AU2006201746A1 - Improved Multi-Cell Heating System - Google Patents

Description

P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION PATENT OF ADDITION Name of Applicant: Actual Inventor: Address for service is: Hatch Associates Pty Ltd Dirk Jacob De Boer WRAY ASSOCIATES Level 4, The Quadrant 1 William Street Perth, WA 6000 Attorney code: WR Invention Title: Improved Multi-Cell Heating System The following statement is a full description of this invention, including the best method of performing it known to me:- -2- "IMPROVED MULTI-CELL HEATING SYSTEM" The present invention relates to a multi-cell heating system. The heating system is particularly useful for controlling the deposition of heater scale in a manner such that the adverse effects of scale deposition are minimised. The heating system of the present invention is also particularly useful for increasing the temperature of a slurry in which the deposition of scale within heating cells is an important factor on the amount and size of equipment that needs to be installed for a given duty.

The heater system of this invention has been primarily developed for flash evaporation equipment in the alumina refining industry. However, the useful application of the invention is not limited to that industry and may be extended to all branches of industry that encounter similar or identical processing problems to those mentioned below.

In order to understand the relevance of the advantages provided by the present invention, particularly in relation to the alumina refining industry, it is helpful to explain some of the problems encountered in that industry. To this end, a brief description of the processes involved will now be provided.

Alumina, or aluminium oxide, is chemically designated as AI20 3 It is an important mineral used industrially to manufacture a wide range of products from abrasives to aluminium metal. Its occurrence in commercial quantities is mainly as bauxite ore, in which alumina is present in the form of hydrates and silicates.

Of these, the hydrates, which occur as both alumina monohydrate and alumina trihydrate, are the only compounds that are extracted and these must be separated from the remainder of the ore. A typical commercial bauxite ore ranges from about 30% to about 60% extractable alumina.

Industrially, the extraction of alumina is accomplished by the Bayer process, so called after the Austrian chemist K J Bayer who developed the process in 1888.

-3- In this process, finely ground bauxite is mixed with aqueous caustic soda solution and heated. This causes the alumina hydrates to go into solution, which allows them to be separated from the residual solids. Commercially this latter step is carried out by a combination of sedimentation and filtration.

Cooling of the filtered liquid reverses the effects of the heating process and the dissolved alumina hydrates are precipitated while the remaining liquid reverts to its initial state and can be reused to repeat the process. The temperature range of the process depends upon the quality of the bauxite. In this respect, the digestion of trihydrate ores normally requires temperatures less than 150 0

C,

whereas the digestion of monohydrate ores requires temperatures ranging to as high as 300 0

C.

In commercial plants the extraction of alumina is usually carried out in a continuous process and on a large scale. The caustic liquor stream is continuously recirculated and alternately heated and cooled in accordance with the requirements of the Bayer process. However, because of the large scale of operation the energy content of the liquor stream is very high, especially in high temperature refineries, and efficient energy management is essential to the economy and viability of the plant. A large proportion of the equipment installed in alumina refineries is therefore dedicated to heat recovery.

The main area for heat recovery is in the digestion section of the refinery where heat is transferred from the outgoing hot Bayer solution to the incoming cold bauxite slurry. This is accomplished in tubular heaters, wherein cold slurry flows through the inside of the tubes and hot flash vapour flows on the outside of the tubes. The vapour condenses on the cold(er) tubes, which causes it to release its heat of evaporation, which is then absorbed by the cold slurry stream.

This heating process is usually carried out in a number of stages.

Thermodynamic theory shows that the efficiency of this heat recovery process increases with the number of heating stages that are employed. In practice, the -4number of stages is limited by economic considerations. In particular, each additional stage requires more equipment and there is thus a point beyond which the marginal increase in efficiency does not warrant the additional investment.

It is in the tubular heaters of the digestion section of a Bayer process plant, particularly those in high temperature Bayer plants, that a number of problems arise that add considerably to the capital cost for equipment, as well as to the cost for operating and maintaining the plant.

In particular, to operate the Bayer process on a bauxite ore that contains both alumina monohydrate and alumina trihydrate, the slurry of bauxite and aqueous caustic soda must be heated to temperatures as high as 3000C in order to successfully dissolve the monohydrates. However, and as will be described in more detail below, this requires the slurry to be taken to temperatures where the deposition of scale is extreme, compared with most processes in which heat exchangers are normally employed.

Even when the slurry only has to be heated to temperatures of approximately 1500C for trihydrate digestion, the deposit of scale is significant.

In this respect, as the slurry temperature rises, the trihydrate goes readily into solution, but a portion of the trihydrate is converted to monohydrate. The monohydrate is not readily soluble until it reaches a higher temperature and thus monohydrate scale tends to precipitate and deposit on the walls of the heaters.

