发明内容：
[0011]Some variations of the invention provide a nanocellulose-slurry dewatering system comprising:[0012]a nanocellulose slurry feed sub-system, wherein the nanocellulose slurry comprises nanocellulose and water;[0013]an inlet for a dispersion/drying agent;[0014]a twin-screw extruder in flow communication with the nanocellulose slurry feed sub-system, wherein the twin-screw extruder is configured for intimately mixing the nanocellulose slurry and the dispersion/drying agent, and wherein the twin-screw extruder is configured with one or more extruder vents to remove at least a portion (such as all) of the water from the nanocellulose slurry;[0015]an extruder outlet for recovering a nanocellulose-dispersion concentrate comprising (i) the nanocellulose, (ii) the dispersion/drying agent, and (iii) residual water, if any; and[0016]an optional milling device configured to generate a powder containing the nanocellulose-dispersion concentrate.
[0017]In some embodiments, such as when processing dilute slurries, the nanocellulose slurry feed sub-system is configured with an internal rotating agitator and/or wiping blade to mix the nanocellulose slurry.
[0018]In some embodiments, the nanocellulose-slurry dewatering system comprises a nanocellulose slurry pre-concentration unit configured to remove at least a portion of the water from the nanocellulose slurry prior to water removal in the extruder. For example, the nanocellulose slurry pre-concentration unit may be a centrifuge or a filtration device.
[0019]In some embodiments, the nanocellulose-slurry dewatering system comprises a mixing unit configured for mixing the nanocellulose slurry with the dispersion/drying agent.
[0020]In some embodiments, the inlet for the dispersion/drying agent is an inlet to the nanocellulose slurry feed sub-system. In other embodiments, the inlet for the dispersion/drying agent is an inlet directly to the twin-screw extruder. There also may be inlets for the dispersion/drying agent both to the nanocellulose slurry feed sub-system as well as to the twin-screw extruder. The dispersion/drying agent may be combined with the nanocellulose slurry prior to adding to pre-concentration unit, or after the nanocellulose slurry has been pre-concentrated in a pre-concentration unit, if any. Preferably, the nanocellulose slurry and dispersion/drying agent are mixed together, such as with an agitated mix tank or in-line mixer, before adding the mixture to the nanocellulose slurry feed sub-system.
[0021]The twin-screw extruder may be a co-rotating twin-screw extruder, a counter-rotating twin-screw extruder, or another type of twin-screw extruder.
[0022]When the optional milling device is present, the milling device may be selected from a hammer mill, a ball mill, a jet mill, an impact crusher, a pulverizer, a cage mill, a grinder, or an extruder, for example.
[0023]Some variations do not necessarily employ a dispersion/drying agent. In some of these variations, a nanocellulose-slurry dewatering system comprises:[0024]a nanocellulose slurry feed sub-system, wherein the nanocellulose slurry comprises nanocellulose and water;[0025]a twin-screw extruder in flow communication with the nanocellulose slurry feed sub-system, wherein the twin-screw extruder is configured for shearing the nanocellulose slurry, and wherein the twin-screw extruder is configured with one or more extruder vents to remove at least a portion of the water from the nanocellulose slurry;[0026]an optional milling device configured to generate a powder containing the nanocellulose slurry; and[0027]an extruder outlet for recovering dewatered nanocellulose.
[0028]Some variations provide a nanocellulose-slurry dewatering system comprising:[0029]a nanocellulose slurry feed sub-system, wherein the nanocellulose slurry comprises nanocellulose and water;[0030]an inlet for a dispersion/drying agent;[0031]a twin-rotor mixer in communication with the nanocellulose slurry feed sub-system, wherein the twin-rotor mixer is configured for intimately mixing the nanocellulose slurry and the dispersion/drying agent, and wherein the twin-rotor mixer is configured with one or more mixer vents to remove at least a portion of the water from the nanocellulose slurry; and[0032]an optional milling device configured to generate a powder containing the nanocellulose-dispersion concentrate.
[0033]In some embodiments, the nanocellulose-slurry dewatering system comprises a mixing unit configured for mixing the nanocellulose slurry with the dispersion/drying agent.
[0034]In some embodiments, the inlet for the dispersion/drying agent is an inlet to the nanocellulose slurry feed sub-system. In some embodiments, the inlet for the dispersion/drying agent is an inlet to the twin-rotor mixer.
