WO2014134000A1

WO2014134000A1 - Chemical activation of carbon via an entrained stream method

Info

Publication number: WO2014134000A1
Application number: PCT/US2014/018199
Authority: WO
Inventors: James Gerard Fagan
Original assignee: Corning Incorporated
Priority date: 2013-02-28
Filing date: 2014-02-25
Publication date: 2014-09-04
Also published as: CN105492386A; TW201438992A

Abstract

The disclosure relates to methods for forming activated carbon comprising providing a feedstock mixture comprising a carbon feedstock and at least one chemical activating agent, introducing the feedstock mixture into a reactor, rapidly heating the feedstock mixture to at least the solidification temperature by introducing a hot stream into the reactor, introducing the heated feedstock mixture into a reaction vessel, and holding the heated feedstock mixture in the reaction vessel at a temperature and for a time sufficient to react the carbon feedstock with the at least one chemical activating agent to form activated carbon, wherein rapidly heating the feedstock mixture comprises heating the mixture within a time period sufficient to maintain the feedstock mixture in a substantially solid state throughout the rapid heating stage.

Description

CHEMICAL ACTIVATION OF CARBON VIA AN ENTRAINED STREAM METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. § 1 19 of U.S. Application Serial No. 61/770,491 filed on February 28, 2013, the entire content of which is hereby incorporated by reference.

FIELD OF THE DISCLOSURE

[0002] The present disclosure relates generally to methods for forming activated carbon, and more particularly to chemical activation of carbon via an entrained stream method.

BACKGROUND

[0003] Energy storage devices such as ultracapacitors may be used in a variety of applications such as where a discrete power pulse is required. Example applications range from cell phones to hybrid vehicles. Ultracapacitors have emerged as an alternative to batteries in applications that require high power, long shelf life, and/or long cycle life. Ultracapacitors typically comprise a porous separator and an organic electrolyte sandwiched between a pair of carbon-based electrodes. The energy storage is achieved by separating and storing electrical charge in the

electrochemical double layers that are created at the interfaces between the electrodes and the electrolyte. Important characteristics of these devices are the energy density and power density that they can provide, which are both largely determined by the properties of the carbon that is incorporated into the electrodes.

[0004] Carbon-based electrodes suitable for incorporation into energy storage devices are known. Activated carbon is widely used as a porous material in ultracapacitors due to its large surface area, electronic conductivity, ionic

capacitance, chemical stability, and/or low cost. Activated carbon can be made from natural precursor materials, such as coals, nut shells, and biomass, or synthetic materials such as phenolic resins. With both natural and synthetic precursors, the activated carbon can be formed by carbonizing the precursor and then activating the intermediate product. The activation can comprise physical (e.g., steam or C0₂) or chemical activation at elevated temperatures to increase the porosity and hence the surface area of the carbon.

[0005] Both physical and chemical activation processes typically involve large thermal budgets to heat and react the carbonized material with the activating agent. In the case of chemical activation, corrosive by-products can be formed when a carbonized material is heated and reacted with a chemical activating agent such as KOH. Additionally, phase changes that may occur during the heating and reacting of the carbonized material and chemical activating agent can result in agglomeration of the mixture during processing. These drawbacks can add complexity and cost to the overall process, particularly for reactions that are carried out at elevated

temperatures for extended periods of time.

[0006] Significant issues have been reported when caustics, such as KOH, are used for the chemical activation of carbon. For example, when rotary kilns are used in carbon activation, it is often required that the feedstock undergoes calcination and/or drying and/or dehydration prior to treatment at activation temperatures.

Agglomeration tends to pose significant issues, such as increased process complexity and/or cost, in continuous processes, for instance, processes employing screw kneaders. As a means to avoid agglomeration issues, other technologies such as roller hearths, have been employed wherein trays are loaded with activation mix material and passed through a multiple zone tunnel furnace. Such furnaces may be costly in operation and may have limited throughput since only one tray level is passed through the furnace at a time. The furnace width is also limiting factor for roller hearths on throughput since roller length spanning across the furnace is limited by material availability and strength at service temperature.

[0007] Accordingly, it would be advantageous to provide activated carbon materials and processes for forming activated carbon materials using a more economical chemical activation route while also minimizing the technical issues of corrosion and/or agglomeration. The resulting activated carbon materials can possess a high surface area to volume ratio and can be used to form carbon-based electrodes that enable efficient, long-life and high energy density devices.

