AU2005237179A1

AU2005237179A1 - Process and plant for producing metal oxide from metal compounds

Info

Publication number: AU2005237179A1
Application number: AU2005237179A
Authority: AU
Inventors: Roger Bligh; Bernd Kerstiens; Michael Missalla; Guenter Schneider; Werner Stockhausen; Michael Stroeder
Original assignee: Outokumpu Oyj; Outokumpu Technology Oyj
Current assignee: Metso Corp
Priority date: 2005-11-25
Filing date: 2005-11-25
Publication date: 2007-06-14
Anticipated expiration: 2025-11-25
Also published as: AU2005237179B2; CN1990384B; CN1990384A; SA06270433B1

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): OUTOKUMPU TECHNOLOGY Oy Invention Title: PROCESS AND PLANT FOR PRODUCING METAL OXIDE FROM METAL COMPOUNDS The following statement is a full description of this invention, including the best method of performing it known to me/us: -1A- 0 SPROCESS AND PLANT FOR PRODUCING METAL OXIDE FROM METAL COMPOUNDS Technical Field c S The present invention refers to a process for producing for example a metal oxcN ide from metal compounds, in particular metal hydroxide, in which the metal compound is calcined in a reactor into metal oxide, wherein the exhaust gases produced in said process contain for example more than 30% water vapor, as well as to a corresponding plant.

Such a process for producing aluminium oxide (alumina) from aluminium hydroxide is known from EP 0 861 208 B1 using a circulating fluidized bed in which the aluminium hydroxide is first dried in several preheating stages, partly calcined and in the preheated condition introduced into a fluidized bed reactor in which alumina is generated, The exhaust gas of the fluidized bed reactor which is obtained during the combustion in the fluidized bed reactor is supplied to the preheating stages and its heat is utilized.

Especially in dry areas it is difficult and expensive to provide water used in such a process, e. g. the Bayer process for bauxite, for various purposes, e. g. upstream leaching andlor washing. For example metal hydroxide is usually provided as a filter cake which has to be washed with a considerable amount of clean process water.

From EP 1 063 472 Al there is known a method for cooling flue gases in the combustion of fuels. The flue gas is initially cooled in a condenser to form a condensate and further cooled in a heat exchanger so that the flue gas is utilized energetically. The condensate is disposed directly out into the sewage system.

Thus, the condensate is not used In the plant as process water.

-2- 0 Description of the Invention Therefore, it is the object of the present invention to provide a process and plant N 5 for example for the calcination of metal compounds optimizing the use of re- O sources of water and the use of thermal energies.

In accordance with the invention, this object is solved by a process as mentioned above in which the exhaust gas of the reactor and/or of a preheating stage being provided upstream of the reactor is condensed in at least one heat exchanger producing condensed water which is used in the process. Thus, by re-using the condensed water of the water vapor content in the exhaust gas of the plant the amount of fresh water used in the inventive process may be reduced. Hence, especially in areas with low resources of water and/or high costs for the water supply, the consumption of water may be reduced by avoiding the use of considerable amounts of fresh water. Further, by the condensation of water from the water vapor of the exhaust gas the thermal energy is used in heat exchanger(s).

Although this invention is described with reference to a process and plant for the calcination of metal compounds it is to be understood that the inventive concept of regaining water from water vapor of the exhaust gas may be used in different processes or plants having exhaust gases with a high content of water vapor for example more than 30% water vapor. For example the inventive concept may be used in general calcining processes, for drying of crystals, leaching residues, biomasses or burning processes of hydrogen or biomasses, prior to subsequent treatment, without leaving the scope of the present invention. The described process can be operated at atmospheric pressure but also at lower pressure below 1 bar and higher pressure from about 1 to 50 bar. It is also possible to l In accordance with a preferred embodiment of the inventive idea it Is proposed to N 5 cool the exhaust gas of the reactor in a first heat exchanger to a temperature 0 above the water vapor dew point, preferably to about 1100 C, and to further cool q-3- 0 the exhaust gas in a second heat exchanger, preferably a quench or spray coodecrease, to a the pressure of the gases or compress the water vapor dew point, preferably to about before effecting the treatment according to this invention.

