WO2022027100A1 - Method for carbon reduction - Google Patents

Method for carbon reduction Download PDF

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Publication number
WO2022027100A1
WO2022027100A1 PCT/AU2021/050855 AU2021050855W WO2022027100A1 WO 2022027100 A1 WO2022027100 A1 WO 2022027100A1 AU 2021050855 W AU2021050855 W AU 2021050855W WO 2022027100 A1 WO2022027100 A1 WO 2022027100A1
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WO
WIPO (PCT)
Prior art keywords
flash
refinery
water vapour
controlling
accordance
Prior art date
Application number
PCT/AU2021/050855
Other languages
French (fr)
Inventor
Raymond Chatfield
Original Assignee
Alcoa Of Australia Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from AU2020902780A external-priority patent/AU2020902780A0/en
Application filed by Alcoa Of Australia Limited filed Critical Alcoa Of Australia Limited
Priority to CN202180057685.1A priority Critical patent/CN116601115A/en
Priority to AU2021322842A priority patent/AU2021322842A1/en
Publication of WO2022027100A1 publication Critical patent/WO2022027100A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
    • C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
    • C01F7/062—Digestion
    • C01F7/064—Apparatus for digestion, e.g. digestor vessels or heat exchangers
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
    • C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D1/00—Evaporating
    • B01D1/0011—Heating features
    • B01D1/0058—Use of waste energy from other processes or sources, e.g. combustion gas
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D1/00—Evaporating
    • B01D1/28—Evaporating with vapour compression
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D5/00—Condensation of vapours; Recovering volatile solvents by condensation
    • B01D5/0033—Other features
    • B01D5/0054—General arrangements, e.g. flow sheets
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
    • C01F7/14—Aluminium oxide or hydroxide from alkali metal aluminates
    • C01F7/144—Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
    • C01F7/14—Aluminium oxide or hydroxide from alkali metal aluminates
    • C01F7/144—Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process
    • C01F7/147—Apparatus for precipitation
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D21/00—Heat-exchange apparatus not covered by any of the groups F28D1/00 - F28D20/00
    • F28D21/0001—Recuperative heat exchangers
    • F28D21/0003—Recuperative heat exchangers the heat being recuperated from exhaust gases
    • F28D21/001—Recuperative heat exchangers the heat being recuperated from exhaust gases for thermal power plants or industrial processes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
    • Y02P20/10—Process efficiency
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
    • Y02P20/10—Process efficiency
    • Y02P20/133—Renewable energy sources, e.g. sunlight
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
    • Y02P20/50—Improvements relating to the production of bulk chemicals
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/10—Greenhouse gas [GHG] capture, material saving, heat recovery or other energy efficient measures, e.g. motor control, characterised by manufacturing processes, e.g. for rolling metal or metal working

Definitions

  • the Bayer process is a cyclic process where bauxite ore is digested in a hot caustic solution to dissolve the alumina bearing minerals. Undissolved solids are separated and the liquor cooled to precipitate aluminium hydroxide. The precipitated material is removed and calcined at approximately 1000°C to form alumina. The cooled Bayer liquor is reheated and recycled to the digestion stage. As much as possible, energy is captured and reused in the process to encourage energy efficient alumina production.
  • Alumina refining is energy intensive, on average approximately 10.7GJ of energy is required to produce 1 tonne of alumina globally. Approximately 70% of this energy is consumed in the dissolution and precipitation portion of the process with the remaining 30% being consumed in calcination. Therefore, energy is used as efficiently as possible to reduce costs and carbon dioxide emissions.
  • energy is provided to alumina refineries through steam generated in boilers, this steam may be also be used to generate electricity for the refinery.
  • the steam is generated by combustion of fossil fuels, commonly coal, but also natural gas and less commonly oil, contributing to carbon dioxide emissions.
  • Alumina refineries are known to use flash vessels to decrease the temperature of liquor streams within the refinery.
  • a liquor stream will enter a flash vessel and the pressure reduced, generating a cooled liquor stream and pressurised water vapour.
  • flash vessels are arranged in series in banks or trains.
  • In fluid communication with the bank of flash vessels is generally a corresponding bank of heaters. Vapour flashed from a flash vessel is passed to a corresponding heater as is known in the art to re-use the energy present in the pressurised water vapour.
  • Flash vessels and banks thereof are known to be used at numerous locations in alumina refineries including: a. cooling of liquor from digestion prior to clarification; b. cooling of clarified liquor prior to precipitation; c. cooling of liquor from an evaporation circuit; and d. cooling within a precipitation circuit.
  • Alumina production using natural gas typically produces approximately 0.55 tonnes of carbon dioxide per tonne of alumina while alumina produced from an equivalent refinery using coal typically produces approximately twice the mass of carbon dioxide per tonne of alumina. Approximately 132 million tonnes of alumina were produced globally in 2019.
  • solution or variations such as “solutions”, will be understood to encompass slurries, suspensions and other mixtures containing undissolved and/or dissolved solids.
  • a method for controlling the carbon output from an alumina refinery comprising the steps of: capturing water vapour from at least one location in the refinery; compressing the captured water vapour and; utilising the energy in the captured water vapour to heat a process stream within the alumina refinery, wherein the captured water vapour is compressed in an electrically powered compressor wherein the electricity has a carbon intensity of less than 0.5 t CO 2e /MWhr.
  • a method for the preparation of low carbon alumina from an alumina refinery comprising the steps of: capturing water vapour from at least one location in the refinery; compressing the captured water vapour; and utilising the energy in the captured water vapour to heat a process stream within the alumina refinery, wherein the captured water vapour is compressed in an electrically powered compressor wherein the electricity has a carbon intensity of less than 0.5 t CO 2e /MWhr.
  • a method of utilising low grade heat in an alumina refinery comprising the steps of: capturing water vapour from at least one location in the refinery; compressing the captured water vapour; and utilising the energy in the captured water vapour to heat a process stream within the alumina refinery.
  • the method of utilising low grade heat in an alumina refinery may comprise the additional step of compressing the captured water vapour in an electrically powered compressor wherein the electricity has a carbon intensity of less than 0.5 t CC e/MWhr.
  • a method for controlling the carbon output from an alumina refinery comprising the steps of: capturing water vapour from a flash train in the refinery; compressing the captured water vapour and; utilising the energy in the captured water vapour to heat a process stream within the alumina refinery.
  • controlling shall be understood to include reducing the carbon output from an alumina refinery.
  • reducing shall be understood to encompass reducing the carbon output relative to an equivalent alumina refinery utilising fossil fuel fired boiler steam as is known in the art.
  • reducing shall also be understood to encompass the retrofit of an existing alumina refinery with the present invention as well as the construction of a new alumina refinery with the present invention.
  • carbon output shall be considered to refer to tonnes of carbon dioxide per tonne of alumina produced by the refinery.
  • the method of the invention decreases the carbon output of the alumina refinery by 10 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 20 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 30 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 40 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 50 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 60 %.
  • the method of the invention decreases the carbon output of the alumina refinery by 70 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 80 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 90 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 95 %.
  • the electricity is generated from a renewable power source.
  • Renewable power sources may include wind, solar, hydro, tidal, geothermal and biomass.
  • the electricity is generated from a zero carbon power source such as hydrogen or nuclear.
