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1. WO2006092005 - HEAT EXCHANGE APPARATUS

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

"Heat Exchange Apparatus"

Field of the Invention

The present invention relates to a heat exchange apparatus. More particularly, the heat exchange apparatus of the present invention has application in the transfer of heat to or from slurries.

Background Art

Heat exchange is a commonly employed process step in a wide variety of engineering and chemical engineering applications. To achieve heat exchange between two fluids one or more heat exchange apparatus are utilised. There are a wide variety of such heat exchange apparatus presently available to the engineer depending upon the specific conditions of the application in which such is to be employed.

In the context of the refining of alumina using the Bayer process there are a number of instances in which heat exchange apparatus are typically employed. Such applications include the use of live steam heaters in the evaporation and digestion stages wherein heat from the digestion and evaporation flash stages is recovered and used to preheat liquor for the digestion and evaporation stages.

The form of heat exchanger typically used for the above described indirect heating applications are shell and tube heat exchangers in which Bayer process liquor passing through an array of tubes arranged within the shell are heated by steam piped into the shell and which subsequently condenses. This form of heat exchange is typically applied to the liquor within the Bayer process, whether it be pregnant or spent liquor.

Several of the process steams within the Bayer process are best described as a slurry, being a solid-liquid mixture. Examples of such slurries are a bauxite slurry produced by the grinding of bauxite with spent liquor. A further example is the slurry leaving the digestion stage, being a slurry of sand and mud in liquor, the liquor containing the dissolved alumina. There are various stages within the Bayer process in which it would be advantageous to heat slurries. However, it is the nature of a slurry that it contains a high proportion of solids which, when combined with the relatively slow rate of passage of fluid streams through heat exchange apparatus, may lead to blockages.

The heat exchange apparatus of the present invention has as one object thereof to overcome substantially the abovementioned problems associated with the prior art.

The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia or any other country or region as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. The terms "slurry" or "slurries" is to be understood to refer to a solid-liquid mixture sufficiently fluid to flow viscously.

Disclosure of the Invention

In accordance with the present invention there is provided a heat exchange apparatus comprising an arrangement of relatively small diameter tubes extending substantially longitudinally through a shell, the shell having provided an inlet and an outlet for the ingress and egress of a material from which heat is to be transferred, and the tubes being provided with both an inlet and an outlet for a material to which heat is to be transferred, wherein there are at least two such shell and tube arrangements provided and between which the material to be heated is to be transferred, and a transfer line connecting the at least two shell and tube arrangements, characterised in that the transfer line is provided in a form that discourages settling of solids entrained in the material to be heated.

Preferably, the transfer line is provided in a form of a gentle arc that facilities the transfer of material therethrough at a velocity higher than the settling velocity of that material.

In one form of the present invention the transfer line is provided in the form of two 90° elbow fittings provided on neighbouring shell and tube arrangements, between which is provided a spacing member so as to minimise interference of the flow of the material between the shell and tube arrangements.

Preferably, the material to be heated contains solids at the level of about 200 g/l.

Still preferably, the material to be heated has a settling velocity of about 1.5 m/s.

In accordance with the present invention there is further provided a process for the heating of a slurry, the process comprising the method steps of passing the slurry through a series of at least two heat exchange apparatus, the transfer from one apparatus to another being conducted at a velocity above the settling velocity of the slurry.

In one form of the present invention the settling velocity of the slurry is about 1.5 m/s. Further, the solids content of the slurry is about 200 g/l.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which:

Figure 1 is an elevational side view of a heat exchange apparatus in accordance with the present invention;

Figure 2 is a cross-sectional detail view of a portion of the inlet and outlet ends of heat exchange apparatus of Figure 1 ;

Figure 3 is a cross-sectional plan view through the shell and tube arrangement of the heat exchange apparatus of Figure 1 ; and

Figure 4 is a plan view of an array of the heat exchange apparatus of

Figure 1 such as may be employed in use.

Best Mode(s) for Carrying Out the Invention

In Figure 1 there is shown a heat exchange apparatus 10 in accordance with the present invention, as may be employed to transfer heat from steam or water vapour to a slurry. Such heat exchange apparatus may be employed in the Bayer process.

The heat exchange apparatus 10 comprises an elongate shell 12 defining therein a bore 14, best seen in Figures 2 and 3. The shell 12 is provided with an inlet 16 and an outlet 18.

Provided at either end of the shell 12 are an inlet manifold 20 and an outlet manifold 22. The inlet manifold 20 is fed by way of a first 90° elbow fitting 24 and a second 90° elbow fitting 26. Between the elbow fittings 24 and 26 there is provided a spacer fitting 28, the flow path described by the two elbow fittings 24 and 26, and the spacer 28, being generally uninterrupted and without sharp bends that might allow the velocity of a slurry passing therethrough to approach the settling velocity of that slurry. The elbow fitting 26 connects to an outlet manifold 22 provided on an adjacent heat exchange apparatus 10, as may be seen in Figures 1 and 4. The combination of the elbow fittings 24 and 26 with the spacer fitting 28 is referred to as a transfer line.

With reference to Figures 2 and 3, an arrangement of tubes 30 are provided extending through the shell 12 and communicating with the inlet manifold 20 and outlet manifold 22 at either end thereof. The combination of the shell 12 and the multiple tubes 30 extending therethrough will herein be referred to as a shell and tube arrangement.

In Figure 4 there is shown an array of heat exchange apparatus 10 such as may be employed in a heat exchange process between steam or vapour and a slurry such as that employed in the Bayer process.

In use, a Bayer process slurry having a solids content of about 200 g/l is introduced to an array of heat exchange apparatus 10, as shown in Figure 4, in known manner. The slurry is passed through the shell and tube arrangement b.y way of introduction through the inlet manifold 20 and is subsequently passed through the tubes 30, ultimately to the outlet manifold 22. During this passage heat is transferred from steam or vapour introduced at the shell inlet 16, that steam or vapour condensing as it passes through the bore 14, the condensed steam or vapour exiting the shell 12 at the outlet 18. The heated slurry subsequently exits the heat exchange apparatus 10 by way of the outlet manifold 22 and 90° elbow fitting 26. The 90° elbow fitting 26 is of a radius such that it does not impede the flow velocity of the slurry. Similarly, the provision of the spacer fitting 28 and 90° elbow fitting 24 are similarly arranged for a similar outcome. Such an arrangement allows the velocity of the slurry in the transfer lines to remain above the settling velocity of that slurry, understood to be about 1.5 m/s, thereby minimising the opportunity for blockage in any one or more of the heat exchange apparatus 10 provided in the array shown in Figure 4.

In the above described manner the velocity of slurry transfer between adjacent heat exchange apparatus 10 is largely constant and allows that slurry not to settle and contributes to efficient operation of each heat exchange apparatus 10 and the array provided, for example, therefrom and shown in Figure 4.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.