  140  to process material  110 . A wide variety of different types of ions can be used including, but not limited to, protons, hydride ions, oxygen ions, carbon ions, and nitrogen ions. These charged particles can be used under a variety of conditions; parameters such as particle currents, energy distributions, exposure times, and exposure sequences can be used to ensure that the desired extent of separation of the hydrocarbon components from the non-hydrocarbon components in material  110 , and the extent of the chemical conversion processes among the hydrocarbon components, is reached. Suitable systems and methods for exposing material  110  to charged particles are discussed, for example, in U.S. Ser. No. 12/417,699, filed Apr. 3, 2009, U.S. Ser. No. 12/486,436, filed Oct. 5, 2009, as well as the following U.S. Provisional Patent Applications: Ser. No. 61/049,406, filed on Apr. 30, 2008; Ser. No. 61/073,665, filed on Jun. 18, 2008; and Ser. No. 61/073,680, filed on Jun. 18, 2008. The entire contents of each of the foregoing applications is incorporated herein by reference. In particular, charged particle systems such as inductive linear accelerator (LINAC) systems can be used to deliver large doses of energy (e.g., doses of 50 Mrad or more) to material  110 . 
     In the final step of the processing sequence of  FIG. 1 , the processed material  110  is subjected to a separation step  150 , which separates the hydrocarbon products  160  and the non-hydrocarbon products  170 . The separation step includes an extraction process that involves agitating the material  110 . For example, tar sands are processed using a hot water extraction process. After mining, the tar sands are transported to an extraction plant, where the hot water extraction process separates bitumen from sand, water and minerals. Hot water is added to the sand, and the resulting slurry is agitated. The combination of hot water and agitation releases bitumen from the oil sand in the form of droplets. Air bubbles attach to the bitumen droplets, causing the droplets to float to the top of the separation tank. The bitumen is then skimmed off and processed to remove residual water and solids. During this extraction process, agitation is performed using the jet mixing techniques discussed below. 
     A wide variety of other processing steps can optionally be used to further separate and refine the products. Exemplary processes include, but are not limited to, distillation, centrifugation and filtering. 
     The processing sequence shown in  FIG. 1  is a flexible sequence, and can be modified as desired for particular materials  110  and/or to recover particular hydrocarbon products  160 . For example, the order of the various steps can be changed in  FIG. 1 . Further, additional steps of the types shown, or other types of steps, can be included at any point within the sequence, as desired. For example, additional mechanical processing steps, cooling/temperature-cycling steps, particle beam exposure steps, and/or separation steps can be included at any point in the sequence. Further, other processing steps such as sonication, chemical processing, pyrolysis, oxidation and/or reduction, and radiation exposure can be included in the sequence shown in  FIG. 1  prior to, during, and/or following any of the steps shown in  FIG. 1 . Many processes suitable for inclusion in the sequence of  FIG. 1  are discussed, for example, in PCT Publication No. WO 2008/073186 (e.g., throughout the Detailed Description section). 
     Suitable liquids that can be added to material  110 , e.g., during extraction, include, for example, water, various types of liquid hydrocarbons (e.g., hydrocarbon solvents), and other common organic and inorganic solvents. 
     Agitation 
     Jet Mixing Characteristics 
     Various types of mixing devices which may be used during hydrocarbon processing are described below. Other mixing devices having similar characteristics may be used. Suitable mixers have in common that they produce high velocity circulating flow, for example flow in a toroidal or elliptical pattern. Generally, preferred mixers exhibit a high bulk flow rate. Preferred mixers provide this mixing action with relatively low energy consumption. It is also preferred in some cases that the mixer produce relatively low shear and avoid heating of the liquid medium. As will be discussed in detail below, some preferred mixers draw the mixture through an inlet into a mixing element, which may include a rotor or impeller, and then expel the mixture from the mixing element through an outlet nozzle. This circulating action, and the high velocity of the jet exiting the nozzle, assist in dispersing material that is floating on the surface of the liquid or material that has settled to the bottom of the tank, depending on the orientation of the mixing element. Mixing elements can be positioned in different orientations to disperse both floating and settling material, and the orientation of the mixing elements can in some cases be adjustable. 
     For example, in some preferred mixing systems the velocity v o  of the jet as meets the ambient fluid is from about 2 to 300 m/s, e.g., about 5 to 150 m/s or about 10 to 100 m/s. The power consumption of the mixing system may be about 20 to 1000 KW, e.g., 30 to 570 KW, 50 to 500 KW, or 150 to 250 KW for a 100,000 L tank. It is generally preferred that the power usage be low for cost-effectiveness. 
     Jet mixing involves the discharge of a submerged jet, or a number of submerged jets, of high velocity liquid into a fluid medium, in this case the mixture of feedstock and liquid medium. The jet of liquid penetrates the fluid medium, with its energy being dissipated by turbulence and some initial heat. This turbulence is associated with velocity gradients (fluid shear). The surrounding fluid is accelerated and entrained into the jet flow, with this secondary entrained flow increasing as the distance from the jet nozzle increases. The momentum of the secondary flow remains generally constant as the jet expands, as long as the flow does not hit a wall, floor or other obstacle. The longer the flow continues before it hits any obstacle, the more liquid is entrained into the secondary flow, increasing the bulk flow in the tank or vessel. When it encounters an obstacle, the secondary flow will lose momentum, more or less depending on the geometry of the tank, e.g., the angle at which the flow impinges on the obstacle. It is generally desirable to orient the jets and/or design the tank so that hydraulic losses to the tank walls are minimized. For example, it may be desirable for the tank to have an arcuate bottom (e.g., a domed headplate), and for the jet mixers to be oriented relatively close to the sidewalls, as shown in  FIG. 17 . The tank bottom (lower head plate) may have any desired domed configuration, or may have an elliptical or conical geometry. 
     Jet mixing differs from most types of liquid/liquid and liquid/solid mixing in that the driving force is hydraulic rather than mechanical. Instead of shearing fluid and propelling it around the mixing vessel, as a mechanical agitator does, a jet mixer forces fluid through one or more nozzles within the tank, creating high-velocity jets that entrain other fluid. The result is shear (fluid against fluid) and circulation, which mix the tank contents efficiently. 
     Referring to  FIG. 2 , the high velocity gradient between the core flow from a submerged jet and the surrounding fluid causes eddies.  FIG. 2A  illustrates the general characteristics of a submerged jet. As the submerged jet expands into the surrounding ambient environment the velocity profile flattens as the distance (x) from the nozzle increases. Also, the velocity gradient dv/dr changes with r (the distance from the centerline of the jet) at a given distance x, such that eddies are created which define the mixing zone (the conical expansion from the nozzle). 
     In an experimental study of a submerged jet in air (the results of which are applicable to any fluid, including water), Albertson et al. (“Diffusion of Submerged Jets,” Paper 2409, Amer. Soc. of Civil Engineers Transactions, Vol. 115:639-697, 1950, at p. 657) developed dimensionless relationships for v(x) r=0 /v o  (centerline velocity), v(r) x /v(x) r−0  (velocity profile at a given x), Q x /Q o  (flow entrainment), and E x /E o (energy change with x): 
     (1) Centerline velocity, v(x) r=0 /v o : 
     
