PRETREATMENT METHOD OF CELLULOSIC BIOMASS VIA FLOWABILITY CONTROL AND REACTIVE EXTRUSION PROCESS

Abstract

In a pretreatment process for lignocellulosic biomass, lignin is separated from cellulose by a reactive extrusion process with flowability control of the biomass suspension. Flowability control via incorporation of flowability control agents renders biomass suspensions processable using equipment in which relatively high shearing stresses and pressures can be applied. In one embodiment, particulate cellulosic biomass is added to a fully-intermeshing twin-screw extruder with pH adjusters, water and flowability control agents. Upon the incorporation of the flowability control agent, the biomass suspension flows without phase separation and thus can be subjected to high mechanical shearing stresses and chemical reaction in the extruder. The screw configuration creates multiple mixing zones within the confines of the extruder. The flowability control agent may be a water-soluble polymer that allows a hydrogel to be formed or may be a recycled portion of the black liquor from the reactive extrusion process.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Patent Application No. 61/435,502, filed Jan. 24, 2011, which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

FIELD OF THE INVENTION

The present invention relates to systems and methods for pretreatment of lignocellulosic biomass, more particularly, to a system for delignification of cellulosic biomass via flowability control and/or reactive extrusion processes.

BACKGROUND OF THE INVENTION

One of the pressing technological challenges of the present day is to find economical means of converting cellulosic biomass to ethanol and other biofuels. Wood wastes are one form of cellulosic biomass presently being considered as a feedstock for conversion in fermentation processes. Cellulosic biomass comprises three main components: cellulose, hemicellulose and lignin. Cellulose and hemicellulose are, respectively, a polymer of glucose and a heteropolymer of various sugars, and are fermentable. Lignin is a complex polymer of phenylpropane and is not readily fermented. Typically, cellulose is typically present in amounts of about 40-50% of the biomass, hemicellulose in amounts of about 25-35%, and lignin in amounts of about 15-20%.

Lignin provides structural integrity to the biomass fibers by acting like a glue to bind the cellulose and hemicellulose polymers into fibers. Its presence inhibits the disruption of the biomass structure that is necessary to facilitate biomass fermentation. Upon the disruption of the structure of the biomass and the separation of the lignin binder, the biomass can then be treated by biological means to allow the conversion of the cellulose and hemicellulose to ethanol and biofuels. The removal of the lignin also provides for the production of a more consistent synthesis gas since the lignin concentration changes from one source of cellulosic biomass to another, which renders it difficult to keep the reactor conditions at optimum and to standardize the gas.

Pretreatment is required to disrupt the fibrous structures and separate the lignin, as well as reduce the size of the biomass particles and increase their surface area and pore sizes. Effective pretreatment is thus one of the most important, yet most expensive, steps of the biofuel production process.

There are currently a number of pretreatment methods under development that use physical, chemical and/or biological approaches. Methods such as fine grinding and size reduction, solvation, microfibrillation, irradiation, ultrasonication, explosive depressurization, freeze explosion, chemical derivatization and wood pulping have been proposed for the separation of the biomass components from each other to render the cellulosic substrates readily hydrolysable. Other methods include steam explosion, wet oxidation under alkaline conditions, super critical carbon dioxide pretreatment, mild and concentrated acid hydrolysis, and solvent extractions.

The cost of pretreatment depends largely on the particle size that is to be attained and the physical characteristics of the biomass. Overall, there is no well established and universally recognized pretreatment method that is economically feasible for the pretreatment of cellulosic biomass. Elevated temperatures need to be used in hydrothermal pretreatment and such high temperatures require large energy inputs and usually lead to degradation of carbohydrates. Chemical pretreatment using acids or bases promote the chemical hydrolysis of the biomass components and improve enzymatic hydrolysis by removing lignin. The use of even dilute acids necessitates costly equipment and neutralization of the hydrolysate prior to subsequent biological treatment steps. Biological pretreatments employ microorganisms to disrupt the biomass structure, but require a great deal of time to produce a desirable feedstock. Thus, there is a significant need for the development of a viable pretreatment method for large-scale biofuel production from biomass.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a pretreatment method for cellulosic biomass comprises the steps of creating a suspension of particulate cellulosic biomass, adding a flowability control agent to the suspension, adjusting the pH of the suspension to achieve a pH that promotes the separation of lignin from the cellulose into a black liquor phase, mechanically shearing the suspension while controlling the temperature of the suspension. In some embodiments of the invention, the flowability control agent includes a pH-insensitive water-soluble polymer that can form a hydrogel.

In other embodiments of the invention, the flowability control agent includes a portion of the black liquor in a recycle stream. In yet other embodiments of the invention, the flowability control agent includes other polymers which can impart flowability under relatively high shearing stresses and pressures.

