Reaction bonded alumina filter and membrane support

Details Reaction bonded alumina filter and membrane support Reaction bonded alumina filter and membrane support

2002-03-13

2004-02-24

Mai, Ngoclan (Department: 1742)

Liquid purification or separation

Filter

Material

C419S002000, C419S019000, C428S116000, C428S188000, C428S304400

Reexamination Certificate

active

06695967

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an improved porous alumina filter body formed from an extruded monolith substrate. The body is formed by sintering mixtures containing aluminum and alumina powders in an oxidizing atmosphere (e.g., air). During sintering of the body, oxidation of the metal occurs with a concomitant expansion that counteracts the shrinkage caused by sintering, giving an overall volume change that is negligibly small or zero. The resulting body is highly permeable to gases and liquids, and may be used for filtration purposes or as a support for a semi-permeable membrane.
BACKGROUND OF INVENTION
Ceramic Honeycomb Monoliths. Extruded ceramic honeycomb monoliths were initially developed as catalyst supports for automotive catalytic converters, environmental catalyst supports for fixed site installations, and diesel particulate filters. These monoliths have a multiplicity of passageways that extend from one end face to an opposing end face. The cell structure is formed by the extrusion process, with a cell density as low as low as 9 cells per square inch (cpsi) to as high as 1400 cpsi. For monoliths with circular cross sections, diameters can be as large as 12 inches, or greater. The length of such monoliths in extrusion can be over 6 feet, and is limited by such factors as the available facilities for uniform drying and sintering. Numerous patents exist for such monoliths produced from cordierite (e.g., Lachman and Lewis in U.S. Pat. No. 3,885,997, and Frost and Holleran in U.S. Pat. No. 3,899,326) and silicon carbide (e.g., Stobbe in U.S. Pat. No. 5,195,319 and U.S. Pat. No. 5,497,620). In general, such monoliths, especially those with larger diameters and longer lengths, are difficult to produce from most ceramics. Cordierite has been produced relatively readily because it has a low coefficient of thermal expansion (CTE) of about 2×10
−7
/° C. to 1×10
−6
/° C. This low CTE minimizes thermal and mechanical stresses during the sintering and cool-down process, allowing sintering of such monoliths and avoiding fracture during sintering. Silicon carbide monoliths with a higher CTE of about 3.5-4×10
−6
/° C. have superior thermal and mechanical properties that permit their sintering and cooling without fracturing. First, the relatively high thermal conductivity of silicon carbide (e.g., >5W/m-K) relieves thermal gradients within the monolith. Second, the relatively high mechanical strength allows greater stress tolerance during sintering and cool-down. Third, and relevant to the subject of this invention, the volume change during sintering is very small, typically about 1-2% shrinkage. This “near net shape” property is important for sintering ceramic bodies in a way that minimizes internal stresses during the sintering cycle, thereby reducing the risk of mechanical failure.
In summary, present methods for manufacturing honeycomb-structured monoliths involve extrusion of suitably plastic batch materials through a die, followed by drying and sintering at an appropriate temperature to produce the final monolith. The choice of materials currently available for monolith fabrication is restricted to those that have a very low CTE or exhibit negligible (<2-5% linear) shrinkage during sintering in order to prevent deformation and/or cracking of the monolith channels during sintering and subsequent cooling. At present, honeycomb-structured monoliths are only commercially available in relatively low CTE materials, such as cordierite, mullite and silicon carbide. Although cordierite and mullite are relatively inexpensive materials, their chemical durabilities are inferior to those of silica-free oxide ceramics. The chemical durability of silicon carbide is significantly greater, but the relatively high fabrication cost associated with sintering at elevated temperatures (>2000° C.) in an inert atmosphere make the use of silicon carbide an expensive proposition for many engineering applications. Also, when used for certain applications, including those involving membrane coatings, silicon carbide monoliths may suffer from a chemical durability limitation. Specifically, the surface of silicon carbide is readily oxidized to silica. The bond between an overlying membrane coating and this silica interface may be subject to chemical attack, especially by alkaline solutions.
