Field of the Invention

This invention relates to a heating system used in processing of dense slurries and similar types of viscous fluids. In particular, this invention relates to a slurry heating system suitable for heating dense slurries handled in the Bayer digestion process, in which aluminium hydroxide is extracted from bauxite.

Background Art

In the Bayer process, bauxite is usually wet milled with a caustic soda solution, usually referred to as spent liquor. The resulting bauxite slurry (referred to as thick bauxite slurry) must be heated prior to further dilution with hot spent liquor, after which it is conveyed to the digestion plants.

Various methods of heating thick bauxite slurry have at times been utilised, but all suffer from certain disadvantages:

There are two basic modes of heating that can be employed. They are termed indirect heating and direct heating. In indirect heating the increase in the slurry temperature is achieved by heat transfer equipment. In direct heating steam or vapour is injected directly into the slurry. The latter form of heating is often employed because the equipment required is extremely simple and reliable and avoids a number of serious problems associated with the more complex conventional heat transfer equipment utilised in previous attempts at indirect heating.

The major disadvantage of direct heating is that it adds the condensate created by the injected vapour to the liquor stream and thereby dilutes the process liquor stream. The process liquor stream is a recirculating flow that must be maintained at a certain concentration. Any water added to the process will steadily dilute the process liquor stream and is therefore removed by forced evaporation in other parts of the Bayer process, before it is returned to the digestion plant. Forced evaporation is an expensive process and there are significant advantages in elimination of direct injection in favour of suitable and reliable heat exchange equipment. There are several problems associated with the heating of thick bauxite slurry. The slurry is essentially viscous. It has a shear stress at zero flow and exhibits marked non-Newtonian behaviour. Its solids content has a tendency to settle and has the potential to obstruct flow. Its semi-viscous behaviour causes the flow to be near the laminar regime at the velocities employed in conventional heat transfer equipment and this causes such equipment to attain only moderate heat transfer coefficients. Heat transfer equipment with proportionately larger heating areas is therefore required, which adds considerably to the cost of such equipment. The slurry's tendency to block tubes makes the use of conventional heat exchange equipment problematical, both in regard to its operating reliability and to the difficulty it presents during the frequent cleaning and descaling it requires.

Different types of heat transfer equipment have at various times been employed by the industry:

Spiral heat exchangers of stainless steel construction have been utilised to some extent. These have relatively small unit heat transfer areas combined with high capital and maintenance cost. In large commercial applications their small heating area require a number of such heaters to be assembled to make-up a suitable process unit. These assemblies require appropriate distribution manifolds at both the inlet and outlet of each heater. This distribution pipe work is complex and extensive, because, despite their relatively small heat transfer area, spiral heaters are relatively bulky pieces of equipment and also require considerable space for maintenance access. These factors have proven spiral heat exchangers to be unattractive and uneconomical for thick bauxite slurry heating.

Shell and tube heat exchangers of conventional design have been tried, but in regard to thick slurry heating they experience a number of significant problems. In shell and tube heat exchangers of conventional design, the flow path offered by conventional 25 or 38 mm diameter heat exchanger tubes is insufficient to guarantee blockage free operation. Viscous fluids in general are difficult to heat using shell and tube heaters. Shell and tube heaters normally operate in the turbulent flow regime where heat transfer coefficients are considerably greater than those attainable in the laminar flow regime. High viscosities and high shear values position the flow in the laminar regime (low Reynolds number). In the laminar regime there is a reduction or absence of turbulence, which results in poor mixing between the outer layers and the core of the flow. This causes a high film temperature, which is disproportionate to the mean fluid temperature and which effectively reduces the temperature difference (LMTD) between the heating medium and the fluid being heated at the tube wall. Overall heat transfer is therefore low, while at the same time there is a danger that at the tube wall itself the fluid may over-heat. This may cause the fluid at the wall to reach its boiling point, where, by evaporation of its liquid content, it may cause scaling of the tube wall, resulting in a further loss of heat transfer capability. Viscous flow therefore requires considerably larger heater areas than for the same flow in the turbulent regime.

For thick bauxite slurry, which exhibits highly non-Newtonian behaviour, the problem is compounded. High-density bauxite slurry is also termed a 'power-law' fluid. This means that the velocity distribution across the tube diameter does not adhere to the same exponential functions that rule the velocity distribution in pure fluids. This causes slurry flow to be less turbulent than the pure fluids in laminar flow, resulting in even less exchange of heat between the fluid temperature attained at the tube wall and the core of the flow area.

