WO2009017688A2

WO2009017688A2 - Carbon pore formers for dielectric drying

Info

Publication number: WO2009017688A2
Application number: PCT/US2008/009074
Authority: WO
Inventors: William P Addiego; Steven B Dawes; Kishor P Gadkaree; Youchun Shi; Gary G Squier; Elizabeth M Vileno
Original assignee: Corning Inc
Current assignee: Corning Inc
Priority date: 2007-07-31
Filing date: 2008-07-25
Publication date: 2009-02-05
Anticipated expiration: 2010-01-31
Also published as: WO2009017688A3

Abstract

This invention describes a composition and article for a honeycomb body using dielectric drying. The composition comprises a refractory material and a pore former comprising activated carbon. The activated carbon pore former improves efficiency of dielectric drying, increases penetration depth, accelerates volatilization of the pore former, and reduces cracking of the green body.

Description

CARBON PORE FORMERS FOR DIELECTRIC DRYING

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S. C. § 119(e) of

U.S. Provisional Application Serial No. 60/962719 filed on July 31, 2007.

TECHNICAL FIELD

[0002] The present invention relates to a composition and article for honeycomb bodies, and more particularly to a composition and article for use in the removal of particles from exhaust gas.

BACKGROUND

[0003] Particulate filters, such as diesel particulate filters (DPF), remove solids from the exhaust stream of engines. These filters often include honeycomb bodies that possess a combination of high thermal shock resistance, chemical and mechanical durability, and good filtration efficiency.

[0004] The particulate-laden exhaust stream enters the filter, which traps the particles in its pores and along its walls. The exhaust gas then permeates through the walls and escapes into the air. The accumulation of particles can clog the filter, which must then be replaced or regenerated by volatilizing the particulates at high temperature.

[0005] The process of producing a ceramic filter can comprise mixing a pore former with the refractory ceramic and a plasticizer, such as water. The mixture is formed into a green body, and the green body is dried to remove excess plasticizer that could cause cracking during firing. The dried green body is fired to produce the filter. Drying may occur by any convenient means, such as by using hot-air, infrared, or dielectric methods. Hot air and infrared methods dry the green body from the outside, which is a slow process given the thermal insulating properties of refractory ceramics. In contrast, dielectric drying generates heat within the green body, is relatively fast, and can be highly selective in the area to be heated. Suitable radiation includes, for example, microwave or radio frequencies.

[0006] Unfortunately, carbonaceous pore formers, like graphite, often absorb the radiation from dielectric drying. This dramatically reduces the efficiency of dielectric heating, both in terms of energy efficiency and drying time. Absorption may also restrict penetration of the radiation into the green body and cause cracking by localized heating. Graphite, in particular, because of its high electrical conductivity and dielectric properties can cause localized overheating and cracking of the surrounding refractory ceramic. Other carbonaceous pore formers include thermoplastic resins, such as polyacrylates and polyolefins. Although these materials are poor conductors and slow to transfer heat, thermoplastics can soften or melt when dielectrically heated, thereby potentially distorting the green body and the size and shape of the pores. SUMMARY

[0007] The present invention describes a composition and an article produced from the composition that is less susceptible to cracking during dielectric drying than prior art using other carbonaceous pore formers. The article may include a diesel particulate filter.

[0008] The composition includes a refractory material, pore former particles, and a plasticizer. The pore former consists essentially of an activated carbon that, during dielectric drying, retains it shape and allows better penetration of microwave and radio frequencies into the surrounding refractory ceramic matrix. The activated carbon will have an apparent density of less than about 1.7 g/cm³ and will comprise a majority of an amorphous phase. The low apparent density and amorphous phase reduce both absorption of radiation and transfer of any irradiation-generated heat to the refractory ceramic matrix.

[0009] In one embodiment, the composition includes activated carbon particles having a median size (dso) from about 10-50 microns. The activated carbon is combined with refractory ceramic grains, such as cordierite, and a sufficient amount of water which functions as a plasticizer. The mixture is extruded as a honeycomb green body and dried using dielectric drying. The low density activated carbon absorbs little radiation and does not generate significant heat. Advantageously, the decrease in absorption permits the radiation to penetrate more deeply in the green body, thereby drying the green body more efficiently. The lack of heat generated by the activated carbon reduces the chance of localized overheating and cracking of the refractory ceramic.