In accordance wth a preferred embodiment of the inventive idea it Is proposed to CI 5 cool the exhaust gas of the reactor in a first heat exchanger to a temperature S above the water vapor dew point, preferably to about 110° C, and to further cool c-i the exhaust gas in a second heat exchanger, preferably a quench or spray cooler, to a temperature below the water vapor dew point, preferably to about 60° C to 90" C, for example to about 80' C, The exhaust gas of a reactor and/or a heat exchanger may contain between 40% and 100% water vapor, preferably between about 60% and about 100% water vapor, usually about 60 and 80% water vapor, more preferably between 80 and 100 with the water vapor dew point being preferably between 75 °C and 85"C at atmospheric pressure. The first heat exchanger may be a recuperator or a regenerator in which the exhaust gas of the second heat exchanger is re-heated to a temperature of preferably about 145° C such that the condensation of exhaust gas in a stack or directly above the stack may be avoided. As an alternative, the exhaust gas may be (re-)heated by a burner, by adding dry air and/or by adding pre-heated air.

To ensure that the condensed water is of sufficient purity for the reuse in the process for filtering or the like the condensed water and/or the condensed water together with the cooling water of the spray cooler is preferably subjected to a stripping process by introducing an air or gas jet to reduce the CO2 and/or SO 2 content in the condensed water. However, if the CO2 and/or SO 2 content is low enough to use the condensed water directly for washing in a hydrate filter the stripping process may be omitted. The necessity for the stripping process Is dependent on the fuel used.

Prior to the subsequent use of the condensed water, the water may be subjected to a cleaning or purification process, e. g. ion exchange or physical treat- -4- 0 c ments. Such a process may include treatment of the water in a filtration stage having a filter made from aluminium hydrate, alumina, activated carbon or the like. Further, the cleaning or purification process may include a treatment in a biological or chemical cleaning treatment, like oxidation with the aid of H 2 0 2 N 5 precipitation and the like. As an additional or as an alternative step, a physical O or other treatment may be used, like irradiation with ultraviolet light, x-Ray, elec- C trons etc. or e. g. magnetic fields or magnetic separation, electrolysis, ultrafiltration, electrophoresis. In addition or as an alternative, the condensed water may be subjected to a neutralization or an adjustment of pH-value with the aid of alkaline or acid components and/or an add-on of substances e. g. blozide or auxiliary materials for a processing like salts.

At least a part of the condensed water may be fed as a cooling agent in a heat exchanger, preferably a fluidized bed cooler. In this fluidized bed cooler the condensed water may be heated to a temperature suitable for downstream conveying equipment e. g. hydrate filtration, preferably to about 800 C. Whereas cooling water is required for the last cooling step, the condensed water may also be used to cool the alumina.to an intermediate temperature which may be around 120' C by indirect heat transfer to the condensed water which may at the same time be heated to a temperature between 90" C and 1000 C thus providing the best possible energy recovery from the alumina cooling and reducing the steam requirement for filter wash water preheating. After leaving the cooler, the heated condensed water may be used as washing water for the hydrate filtration.

In accordance with a further embodiment of the invention at least a part of the condensed water is fed as cooling, agent in a heat exchanger, preferably a countercurrent cooler, in which the exhaust gas of the reactor is additionally cooled.

The countercurrent heat exchanger is located downstream of said heat exchanger and upstream of the second heat exchanger, which is preferably a quench or spray cooler. The exhaust gas is cooled to about 90' C, but above 0 c the water vapor condensation temperature while the condensed water is further heated to about 95 "C in a separate heat exchanger. Thus, the exhaust gas of the reactor may be cooled in three cooling stages; A first heat exchanger which may be a recuperator, the cooling stage using the condensed water as cooling c- 5 agent and a third heat exchanger, preferably a spray cooler, to reduce the tem- 0 perature of the exhaust gas below the water vapor dew point. Thus, the cooling c capacity of the spray cooler may be reduced so that the cooling surface of an air cooler which may be provided for cooling the cooling agent of the spray cooler can be reduced, too.

In accordance with a further embodiment of this invention the water vapor containing exhaust gas from the reactor is cooled by a heat exchanger, e, g. a recuperator or regenerator, to a temperature below the dew point of the exhaust gas. This cooling can be effected by ambient air or other suitable heat exchange medias. In this heat exchanger water is condensed from the gas and may be further used for processing.

