WO2016172758A1 - Processing plant arrangement - Google Patents

Abstract

A processing plant (10) comprising at least two process units (12, 14), each process unit (12, 14) in turn comprising a stack of process trains {18} that in turn comprise a plurality of process cells (20), wherein the process units (12, 14) are arranged in a spaced apart manner defining therebetween a substantially clear extraction zone (18) into which a process cell (20) from either process unit (12, 14) may be withdrawn from its process cell train (18) and stack. Also described is a method for the laying out or construction of such a processing plant (10).

Description

"Processing Plant Arrangement" Field of the Invention

[0001] The present invention relates to a processing plant. More particularly, the processing piant of the present Inventio is arranged in a manner intended to facilitate the efficient and effective maintenance or at least a portion thereof.

[0002] The arrangement of the processing plant of the present invention is envisaged to have partieuiar application in processing plants that utilis multiple banks, stacks or trains of equipment throughout the plant, wherein that equipment ma need to be removed for servicing, replacement or some similar activity during the lifetime of the plant.

[0003] it is further envisaged that the arrangement of the processing plant of the present invention has still more partieuiar appiication in plants operating the Bayer process for the extraction of alumina from bauxite ore, those plants in turn utilising tube digestion technology.

Background Art

[0004] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an

acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0005] The Applicants are the registered proprietors of several prior patents directed to multi-eel! heating systems, including Australian Patents 676920, 697381 and

2006201746. The heater systems of these prior patents had been primarily developed for flash evaporation equipment utilised in the aiumina refining industry. However, the useful appiication of the inventions described in those patents is not limited to that particular industry and may be extended to ali branches of industry that encounter similar or identical processing problems to those mentioned below.

[0006] In order to understand the relevance of the advantages provided by the Applicant's prior inventions, the subject of the abovementioned patents, and the further advantages provided by the present invention, it is helpful to explain some of the problems encountered, To this end, a brief description of the processes involved will now foe provided.

[0007] Alumina, or aluminium oxide, is chemically designated as AI₂O₃. 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% extra ctable afumina,

[0008] industrially, the extractio of alumina is accomplished by the Bayer process, so called after the Austrian chemist J Bayer who developed the process in 1888. 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.

[0009] 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 iess than 150*0, whereas the digestion of monohydrate ores requires temperatures ranging to as high as 3QQ°C.

[0010] 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, especiall 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. [0011] 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.

[0012] 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 number of stages is limited by economic considerations. In particular, eac additional stage requires more equipment and there is thus a point beyond which the marginal increase in efficiency does not warrant the additional investment.

[0013] it is in the tubula 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 capita! cost for equipment, as well as to the cost for operating and maintaining th plant.

[0014] 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 300^aC 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 normall employed.

[0015] Even when the slurry only has to be heated to temperatures of approximately 150°C for trihydrate digestion, the deposit of scale is significant.

[0016] In this respect, as the slurry temperature rises, the trihydrate goes readil 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. [0017] The major problem with the buildup 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 de-fouling (the cleaning cycle), generous safety factors are usually provided during the initial design.

Consequently, the heaters are provided wit 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.

[0018] Clearly, these design allowances have a cumulative effect and have a large impact on the capital and operating costs of the plant equipment.

[0019] Furthermore, despite the significant safety factors built into the design and th expense of the equipment, its service life remains limited, it still requires a considerabl period of dow time to de-scale 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.

[0020] 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.

[0021] Not surprisingly, the industr has been engaged for man years in actively deveioping improvements in the digestion plant. These improvements have been in the way of process improvements, equipment improvements, operating improvements, or a combination thereof.

[0022] 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.

[0023] 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 modem alumina refineries, it did not therefore specifically address the problems that now remain in modern large scale plants.

[0024] Following that, a somewhat similar design was developed with a similar aim of finding a suitable alternative to the old style autoclave. Since sca!lng-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 digestio slurry of the requisite consistency. The flow through the tubes was periodicall 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 under operating conditions (i.e. 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 seale that were insoluble in spent liquor or that remained undissolved, and this cleaning process could not be carried out under operating conditions.