Similarly, other types of scales, for example silicates and titanates, are also deposited.

The major problem with the build up of scale on any heater unit is that it seriously effects the heat transfer coefficient. The scale deposits also increase resistance to fluid flow and thus add considerably to the hydraulic gradient necessary to maintain the required flow through the apparatus. In order to provide at least a minimum acceptable time span between heater down time for defouling (the cleaning cycle), generous safety factors are usually provided during the initial design. Consequently, the heaters are provided with surface areas several times higher than comparable heaters would be in industries where the deposition of scale is not a significant factor. This adds considerably to the total length of heater tubing. Pumping heads are therefore high which in turn requires the equipment and its connecting pipe work to withstand considerably higher pressures than would otherwise be the case.

Clearly, these design allowances have a cumulative effect that has a large impact on the capital and operating costs of the plant equipment.

Furthermore, despite the large safety factors built into the design and the expense of the equipment, its service life remains limited. It still requires a considerable period of down time to descale tubing and carry out associated maintenance, such as the replacement of blocked tubes and the like. It is therefore standard practice in the industry to provide ample spare equipment, so that cleaning can be carried out on a rotating basis without effecting plant production or the continuity of operations. Indeed, in large conventional refineries it is not unusual to have from 30% to 50% spare equipment in the heat recovery section.

Finally, the thermal performance of the heaters is directly related to the rate of flash steam generation in the evaporator vessels and this has an important bearing on the quality and utility of the condensate that is collected from the heater train.

Not surprisingly, the industry has been engaged for many years in actively developing improvements in the digestion plant. These improvements have been in the way of process improvements, equipment improvements, operating improvements, or a combination thereof.

-6- One such improvement utilised a system of tubes provided with heating jackets, instead of conventional shells, where the tubes are jacketed in small groups.

The tubes are large in diameter compared with standard heat exchanger tubes and essentially continue uninterrupted throughout the length of the entire heater system. The jackets are not continuous, but are applied intermittently in accordance with the number of evaporators and to suit dismantling of heater elements for cleaning and maintenance. Regarding thermal design, there is essentially no difference between such tube heaters and conventional heaters; but mechanically the differences are significant.

However, this design was primarily aimed at eliminating the old fashioned autoclave type digester in which the build-up of scale was quite out of proportion with what is considered bad scale build-up in modern alumina refineries. It did not therefore specifically address the problems that now remain in modern large scale plants.

Following that, a somewhat similar design was developed with a similar aim of finding a suitable alternative to the old style autoclave. Since scaling-up of tube digesters remained a problem, a heater design was adopted that contained three tubes within each jacket. Two of these tubes conveyed bauxite slurry, the third conveyed spent liquor. At the end of the heater system, and prior to digestion, the three flows were combined to provide a digestion slurry of the requisite consistency. The flow through the tubes was periodically switched, so that each of three lines are in turn subjected to spent liquor flow, with the aim of dissolving the scale. This procedure was carried out in-situ and under operating conditions (ie at temperature), and removed at least a portion of the scale. However, the equipment required periodic cleaning with acid, to remove the components of the scale that were insoluble in spent liquor or that remained undissolved, and this cleaning process could not be carried out under operating conditions.

The metallurgical implications were more serious, as the spent liquor generated by this process was above 140 0 C and could not be contained in carbon steel.

-7- This required alloys that are either extremely expensive, or, if only moderately expensive, such as ferritic stainless steels, were difficult to weld.

Another development has been to retain conventional shell and tube heat exchangers and install them with the tube bundles vertically instead of horizontally. This has proved to be reasonably effective in limiting the growth of scale within the tubes and demonstrates the effect of gravity on precipitation and sedimentation/cementation on the tube wall.

An aim of the present invention is to provide a multi-cell heating system that allows individual heater cell trains to be cleaned while the remainder of the equipment remains in operation.

The present invention provides a multi-cell heating system for increasing the temperature of a tri-hydrate bauxite ore slurry to a temperature of up to 1600C through a number of heater cell trains, the heating system comprising an array of heater cells, the array comprising a plurality of heater cell trains and a plurality of heater cell stacks, each stack being associated with and being in fluid communication with a respective heat source, and each train being defined by aligned individual heater cells in adjacent stacks such that the slurry may be split to flow through two or more of the trains in order to be heated thereby, the array being configured such that there is an inlet temperature at one side thereof and an outlet temperature at the other side thereof, wherein the interconnection of trains, stacks, cells and heat sources is such that each heater cell train, and each heater cell, is able to be isolated from the heating medium. In a preferred form, the heat sources are evaporators and the heat medium is vapour or steam.

The heat source may also be condensed vapour (condensate).

In use of the system of the invention, the maximum temperature that the slurry is to be increased to is about 1600C, or in most situations, including those where scale deposition proves a problem at higher temperatures, the temperature may be increased to a maximum of from about 1450C to about 150 0

C.