  • the electricity may be stored in a battery after generation and prior to use.
  • the captured water vapour is a source of water vapour that would not otherwise be utilised as an energy source in the alumina refinery.
  • Some water vapour streams are close to ambient conditions and do not have the temperature differential to do useful work within a refinery. Some water vapour streams are conventionally emitted to atmosphere due to a localised energy imbalance. Such water vapour streams are particularly suitable for the present invention. [0025] The present invention enables the energy in what was previously considered to be waste streams is to be harvested.
  • An alumina refinery contains numerous potential low energy or waste water vapour streams. Attractive water vapour streams are those from flash trains and in particular, water vapour streams from the final flash vessel in a flash train such as those found in the digestion circuit, the heat exchange circuit and the evaporation circuit.
  • the water vapour is captured from a flash vessel in the flash train.
  • the flash train comprises a plurality of flash vessels.
  • water vapour is sourced from a flash vessel in a flash train, it is preferably sourced from the final or penultimate flash vessel in the train.
  • the process stream is a heater in a flash train.
  • the flash train may be the same flash train from which the water vapour was sourced.
  • the method comprises the additional step of feeding the heated process stream to a flash train.
  • the step of feeding the heated process stream to a flash train comprises feeding at least a portion of the heated process stream to the flash train.
  • the train of heaters precedes the flash train.
  • the flash train is a heat recovery flash train.
  • the flash train comprises at least two flash vessels.
  • the water vapour is sourced from the final and/or penultimate flash vessel in the flash train.
  • the process stream is the final and/or penultimate heater in the train.
  • the train of heaters comprises one or more heaters.
  • the water vapour may be sourced from a liquor flash system or a water flash system.
  • Liquor flash systems may include digestion, heat interchange, evaporation and precipitation.
  • Water flash systems may include precipitation external cooling and calcination flue gases.
  • the flash train may form part of the digestion circuit, the heat interchange circuit, the evaporation circuit, the precipitation circuit or a calcination heat recovery system in the alumina refinery.
  • the method comprises the further step of exporting excess compressed water vapour out of the flash train.
  • calciner flue gas temperatures and compositions may occur; and it is estimated that approximately 35% to 45% by mass % of the gas leaving calcination ('calciner flue gas') is water, with other components including carbon dioxide and volatile organic carbon compounds. Additionally, the calciner flue gas may contain entrained particulate alumina.
  • a method utilising low grade heat in an alumina refinery comprising the steps of: heating a fluid stream with calciner flue gas to provide a heated fluid stream; passing the heated fluid stream to a flash train; capturing water vapour from the flash train; compressing the captured water vapour; and utilising at least a portion of the energy in the captured water vapour to heat a process stream within the alumina refinery.
  • the water vapour is captured from a flash vessel in the flash train.
  • the flash train comprises a plurality of flash vessels.
  • the flash train comprises one flash vessel.
  • the step of heating a process stream comprises passing the process stream through at least one heater in a train of heaters.
  • the water vapour is sourced from the final and/or penultimate flash vessel in the flash train.
  • the process stream is the final and/or penultimate heater in the train.
  • the train of heaters comprises one or more heaters.
  • the present invention also reduces water consumption in the alumina refinery.
  • the step of compressing the captured water vapour may be repeated. There may be a plurality of compression steps to attain the desired water vapour condensing temperature.
  • a low temperature alumina refinery requiring water vapour to be compressed from about 50 °C to about 170 °C condensing temperature about 13 low speed centrifugal compressors may be required in series.
  • a high temperature alumina refinery requiring water vapour to be compressed from about 50 °C to about 300 °C condensing temperature about 20 low speed centrifugal compressors may be required.
  • a high temperature alumina refinery compressing waste blow off vapour at about 100 °C to 300 °C about 15 low pressure compressors are required.
  • the water vapour from the flash train is less than 80 °C.
  • the step of compressing the captured water vapour is repeated to attain the desired water vapour condensing temperature.
  • the step of compressing the captured water vapour is performed by mechanical vapour recompression.
  • mechanical vapour recompression reduces reliance on fossil fuel generated sources of heat to provide high condensing temperature steam.
  • Mechanical vapour compression may be performed by centrifugal compressors, axial flow compressors or turbo compressors. Centrifugal compressors may be classified as high speed or low speed compressors. Low speed centrifugal compressors typically around 3300 rpm. High speed centrifugal compressors typically operate around 6000 rpm and higher. High speed compressors generally provide a higher compression ratio. [0061] In one form of the invention, there are provided a plurality of mechanical vapour compressors in series wherein compressed steam from the first compressor in the series is passed to the second compressor for further compression. This process may be repeated until the steam is at the desired condensing temperature.
  • the captured water vapour is compressed in an electrically powered compressor wherein the electricity has a carbon intensity of less than 0.5 t CC MWhr.
  • the electricity has a carbon intensity of less than 0.4 t CO2e/MWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.3 t CO2e/MWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.2 t CO2e/MWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.1 t GC MWhr.
  • the electricity is generated from a renewable power source.
  • the electricity is generated from a zero carbon power source such as hydrogen or nuclear.
  • the electricity is stored in a battery after generation and prior to use.
  • the method of the invention may comprise the further step of: removing particulate matter from the captured water vapour, prior to the step of: compressing the captured water vapour.
  • Particulate matter may abrade the impeller of a high speed vapour compressor. This may be addressed by the use of vapour cleaning methods such as mist eliminators, scrubbers and/or cyclones optionally fitted with online washing system utilising a caustic solution to dissolve gibbsitic scale. Such an online wash system could be continuous or intermittent.
  • the water vapour may contain caustic liquid droplets, water droplets and mist which can include small solid matter adhered to liquid droplets.
  • water vapour sources may contain different particulate matter.
  • water vapour from digestion may contain, amongst others, bauxite residue, undigested bauxite
  • water vapour from precipitation may contain, amongst others, hydrate
  • water vapour from spent liquor may contain, amongst others, fine alumina and scale products
  • water vapour from calcination may contain, amongst others, alumina.
  • the present invention may remove the need for the alumina refinery to rely upon fossil fuel generated heat sources.
  • Figure 1 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised
  • Figure 2 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised
  • Figure 3 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised
  • Figure 4 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised
  • Figure 5 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised
  • Figure 6 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised
  • Figure 7 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised
  • Figure 8 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised.
  • Figure 9 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised.
  • Figure 1 presents a flowsheet for an alumina refinery with a low digestion temperature in the range of 145° C to 190 °C.
  • a high temperature digestion refinery in the range of 191 °C to 290 °C is the same as the low temperature digestion refinery but has more flash stages to achieve the same output liquor temperature.
  • Some refineries heat the bauxite slurry with the liquor in the heat exchangers, whereas some heat the bauxite slurry and liquor separately.
  • the digestion type does not change the application of invention; however, a higher compressor exit pressure is required to achieve the higher digestion temperatures.
  • Milled bauxite 12 is fed to a digester 14 with caustic solution and digested at 145° C to 190 °C in the known manner.
  • the green liquor 16 is passed to a series of flash vessels 18 to gradually decrease the temperature and pressure.
  • Figure 1 there are depicted four flash vessels in the series. It will be appreciated that in practice, the number of flash stages required will depend upon the digestion temperature.
