Heat exchange – Recirculation
Patent
1993-11-03
1996-04-02
Ford, John K.
Heat exchange
Recirculation
1651091, 366147, 422227, 422201, F28D 102, F28D 2100, B01F 1506, B01F 716
Patent
active
055032206
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the heating and/or cooling of vessels and in particular vessels which are agitated by a draft tube.
Vessels containing slurries must be agitated to prevent settling of the solid phase. This is frequently achieved by pumping the slurry up or down a draft tube.
In addition to agitation, temperature sensitive processes require a heat source or sink to regulate the temperature by cooling or heating the process slurry. For example, in a multi-stage precipitation process which utilises a series of vessels known as precipitation tanks, it is necessary to maintain the process liquor in a state of a super-saturation so that precipitate will continuously form onto seed crystals. To maintain the process liquor in each stage at a supersaturated state, heat must be continually removed from the slurry as it moves downstream.
In small scale operations heat loss due to radiation and convection from the surface of the vessels is sufficient to propagate the precipitation process without resulting in excessive residence times. However, in large scale operations, such as those found in commercial plants using the Bayer process for the precipitation of alumina hydrate from bauxite, total reliance on radiation and convection from the surfaces of the precipitation tanks to achieve any substantial overall temperature drop results in a high residence time for the process and consequently a large number of tanks. For a typical commercial size vessel of about 4 million liters, heat loss based on radiation and convection, although dependent on atmospheric conditions, is typically 0.5.degree. C. to 1.0.degree. C. per vessel. Thus at a temperature drop from 80.degree. C. to 50.degree. C., about forty vessels are required.
Description and Related Art
While the slow cooling rate has many process advantages, the temperature drop per vessel due to radiation and convection losses is low resulting in high capital expenditure. As a compromise, it has become normal practice to rely on interstage cooling in the precipitation circuit, to decrease the number of tanks. In interstage cooling the process slurry is cooled between stages of the precipitation circuit in an external cooling system by flash cooling or in a heat exchanger.
Conventionally, commercially operating interstage cooling systems provide cooling of the process slurry by 5.degree. C. to 15.degree. C. at only two to three selected points in the precipitation circuit. However, the temperature profile achieved in precipitation processes with temperature drops of this magnitude are far from optimum, resulting in loss of yield and inferior product quality.
To maintain a high level of control over the quality and yield of precipitate forming, it is desirable to have a control over the temperature drop between each stage and the cooling rate within each stage. However, if conventional cooling were used in this manner, the capital cost of the additional pumping system and the heat exchangers would be substantial.
Disclosure of the Invention
It is an objective of the present invention to provide an apparatus for heating or cooling material in a process vessel which is agitated by a draft tube.
The invention provides an apparatus for heating or cooling a particulate suspension in a process vessel comprising
a draft tube having a draft section and an elongate cylindrical section,
a means in said draft section for circulating suspension through said draft tube at a velocity sufficient to substantially maintain the particulate suspension in the vessel, and
a heat exchange means in a substantial length of said cylindrical section of said draft tube to remove or supply heat to the suspension circulating through the tube, thereby cooling or heating the suspension in the vessel.
As discussed earlier, to prevent settling of a solid phase in a process vessel, slurry is pumped through the draft tube at high velocities typically in excess of 1 meter per second. In many cases the velocity is sufficiently high to provide turbulent conditions in the
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patent: 3517732 (1970-06-01), Brebant
patent: 3962042 (1976-06-01), Malick
patent: 4304753 (1981-12-01), Klaren
Shaw, John A. "The Design of Draft Tube Circulators", Proc. Australas. Inst. Min. Metall. No. 283, Sep. 1982 pp. 47-58.
Japan Utility Model No. Sho 60-18415 (Nippon Light Metals Co., Ltd.), Jun. 1985.
Bursle Arthur J.
Crisp Anthony J.
Wood David G.
Comalco Aluminium Limited
Ford John K.
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