Porous Ceramic Monoliths as Membrane Supports. Porous ceramic monoliths are widely used as supports for filter bodies and ceramic membrane filter devices, and the patent art contains descriptions of monoliths produced from many different materials. Perhaps the earliest disclosure was in the French Patent Publication 2,061,933, filed Oct. 3, 1969 by the Commissariat a L'Energie Atomique, which describes a multichannel &agr;-alumina monolith as a support for an &agr;-alumina ultrafiltration membrane. In 1978 Hoover and Roberts (U.S. Pat. No. 4,069,157) described the use of cordierite honeycomb monoliths as supports for dynamically formed membranes. In 1984, Gillot, et al., presented a paper “New Ceramic Filter Media for Cross-Flow Microfiltration and Ultrafiltration” (
Filtra
1984
Conference, Oct.
2-4, 1984) that described the use of sintered &agr;-alumina membranes deposited on sintered &agr;-alumina monoliths supports, closely related to the CEA French patent mentioned above. Abe, et al. (U.S. Pat. No. 4,894,160) disclosed the use of clay-bonded &agr;-alumina as a honeycomb monolith support. In 1993 Faber and Hudgins described the use of titania as a monolith membrane support (U.S. Pat. No. 5,223,318). In 1995 Castillon and Laveniere (U.S. Pat. No. 5,415,775) disclosed the use of a mixture of titania/&agr;-alumina monoliths as membrane supports. Grangeon and Lescoche describe metal oxide monolith supports containing titania in combination with other metal oxides, especially alumina (U.S. Pat. No. 5,607,586 and U.S. Pat. No. 5,824,220).
In general, porous &agr;-alumina, configured in tubular and monolith structures, is the most common material used as a support for ceramic membranes. Such porous &agr;-alumina materials are most commonly produced by sintering a monodisperse alumina at temperatures of 1600° C. to 1800° C. The use of clay, or other metal oxides, or fine &agr;-alumina reactive binders can reduce the sintering temperature needed.
Large diameter honeycomb monoliths have been used for membrane supports for crossflow membrane devices. For example, the patents of Hoover and Roberts (U.S. Pat. No. 4,069,157), Hoover and Iler (U.S. Pat. No. 4,060,488), Goldsmith (U.S. Pat. No. 4,781,831, U.S. Pat. No. 5,009,781, and U.S. Pat. No. 5, 108,601), Faber and Frost (U.S. Pat. No. 5,641,332), Yorita, et al., (U.S. Pat. No. 5,855,781), and Rajnik, et al. U.S. Pat. No. 6,077,436) disclose such devices.
Similar large diameter monoliths have been used as dead end filters, especially for diesel exhaust gas filtration. Early diesel filter devices are described by Outland (U.S. Pat. No. 4,276,071), Higuchi, et al. (U.S. Pat. No. 4,293,357, U.S. Pat. No. 4,340,403 and U.S. Pat. No. 4,364,760), Berg, et al., (U.S. Pat. No. 4,364,761), Pitcher (U.S. Pat. No. 4,329,162 and U.S. Pat. No. 4,417,908), and other extensive patent art.
Similar monolith structures have been used as membrane supports for dead end membrane filters in which the monolith passageways are coated with a membrane and the passageway ends are plugged, for example, in an “alternate, adjacent checkerboard pattern” typical of diesel exhaust filters, Goldsmith, et al. (U.S. Pat. No. 5,114,581). These filters can be used for the removal of particulates from a gas or a liquid.
The above large diameter monoliths used as membrane supports (or filter bodies) have all been conceptual designs or made from ceramic materials (cordierite, mullite or silicon carbide) that can be successfully extruded, dried and sintered in large diameter parts while maintaining mechanical integrity. The decisive disadvantage of ceramics and ceramic composites formed by such a process is the normally high linear shrinkage that o

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