[0035]In some embodiments, the nanocellulose-slurry dewatering system comprises a nanocellulose slurry pre-concentration unit configured to remove at least a portion of the water from the nanocellulose slurry prior to water removal in the twin-rotor mixer. The nanocellulose slurry pre-concentration unit may be a centrifuge or filtration device, for example.
[0036]In some embodiments, the inlet for the dispersion/drying agent is an inlet to the nanocellulose slurry pre-concentration unit.
[0037]The twin-rotor mixer may be a co-rotating twin-rotor mixer or a counter-rotating twin-rotor mixer.
[0038]The present invention also provides a method to dewater and optionally dry a nanocellulose slurry, the method comprising:[0039](a) providing a nanocellulose slurry comprising nanocellulose and water;[0040](b) providing a dispersion/drying agent that is selected for compatibility with the nanocellulose;[0041](c) in a twin-screw system (e.g., a twin-screw extruder or a twin-rotor mixer), intimately mixing the nanocellulose slurry and the dispersion/drying agent;[0042](d) in the twin-screw system, removing at least a portion of the water from the nanocellulose slurry via one or more system vents, to generate a nanocellulose-dispersion concentrate;[0043](e) optionally milling the nanocellulose-dispersion concentrate to generate a powder, which may or may not have residual moisture; and[0044](f) recovering the nanocellulose-dispersion concentrate in solid form or liquid form.
[0045]In some embodiments, the nanocellulose slurry is pre-concentrated in a nanocellulose slurry pre-concentration step to remove at least a portion of the water from the nanocellulose slurry, prior to step (c). The nanocellulose slurry pre-concentration step may be centrifugation and/or filtration, for example. The dispersion/drying agent may be combined with the nanocellulose slurry prior to or after pre-concentrating, if a pre-concentration step is performed.
[0046]The dispersion/drying agent may be selected from the group consisting of waxes, polyolefins, olefin-maleic anhydride copolymers, olefin-acrylic acid copolymers, polyols, fatty acids, fatty alcohols, polyol-glyceride esters, polydimethylsiloxanes, polydimethylsiloxane-alkyl esters, polyacrylamides, starches, cellulose derivatives, particulates, and combinations or reaction products thereof, for example.
[0047]In some embodiments, the dispersion/drying agent is added to the nanocellulose slurry prior to step (c). The dispersion/drying agent may be added directly to the twin-screw system, such as via an additive inlet port. Preferably, the nanocellulose slurry and dispersion/drying agent are mixed together, such as with an agitated mix tank or in-line mixer, before adding the mixture to the twin-screw system. In certain embodiments, the nanocellulose slurry feed sub-system is configured to agitate the nanocellulose slurry and dispersion/drying agent.
[0048]The twin-screw system may be a co-rotating twin-screw extruder, a counter-rotating twin-screw extruder, or another type of twin-screw extruder that is typically operated continuously. The twin-screw system may be a twin-rotor mixer operated in batch or semi-batch mode.
[0049]The twin-screw system may be operated with an average system temperature from about 120° C. to about 300° C., for example. The twin-screw system is preferably operated with a maximum system temperature that is less than the thermal-decomposition onset temperature of the nanocellulose, and that is also preferably less than the thermal-decomposition onset temperature of the dispersion/drying agent.
[0050]In some embodiments, the twin-screw system is a twin-screw extruder that contains a plurality of extruder zones, wherein zone temperatures for each of the extruder zones are independently controlled. In certain embodiments, the zone temperatures increase along the length of the twin-screw extruder.
[0051]The twin-screw system may be heated with a heat-transfer medium selected from the group consisting of steam, hot oil, electrical-heating elements, and combinations thereof. The twin-screw system may be cooled with a heat-transfer medium selected from the group consisting of cooling water, air, oil, and combinations thereof. Heating and cooling configurations may be designed based on the desired temperature profile along the length of the twin-screw system, the throughput, the materials present, shear rates, screw or rotor design, and other parameters.
[0052]The twin-screw system may be operated with an average nanocellulose residence time from about 30 seconds to about 30 minutes, for example. In a batch twin-rotor mixer, the residence time is the batch time.
[0053]In some embodiments, at least one of the system vents is operated under vacuum. When step (d) utilizes multiple system vents, each of the system vents may be operated under vacuum, or less than all of the system vents may be operated under vacuum.