Bauxite slurry is also a 'shear-thinning' fluid. This means that in fluid elements subject to high shear rates, such as along the tube wall, the 'apparent viscosity' may reduce significantly. At extreme solid densities this shear thinning nature of thick bauxite slurry may cause plug flow. In plug flow, the velocity profile across the tube is essentially flat, with all fluid elements being transported at the same velocity. The only relative movement then occurs at a high shear lubricating laminar sub-layer along the tube wall. At this extreme flow regime there is effectively no mixing and indirect slurry heating is for all practical purposes impossible. - A -

lndirect heating of thick slurry therefore requires slurry densities that avoid plug flow, high shear rates and extreme laminar flow conditions. The Reynolds number determines the flow regime; the lower the Reynolds number the nearer the flow is to the laminar flow regime. Low Reynolds numbers must therefore be avoided. The Reynolds number is proportional to tube diameter, so that smaller heater tubes show a greater tendency toward laminar flow. In conventional shell and tube heaters, which have relatively small tubes, this causes heat transfer coefficients to be lower than would otherwise be the case.

The tendency of the solids contained in thick slurry to settle-out and plug, will increase as tube diameters are reduced and the number of tubes over which the slurry flow is distributed is increased. Conventional shell and tube heat exchangers have a large number of small tubes, with multiple passes within a single large shell. Within the shell these passes are connected by large channel sections. Large channel sections have a tendency for solids to settle-out and collect at the bottom, which further increases the likelihood of blockages in the tubes fed from the bottom of these channels. All indirect heating equipment in thick slurry service will require frequent cleaning and descaling. In conventional shell and tube heat exchangers this cleaning and descaling will require the removal of large bolted channel sections and the possible replacement of tubes in a major labour intensive maintenance operation.

For fluids containing solids there is a limit to the range of velocities that can be permitted through the tubes. The lower limit is determined by preventing Reynolds numbers that are too low. The upper limit is determined by preventing excessive tube wear. The preferred velocity is about 2 m/s, which, in the range of sizes likely to be utilised in tubular heating equipment, is independent of tube size. There is therefore no degree of freedom to significantly vary velocities for the particular tube size that has been employed. Given the velocity, smaller tubes will require a higher driving force in order to maintain flow. This higher pressure must be added to the back-pressure that must be maintained on the heater to prevent boiling of the slurry at the laminar sub-layer in the tubes. This adds significantly to both capital and operating (power) costs for pumps, pumping systems, the heat exchange equipment itself as well as any downstream equipment.

There are therefore a number of important reasons that render conventional shell and tube heaters unsuitable for thick slurry heating.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, 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 anywhere else, as at the priority date of the application

It is the object of the present invention to provide an alternative heating system for indirect heating of thick slurries generally.

Throughout the following 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.

Disclosure of the Invention

In accordance with the invention there is provided a slurry heating system comprising a plurality of tube heat exchanger units, each of said units comprising a plurality of inner tubes arranged in parallel for single-pass operation, contained within an outer tube, each said unit having an internal wall located proximal to each opposed end of said inner tubes to fluidly isolate a heating volume from a slurry containing volume; said heating volume being located between said walls, said outer tube and said inner tube, and said slurry containing volume being located within said inner tubes and beyond either end of said inner tubes bound by said walls; each said unit having_a removable cover at least at one end to allow physical access to open ends of said inner tubes for cleaning purposes, said plurality of units being arranged in series with adjacent units in the series being connected by an interconnecting pipe or other interconnecting system that provides some turbulence in slurry flow between adjacent said units. This arrangement provides for the slurry to be remixed after passing through each unit. The remixing provides an element of turbulent flow, which compensates for uneven heating brought about due to laminar flow within the inner tubes in each unit. This effect will also be observed where remixing occurs in multi-pass units.

Preferably said interconnecting pipe intersects said outer tube between at least one said wall and adjacent said removable cover.

Preferably the volume between at least one said wall and adjacent said removable cover is minimised, to avoid low flow rate areas and attendant settling out of slurry.

The heat source can be any fluid medium, but most preferably is either steam or hot condensate.

Preferably in each of said units, said plurality of inner tubes are arranged for single-pass operation for parallel flow of slurry.