[0025] The metallurgical implications were more serious, as the spent liquor generated by this process was above 140°C and could not be contained in carbon steel. This required alloys that are either extremely expensive, or, if only moderately expensive, such as ferritsc stainless steels, were difficult to weld.

[0026] The Applicant's prior inventions, those being the subject of the

abovementioned patents, have been directed to providing a multi-cell heating system that allows individual heater cell trains to be cleaned while the remainder of the equipment remains in operation. For example, Australian Patent of Addition

2006201746 provides a multi-ceil heating system for increasing the temperature of a tri- - 8 - hydrate bauxite ore slurry to a temperature of up to 160°C through a number of heater cell trains, the heating system comprising an array of heater ceils, 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 a!igned individual heater celis 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).

[0027] The isolation referred to above provides two significant ways in which the deposition of scale within the heater ceils 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 ceils, 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.

[0028] 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. [0029] Furthermore, the multi-ceil 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 profsie 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 the vapour and

condensate are highly dependent on a relatively steady pressure/temperature profsie. 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 stead 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.

[0030] In this respect, in prior art techniques the heater units that subject slurry to a increase in temperature each 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. I 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.

[0031] The multi-cell heater system of the Applicant's prior patents referred to above do not require such excess heat transfer area, and control their operation by

maintaining an average scale deposition at a steady state. [0032] 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 affected heaters declines, the

pressure/temperature profile through the heat recovery system changes, and the thermal duty is gradually transferred to the lesser sealed 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 basicaiiy 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 usefu! stages has effectively been reduced, the heater system as a whole can no longer perform and rapidly declines in efficiency.

[0033] 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 fou! 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 energy it contains. There may also be an increase in plant water consumption to compensate for the !oss of boiler make up water,

[0034] 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: a) 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; b) because it has reached the limit of its hydraulic (pressure) capacity due to

localised constrictions of the flow area; and c) because it produces bad condensate.

[0035] These limitations are addressed by th multi-cell, heating system of the Applicant's prior patents noted above. That multi-cell heating system 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 condensat 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.

[0036] in a preferred form of the Applicant's prior invention, the heater ceils 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.

[0037] 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 expansio 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 she!!.

[0038] The number of passes to be employed is a design consideration, i which tube size, shell diameter, heater length and required heat transfer area are weighed up to provide the most cost-effective unit.

[0039] At the tube plate, all tubes ma 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,

[0040] 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.

[0041] The number of cells in each 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,

[0042] However, a heater stack of five smaller cells - being four operating cells and one spare ceil - provides a more suitable arrangement. Four operating cells allow the heat transfer area per stage to be redistributed at 25% increments, the spare ceil 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.

[0043] 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 mdsvidualiy valved and can have its steam supply individually connected, varied or isolated as required.

[0044] To heat slurry to about 180^QC, the number of stacks is generally 3 to 6 and preferabi 4 to 5.

[0045] 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 heate ceils is also distributed vertically, in which case the slurry flow runs horizontally, arranged i tiers or trains. Thus, ail 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.

[0046] 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 capacity 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.

[0047] Firstly, each stream may be pumped individually. However, this generally only is practical in larg 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. [0048] 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 shouid 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 he 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.

[0049] 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.

[00 SO] 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 equall applicable to low temperature plants.

[0051] 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 shouid 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 info account in the thermal design.

[0052] 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 first 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.

[0053] The heating system of the Applicant's prior art results in an extremely compact and flexible physical arrangement in which each heater cell is effectively a nod on a network formed by the slurry and vapour distribution systems. In particular, the arrangement facilitates th 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 siurry streams. It also provides control over the distribution of vapour flow, both between evaporators and between heater cells, and over the quality of condensate.

[0054] The Applicant's prior art 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 buildup 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-ceil heating system also has significant operational advantages.