In another aspect of the invention, lignin is separated from a particulate cellulosic biomass by a reactive extrusion process performed on suspensions of the biomass with incorporated flowability control agents. In some embodiments of this aspect of the invention, the separation is performed as a continuous process within a fully-intermeshing twin-screw extruder. In further embodiments of the invention, the screws of the extruder are configured to create mixing zones separated by suspension seals. In some such further embodiments, the temperatures of the mixing zones are controlled such that a first mixing zone has a temperature below the boiling temperature of water and a second mixing zone has a temperature above the boiling temperature of water.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is made to the following detailed description of an exemplary embodiment considered in conjunction with the accompanying drawings, in which:

FIG. 1 is a plot of the typical load versus time generated during an attempt to force a concentrated suspension of cellulosic biomass treated according to a delignification method of the prior art through a capillary;

FIG. 2 is a photograph of a biomass mat formed within the extruder during the delignification method of FIG. 1, which does not include the use of a flowability control agent;

FIG. 3 is a microphotograph showing the fibrous structure of the biomass mat of FIG. 2;

FIG. 4 is a schematic illustration of a fully-intermeshing twin-screw extruder used in a first delignification process according to an embodiment of the present invention;

FIG. 5 is a microphotograph of a sample of cellulosic biomass recovered from the first delignification process of FIG. 4 under a first set of process conditions;

FIG. 6 is a microphotograph of a sample of cellulosic biomass recovered from the first delignification process of FIG. 4 under a second set of process conditions;

FIG. 7 is a microphotograph of a sample of cellulosic biomass recovered from the first delignification process of FIG. 4 under a third set of process conditions;

FIG. 8 is a microphotograph of a sample of cellulosic biomass recovered from the first delignification process of FIG. 4 under a fourth set of process conditions;

FIG. 9 is a flowchart of a second delignification process according to an embodiment of the present invention;

FIG. 10 is a plot of the results of a dynamic frequency sweep test conducted on a white liquor with a flowability control agent (carboxy methyl cellulose) used in the first delignification process of FIG. 4;

FIG. 11 is a plot of the results of a dynamic sweep frequency test conducted on black liquor prepared according to an embodiment of the present invention;

FIG. 12 is a plot of the magnitude of complex viscosity of the black liquor of FIG. 11 as a function of water concentration;

FIG. 13 is a schematic illustration of a fully-intermeshing twin-screw extruder used in a second delignification process according to another embodiment of the present invention;

FIG. 14 is a microphotograph of a biomass sample recovered from the second delignification process of FIG. 13 under a fifth set of process conditions;

FIG. 15 is a scanning electron microscopy (SEM) image of the biomass sample of FIG. 14;

FIG. 16 is a microphotograph of a biomass sample recovered from the second delignification process of FIG. 13 under a sixth set of process conditions;

FIG. 17 is a scanning electron microscopy (SEM) image of the biomass sample of FIG. 16;

FIG. 18 is a schematic depiction of the extruder of FIG. 13 after the second delignification process, showing locations where biomass samples were collected;

FIG. 18A is a microphotograph of a first biomass sample collected from the extruder of FIG. 18;

FIG. 18B is a microphotograph of a second biomass sample collected from the extruder of FIG. 18;

FIG. 18C is a microphotograph of a third biomass sample collected from the extruder of FIG. 18;

FIG. 18D is a microphotograph of a fourth biomass sample collected from the extruder of FIG. 18;

FIG. 19 is an SEM image of the biomass sample of FIG. 18A;

FIG. 20 is an SEM image of the biomass sample of FIG. 18B;

FIG. 21 is an SEM image of the biomass sample of FIG. 18C; and

FIG. 22 is an SEM image of the biomass sample of FIG. 18D.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect of the present invention, cellulosic biomass is pretreated and processed to allow the efficient separation of lignin and hemicellulose from the cellulose fibers of the biomass. In another aspect of the present invention, the pretreatment is performed as a continuous process that combines the application of high shearing stresses and chemical treatment in the confines of a single fully-intermeshing co-rotating apparatus that is also utilized to convey the biomass. Both aspects include flowability control by incorporating a flowability control agent in the biomass suspension, preferably at relatively low concentrations, to allow the biomass suspension to flow in the extruder, especially without phase separation.