Preferably the cross-sectional area of the interconnecting pipe is less than the combined cross-sectional area of the inner tubes in each unit.

Preferably the cross-sectional area of the interconnecting pipe is less than 0.8 times the combined cross-sectional area of the inner tubes in each unit.

Preferably the cross-sectional area of the interconnecting pipe is from about 0.5 to 0.75 times the combined cross-sectional area of the inner tubes in each unit.

Preferably the interconnecting pipe has a nominal bore diameter of from 200mm to 300mm.

Preferably the outer tube is circular in cross-section, and has a nominal bore diameter of from 400mm to 750mm.

Preferably said plurality of inner tubes comprises from 10 to 60 individual inner tubes.

Preferably said plurality of inner tubes comprises from 30 to 60 individual inner tubes. Preferably each end of each of said units is provided with a said removable end cover.

Preferably said removable end cover comprises a quick-opening blind.

Preferably said slurry heating system has from three to twelve said units arranged in series.

Preferably said slurry heating system has from three to ten said units arranged in series.

Preferably said slurry heating system has from three to seven said units arranged in series.

Preferably said slurry heating system has from four to six said units arranged in series.

Preferably said slurry heating system has five said units arranged in series.

Preferably a plurality of said slurry heating systems can be arranged in parallel (so as to form an array). The number of slurry heating systems arranged in parallel depends on the total plant production requirement.

Preferably said slurry heating system has a restrictor or pressure controller to maintain a higher pressure of said slurry within said slurry heating system. This arrangement will prevent boiling of said slurry, to prevent scale formation within said inner tubes.

Preferably said restrictor comprises a valve.

The most preferred embodiment of the invention would have the following attributes:

• Heating systems must be single pass.

• They must operate at the highest practicable Reynolds number.

• They must mix the flow at frequent intervals to counter the negative effect of differential rates of heating within the inner tubes. • The heat flux must be relatively high in order to induce some turbulence through convection, in order to supplement the limited turbulence associated with the unavoidable near laminar flow.

• Inner tubes should be as few and as large as possible, compatible with the requirements of maximum permissible inner tube velocities and the required heat transfer area.

• The size of channel sections must be minimised.

• Inner tube erosion must be minimised to prevent inner tubes having to be replaced.

• Heaters must be simple to operate and easy to dismantle and maintain.

Brief Description of the Drawings

A preferred embodiment of the invention will now be described in the following description made with reference to the drawings in which: Figure 1 is a plan view of a single slurry heating system showing a series of tube heat exchanger units according to the embodiment; Figure 2 is an end elevation of the series shown in figure 1 ; Figure 3 is a side elevation of the series shown in figures 1 and 2; and Figure 4 is a cross section through one of the tube heat exchanger units.

Best Mode(s) for Carrying Out the Invention

The embodiment of the invention provides a thick bauxite slurry heating system for use in heating slurry for processing in the Bayer process.

Each heating system as illustrated in the drawing figures is designed for a throughput of at least 200 t/h, typically 500 to 700 t/h, and up to 1000 t/h of slurry containing approximately 40% to 50% solids by weight. These are the approximate quantities of thick slurry required for digestion plants with capacity equivalent to 350 000 tpa (typically 700 000 tpa to 1 000 000 tpa) and 1 400 000 tpa of alumina respectively when extracted from bauxite containing approximately 50% available alumina. Each heating system consists of tube heat exchanger units 11 , 12, 13, 14, and 15, combined into a series by interconnecting pipes in the form of flanged pipe connections 21 , 22, 23, 24 arranged either horizontally or at an inclination equal to the hydraulic gradient for the same slurry flow under gravity. Each tube heat exchanger unit 11 , 12, 13, 14, and 15 is supported on a common structural support 25.

Each tube heat exchanger unit 11 , 12, 13, 14, and 15 is single pass, comprising 34 inner tubes 30 of 50mm nominal bore (ID), enclosed in a 600mm nominal bore (ID) outer tube 32. The inner tubes 30 are held in place by and sealed into a wall in the form of a 38mm thick steel sheet 35 located near each end of each outer tube 32, and at opposed ends of the inner tubes 30. The inner tubes 30 are located in a triangular configuration relative to each other, and are spaced apart by a minimum of about 10mm. These sheets 35 effectively create slurry chambers 41 to 50 (inclusive) that distribute the slurry into and receive slurry from the individual inner tubes 30.