According to a further embodiment of this invention the exhaust gas may be cooled in a first step with heat transfer media to gain the higher valued energy, e. g. for production of low pressure steam or electricity. In a subsequent step cooling may be effected by one or more heat pumps and/or other heat exchangers as mentioned before.

According to a further embodiment of this invention the exhaust gas may be cooled in a first step by a heat pump or several subsequent heat pumps with different temperatures to gain higher valued energy, e. g. for production of low pressure steam or electricity. In these heat pumps no or nearly no water vapor condenses. The further cooling of the exhaust gas below the dew point may be effected by a heat exchanger as mentioned before.

0 -6- In cI In a further preferred embodiment a second heat exchanger, e. g. a spray cooler or a countercurrent cooler is used, which condenses a further part of the water vapor in the gas. This second condensate, which is usually of higher purity than Sthe water condensed in the first heat exchanger, may be mixed with the first 5 condensate or used separately for other processes.

SIn every embodiment of the present invention one or more heat exchangers may be used which condense more than one fraction of water with different purity and use the different condensates for different purposes. Especially in the embodiment with three heat exchangers it is possible, to condense a part of the water vapor in the second heat exchanger and another part in the third heat exchanger.

In addition or as an alternative to producing condensed water from exhaust gas of the reactor condensed water may be produced from the exhaust gas of a preheating or drying stage being provided upstream of said reactor. This preheating or drying stage may be a fluidized bed heat exchanger for drying and fluidizing hydrate by introducing a gas or a gas mixture, e. g. air, or steam through a nozzle grade in the preheating or drying stage such that water is evaporated and discharged together with exhaust gas from the preheating or drying stage. As the percentage of water in the exhaust gas of this preheating or drying stage may be between about 60% and about 100 the water vapor may be condensed with decreased heat exchange areas compared to the method described above. As an additional benefit of this preferred embodiment the energy consumption of the calcining process may be reduced as the hydrate is dried and preheated. As an example the process of heating and calcining of aluminium hydroxide described in Australian Patent 585 156 B (EP 0 245 751), may be improved by the present invention. Further, the temperature of the exhaust gas of the calcining reactor may be increased allowing the use of a further preheating -7- 0 c-I stage for the hydrate in the calcining plant. Thus, the specific energy consumption of the whole plant may be further reduced.

In accordance with a development of this inventive idea it is proposed that the S 5 exhaust gas of the preheating or drying stage is fed in at least one heat ex- 0 changer, preferably a countercurrent cooler, in which the exhaust gas of the preheating stage is cooled to a temperature below the water vapor dew point such that at least a part of the water vapor is condensed. In this heat exchanger process water for the hydrate filtration may be used as a cooling agent which is heated in the heat exchanger to about 950 C. As an alternative boiler feed water for the boiler plant which may be heated to about 60° C, hydrate or other suitable materials which can be heated may be used as a cooling agent, e. g. heat transfer media. The exhaust gas of the preheating or drying stage may then be fed to an exhaust gas cleaning stage, preferably an electrostatic filter and/or bag house filter, which can also be part of the main process.

It should be noted that according to the present invention in any of the above described embodiments at least one heat pump may be used instead of or in addition to a heat exchanger, e, g. to cool down the gases and/or to heat water to a necessary temperature. A heat pump may also be used to produce low pressure steam or electricity to use the water vapor and/or the hot gases energetically. Further, it is possible to replace one heat exchanger by several heat exchangers, By means of the process In accordance with the invention all kinds of metal hydroxide can be exposed to an effective heat treatment, in order to produce in particular metal oxides. The process and plant Is particularly useful for producing aluminium oxide (alumina) by calcining aluminium hydroxide and/or for producing magnesium oxide (magnesia) by calcining magnesium hydroxide.