The maximum solids concentration of biomass in traditional pretreatment processes is around 12-15% by weight. One reason that this concentration cannot be higher is that the shear viscosity of a biomass/water suspension is very high. In fact, aqueous suspensions of lignocellulosic biomass are sometimes referred to as “damp solids.” The torque required to agitate the water/biomass suspension is thus very high, and is reported to being proportional to the cube of the solids content from about 2-8%, and increasing even more rapidly above a concentration of about 8%. Specialized equipment is required to handle suspensions having such high viscosity, therefore, and it is not possible to increase the concentration of the biomass phase to greater than about 12-15% using only conventional equipment and methodologies. Aqueous biomass suspensions with relatively high biomass concentrations give rise to the ready separation of the biomass from the liquid phase under pressure and shear and thus cannot be subjected to relatively high shearing stresses and pressures.

The commercial prior art does not include flowability control processes for biomass prior to or during pretreatment. For example, in the use of extrusion technologies for pretreatment of biomass suspensions, no commercial prior art exists for the control of the flowability of biomass suspensions during or prior to extrusion so that the application of high shearing stresses within sealed mixing sections of the extruder become possible. Generally, simple right-handed Archimedean screws can pump the biomass at relatively low rates and mix it with chemical treatment agents. However, such devices are limited with regard to the range of shearing stresses that can be applied and the degree of backmixing that can be achieved. Flowability control of the biomass suspensions during chemical treatment allows the application of high shearing stresses at relatively high rates and backmixing of the reaction medium. In embodiments of the present invention, the use of various types of flowability control agents including a polymeric gelling agent, a polymer or recycling-based use of the black liquor that is separated as part of the process facilitate the delignification process. Further, embodiments of the present invention allow biomass to be processed at relatively low temperatures, reducing energy consumption and biomass degradation that result from conventional methods.

An exemplary method for the pretreatment and processing of a cellulosic biomass, according to an embodiment of the present invention, enables the efficient removal of the lignin and hemicellulose from the cellulose fibers of the cellulosic biomass. In such a method, the cellulosic biomass is fed into a fully-intermeshing co-rotating twin screw extruder, followed by addition of chemical treatment and the flowability control agents (e.g., NaOH solution and carboxy methyl cellulose, CMC, or black liquor, respectively). The screw configuration is arranged such that multiple mixing zones with suspension seals may form in the extruder and such that kneading blocks may apply high shearing stresses in the space between the blocks and between the kneading blocks and the extruder barrel. The suspension, consisting of the biomass, chemical treatment agents and flowability control agents, is flowable, and thus can be subjected to high shearing stresses during the extrusion process. This allows suspension seals to form, inhibiting the steam generated during extrusion from readily escaping the extruder. Thus, the reactive extrusion process occurs under relatively high shearing stresses and temperatures with steam in abundance. At the discharge end of the extruder, the temperature of the extruder barrel is lowered to allow the removal of the reaction products. Subsequent to removal from the extruder, the reaction products are filtered and washed to separate the cellulose from a solution of lignin and hemicelluloses (i.e., the “black liquor”), a portion of which may be recycled back into the extruder to impart flowability to the biomass suspension.

Exemplary Embodiments of the Invention

The embodiments of the invention discussed herein are exemplary in nature and are not intended to limit the scope of the invention. A person having ordinary skill in the relevant arts, and having possession of the present disclosure, will recognize that numerous modifications and variations may be made to the examples presented herein without departing from the scope and spirit of the present invention.

Materials and Procedures for the Exemplary Embodiments

Random mixtures of quercus, pinus and abies hardwoods with particle sizes of approximately 1 mm (i.e., saw dust) were used as the biomass stock to demonstrate exemplary embodiments of the invention. Commercial lye with 98.5% sodium hydroxide (“NaOH”) was purchased from Boyer Corporation, IL. Carboxy methyl cellulose (CMC) type 12M31XP was obtained from Hercules, Va. The white liquor used for the first part of the study contained 14% sodium hydroxide, and was gelled with 4% CMC in the confines of the extruder. Excess black liquor was also produced from biomass and white liquor using a Parr reactor, and fed to the extruder to simulate a recycle black liquor stream. Upon separating the excess black liquor from the cellulose, the water content of the excess black liquor was adjusted to match the flowability of white liquor containing 4% CMC, and the molarity of NaOH in the excess black liquor was adjusted to match the molarity of NaOH in the white liquor (i.e., 4.16 M).

To determine the lignin recovery efficiency, the black liquor from the extruder was separated from the pretreated biomass, and lignin was precipitated from the black liquor at low pH. To do this, a sample of pretreated biomass was placed on filter paper and washed with hot water. The sample remaining on the filter paper was dried in an oven at a temperature in the range of about 80-90° C. for 30 min and weighed. Black liquor separated from the pretreated biomass was titrated to a pH of 1.8 with sulfuric acid and heated to 85° C., then centrifuged at 7500 rpm for 45 min at about 23° C. The liquid phase was decanted and the precipitated lignin was weighed upon drying.