[0054]In the nanocellulose-dispersion concentrate generated in step (d), nanocellulose may be present at a concentration of about 10 wt % to about 90 wt %, for example. The dispersion/drying agent may be present at a concentration of about 5 wt % to about 65 wt %, for example, in the nanocellulose-dispersion concentrate. In some embodiments, the weight ratio of the nanocellulose to the dispersion/drying agent is selected from about 0.5 to about 2 in the nanocellulose-dispersion concentrate.
[0055]The nanocellulose may include cellulose nanocrystals, cellulose nanofibrils, microfibrillated cellulose, or a combination thereof. In some embodiments, the nanocellulose includes lignin-containing nanocellulose. In certain embodiments, the nanocellulose includes lignin-coated nanocellulose.
[0056]Some methods do not necessarily employ a dispersion/drying agent. In some of these methods, a method to dewater and optionally dry a nanocellulose slurry comprises:[0057](a) providing a nanocellulose slurry comprising nanocellulose and water;[0058](b) optionally pre-concentrating the nanocellulose slurry, such as via centrifugation or filtration;[0059](c) in a twin-screw system, shearing the nanocellulose slurry and removing at least a portion of the water from the nanocellulose slurry via one or more system vents, to generate dewatered nanocellulose;[0060](d) optionally milling the nanocellulose-dispersion concentrate to generate a powder; and[0061](e) recovering the dewatered nanocellulose in solid form or liquid form.
[0062]In the dewatered nanocellulose recovered in step (d), nanocellulose may be present at a concentration of about 10 wt % to about 25 wt %, for example.
具体实施方式：
[0077]This description enables one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives, and uses of the invention. These and other embodiments, features, and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following detailed description of the invention in conjunction with any accompanying drawings.
[0078]As used in this specification and the appended claims, the singular forms “a,”“an,” and “the” include plural referents unless the context clearly indicates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. All composition numbers and ranges based on percentages are weight percentages, unless indicated otherwise. All ranges of numbers or conditions are meant to encompass any specific value contained within the range, rounded to any suitable decimal point.
[0079]Unless otherwise indicated, all numbers expressing parameters, reaction conditions, concentrations of components, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending at least upon a specific analytical technique.
[0080]The term “comprising,” which is synonymous with “including,”“containing,” or “characterized by” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. “Comprising” is a term of art used in claim language which means that the named claim elements are essential, but other claim elements may be added and still form a construct within the scope of the claim.
[0081]As used herein, the phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of” (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phrase “consisting essentially of” limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter.
[0082]With respect to the terms “comprising,”“consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of” or, alternatively, by “consisting essentially of.”
[0083]The present invention, in some variations, is predicated on dewatering and drying aqueous nanocellulose slurries using a twin-screw system (e.g., a twin-screw extruder) configured to allow release of water vapor. The present invention, in some variations, is also predicated on the selection and incorporation of a dispersion/drying agent for nanocellulose, wherein the dispersion/drying agent is added to the nanocellulose slurry which is dewatered in a twin-screw system. It has been discovered that a twin-screw system with one or more system vents, in conjunction with a dispersion/drying agent, works surprisingly well to dewater a nanocellulose slurry. As water is removed from the system vent(s), the dispersion/drying agent prevents the nanocellulose from agglomerating and irreversibly bonding with itself.
[0084]As explained in the Background, it is often desirable for composite products to incorporate distinct nanocellulose particles and prevent those particles from bonding together (agglomerating) during production or use. Nanocellulose is typically available as an aqueous dispersion, as produced from cellulosic biomass or through bacterial synthesis. In dilute aqueous dispersions, the nanocellulose particles remain non-agglomerated or reversibly agglomerated. For most polymer systems, for example, the aqueous dispersion itself cannot be introduced into a polymer matrix—the water needs to first be removed. Even for aqueous systems, additive products containing as little water as possible are preferred to minimize product delivery costs and the amount of water introduced to the end-use product system with the additive. It is generally unacceptable to introduce excess water into a product system along with the additive such that the product must then be dewatered or dried beyond normal levels.
[0085]The present invention is a breakthrough that allows the production of nanocellulose in dry or dewatered (concentrated) form, enabling incorporation in a wide range of plastics, elastomers, and adhesives, as well as non-polymer matrices including electronic inks, sealants, and other non-water-based applications. The present invention also provides methods to concentrate nanocellulose for shipping and storage purposes for dispersion in aqueous systems.