       
         
           
             
               
                 
                   v 
                   ⁡ 
                   
                     ( 
                     
                       r 
                       = 
                       0 
                     
                     ) 
                   
                 
                 
                   v 
                   o 
                 
               
               ⁢ 
               
                 x 
                 
                   D 
                   o 
                 
               
             
             = 
             6.2 
           
         
       
     
     (2) velocity profile at any x, v(r) x /v(x) r=0 : 
     
       
         
           
             
               log 
               ⁡ 
               
                 [ 
                 
                   
                     
                       
                         v 
                         ⁡ 
                         
                           ( 
                           r 
                           ) 
                         
                       
                       x 
                     
                     
                       v 
                       o 
                     
                   
                   ⁢ 
                   
                     x 
                     D 
                   
                 
                 ] 
               
             
             = 
             
               0.79 
               - 
               
                 33 
                 ⁢ 
                 
                   
                     r 
                     2 
                   
                   
                     x 
                     2 
                   
                 
               
             
           
         
       
     
     (3) Flow and energy at any x: 
                       Q   x       Q   o       =     0.32   ⁢     x     D   o                 (   10.21   )                   E   x       E   o       =     4.1   ⁢       D   o     x               (   10.22   )               
where:
     v(r=0)=centerline velocity of submerged jet (m/s),   v o =velocity of jet as it emerges from the nozzle (m/s),   x=distance from nozzle (m),   r=distance from centerline of jet (m),   D o =diameter of nozzle (m),   Q x =flow of fluid across any given plane at distance x from the nozzle (me/s),   Q u =flow of fluid emerging from the nozzle (m3/s),   E=energy flux of fluid across any given plane at distance x from the nozzle (m 3 /s),   E o =energy flux of fluid emerging from the nozzle (m 3 /s).   