-8-

I

A plant in accordance with the invention which is suited in particular for performing the above described process has a reactor in which water vapor containing gas is produced, e.g. a drying stage for moist hydrate, biomass or a stage for calcining a metal compound into metal oxide. Said reactor and/or a preheating C 5 stage being provided upstream of said reactor has an exhaust gas conduit connected to at least one heat exchanger for cooling the exhaust gas to a temperai ture below the water vapor dew point, wherein the heat exchanger has a conduit connected to the plant for reintroducing condensed water in the process. In other words, the conduit reintroduces the condensed water for further use as for example as boiler feed water Into a boiler plant or as process water into a filtration plant. Thus, the consumption of fresh water of this plant may be reduced which has especially in dry areas considerable advantages. At the same time the exhaust gas may be utilized energetically.

In a preferred embodiment hydrate surface moisture entering at a first pre-drying stage of the calcining process is reduced. This leads to a higher exhaust gas temperature which may be utilized for the further reduction of the specific energy consumption of the plant. Therefore a further pre-drying stage may be introduced in the process to adjust the exhaust temperature and to preheat the hydrate alumina.

To provide for a reliable cooling of the exhaust gas the exhaust gas conduit of the reactor is connected to a first cooling stage, preferably a recuperator or a regenerator, which is connected to a further cooling stage, preferably a spray cooler, which is connected to the conduit for reintroducing condensed water in the process. As an alternative the exhaust gas conduit of the reactor may be connected to a first cooling stage, preferably a recuperator, which is connected to a second cooling stage, which is connected to a further cooling stage, preferably a spray cooler, which is connected to a conduit for reintroducing condensed water as a cooling agent in the cooling stage, -9- 0 In In accordance with a preferred embodiment a conduit for condensed water leads from the heat exchanger to a fluidized bed cooler for heating the condensed water for filtration and at the same time cooling a metal oxide. Thus, hot oxide disc- 5 charged from the reactor may be cooled and in a further stage, hydrate at the filtration plant may be washed by use of the condensed water. Further, a stripc-I per may be provided upstream of the fluidized bed cooler and downstream of the heat exchanger to reduce the CO 2 and/or SO 2 content in the condensed water. In accordance with another embodiment a water cleaning stage may be used instead of or in addition to the stripper. This condensate cleaning stage may provide for a chemical, biological, mechanical or other treatment.

To provide for an effective condensation of the water vapor in the exhaust gas a further conduit for condensed water and/or cooling water leads from the heat exchanger via a pump and a cooling stage, preferably an air cooler, to the cooling agent inlet of the heat exchanger. Preferably, a mist collector is provided In said heat exchanger.

In accordance with the invention, an exhaust pipe leads from the spray cooler to the recuperator using the exhaust gas from the spray cooler as a cooling agent in the recuperator or regenerator. In addition to cooling the exhaust gas of the reactor this plant has the benefit of heating the exhaust gas from the spray cooler to reduce condensation of the exhaust gas in the stack or directly above the stack. This heating may also be effected e. g. by a bypass conduit of the exhaust gas to the stack without passing the condensation device with recuperator or regenerator, This may also be achieved by providing a burner upstream of or in the stack to heat the exhaust gases or by mixing heated air from the air cooler or dry air with the exhaust gases.

S c-io 0 SIf the inventive plant has three cooling stages for cooling the exhaust gas of the reactor it is preferred that a conduit for condensed water leads from the second cooling stage to hydrate filtration stage which is provided downstream of the reactor. Thus, the condensed water may not only be used to cool down the ex- C 5 haust gas of the reactor but also in a hydrate filtration stage.

In accordance with a preferred embodiment of the invention a fluidized bed heat exchanger is provided upstream of the reactor for preheating and drying hydrate wherein the exhaust pipe of the heat exchanger leads to a further heat exchanger using process water for the hydrate filtration as a cooling agent. In this heat exchanger the off gases from the fluidized bed are condensed. The exhaust gas pipe of this heat exchanger may be connected to an exhaust gas cleaning stage, preferably an electrostatic filter or a bag house filter.

Developments, advantages and possibilities for applying the present invention can also be taken from the following description of embodiments and from the drawings. All described and/or illustrated features per se or in any combination form the subject matter of the invention, independent of their inclusion in the claims or their back reference.