-8- Typical applications will have the slurry enter the system of the invention from about 750C to about 1050C, and often from about 800C to about 850C.

The isolation referred to above provides two significant ways in which the deposition of scale within the heater cells and the performance of the heating system as a whole can be controlled: a) isolation of a single train of heater cells allows that single train to be cooled, drained and cleaned while the remaining trains stay in operation. By thus cleaning each train in turn and at regular intervals, the overall deposition of scale can be maintained at an average level; and b) isolation of individual heater cells allows evaporator vapour flows to be controlled and distributed so as to maintain the desired pressure/temperature profile through the evaporators. Such isolation or, if necessary, modulation of the vapour flows to individual heater cells, also provides a means of controlling the rate at which the temperature increases along the heater tube, ie. the heat flux. This is an advantageous feature of the invention, in that the rate of deposition of scale is a function of both temperature and heat flux.

Thus, the manner in which the heater cells are arranged and connected provides a useful measure of control over the relative thermal performance between heater cell trains as well as over the thermal gradient within heater cell trains. It allows the surface area required for heat transfer to be minimised and plant performance to be optimised.

Furthermore, the multi-cell system allows the deposition of scale to be distributed in a manner that will substantially reduce the amount of equipment that would otherwise be required. The system also provides a large measure of control over the pressure/temperature profile through the heating system. This is particularly advantageous in process plants in which the heating system forms part of a heat recovery system in which the heating medium consists of process vapour extracted from flash evaporating vessels. In such plants, the quality of -9the vapour and condensate are highly dependent on a relatively steady pressure/temperature profile. A steady pressure/temperature profile is also required to maintain the driving force between evaporators. This is the force or pressure required between adjacent evaporators to ensure that the designated evaporating liquid flow can be maintained. This force or pressure is directly related to temperature and should therefore be maintained steady throughout the operational cycle of the plant, irrespective of the increasing build-up of scale and the concomitant decline of the capacity of individual heater cells to condense/extract vapour from the evaporator vessels.

In this respect, in prior art techniques heater units which subject slurry to an increase in temperature contain a number of tubes that run parallel through a single line of heater shells. Thus, individual tubes cannot be isolated from the vapour flows. This prevents individual tubes from being descaled while the heating plant remains in operation and thereby prevents the scale deposition within a single heating unit from being maintained at average conditions.

Descaling in such prior art techniques, in which a number of tubes run through a single line of shells, takes place when the operational heating unit is fully scaled and requires the heating unit to be taken out of operation. In order to maintain the required pressure temperature/profile between when all tubes are clean and all tubes are fully scaled, such prior art provides excess heat transfer area, which is initially flooded with condensate to render the tubes temporally ineffective. As tubes scale, the condensate level is reduced in order to expose more tube area to vapour or other heating medium.

However, the multi-cell heater system of the present invention does not require such excess heat transfer area, and controls its operation by maintaining an average scale deposition at a steady state.

Indeed, in prior art systems the localisation of heater scale is particularly troublesome. It causes the thermal performance of heaters in the high temperature range to decline much more rapidly than the performance of the heaters outside that range. As the performance of the most effected heaters declines, the pressure/temperature profile through the heat recovery system changes, and the thermal duty is gradually transferred to the lesser scaled heaters. The temperature intervals between these cleaner heaters increase and effectively this reduces the number of heater stages that are actually utilised.

This in turn reduces the overall efficiency of the heat recovery process and limits full utilisation of the heat transfer area in those heaters in which the deposition of scale is basically still within acceptable limits; in other words a point is reached where the total available heater surface area would still be serviceable if it would be more evenly divided over the number of stages, but where, because the number of useful stages has effectively been reduced, the heater system as a whole can no longer perform and rapidly declines in efficiency.

Furthermore, the performance of the evaporation vessels is directly related to the troublesome localisation of heater scale. In a conventional heat recovery system, evaporator duty is always equal to the condensing capacity of its corresponding heaters. The evaporators are sized to keep the upward vapour velocity below a certain limit. This is in order to prevent any caustic or solid or other contaminating matter, contained in the evaporator fluid, from being carried along with the vapour flow. When heaters foul at different rates and the pressure/temperature profile and thermal capacities change, the vapour generating rates of the evaporator vessels must change accordingly. The vapour rates in the cleaner stages will gradually increase and their upward vapour velocities may eventually exceed allowable values. Impurities will then be carried along by the vapour stream and effect the process in two ways: a) impurities will be deposited on the outside of the heater tubes and this will further diminish heat transfer capability. Moreover, scale on the outside of the tubes is difficult to detect and is also much more difficult to remove than scale deposited on the inside of the tubes; and b) the impurities will pollute the condensate which is then no longer fit to be returned to the steam plant. The condensate must then be used for secondary purposes and this generally results in the loss of much of the -11 energy it contains. There may also be an increase in plant water consumption to compensate for the loss of boiler make up water.