  • the green liquor 20 exiting the final flash vessel 24, a blow off vessel, is near atmospheric pressure and about 110 °C. Flashed vapour 26 from each flash vessel is used to heat a corresponding heater (see below). Excess vapour 28 from the blow off vessel 24 cannot be used due to insufficient condensing capability in its associated heater(s).
  • the flashed green liquor 20 is sent to a clarification stage 30 by which time it is cooled below the boiling point after which it passes to a heat interchange stage 32 to further reduce the temperature.
  • the liquor enters 34 the heat interchange stage at about 103 °C and exits 36 at about 80 °C.
  • the heat interchange stage 32 also comprises a series of flash vessels 38. In Figure 1 there are depicted four flash vessels in series. It will be appreciated that in practice, fewer or more flash vessels may be used.
  • the liquor exiting 36 the heat interchange stage 32 is at about 80 °C and passes to the precipitation stage 40 for precipitation of aluminium hydroxide.
  • the liquor passes through a series of precipitators where the liquor temperature gradually decreases. This can be achieved by way of liquid/liquid heat exchangers, for example, plate heat exchangers 42, 44, two being shown in Figure 1 , although more or fewer may be utilised in practice.
  • Cooling water 46 is used to cool the last heat exchanger 34.
  • Aluminium hydroxide precipitates from the cooled liquor in the precipitation circuit 40 and the solids removed (not shown) to provide a spent liquor 48.
  • the spent liquor 48 is at about 60 °C and is recycled to digest further bauxite.
  • the temperature of the spent liquor 48 Prior to recycling, the temperature of the spent liquor 48 must be increased up to the desired digestion temperature through a number of heating stages. It will be appreciated that different alumina refineries may operate different numbers or different types of heating stages. However, all alumina refineries recycle spent liquor and must increase its temperature.
  • the first stage shown in Figure 1 is in the heat interchange stage 32. Flashed vapour 50 from the flash vessels 38 in the heat interchange stage 32 is used to heat a corresponding heater 52.
  • the spent liquor 48 entering the heat interchange stage 32 is about 62 °C and exits 54 at about 83 °C. Importantly, there is a temperature differential of about 20 °C between the green liquor 34 entering the heat interchange stage and the spent liquor 54 exiting the heat interchange stage 32. There is also a temperature differential of about 20 °C between the green liquor 36 exiting the heat interchange stage and the spent Iiquor48 entering the heat interchange stage.
  • the spent liquor 54 exiting the heat interchange stage is sent to the evaporation stage 60 to increase its caustic content prior to recycling to digestion 14 as is known in the art.
  • the evaporation stage 60 also comprises a series of heaters 62 and flash vessels 64. Their interrelationship is the same as described above for the digestion blow off stage 18 and the heat interchange stage 32. That is, flash vapour 66 from the flash vessel 64 is used to heat a corresponding heater 62.
  • the final heater 68 in the evaporation stage uses a source of live steam 70 to attain the desired temperature. This is the only way to heat the liquor as the there is no flash vapour in any of the flash vessels that can heat the liquor to the desired condensing temperature. Live steam 70 is generated from fossil fuels and contributes to the carbon footprint of the refinery.
  • the temperature of the concentrated liquor exiting 72 the evaporation stage 60 is approximately equal to the liquor temperature entering the evaporation circuit 60 and enters the final heater 80 to attain the temperature necessary to digest further bauxite.
  • the final heater 80 heats the liquor as described above. Flash vapour 28 from the flash vessel 24 is used to heat a corresponding heater 82.
  • Flash vapour 84 from the first flash vessel 86 can only heat the liquor to about 25 °C less than the digestion temperature. It is necessary to rely on live steam 88 to heat the liquor in the last heater 90 and attain the desired digestion temperature. Generation of live steam 88 with fossil fuels consumes energy and contributes to the carbon footprint of the refinery.
  • the final liquor 92 is at about 160 °C (for a low temperature refinery) and enters the digester 14. It will be appreciated that for a high temperature refinery, a greater number of heaters and corresponding flash vessels would be required.
  • the spent liquor 48 is fed directly to the evaporation stage 60 instead of the heat interchange stage 32 and the evaporator feed 54 enters the coldest heater 94 and leaves the last flash vessel 48.
  • the present invention can be utilised in both existing refineries as described above and new refineries. It will be appreciated that use of the invention in a new refinery may provide more flexibility than retrofitting the invention in an existing refinery. In the former, there may be chemical and thermodynamic and even spatial limitations to the incorporation of the invention.
  • FIG. 2 presents a flow sheet demonstrating how the present invention could be utilised in the design of a new refinery.
  • a bank of mechanical vapour recompressors 100 are installed to compress the vapour from the flash vessel blow off 28. While only one compressor is depicted in Figure 2, in practice a bank of compressors in series would be utilised.
  • the compressors compress the water vapour and provide a temperature increase of about 8-15 °C across each compressor for a typical low speed centrifugal compressor.
  • the vapour 102 exiting the final compressor may be sent to the live heater 90 or the penultimate heater 91 . In some digestion processes, the vapour 102 exiting the final compressor may be directed to the digester.
  • vapour could be compressed to a lower condensing temperature and directed to a heater prior to the heater 90 to save on compressor capital and operating costs.
  • the digested liquor 20 is sent to clarification as described above. Clarified liquor 34 at about 103 °C is sent to the heat exchange stage which in this case utilises liquid/liquid heat exchangers 104 instead of liquor flashing to cool the liquor.
  • the cooled liquor 36 is sent to precipitation as described above.
  • the precipitation stage comprises a series of precipitators. There is one or more cooling stages in precipitation in which waste heat is produced.
  • one or more flash vessels 106 in series flashes the liquor releasing cooled water vapour.
  • This water vapour 108 (which would otherwise not be utilised) is passed through a series of mechanical vapour compressors 110 to increase its condensing temperature.
  • the water vapour 1 12 exiting the final compressor is sent to the final heating stage 90 of the spent liquor being sent to digestion. This contributes to the ability to heat the spent liquor to digestion temperature by recovering energy available in the process and reducing or eliminating the requirement to provide additional boiler generated steam.
  • Figure 3 depicts how the present invention may be utilised in the evaporation circuit 60 of an existing alumina refinery. Depicted are a series of ten heaters 62 and ten flash vessels 64, although it will be appreciated that more or less may be used depending on the requirements of the refinery.
  • Alumina refinery flash evaporation systems typically consist of 5 to 12 heating stages followed by the same number of flash stages.
  • the last heating stage 68 typically uses live steam 70 in a low temperature digestion refinery, and low pressure steam sourced from digestion flash vessels in a high temperature digestion refinery.
  • the final heating stage typically provides an increase of 18 °C to 25 °C.
  • the last flash stage 67 typically is a barometric flash stage where the vapour is condensed with cooling water 69 (see Figure 1 ). Its temperature drop is typically, but not always, about the same as the final heating stage. The condensing temperature in the last flash stage 67 is typically around 50 °C, governed by the cooling water circuit.
  • vapour stream 120 from the final flash vessel 67 is passed to a series of mechanical vapour compressors 122.
  • the stream 124 from the final compressor is at about 170 °C condensing temperature and is used to heat the final heater 68, reducing or eliminating the requirement to provide additional boiler generated steam 70 in this part of the refinery.