     (“Water Treatment Unit Processes: Physical and Chemical,” David W. Hendricks, CRC Press 2006, p. 411.) 
     Jet mixing is particularly cost-effective in large-volume (over 1,000 gal) and low-viscosity (under 1,000 cPs) applications. It is also generally advantageous that in most cases a jet mixer has no moving parts submerged, e.g., when a pump is used it is generally located outside the vessel. 
     One advantage of jet mixing is that the temperature of the ambient fluid (other than directly adjacent the exit of the nozzle, where there may be some localized heating) is increased only slightly if at all. For example, the temperature may be increased by less than 5° C., less than 1° C., or not to any measureable extent. 
     Jet-Flow Agitators 
     One type of jet-flow agitator is shown in  FIGS. 3-3A . This type of mixer is available commercially, e.g., from IKA under the tradename ROTOTRON™. Referring to  FIG. 3 , the mixer  200  includes a motor  202 , which rotates a drive shaft  204 . A mixing element  206  is mounted at the end of the drive shaft  204 . As shown in  FIG. 3A , the mixing element  206  includes a shroud  208  and, within the shroud, an impeller  210 . As indicated by the arrows, when the impeller is rotated in its “forward” direction, the impeller  210  draws liquid in through the open upper end  212  of the shroud and forces the liquid out through the open lower end  214 . Liquid exiting end  214  is in the form of a high velocity stream or jet. If the direction of rotation of the impeller  210  is reversed, liquid can be drawn in through the lower end  214  and ejected through the upper end  212 . This can be used, for example, to suck in solids that are floating near or on the surface of the liquid in a tank or vessel. (It is noted that “upper” and “lower” refer to the orientation of the mixer in  FIG. 3 ; the mixer may be oriented in a tank so that the upper end is below the lower end.) 
     The shroud  208  includes flared areas  216  and  218  adjacent its ends. These flared areas are believed to contribute to the generally toroidal flow that is observed with this type of mixer. The geometry of the shroud and impeller also concentrate the flow into a high velocity stream using relatively low power consumption. 
     Preferably, the clearance between the shroud  208  and the impeller  210  is sufficient so as to avoid excessive milling of the material as it passes through the shroud. For example, the clearance may be at least 10 times the average particle size of the solids in the mixture, preferably at least 100 times. 
     In some implementations, the shaft  204  is configured to allow gas delivery through the shaft. For example, the shaft  204  may include a bore (not shown) through which gas is delivered, and one or more orifices through which gas exits into the mixture. The orifices may be within the shroud  208 , to enhance mixing, and/or at other locations along the length of the shaft  204 . 
     The impeller  210  may have any desired geometry that will draw liquid through the shroud at a high velocity. The impeller is preferably a marine impeller, as shown in  FIG. 3A , but may have a different design, for example, a Rushton impeller as shown in  FIG. 3B , or a modified Rushton impeller, e.g., tilted so as to provide some axial flow. 
     In order to generate the high velocity flow through the shroud, the motor  202  is preferably a high speed, high torque motor, e.g., capable of operating at 500 to 20,000 RPM, e.g., 3,000 to 10,000 RPM. However, the larger the mixer (e.g., the larger the shroud and/or the larger the motor) the lower the rotational speed can be. Thus, if a large mixer is used, such as a 5 hp, 10 hp, 20 hp, or 30 hp or greater, the motor may be designed to operate at lower rotational speeds, e.g., less than 2000 RPM, less than 1500 RPM, or even 500 RPM or less. For example, a mixer sized to mix a 10,000-20,000 liter tank may operate at speeds of 900 to 1,200 RPM. The torque of the motor is preferably self-adjusting, to maintain a relatively constant impeller speed as the mixing conditions change over time. 
     Advantageously, the mixer can be oriented at any desired angle or location in the tank, to direct the jet flow in a desired direction. Moreover, as discussed above, depending on the direction of rotation of the impeller the mixer can be used to draw fluid from either end of the shroud. 
     In some implementations, two or more jet mixers are positioned in the vessel, with one or more being configured to jet fluid upward (“up pump”) and one or more being configured to jet fluid downward (“down pump”). In some cases, an up pumping mixer will be positioned adjacent a down pumping mixer, to enhance the turbulent flow created by the mixers. If desired, one or more mixers may be switched between upward flow and downward flow during processing. It may be advantageous to switch all or most of the mixers to up pumping mode during initial dispersion of the feedstock in the liquid medium, as up pumping creates significant turbulence at the surface. 
     Suction Chamber Jet Mixers 
     Another type of jet mixer includes a primary nozzle that delivers a pressurized fluid from a pump, a suction inlet adjacent the primary nozzle through which ambient fluid is drawn by the pressure drop between the primary nozzle and the wider inlet, and a suction chamber extending between the suction inlet and a secondary nozzle. A jet of high velocity fluid exits the secondary nozzle. 
     An example of this type of mixer is shown in  FIG. 4 . As shown, in mixer  600  pressurized liquid from a pump (not shown) flows through an inlet passage  602  and exits through a primary nozzle  603 . Ambient liquid is drawn through a suction inlet  604  into suction chamber  606  by the pressure drop caused by the flow of pressurized liquid. The combined flow exits from the suction chamber into the ambient liquid at high velocity through secondary nozzle  608 . Mixing occurs both in the suction chamber and in the ambient liquid due to the jet action of the exiting jet of liquid. 