Depending on the degree and exact location of scale deposition, unequal heater fouling will ultimately limit the capacity of a heat recovery system in one of three possible ways: i. because the heater train has reached the limits of its thermal capacity, while a significant proportion of the total heat transfer area may still only be moderately fouled; ii. because it has reached the limit of its hydraulic (pressure) capacity due to localised constrictions of the flow area; and iii. because it produces bad condensate.

These limitations, as outlined in ii., and iii. above, are addressed by the multicell heating system of the present invention. The present invention allows the available heat transfer area to be more efficiently utilised. It enables the pressure/temperature profile through the evaporators to be controlled, while minimising the required heat transfer area. Vapour generation rates therefore remain uniform and the likelihood of impurities contaminating the condensate is lessened. More importantly, because the temperature intervals between stages remain constant, there is no reduction in the number of effective stages, when heaters become scaled, and there is therefore no concomitant reduction in the overall efficiency of the heat recovery process.

In a preferred form of the invention, the heater cells are multiple pass heaters.

However, each pass preferably comprises only a single tube. Preferably, such single tube heaters resemble conventional heaters in that they have a shell side, a tube side, a tube plate and tube passes, although unlike a conventional heater, they have no channel section.

-12- While each heater cell may contain one or more passes, again each pass preferably comprises only a single tube. In its most convenient arrangement, which does not require provisions to be made for differential expansion between tubes and shell, the tubes are arranged with return bends within the shell. Thus, externally to the tube plate, individual passes may be affected by means of return bends to provide a single continuous tube within each shell.

The number of passes to be employed is a design consideration, in which tube size, shell diameter, heater length and required heat transfer area are weighed up to provide the most cost-effective unit.

At the tube plate, all tubes may be flanged to provide access for manual descaling. However, cleaning flanges generally need not be provided for internal bends, which preferably have sufficient radius to allow standard cleaning equipment to be effective. At the opposite end to the tube plate the shell may be provided with a flanged cap to allow for inspection of the internal bends.

The shell diameter is generally determinable by tube bundle geometry. The shell preferably also has sufficient volume, clear from the heat transfer area it provides, to act as condensate receiver. In this respect, condensate collection inside the heater shell eliminates the requirement for a condensate receiver for each individual cell or stack of cells. It also simplifies the condensate pipe work connecting the cells and helps equalising vapour flow rates from evaporators.

The number of cells per stack is generally determined by economic and operational considerations. Consistent with the principles set out in the foregoing, to accommodate two different vapour streams, at least two heater cells per stack are required. However, an additional cell should be provided for descaling. In practice, the most suitable number of cells per heater stack ranges from three to seven.

-13- However, a heater stack of five smaller cells being four operating cells and one spare cell provides a more suitable arrangement. Four operating cells allow the heat transfer area per stage to be redistributed at 25% increments, the spare cell then amounting to only 25% stand by equipment which is reasonably representative of the ratio between the length of the operating cycle and the length of the descaling cycle.

As described above, a stack of heater cells is preferably installed opposite each flash evaporator, and each heater cell stack is preferably capable of having vapour or steam fed to it from its associated flash evaporator. Thus, each heater cell in a stack is preferably individually valved and can have its steam supply individually connected, varied or isolated as required.

To heat slurry to about 160 0 C, the number of stacks is generally 3 to 6 and preferably 4 to Preferably, the flow of slurry is at right angles to the flow of steam or vapour. If each stack is considered to be arranged vertically, the steam or vapour flow to the heater cells is also distributed vertically, in which case the slurry flow runs horizontally, arranged in tiers or trains. Thus, all of the uppermost units in each stack are connected on the tube side to form one single continuous slurry stream similarly for the second unit in each stack, all being connected to provide a single uninterrupted stream from the slurry inlet to the end of the respective heater cell train.

With regard to the manner in which the array of heater cells are connected, there are various aspects to be considered. In particular, scale growth has a significant effect on hydraulic resistance. It may also seriously effect flow distribution between parallel streams propelled from a common pressure source.

In particular, such streams foul at different rates, flows being distributed in accordance with the hydraulic resistance of each individual stream. However, this distribution does not necessarily coincide with the comparative thermal -14capacity of each stream and overall thermal performance will thus be impaired.

Therefore, heater cell trains are preferably individually controlled, and this may be achieved in two ways.

