Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Fluidized bed
Reexamination Certificate
1999-03-25
2002-11-19
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Fluidized bed
C422S139000, C422S198000, C422S200000, C423S498000
Reexamination Certificate
active
06482366
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to calcination and, more particularly, to indirectly heated calcination. Calcination is the process of subjecting a material to prolonged heating at fairly high temperatures.
BACKGROUND OF THE INVENTION
In directly heated calcination, the material to be calcined is exposed to the source of heat, for example, the calcination of Al(OH)
3
to Al
2
O
3
in which Al(OH)
3
is directly heated by combustion of oil, gas or coal. In indirectly heated calcination, the material to be calcined is isolated from the source of heat. Typically the material to be calcined is contained within a cylindrical retort which is rotated within a stationary refractory lined cylindrical furnace with combustion of fuel occurring within the annular ring between the retort and the furnace. Such calciners have been used for activating wood charcoal, reducing mineral high oxides to low oxides, drying fluoride precipitates in a hydrogen fluoride atmosphere, calcination of silica gel, drying and removal of sulphur from cobalt, copper and nickel, reduction of metal oxides, oxidising of organic impurities, and reclamation of foundry sand.
The present invention is not concerned with directly heated calcination and, in contrast to known indirectly heated calcination processes, the present invention is concerned with indirectly heated calcination in which the material to be calcined is contained within a calcination vessel and is heated by heat transferred from a liquid metal flowing through a heat exchanger within the calcination vessel. The present invention is applicable to both processes in which a fluidised bed of the material to be calcined is formed within the calcination vessel and to processes in which fluidisation is not utilised, for example, indirectly heated rotating drum calcination. The present invention is also applicable to processes in which the pressure within the calcination vessel is atmospheric, greater than atmospheric, or less than atmospheric.
SUMMARY OF THE INVENTION
In a first aspect, the present invention provides an apparatus for calcining a material, the apparatus comprising a calcination vessel which houses a heat exchanger arranged to transfer heat to the material from a liquid metal heat exchange fluid arranged to flow through the heat exchanger.
The apparatus according to the first aspect of the present invention may comprise a single calcination vessel. Alternatively, the apparatus may comprise a series of calcination vessels, each of the series of calcination vessels being arranged to partially calcine the material. Typically, the series of calcination vessels will comprise two or three calcination vessels.
In a second aspect, the present invention provides a process for calcining a material in an apparatus according to the first aspect of the present invention, the process comprising transferring heat to the material from a liquid metal heat exchange fluid flowing through the heat exchanger within the calcination vessel.
Where the apparatus according to the first aspect of the present invention comprises a single calcination vessel, the process according to the second aspect of the present invention is a single-stage calcination process and where the apparatus comprises a series of calcination vessels, the process is a multi-stage calcination process.
In a third aspect, the present invention provides material calcined by a process according to the second aspect of the present invention.
Liquid metals suitable for use in the present invention are characterised by having relatively low melting points, relatively high boiling points, relatively high heat transfer coefficients, relatively high specific heats and relatively low viscosities. Such liquid metals include sodium, potassium, magnesium, lead, tin, mercury and alloys thereof. A sodium-potassium alloy comprising 22% by weight sodium and 78% by weight potassium is an example of a suitable liquid metal alloy.
The heat exchanger(s) housed within the calcination vessel(s) may form part of a closed loop with the liquid metal heated externally of the calcination vessel(s) by heating means.
Liquid metals such as sodium and potassium are very reactive and hence for safety reasons the liquid metal is isolated from the atmosphere and other sources of reactant. The liquid metal may therefore be caused to flow through the heat exchange loop by use of one or more mechanical or electromagnetic pumps.
Liquid metals are electrical conductors and hence can be forced to flow under the influence of a magnetic field when a current is passed through the liquid metal normal to the direction of the magnetic field. Force is exerted on the liquid metal in a direction normal to both the magnetic field and the current flow. For example, a portion of the heat exchange loop may be passed horizontally between the poles of an electromagnet (arranged to impart a vertically orientated magnetic field) with an externally sourced current passed horizontally across the liquid metal in the magnetic field in a direction normal to the desired direction of flow of the liquid metal. Electromagnetic pumps are advantageous because they do not have any moving parts.
Preferably, the liquid metal is caused to flow through the heat exchange loop by being passed through one or more centrifugal pumps. Centrifugal pumps are preferred to electromagnetic pumps because centrifugal pumps are more efficient and are capable of pumping greater volumes. However, high operating temperatures necessitate careful design of centrifugal pumps used for pumping liquid metal. Factors to be considered in the design of a centrifugal pump for pumping liquid metal include the dissipation of heat from the pump, the expansion of components of the pump, the critical speed of rotation of the shaft, operation of the bearing in liquid metal, and sealing of the shaft to prevent leakage of liquid metal.
In either case, it is preferred that the pump or pumps are located at the coolest point in the heat exchange loop, for example, between the exit of the heat exchanger of a single-stage calcination process and the point where the liquid metal is heated.
As an alternative to a pump, a thermosiphon may be used to induce flow of a liquid metal through the heat exchange loop. Thermosiphon circulation can be induced provided that there is a sufficient difference in density between the hot and cool portions of the liquid metal.
The heat exchanger may take a variety of forms. The heat exchanger may simply be a pipe passing through the calcination vessel. However, to increase the transfer of heat to the material within the calcination vessel, it is preferred that the heat exchanger is arranged to maximise the surface area for heat transfer. The heat exchanger may take the form of a pipe or pipes having a serpentine passage through the calcination vessel. Alternatively, the heat exchanger may take the form of a series of pipes which are connected by manifolds or pigtails.
As an alternative to a heat exchange loop through which the liquid metal is pumped, the liquid metal may be contained within one or more heat pipes. Each pipe is part of a closed, normally evacuated, system which protrudes within the calcination vessel as the heat exchanger. Heat is supplied to a portion of the system external to the calcination vessel. For example, a heat pipe may take the form of a connecting pipe passing through the bottom of the calcination vessel which joins a base portion to a heat exchange portion. Heat may be applied to the base portion from an external source, for example, by combustion of gas or the like, resulting in heating of the contained liquid metal so as to generate a vapour and passage of the metal vapour through the connecting pipe to the heat exchange portion where the vapour condenses on the walls of the heat exchange portion with heat transferred to the material within the calcination vessel. On cooling, the liquid metal in the heat exchange portion returns through the connecting pipe to the base portion where it is again heated to vapour. A conv
Frost Malcolm Timothy
Koenig Raymond Louis
Mirovics Ivars
Tait Peter James
Commonwealth Scientific and Industrial Research Organisation
Rodman & Rodman
Tran Hien
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