Brief description of the drawings Fig. 1 shows a process diagram of a process and a plant in accordance with a first embodiment of the present invention, Fig. 2 shows a process diagram of a process and a plant in accordance with a second embodiment of the present invention, Fig. 3 shows a process diagram of a process and a plant in accordance with a third embodiment of the present invention, In 0 Fig. 4 shows a process diagram of a process and a plant in accordance with a fourth embodiment of the present invention and c 5 Fig. 5 shows a process diagram of a process and a plant in accordance with a S fifth embodiment of the present invention.

O

Detailed description of the preferred embodiments.

For calcining metal hydroxide like aluminium hydroxide or magnesium hydroxide the plant 1 shown in Figure 1 has a reactor 2. Exhaust gas of said reactor 2 is fed via conduit 3 in a recuperator or regenerator 4 acting as a first cooling stage.

After leaving the recuperator 4 the exhaust gas of the reactor 2 is fed into a spray cooler 5 constituting a further cooling stage for the exhaust gas. In said spray cooler the exhaust gas is cooled to a temperature below the water vapor dew point such that condensed water is produced. In the spray cooler 5 exhaust gas of a stripper 6 is introduced which is provided downstream of the spray cooler in a conduit for the condensed water. The exhaust gas of the spray cooler is introduced as a cooling agent in recuperator 4. Thus, the exhaust gas of the spray cooler is (re-)heated prior to being discharged via stack 7. The heating of the exhaust gas prevents a visible plume at the stack. This can be further supported by installing a burner 19 in the stack or by adding heated gas 21 from air cooler 9 or by adding ambient air 20 from the environment.

In the stripper 6 ambient air or other gas is Introduced to reduce the CO 2 and/or

SO

2 content in the condensed water. The need for the installation of a stripper 6 Is defined by the fuel fed to reactor 2. Downstream of said stripper 6 the net amount of condensed water is fed via a water treatment stage 31 for adjusting the pH of the water as a cooling agent in a fluidized bed heat exchanger 18. Wa- S-12- 0 c N ter which is used as a cooling agent in the spray cooler 5 is reintroduced in the spray cooler via a pump 8 and a cooling stage which Is depicted as an air cooler 9 in Figure 1. In the spray cooler 5, there is provided a mist collector CI 5 In the fluidized bed heat exchanger 18 a fluidized bed is provided in which hot metal oxide like alumina or magnesia is introduced via line 24 from reactor 2. A c-I pre-cooling stage (not shown) may be provided in line 24, The metal oxide is cooled in the fluidized bed heat exchanger 18. At the same time the condensed water is heated in the heat exchanger to a temperature suitable for hydrate filtration or suitable to serve as boiler feed water. After leaving the fluidized bed heat exchanger 18 the condensed water may be used as a washing agent for hydrate filtration, As an alternative to the heat exchanger 9 a heat pump may be used to cool down the circulating water used in the spray cooler and to provide heat for the preheating of the water used in the heat exchanger 18.

The reactor 2 is provided with supply lines for hydrate 25, fuel 29 and combustion gas 30. The combustion gas 30 can be preheated, e. g. in the alumina cooling device (heat exchanger) or other devices. The fluidizing gas 32 used for the fluidized bed cooler 18 may also be used as combustion gas and may therefore be introduced in the reactor 2. In other words, the exhaust gas of the fluidized bed heat exchanger 18 may be introduced into reactor 2 as preheated combustion air via line Turning now to Figure 2, a plant 1' is depicted which defers from plant 1 as shown in Figure 1 in that a further cooling stage 11 is provided interposed between recuperator or regenerator 4 and spray cooler 5. In this second cooling stage 11 which is a countercurrent cooler exhaust gas from the reactor 2 is further cooled to a temperature which is however above the water vapor dew point.

Thus, after further cooling of the exhaust gas of the reactor 2 to a temperature below the water vapor dew point In spray cooler 5 condensed water is produced.

-13- 0 c After leaving the spray cooler 5 and an optional stripper 6 the net amount of condensed water is used as a cooling agent in the second cooling stage 11.

Downstream of the countercurrent cooler 11, the condensed water may be further heated and used as washing agent for hydrate filtration. Preferably the conc 5 densed water is heated in a fluidized bed cooler as described with respect to Figure 1. In this embodiment the recuperator 4 and/or heat exchanger 11 may cI be substituted by a heat pump to produce energy of a higher level e. g. for producing electricity.