Firstly, each stream may be pumped individually. However, this generally only is practical in large plants in which the flow through each individual train of heater cells is large enough to warrant a dedicated pump, or, in the case of multiple chamber positive displacement pumps, to warrant a set of dedicated pump chambers. Secondly, all streams may be pumped from a common source and a flow control valve may be installed in each line of heaters. Scale growth is a gradual process and while the rates of growth may vary, there are no sudden fluctuations in the way scale growth effects the heaters. Manual control, by means of throttling valves, is therefore quite satisfactory. Furthermore, both manual control and automatic control may be activated by the outlet temperature of the slurry heater streams.

With regard to the collection and transmission of condensate, individual heater cells may operate at different temperatures. Their condensates should then preferably not be collected in a common receiver vessel installed at each stack as in conventional plants, but each train of heater cells should have its condensate collected and transmitted separately. Effectively, this divides the condensate system into a number of parallel streams running from the high pressure end to the low pressure end of the plant. In this respect, individual streams are small and this allows condensate to be collected inside each heater cell. Separate condensate pots at each heater stack are therefore not required, except where accumulated condensate flows through the heater shells are likely to be detrimental to effective tube areas The requirement for condensate pots, reflux, vapour lines and steam traps, or flow control valves may therefore be minimised or eliminated provided the condensate piping is arranged in the preferred manner.

The preferred arrangement relies on orifice plates and allows steam to by-pass when the condensate flow rate declines, This may occur when plant throughput is low and/or when condensing capacity declines due to scale deposition. Such by-pass steam is not detrimental to heater train operation provided it reaches the next heater shell in a saturated state at the downstream pressure condition.

Within the range of plant operating conditions that can be expected in practice, such will always be the case. Moreover, in the case of reduced condensate flow due to heater tube scaling, by-pass of steam actually enhances thermal performance. Indeed, it distributes vapour to a downstream heater stage without a concomitant increase in upstream evaporator pressure and thus helps to maintain the desired pressure/temperature profile.

The resulting condensate system is relatively simple and provides additional advantages over systems utilised in conventional plants. Its advantages are not confined to high temperature plants, but are equally applicable to low temperature plants.

With regard to the collection of the non-condensable gases entrained in the vapour stream, these are separated in the condensation process and are collected in the heaters. However, these gases are detrimental to heater performance and thus should preferably be removed. Each cell in the heater train is therefore preferably connected to a non- condensable vent system.

While the individual vent streams are small, each stream is saturated with water vapour and collectively they represent a significant amount of energy, as well as condensate. Water vapour can be separated from non-condensable gases by cooling, and this is preferably carried out in two low pressure heater cells.

Energy is thus retrieved by the incoming slurry stream. The heat transfer area required for this operation should be taken into account in the thermal design.

There are significant design, constructional, operational and maintenance advantages to making all heater cells identical. Thermal design should therefore preferably be arranged to suit heater cells of equal transfer area, except for the -16first two heater stacks. The first two stacks are preferably designed so that a smaller area is dedicated to condensing evaporator steam, wherein the difference in area is preferably to allow one entire cell in each of the first two heater stacks to be dissociated from the evaporator vessels and to be utilised for non-condensable cooling.

The heating system of the present invention results in an extremely compact and flexible physical arrangement in which each heater cell is effectively a node on a network formed by the slurry and vapour distribution systems. In particular, the arrangement facilitates the descaling operation while the plant is in operation.

This considerably reduces the amount of equipment that has to be taken out of service for descaling at any one time and provides large savings in the amount of spare equipment that needs to be installed. The arrangement also provides a great deal of operator control over the deposition of scale, both within slurry streams as well as between slurry streams. It also provides control over the distribution of vapour flow, both between evaporators and between heater cells, and over the quality of condensate.

The present invention thus advantageously provides a heating system that allows individual heater cell trains to be cleaned, while the remainder of the equipment remains in operation. This provides a means of controlling the overall scale growth within a single operating heating system in such a way that the total of accumulated scale deposition within this single operating heating system is always maintained at an average value (steady state). In effect, this means that heat transfer areas may then be determined for average conditions of scale formation, rather than for maximum conditions. Since the build up of scale is the most significant factor in determining the amount of heat transfer area that needs to installed, the multi-cell heating system will allow large savings in capital expenditure to be made. By providing means of controlling vapour flows and thereby allow the designated pressure/temperature profile of the plant to be maintained, the multi-cell heating system also has significant operational advantages.

-17- The present invention will now be described in relation to an example that will be described in conjunction with two embodiments as illustrated in the accompanying drawings. In the drawings: Figure 1 is a schematic representation of a conventional heating system; Figure 2 is a schematic representation of a multi-cell heating system according to a first preferred embodiment of the present invention; Figures 3 and 4 are schematic representations of a multi-cell heating system according to a second preferred embodiment of the present invention; Figures 5a, 5b, 5c and 5d are elevation, plan and section views respectively of a stack of heater cells as used in the embodiment of Figures 3 and 4; Figures 6a, 6b and 6c are elevation, plan and section views respectively of a stack of alternate heater cells as used in the embodiments of Figures 3 and 4; Figures 7a, 7b and 7c are elevation and isometric views respectively of a stack of alternate heater cells as used in the embodiments of Figures 3 and 4; Figures 8 and 9 are schematic representations of additional connections for the embodiments of Figure 2 and Figures 3 and 4.