[0055] Traditional plant refinery layouts have not made provision for the removal of the long, in many cases in the order of 60 to 65 m Song, jacketed pipe heaters for cleaning and maintenance. As noted above, traditional thinking has suggested that careful operation of the process plant refineries would minimise any contaminatio and/or mechanical damage to the heaters. Accordingly, the intent has been that the heaters would not need to b replaced, or removed from the digestion unit for maintenance, over a 30 year or longer design life. However, this has not been the experience of those operating these piants/refineries. Rather, many heaters have been rendered inoperable, due largely to shell side contamination that prevents condensate extraction. In addition, heater tubes have exhibited pin holes thought to be the result of acid pitting and mechanical hydrob!asting during in ss^'iu maintenance activities. This results in, inter alia, poor plant/refinery energy consumption.

[0056] The layout of a plant/refinery can require a large footprint, particularly if the plant/refinery is to operate with a large capacity, increases in capacity require more infrastructure. In the example of an alumina refinery, as described in detail above, if capacities are to be increased then larger stacks of heater cell trains will be required to accommodate greater capacity in digestion. However, there are limits to what can reasonabl be accommodated given the scale of the components being utiised. As noted above, the heater cells utilised in digestion in high temperature alumina refineries may be in the order of 60 to 65 m long. Vertical stacks of these heaters, themselves arranged horizontally, are positioned within a superstructure or 'heater building' that support them and the various additional items that are required for their operation, including the condensate system that feeds the shell side of the heater cells.

[0057] If heater cells are to be removed from the superstructure that supports them, given their horizontal orientation and the fact that they're stacked vertically, then it requires a significant space into which the heater cell can be withdrawn safely. Such space is not typically available within the layout of a plant/refinery given how valuable the useable space is within a plant refinery. Any increase in capacity of the

plant refinery that requires additional stacks of heater cells magnifies this problem,

[0058] Internationa! Patent Application PGT/CA93/Q0291 (WO 94/03396} in the name of Alcan International Limited describes a process in which a double digestion process can be 'retrofitted' into an existing single digest circuit. The aim of this process is to increase spent liquor caustic concentration while also increasing the average ratio leaving digestion as a result of the high ratio which can be achieved in the gibpsite digester. This is said to provide in turn an increase in digestion production and digeste productivity. This is said to be able to be achieved with a relatively simple modification of a conventional digestion apparatus. However, it is noted that three additional vessels must be installed. It is further noted that whilst sparing may be provided, it is also said to be unnecessary as the additional vessels will be removed from service for any maintenance necessary whilst the operators revert to the conventional process (ie, the single digest circuit). Accordingly, this document does not disclose a mechanism by which a plant or refinery layout may be organised or arranged so as to facilitate maintenance of items such as horizontally arranged Jacketed pipe heaters that may need to be withdrawn from a vertical stack or train, whilst minimising the footprint of the plant or refinery.

[0059] It is one object of th present invention to overcome substantially at least one o more of the abovemenfioned problems associated with the prior art, or to at least provide a useful alternative thereto.

[0060] Throughout this 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- Summary of the Invention

[0061] In accordance with the present invention there is provided a processing plant comprising at least two process units, each process unit in turn comprising a stack of process trains that in turn comprise a plurality of process ceils, wherein the process units are arranged in a spaced apart manner defining therebetween a substantially clea extraction zone into which a process cell from either process unit may be withdrawn from its process eel! train and stack.

[0082] In one form of the present invention the process units are provided in the form of digestion units, the process trains are provided in the form of heater cell trains and in turn comprise a plurality of heater cells.

[00633 Preferably, the digestion units are arranged so as to be spaced apart in a substantially parallel manner such that the extraction zone is of substantially the same width along the full length of the digestion units bordering that extraction zone.

[0064] Still preferably, the heater cells within the heater cell trains of each digestion unit are arranged in a parallel manner such that each may be withdrawn longitudinally from within its heate cell train and the digestion unit into the extraction zone without interfering with neighbouring heater cells.

[0065] The heater ceils within the heater cell trains are preferably each

interconnected to one another by one or more interconnection means that facilitate their ready diseonnection from one another thereby allowing their withdrawal from within their heater cell train and the digestion unit. In one form this interconnection means is provided i the form of one or more interconnecting pipes.