In addition or as an alternative to the production of condensed water as described above, condensed water may be produced in a preheating stage 12 as shown in Figure 3. Said preheating stage 12 comprises a fluidized bed heat exchanger 13 which is provided upstream of reactor 2 for heating and drying hydrate. In said fluidized bed heat exchanger 13 hydrate which is introduced via line 25 is fluidized by Introducing air, steam or a gas as depicted by arrows 32 in Figure 3. Further, the hydrate is heated by a tubular type, plate type or bundle type heat exchanger which is provided within the fluldized bed heat exchanger 13. Thus, hydrate is heated such that water vapor is produced which is discharged together with the fluidization gas via conduit 14 from the fluidized bed heat exchanger 13. To control the temperature of water vapor in conduit 14 a part of the hydrate stream introduced via line 25 may be bypassed via stream A heat transfer medium 28, e. g. water or oil, used in the heat exchanger 13 is preferably heated up in a fluidized bed cooling system 26 for cooling down alumina which is located downstream of said reactor 2.

Said conduit 14 leads the exhaust gas of the fluidized bed heat exchanger 13 into a further heat exchanger 15 which is a countercurrent heat exchanger in a preferred embodiment. In this heat exchanger 15 the exhaust gas of the preheating stage 12 is cooled to a temperature below the water vapor dew point such that condensed water is discharged via conduit 16. The exhaust gas is 0 O -14cN then directed to an exhaust gas cleaning stage (not shown), preferably an electrostatic filter and/or a bag filter, via exhaust gas pipe 17. As a cooling agent process water, for example boiler feed water, may be used. As an alternative, air may be used as a cooling agent in heat exchanger 15. The condensed water C 5 supplied by conduit 16 may be used as washing water for hydrate filtration.

SThe dried and preheated hydrate flows via a conduit 24 to a first preheating stage 22 into which an exhaust gas is introduced, The preheating stage 22 may comprise a venturi dryer, an annular fluidized bed, a pneumatic conveyor or a heat exchange cyclone. Gas and solids discharged from the first preheating stage 22 are separated in a gas cleaner 23 e. g. a cyclone, an electrostatic precipitator and/or a bag filter, and the exhaust gas is then directed to a stack 7.

The exhaust gas may preferably be directed to stack 7 via at least one heat exchanger and/or at least one heat pump in which the exhaust gas is condensed to produce condensed water. The preheated hydrate flows through conduit 24' into a second preheater 22' and to a second gas cleaner or separation device 23' g. cyclone or electrostatic precipitator and/or bag filter) from which the solids are fed into the reactor 2. In the reactor 2 the preheated solids are calcined and fed via a further gas cleaner 23" into the alumina cooling stage 26 of the plant. The cooled alumina leaves the plant via conduit 27. The exhaust gas of the further gas cleaner 23" is introduced via conduit 3" into the second preheating stage 22', whereas the exhaust gas of the second gas cleaner 23' is introduced into the first preheating stage 22.

As an alternative to the embodiment shown in Figure 3 the preheating of hydrate may be effected in three preheating stages as shown In Figure 4. Further to the drying stage 12, the first preheating stage 22 with gas cleaner 23 and the second preheating stage 22' with gas cleaner 23' a third preheating stage 23" is provided together with a third gas cleaner 23".

0 c The third preheating stage 22" is located downstream of the second gas cleaner 23' such that solids are fed via conduit 24" into the third preheating stage 22".

Downstream of the third preheating stage 22" there is provided the third gas cleaner 23" from which preheated solids are introduced into reactor 2. The ex- N S haust gas of the reactor 2 is fed together with entrained solids into the further O gas cleaner 23"' from which exhaust gas is fed via conduit into the third pre- CN heating stage 22".