SUMMARY

[0008] The disclosure relates, in various embodiments, to methods for forming activated carbon comprising providing a feedstock mixture comprising a carbon feedstock and at least one chemical activating agent, introducing the feedstock mixture into a reactor, rapidly heating the feedstock mixture to at least the solidification temperature by introducing a hot stream into the reactor, introducing the heated feedstock mixture into a reaction vessel, and holding the heated feedstock mixture in the reaction vessel at a temperature and for a time sufficient to react the carbon feedstock with the at least one chemical activating agent to form activated carbon.

[0009] According to various embodiments, rapidly heating the feedstock mixture comprises heating the mixture within a time period sufficient to maintain the feedstock mixture in a substantially solid state throughout the heating stage.

[0010] In certain embodiments, the mass ratio of chemical activating agent to carbon feedstock in the feedstock mixture ranges from about 0.5:1 to about 5:1. The feedstock mixture may, in various embodiments, include a particulate mixture of the carbon feedstock and the at least one chemical activating agent, i.e., a powder or granular mixture of dry or substantially dry constituents. The reactor may, in various embodiments, be an entrained stream reactor, for example, such that the feedstock mixture is entrained in the hot stream in the reactor. In some non-limiting

embodiments, the at least one chemical activating agent comprises KOH and the hot stream comprises steam or a mixture of gases such as steam and nitrogen.

[0011] Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. [0012] It is to be understood that both the foregoing general description and the following detailed description present various embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The following detailed description can be best understood when read in conjunction with the following drawings, where like structures are indicated with like reference numerals and in which:

[0014] FIG. 1 is an estimated equilibrium phase diagram for KOH and carbon in a closed system; and

[0015] FIG. 2 is a schematic illustration of a system for preparing activated carbon according one embodiment of the disclosure.

DETAILED DESCRIPTION

[0016] Disclosed herein is a method for making activated carbon comprising providing a feedstock mixture comprising a carbon feedstock and at least one chemical activating agent, introducing the feedstock mixture into a reactor, rapidly heating the feedstock mixture to at least the solidification temperature by introducing a hot stream (e.g., gas stream) into the reactor, introducing the heated feedstock mixture into a reaction vessel, and holding the heated feedstock mixture in the reaction vessel at a temperature and for a time sufficient to react the carbon feedstock with the chemical activating agent to form activated carbon.

[0017] As used herein, the term "solidification temperature" and variations thereof are intended to denote a temperature at which at least one liquid to solid

transformation results in a substantially liquid-free, i.e., substantially solid bulk mixture, wherein the liquid to solid transformation is associated with an increase in temperature. Similarly, the term "fluxing temperature" and variations thereof are intended to denote a temperature at which at least one solid to liquid transformation results in the introduction of at least one liquid phase in the bulk mixture, wherein the solid to liquid transformation is associated with an increase in temperature.

[0018] According to various embodiments, the carbon feedstock may comprise a carbonized material such as coal or a carbonized material derived from a carbon precursor. Example carbon precursors include natural materials such as nut shells, wood, biomass, non-lignocellulosic sources, and synthetic materials, such as phenolic resins, including polyvinyl alcohol) and (poly)acrylonitrile. For instance, the carbon precursor can be comprise edible grains such as wheat flour, walnut flour, corn flour, corn starch, corn meal, rice flour, and potato flour. Other carbon precursors include coconut husks, beets, millet, soybean, barley, and cotton. The carbon precursor can be derived from a crop or plant that may or may not be genetically-engineered.

[0019] Further exemplary carbon precursor materials and associated methods of forming carbon feedstock are disclosed in commonly-owned U.S. Patent Application Nos. 12/335,044, 12/335,078, 12/788,478 and 12/970,073, which are incorporated herein by reference in their entireties.

[0020] Carbon precursor materials can be carbonized to form carbon feedstock by heating in an inert or reducing atmosphere. Example inert or reducing gases and gas mixtures include one or more of hydrogen, nitrogen, ammonia, helium and argon. In an example process, a carbon precursor can be heated at a temperature from about 500°C to 950°C (e.g., 500, 550, 600, 650, 700, 750, 800, 850, 900 or 950 °C, and all ranges and subranges therebetween) for a predetermined time (e.g., 0.5, 1 , 2, 4, 8 or more hours, and all ranges and subranges therebetween) and then optionally cooled. During carbonization, the carbon precursor may be reduced and decomposed to form carbon feedstock.