The volume outside of the inner tubes 30 and inside the outer tube 32, bound by the walls formed by the steel sheets 35 defines a heating volume, which will be discussed later.

The inner tubes 30 are approximately 6 m long. The interconnecting pipes forming the flanged pipe connections 21 , 22, 23, 24 have a nominal diameter of 250mm. The area formed by the diameter of the pipes forming the flanged pipe connections 21 , 22, 23, 24 is about 0.75 times the sum of the area formed by the diameter of the inner tubes 30. This change in area gives rise to a relative difference in velocity of the slurry in the interconnecting pipes forming the flanged pipe connections 21, 22, 23, 24 compared with the velocity in the inner tubes 30. This assists in creating conditions of turbulent flow, and significantly increases heat transfer efficiency in the next pass.

Each outer tube 32 is furnished with a removable end cover in the form of a quick opening blind 60 at each end. This provides access into the slurry chambers 41 to 50 (inclusive) and into the insides of the inner tubes 30, allowing any build-up inside the inner tubes to be cleaned, to maintain the heat transfer efficiency of the unit.

The five tube heat exchanger units 11 , 12, 13, 14, and 15 can be made from standard piping materials. The outer tubes 32 and inner tubes 30 are manufactured from standard pipe, which has the advantage of being a standard off-the-shelf item, reducing the cost of fabrication of the slurry heater.

The series of tube heat exchanger units 11 , 12, 13, 14, and 15 has a slurry inlet 70 where slurry enters, and a slurry outlet 80 where heated slurry exits the series.

In use, an array of a plurality of heating systems in parallel, each consisting of the series of tube heat exchanger units 11 , 12, 13, 14, and 15 can be provided. The number of heating systems required depends on total plant production. For a large plant, particularly envisaged is an array comprising five parallel streams fed by five feed pumps, four of which (streams and pumps) are in use while one stream and pump is on standby and available if any stream comes off-line for maintenance etc.

In order to prevent boiling within the inner tubes 30, which would cause scale formation and corrosion, backpressure control for each stream is provided in the form of a valve 90, operation of which is controlled by a pressure sensor (not shown). The valve is controlled to ensure that the pressure within the heater pipes is above the equilibrium vapour pressure of the maximum temperature that can be reached on the inner surface of the heater pipes. The theoretical limit of this minimum required back-pressure is the equilibrium vapour pressure at the temperature of the heating medium. The design heat transfer coefficient, based on maximum scale conditions, is 300 WVm2 0C to 500 W/m2 0C, or 260 kcal/m2h°C to 430 kcal/m2h°C. The pressure in the slurry is controlled dependent on local temperatures generated in the slurry at the inner wall of the inner tube, to a level equal to or just greater than the vapour pressure of the heating medium. This precaution is taken because the limited turbulence in the inner tubes will cause the temperature of the slurry at the inner tube walls to be noticeably higher than for fully turbulent flow. The outer tubes 32 each have a 150mm nominal bore heating fluid injection inlet 92 and a 50mm nominal bore condensate outlet 94 connecting respectively to a 250mm nominal bore heating fluid header 100 and 50mm nominal bore condensate drains 110. The condensate from the drains 110 is collected in a receiver vessel and returned to other plant operations. The heating fluid injection inlet 92 can receive live steam or hot condensate, depending on the required temperature. The heating fluid injection inlet 92 and a condensate outlet 94 communicate with the heating volume as defined above, to heat slurry within the inner tubes 30.

The intent of the arrangement of the five tube heat exchanger units 11 , 12, 13, 14, and 15 is to eliminate complex channel sections, while still allowing the fluid to be mixed at relatively small length to diameter ratio (L/D) intervals. This mixing is desirable owing to the near laminar flow conditions through the inner tubes 30, which may result in an uneven temperature distribution. The use of relatively large diameter inner tubes 30 in the tube heat exchanger units 11 , 12, 13, 14, and 15 minimises the pressure drop along the series of units, and also avoids inner tube blockage.

The five tube heat exchanger units 11 , 12, 13, 14, and 15, with their small size and quick opening blinds 60 allow easy heater cleaning, without the need for removing heavy covers or channel sections. Cleaning can be accomplished by water hose at normal plant water pressure, although high-pressure water jet cleaning is not precluded.