Figure 5 depicts another embodiment of the invention which differs from the embodiment shown in Figure 1 in that the exhaust gas of a mist collector 10 is fed via conduit 14 directly into stack 7. Ambient air which is introduced via conduit 20 into heat exchanger 4 for preheating is mixed downstream of the heat exchanger 4 with the exhaust gas of the mist collector 10 to prevent a plume in or directly above the stack. The condensed water may be fed through an optional stripper 6 and/or an optional water treatment stage 31, e. g. a biological waste water treatment with diaphragm sewage plant, Example I (production of alumina) Preheated and dried hydrate is introduced In the reactor 2 of Figure 1 in which the aluminium hydroxide is calcined into alumina. Hot alumina is discharged to the fluidized bed heat exchanger 18 via conduit 24 and cooled to a temperature of about 95" C whereas at the same time water is heated in the heat exchanger 18 to a temperature suitable for hydrate filtration.

The exhaust gas of reactor 2 having a temperature of about 162" C and usually to 95 water vapor, preferably 80 to 95 water vapor, is cooled in recuperator 4 to a temperature of about 110° C, The exhaust gas is further cooled in spray cooler 5 to a temperature of about 73" C, i.e. to a temperature below the water vapor dew point such that a required amount of condensed water Is pro- -16- 0 c duced. As a cooling agent water having a temperature of about 68" C is introduced in spray cooler 5. The spray cooler may be equipped with carrier material to improve the separation/condensation of water.

C 5 The exhaust gas of the spray cooler 5 is discharged at a temperature of about 0 73' C and introduced as a cooling agent in recuperator 4. In recuperator 4 the N exhaust gas of the spray cooler 5 is heated to a temperature of about 145" C such that condensation of the exhaust gas in stack 7 is avoided.

The condensed water produced in spray cooler 5 is fed in the stripper 6 to reduce the CO 2 and/or SO 2 content by introducing a jet of ambient air or gas.

Downstream of said stripper 6 the net amount of condensed water is fed to a water treatment plant 31 which cleans the condensed water with an ion exchange resin and adjusts the pH to 8. Then the cleaned water is fed as a cooling agent in the fluidized bed heat exchanger 18. The condensed water heated in the fluidized bed heat exchanger 18 may be used as a washing agent in a hydrate filtration stage.

The waste gas from the recuperator 4 can optionally be mixed with hot air from the air cooler 9 or ambient air to reduce the dew point of the waste gas in the stack 7. Furthermore, the waste gas may also be heated up by a burner 19 to prevent a visible plume at the stack.

Example 2 Referring now to Figure 2, waste gas with a temperature of about 1500 C to about 170* C and a water content of about 40 to about 70, usually of about 50 to about 60 vol-% is introduced via conduit 3 in recuperator 4. In recuperator 4 the waste gas is cooled down to a temperature of about 100* C to about 130° C by exchanging heat with the waste gas from the spray cooler 5. Thus, the waste -17- 0 C gas from the spray cooler 5 is heated up to a temperature of about 130" C to about 150° C.

The waste gas cooled in recuperator 4 is then further cooled down in heat exchanger 11 to a temperature of about 90* C to about 1200 C by exchanging heat with condensate which is fed in the heat exchanger 11 via pump 8. Thus, said C condensate is heated up in the heat exchanger to a temperature of about 85° C to about 950 C.

Waste gas from the heat exchanger 11 is introduced in the spray cooler 5. In the spray cooler 5 this waste gas is cooled down to the dew point temperature of about 70" C to about 75" C. In the spray cooler 5 the net condensate rate is about 0.18 to about 0,22 kg (Nm 3 waste gas introduced in recuperator Further, the water content in the waste gas is removed in the spray cooler 5 to about 30 to about 40 vol-%.

The condensate from the spray cooler 5 has a temperature of about 80" C to about 850 C. Dissolved gases like CO 2

SO

2 or NOx in the condensate may optionally be stripped out in stripper 6 and/or to a water treatment device. The stripping gas flow is about 0 to 0,02 Nm 3 (Nm 3 waste gas introduced in recuperator 4).

The condensate flow into the spray cooler is about 9 to 11 kg (Nm 3 waste gas Introduced in recuperator Said condensate is cooled down in air cooler 9 from a temperature of about 80' C to about 85" C to a temperature of about 3" C to about 6* C below the temperature of the waste gas discharged from the spray cooler The waste gas from the recuperator 4 may optionally be mixed with hot air from the air cooler 9 or ambient air to reduce the dew point of the waste gas in the -18- 0 stack 7. The hot air flow may be about 0 to 1 Nm 3 (Nm 3 waste gas introduced in recuperator 4) and the resulting dew point is about 75' C to about 55" C, Furthermore, the waste gas may also be heated up by a burner to prevent a visible plume at the stack, C- Example 3 Surface moist hydrate (aluminium hydroxide 5 10 wt-% moisture) is introduced in fluidized bed heat exchanger 13 of preheating stage 12 depicted in Figure 3.