[0021] In various embodiments, the carbonization may be performed using a conventional furnace or by heating within a microwave reaction chamber using microwave energy. For instance, a carbon precursor can be exposed to microwave energy such that it is heated and reduced to char within a microwave reactor to form carbon feedstock that is then combined with a chemical activating agent to form a feedstock mixture. It is envisioned that a single carbon precursor material or combination of precursor materials could be used to optimize the properties of the activated carbon product.

[0022] The feedstock mixture may be prepared by combining the carbon feedstock with at least one chemical activating agent. The at least one chemical activating agent may, in certain embodiments, be chosen from KOH, NaOH, H3P0₄, Na₂C03, KCI, NaCI, MgCI₂, KOH, AICI₃, P₂0₅, K₂C0₃, and/or ZnCI₂. According to various non- limiting embodiments, the at least one chemical activating agent may be chosen from alkali metal salts, for instance, alkali hydroxide salts such as sodium hydroxide, lithium hydroxide, and potassium hydroxide.

[0023] In certain embodiments, the carbon feedstock may be combined with a solution of the at least one chemical activating agent. For example, an aqueous solution may be used, and the concentration of chemical activating agent in the solution may range from about 10 to about 90 wt%. In such embodiments, the wet feedstock mixture can optionally be dried during and/or after mixing to provide a substantially dry feedstock mixture. In further embodiments, the carbon feedstock can be combined with the at least one chemical activating agent to form a dry feedstock mixture, i.e., without the use of any liquid or solvent.

[0024] The carbon feedstock and the at least one chemical activating agent may be combined in any suitable ratio to form the feedstock mixture and to bring about chemical activation of the carbon. The specific value of a suitable ratio may depend, for example, on the physical form and type of the carbon feedstock and the chemical activating agent and the concentration, if one or both are in the form of a mixture or solution. A ratio of chemical activating agent to carbon feedstock on the basis of dry material weight can range, for example, from about 0.5:1 to about 5:1. For example, the ratio can range from about 1 :1 to about 4:1 or from about 2:1 to about 3:1 , including all ranges and subranges therebetween. In certain embodiments, the mass ratio of chemical activating agent to carbon feedstock may be about 1 :1 , 2:1 , 3: 1 , 4:1 , or 5:1 , including all ranges and subranges therebetween. According to other embodiments, the mass ratio of chemical activating agent to carbon feedstock may be less than about 12:1 , for instance, less than about 1 1 :1 , less than about 10:1 , or less than about 8:1 , including all ranges and subranges therebetween.

[0025] It is known that certain alkali chemical activating agents may be mixed with carbonaceous material as a means for catalyzing gasification reactions, for example, as disclosed in European Patent No. 0 007 247. However, it is noted that the amount of chemical activating agent suitable for catalyzing such reactions is not sufficient for the chemical activation of carbon as disclosed herein.

[0026] The feedstock mixture suitable for activation may be further prepared by milling or grinding the mixture. For example, prior to mixing, the carbon feedstock and/or the at least one chemical activating agent may be separately milled and then mixed together. In other embodiments, the feedstock mixture may be

simultaneously milled during mixing of the carbon feedstock and at least one chemical activating agent. According to further embodiments, the feedstock mixture may be milled after the carbon feedstock and at least one chemical activating agent are mixed together.

[0027] Feed granulation methods may involve mixing the carbon feedstock with the at least one chemical activating agent, optionally with heating, by way of roll compaction, drum pelletization, vacuum drying, freeze drying, and/or any other means suitable for mixing and/or granulating the feedstock mixture. Additionally, granulation may be accomplished using binder additives such as carbowax, a paraffin wax which may decompose with little or no residue contamination of the activated carbon. Use of such binders may also be employed in granulation methods including, but not limited to, pelletizing via roll compaction, drum pelletizing, and/or extrusion mixing and/or grating.

[0028] By way of non-limiting example, the feedstock mixture and/or carbon feedstock may be milled to an average particle size of less than about 100 microns, for instance, less than about 100, 50, 25, 10, or 5 microns, and all ranges and subranges therebetween. In various embodiments, the feedstock mixture can have an average particle size of less than about 5 microns, such as less than about 4, 3, 2, or 1 microns, and all ranges and subranges therebetween. In further embodiments, the particle size of the carbon feedstock mixture may range from about 0.5 to about 25 microns, such as from about 0.5 microns to about 5 microns.