     A mixing system that operates according to a similar principle is shown in  FIG. 4A . Mixers embodying this design are commercially available from ITT Water and Wastewater, under the tradename Flygt™ jet mixers. In system  618 , pump  620  generates a primary flow that is delivered to the tank (not shown) through a suction nozzle system  622 . The suction nozzle system  622  includes a primary nozzle  624  which functions in a manner similar to primary nozzle  603  described above, causing ambient fluid to be drawn into the adjacent open end  626  of ejector tube  628  due to the pressure drop induced by the fluid exiting the primary nozzle. The combined flow then exits the other end  630  of ejector tube  628 , which functions as a secondary nozzle, as a high velocity jet. 
     The nozzle shown in  FIG. 5 , referred to as an eductor nozzle, operates under a similar principle. A nozzle embodying this design is commercially available under the tradename TeeJet®. As shown, in nozzle  700  pressurized liquid flows in through an inlet  702  and exits a primary nozzle  704 , drawing ambient fluid in to the open end  706  of a diffuser  708 . The combined flow exits the opposite open end  710  of the diffuser at a circulation flow rate A+B that is the sum of the inlet flow rate A and the flow rate B of the entrained ambient fluid. 
     Jet Aeration Type Mixers 
     Another type of jet mixing system that can be utilized is referred to in the wastewater industry as “jet aeration mixing.” In the wastewater industry, these mixers are typically used to deliver a jet of a pressurized air and liquid mixture, to provide aeration. However, in the present application in some cases the jet aeration type mixers are utilized without pressurized gas, as will be discussed below. The principles of operation of jet aeration mixers will be initially described in the context of their use with pressurized gas, for clarity. 
     An eddy jet mixer, such as the mixer  800  shown in  FIGS. 6-6B , includes multiple jets  802  mounted in a radial pattern on a central hub  804 . The radial pattern of the jets uniformly distributes mixing energy throughout the tank. The eddy jet mixer may be centrally positioned in a tank, as shown to provide toroidal flow about the center axis of the tank. The eddy jet mixer may be mounted on piping  806 , which supplies high velocity liquid to the eddy jet mixer. In the embodiment shown in  FIG. 6B , air is also supplied to the eddy jet mixer through piping  812 . The high velocity liquid is delivered by a pump  808  which is positioned outside of the tank and which draws liquid in through an inlet  810  in the side wall of the tank. 
       FIGS. 7 and 8  show two types of nozzle configurations that are designed to mix a gas and a liquid stream and eject a high velocity jet. These nozzles are configured somewhat differently from the eddy jet mixer shown in  FIGS. 6 and 6A  but function in a similar manner. In the system  900  shown in  FIG. 7 , a primary or motive fluid is directed through a liquid line  902  to inner nozzles  904  through which the liquid travels at high velocity into a mixing area  906 . A second fluid, e.g., a gas, such as compressed air, nitrogen or carbon dioxide, or a liquid, enters the mixing area through a second line  908  and entrained in the motive fluid entering the mixing area  906  through the inner nozzles. In some instances the second fluid is nitrogen or carbon dioxide so as to reduce oxidation of the enzyme. The combined flow from the two lines is jetted into the mixing tank through the outer nozzles  910 . If the second fluid is a gas, tiny bubbles are entrained in the liquid in the mixture. Liquid is supplied to the liquid line  902  by a pump. Gas, if it is used, is provided by compressors. If a liquid is used as the second fluid, it can have the same velocity as the liquid entering through the liquid line  902 , or a different velocity. 
       FIG. 8  shows an alternate nozzle design  1000 , in which outer nozzles  1010  (of which only one is shown) are positioned along the length of an elongated member  1011  that includes a liquid line  1002  that is positioned parallel to a second line  1008 . Each nozzle includes a single outer nozzle  1010  and a single inner nozzle  1004 . Mixing of the motive liquid with the second fluid proceeds in the same manner as in the system  900  described above. 
       FIGS. 9 and 10  illustrate examples of jet aeration type mixing systems in which nozzles are positioned along the length of an elongated member. In the example shown in  FIG. 9 , the elongated member  1102  is positioned along the diameter of the tank  1104 , and the nozzles  1106  extend in opposite directions from the nozzle to produce the indicated flow pattern which includes two areas of generally elliptical flow, one on either side of the central elongated member. In the example shown in  FIG. 10 , the tank  1204  is generally rectangular in cross section, and the elongated member  1202  extends along one side wall  1207  of the tank. In this case, the nozzles  1206  all face in the same direction, towards the opposite side wall  1209 . This produces the flow pattern shown, in which flow in the tank is generally elliptical about a major axis extending generally centrally along the length of the tank. In the embodiment shown in  FIG. 10 , the nozzles may be canted towards the tank floor, e.g., at an angle of from about 15 to 30 degrees from the horizontal. 