Equipment

The pretreatment process was developed as a twin-screw extrusion process in the fully-intermeshing co-rotating mode. Such a twin-screw extruder has a relatively high surface-to-volume ratio and is ideal for processing complex fluids, including highly-filled suspensions. Using such equipment, it is possible to impose high shearing and extensional stresses under accurate temperature-controlled conditions. Furthermore, this type of extruder can be utilized as a drag flow reactor. In a conventional batch reactor, mixing becomes a significant issue for suspensions which are very viscous. However, there are no such viscosity limitations in a drag flow device like the twin-screw extruder, since significant pressures can be generated and the viscous suspension can be readily mixed within the high surface-to-volume ratio associated with the mixing volume of the extruder. At similar production rates, the continuous twin-screw extrusion process has only a few pounds of material in its mixing volume at any given time. A batch reactor typically would need to hold significantly higher volumes of biomass than a twin-screw reactor to produce at the same rate, and would have difficulties in providing adequate temperature control and torque.

The pretreatment studies discussed herein were carried out using a 50.8 mm co-rotating fully intermeshing APV Baker Perkins twin screw extruder, which is represented in several of the figures of the present application, as discussed subsequently herein. The extruder was equipped with a modular screw which allowed the testing of different screw configurations with varying pressure generation capabilities and myriad stress histories. Combinations of pressurization elements and kneading blocks configured at different forward and reverse stagger angles were utilized, as discussed further hereinbelow. The extruder was furnished with multiple feed ports and temperature control zones which allowed implementation of precise thermal histories. The twin-screw extruder was run both with and without a rectangular slit die with an infinitely adjustable gap.

To implement the exemplary treatments discussed herein, the screw speed was maintained at 50 rpm and barrel zone temperatures were adjusted to from about 70-120° C., increasing in the down-stream direction, and the discharge was maintained at 90° C. The mixed hardwood feedstock discussed above was introduced to the extruder at a first feed port using a Brabender DDW-H20 loss-in-weight feeder in coarse powder form. White liquor and recycled black liquor were fed through a second feed port using a Zenith gear pump, and CMC was fed through a mini K-Tron K-CL-MT12 loss-in-weight feeder. The total inlet flow rate to the extruder (i.e., hardwood and white and/or black liquors) was maintained at about 10-15 lb/h. In cases where recycle black liquor was used as the flowability control agent, the ratios of hardwood feed to white/black liquor feed were varied from 0.36 to 0.89 to find an optimum recycle ratio for black liquor.

An ARES rotational rheometer (TA Instruments, New Castle, Del.) was used in conjunction with 25 mm parallel plate fixtures for rheological characterization of the white and black liquors. Thermo gravimetric analysis (TGA) was used to determine the water content of the black liquor by using a Q50 TGA (TA Instruments, New Castle, Del.). The treated biomass was filtered to remove the black liquor, washed and dried prior to examination by optical microscopy (Optiphot2-POL polarizing microscope, Nikon) and by scanning electron microscopy (Leo Gemini 982 SEM, SPI module spotter gold coater).

Results

The exemplary processes of the present invention delignify biomass within the confines of a twin-screw extruder at high shearing stresses, low temperatures and small residence times. This combination of features is achieved by controlling the flowability of the biomass suspension. Controlling flowability via the incorporation of flowabiity control agents makes it possible to exert the high shearing forces needed to break up the biomass structure and expose the cellulose fibers, to a degree that is not achieved by conventional pretreatment methods.

Initial studies were carried out in a batch reactor under conditions resembling the commercial Kraft process using NaOH solution (i.e., “white liquor”). The flowability of the biomass suspension fed to the batch reactor was tested by capillary rheometry, wherein an attempt was made to extrude the suspension through a capillary die with a relatively large internal diameter (D=0.0984 in, L/D=40) using an Instron capillary rheometer. FIG. 1 is a plot of the force (“load”) needed to force the suspension into the capillary over time. It was observed that, initially, a liquid-rich phase was extruded through the capillary under low pressures, followed by a sharp increase in the pressure needed to extrude the remaining material. This is indicative of a filtering of the liquid phase (i.e., phase separation). As the liquid phase was filtered, the remaining material in the barrel eventually formed a packed solid bed having no flowability. The material remaining in the capillary was the compacted biomass, free of the liquid phase that was incorporated into it.

An attempt was also made to process the same mixture of biomass with white liquor in the twin-screw extruder without the benefit of a flowability control agent. Consistent with the results of the capillary rheometry experiments, the biomass/white liquor suspension could not be processed in the twin-screw extruder. The biomass suspension phase separated in the extruder, and was observed to be compacted to a rather dry state at the reversely configured screw sections. FIG. 2 is a photograph of a sample of a solid biomass mat formed on the fully-flighted reversely-configured screw sections and kneading blocks of the extruder. FIG. 3 is a microphotograph showing the fibrous structure of the biomass mat.