[0086] Preferably, the heater cell trains are arranged vertically with respect to one another to allow sufficient space for the withdrawal of heater cells without interference from adjacent heater cell trains.

[0067] The digestion units preferably further comprise a superstructure that supports the stacked heate cell trains in their substantially horizontal and spaced apart arrangement.

[0068] Preferably, the superstructure is arranged so as to facilitate the extraction of the heater cells therefrom. Further preferably, the superstructure is arranged so as to facilitate the removal therefrom of any interconnection means used to connect the individual heater cells. [0069] The digestion units each further comprise a heating medium distribution and collection system arranged to service the heater cells of each heater ceil train.

[0070] Preferably, the heating medium distribution and collection system is arranged to be supported from above their respective heater cells and heater ceil trains by a structural tier of the superstructure.

[0071] Each structural tier of the superstructure preferably is arranged so as to be capable of supporting not only the heater ceil train it accommodates directly, but also of supporting any heater cell provided therebeiow should such heater ceil need to be disconnected from th remainder of its heater cell train and withdrawn from the digestion unit.

[0072] In accordance with the present invention there is further provided a method for the laying out or construction of a processing plant, the processing plant being as described hereinabove.

Brief Description of the Drawings

[0073] Further features of the present invention are more fully described in the following description of a non-limiting embodiment thereof. This description is included solely for the purposes of exemplifying the present invention, it should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which: -

Figure 1 is a upper plan diagrammatic or stylised view of an arrangement of a digestion portion of a processing plant, comprising two digestion units (one shown so as to highlight the superstructure supporting the heater cell trains), for the treatment of Bauxite ores and the resulting slurries and liquors in the alumina refining process, in accordance with the present invention, showing the spaced apart relationship of the digestion units and the provision of the substantially clear extraction zone therebetween; and

Figure 2 is a partial upper perspective diagrammatic or stylised view of one digestion unit of the processing plant arrangement of Figure 1 , showing the stacked arrangement of the five heater cell trains and the plurality of heater cells that make them up, each within the superstructure that supports them.

Best Mode s) for Carrying Out the Invention

[0074] The present invention provides a processing plant arrangement comprising at least two process units, eac process unit in turn comprising a stack of process trains that in turn comprise a plurality of process ceils, wherein the process units are arranged in a spaced apart manner defining therebetween a substantially clear extraction zone into which a process eel! from either process unit may be withdrawn from Its process cell train and stack.

[0075] Whilst it is to be understood that the process of the present invention is of general applicability to process engineering plants arranged to implement industrial processes, the Applicants understand thai the process of the present invention has specific and particular application in plants operating the Bayer process for the extraction of alumina from bauxite ore, those plants in turn utilising tube digestion technology.

[0076] In a preferred form, the present invention comprises a processing plant arrangement comprising at least two digestion units, each digestion unit in turn comprising a stack of two or more heater cell trains that in turn comprise a plurality of heater cells, wherein the digestion units are arranged in a spaced apart manner defining therebetween a substantially clear extraction zone into which a heater cell from either digestion unit may b withdrawn from its heater ceil train and stack.

[0077] The digestion units are arranged so as to be spaced apart in a substantially parallel manner such that the extraction zone is of substantially the same width along the full length of the digestion units bordering that extraction zone. Further, the heater ceils within the heater ceil trains of each digestion unit ar arranged in a parallel manner such that each may be withdrawn longitudinally from within its heater cell train and the digestion unit into the extraction zone without interfering with neighbouring heater ceils.

[0078] The heater cells within the heater ceil trains are each interconnected to one another by one or more interconnection means that facilitate their ready disconnection from one another thereby allowing their withdrawal from within their heater cell train and the digestion unit, in one form this interconnection means is provided i the form of one or more interconnecting pipes.

[0079] The heater ceil trains are arranged vertically with respect to one another to allow sufficient space for the withdrawal of heater ceils without interference from adjacent heater cell trains.

[0080] The digestion units further comprise a superstructure that supports the stacked heater cell trains in. their substantialiy horizontal and spaced apart arrangement. The superstructure is arranged so as to facilitate the extraction of the heater ceils therefrom. Further, the superstructure is arranged so as to facilitate the removal therefrom of any interconnection means used to connect the individual heater ceils.