     In another embodiment, shown in  FIG. 11 , the nozzles  1302 ,  1304 , and suction inlet  1306  are arranged to cause the contents of the tank to both revolve and rotate in a toroidal, rolling donut configuration around a central vertical axis of the tank. Flow around the surface of the toroid is drawn down the tank center, along the floor, up the walls and back to the center, creating a rolling helix pattern, which sweeps the center and prevents solids from settling. The toroidal pattern is also effective in moving floating solids to the tank center where they are pulled to the bottom and become homogenous with the tank contents. The result is a continuous helical flow pattern, which minimizes tank dead spots. 
     Backflushing 
     In some instances, the jet nozzles described herein can become plugged, which may cause efficiency and cost effectiveness to be reduced. Plugging of the nozzles may be removed by reversing flow of the motive liquid through the nozzle. For example, in the system shown in  FIG. 12 , this is accomplished by closing a valve  1402  between the pump  1404  and the liquid line  1406  flowing to the nozzles  1408 , and activating a secondary pump  1410 . Secondary pump  1410  draws fluid in through the nozzles. The fluid then travels up through vertical pipe  1412  due to valve  1402  being closed. The fluid exits the vertical pipe  1412  at its outlet  1414  for recirculation through the tank. 
     Mixing in Transit/Portable Mixers 
     In some cases processing can take place in part or entirely during transportation of the mixture, e.g., between a first processing plant for treating the feedstock and a second processing plant for production of a final product. In this case, mixing can be conducted using a jet mixer designed for rail car or other portable use. The mixer can be operated using a control system that is external to the tank, which may include for example a motor and a controller configured to control the operation of the mixer. Venting (not shown) may also be provided. 
     Minimizing Hold Up on Tank Walls 
     In some situations, in particular at solids levels approaching a theoretical or practical limit, material may accumulate along the side wall and/or bottom wall of the tank during mixing. This phenomenon, referred to as “hold up,” is undesirable as it can result in inadequate mixing. Several approaches can be taken to minimize hold up and ensure good mixing throughout the tank. 
     For example, in addition to the jet mixing device(s), the tank can be outfitted with a scraping device, for example a device having a blade that scrapes the side of the tank in a “squeegee” manner. Such devices are well known, for example in the dairy industry. Suitable agitators include the side and bottom sweep agitators and scraper blade agitators manufactured by Walker Engineered Products, New Lisbon, Wis. As shown in  FIG. 14 , a side and bottom sweep agitator  1800  may include a central elongated member  1802 , mounted to rotate about the axis of the tank. Side wall scraper blades  1804  are mounted at each end of the elongated member  1802  and are disposed at an angle with respect to the elongated member. In the embodiment shown, a pair of bottom wall scraper blades  1806  are mounted at an intermediate point on the elongated member  1802 , to scrape up any material accumulating on the tank bottom. These scrapers may be omitted if material is not accumulating on the tank bottom. As shown in  FIG. 14A , the scraper blades  1804  may be in the form of a plurality of scraper elements positioned along the side wall. In other embodiments, the scraper blades are continuous, or may have any other desired geometry. 
     In other embodiments, the jet mixer itself is configured so as to minimize hold up. For example, the jet mixer may include one or more movable heads and/or flexible portions that move during mixing. For example, the jet mixer may include an elongated rotatable member having a plurality of jet nozzles along its length. The elongated member may be planar, as shown in  FIG. 15 , or have a non-planar shape, e.g., it may conform to the shape of the tank walls as shown in  FIG. 16 . 
     Referring to  FIG. 15 , the jet mixer nozzles may be positioned on a rotating elongated member  1900  that is driven by a motor  1902  and shaft  1904 . Water or other fluid is pumped through passageways in the rotating member, e.g., by a pump impeller  1906 , and exits as a plurality of jets through jet orifices  1908  while the member  1900  rotates. To reduce hold up on the tank side walls, orifices  1910  may be provided at the ends of the member  1900 . 
     In the embodiment shown in  FIG. 16 , to conform to the particular shape of the tank  2000  the elongated member includes horizontally extending arms  2002 , downwardly inclined portions  2004 , outwardly and upwardly inclined portions  2006 , and vertically extending portions  2008 . Fluid is pumped through passageways within the elongated member to a plurality of jet orifices  38 , through which jets are emitted while the elongated member is rotated. 
     In both of the embodiments shown in  FIGS. 15 and 16 , the jets provide mixing while also washing down the side walls of the tank. 
     In some implementations, combinations of the embodiments described above may be used. For example, combinations of planar and non-planar rotating or oscillating elongated members may be used. The moving nozzle arrangements described above can be used in combination with each other and/or in combination with scrapers. A plurality of moving nozzle arrangements can be used together, for example two or more of the rotating members shown in  FIG. 15  can be stacked vertically in the tank. When multiple rotating members are used, they can be configured to rotate in the same direction or in opposite directions, and at the same speed or different speeds. 
     Physical Treatment of Feedstock 