Pretreatment Upon Gelling of the Biomass with CMC:

Several flowability control agents were considered for imparting flowability to the biomass/white liquor mixture to provide a consistently extrudable suspension. Suitable gelling agents for use as flowability control agents in the present invention would include water-soluble polymers, preferably those that form hydrogels. Such gelling agents include, without limitation, CMC (discussed above), polyacrylic acid and its salts, chitosan, polyethylene glycol (PEG), polyethylene oxide (PEO), alginate, sodium alginate, and polyvinyl alcohol (PVA). CMC was selected as the flowability control agent for the first set of experiments discussed herein due to its environmental inertness and its stability in the high temperature and high pH environment of the chemical pretreatment process.

FIG. 4 illustrates the twin-screw arrangement used in the first set of experiments. The twin-screw extruder 10 is shown in an open-barrel condition. In operation, the extruder body 12 would be closed such that the screws would be enclosed in a barrel 14, parts of which are visible. One of the two screws is visible (i.e., screw 16). The other screw is obscured by the screw 16, but extends parallel to the screw 16 and consists of screw elements and kneading blocks similar to those of screw 16, as described herein. Screw 16 was assembled to create first and second mixing zones 18, 20, and a discharge zone 22. The first mixing zone 18 comprised regular-flighted right-handed screw elements 24 connected to a set of four pairs (2″ axial length) of 90° neutral kneading blocks 26. The second mixing zone 20 was immediately downstream of and adjacent to the first mixing zone 18. The second mixing zone 20 comprised regular-flighted right-handed screw elements 28 connected to a 30° forwarding set of kneading blocks 32 (3″ axial length) followed by 5.5″ of 30° reversely-configured kneading blocks 34. The discharge zone 22 comprised regular-flighted right-handed screw elements 36, leading to the discharge end 38 of the extruder 10.

The CMC was introduced with the biomass feed 40 (i.e., the mixed hardwood particles, or “saw dust”) at the inlet end 42 of the extruder 10 to achieve 4% CMC by weight of the white liquor (14% sodium hydroxide in water), which was introduced as a separate liquid stream 44 near the biomass feed 40.

In embodiments of the present invention, the temperature in the first mixing zone is maintained at a temperature below the boiling temperature of water. In the present exemplary embodiment, the temperature in the first mixing zone 18 was maintained at 70° C. Thus, water evaporation was minimal while the temperature was sufficiently high to allow rapid gelling of the white liquor/CMC mixture. The presence of the neutrally-configured kneading blocks 26 allowed the formation of a suspension seal (i.e., a location at which sufficient pressure was generated to allow the sealing of the second mixing zone 20 from the first mixing zone 18). Thus, the extruder 10 was no longer open towards the inlet end 42 of the extruder 10. It should be noted that without the presence of the CMC to impart flowability to the biomass suspension, the biomass suspension would not have traversed past the kneading blocks 26, and the screw elements 24 would have been jammed by biomass. The barrel temperature of the second mixing zone 20 was maintained at 120° C., and was reduced to about 90° C. in the discharge zone 22.

The flow rate of biomass suspension was maintained at 10 lb/hr, but the relative concentrations of the biomass and white liquor feed streams were varied systematically. The experiments started with a biomass suspension with 27% biomass by weight at a feed rate of 10 lb/hr, and proceeded to a suspension with 46% biomass by weight. The formulations tested are shown below in Table 1.

TABLE 1
Feed stream compositions for pretreatment tests (weight %)
	Condition 1	Condition 2	Condition 3	Condition 4
Water	60.2%	52.6%	48.2%	44.5%
NaOH	10.4%	9.1%	8.3%	7.7%
CMC	2.4%	2.1%	1.9%	1.8%
Biomass	27.0%	36.3%	41.6%	46.0%
(mixture of
hardwoods)

As the biomass feed rate was increased, white liquor and CMC feed rates were decreased to maintain a constant total mass flow rate. The process was maintained at steady state, as demonstrated by the stabilization of torque and pressure readings during the tests (data not shown). When the system reached steady-state conditions, samples were collected for analysis from the discharge end 38 of the extruder 10 (see FIG. 4). Under Condition 1, mixing and extrusion generated only 20-40 psi of pressure at the 30° reversely-configured kneading blocks 34, and torque of 17% of the total torque capacity of the extruder 10. Upon increasing the biomass concentration to 36.3%, which is Condition 2, the pressure at the kneading blocks 34 increased to 25-71 psi, with a torque of about 27% of the total torque capacity of the extruder 10. Under Condition 3, with 41.6% biomass concentration at the extruder inlet end 42, the pressures in the second mixing zone 20 varied between 40-90 psi, and the torque increased to about 36% of the total torque capacity of the extruder 10. At the maximum biomass concentration of 46%, which is Condition 4, the pressures were as high as 110 psi and the torque leveled off at 41% of the total torque capacity of the extruder 10. The torque readings were directly proportional to the specific energy input and, hence, the shear stress applied on the biomass suspension during extrusion.