[0029] In additional embodiments, the feedstock mixture may be further prepared by pre-heating the mixture. By way of non-limiting example, the feedstock mixture may be pre-heated during and/or after mixing and/or milling the mixture. In these embodiments, the feedstock mixture may be pre-heated to any temperature below the fluxing temperature. For instance, the feedstock may be heated to a temperature of less than about 400°C, such as less than about 350, 300, 250, 200, or 100°C, and all ranges and subranges therebetween. According to various embodiments, the feedstock mixture may be heated to a temperature ranging from about 50°C to about 400°C, such as from about 50°C to about 150°C, from about 90°C to about 120°C, from about 200°C to about 400°C, or from about 300°C to about 400°C, including all ranges and subranges therebetween.

[0030] Subsequent to mixing and optionally milling and/or heating the feedstock mixture, the feedstock mixture is introduced into a reactor, in which it is rapidly heated by contact with a stream of hot gas. In certain embodiments, the reactor is an entrained stream reactor, in which the feedstock mixture and hot stream are introduced in such a manner so as to entrain the feedstock mixture particles in the hot stream. By way of non-limiting example, the feedstock mixture and hot stream may be introduced at the bottom of the reactor and the feedstock mixture particles may be entrained in the hot stream and transported from the bottom of the reactor to the top of the reactor.

[0031] In certain embodiments, the feedstock mixture may be introduced into the reactor by injecting, spraying, or atomizing the particles into the reactor. The feed rate of the feedstock mixture may vary depending on the reactor size, nature of the feedstock, granule particle size, and/or velocity of the entrained stream. The feed rate may range, for example, from about 0.1 kg/min to about 100 kg/min. It is within the ability of one skilled in the art to select the feed rate appropriate for the desired operation and result. According to other non-limiting embodiments, the hot stream may comprise steam. For instance, the hot stream may be chosen from steam, air, carbon dioxide, nitrogen, argon, and mixtures thereof. [0032] It will be appreciated that alkali metals, for example, sodium and potassium, may spontaneously combust upon exposure to air. Formation of alkali metals can be prevented or alleviated by using steam as the hot stream in the reactor. The water vapor will scavenge alkali atoms and react to form non-combustible alkali oxides or hydroxides. Thus, in certain embodiments, during the rapid heating step, steam or water vapor entrained in an inert gas, such as nitrogen or argon, can be introduced into the reactor.

[0033] According to various embodiments, the feedstock mixture is rapidly heated by contact with the hot gas stream. The velocity of the hot gas may also vary depending on the reactor size, nature of the feedstock, granule particle size, and/or feedstock feed rate. The stream velocity may range for example, from about 0.5 m/sec to about 2000 m/sec, such as from about 1 m/sec to about 1000 m/sec, from about 2 m/sec to about 100 m/sec, from about 5 m/sec to about 20 m/sec, or from about 5 m/sec to about 15 m/sec, including all ranges and subranges therebetween. It is within the ability of one skilled in the art to select the stream velocity appropriate for the desired operation and result. The temperature of the hot gas stream may range, for instance, from about 600°C to about 900°C, such as from about 650°C to about 800°C, or from about 680°C to about 750°C, including all ranges and subranges therebetween. In other embodiments, the reactor may be heated by an additional heating means, for example, by way of resistance, microwave, or dielectric (RF) heating.

[0034] As used herein, the term "rapid heating" is used to denote heating of the feedstock mixture to at least its solidification temperature within a period of time sufficient to maintain the feedstock mixture in a substantially solid state. As used herein, the term "substantially solid state" and variations thereof are intended to denote that the feedstock mixture is comprised essentially or totally of solid particles. For instance, the feedstock mixture may comprise 100% by weight of solid particles or, in other embodiments, the feedstock mixture may comprise greater than about 99.9% by weight of solid particles, such as greater than about 99.5%, greater than about 99%, greater than about 98%, greater than about 97%, greater than about 96%, or greater than about 95% by weight of solid particles. [0035] Referring to FIG. 1 , which illustrates an estimated equilibrium phase diagram for KOH and carbon in a closed system, it is noted that the exemplified feedstock mixture undergoes several phase changes at different theoretical temperatures and carbon/KOH ratios. The different phases are depicted in FIG. 1 , with reference to Regions A-l described below in Table I:

Table I: Theoretical Regions A-l

[0036] For instance, in the case of KOH, for the illustrated compositions in theoretical equilibrium, the closed system exists as two solid phases (Region A) up to approximately 375°C. Above about 375°C, which is the approximate fluxing temperature, KOH melts and a plurality of liquid phases are prominent in each of Regions B, C, and D. Typically carbon activation occurs at conditions within Region H of FIG. 1 (i.e., for compositions having a C(C+KOH) mass ratio of at least about 0.08 and at a temperature ranging from about 680°C to about 800°C).

[0037] According to various embodiments herein, when KOH is the chemical activating agent, the feedstock mixture is rapidly heated to at least about the theoretical solidification temperature (approximately 680°C) in a time period sufficient to maintain the feedstock mixture in a substantially solid state. In other words, the feedstock mixture is rapidly heated through the theoretical KOH fluxing temperature range (Regions D and G, approximately 375-680°C), in a time period sufficient so as to avoid the formation of a liquid phase. According to various embodiments, the time period may be less than about 10 seconds, for instance, less than about 5 seconds, less than about 1 second, less than about 0.5 seconds, or less than about 0.1 seconds. In other embodiments, the rapid heating of the feedstock mixture may occur within milliseconds, for example, the time period may range from about 0.01 to about 0.09 seconds.

[0038] The intersecting dashed lines in FIG. 1 serve to illustrate an exemplary set of process parameters, i.e., T = 730°C and C/(C+KOH) = 0.33 (or KOH:C = 2:1 ), which may be employed in the methods disclosed herein and are not intended to be limiting in any way. It is to be understood that the fluxing and solidification temperatures may vary depending on the chemical activating agent or mixture of agents used. It is within the ability of one skilled in the art to identify these temperatures for any feedstock mixture as defined herein.

[0039] It will also be understood by those skilled in the art that the methods and processes disclosed herein may, in various embodiments, function under non- equilibrium conditions. In such cases, it is noted that the actual or observed fluxing and/or solidification temperatures may vary from those predicted by the model in FIG. 1 . For instance, as indicated in Table II below, the experimentally observed values for the KOH/carbon system may be lower than those predicted by the theoretical model. The experimental data provided below is for exemplary purposes only and is not intended to limit or otherwise define the scope of the instant disclosure. It is within the ability of those skilled in the art to obtain similar experimental values for other activating agents disclosed herein and mixtures of such activating agents.

Table II: Experimental Regions A-G

[0040] As illustrated in Table II above, the observed experimental fluxing temperature of a KOH/carbon system can be as low as 120°C, at which point the KOH begins melting or undergoing a phase transformation from solid to liquid.

Similarly, the solidification temperature of the KOH/carbon system can be as low as 500°C, at which point the feedstock mixture undergoes at least one liquid to solid transformation which results in a substantially liquid-free, i.e., substantially solid bulk mixture. Activation may likewise occur at temperatures lower than theoretical values, such as greater than about 500°, or greater than about 600°C. According to various embodiments herein, when KOH is the chemical activating agent, the feedstock mixture is rapidly heated to at least about the experimental solidification temperature (approximately 500°C) in a time period sufficient to maintain the feedstock mixture in a substantially solid state. In other words, the feedstock mixture is rapidly heated through the observed KOH fluxing temperature range (Regions B- E, approximately 120-500°C), in a time period sufficient so as to avoid the solid-liquid transformation and the formation of a liquid phase.

[0041] Without wishing to be bound by theory, it is believed that a sufficiently short residence time in the fluxing temperature range will retain the feedstock mixture in a substantially dry, i.e., substantially solid state throughout the heating stage. For example, the rapid heating may avoid the regions in which the activating agent, such as KOH, melts or undergoes a phase transition from solid to liquid. In the case of KOH as activating agent, the feedstock mixture may be optionally pre-heated up to approximately 375°C (the theoretical fluxing temperature) and rapidly heated up to at least about 680°C (the theoretical solidification temperature), followed by a holding time at a temperature sufficient to activate the carbon, as discussed herein. In other embodiments, the feedstock mixture may be optionally pre-heated up to

approximately 120°C (the experimental fluxing temperature) and rapidly heated up to at least about 500°C (the experimental solidification temperature), followed by a holding time at a temperature sufficient to activate the carbon, as discussed herein.