[0086]A “dispersion/drying agent” as intended herein is a chemical, or combination of chemicals, that functions to prevent irreversible agglomeration of nanocellulose while it is being dried or dewatered. The dispersion/drying agent disclosed herein is selected to retain distinct nanocellulose particles by preventing bonding between nanocellulose particles while the aqueous dispersion is being dried or dewatered (water removal). Without an effective dispersion/drying agent, irreversible bonding between nanocellulose particles has been observed through drying with heat to as low as about 20 wt % solids slurries. The dispersion/drying agent also retains distinct nanocellulose particles while the nanocellulose is being incorporated into a composite product, and effectively and easily releases the individual nanocellulose particles during composite product formulating so that the effectiveness of the nanocellulose is maximized. To reduce or prevent nanocellulose from bonding to itself during drying or dewatering, a dispersion/drying agent may be selected to interact sufficiently with the surface of the nanocellulose and/or distribute uniformly between nanocellulose particles, thereby reducing or preventing nanocellulose agglomeration.
[0087]In this patent application, “dewatering” means removal of liquid water from a nanocellulose slurry. “Drying” typically refers to a relatively high extent of dewatering, up to and including the removal of all water from the nanocellulose, using thermal energy.
[0088]In the specification, a “nanocellulose-dispersion concentrate” refers to a composition containing at least nanocellulose and a dispersion/drying agent. “Dewatered nanocellulose,”“dried nanocellulose,”“dewatered nanocellulose slurries,” and the like refer to compositions containing nanocellulose and optionally containing a dispersion/drying agent.
[0089]Exemplary embodiments of the invention will now be described. These embodiments are not intended to limit the scope of the invention as claimed. The order of steps (or unit operations of a system) may be varied in any logical order, some steps or units may be omitted, and/or other steps or units may be added. Reference herein to first step, second step, etc. is for purposes of illustrating some embodiments only. Also, the locations of steps or units may vary, at one or multiple sites. Also, it should be understood that all references to “embodiments” are non-limiting and are considered to also be options with respect to any other disclosed embodiment, unless the context clearly dictates otherwise. In the drawings, dotted lines denote optional unit operations or streams.
[0090]Some variations of the invention provide a nanocellulose-slurry dewatering system comprising:[0091]a nanocellulose slurry feed sub-system, wherein the nanocellulose slurry comprises nanocellulose and water;[0092]an inlet for a dispersion/drying agent;[0093]a twin-screw extruder in flow communication with the nanocellulose slurry feed sub-system, wherein the twin-screw extruder is configured for intimately mixing the nanocellulose slurry and the dispersion/drying agent, and wherein the twin-screw extruder is configured with one or more extruder vents to remove at least a portion of the water from the nanocellulose slurry;[0094]an extruder outlet for recovering a nanocellulose-dispersion concentrate comprising the nanocellulose and the dispersion/drying agent; and[0095]an optional milling device configured to generate a powder containing the nanocellulose-dispersion concentrate.
[0096]In some embodiments, such as when processing dilute slurries (e.g., about 8 wt % solids or less), the nanocellulose slurry feed sub-system is configured with an internal rotating agitator and/or wiping blade to mix the nanocellulose slurry prior to the slurry being fed to the twin-screw extruder. A control valve may be used to meter the rate of addition of nanocellulose slurry from the feed sub-system to the twin-screw extruder.
[0097]In some embodiments, the nanocellulose-slurry dewatering system comprises a nanocellulose slurry pre-concentration unit configured to remove at least a portion of the water from the nanocellulose slurry prior to water removal in the extruder. Pre-concentrating the slurry is advantageous to reduce the overall volume being fed to the twin-screw extruder, which gives a higher capacity on a nanocellulose basis. The nanocellulose slurry pre-concentration unit may employ mechanical separation of water rather than thermal separation (water vaporization), to reduce overall energy costs. Mechanical separation means that separation is achieved using mechanical forces such as centrifugal or centripetal forces, or physical forces such as pressure causing water permeation through filtration media or a membrane.
[0098]For example, the nanocellulose slurry pre-concentration unit may be a centrifuge or a filtration device. An exemplary centrifuge is a decanter centrifuge employing high-speed rotation with centrifugal forces to separate nanocellulose, which has a higher density that that of water, from the water which is continuously removed (decanted). An exemplary filtration device is a pressure filter employing high-pressure air (or another inert gas) to create a mat of nanocellulose and a water-rich filtrate. Filtration devices include filter presses, belt presses, and the like.