Upon washing and filtering the pretreated biomass exiting the discharge zone 22, specimens of the recovered biomass were investigated using optical microscopy. FIGS. 5-8 are microphotographs of the biomass recovered under Conditions 1-4, respectively. Condition 1 samples, represented by FIG. 5, did not undergo significant size reduction and no major fibrillation was evident. Under Condition 2, particle size reduction and emergence of the fibrous structure was observed (see FIG. 6). Under Conditions 3 and 4 (see FIGS. 7 and 8, respectively), large particles were completely absent, and washed biomass samples were primarily comprised of fibrous cellulose structures. The recovered biomass transitioned from brown to white in color from Conditions 1 to 4. This change in color and the observed increase in fibrillation are indicative of delignification. The rate of structural disruption necessary to release the lignin binder of the biomass was favorably affected by the increased shear viscosity and the elasticity of the biomass/CMC/white liquor suspension. Upon the successive increases of the biomass concentration going from Condition 1 to Condition 4, the shear viscosity of the suspension increased, as is evident from the increases in the torque and pressure values. Consequently, the shearing stresses that are applied at the kneading block sections 26, 32, 34 increased correspondingly. In Conditions 3 and 4, the material coming out of the extruder 10 could be formed into shapes while Conditions 1 and 2 generated softer suspensions which did not have enough elasticity to hold their shapes. The favorable degree of delignification that was achieved under Conditions 3 and 4 can thus be attributed to the much greater specific energy input and stress magnitudes that could be applied during extrusion process, which effectively disrupted the biomass. This is again evident from a comparison of the torque and pressure readings obtained during extrusion under Conditions 1 and 2 with those obtained under Conditions 3 and 4.

Under Condition 4, it was more difficult to wash the black liquor from the recovered biomass than from biomass treated under Conditions 1-3. This is consistent with the more highly concentrated black liquor obtained under Condition 4, which surpassed the concentration of biomass used in the commercial Kraft process, and, hence, exhibited significantly greater shear viscosity than the black liquor generated under Conditions 1 and 2. As noted above, the accompanying elasticity even allowed the recovered biomass to be formed into the shape of a rectangular slit die at the discharge end 38 of the extruder 10. The white color of cellulose could still be obtained from Conditions 3 and 4, however, a more rigorous washing technique was required.

Hardwood biomass typically has about 45% cellulose, about 20% lignin and about 35% hemicellulose, which corresponds to a ratio of solid cellulose to lignin of typically about 2.25. The theoretical ratio indicating complete removal of lignin and hemicellulose from a biomass sample is therefore about 2.25, with the lignin measured in the recovered black liquor. If the lignin and hemicellulose separation is not fully achieved, the ratio of cellulose in the solid phase (i.e., cellulose plus unseparated lignin and hemicellulose) to recovered lignin will be higher than 2.25, approaching infinity for completely unsuccessful separation. The ratio of solid cellulose to recovered lignin in samples recovered from the exemplary pretreatment process of the present invention was estimated to be 2.64, indicating that this embodiment of the present invention resulted in successful pretreatment and separation of lignin and hemicellulose from the cellulose.

Pretreatment Using Recycled Black Liquor as the Flowability Control Agent:

In the first set of experiments (discussed above), CMC was used as the flowability control agent in the white liquor to provide elasticity and flowability to the biomass suspension, which would otherwise have separated into its two phases (i.e., liquor and biomass). Although the concentration of CMC that was used was relatively low, it still would increase the cost of pretreatment. Further experiments were conducted to demonstrate that, in other embodiments of the present invention, black liquor recycle could be used to maintain the flowability of the biomass suspension.

FIG. 9 is a simplified flow chart of a pretreatment process performed with black liquor recycle according to an embodiment of the present invention. Biomass 46 and white liquor 48 are fed to a reactive extruder 50 through its solids inlet 52 and liquids inlet 54, respectively. The reacted biomass suspension 56 is transferred to filtering and washing units 58, where the cellulose 60 and black liquor 62, 64 are separated. A portion 64 of the black liquor is recycled into the reactive extruder 50 at the liquids inlet 54.