Operation of the series of tube heat exchanger units 11 , 12, 13, 14, and 15 will now be described. Slurry enters at slurry inlet 70, and flows into the inlet slurry chamber 41 of the first heat exchanger unit 11. Within the first heat exchanger unit 11 , the slurry passes through the inner tubes 30 to outlet slurry chamber 42 of element 11. Slurry then flows across flanged pipe connection 21 to the inlet chamber 43 of the second heat exchanger unit 12. Slurry then flows through heat exchanger units 12 to 15 in the same manner as for heat exchanger unit 11 and eventually exits the heater via slurry outlet 80. Typically the slurry flow takes place at 2m/s and is arranged to occur at Reynolds numbers of about 6000 to 10000. For the rheology of typical thick bauxite slurry, this Reynolds number locates the flow in the semi-turbulent regime and indicates an acceptable and desirable degree of flow mixing.

It has been shown in practice that the slurry needs to be heated with a reasonably large difference between the temperature of the slurry to be heated and the temperature of the heating fluid (approach temperature). The resulting temperature difference between the inner and outer surfaces of the inner tubes 30 wall, when transferred to the semi-laminar flow of the outer layers of slurry adjacent to the inner surface of the inner tubes 30, will cause a measurable degree of turbulence due to convection. This convection, together with the limited turbulence due to flow, will enhance the mixing of the hot and cold regions of slurry across the inner tubes 30 section.

The lack of complete turbulence may cause the temperature in the outer layers of the slurry flow to be considerably higher than the mean temperature. Together with the low vapour pressure of the product being heated, the outer layers of slurry can therefore easily reach boiling point. This would lead to evaporation of the liquor content and to baking of the solids onto the inner surface of the inner tubes 30. This would cause rapid heater pipe-side fouling and would prevent effective heat transfer. Use of the backpressure control to raise the pressure of the slurry assists in preventing this problem from occurring.

Within low-pressure bauxite digestion plants, the available flash vessel vapour temperatures are normally insufficient to provide the approach temperature necessary for thick slurry heating. Heating is therefore achieved by means of live steam via inlet heating fluid header 100. Condensate formed by condensation of the steam exits the heater by condensate outlets 110.

Within high pressure, high temperature digestion plants, available flash vessel vapour temperatures and hot heater condensate temperatures are both sufficient to provide the approach temperatures necessary for indirect thick slurry heating. In this situation, heating can be achieved by means of flash vapour or hot condensate, via inlet heating fluid header 100. Condensate formed by condensation of the vapour exits the heater by condensate outlets 110. If "hot" condensate is used for heating, "cooled" condensate again exits the heater by condensate outlets 110.

The effects of the low overall heat transfer coefficients on required heat transfer area are mitigated to an extent by the substantial temperature driving force which exists between the high temperature (178 0 C typical) 950 kPa live steam from the powerhouse and the comparatively cool ( 73 0 C typical) ground bauxite slurry, in the low temperature digestion application. This high temperature difference causes convection currents that enhance turbulence and fluid mixing. Similar temperature driving forces can be achieved with flash vapour and hot condensate in high temperature digestion applications.

To simplify the arrangement of the series of tube heat exchanger units 11 , 12, 13, 14, and 15 within an individual heating system, and to minimise the pressure drop, preferably there are no intermediate isolation valves between adjacent units in a heating system.

The invention has a number of advantages over hitherto known methods for heating bauxite slurry. The invention achieves process benefits of indirect heating, avoiding unwanted dilution that causes lower caustic concentrations in slurry leaving digestion and hence consequent constraints on digester productivity. The introduction of turbulent zones avoids need for large heat exchange areas, usually resulting from inherently low slurry film heat transfer coefficients in high bauxite slurry viscosities with inherent low Reynolds numbers. Thus a benefit of smaller heat transfer apparatus is reduced capital cost and in addition, lower operating cost due to reduced de-scaling requirements. The invention achieves high heat fluxes without risk of slurry boiling in proximity to the inner tube walls (and resulting scale generation).

In the particular embodiment described herein, there is also a much reduced risk of blockage of heater inner tubes, compared with 25 mm and 38 mm tubes utilised in known shell and tube heaters.

It should be appreciated that changes may be made to various features in the above described embodiment without departing from the spirit and scope of the invention, and that the invention is not limited to the specific embodiment described herein.