  • the invention is equally applicable to other locations in the Bayer circuit that use steam from a boiler that are characterised by having a heat input at a temperature of 130 °C or hotter, and waste heat rejection at 70 °C or colder.
  • This invention also applies to these evaporators whereby waste heat vapour can be recovered from the cold end, compressed and utilised at the hot end.
  • Figure 4 depicts how the present invention may be utilised in the evaporation circuit 60 of an existing alumina refinery with some optimisations over the embodiment depicted in Figure 3.
  • compressed water vapour may be exported from an intermediate stage in the compressor train for use in heating duties that only require a low final temperature such as caustic wash duties, causticisation duties and oxalate seed heating.
  • the present invention permits the utilisation of at least one additional stage of heating 130 before the last heater 68 further minimising capital and operating costs as shown in Figure 4.
  • the present invention further permits the utilisation of at least one additional flash vessel 132 before the last flash vessel 67 further minimising capital and operating costs.
  • vapour flow requiring compression 120 in the final stage is reduced, thus saving power and ultimately carbon emissions while maintaining the same final stage compressor output conditions.
  • the evaporator circuit is likely to produce plant steam in excess of requirements. Plant steam can be exported or imported to balance steam requirements.
  • the evaporators typically used in a refinery are fed at the coldest heater and discharge from the last flash vessel , however many evaporators are fed part way up the heater train and discharge part way up the heater train just before the inlet.
  • Figure 5 depicts how the present invention may be retrofitted into an existing alumina refinery at the precipitation stage 40.
  • the precipitation system typically requires cooling where the energy goes to waste either before the precipitators or part way down the precipitators.
  • the temperature drop is typically in the order of 20 °C. It is common to have one or more cooling stages within a precipitator bank. Cooling is typically by liquid slurry/liquid heat exchange as shown in Figure 1 .
  • the final heat exchange 44 and cooling tower 46 are replaced by at least one flash vessel in series 106 as described previously.
  • Water vapour 108 from at least one flash vessel 106 is compressed in a bank of mechanical vapour compressors 110 and used elsewhere in the refinery.
  • Figure 6 depicts further ways in which the present invention may be implemented. As an alternative to Figure 5, it is possible to flash the cooling water 142 circuit in a plurality of flash vessels to recover waste heat which is then compressed 144.
  • the digestion blow off heater can be made redundant.
  • the digestion blow off heater 162 can be made redundant. Reconfiguration of the heater train could enable an additional heater 164 before the last heater 90 heater.
  • the present invention can use the upstream components of calcination stack 170 heat recovery to generate hot water as shown in Figure 9.
  • the heat recovery process is described in Australian Patent 2009225953 the contents of which are incorporated herein by reference.
  • the water 172 from the scrubber 174 is flashed down in one or more flash tanks 176 to create water vapour 178 which is then compressed 180 to make plant steam 182 as shown in Figure 9.
  • the water can be flashed in one or more stages as previously described to minimise MVR capital and operating costs. Warm water 184 is also recovered.
  • the sum of all the vapour sources in the refinery can exceed the vapour required to be compressed to make sufficient plant steam for the refinery. If there is an excess, cooling water can be applied to a last stage flash vessel (typically the barometric flash vessel in evaporation) to remove excess waste heat vapour.
  • a last stage flash vessel typically the barometric flash vessel in evaporation
  • the refinery can be configured to not capture all waste heat vapour and the refinery balance is achieved by an alternative heating source, most likely a boiler.
  • Power load modulation capability is important to manage power costs, particularly in a high renewables grid.
  • the compressors draw a high load and significant costs can be saved if load is appropriately modulated.
  • Load modulation types a. Load reduction (partial or completely off) for periods of 5 minutes to 4 hours at a time to avoid peak load charges. Peak load charges generally apply when the grid is near peak power consumption.
  • the high inertial of the MVR compressors will allow “Raise Contingency Service” participation for short term events of up to 15 around seconds. For these events, the power load is increased or decreased for up to 15 s to assist in maintain power grid stability.

Abstract

A method for controlling the carbon output from an alumina refinery, the method comprising the steps of capturing water vapour from a flash train in the refinery, compressing the captured water vapour and utilising at least a portion of the energy in the captured water vapour to heat a process stream within the alumina refinery.

Description

Method for carbon reduction
TECHNICAL FIELD
[0001] A method for controlling carbon output from an alumina refinery
BACKGROUND ART
[0002] The Bayer process is a cyclic process where bauxite ore is digested in a hot caustic solution to dissolve the alumina bearing minerals. Undissolved solids are separated and the liquor cooled to precipitate aluminium hydroxide. The precipitated material is removed and calcined at approximately 1000°C to form alumina. The cooled Bayer liquor is reheated and recycled to the digestion stage. As much as possible, energy is captured and reused in the process to encourage energy efficient alumina production.
[0003] Alumina refining is energy intensive, on average approximately 10.7GJ of energy is required to produce 1 tonne of alumina globally. Approximately 70% of this energy is consumed in the dissolution and precipitation portion of the process with the remaining 30% being consumed in calcination. Therefore, energy is used as efficiently as possible to reduce costs and carbon dioxide emissions. Typically, energy is provided to alumina refineries through steam generated in boilers, this steam may be also be used to generate electricity for the refinery. The steam is generated by combustion of fossil fuels, commonly coal, but also natural gas and less commonly oil, contributing to carbon dioxide emissions.
[0004] Alumina refineries are known to use flash vessels to decrease the temperature of liquor streams within the refinery. A liquor stream will enter a flash vessel and the pressure reduced, generating a cooled liquor stream and pressurised water vapour. Typically, flash vessels are arranged in series in banks or trains. In fluid communication with the bank of flash vessels is generally a corresponding bank of heaters. Vapour flashed from a flash vessel is passed to a corresponding heater as is known in the art to re-use the energy present in the pressurised water vapour.
[0005] Flash vessels and banks thereof are known to be used at numerous locations in alumina refineries including: a. cooling of liquor from digestion prior to clarification; b. cooling of clarified liquor prior to precipitation; c. cooling of liquor from an evaporation circuit; and d. cooling within a precipitation circuit.
[0006] Common with these systems is that the liquor cools as it proceeds along the flash train. The flashed vapour exiting the final flash vessel in the series is generally too cool to heat any of the heaters and is considered unusable.
[0007] As a consequence of this lost energy and the reliance on fossil fuel generated steam to provide energy, fossil fuels represent a significant portion of total alumina production cost and produce significant greenhouse gas emissions. Alumina production using natural gas typically produces approximately 0.55 tonnes of carbon dioxide per tonne of alumina while alumina produced from an equivalent refinery using coal typically produces approximately twice the mass of carbon dioxide per tonne of alumina. Approximately 132 million tonnes of alumina were produced globally in 2019.
[0008] The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.
[0009] Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
[0010] Throughout the specification, unless the context requires otherwise, the word "solution" or variations such as "solutions", will be understood to encompass slurries, suspensions and other mixtures containing undissolved and/or dissolved solids.
[001 1] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referenced to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.
[0012] The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein. The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference.