[0099]When the nanocellulose slurry is pre-concentrated through the pre-concentration unit, to provide nanocellulose with higher than about 15 wt % solids, the nanocellulose feed sub-system may essentially consist of a traditional hopper feeding system. Hoppers are well-known for feeding moist solids to extruders. If the moist solids are not a free-flowing material, the hopper may be designed with a rotating agitator and/or wiping blade, or with hopper vibration, to prevent bridging of feed material.
[0100]In some embodiments, the inlet for the dispersion/drying agent is an inlet to the nanocellulose slurry feed sub-system, which may be the same inlet as the feed stream for the nanocellulose slurry or may be a separate inlet, as depicted in FIG. 1. In certain embodiments, the nanocellulose slurry and dispersion/drying agent are combined before feeding to the feed sub-system, such as following production of the nanocellulose slurry or even integrated with nanocellulose production. In other embodiments, the inlet for the dispersion/drying agent is an inlet directly to the twin-screw extruder, such as via a feed port (e.g., see FIG. 1) or a side port. There also may be inlets for the dispersion/drying agent both to the nanocellulose slurry feed sub-system as well as to the twin-screw extruder.
[0101]As intended herein, a “twin-screw system” is a machine that utilizes at least two solid screws or rotors that rotate (radially) with preferably small gaps between the screws or rotors, imparting significant shear forces onto a material being processed. In the present case of the material being a nanocellulose slurry, the high shear and preferably small gaps create a thin material layer with high surface area that is exposed for vaporization of water. The high shear forces enable intimate mixing of the nanocellulose slurry with the dispersion/drying agent. Also, in some cases, depending on the dispersion/drying agent used, as water is released, the intimate mixing provided by the twin screws or rotors leads to particle tumbling and grinding so that particle drying is relatively uniform.
[0102]The twin-screw system may be a continuous system, a semi-continuous system, a semi-batch system, or a batch system. When the twin-screw system is continuous or semi-continuous twin-screw extruder, material is continually conveyed axially through the system, from a feed sub-system to an extruder outlet, for some period of time. When the twin-screw system is a batch or semi-batch twin-rotor mixer, material is initially added to the system and then the system is typically closed, except for vents or water vapor removal, and then subjected to intimate mixing via high-shear forces caused by two rotors rotating radially with preferably small gaps between the rotors. After a period of time (batch time), the system is opened up and the processed material is recovered. In a semi-batch system, the system may be operated for a period of time and then material may be periodically added to, or withdrawn from, the system (e.g., water release may be intermittent, or dewatered nanocellulose may be periodically recovered through a valve). In the case of batch or semi-batch, the feed sub-system may be the vessel itself that may be initially loaded, and the system outlet may also be the vessel itself, following batch operation. See Examples 1-8 herein for examples of semi-batch twin-rotor mixers, and Examples 9-11 for examples of continuous twin-screw extruders.
[0103]In the case of the twin-screw system being a twin-screw extruder, the solid screws are typically fabricated from metals or metal alloys, such as stainless steel, optionally with ceramic coatings, such as chromium carbide. A screw may be fabricated as a single piece or may be segmented and assembled on a shaft. The screws may be arranged parallel to each other, or in a conical arrangement in which the screw axes are not parallel to each other but rather converge along the length of the extruder.
[0104]A twin-screw extruder may be a co-rotating twin-screw extruder, a counter-rotating twin-screw extruder, or another type of twin-screw extruder (e.g., a gear pump extruder). When two screws are designed to rotate in the same radial direction (co-rotation), the twin screws are co-rotating screws. When two screws are designed to rotate in the opposite radial direction, the twin screws are counter-rotating screws. The flights of the screws may be designed such that two screws intermesh with each other (intermeshed screws) or do not fully intermesh with each other (non-intermeshed screws). The screws are contained within one or more barrels that form external walls around the screws, thereby containing the material during processing.
[0105]For continuous systems, a counter-rotating twin-screw extruder has beneficial material feed and conveying characteristics. The residence time and material temperature control in a counter-rotating twin-screw extruder are also relatively uniform. However, air entrapment, generation of high pressure, and low maximum screw speed may be disadvantages. The advantages of a co-rotating twin-screw extruder are that the screws wipe each other clean (self-wiping), and high screw speeds and high outputs may be realized, along with good mixing. Co-rotating twin-screw extruders may also be desirable for reduced screw and barrel wear.