[0010] The article includes an engine exhaust filter, more particularly, a diesel engine particular matter filter, that is produced from a composition comprising refractory ceramic grains, an activated carbon, and a plasticizer. The composition is formed into a green body and dried using dielectric heating. The dried body is then fired at a temperature sufficient to volatilize the activated carbon and bond the ceramic grains. [0011] Some embodiments disclosed herein describe the use of a carbonaceous pore former during dielectric heating; in some of these embodiments, the pore former retains its shape. Such a pore former allows better microwave and radio frequency penetration into the ceramic matrix than that for a comparable graphite pore former. DETAILED DESCRIPTION

[0012] Typical batch compositions for producing honeycomb bodies include carbonaceous pore formers that can interfere with dielectric drying of green bodies. Pore formers may absorb radiation, thereby reducing the efficiency of dielectric drying, contributing to localized overheating of the ceramic matrix, and distorting the pores from their expected shape, size or density.

[0013] The inventors have discovered that activated carbon having a high porosity, low apparent density, and substantially amorphous morphology can substantially reduce absorption of radiation and result in more uniform drying. Other benefits include reduced drying times, lower power requirements, deeper penetration of dielectric heating, and reduced risk of cracking. Compared to other carbonaceous pore formers, activated carbon produces the same pore density with only ¹A to V₆ the mass. Additionally, activated carbon resists deformation on drying and, at least partially because of its low mass, volatilizes more easily during firing. [0014] The starting batch composition of the invention includes inorganic ceramic forming components including a refractory material, pore former consisting essentially of activated carbon, and a plasticizer. The relative ratios of the constituents will depend on the application. For a DPF, the starting batch composition may include from 60-90 wt.% refractory material, from 1-40 wt.% pore former, and from 0.5-25 wt.% plasticizer. Preferably, the composition includes 4-20 wt.% pore former. The composition may also include a binder or lubricant. Binders are well known by those skilled in the art and typically comprise 2-6 wt.% of the composition. Lubricants facilitate processing and include oils. Typical amounts include up to about 2 wt.% of lubricant, such as tall oil. Additionally, other pore formers in addition to the activated carbon may be included in the starting batch compositions; for example starch and nut flours and the like may used [0015] The refractory material may include any suitable compound which exhibits the requisite properties for an engine exhaust filter. That said, suitable material for this engine exhaust filter embodiment would combine a low coefficient of thermal expansion, chemical inertness, and resistance to high temperatures. Although, this invention is not limited to any particular refractory material, typical compounds include refractory ceramics such as, for example, cordierite, alumina, aluminum titanate, and silica. The plasticizer includes any medium that facilitates mixing of the refractory material and can later be removed. The plasticizer is typically water. Other plasticizers include various alcohols, glycerin, acrylates, glycols, and combinations thereof. A sufficient amount of plasticizer will be used to permit forming the composition into the green body. The amount will depend on various factors known to one skilled in the art, including the article to be produced, the forming process, the size, shape and composition of the refractory grains, and the amount of pore former.