[0042] A chemical activating agent such as KOH can interact and react with carbon such that the potassium ion is intercalated into the carbon structure and potassium carbonate is formed. The reaction kinetics for both of these processes is believed to increase at elevated temperatures, which can lead to a higher rate of activation. As used herein, the term "activation" and variations thereof refer to a process whereby the surface area of carbon is increased such as through the formation of pores within the carbon.

[0043] While the carbon in the feedstock mixture may become activated to some degree during the rapid heating stage, generally this stage is not of a sufficient duration to achieve an adequate level of activation. Thus, subsequent to the rapid heating stage, the heated feedstock mixture is introduced into a reaction vessel, where it is held at a temperature and for a time sufficient to react the carbon feedstock with the at least one chemical activating agent to form activated carbon. According to various embodiments, the feedstock is held for a time ranging from about 5 minutes to about 6 hours, for instance, from about 5 minutes to about 1 hour, or from about 10 minutes to about 40 minutes, including all ranges and subranges therebetween. The temperature in the reaction vessel may range, for example, from about 600°C to about 900°C, such as from about 700°C to about 900°C, or from about 680°C to about 800°C, including all ranges and subranges therebetween. [0044] The reaction vessel may be chosen, for example, from fluid bed reactors, rotary kiln reactors, tunnel kiln reactors, crucibles, microwave reaction chambers, or any other reaction vessel suitable for heating and maintaining the feedstock at the desired temperature for the desired period of time. Such vessels can operate in batch, continuous, or semi-continuous modes. In at least one embodiment, the reaction vessel operates in continuous mode, which may provide certain cost and/or production advantages. Because the feedstock mixture is in a substantially solid state, it is believed that the potential for agglomeration will be significantly decreased, thereby impacting material flowability to a much smaller degree versus other conventional processes.

[0045] Microwave heating can also be employed to heat the reaction vessel. A microwave generator can produce microwaves having a wavelength from 1 mm to 1 m (frequencies ranging from 300 MHz to 300 GHz), though particular example microwave frequencies used to form activated carbon include 915 MHz, 2.45 GHz, and microwave frequencies within the C-band (4-8 GHz). Within a microwave reaction chamber, microwave energy can be used to heat a feedstock mixture to a predetermined temperature via a predetermined thermal profile.

[0046] In the case of microwave heating, batch processes can include loading the feedstock mixture into a crucible that is introduced into the microwave reaction chamber. Suitable crucibles include those that are compatible with microwave processing and resistant to alkali corrosion. Exemplary crucibles can include metallic (e.g., nickel) crucibles, silicon carbide crucibles or silicon carbide-coated crucibles such as silicon carbide-coated mullite. Continuous feed processes, may include, for example, fluid bed, rotary kiln, tunnel kiln, screw-fed, or rotary-fed operations. Carbon material in the form of a feedstock mixture can also be activated in a semi-continuous process where crucibles of the feedstock mixture are conveyed through a microwave reactor during the acts of heating and reacting.

[0047] As the activated carbon exits the reaction vessel, it can be held in a quench tank where it is cooled to a desired temperature. For instance, the activated carbon may be quenched using a water bath or other liquid or gaseous material. An additional benefit to quenching with water may include potential neutralization of unreacted alkali metals to minimize potential corrosion and/or combustion hazards. A rotary cooling tube or cooling screw may also be used prior to the quench tank.

[0048] After activation and quenching, the activated carbon can be optionally ground to a desired particle size and then washed in order to remove residual amounts of carbon, retained chemical activating agents, and any chemical byproducts derived from reactions involving the chemical activating agent. As noted above, the activated carbon can be quenched by rinsing with water prior to grinding and/or washing. The acts of quenching and washing can, in some embodiments, be combined.