[0106]For continuous systems, screws may be designed to incorporate different screw elements along the screw length. Such screw elements may include, but are not limited to, flighted elements, mixing elements, and zoning elements. Flighted elements forward material past barrel ports, through mixers, and out of the extruder through a die. Mixing elements facilitate the mixing of the various components being processed. Zoning elements isolate two operations. Some elements may be multifunctional.
[0107]For continuous systems, the mixing efficiency of a twin-screw extruder can be increased by incorporating many mixing elements along the screws. These and other elements may be slotted onto a central shaft to build up a screw section. Preferably, the length, number, and form of the elements may be easily changed. The elements may take various forms such as reversed screw flights, kneading discs, pins, rotors, slotted vanes, blister rings, etc. Mixing elements may be designed to cause extensional mixing and planar shear to be imparted into the materials being processed, to facilitate dispersive mixing. Mixing elements may be designed to result in divisions and recombinations of the stream, to facilitate distributive mixing.
[0108]For continuous systems, the outside screw diameter, inside screw diameter, and channel depth are important twin-screw extruder design parameters as these parameters dictate the available free volume and torque (and thus radial shear forces). As the channel depth increases, the inside screw diameter decreases and results in less attainable shaft torque.
[0109]Generally speaking, screw design will be tailored to the specific application (e.g., type of nanocellulose, dispersion/drying agent, extent of dewatering desired, throughout, etc.). A skilled artisan in twin-screw extruders will be able to customize screw design using known principles and calculations.
[0110]This specification hereby incorporates by reference Goff et al., The Dynisco Extrusion Processors Handbook, 2nd edition, 2000, for its teachings of the principles and design parameters of twin-screw extruders. This specification also hereby incorporates by reference Martin, “Twin Screw Extruders as Continuous Mixers for Thermal Processing: a Technical and Historical Perspective”, AAPS PharmSciTech, Vol. 17, No. 1, February 2016, for its teachings of various design and operation principles of twin-screw extruders.
[0111]Extruders employing three or more screws are within the scope of twin-screw extruders herein. In a three-screw extruder, all three screws may be co-rotating, or two screws may be co-rotating and one screw counter-rotating in relation to the radial direction of the other screws.
[0112]A gear pump extruder is a simple twin-screw extruder that moves a material through the action of two intermeshing gears, which are essentially screws or rotors with relatively short lengths. The gears are typically fabricated from metals or metal alloys, such as stainless steel, optionally with ceramic coatings, such as chromium carbide. When the two gears are designed to rotate in the same radial direction (co-rotation), the gear pump extruder is a co-rotating gear pump extruder. When the two gears are designed to rotate in the opposite radial direction, the gear pump extruder is a counter-rotating gear pump extruder. The flights of the gears may be designed such that the two gears intermesh with each other or do not fully intermesh with each other.
[0113]In some embodiments, the twin-screw system is a gear pump extruder that is operated in batch rather than continuously. The intermeshing gears may be designed to co-rotate or counter-rotate, and the flights of the gears may be designed such that the two gears intermesh with each other or do not fully intermesh with each other. The gear pump extruder is initially loaded with the nanocellulose slurry and optionally a dispersion-drying agent. The batch gear pump is operated for a period of time (batch time) to allow intimate, high-shearing mixing and release of water vapor out of a vent.
[0114]In some embodiments, the twin-screw system is a batch or semi-batch twin-rotor mixer. The twin-rotor mixer is configured with two rotors that may be intermeshing or non-intermeshing. The two rotors rotate (radially), imparting significant shear forces onto a material being processed. The high shear forces enable intimate mixing of the nanocellulose slurry with the dispersion/drying agent. The rotors are typically fabricated from metals or metal alloys, such as stainless steel, optionally with ceramic coatings, such as chromium carbide. The two rotors may be designed to rotate in the same radial direction (co-rotation) or in the opposite radial direction (counter-rotation). Examples 1-8 of this specification utilize semi-batch twin-rotor mixers. In principle, such twin-rotor mixers may be scaled up to commercial scale.
[0115]A twin-rotor mixer contains a cavity (or mixing chamber) with vaned rotors, where vanes pump and intimately mix material in opposite directions. The rotors of a twin-rotor mixer may be designed such that the number, shape, and angles of the vanes are optimized for the particular application. Rotors generally have two or four flights, but other numbers of flights are possible. The rotors of a twin-rotor mixer may be selected from tangential rotors, roller rotors, delta rotors, cam rotors, Sigma rotors, Banbury rotors, or other industrially available rotors, for example. In some embodiments of twin-rotor mixers, two rotors rotate toward each other at slightly different speeds. Each rotor has a blade that extends along the length of the rotor roughly in the form of a spiral. Each rotor may be cored to permit cooling or heating by the passage of water or of an appropriate heating agent. An example of a commercial-scale two-wing rotor is the well-known Banbury design.