SUMMARY OF INVENTION
[0013] In accordance with the present invention, there is provided a method for controlling the carbon output from an alumina refinery, the method comprising the steps of: capturing water vapour from at least one location in the refinery; compressing the captured water vapour and; utilising the energy in the captured water vapour to heat a process stream within the alumina refinery, wherein the captured water vapour is compressed in an electrically powered compressor wherein the electricity has a carbon intensity of less than 0.5 t CO2e/MWhr.
[0014] In accordance with the present invention, there is provided a method for the preparation of low carbon alumina from an alumina refinery, the method comprising the steps of: capturing water vapour from at least one location in the refinery; compressing the captured water vapour; and utilising the energy in the captured water vapour to heat a process stream within the alumina refinery, wherein the captured water vapour is compressed in an electrically powered compressor wherein the electricity has a carbon intensity of less than 0.5 t CO2e/MWhr.
[0015] In accordance with the present invention, there is provided a method of utilising low grade heat in an alumina refinery, the method comprising the steps of: capturing water vapour from at least one location in the refinery; compressing the captured water vapour; and utilising the energy in the captured water vapour to heat a process stream within the alumina refinery.
[0016] The method of utilising low grade heat in an alumina refinery may comprise the additional step of compressing the captured water vapour in an electrically powered compressor wherein the electricity has a carbon intensity of less than 0.5 t CC e/MWhr.
[0017] In accordance with the present invention, there is provided a method for controlling the carbon output from an alumina refinery, the method comprising the steps of: capturing water vapour from a flash train in the refinery; compressing the captured water vapour and; utilising the energy in the captured water vapour to heat a process stream within the alumina refinery.
[0018] In the context of the present specification, the term controlling shall be understood to include reducing the carbon output from an alumina refinery. The term reducing shall be understood to encompass reducing the carbon output relative to an equivalent alumina refinery utilising fossil fuel fired boiler steam as is known in the art. The term reducing shall also be understood to encompass the retrofit of an existing alumina refinery with the present invention as well as the construction of a new alumina refinery with the present invention. [0019] In the context of the present specification, the term carbon output shall be considered to refer to tonnes of carbon dioxide per tonne of alumina produced by the refinery.
[0020] In one form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 10 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 20 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 30 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 40 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 50 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 60 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 70 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 80 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 90 %. In an alternate form of the invention, the method of the invention decreases the carbon output of the alumina refinery by 95 %.
[0021] In one form of the invention, the electricity is generated from a renewable power source. Renewable power sources may include wind, solar, hydro, tidal, geothermal and biomass. In an alternate form of the invention, the electricity is generated from a zero carbon power source such as hydrogen or nuclear. The electricity may be stored in a battery after generation and prior to use.
[0022] It will be appreciated that the step of compressing the water vapour will increase both its temperature and pressure.
[0023] In one form of the invention, the captured water vapour is a source of water vapour that would not otherwise be utilised as an energy source in the alumina refinery.
[0024] Some water vapour streams are close to ambient conditions and do not have the temperature differential to do useful work within a refinery. Some water vapour streams are conventionally emitted to atmosphere due to a localised energy imbalance. Such water vapour streams are particularly suitable for the present invention. [0025] The present invention enables the energy in what was previously considered to be waste streams is to be harvested.
[0026] An alumina refinery contains numerous potential low energy or waste water vapour streams. Attractive water vapour streams are those from flash trains and in particular, water vapour streams from the final flash vessel in a flash train such as those found in the digestion circuit, the heat exchange circuit and the evaporation circuit.
[0027] In one form of the invention, the water vapour is captured from a flash vessel in the flash train.
[0028] In one form of the invention, the flash train comprises a plurality of flash vessels.
[0029] Where the water vapour is sourced from a flash vessel in a flash train, it is preferably sourced from the final or penultimate flash vessel in the train.
[0030] In one form of the invention, the process stream is a heater in a flash train. The flash train may be the same flash train from which the water vapour was sourced.
[0031] In one form of the invention, the method comprises the additional step of feeding the heated process stream to a flash train. Preferably, the step of feeding the heated process stream to a flash train comprises feeding at least a portion of the heated process stream to the flash train.
[0032] In one form of the invention, the train of heaters precedes the flash train.
[0033] In one form of the invention, the flash train is a heat recovery flash train.
[0034] In one form of the invention, the flash train comprises at least two flash vessels.
[0035] In one form of the invention, the water vapour is sourced from the final and/or penultimate flash vessel in the flash train.
[0036] In one form of the invention, the process stream is the final and/or penultimate heater in the train. [0037] In one form of the invention, the train of heaters comprises one or more heaters.
[0038] The water vapour may be sourced from a liquor flash system or a water flash system. Liquor flash systems may include digestion, heat interchange, evaporation and precipitation. Water flash systems may include precipitation external cooling and calcination flue gases.
[0039] The flash train may form part of the digestion circuit, the heat interchange circuit, the evaporation circuit, the precipitation circuit or a calcination heat recovery system in the alumina refinery.
[0040] In one form of the invention, the method comprises the further step of exporting excess compressed water vapour out of the flash train.
[0041] In one form of the invention, there are at least two heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least three heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least four heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least five heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least six heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least seven heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least eight heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least nine heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least ten heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least eleven heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least twelve heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least thirteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least fourteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least fifteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least sixteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least seventeen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least eighteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least nineteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are at least twenty heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery.
[0042] In one form of the invention, there are two heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are three heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are four heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are five heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are six heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are seven heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are eight heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are nine heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are ten heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are eleven heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are twelve heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are thirteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are fourteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are fifteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are sixteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are seventeen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are eighteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are nineteen heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery. In an alternate form of the invention, there are twenty heat recovery liquor flash and heating stages between the water vapour source and the process stream in the refinery.
[0043] Production of alumina from the Bayer process requires calcining precipitated aluminium hydroxide to aluminium oxide as shown below:
2AI(OH)3 + heat
Figure imgf000011_0001
[0044] Different types of calciner designs are used commercially with different fuel types (e.g. oil, gas, coal) and operating conditions such as excess air. Thus, a range of calciner flue gas temperatures and compositions may occur; and it is estimated that approximately 35% to 45% by mass % of the gas leaving calcination ('calciner flue gas') is water, with other components including carbon dioxide and volatile organic carbon compounds. Additionally, the calciner flue gas may contain entrained particulate alumina.
[0045] A significant quantity of heat is lost from the Bayer process by way of the calciner flue gas. It is estimated that the majority of the available heat is low-grade sensible heat or latent heat released when the water vapour in the flue gas is condensed. However, the latter can only be recovered as low grade heat at atmospheric conditions as the dew point is less than 100 °C, typically 80 to 83 °C. However, although significant in quantity, the low grade heat is conventionally understood to be of limited utility in the Bayer process.
[0046] In accordance with the present invention, there is provided a method utilising low grade heat in an alumina refinery, the method comprising the steps of: heating a fluid stream with calciner flue gas to provide a heated fluid stream; passing the heated fluid stream to a flash train; capturing water vapour from the flash train; compressing the captured water vapour; and utilising at least a portion of the energy in the captured water vapour to heat a process stream within the alumina refinery.
[0047] In one form of the invention, the water vapour is captured from a flash vessel in the flash train.
[0048] In one form of the invention, the flash train comprises a plurality of flash vessels.