[0049] The activated carbon may be washed and/or filtered in a batch, continuous, or semi-continuous manner and may take place at ambient temperature and pressure. For example, washing may comprise rinsing the activated carbon with water, then rinsing with an acid solution, and finally rinsing again with water. Such a washing process can reduce residual alkali content in the carbon to less than about 200 ppm (0.02 wt%). In certain embodiments, after quenching and/or rinsing, the activated carbon is substantially free of the at least one chemical activating agent, its ions and counterions, and/or its reaction products with the carbon. For instance, in the case of KOH as the chemical activating agent, the activated carbon is

substantially free of KOH, K⁺, OH^", and K2CO3. Accordingly, it is believed that the chemical activating agent intercalates into the carbon and is then removed, leaving behind pores, i.e., increasing the surface area and activating the carbonaceous feedstock.

[0050] The activated carbon can comprise micro-, meso- and/or macroscale porosity. As defined herein, microscale pores have a pore size of about 2 nm or less and ultra-microscale pores have a pore size of about 1 nm or less. Mesoscale pores have a pore size ranging from about 2 to about 50 nm. Macroscale pores have a pore size greater than about 50 nm. In one embodiment, the activated carbon comprises a majority of microscale pores.

[0051] As used herein, the term "microporous carbon" and variants thereof means an activated carbon having a majority (i.e., at least 50%) of microscale pores. A microporous, activated carbon material can comprise greater than 50% microporosity (e.g., greater than about 50, 55, 60, 65, 70, 75, 80, 85, 90 or 95% microporosity). According to certain embodiments, the activated carbon may have a total porosity of greater than about 0.2 cm³/g (e.g., greater than about 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65 or 0.7 cm³/g). The portion of the total pore volume resulting from micropores (d < 2 nm) can be about 90% or greater (e.g., at least about 90, 94, 94, 96, 98 or 99%) and the portion of the total pore volume resulting from micropores (d < 1 nm) can be about 50% or greater (e.g., at least about 50, 55, 60, 65, 70, 75, 80, 85, 90 or 95%).

[0052] FIG. 2 illustrates an exemplary system operable for carrying out a method according to the present disclosure. In this embodiment, a carbon feedstock 100 and at least one chemical activating agent 105 are introduced into a mixer 110, wherein they are combined to form a feedstock mixture 115. The feedstock mixture is then introduced into a granulator 120, which is optionally heated. While the granulator 120 is depicted as a separate vessel from the mixer 1 10, it is to be understood that variations of this configuration are possible and within the scope of the disclosure. The granulated feedstock mixture 125 is then introduced into an entrained stream reactor 130. While FIG. 2 illustrates the feedstock mixture being fed into the bottom of the reactor, it is to be understood that variations of this configuration are possible and within the scope of the disclosure. A boiler 140 produces steam 145, which is also introduced into reactor 130. While FIG. 2 employs steam as the hot stream, it is to be understood that various hot gases and mixtures thereof can be employed in accordance with the instant disclosure. The feedstock mixture 125 is entrained in the steam 145 and carried up the reactor 130, during which time it is rapidly heated by the steam.

[0053] In the illustrated embodiment, the heated feedstock and steam mixture 135 is then introduced into a cyclone 150, where the gases 185 are separated from the heated feedstock 155. The heated feedstock is subsequently introduced into a reaction vessel 160, where it is held at a temperature and for a time sufficient to produce activated carbon 165. The activated carbon is sent to a quench tank 170, where it is cooled to a desired temperature. The quenched activated carbon 175 is then optionally subjected to additional processing steps such as rinsing and/or filtration. An optional recycle loop may be employed, wherein the gases 185 are condensed in condenser 180, purified in a gas/liquid separator 190, and pumped back into the boiler 140 by pump 200. The boiler then introduces the partially recycled stream 145 into the reactor 130, and releases any flu gases 195.

[0054] It will be appreciated that the various disclosed embodiments may involve particular features, elements or steps that are described in connection with that particular embodiment. It will also be appreciated that a particular feature, element or step, although described in relation to one particular embodiment, may be interchanged or combined with alternate embodiments in various non-illustrated combinations or permutations.

[0055] It is also to be understood that, as used herein the terms "the," "a," or "an," mean "at least one," and should not be limited to "only one" unless explicitly indicated to the contrary. Thus, for example, reference to "a chemical activating agent" includes examples having two or more such "chemical activating agents" unless the context clearly indicates otherwise.

[0056] Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, examples include from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0057] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred.

[0058] It is also noted that recitations herein refer to a component of the present invention being "configured" or "adapted to" function in a particular way. In this respect, such a component is "configured" or "adapted to" embody a particular property, or function in a particular manner, where such recitations are structural recitations as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is "configured" or "adapted to" denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component.