The following example is specific to the alumina refining industry, and in particular to the extraction of alumina as achieved by the Bayer process.

-18- However, and as explained above, the invention is not to be limited to use only with the Bayer process.

Illustrated in Figure 1 is a conventional flash vapour/heating system having a plurality of conventional tubular heat exchangers 10 arranged in three lines (12, 14 and 16) and four stages (18, 20, 24 and 26). The top two lines 12 and 14 would be operating lines while the third line 16 would be a spare line.

The system illustrated is a conventional digestion system of an alumina refinery where the majority of the heat recovery takes place. The heat recovery primarily takes place by exchanging heat between the outgoing hot Bayer solution (moving through the evaporators 28 via the slurry stream 30) and the incoming cold bauxite slurry of the streams 32 to the various heater lines (12,14 and 16).

The heat recovery is accomplished in conventional tubular heat exchangers wherein the cold slurry flows through the inside of the tubes and hot flash vapour flows on the outside of the tubes. The vapour condenses on the cold(er) tubes, which causes it to release its heat of evaporation, which is then absorbed by the cold slurry stream.

Referring to Figure 2, a first preferred embodiment of the present invention is illustrated wherein a bauxite slurry is split into four streams and is pumped into four heater cell trains (referenced as A, B, C, and D) through four heater stacks (referenced as 48, 48, 46 and 46) opposite the four flash stages only that are required to increase the temperature thereof from the inlet temperature of about 850C to an outlet temperature of about 1100C by utilising a multi-cell heating system in accordance with the present invention.

After completion of the flash vapour heating process, the heated bauxite slurry may be subjected to further heating either by live steam or some other heating medium to increase the slurry to its final operating temperature, which is followed by blending, digestion and flashing to relieve the vapour pressure and -19remove the heat (energy) therefrom. The normal clarification and precipitation processes may then follow to clarify and precipitate the alumina as required.

The further heating may be any type of heating as necessary, although the heating is most commonly steam heating. This heating may also be carried out using a multicell heating system in accordance with this invention.

In commercial operations the extraction of alumina is carried out in large volumes. The caustic liquor stream is continuously recirculated and alternately heated and cooled in accordance with the Bayer process to produce the alumina. However, because of the large size of operation, the energy content of the liquor stream is very high, especially in high temperature refineries such as that described herein, and efficient energy management is important to the economy and viability of the plant. Thus, and as indicated above, a large proportion of the equipment installed in alumina refineries is dedicated to heat recovery.

In the first preferred embodiment illustrated in Figure 2, the main heat recovery takes place in the heater cells 42 by exchanging heat between the outgoing hot Bayer solution (not shown) and the incoming cold bauxite slurry entering trains A, B, C and D. Heat is extracted from the hot stream by flash evaporation taking place in a number of stages in flash evaporators. In this respect, a flash evaporator 44 is illustrated at the top of each heater stack (46 and 48).

Heating the cold slurry stream is accomplished by condensing the flash vapour in the heater cells 42 wherein the vapour is condensed on the outside of the tube, while the bauxite slurry to be heated flows inside the tubes. The number of stages of flash evaporators depends on the type of plant, and the rise in operating temperature of the slurry, and for example in low temperature plants used for digestion of trihydrate bauxite ore, the number may range from 3 to about 7. In this respect, four flash evaporators 44 have been illustrated in Figure 2 for ease of understanding. The heating stages can also be connected with one or several pressure vessels in which the slurry is retained for a certain time to allow complete dissolution. The slurry in the pressure vessel can be directly or indirectly heated to the maximum desired temperature.

As can be seen from Figure 2, the multi-cell heating system of the first preferred embodiment of the present invention includes an array of heater cells 42, essentially arranged in rows and columns. In this respect, the rows designate the four heater cell trains A, B, C, and D, and the columns designate the four heater cell stacks (46 and 48). Of course, the number of heater cells in each of the trains and stacks may be altered as necessary, as will be apparent in the second preferred embodiment illustrated in Figures 3 to 9.

The system illustrated in Figure 2 includes pumps 50 which may be any type of suitable pump such as a centrifugal or positive displacement pump, and each of the streams 52 of the trains A, B, C and D may be pumped separately.

Alternatively, there may be a common pump with a valve in each stream or a multiple chamber pump, with one or more chambers feeding into each stream.

Of course, the control for these streams may be automatic or manual.