[0116]For batch or semi-batch systems, the mixing efficiency of a twin-rotor mixer can be increased by incorporating many mixing elements in the rotors. The rotor elements may take various forms. Rotor mixing elements may be designed to cause extensional mixing and planar shear to be imparted into the materials being processed, to facilitate dispersive mixing. Rotor mixing elements may be designed to result in divisions and recombinations of the stream, to facilitate distributive mixing. Generally speaking, rotor design will be tailored to the specific application (e.g., type of nanocellulose, dispersion/drying agent, extent of dewatering desired, throughout, etc.). A skilled artisan in twin-rotor mixers will be able to customize rotor design using known principles and calculations.
[0117]The twin-screw system may be designed to operate with an average system temperature from about 120° C. to about 300° C., for example. The twin-screw system is preferably designed to operate with a maximum system temperature that is less than the thermal-decomposition onset temperature of the nanocellulose being dewatered or dried and that is preferably less than the thermal-decomposition onset temperature of the dispersion/drying agent. In some embodiments, when the twin-screw system is a twin-screw extruder, there is a plurality of extruder zones, wherein zone temperatures for each of the extruder zones may be independently controlled, such as via a control panel or computer interface. When the twin-screw system is a batch or semi-batch twin-rotor mixer, the mixer may be operated at a single temperature or at a time-varying temperature, if desired.
[0118]The twin-screw system may be heated with a heat-transfer medium selected from the group consisting of steam, hot oil, electrical-heating elements, and combinations thereof. The twin-screw system may be cooled with a heat-transfer medium selected from the group consisting of cooling water, air, oil, and combinations thereof. In the case of a twin-screw extruder, heating and cooling configurations may be designed based on the desired temperature profile along the length of the twin-screw extruder, the throughput, the materials present, shear rates, screw design, and other parameters.
[0119]In some embodiments, the twin-screw system is electrically heated using resistance coils, bands, or cuffs that are strapped or bolted around a barrel or mixer bowl. Upon demand, such as initiated by a thermocouple, electrical current is passed through the resistance wire, inside the coil. The resistance produces heat that increases the barrel or mixer bowl temperature, and the system internal temperature via heat transfer. For a twin-screw extruder, the resistance settings required to achieve a desired temperature will depend on the screw rotational speed, the pressure within the system, and the throughput.
[0120]In some embodiments, a twin-screw extruder is configured with steam heating for one or more barrels. Upon demand, such as initiated by a thermocouple, steam is introduced to the barrel surface for indirect heating (i.e., steam is not introduced directly into the extruder), thereby increasing the barrel temperature and the extruder internal temperature via heat transfer. For a twin-screw extruder, the steam pressure and flow rate to achieve a desired temperature will depend on the screw rotational speed, the pressure within the system, and the throughput.
[0121]In some embodiments, a twin-screw extruder is equipped with an air-cooling system to reduce temperature when too much heating has occurred (e.g., electrical heating exceeds set point, or shear heating has become excessive). An exemplary air-cooling system consists of fans that circulate air around the barrel on demand.
[0122]In some embodiments, a twin-screw extruder is equipped with a liquid cooling system, such as a closed-loop heat exchanger using cooling water contained inside a sealed coil surrounding the barrel. When a set-point temperature is exceeded, the vapor from this cooling water is cooled by water flow so that the cooling water vapor condenses to absorb more heat.
[0123]The twin-screw system is preferably configured with one or more system vents to remove at least some of the water from the nanocellulose slurry. A “vent” is a port, adjustable valve, pressure-relief valve, pressure-relief disk, water-permeable membrane, or other device that allows water vapor to be released from the system. A vent may be normally-open or normally-closed, e.g. the vent may be designed only to open when there is sufficient outward vapor pressure or upon demand. In the case of a twin-screw extruder, a vent may be referred to as an extruder vent. In the case of a twin-rotor mixer, a vent may be referred to as a mixer vent.
[0124]In some embodiments of a twin-screw extruder, an extruder vent is built into a barrel. In these or other embodiments, an extruder vent may be configured in a vent ring or other vent element between axially adjacent barrels. In the case of a gear pump extruder, an extruder vent may be built into gear box, the case seal, or the suction port, for example.