Lignin in the black liquor has the ability to form microgels. Presumably, a significant increase in viscosity and elasticity can be achieved at a critical lignin concentration, especially at high pH values. In embodiments of the present invention, the criterion for selecting an effective black liquor recycle rate is to match the rheological behavior of the black liquor to that which could be achieved with a polymer/white liquor stream.

The elasticity and the viscosity of a 4% CMC-containing white liquor was characterized under oscillatory shear flow using an ARES rotational rheometer. Frequency sweep tests revealed that the material was Newtonian-like, exhibiting a constant viscosity (shear rate invariant) at low frequencies and becoming shear-thinning at frequencies greater than 5 rps. Referring to FIG. 10, the loss modulus G″ was greater than storage modulus G′, which crossed-over the loss modulus G″ at 100 rps.

The water content of the black liquor was altered systematically until a match with the viscosity of the CMC-containing white liquor of the first set of experiments could be obtained. During these experiments, thermo gravimetric analysis was also carried out to determine the actual water contents of the specimens.

The typical oscillatory shear behavior of the black liquor comprising 41% water by weight is shown in FIG. 11A number of these experiments were performed to generate a master curve of magnitude of complex viscosity versus water concentration. The magnitude of the complex viscosity values, varying with water content, are shown in FIG. 12. The magnitude of the complex viscosity of the black liquor matched that of the magnitude of the complex viscosity of the 4% CMC-containing white liquor at a water content of 45.3%, indicated by the arrow in FIG. 12. The black liquor at 45.3% water content had a NaOH molarity of 3.07 M, while white liquor used in the CMC experiments discussed above had a NaOH molarity of 4.16 M. Thus, fresh sodium hydroxide had to be incorporated into the black liquor prior to feeding it into the extruder.

In the pretreatment experiments using black liquor addition, the screw configuration was altered slightly to a more conservative and less aggressive one than was used in the CMC experiments to avoid torque and pressure overloads. FIG. 13 illustrates the screw configuration used in this second set of experiments. Elements of extruder 110 of FIG. 13 which correspond to elements of the extruder 10 of FIG. 4 are indicated by the same reference numbers used in FIG. 4, incremented by 100. The most significant modification to the screw configuration of FIG. 4 was to replace the set of kneading blocks 32, 34 of the screw 16 of FIG. 4 with six pairs of 30° forwardly staggered kneading block elements 132 (axial length of 3″), followed by six pairs of 60° reversely-configured kneading blocks 134 (axial length of 3″). During the experiments using black liquor as the gelling agent, the total flow rate of the biomass suspension was increased over the flow rate used during the reactive extrusion runs with CMC. The flow rate of the replenished black liquor was 10.4 lb/h and the biomass feed rate was 4.7 lb/h during the first condition (“Condition 5”) and 5.5 lb/h during the second condition (“Condition 6”). The temperature of the first mixing zone 118 was maintained at 70° C. in the first mixing zone 118, and the temperature in the second mixing zone 120 was maintained at 120° C.

Under Condition 5, the pressure was 32 psi at the kneading block section 132, 134 of the second mixing zone 120 and torque was 40-42% of the total torque capacity of the extruder. Upon increasing the biomass flow rate to 5.5 lb/h (i.e., Condition 6), while targeting better mixing in conjunction with greater shear stresses, pressure and torque increased to 35 psi and 52%, respectively. The temperature of the biomass suspension in the discharge zone 122 reached 101.2° C. with a local maximum of 105° C. at discharge.

After washing, filtering and drying the biomass samples recovered under Conditions 5 and 6, the biomass samples were examined under both optical and scanning electron microscopy (SEM). FIGS. 14 and 15 are a microphotograph and SEM image, respectively, of a sample of biomass pretreated under Condition 5. FIGS. 16 and 17 are a microphotograph and SEM image, respectively, of a sample of biomass pretreated under Condition 6. Both samples were white and fibrous, indicating that delignification could be achieved equally well with black liquor recycle as by the addition of CMC.

Following steady-state extrusion of biomass suspension under Condition 6, the barrel halves 114 were opened and samples of biomass were collected from four different locations on the screw 116. Referring to FIG. 18, samples were collected in the areas of 90° neutral kneading blocks 126 (“sample A”), 30° forwardly staggered kneading blocks 132 (“Sample B”), reversely-configured kneading blocks 134 (“Sample C”), and discharge end 138 (“Sample D”). The samples were then washed and dried, and analyzed under an optical microscope. FIGS. 18A-18D are microphotographs of Samples A-D, respectively. As had been observed for reactive extrusion with CMC as the gelling agent, there was no significant biomass particle size reduction and hence no significant delignification occurred at the first set of 90° neutral kneading blocks 126 (Sample A, FIG. 18A). Diminution in the biomass particle size and emergence of a fibrous structure were evident at the 30° forward configured kneading blocks 132 (Sample B, FIG. 18B). Large biomass particles were completely absent and substantial delignification was achieved at the 60° reversely configured kneading blocks 134 (Sample C, FIG. 18C), and the emergent fibers were even finer at the extruder discharge 138 (Sample D, FIG. 18D).