[0059] While various features, elements or steps of particular embodiments may be disclosed using the transitional phrase "comprising," it is to be understood that alternative embodiments, including those that may be described using the transitional phrases "consisting" or "consisting essentially of," are implied. Thus, for example, implied alternative embodiments to a carbon feedstock that comprises a carbonized material include embodiments where a carbon feedstock consists of a carbonized material and embodiments where a carbon feedstock consists essentially of a carbonized material.

[0060] It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Since modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A method for forming activated carbon, comprising:

providing a feedstock mixture comprising a carbon feedstock and at least one chemical activating agent;

introducing the feedstock mixture into a reactor;

rapidly heating the feedstock mixture to at least the solidification temperature by introducing a hot stream into the reactor;

introducing the heated feedstock mixture into a reaction vessel; and holding the heated feedstock mixture in the reaction vessel at a temperature and for a time sufficient to react the carbon feedstock with the at least one chemical activating agent to form activated carbon;

wherein rapidly heating the feedstock mixture comprises heating within a time period sufficient to maintain the feedstock mixture in a substantially solid state.

2. The method according to claim 1 , wherein providing the feedstock mixture comprises providing a carbon feedstock and mixing the carbon feedstock with at least one chemical activating agent to produce a feedstock mixture.

3. The method according to claim 2, wherein providing the carbon feedstock comprises carbonizing at least one carbonaceous material in an inert atmosphere at a temperature ranging from about 500 to 950°C.

4. The method according to claim 1 , wherein the at least one chemical activating agent is chosen from KOH, H₃P0₄, NaOH, Na₂C0₃, NaCI, MgCI₂, AICI₃, P2O5, K₂C0₃, KCI, ZnCI₂, and mixtures thereof.

5. The method according to claim 1 , wherein the at least one chemical activating agent is chosen from alkali hydroxide salts.

6. The method according to claim 1 , wherein the mass ratio of chemical activating agent to carbon feedstock in the feedstock mixture ranges from about 0.5:1 to about 5:1 .

7. The method according to claim 1 , further comprising milling the feedstock mixture to an average particle size ranging from about 0.5 to about 25 mm.

8. The method according to claim 2, wherein providing the feedstock mixture further comprises heating at a temperature of 400°C or less while mixing the carbon feedstock with the at least one chemical activating agent.

9. The method according to claim 1 , wherein the feedstock mixture is a wet or dry mixture.

10. The method according to claim 1 , wherein the reactor is an entrained stream reactor.

1 1 . The method according to claim 10, wherein the feedstock mixture is entrained in the hot stream within the reactor.

12. The method according to claim 10, wherein the feedstock mixture is introduced into the entrained stream reactor by injecting, spraying, or atomizing the feedstock mixture.

13. The method according to claim 10, wherein the hot stream is introduced into the entrained stream reactor at a rate ranging from about 0.5 m/sec to about 2000 m/sec.

14. The method according to claim 1 , wherein the hot stream comprises steam or a mixture of steam with at least one other gas chosen from nitrogen or argon.

15. The method according to claim 1 , wherein the rapid heating occurs within a time period of 10 seconds or less.

16. The method according to claim 1 , wherein rapidly heating the feedstock mixture comprises heating to a temperature of at least 500°C in a time period of 5 seconds or less.

17. The method according to claim 1 , wherein the heated feedstock mixture is held in the reaction vessel at a temperature ranging from about 600°C to about 900°C and for a time ranging from about 5 minutes to about 6 hours.

18. The method according to claim 1 , further comprising at least one step chosen from cooling the activated carbon and/or rinsing the activated carbon with water.

19. A method for forming activated carbon, comprising:

providing a feedstock mixture comprising a carbon feedstock and at least one alkali hydroxide salt;

introducing the feedstock mixture into an entrained stream reactor;

rapidly heating the feedstock mixture by introducing steam into the reactor; introducing the heated feedstock mixture into a reaction vessel;

holding the heated feedstock mixture in the reaction vessel at a temperature and for a time sufficient to react the carbon feedstock with the at least one alkali hydroxide salt to form activated carbon; and

cooling the activated carbon,

wherein rapidly heating the feedstock mixture comprises heating to at least 500°C in a time period of 10 seconds or less.

20. The method according to claim 19, wherein the at least one alkali hydroxide salt is KOH.