[0016] As already stated, the pore former comprises activated carbon consisting essentially of carbon. Activated carbon means a material consisting essentially of a porous carbon with a correspondingly low apparent density and substantial non- crystalline or amorphous phase. The amorphous phase may form a continuous matrix. The apparent density of activated carbon is typically less than aboutl.7 g/cm³ and preferably less than 1.5 g/cm³. Apparent density means the mass per unit volume of a material including voids inherent in the material as tested. The porosity of activated carbon often inherently results in a material with high surface area per unit mass. Low density corresponds to electrically insulating properties and a low mass per unit volume. In contrast, graphite has a density of about 2.2 g/cm³, is crystalline, is not inherently porous, has a relatively low surface area, and is semi-conductive. Without intending to be bound by this explanation, the inventors believe activated carbon's amorphous structure and lower density significantly reduce the electrical conductivity and radiation absorption of green bodies during dielectric drying. This dramatically enhances the efficiency of the dielectric drying process. [0017] Activated carbon can be manufactured by any know technique including coking or extraction from coal and wood chars. Coke is an amorphous or poorly crystallized carbon that is generally made from petroleum refinery products or coal. Chars are similar to coke but may be derived from wood, for example, coconut shell, walnut shell, pecan shell, hardwood, and softwood, and other cellulose-bearing stock. Unlike coke, chars do not pass through a fluid phase during pyrolysis and carbonization. Activated carbon can also be manufactured by carbonizing organic polymers, such as phenolic resins, and subsequently activating the carbon. The activated carbon preferably has an ash content of less than 6%, more preferably less than 3%, and most preferably less than 1%. [0018] Activated carbon has a lower density than most carbonaceous pore formers, so it will typically absorb less radiation per volume than other carbonaceous pore formers such as graphite. Radiation can then penetrate more deeply into the green body and the activated carbon transmits less energy to surrounding refractory ceramic matrix. Further, even ignoring the density of the compounds, activated carbon has a lower electrical conductivity than graphite. Graphite has delocalized π bonds and an ordered lamellar structure, which facilitate absorption of radiation. The amorphous phase of activated carbon substantially lacks π bonding and the disordered morphology of activated carbons interferes with electrical conductivity, so that activated carbons absorb less energy and radiate less of that absorbed energy to the surrounding refractory material. Electrical conductivity of the pore former may be further reduced by removing carbon fines or coating the activated carbon with a non- conductive material. Carbon fines means particles having a median size (dso) less than about 15 microns. Electrical conductivity can be reduced by limiting carbon fines to less than about 10 wt.%. Non-conductive material means a material having an electrical conductivity less about 10^~2 Sm^"1, that is less than that of activated carbon. Non-conductive materials includes certain ceramic materials and polymers. [0019] A pore former consisting essentially of activated carbon may comprise one or more non-graphitic carbons that are either graphitizable (anisotropic), such as cokes, or non-graphitizable (isotropic), such as chars. Activated carbons may also include porous molecular sieve carbons, with poorly organized, disrupted, or imperfect lamellae structures, and with apparent densities less than graphite. Similarly, activated carbons may also include partially or fully carbonized carbonaceous materials such as woods, starches, polymers and petroleum refinery products.

[0020] The honeycomb or porous ceramic article of the present invention is produced by forming the composition into a green body, drying using dielectric techniques (microwave or RF drying), and firing at a sufficiently high temperature to volatilize the activated carbon. In particular, process involves the following steps: (1) mixing inorganic ceramic- forming components, a plasticizer, and a pore former consisting essentially of an activated carbon, to form a batch composition; (2) shaping the batch composition into a green body; (3) drying the green body via dielectric drying; and, (4) heating the green body at a temperature and for a time to form the porous ceramic article.

[0021] The article may be shaped into a green body by extrusion, pressing or any other method known to one skilled in the art. The refractory material will typically form a continuous phase around the pore former. Dielectric drying, may be accomplished by any known means, for example, microwave, radio frequency (which includes induction drying); in this type of drying the microwaves and the RF waves, interact with molecules (including, for example water) and certain polarizable species, to quickly generate heat within a honeycomb body, as opposed to conventional heating where heat is applied externally. The particular drying parameters, such as irradiation power and time, will be determined by the particular application. Firing will occur at a temperature sufficient to volatilize the pore former and bond the refractory material. To this end, firing is typically above about 500 C and more typically is above 700 C. [0022] The composition, porosity, and pore size will depend on the intended use of the article. For a DPF, a porosity of 40-75% is preferred, and 50-70% is more preferred. The lower porosity limit depends on the allowable pressure drop of the exhaust gas across the filter. Lower porosity increases the pressure drop. The upper porosity limit depends on the permissible lower limit of strength. Higher porosity reduces strength. In a DPF, the pore size affects filtration efficiency and the filter strength. Preferably, the pore size is 1-75 microns and more preferably is 5-30 microns. EXAMPLE

[0023] Four mixtures were made comprising an 80.5 vol% aluminum titanate ceramic and 19.5 vol% pore former. A sufficient amount of water was added to provide plasticity for extrusion. The pore formers included activated carbon, graphite, a calcined petroleum coke, and a calcined petroleum coke in which fine particles were removed. The mixtures were extruded as green body rods having a diameter of 15.9 mm. The rods were allowed to air dry, and were core drilled to a depth of 12.4 mm.