     In some implementations, the feedstock is physically treated, e.g., to change its molecular structure. Physical treatment processes can include one or more of any of those described herein, such as mechanical treatment, chemical treatment, irradiation, sonication, oxidation, pyrolysis or steam explosion. Treatment methods can be used in combinations of two, three, four, or even all of these technologies (in any order). When more than one treatment method is used, the methods can be applied at the same time or at different times. Other processes that change a molecular structure of a feedstock may also be used, alone or in combination with the processes disclosed herein. 
     Mechanical Treatments 
     In some cases, methods can include a mechanical treatment. Mechanical treatments include, for example, cutting, milling, pressing, grinding, shearing and chopping. Milling may include, for example, ball milling, hammer milling, rotor/stator dry or wet milling, or other types of milling. Other mechanical treatments include, e.g., stone grinding, cracking, mechanical ripping or tearing, pin grinding or air attrition milling. 
     In some implementations, the feedstock material can first be physically treated by one or more of the other physical treatment methods, e.g., chemical treatment, radiation, sonication, oxidation, pyrolysis or steam explosion, and then mechanically treated. This sequence can be advantageous since materials treated by one or more of the other treatments, e.g., irradiation or pyrolysis, tend to be more brittle and, therefore, it may be easier to further change the molecular structure of the material by mechanical treatment. 
     Feed preparation systems can be configured to produce streams with specific characteristics such as, for example, specific maximum sizes or specific surface areas. 
     Radiation Treatment 
     Irradiation can reduce the molecular weight and/or crystallinity of feedstock. In some embodiments, energy deposited in a material that releases an electron from its atomic orbital is used to irradiate the materials. The radiation may be provided by 1) heavy charged particles, such as alpha particles or protons, 2) electrons, produced, for example, in beta decay or electron beam accelerators, or 3) electromagnetic radiation, for example, gamma rays, x rays, or ultraviolet rays. In one approach, radiation produced by radioactive substances can be used to irradiate the feedstock. In some embodiments, any combination in any order or concurrently of (1) through (3) may be utilized. In another approach, electromagnetic radiation (e.g., produced using electron beam emitters) can be used to irradiate the feedstock. The doses applied depend on the desired effect and the particular feedstock. For example, high doses of radiation can break chemical bonds within feedstock components. In some instances when chain scission is desirable and/or polymer chain functionalization is desirable, particles heavier than electrons, such as protons, helium nuclei, argon ions, silicon ions, neon ions, carbon ions, phosphorus ions, oxygen ions or nitrogen ions can be utilized. When ring-opening chain scission is desired, positively charged particles can be utilized for their Lewis acid properties for enhanced ring-opening chain scission. For example, when maximum oxidation is desired, oxygen ions can be utilized, and when maximum nitration is desired, nitrogen ions can be utilized. 
     Ionizing Radiation 
     Each form of radiation ionizes the carbon-containing material via particular interactions, as determined by the energy of the radiation. Heavy charged particles primarily ionize matter via Coulomb scattering; furthermore, these interactions produce energetic electrons that may further ionize matter. Alpha particles are identical to the nucleus of a helium atom and are produced by the alpha decay of various radioactive nuclei, such as isotopes of bismuth, polonium, astatine, radon, francium, radium, several actinides, such as actinium, thorium, uranium, neptunium, curium, californium, americium, and plutonium. 
     When particles are utilized, they can be neutral (uncharged), positively charged or negatively charged. When charged, the charged particles can bear a single positive or negative charge, or multiple charges, e.g., one, two, three or even four or more charges. In instances in which chain scission is desired, positively charged particles may be desirable, in part due to their acidic nature. When particles are utilized, the particles can have the mass of a resting electron, or greater, e.g., 500, 1000, 1500, 2000, 10,000 or even 100,000 times the mass of a resting electron. For example, the particles can have a mass of from about 1 atomic unit to about 150 atomic units, e.g., from about 1 atomic unit to about 50 atomic units, or from about 1 to about 25, e.g., 1, 2, 3, 4, 5, 10, 12 or 15 amu. Accelerators used to accelerate the particles can be electrostatic DC, electrodynamic DC, RF linear, magnetic induction linear or continuous wave. For example, cyclotron type accelerators are available from IBA, Belgium, such as the Rhodotron® system, while DC type accelerators are available from RDI, now IBA Industrial, such as the Dynamitron®. Ions and ion accelerators are discussed in Introductory Nuclear Physics, Kenneth S. Krane, John Wiley &amp; Sons, Inc. (1988), Krsto Prelec, FIZIKA B 6 (1997) 4, 177-206, Chu, William T., “Overview of Light-Ion Beam Therapy” Columbus-Ohio, ICRU-IAEA Meeting, 18-20 Mar. 2006, Iwata, Y. et al., “Alternating-Phase-Focused IH-DTL for Heavy-Ion Medical Accelerators” Proceedings of EPAC 2006, Edinburgh, Scotland and Leaner, C. M. et al., “Status of the Superconducting ECR Ion Source Venus” Proceedings of EPAC 2000, Vienna, Austria. 
     Gamma radiation has the advantage of a significant penetration depth into a variety of materials. Sources of gamma rays include radioactive nuclei, such as isotopes of cobalt, calcium, technicium, chromium, gallium, indium, iodine, iron, krypton, samarium, selenium, sodium, thalium, and xenon. 