[0049] In one form of the invention, the flash train comprises one flash vessel.
[0050] In one form of the invention, the step of heating a process stream comprises passing the process stream through at least one heater in a train of heaters.
[0051] In one form of the invention, the water vapour is sourced from the final and/or penultimate flash vessel in the flash train. [0052] In one form of the invention, the process stream is the final and/or penultimate heater in the train.
[0053] In one form of the invention, the train of heaters comprises one or more heaters.
[0054] Advantageously, the present invention also reduces water consumption in the alumina refinery.
[0055] The step of compressing the captured water vapour may be repeated. There may be a plurality of compression steps to attain the desired water vapour condensing temperature.
[0056] In a low temperature alumina refinery requiring water vapour to be compressed from about 50 °C to about 170 °C condensing temperature, about 13 low speed centrifugal compressors may be required in series. In a high temperature alumina refinery requiring water vapour to be compressed from about 50 °C to about 300 °C condensing temperature, about 20 low speed centrifugal compressors may be required. In a high temperature alumina refinery compressing waste blow off vapour at about 100 °C to 300 °C, about 15 low pressure compressors are required.
[0057] In one form of the invention, the water vapour from the flash train is less than 80 °C.
[0058] In one form of the invention, the step of compressing the captured water vapour is repeated to attain the desired water vapour condensing temperature.
[0059] In one form of the invention, the step of compressing the captured water vapour is performed by mechanical vapour recompression. Advantageously, mechanical vapour recompression reduces reliance on fossil fuel generated sources of heat to provide high condensing temperature steam.
[0060] Mechanical vapour compression may be performed by centrifugal compressors, axial flow compressors or turbo compressors. Centrifugal compressors may be classified as high speed or low speed compressors. Low speed centrifugal compressors typically around 3300 rpm. High speed centrifugal compressors typically operate around 6000 rpm and higher. High speed compressors generally provide a higher compression ratio. [0061] In one form of the invention, there are provided a plurality of mechanical vapour compressors in series wherein compressed steam from the first compressor in the series is passed to the second compressor for further compression. This process may be repeated until the steam is at the desired condensing temperature.
[0062] In one form of the invention, the captured water vapour is compressed in an electrically powered compressor wherein the electricity has a carbon intensity of less than 0.5 t CC MWhr.
[0063] In one form of the invention, the electricity has a carbon intensity of less than 0.4 t CO2e/MWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.3 t CO2e/MWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.2 t CO2e/MWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.1 t GC MWhr.
[0064] In one form of the invention, the electricity is generated from a renewable power source.
[0065] In one form of the invention, the electricity is generated from a zero carbon power source such as hydrogen or nuclear.
[0066] In one form of the invention, the electricity is stored in a battery after generation and prior to use.
[0067] The method of the invention may comprise the further step of: removing particulate matter from the captured water vapour, prior to the step of: compressing the captured water vapour.
[0068] Particulate matter may abrade the impeller of a high speed vapour compressor. This may be addressed by the use of vapour cleaning methods such as mist eliminators, scrubbers and/or cyclones optionally fitted with online washing system utilising a caustic solution to dissolve gibbsitic scale. Such an online wash system could be continuous or intermittent. [0069] The water vapour may contain caustic liquid droplets, water droplets and mist which can include small solid matter adhered to liquid droplets.
[0070] It will be appreciated that different water vapour sources may contain different particulate matter. For example, water vapour from digestion may contain, amongst others, bauxite residue, undigested bauxite, water vapour from precipitation may contain, amongst others, hydrate, water vapour from spent liquor may contain, amongst others, fine alumina and scale products, and water vapour from calcination may contain, amongst others, alumina.
[0071] Advantageously, the present invention may remove the need for the alumina refinery to rely upon fossil fuel generated heat sources.
BRIEF DESCRIPTION OF THE DRAWINGS
[0072] Further features of the present invention are more fully described in the following description of several non-limiting embodiments 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 schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;
Figure 2 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;
Figure 3 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;
Figure 4 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;
Figure 5 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;
Figure 6 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised; Figure 7 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;
Figure 8 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised; and
Figure 9 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised.
DESCRIPTION OF EMBODIMENTS
[0073] Those skilled in the art will appreciate that the invention described herein is amenable to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.
[0074] Figure 1 presents a flowsheet for an alumina refinery with a low digestion temperature in the range of 145° C to 190 °C. A high temperature digestion refinery in the range of 191 °C to 290 °C is the same as the low temperature digestion refinery but has more flash stages to achieve the same output liquor temperature. Some refineries heat the bauxite slurry with the liquor in the heat exchangers, whereas some heat the bauxite slurry and liquor separately. The digestion type does not change the application of invention; however, a higher compressor exit pressure is required to achieve the higher digestion temperatures.
[0075] Milled bauxite 12 is fed to a digester 14 with caustic solution and digested at 145° C to 190 °C in the known manner. The green liquor 16 is passed to a series of flash vessels 18 to gradually decrease the temperature and pressure. In Figure 1 there are depicted four flash vessels in the series. It will be appreciated that in practice, the number of flash stages required will depend upon the digestion temperature. The green liquor 20 exiting the final flash vessel 24, a blow off vessel, is near atmospheric pressure and about 110 °C. Flashed vapour 26 from each flash vessel is used to heat a corresponding heater (see below). Excess vapour 28 from the blow off vessel 24 cannot be used due to insufficient condensing capability in its associated heater(s). [0076] The flashed green liquor 20 is sent to a clarification stage 30 by which time it is cooled below the boiling point after which it passes to a heat interchange stage 32 to further reduce the temperature. The liquor enters 34 the heat interchange stage at about 103 °C and exits 36 at about 80 °C. The heat interchange stage 32 also comprises a series of flash vessels 38. In Figure 1 there are depicted four flash vessels in series. It will be appreciated that in practice, fewer or more flash vessels may be used.
[0077] The liquor exiting 36 the heat interchange stage 32 is at about 80 °C and passes to the precipitation stage 40 for precipitation of aluminium hydroxide. The liquor passes through a series of precipitators where the liquor temperature gradually decreases. This can be achieved by way of liquid/liquid heat exchangers, for example, plate heat exchangers 42, 44, two being shown in Figure 1 , although more or fewer may be utilised in practice. Cooling water 46 is used to cool the last heat exchanger 34.
[0078] Aluminium hydroxide precipitates from the cooled liquor in the precipitation circuit 40 and the solids removed (not shown) to provide a spent liquor 48. The spent liquor 48 is at about 60 °C and is recycled to digest further bauxite. Prior to recycling, the temperature of the spent liquor 48 must be increased up to the desired digestion temperature through a number of heating stages. It will be appreciated that different alumina refineries may operate different numbers or different types of heating stages. However, all alumina refineries recycle spent liquor and must increase its temperature. The first stage shown in Figure 1 is in the heat interchange stage 32. Flashed vapour 50 from the flash vessels 38 in the heat interchange stage 32 is used to heat a corresponding heater 52. The spent liquor 48 entering the heat interchange stage 32 is about 62 °C and exits 54 at about 83 °C. Importantly, there is a temperature differential of about 20 °C between the green liquor 34 entering the heat interchange stage and the spent liquor 54 exiting the heat interchange stage 32. There is also a temperature differential of about 20 °C between the green liquor 36 exiting the heat interchange stage and the spent Iiquor48 entering the heat interchange stage.