The heater cells 42 illustrated in Figure 2 are in the form of multiple pass tube heaters as described above. These multiple pass tube heaters are typically in the order of 20 to 30 metres long and may comprise shells having diameters of 400mm to 1000mm.

The interconnections between a flash evaporator 44 and the heater stack (46 or 48) above which it is aligned are such as to allow the amount of flash vapour passed to any of the heater cells in a particular stack to be altered modulated or isolated.

In this way, the heating system can be controlled to keep both the total accumulation of scale within the tubes as well as the pressure/temperature profile through the evaporators in a steady state at average values. Thus, by -21 monitoring and controlling the distribution of scale by cleaning individual trains while the plant is in operation, it is possible to limit its thickness in any one location. It also results in lower pressure drop through the system and allows the heat flux to be directed to that part of the heating system most able to efficiently handle it. In particular, it can be seen that by altering the flow of flash vapour to various of the cells in a particular heater cell stack, the operator of the system is able to control the temperature profile and affect the rates of heating and thereby the rates of scaling along individual heater trains.

In this respect, and as indicated earlier, it has been found that the rate of growth of scale increases exponentially with temperature and heat flux. The rate of scaling may peak at a certain temperature, but this depends entirely on the type of bauxite being processed and its mineral constituents.

By utilising the heating system of the present invention it is possible to monitor scale growth by measuring heat transfer and by a regular cycle of cleaning to maintain overall plant scale growth at a steady average state. By controlling vapour flows to individual cells it is also possible to influence heat flux and to control and distribute vapour flows to maintain condensate quality and evaporator driving force. Such a consistent level of average scale deposition also results in lower pressure drops through the system.

Illustrated in Figures 3 and 4 is a multi-cell heating system which comprises an array of heater cells 62 arranged so as to provide a plurality of heater cell trains (referenced as trains A, B, C, D, E and F) and a plurality of heater cell stacks (being first, second, third, and fourth stages designated by reference numerals 64, 66, 70 and 72.) Thus, it is evident that various of the heater cells may be referred to as l a, lb, 2a, 2b, 3a etc. Each stage of heater cells (being the stacks referred to above) is associated with a respective flash evaporator 74.

The hot Bayer liquor stream 78 shows passage of the hot Bayer liquor through the evaporators in order to cool that liquor. However, it will be appreciated that -22 the streams of cold bauxite slurry being heated are not shown in their entirety, although the stream 76 for heater train F is shown in detail.

In this embodiment, the vapour piping is arranged so that vapour (in respective lines 80) from each evaporator 74 is separately valved to each heater cell in its corresponding heater stack. As can be seen, this allows detailed operator control of the passage of vapour to each of the cells and thus enables the control of the temperature of each cell. While more elaborate distribution of the vapour is possible, this generally adds to the complexity of the interconnecting pipework while providing only marginal benefits.

Illustrated in Figures 5 to 9 are various aspects of the first and second preferred embodiments of the present invention. These drawings better illustrate the various aspects of heater connection as will be described.

Illustrated in Figures 5a, 5b, 5c and 5d are elevation, plan and section views of one of the stages (otherwise referred to as a stack) of heater cells of Figures 3 and 4. Thus, this stage of heater cells includes six cells 62 arranged one above the other having vapour inlets 82, flash condensate inlets 84, non-condensable outlets 86, and condensate outlets 88. The cells comprise four passes of a single tube such that the slurry may enter the cell via inlet 90, may pass the full length of the shell, then may travel via a return bend back to the inlet end of the shell to make a second pass of the shell, then by a second return bend back to the inlet to make a third pass of the shell, then via a third return bend once more back to the inlet to make its fourth and final pass of the full length of the shell, after which it exits through outlet 92 of the cell.

With reference to the configurations shown in Figures 6a, 6b and 6c and Figures 7a, 7b and 7c, like reference numerals have been used to identify like features as described in relation to Figures 5a, 5b, 5c and 5d. However, it will of course be appreciated that Figures 7b and 7c show six tubes rather than the four tubes of Figures 5c and 6c.

23 The schematic arrangement in Figure 8 illustrates for the first and second preferred embodiment a preferred arrangement of condensate piping. This preferred arrangement relies on orifice plates and allows steam to by-pass when the condensate flow rate declines. Such by-pass steam is not detrimental to the operation of the heating system provided it reaches an adjacent heater cell in a saturated state at the downstream pressure condition. Moreover, in the case of reduced condensate flow due to scale deposition any by-pass of steam actually enhances thermal performance. Indeed, this distributes vapour to adjoining evaporation stages without a concomitant redistribution of evaporated temperatures and pressures and thereby augments the vapour distribution pipework.

This arrangement does not require separate condensate pots except at the first stage of heaters and where accumulated condensate flows through the heater shells are likely to be detrimental to effective tube areas. The need for condensate pots, reflux vapour lines, steam traps, or flow control valves may therefore be minimised or eliminated.

