Plastic and nonmetallic article shaping or treating: processes – Carbonizing to form article – From cellulosic material
Reexamination Certificate
2002-02-05
2004-01-27
Lechert, Jr., Stephen J. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Carbonizing to form article
From cellulosic material
C264S029600, C264S029700, C264S109000, C264S122000, C264S126000
Reexamination Certificate
active
06682667
ABSTRACT:
FIELD OF INVENTION
The present invention relates to methods for producing self-supporting activated carbon structures and, more particularly, to the production of self-supporting activated carbon structures using inexpensive, readily available starting materials, such as granular carbon.
BACKGROUND OF THE INVENTION
Self-supporting activated carbon structures have a high degree of utility. For example, self-supporting activated carbon structures have been used as filters, catalyst supports, adsorbents, and electrodes. In some applications, electrical conductivity of the activated carbon structure is desirable for regenerative or electrochemical purposes. The shape or form of the activated carbon structure has been selected to compliment the intended utility. These shapes or forms have included cylinders, rectangular or square blocks, honeycombs, plates, thin sheets, and various other three dimensional forms.
The known methods for producing activated carbon structures can be generally classified into three groups: shaping and activating organic materials; coating a substrate with activated carbon; or combining activated carbon particles with binder to form a structure of desired shape.
In the first group, self-supporting activated carbon structures are formed by shaping selected organic materials into a structure, after which the shaped structure is carbonized and activated. Generally, these organic materials are solids or semi-solids capable of producing appreciable amounts of carbonaceous char when carbonized. Such materials tend to be synthetic, such as thermosetting resins, and thus are relatively expensive when compared to the naturally occurring feed stocks used to produce commercial granular and pulverized activated carbons. Limited amounts of activated carbon particles may be added to the organic material prior to shaping to improve the adsorption properties (i.e. “activity”) of the resultant formed structure. Various other fillers can also be added to the organic material prior to shaping to provide additional strength or porosity to the resultant formed structure.
Shaping of the structure can be performed by pressing, molding, or extrusion, for example. The carbonization and activation are practiced using methods known in the art. It is expected that carbonization and activation of such structures would have to be conducted at relatively slow rates (i.e., low temperatures or slow changes in temperature) to prevent cracking of the formed structure and to insure that a uniform degree of activity (i.e., adsorptive pore volume or adsorptive capacity) is developed throughout the structure. Slow process rates, due to slow heating or slow activation of the structure, are inherently more costly as longer furnace residence times are required. Longer furnace residence times inherently result in higher product costs. Even using slow rates, it is expected that it would be very difficult to produce a structure that has a uniform degree of activity (i.e., adsorptive pore volume or adsorptive capacity) throughout the resultant carbon structure because the requisite uniform heat and mass transfer throughout the structure is exceedingly difficult to establish and/or maintain at temperatures sufficient to provide other than very slow activation rates. Also, the properties of the resultant activated carbon are restricted to those that can be developed from the starting organic material and to the limited degree of modification imparted by the addition of activated carbon particles or other fillers. The economics of producing activated carbon structures by this method are very unfavorable even though electrical conductivity of these structures could be potentially high.
In the second group, activated carbon structures are formed from a non-carbon material substrate which is subsequently coated with activated carbon. Typically, the activated carbon coating is applied both on and throughout the structure, including intrastructural surfaces that define larger pores and void volumes. Several types of approaches have been used to achieve the desired activated carbon coating.
In one approach, a binder/activated carbon mixture can be coated onto the surface of a support structure. Useful binders are those that adhere well to both the support material and the activated carbon. For example, thermoplastic resins and hot melt adhesives can be used to fix an activated carbon coating to a support material. However, caution has to be exercised in selecting the binder so that the binder does not fill or prevent access to the activated carbon adsorption pore space. Alternatively, in another approach, an organic material capable of producing appreciable amounts of carbonaceous char when pyrolyized is coated onto the surface of the structure. Following this coating, the organic material is carbonized and activated. It is generally desired to produce a uniform, continuous layer of carbon on the surface of the structure using this approach. Such a layer has been found to provide good electrical conductivity and structural integrity. In another approach, the organic material of the previous approach is combined with activated or non-activated carbon particles that are typically of very small size. The surface coating is then carbonized and activated. For all three approaches, coatings can generally be applied to support structures that can tolerate the selected process conditions and coating materials without significant degradation. Typically, the support structures are ceramic.
Although these approaches may result in uniform carbon coatings of good activity and utility, the mass and volume based adsorptive capacity of the structure is limited. That is, activated carbon is present as only a surface coating and no adsorptive capacity is claimed for the underlying support material. Therefore, both the mass and volume based adsorptive capacity of the structure are limited by the support material which adds both appreciable volume and mass to the structure without corresponding increases in the structure activity. Moreover, thermosetting resins, or precursors to such resins, are favored organic coating materials. Such resins may have toxicological and/or economic issues associated with their use that can make their application difficult and/or expensive. Additionally, the range of properties of the activated carbons derived from these materials is limited as such properties are largely related to the starting materials. Also, the requisite activation of these materials typically would be best performed under very mild conditions to ensure uniform activation throughout the structure. That is, mild activation conditions are required to prevent the formation of a highly activated carbon on the outer surface of the structure while the carbon in the interior of the structure is only marginally, if at all, activated. The use of such mild conditions increases activation furnace residence times, decreases production rates and leads to substantially increased production costs.
Furthermore, the support material used in preparing activated carbon coated structures of the second group is generally not electrically conductive. Therefore any electrical conductivity associated with these structures is dependent on the conductivity of the carbon coating. Obviously, if high electrical conductivity is desired, care must be exercised that this coating provides for continuous carbon contact throughout its surface coverage.
In the third group, activated carbon structures are produced by bonding activated carbon particles into the desired shape by use of a binder. The method of this group can offer inherent advantages over the methods taught in the first two groups including significantly improved production economics, a greater range of usable and achievable activated carbon activities, greater volumetric structural adsorption capacity, greater structure mass-based adsorption capacity and, sometimes, improved structure electrical conductivity.
Generally, this method utilizes powdered activated carbons, but fibrous activated carbons
Calgon Carbon Corporation
Cohen & Grigsby P.C.
Lechert Jr. Stephen J.
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