[0079] The spent liquor 54 exiting the heat interchange stage is sent to the evaporation stage 60 to increase its caustic content prior to recycling to digestion 14 as is known in the art. The evaporation stage 60 also comprises a series of heaters 62 and flash vessels 64. Their interrelationship is the same as described above for the digestion blow off stage 18 and the heat interchange stage 32. That is, flash vapour 66 from the flash vessel 64 is used to heat a corresponding heater 62. The final heater 68 in the evaporation stage uses a source of live steam 70 to attain the desired temperature. This is the only way to heat the liquor as the there is no flash vapour in any of the flash vessels that can heat the liquor to the desired condensing temperature. Live steam 70 is generated from fossil fuels and contributes to the carbon footprint of the refinery.
[0080] The temperature of the concentrated liquor exiting 72 the evaporation stage 60 is approximately equal to the liquor temperature entering the evaporation circuit 60 and enters the final heater 80 to attain the temperature necessary to digest further bauxite. The final heater 80 heats the liquor as described above. Flash vapour 28 from the flash vessel 24 is used to heat a corresponding heater 82.
[0081] Flash vapour 84 from the first flash vessel 86 can only heat the liquor to about 25 °C less than the digestion temperature. It is necessary to rely on live steam 88 to heat the liquor in the last heater 90 and attain the desired digestion temperature. Generation of live steam 88 with fossil fuels consumes energy and contributes to the carbon footprint of the refinery. The final liquor 92 is at about 160 °C (for a low temperature refinery) and enters the digester 14. It will be appreciated that for a high temperature refinery, a greater number of heaters and corresponding flash vessels would be required.
[0082] In some refineries, the spent liquor 48 is fed directly to the evaporation stage 60 instead of the heat interchange stage 32 and the evaporator feed 54 enters the coldest heater 94 and leaves the last flash vessel 48.
[0083] The present invention can be utilised in both existing refineries as described above and new refineries. It will be appreciated that use of the invention in a new refinery may provide more flexibility than retrofitting the invention in an existing refinery. In the former, there may be chemical and thermodynamic and even spatial limitations to the incorporation of the invention.
[0084] Figure 2 presents a flow sheet demonstrating how the present invention could be utilised in the design of a new refinery. A bank of mechanical vapour recompressors 100 are installed to compress the vapour from the flash vessel blow off 28. While only one compressor is depicted in Figure 2, in practice a bank of compressors in series would be utilised. The compressors compress the water vapour and provide a temperature increase of about 8-15 °C across each compressor for a typical low speed centrifugal compressor. The vapour 102 exiting the final compressor may be sent to the live heater 90 or the penultimate heater 91 . In some digestion processes, the vapour 102 exiting the final compressor may be directed to the digester.
[0085] Alternately the vapour could be compressed to a lower condensing temperature and directed to a heater prior to the heater 90 to save on compressor capital and operating costs.
[0086] The digested liquor 20 is sent to clarification as described above. Clarified liquor 34 at about 103 °C is sent to the heat exchange stage which in this case utilises liquid/liquid heat exchangers 104 instead of liquor flashing to cool the liquor. The cooled liquor 36 is sent to precipitation as described above. In Figure 2, the precipitation stage comprises a series of precipitators. There is one or more cooling stages in precipitation in which waste heat is produced. In this example one or more flash vessels 106 in series (with only one being depicted in Figure 2) flashes the liquor releasing cooled water vapour. This water vapour 108 (which would otherwise not be utilised) is passed through a series of mechanical vapour compressors 110 to increase its condensing temperature. The water vapour 1 12 exiting the final compressor is sent to the final heating stage 90 of the spent liquor being sent to digestion. This contributes to the ability to heat the spent liquor to digestion temperature by recovering energy available in the process and reducing or eliminating the requirement to provide additional boiler generated steam.
[0087] Figure 3 depicts how the present invention may be utilised in the evaporation circuit 60 of an existing alumina refinery. Depicted are a series of ten heaters 62 and ten flash vessels 64, although it will be appreciated that more or less may be used depending on the requirements of the refinery. Alumina refinery flash evaporation systems typically consist of 5 to 12 heating stages followed by the same number of flash stages. The last heating stage 68 typically uses live steam 70 in a low temperature digestion refinery, and low pressure steam sourced from digestion flash vessels in a high temperature digestion refinery. The final heating stage typically provides an increase of 18 °C to 25 °C. The last flash stage 67 typically is a barometric flash stage where the vapour is condensed with cooling water 69 (see Figure 1 ). Its temperature drop is typically, but not always, about the same as the final heating stage. The condensing temperature in the last flash stage 67 is typically around 50 °C, governed by the cooling water circuit.
[0088] The vapour stream 120 from the final flash vessel 67 is passed to a series of mechanical vapour compressors 122. The stream 124 from the final compressor is at about 170 °C condensing temperature and is used to heat the final heater 68, reducing or eliminating the requirement to provide additional boiler generated steam 70 in this part of the refinery.
[0089] By comparison with Figure 1 , no cooling tower 69 is necessary. Water vapour 120 exiting the final flash vessel 67 that is normally not utilised and treated via the cooling tower 69, is instead passed to a bank of mechanical vapour recompressors 120 (only one being shown). The water vapour exiting the final compressor is at live steam pressure and may actually be in excess of the requirements to heat the final heater. It is expected that about 90 % of the water vapour 126 will be used for heating the final heater 68 and the remaining 10 % recycled elsewhere in the refinery 128. The recycled vapour may need further compression.
[0090] The invention is equally applicable to other locations in the Bayer circuit that use steam from a boiler that are characterised by having a heat input at a temperature of 130 °C or hotter, and waste heat rejection at 70 °C or colder. This invention also applies to these evaporators whereby waste heat vapour can be recovered from the cold end, compressed and utilised at the hot end.
[0091] Figure 4 depicts how the present invention may be utilised in the evaporation circuit 60 of an existing alumina refinery with some optimisations over the embodiment depicted in Figure 3.
[0092] It is possible to optimise a compressor train to minimise capital and operating costs by importing and exporting vapour from stages within the compressor train. For example, compressed water vapour may be exported from an intermediate stage in the compressor train for use in heating duties that only require a low final temperature such as caustic wash duties, causticisation duties and oxalate seed heating.
[0093] The present invention permits the utilisation of at least one additional stage of heating 130 before the last heater 68 further minimising capital and operating costs as shown in Figure 4. [0094] The present invention further permits the utilisation of at least one additional flash vessel 132 before the last flash vessel 67 further minimising capital and operating costs.
[0095] By taking vapour from a penultimate flash stage 132, the vapour flow requiring compression 120 in the final stage is reduced, thus saving power and ultimately carbon emissions while maintaining the same final stage compressor output conditions.
[0096] The evaporator circuit is likely to produce plant steam in excess of requirements. Plant steam can be exported or imported to balance steam requirements. The evaporators typically used in a refinery are fed at the coldest heater and discharge from the last flash vessel , however many evaporators are fed part way up the heater train and discharge part way up the heater train just before the inlet.