[0125]Generally, a system vent allows for volatiles, such as water vapor, and entrapped air to be removed from the system. In the present invention, dewatering or drying of the nanocellulose slurry means that at least water is removed. Other components may be removed from a system vent, sometimes unintentionally (e.g., minor entrainment of solids). In some embodiments, the system vent allows removal not only of some water but also removal of components derived from the nanocellulose production process, such as acids, sugar-degradation compounds, or lignin-derived compounds.
[0126]The number of system vents may vary, such as 1, 2, 3, 4, 5, or more, depending on the total length of a twin-screw extruder or the total volume of a twin-rotor mixer, for example. The placement of extruder vents may vary along the length of the twin-screw extruder. For example, when there is an increasing temperature profile along the extruder length, an extruder vent may be placed near the end of the extruder where water vaporization is faster or more thermodynamically favorable.
[0127]A system vent may allow vapors to escape to the atmosphere or may be adapted to a flow line such that the vapors may be captured and potentially reused for other purposes, or analyzed for composition, for example. The flow line may be in communication with a vacuum system, such that the vent is under vacuum. When multiple vents are utilized, each of the vents may be sized the same or differently, i.e. one vent may have a larger opening area (for more vapor release) than another vent.
[0128]In some embodiments, at least one of the system vents is operated under vacuum. When multiple system vents are utilized, each of the system vents may be operated under vacuum, or less than all of the system vents may be operated under vacuum. The vacuum pressure may be from about 0.01 bar to about 0.99 bar (absolute pressure), such as about 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95 bar. When there are multiple system vents, they may be operated at the same pressure or at different pressures. As an illustration, if there are three system vents (as implied in FIG. 1), all three vents may be connected to a common vacuum system that operates at −0.9 bar gauge pressure, which is 0.1 bar absolute pressure. Or, for example, two system vents may be at 0.5 bar pressure while one vent is at atmospheric pressure (1 bar).
[0129]A twin-screw extruder outlet typically is configured with an extruder die. The extruder die is the assembly, located at the end of the extruder, which contains an orifice to allow dewatered nanocellulose to flow out. The extruder die is a block of metal or metal alloy, which may be the same material as the screws and/or the barrels. In certain embodiments, an extruder vent is configured as part of the extruder die at or near the extruder outlet. Also note that when the extruder outlet is open to the atmosphere, an additional amount of water vapor release may take place.
[0130]In addition to number and size of extruder vents and the vacuum levels, other parameters may dictate the efficiency of water removal from a twin-screw extruder. For example, longer residence times specifically in the zone(s) with an extruder vent will help since there is more time for water mass transport to the vent. Screw design may be optimized such that one or more screw elements enhance water flashing and removal out of the extruder vent(s). A higher surface area of the mixture, from high shear rates, may assist by reducing diffusion and/or convection mass-transport limitations. Of course, the temperature and pressure within the extruder, or within extruder zones, will dictate whether water is present in liquid or vapor states, at thermodynamic equilibrium. True equilibrium may or may not be present.
[0131]One skilled in the art of chemical engineering can experiment with the twin-screw system and vary the number and size of vents, the vacuum pressures, screw or rotor design, and the process conditions within the system to determine the number of vents required to achieve a desired extent of dewatering or drying. Alternatively, or additionally, one skilled in the art of chemical engineering can simulate the twin-screw system and vary the number of vents, the vacuum pressures, and the process conditions to calculate or estimate the number of vents required to achieve a desired extent of dewatering or drying. For example, it may be assumed that each vent can be modeled as one equilibrium flash stage with full vapor disengagement and no solids entrainment. In reality, due to high surface area (thin film) in the twin-screw system, there may be more than one equilibrium flash occurring at each vent.
[0132]When the optional milling device is present, the milling device may be connected directly to the system outlet, such that the system outlet (e.g., extruder outlet) is essentially the milling device inlet. Alternatively, the nanocellulose-dispersion concentrate may be collected from the outlet and then, at potentially a later time and/or at a different location, introduced to the milling device. The milling device may be selected from a hammer mill, a ball mill, a jet mill, an impact crusher, a pulverizer, a cage mill, a grinder, or an extruder, for example. The milling device may be operated to reduce the particle size of the nanocellulose-dispersion concentrate, such as to an average size range of about 10 microns to about 1 millimeter, e.g. about 25, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, or 950 microns.
[0133]Some variati