[0024] The half-power depth of heating was determined for each green body rod at a frequency of 915 MHz. The half-power depth (Di_/2p) is the depth at which the forward traveling power is half the entry power, and can be described by the following equations.

u n i / 2 P ^~ - ^(ln _n ²⁾ Equation 1

a Equation 2

tan² δ = I — I Equation 3

[0025] Where X₀ is the wavelength in free space and ε¹ is the dielectric constant of the material, and ε" is the dielectric loss. .

[0026] Table I shows the half-power depths of penetration for the various pore formers. Noticeably, only 4.7 wt.% (of the starting batch material) activated carbon was required to produce a porosity of 47% in the final article. In contrast, graphite and the calcined petroleum cokes required over 25 wt.% to achieve comparable porosity in the final article.

[0027] The half-power depth for the activated carbon green body was 67 cm, which was two orders of magnitudes greater than the article with the graphite pore former. Calcined petroleum coke was over one order of magnitude greater, and removal of the fines more than doubled the half-power depth.

Table I

[0028] Obviously, numerous modifications and variations of the present invention are possible. It is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described. While this invention has been described with respect to certain preferred embodiments, different variations, modifications, and additions to the invention will become evident to persons of ordinary skill in the art. All such modifications, variations, and additions are intended to be encompassed within the scope of this patent, which is limited only by the claims appended hereto.

Claims

Claims:

1. A composition for the production of a honeycomb article using dielectric drying, the composition comprising: a. a refractory material; b. a pore former consisting essentially of an activated carbon; and c. a plasticizer

2. The composition of claim 1, wherein the starting batch composition includes from 60-99 wt.% refractory material, from 1 -40 wt.% carbon pore former, and from 0.5-25 wt.% plasticizer.

3. The composition of claim 1, wherein the starting batch composition includes 4-20 wt.% pore former.

4. The composition of claim 3, wherein the refractory material is a refractory ceramic selected from a group consisting of cordierite, alumina, silica, and combinations thereof.

5. The composition of claim 1, wherein the activated carbon has an apparent density less than 1.7 g/cm³.

6. The composition of claim 1 , wherein the activated carbon has a density of less than 1.5 g/cm³.

7. The composition of claim 1, wherein the activated carbon consists essentially of non-graphitic carbons.

8. The composition of claim 1 , wherein the activated carbon includes a majority amorphous phase.

9. The composition of claim 1, wherein the activated carbon is selected from a group consisting of coke, char, fully carbonized carbonaceous materials, and combinations thereof.

10. The composition of claim 1, wherein the activated carbon consists essentially of particles having a median size (d₅₀) from about 10-50 microns.

11. The composition of claim 1 , wherein the activated carbon includes less than 10 wt.% of fines having a median size (d₅₀) from about 15 microns.

12. A honeycomb article produced from a composition comprising a refractory material a pore former consisting essentially of an activated carbon; and a sufficient amount of plasticizer, where the composition is formed into a green body, dried with dielectric heating, and fired at a temperature sufficient to volatilize the pore former.

13. The honeycomb article of claim 12, wherein the article includes a diesel particulate filter having a porosity of 40-75% and a pore size from 1-75 microns.

14. The honeycomb article of claim 13, wherein the filter has a porosity of 50- 70% and wherein the pore size is from 5-30 microns.

15. The honeycomb article of claim 12, wherein the green body is formed by a method selected from a group consisting of extrusion and pressing, and dried by radiation selected from a group consisting of microwave and radio waves.

16. A method of forming a porous ceramic article comprising mixing inorganic ceramic-forming components including a refractory component, a plasticizer, and a pore former consisting essentially of an activated carbon, to form a starting batch composition; shaping the batch composition into a green body; drying the green body via dielectric drying; heating the green body at a temperature and for a time to form the porous ceramic article.

17. The method of claim 16 wherein the starting batch composition includes from 60-99 wt.% refractory material, from 1-40 wt.% pore former, and from 0.5-25 wt.% plasticizer.

18. The method of claim 16 wherein activated carbon pore former has an apparent density less than 1.7 g/cm³ and wherein the activated carbon is selected from a group consisting of coke, char, fully carbonized carbonaceous materials, and combinations thereof.

19. The method of claim 16 wherein the dielectric drying comprises exposing the green body to microwave energy, RF energy, or both.

20. The method of claim 16 wherein the pore former volatizes during the heating of the green body.