[0097] Figure 5 depicts how the present invention may be retrofitted into an existing alumina refinery at the precipitation stage 40. The precipitation system typically requires cooling where the energy goes to waste either before the precipitators or part way down the precipitators. The temperature drop is typically in the order of 20 °C. It is common to have one or more cooling stages within a precipitator bank. Cooling is typically by liquid slurry/liquid heat exchange as shown in Figure 1 .
[0098] In the present embodiment, the final heat exchange 44 and cooling tower 46 (see Figure 1 ) are replaced by at least one flash vessel in series 106 as described previously. Water vapour 108 from at least one flash vessel 106 is compressed in a bank of mechanical vapour compressors 110 and used elsewhere in the refinery.
[0099] Figure 6 depicts further ways in which the present invention may be implemented. As an alternative to Figure 5, it is possible to flash the cooling water 142 circuit in a plurality of flash vessels to recover waste heat which is then compressed 144.
[00100] It is known that cooling via liquor to liquor heat exchange 150 between digestion and precipitation improves precipitation yield, but the approach temperature of the heat exchanger is typically limited to greater than 18 °C to avoid excess blow off vapour. An extension of this invention is to utilise liquor to liquor heat exchanges with a smaller approach temperature to increase the amount of blow off vapour as shown in Figure 7. It can also be enhanced by including heat recovery from condensate and/or calcination alumina coolers to achieve the lower approach temperature and thus additional blow off vapour.
[00101 ] If sufficient heat is recovered prior to the digestion blow off heaters then the digestion blow off heater can be made redundant.
[00102] Utilising multi stage vapour compressors enables vapour to be extracted for use in various utilities in the refinery without changing the energy balance within the flash train as represented in Figure 8. Extracting vapour from a flash train is possible today but this can upset the temperature profiled down the flash train, reducing efficiency. Extracting vapour at the pressure best matching the downstream utility needs reduces the capital investment in compressors and compression operating cost. As described in the evaporation optimisation, it is possible to utilise one or more additional heaters immediately before the last heater 90 (that is utilising plant steam) with vapour supplied from an intermediate compressor 160 to reduce the capital investment in compressors and compression operating cost.
[00103] As described above, the digestion blow off heater 162 can be made redundant. Reconfiguration of the heater train could enable an additional heater 164 before the last heater 90 heater.
[00104] The present invention can use the upstream components of calcination stack 170 heat recovery to generate hot water as shown in Figure 9. The heat recovery process is described in Australian Patent 2009225953 the contents of which are incorporated herein by reference. The water 172 from the scrubber 174 is flashed down in one or more flash tanks 176 to create water vapour 178 which is then compressed 180 to make plant steam 182 as shown in Figure 9. The water can be flashed in one or more stages as previously described to minimise MVR capital and operating costs. Warm water 184 is also recovered.
[00105] The sum of all the vapour sources in the refinery can exceed the vapour required to be compressed to make sufficient plant steam for the refinery. If there is an excess, cooling water can be applied to a last stage flash vessel (typically the barometric flash vessel in evaporation) to remove excess waste heat vapour.
Alternatively, the refinery can be configured to not capture all waste heat vapour and the refinery balance is achieved by an alternative heating source, most likely a boiler. [00106] Power load modulation capability is important to manage power costs, particularly in a high renewables grid. The compressors draw a high load and significant costs can be saved if load is appropriately modulated. Load modulation types: a. Load reduction (partial or completely off) for periods of 5 minutes to 4 hours at a time to avoid peak load charges. Peak load charges generally apply when the grid is near peak power consumption. b. Ancillary service “Spinning Reserve” participation by substantially reducing loads based on a grid under frequency signal. c. The high inertial of the MVR compressors will allow “Raise Contingency Service” participation for short term events of up to 15 around seconds. For these events, the power load is increased or decreased for up to 15 s to assist in maintain power grid stability.
[00107] Presented below is a summary of calculations demonstrating the benefit of the present invention. a. As a base case, a standard alumina refinery using natural gas to generate steam operating under the conditions of Figure 1 , generates 0.55 tonnes C02e/tonne alumina. b. The embodiment of Figure 2 operated under a present day electricity grid of 0.7 tonnes CC MWhr would generate 0.50 tonnes CC tonne alumina. c. The embodiment of Figure 2 operated under a zero carbon electricity grid would generate 0.15 tonnes CC e/tonne alumina.

Claims

22 CLAIMS
1 . A method for controlling the carbon output from an alumina refinery, the method comprising the steps of: capturing water vapour from a flash train in the refinery; compressing the captured water vapour; and utilising at least a portion of the energy in the captured water vapour to heat a process stream within the alumina refinery.
2. A method for controlling the carbon output from an alumina refinery in accordance with claim 1 , wherein the water vapour is captured from a flash vessel in the flash train.
3. A method for controlling the carbon output from an alumina refinery in accordance with claim 1 , wherein the flash train comprises a plurality of flash vessels.
4. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, comprising the additional step of feeding the heated process stream to a flash train.
5. A method for controlling the carbon output from an alumina refinery in accordance with claim 4, wherein the step of feeding the heated process stream to a flash train comprises feeding at least a portion of the heated process stream to the flash train.
6. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the step of heating a process stream comprises passing the process stream through at least one heater in a train of heaters.
7. A method for controlling the carbon output from an alumina refinery in accordance with claim 6, wherein the train of heaters precedes the flash train.
8. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the flash train is a heat recovery flash train. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the flash train comprises at least two flash vessels. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the water vapour is sourced from the final and/or penultimate flash vessel in the flash train. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the process stream is from the final and/or penultimate heater in the train. A method for controlling the carbon output from an alumina refinery in accordance with any one of claims 6 to 11 , wherein the train of heaters comprises one or more heaters. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the flash train forms part of the digestion circuit, the heat interchange circuit, the evaporation circuit, the precipitation circuit or a calcination heat recovery system in the alumina refinery. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the method comprises the further steps of: heating a fluid stream with calciner flue gas to provide a heated fluid stream; passing the heated fluid stream to a flash train and forming water vapour; A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims comprising the further step of exporting excess compressed water vapour out of the flash train. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the water vapour from the flash train is less than 80 °C. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the step of compressing the captured water vapour is repeated to attain the desired water vapour condensing temperature. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the step of compressing the captured water vapour is performed by mechanical vapour recompression. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein there are provided a plurality of mechanical vapour compressors in series wherein compressed steam from the first compressor in the series is passed to the second compressor for further compression. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, wherein the captured water vapour is compressed in an electrically powered compressor wherein the electricity has a carbon intensity of less than 0.5 t CO2e/MWhr. A method for controlling the carbon output from an alumina refinery in accordance with claim 20, wherein the electricity is generated from a renewable power source. A method for controlling the carbon output from an alumina refinery in accordance with claim 20, wherein the electricity is generated from a zero carbon power source such as hydrogen or nuclear. A method for controlling the carbon output from an alumina refinery in accordance with any one of claims 20 to 22, wherein the electricity is stored in a battery after generation and prior to use. A method for controlling the carbon output from an alumina refinery in accordance with any one of the preceding claims, comprising the further step of removing particulate matter from the captured water vapour prior to the step of compressing the captured water vapour.
PCT/AU2021/050855 2020-08-06 2021-08-05 Method for carbon reduction WO2022027100A1 (en)

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