Illustrated in Figure 9 is the preferred arrangement of non condensable vent piping for the system illustrated in Figures 2 to 4. In this respect, noncondensable gases entrained in the vapour system may be separated out in the condensation process and collected in the heaters. However, these gases are detrimental to the performance of the heaters and require removal. Thus, each heater cell in the system is connected to a non-condensable vent system as shown in Figure 9. While these individual vent streams are small, each stream is saturated with water vapour and collectively represents a significant amount of energy, as well as condensate.

Finally, it will be understood that there may be other modifications and alterations made to the configurations described herein that are also within the scope of the present invention.

Claims (24)

1. A multi-cell heating system for increasing the temperature of a tri-hydrate bauxite ore slurry to a temperature of up to 1600C through a number of heater cells, the heater system comprising an array of heater cells, the array comprising a plurality of heater cell trains and a plurality of heater cell stacks, each stack being associated with and being in fluid communication with a respective heat source, and each train being defined by aligned individual heater cells in adjacent stacks such that the slurry may be split to flow through two or more of the trains in order to be heated thereby, the array being configured such that there is an inlet temperature at one side thereof and an outlet temperature at the other side thereof, wherein the interconnection of trains, stacks, cells and heat sources is such that each heater cell train, and each heater cell, are able to be isolated from the heat medium.

2. A multi-cell heating system according to claim 1, wherein the temperature is increased to about 1550C.

3. A multi-cell heating system according to claim 1, wherein the temperature is increased to about 1500C.

4. A multi-cell heating system according to claim 1, wherein the temperature is increased to a range of from about 1450C to about 1500C.

5. A multi-cell heating system according to any one of claims 1 to 4, wherein the temperature is increased from about 750C.

6. A multi-cell heating system according to any one of claims 1 to 4, wherein the temperature is increased from about 800C.

7. A multi-cell heating system according to any one of claims 1 to 4, wherein the temperature is increased from about 850C.

8. A multi-cell heating system according to any one of claims 1 to 4, wherein the temperature is increased from about

9. A multi-cell heating system according to any one of claims 1 to 4, wherein the temperature is increased from about 950C. A multi-cell heating system according to any one of claims 1 to 4, wherein the temperature is increased from about 1000C.

11. A multi-cell heating system according to any one of claims 1 to 4, wherein the temperature is increased from about 105 0 C.

12. A multi-cell heating system according to any one of the preceding claims, wherein the heat source is an evaporator and the heat medium is vapour, steam or condensed vapour.

13. A multi-cell heating system according to any one of the preceding claims, wherein the heater cells are multiple pass heaters, each pass comprising only a single tube.

14. A multi-cell heating system according to claim 13, wherein each single pass is provided by means of return bends to provide a single continuous tube within each cell and for each heater train. A multi-cell heating system according to any one of the preceding claims wherein the shell of each cell has sufficient volume, clear from the heat transfer area it provides, to act as a condensate receiver.

16. A multi-cell heating system according to any one of the preceding claims wherein the number of cells per stack ranges from three to seven. 26

17. A multi-cell heating system according to claim 16 wherein the number of cells per stack ranges from three to five.

18. A multi-cell heating system according to claim 16 wherein the number of cells per stack is four. 19 A multi-cell heating system according to any one of the preceding claims, wherein each heater cell stack is capable of having vapour or steam fed thereto from its associated evaporator, and wherein each heater cell in a stack is individually valved and can have vapour or steam supply individually connected, varied or isolated as required. A multi-cell heating system according to claim 19 wherein the number of stacks ranges from three to six.

21. A multi-cell heating system according to claim 19 wherein the number of stacks is four

22. A multi-cell heating system according to any one of claims 19 to 21 wherein control is provided by pumping each stream individually.

23. A multi-cell heating system according to any one of claims 19 to 21 wherein control is provided by pumping all streams from a common source, and installing a flow control valve in each train of heater cells.

24. A multi-cell heating system according to claim 22 or claim 23 wherein manual control or automatic control are utilised, being activated by the outlet temperature of the slurry heater streams.

25. A multi-cell heating system according to any one of the preceding claims wherein the condensate of individual heater cells or heater stacks is not -27- collected in a common receiver vessel, but each train of heater cells has condensate collected and transmitted separately.

26. A multi-cell heating system according to any one of the preceding claims wherein each cell in a heater train is connected to a non-condensable vent system.

27. A multi-cell heating system according to any one of the preceding claims capable of use as a heat recovery system in a Bayer process plant.

28. A multi-cell heating system according to claim 1 substantially as herein described in relation to Figure 2. Dated this Twenty Seventh day of April 2006. Hatch Associates Pty Limited Applicant Wray Associates Perth, Western Australia Patent Attorneys for the Applicant