The present invention relates to a thermal insulating device for high temperature reactors and furnaces which utilize highly active chemical gases as reactants. The thermal insulating device includes a shell of resin bonded spiral wound flexible graphite sheet. The device also includes barriers for restriction of thermal transfer.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. Graphites possess anisotropic structures and thus exhibit or possess many properties which are highly oriented, i.e. directional. Graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, i.e. the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. Natural graphites possess a high degree of orientation and hence anisotropy with respect to thermal and electrical conductivity and other properties.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction and thus form an expanded or intumesced graphite structure in which the laminar character is substantially retained.
Natural graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is at least 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated sheets, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is at least 80 times the original xe2x80x9ccxe2x80x9d direction dimension into integrated sheets without the use of any binding material is believed to be possible due to the excellent mechanical interlocking, or cohesion which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of thermal anisotropy. Sheet material can be produced which has excellent flexibility, good strength and is highly resistant to chemical attack and has a high degree of orientation.
Briefly, the process of producing flexible, binderless graphite sheet material comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is at least 80 times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles which generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 5 pounds per cubic foot to about 125 pounds per cubic foot. The flexible graphite sheet material exhibits an appreciable degree of anisotropy, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the sheet surface comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the surfaces comprises the xe2x80x9caxe2x80x9d directions.
The present invention comprises a hollow, self-supporting, cylindrically shaped, vertically extending shell for surrounding a high temperature radiant heat source, e.g. reactors or furnaces in which highly chemically active gases in a reducing atmosphere, are utilized. The hollow shell is used as a heat shield to reflect radiant heat energy back to the reactor and to minimize loss of thermal energy due to conduction. The aforementioned shell is in the form of a spiral wound sheet of anisotropic flexible graphite, formed by roll-pressing and compressing expanded articles of graphite, bonded to an in situ cured phenolic resin coated on both sides of a thin sheet of heat decomposable carbon based material which is co-extensive with the spiral wound sheet of flexible graphite. The thin sheet of heat decomposable carbon based material provides a path for the escape of gases which develop in the course of in situ curing of the phenolic resin; this path, resulting from the aforesaid decomposition, is provided between the sheet of spiral wound flexible graphite and further enables contact between resin applied on both, i.e. the opposite, sides of the sheet of heat decomposable carbon-based material in the course of in situ curing of the phenolic resin. This results in a strong continuous bonding layer of phenolic resin between, and co-extensive with, the spiral wound sheet of flexible graphite. In a further embodiment of the present invention, a second shell essentially identical to the first shell, except for being larger in cross-section, is provided. The second larger shell is positioned to surround the first shell to define an annular chamber therebetween which is filled with individual, uncompressed particles of expanded graphite which essentially fill the annular chamber. The annular chamber can be closed, top and bottom, by sheets of laminated anisotropic flexible graphite resin-bonded to the first and second shells. These will create chambers encapsulating the insulation material. This will have the affect of isolating thermals in specific areas and inhibit transmission over the entire shield.
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