     Sources of x rays include electron beam collision with metal targets, such as tungsten or molybdenum or alloys, or compact light sources, such as those produced commercially by Lyncean. 
     Sources for ultraviolet radiation include deuterium or cadmium lamps. 
     Sources for infrared radiation include sapphire, zinc, or selenide window ceramic lamps. 
     Sources for microwaves include klystrons, Slevin type RF sources, or atom beam sources that employ hydrogen, oxygen, or nitrogen gases. 
     In some embodiments, a beam of electrons is used as the radiation source. A beam of electrons has the advantages of high dose rates (e.g., 1, 5, or even 10 Mrad per second), high throughput, less containment, and less confinement equipment. Electrons can also be more efficient at causing chain scission. In addition, electrons having energies of 4-10 MeV can have a penetration depth of 5 to 30 mm or more, such as 40 mm. 
     Electron beams can be generated, e.g., by electrostatic generators, cascade generators, transformer generators, low energy accelerators with a scanning system, low energy accelerators with a linear cathode, linear accelerators, and pulsed accelerators. Electrons as an ionizing radiation source can be useful, e.g., for relatively thin piles of materials, e.g., less than 0.5 inch, e.g., less than 0.4 inch, 0.3 inch, 0.2 inch, or less than 0.1 inch. In some embodiments, the energy of each electron of the electron beam is from about 0.3 MeV to about 2.0 MeV (million electron volts), e.g., from about 0.5 MeV to about 1.5 MeV, or from about 0.7 MeV to about 1.25 MeV. 
     Electron beam irradiation devices may be procured commercially from Ion Beam Applications, Louvain-la-Neuve, Belgium or the Titan Corporation, San Diego, Calif. Typical electron energies can be 1 MeV, 2 MeV, 4.5 MeV, 7.5 MeV, or 10 MeV. Typical electron beam irradiation device power can be 1 kW, 5 kW, 10 kW, 20 kW, 50 kW, 100 kW, 250 kW, or 500 kW. The level of depolymerization of the feedstock depends on the electron energy used and the dose applied, while exposure time depends on the power and dose. Typical doses may take values of 1 kGy, 5 kGy, 10 kGy, 20 kGy, 50 kGy, 100 kGy, or 200 kGy. 
     Ion Particle Beams 
     Particles heavier than electrons can be utilized to irradiate hydrocarbon-containing materials. For example, protons, helium nuclei, argon ions, silicon ions, neon ions carbon ions, phosphorus ions, oxygen ions or nitrogen ions can be utilized. In some embodiments, particles heavier than electrons can induce higher amounts of chain scission (relative to lighter particles). In some instances, positively charged particles can induce higher amounts of chain scission than negatively charged particles due to their acidity. 
     Heavier particle beams can be generated, e.g., using linear accelerators or cyclotrons. In some embodiments, the energy of each particle of the beam is from about 1.0 MeV/atomic unit to about 6,000 MeV/atomic unit, e.g., from about 3 MeV/atomic unit to about 4,800 MeV/atomic unit, or from about 10 MeV/atomic unit to about 1,000 MeV/atomic unit. 
     In certain embodiments, ion beams can include more than one type of ion. For example, ion beams can include mixtures of two or more (e.g., three, four or more) different types of ions. Exemplary mixtures can include carbon ions and protons, carbon ions and oxygen ions, nitrogen ions and protons, and iron ions and protons. More generally, mixtures of any of the ions discussed above (or any other ions) can be used to form irradiating ion beams. In particular, mixtures of relatively light and relatively heavier ions can be used in a single ion beam. 
     In some embodiments, ion beams for irradiating materials include positively-charged ions. The positively charged ions can include, for example, positively charged hydrogen ions (e.g., protons), noble gas ions (e.g., helium, neon, argon), carbon ions, nitrogen ions, oxygen ions, silicon atoms, phosphorus ions, and metal ions such as sodium ions, calcium ions, and/or iron ions. Without wishing to be bound by any theory, it is believed that such positively-charged ions behave chemically as Lewis acid moieties when exposed to materials, initiating and sustaining cationic ring-opening chain scission reactions in an oxidative environment. 
     In certain embodiments, ion beams for irradiating materials include negatively-charged ions. Negatively charged ions can include, for example, negatively charged hydrogen ions (e.g., hydride ions), and negatively charged ions of various relatively electronegative nuclei (e.g., oxygen ions, nitrogen ions, carbon ions, silicon ions, and phosphorus ions). Without wishing to be bound by any theory, it is believed that such negatively-charged ions behave chemically as Lewis base moieties when exposed to materials, causing anionic ring-opening chain scission reactions in a reducing environment. 
     In some embodiments, beams for irradiating materials can include neutral atoms. For example, any one or more of hydrogen atoms, helium atoms, carbon atoms, nitrogen atoms, oxygen atoms, neon atoms, silicon atoms, phosphorus atoms, argon atoms, and iron atoms can be included in beams that are used for irradiation of hydrocarbon-containing materials. In general, mixtures of any two or more of the above types of atoms (e.g., three or more, four or more, or even more) can be present in the beams. 