[0081] The digestion units each further comprise a heating medium distribution and collection system, for example a condensate system, arranged to service the heater cells of each heater cell train. The heating medium distribution and collection system is arranged to be supported from above their respective heater ceils and heater ceil trains by a structural tier of the superstructure.

[0082] Each structural tier of the su erstructure preferably is arranged so as to be capable of supporting not only the heater ceil train it accommodates directly, but also of supporting any heater cell provided therebe!ow should such heater ceil need to be disconnected from the remainder of its heater cell train and withdrawn from the digestion unit.

[0083] In Figures 1 and 2 there is shown a processing p!ant 10 in accordance with an embodiment of the present invention. The processing plant 10 comprises two process units, for example a first digestion unit 12 and a second digestion unit 14, arranged in a generally opposed and spaced apart manner, defining therebetween a substantially clear extraction zone 16, Each digestion unit 12, 14 in turn comprises a generaliy vertical stack of two or more process trains, for example five heater ceil trains 18. The heater cell trains 18 in turn comprise a plurality of process cells, for example heater cel!s 20. The heater cells 20 from either digestion unit 12, 14 may be withdrawn from their heater cell train 18, and their respective stack, info the extraction zone 16. [0084] The digestion units 12, 14 are arranged so as to be spaced apart in a substantially parallel manner such that the extraction zone 16 is of substantially the same width along the full length of the digestion units 12, 14 bordering that extraction zone 16, Further, the heater cells 20 within the heater cell trains 18 of each digestion unit 12, 14 are arranged in a parallel manner such that each may be withdrawn longitudinally from within its heater cell train 18 and digestion unit 12, 14 into the extraction zone 16 without interfering with neighbouring heater cells 20. The heater cells are, for example, in the order of 80 m long. In such a case , the width of the extraction zone is, for example, 85 m to accommodate the withdrawal into the extraction zone 16 of the heater cell 20. However, in general terms, the extraction zone 18 is provided at a width that is sufficient to accept the withdrawn heater cell 18.

[0085] The heater cells 20 within the heater ceil trains 18 are each interconnected to one another by one or more interconnection means (not shown) that facilitate their ready disconnection from one another, thereby allowing their withdrawal from within their heater eel! train 18 and their respective digestion unit 12 or 14. In one form this interconnection means is provided in the form of one or more interconnecting pipes. Such an arrangement is described i detail in the Applicant's co-pending International Patent Application international Patent Application PCT/AU20 4/000239

(WO 2014/138784), the entire conten of which is explicitly incorporated herein by reference,

[0088] The heater cell trains 18 are arranged vertically with respect to one another to allow sufficient space for the withdrawal of heater cells 20 without interference from adjacent heater cell trains 18. The experience of the Applicant is such that a separation of 2500 mm is appropriate, as compared with a separation of 1600 mm found in the prior art. This represents an increase in spacing in the order of about 56%.

[0087] The digestion units 2, 14 further comprise a superstructure 22 that supports the stacked heater cell trains 18 i their substantially horizontal and spaced apart arrangement. The superstructure 22 comprises a rigid and sufficiently sound structural framework that is best seen in Figure wherein digestion unit 14 is shown without the heater cell trains 18 therein purely for illustrative purposes only. The superstructure 22 is arranged so as to facilitate the extraction of the heater cells 18 therefrom, a part of which includes the facilitation of the removal therefrom of the interconnection means used to connect the individual heater cells 18. Accordingly, structural columns or uprights that form a part of the superstructure 22 and framework are spaced apart sufficiently to allow the removai of any portion of the heater cells 18 that require removal prior to removai of the entire remainder of the heater eel! IS,

[0088] The digestion units 12, 14 each further comprise a heating medium

distribution and collection system, for example a condensate system (not shown), arranged to service the heater cells 20 of each heater cell train 18. The heating medium distributio and collection system is arranged to be supported from above their respective heater ceils 20 and heater ceil trains 8 by a structural tier of the

superstructure 22.