The foregoing discussions of the CMC/white liquor and black liquor recycle experiments show that pretreatment using the chemical and mechanical shearing methods of the exemplary embodiments of the present invention concomitantly accelerate the disruption of the biomass and separation of lignin from cellulose. Black liquor recycle included steps of enriching the black liquor with sodium hydroxide and adjusting its water content. Without being bound by theory, it appears that effective application of the chemical treatment and mechanical shearing may require the introduction of a flowability control agent, whether it is a one-pass addition of a polymer or a black liquor stream adjusted to desirable elasticity and viscosity.

It will be understood that the embodiment described herein is merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. For instance, flowabiity control agents other than CMC or black liquor may be utilized, and chemicals may be utilized (i.e., other than strong base) such as dilute or strong acids or other ionic liquids, or by use of such gelling agents and chemicals at concentrations other than those disclosed in the exemplary embodiments discussed herein. Other types of twin screw extruders as well as screw arrangements, extruder geometries, temperatures, and other operating conditions may also be utilized once the principles of the invention disclosed herein are applied. All such variations and modifications are intended to be included within the scope of the invention described in the claims appended hereto.

Claims

1. A method of pretreatment of lignocellulosic biomass, comprising the steps of:

suspending particles of a lignocellulosic biomass containing cellulose and lignin in a liquid thereby forming a suspension;

adding a flowability control agent to the suspension; and

mechanically shearing the suspension, whereby the size of the biomass particles is reduced and at least some of the lignin is separated into a liquid phase of recoverable black liquor and the cellulose remains in the cellulosic biomass.

2. The method of claim 1, comprising the further step of adjusting the pH of the suspension so as to achieve a pH that promotes separation of the lignin from the cellulose;

3. The method of claim 1, wherein the flowability control agent includes a water-soluble polymer that forms a hydrogel.

4. The method of claim 3, wherein the flowability control agent is selected from the group of polymers consisting of carboxy methyl cellulose, chitosan, polyacrylic acid, a polyacrylate, polyethylene glycol, polyethylene oxide, an alginate and polyvinyl alcohol.

5. The method of claim 1, where the flowability control agent is a polymer that imparts elasticity and viscosity to the biomass suspension such as to inhibit separation of said cellulosic biomass from said suspension under pressure and shear.

6. The method of claim 1, wherein the flowability control agent includes an aqueous solution containing lignin.

7. The method of claim 1, wherein the flowability control agent includes a portion of the black liquor in a recycle stream.

8. The method of claim 7, further including the steps of separating the portion of the black liquor from the suspension, adjusting the pH of the portion of the black liquor to a pH that promotes separation of the lignin from the cellulose, and adjusting the water content of the portion of the black liquor to provide the recycle stream with an elasticity and a viscosity that inhibit separation of the cellulosic biomass from the suspension under pressure and shear upon mixing the portion of the black liquor into the suspension.

9. The method of claim 8, wherein the recycle stream consists essentially of the portion of the black liquor.

10. The method of claim 8, wherein the recycle stream includes NaOH at a molarity of about 4 M NaOH.

11. The method of claim 1, wherein said steps are performed as a continuous process within a fully-intermeshing twin-screw extruder.

12. The method of claim 11, wherein the extruder has intermeshing screws within a barrel, said screws being configured to create at least a first zone and a second zone within the barrel, the first and second zones being created such that the suspension passes first through the first zone then through the second zone.

13. The method of claim 12, wherein the screws are further configured to promote the formation of a suspension seal between the first and second zones

14. The method of claim 12, wherein the mechanical shearing step includes the steps of controlling the temperature of the first zone at a first temperature below the boiling temperature of water and controlling the temperature of the second zone at a temperature above the boiling temperature of water.

15. The method of claim 14, wherein the first temperature is about 70° C. and the second temperature is about 120° C.

16. The method of claim 1, including the further steps of collecting the suspension after the mechanical shearing step and separating the black liquor from the cellulosic biomass.

17. The method of claim 16, including the further step of returning a portion of the separated black liquor to the suspension.

18. The method of claim 1, wherein said method is performed as a continuous process.

19. The method of claim 16, wherein the cellulosic biomass is suitable for fermentation to ethanol and/or biofuels.