Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Patent
1996-10-01
1998-02-17
Cain, Edward J.
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
422169, 422170, 423240S, B01D 5000
Patent
active
057188734
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and an apparatus for gas-solid contacting. The present invention is especially suitable for the removal of hydrogen fluoride from process gas streams by adsorption on alumina and, for convenience, the invention will be described with reference to the removal of hydrogen fluoride from gas streams. However, it is to be understood that the present invention is generally applicable to gas-solid contacting and that the invention should not be considered to be limited to hydrogen fluoride removal.
In the production of aluminium by the electrolytic reduction of alumina dissolved in molten cryolite, off-gases collected from the reduction cells contain appreciable quantities of gaseous fluorides. These fluorides, generally in the form of hydrogen fluoride, are required to be removed from the off-gases prior to venting of the off-gases in order to meet environmental regulations. Recovery of the fluorides in the off-gases is also desirable from an economic viewpoint, as such fluorides may be recycled to the reduction cells.
Gaseous fluorides are often removed from the off-gases of aluminium smelters by a dry scrubbing technique. This technique involves contacting the raw potline off-gases with alumina particles, which results in the fluorides being adsorbed onto the alumina particles. The alumina particles are removed from the off-gas stream in a solids collector, which also collects any entrained particles from the reduction cell present in the off-gas. The alumina and any entrained particles are collected from the baghouse and become feedstock for the reduction cells.
Conventional dry scrubbing systems employed at aluminium smelters may require all of the smelter's alumina feedstock to be used in controlling fluoride emissions. From the metal production point of view, this has at least two implications: impurity loading, the ability of the smelter to produce high purity metal is reduced; and bath chemistry is reduced.
Therefore, it may be desirable to reduce the amount of alumina required for fluoride removal from off-gases whilst maintaining fluoride levels in the vented off-gases at an acceptable level.
The present applicant, in International Patent Application No. PCT/AU91/00342, has proposed a dry scrubbing process in which particulate material is contacted with a high velocity off-gas stream to reduce the resistance to mass transfer attributable to the diffusion boundary layer around the solid particles to provide higher mass transfer rates. It has also been found that the fine particles sized less than 45 .mu.m contain a disproportionate amount of contaminant trace metals from the gas stream and the fine fraction is removed in order to reduce the amount of such trace metals recycled to the reduction cells.
The process of International Patent Application No. PCT/AU91/00342 is carried out in a toroidal bed reactor, such as is described in Australian Patent No. 554,537, in the name of Torftech Limited, the entire contents of which are herein incorporated by reference. This patent describes a method and an apparatus for processing a mass of particulate material. The apparatus includes an annular treatment zone having a number of angled vanes located below the treatment zone. Gas is passed through the angled vanes, which cause the gas to enter the treatment zone with a swirling flow pattern. Solid material is fed to the treatment zone and the swirling gas flow therein produces a turbulent band of solid particulate material in the treatment zone. The apparatus also includes a gas outlet and means to remove solid particulate material from the treatment zone.
In Australian Patent No. 616,406, also in the name of Torftech Limited, a toroidal bed reactor for treating particulate and/or liquid material is described. This reactor includes two or more annular treatment zones. In the case where only two annular treatment zones are used, the first treatment zone is located lowermost in the apparatus. The second annular treatment zone is concentric with the first treatment zone, but
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The Torbed Process--A Small Revolution in Heat and Mass Transfer.
Groszek Martin A.
Liddy Matthew Ion
Wellwood Grant A.
Cain Edward J.
Comalco Aluminium Limited
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