[0089] Each structural tier of the superstructure 22 is arranged substantially horizontally so as to be capable of supporting not only the heater cell train 18 it accommodates directly, but also of supporting any heater ceil 20 provided fherebeiow should such heater ceii 20 need to be disconnected from the remainder of its heater eel! train 18 and withdrawn from the digestion unit 12, 14.

[0090] As ca be best seen in Figure 1 , in addition to the digestion units 12 and 14 there is provided an area of supporting infrastructure 24 for the digestion portion of the processing plant of which the processing plant 10 forms a part. The supporting infrastructure 24 is arranged between the digestion units 12, 14 so as to be bordering and not impinging upon the extraction zone 18.

[0091] As can be seen with reference to the forgoing description, the processing plant arrangement of the present invention provides a processing plant, or portion thereof, that facilitates the provision of a plant of an increased capacity on the same or similar footprint relative to the prior art and which includes the facility for the ongoing maintenance of that processing plant and its component parts. The significant size of the component parts is accommodated efficiently through th allowance for a common extraction zone between similar portions of the plant, being the digestion units 12 and 14 described above.

[0092] it is envisaged that the extraction of the heater cells from the digestion units 12, 14 and its superstructure 22 may require specifically adapted hoists and related equipment. [0093] Modifications and variations such as wouid be apparent to the skiiled addressee are considered to fail within the scope of the present invention.

Claims

Claims:

1. A processing plant comprising at least two process units, each process unit in turn comprising a stack of process trains that in turn comprise a plurality of process Celis, wherein the process units are arranged in a spaced a art manner defining therebetween a substantially clear extraction zone into which a process cell from either process unit may be withdrawn from its process cell train and stack.

2. The processing plant of claim 1 , wherein the process units are provided in the form of digestion units, the process trains are provided in the form of heater cell trains and in turn comprise a plurality of heater cells.

3. The processing plant of claim 2, wherein the digestion units are arranged so as to be spaced apart in a substantially parallel manner such that the extraction zone is of substantially the same width along the full length of the digestion units bordering that extraction zone.

4. The processing plant of claim 3, wherein the heater ceils within the heater eel! trains of each digestion unit are arranged in a parallel manner such that each may be withdrawn longitudinally from within its heater cell train and the digestion unit into the extraction zone without interfering with neighbouring heater cells.

5_ The processing plant of any one of claims 2 to 4, wherein the heater cells within the heater cell trains are each interconnected to one another by one or more

interconnection means that facilitate their ready disconnection from one another thereby .allowing their withdrawal from withi their heater cell train and the digestion unit.

The processing plant of claim 5, wherein the interconnection means is provided the form of one or more interconnecting pipes.

7. The processing plant of any one of claims 2 to 6, wherein the heater celt trains are arranged vertically with respect to one another to allow sufficient space for the withdrawal of heater cells without interference from adjacent heate cell trains.

8. The processing plant of any one of claims 2 to 7, wherein the digestion units further comprise a superstructure that supports the stacked heater cell trains In their substantially horizontal and spaced apart arrangement.

9. The processing plant of claim 8, wherein the superstructure is arranged so as to facilitate the extraction of the heater cells therefrom.

10. The processing plant of claim 8 or 9, wherein the superstructure is arranged so as to facilitate the removal therefrom of any interconnection means used to connect the individual heater cells.

11 . The processing plant of any one of claims 2 to 10, wherein the digestion units each further comprise a heating medium distribution and collection system arranged to service the heater ceils of each heater cell train.

12. The processing plant of claim 1 1 , wherein the heating medium distribution and

collection system is arranged to be supported from above their respective heater cells and heater cell trains by a structural tier of the superstructure.

13. The processing plant of claim 12, wherein each structural tier of the superstructure is arranged so as to be capable of supporting not only the heater cell train it accommodates directly, but also of supporting any heater ceil provided therebelow should such heater cell need to be disconnected from the remainde of its heater cell train and withdrawn from the digestion unit.

14. A method for the laying out or construction of a processing plant, the processing plant according to any one of claims 1 to 13.