     In certain embodiments, ion beams used to irradiate materials include singly-charged ions such as one or more of H + , H − , He + , Ne + , Ar + , C + , C − , O + , O − , N + , N − , Si + , Si − , P + , P − , Na + , Ca + , and Fe + . In some embodiments, ion beams can include multiply-charged ions such as one or more of C 2+ , C 3+ , C 4+ , N 3+ , N 5+ , N 3− , O 2+ , O 2− , O 2   2− , Si 2+ , Si 4+ , Si 2− , and Si 4− . In general, the ion beams can also include more complex polynuclear ions that bear multiple positive or negative charges. In certain embodiments, by virtue of the structure of the polynuclear ion, the positive or negative charges can be effectively distributed over substantially the entire structure of the ions. In some embodiments, the positive or negative charges can be somewhat localized over portions of the structure of the ions. 
     Electromagnetic Radiation 
     In embodiments in which the irradiating is performed with electromagnetic radiation, the electromagnetic radiation can have, e.g., energy per photon (in electron volts) of greater than 10 2  eV, e.g., greater than 10 3 , 10 4 , 10 5 , 10 6 , or even greater than 10 7  eV. In some embodiments, the electromagnetic radiation has energy per photon of between 10 4  and 10 7 , e.g., between 10 5  and 10 6  eV. The electromagnetic radiation can have a frequency of, e.g., greater than 10 16  H z , greater than 10 17  Hz, 10 18 , 10 19 , 10 20 , or even greater than 10 21  Hz. In some embodiments, the electromagnetic radiation has a frequency of between 10 18  and 10 22  Hz, e.g., between 10 19  to 10 21  Hz. 
     Doses 
     In some embodiments, the irradiating (with any radiation source or a combination of sources) is performed until the material receives a dose of at least 0.25 Mrad, e.g., at least 1.0 Mrad, at least 2.5 Mrad, at least 5.0 Mrad, or at least 10.0 Mrad. In some embodiments, the irradiating is performed until the material receives a dose of between 1.0 Mrad and 6.0 Mrad, e.g., between 1.5 Mrad and 4.0 Mrad. 
     In some embodiments, the irradiating is performed at a dose rate of between 5.0 and 1500.0 kilorads/hour, e.g., between 10.0 and 750.0 kilorads/hour or between 50.0 and 350.0 kilorads/hours. 
     In some embodiments, two or more radiation sources are used, such as two or more ionizing radiations. For example, samples can be treated, in any order, with a beam of electrons, followed by gamma radiation and UV light having wavelengths from about 100 nm to about 280 nm. In some embodiments, samples are treated with three ionizing radiation sources, such as a beam of electrons, gamma radiation, and energetic UV light. 
     Sonication, Pyrolysis and Oxidation 
     In addition to radiation treatment, the feedstock may be treated with any one or more of sonication, pyrolysis and oxidation. These treatment processes are described in U.S. Ser. No. 12/417,840, the disclosure of which is incorporated by reference herein. 
     Other Processes 
     Any of the processes of this paragraph can be used alone without any of the processes described herein, or in combination with any of the processes described herein (in any order): steam explosion, acid treatment (including concentrated and dilute acid treatment with mineral acids, such as sulfuric acid, hydrochloric acid and organic acids, such as trifluoroacetic acid), base treatment (e.g., treatment with lime or sodium hydroxide), UV treatment, screw extrusion treatment (see, e.g., U.S. Patent Application Ser. No. 61/073,530, filed Nov. 18, 2008, solvent treatment (e.g., treatment with ionic liquids) and freeze milling (see, e.g., U.S. patent application Ser. No. 61/081,709). 
     OTHER EMBODIMENTS 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. 
     For example, the jet mixers described herein can be used in any desired combination, and/or in combination with other types of mixers. 
     The jet mixer(s) may be mounted in any desired position within the tank. With regard to shaft-mounted jet mixers, the shaft may be collinear with the center axis of the tank or may be offset therefrom. For example, if desired the tank may be provided with a centrally mounted mixer of a different type, e.g., a marine impeller or Rushton impeller, and a jet mixer may be mounted in another area of the tank either offset from the center axis or on the center axis. In the latter case one mixer can extend from the top of the tank while the other extends upward from the floor of the tank. Moreover, as shown in  FIG. 13 , two or more jet mixers can be mounted in a multi-level arrangement at different heights within the tank. 
     In any of the jet mixing systems described herein, the flow of fluid (liquid and/or gas) through the jet mixer can be continuous or pulsed, or a combination of periods of continuous flow with intervals of pulsed flow. When the flow is pulsed, pulsing can be regular or irregular. In the latter case, the motor that drives the fluid flow can be programmed, for example to provide pulsed flow at intervals to prevent mixing from becoming “stuck.” The frequency of pulsed flow can be, for example, from about 0.5 Hz to about 10 Hz, e.g., about 0.5 Hz, 0.75 Hz, 1.0 Hz, 2.0 Hz, 5 Hz, or 10 Hz. Pulsed flow can be provided by turning the motor on and off, and/or by providing a flow diverter that interrupts flow of the fluid. 
     While tanks have been referred to herein, jet mixing may be used in any type of vessel or container, including lagoons, pools, ponds and the like. If the container in which mixing takes place is an in-ground structure such as a lagoon, it may be lined. The container may be covered, e.g., if it is outdoors, or uncovered. 
     While hydrocarbon-containing feedstocks have been described herein, other feedstocks and mixtures of hydrocarbon-containing feedstocks with other feedstocks may be used. For example, some implementations may utilize mixtures of hydrocarbon-containing feedstocks with biomass feedstocks such as those disclosed in U.S. Provisional Application No. 61/218,832, filed Jun. 19, 2009, the full disclosure of which is incorporated by reference herein. 
     Accordingly, other embodiments are within the scope of the following claims.