An amount of 0 to 5 Nm3/(t hydrate) of a gas, steam or a gas mixture 32 is introduced in the fluidized bed nozzle grade of the fluidized bed heat exchanger 13.

The hydrate is preheated in preheating stage 12 such that exhaust gas having a temperature of about 100" C to about 110" C, in particular about 1030 C, and containing more than 60%, in particular between about 90% and about 100% water vapor, is discharged via conduit 14. Depending on the plant load and/or the hydrate moisture a part stream may be by-passed to achieve the required exhaust temperature.

About 0,4 to 0,6 t/(t hydrate) water vapor and air are introduced via conduit 14 in the heat exchanger 15. As a cooling agent process water having a temperature of 55" C is introduced via line 33 in the heat exchanger 15 and heated to a temperature of about 95" C. By cooling the exhaust gas of the preheating stage 12 to a temperature below the water vapor dew point condensed water having a temperature of about 800 C to about 1000 C is produced and discharged via conduit 16. The condensed water is used in a hydrate filtration stage. The temperature of the off gas discharged from heat exchanger 15 via line 17 may be controlled by amending the amount of water Introduced in line 33 and the flow rate of condensed water in conduit 16.

0 cExample 4 The off gas from reactor 2 of Figure 5 with 50 to 90 vol% water vapor, usually vol% water vapor, is entering the recuperator 4 with a temperature of 162" C N 5 where the off gas is cooled to 73° C, which is below the water dew point in this O example. The off gas is cooled with ambient air 20 which is introduced into the recuperator at a temperature of about 35* C and leaves the recuperator at a temperature of about 140* C which may be controlled by amending the amount of air which Is blown through the recuperator.

The condensed water in the recuperator 4 is separated from the off gases and then pumped through a heat exchanger bundle, preferably in a fluid bed cooler 18, where it is heated up to 950 C by hot alumina. Then the condensed water is used e, g. for washing purposes in the hydrate filtration (not shown).

After separation of the condensed water including droplets from the off gas, the exhaust off gases are introduced into the stack together with the ambient air used for cooling the off gas in the recuperator 4. The off gases leave the stack with a temperature of about 80 *C to about 120 Due to the dilution with the ambient air condensation of the exhaust gases in the stack or directly above the stack is prevented.

m 19A 0 z In the claims which follow and in the preceding description of the invention, Ivn except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or S"comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features Sin various embodiments of the invention.

in It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

List of numerals 1,1' 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22, 23, 24 25' plant reactor exhaust gas conduit recuperator spray cooler stripper stack pump air cooler mist collector second cooling stage (countercurrent cooler) preheating stage fluidized bed heat exchanger gas conduit heat exchanger liquid conduit exhaust gas pipe fluidized bed heat exchanger burner dry air inlet conduit for preheated gas preheater gas cleaner solid conduit hydrate inlet bypass stream 22', 22" 23', 23" -21-

O

c 26 alumina cooling stage 27 alumina outlet 28 heat transfer media 29 fuel inlet c 5 30 oxygen containing gas line 0 31 water treatment stage cN 32 fluidizing gas 33 cold condensate (water) flow 34 warm condensate (water) flow 35 solid conduit for alumina 36 gas conduit for stripper 37 water conduit

Claims

2. The process as claimed in claim 1, characterized in that exhaust gas of the reactor containing more than 30%, preferably between 40% and water vapor is cooled in a first heat exchanger to a temperature above the water vapor dew point, preferably to about 1100 C, and that the exhaust gas is further cooled in a second heat exchanger preferably a quench or spray cooler to a temperature below the water vapor dew point, preferably to about 800 C.
3. The process as claimed in claim 1 or 2, characterized in that the con- densed water and/or the condensed water together with cooling water of a spray cooler is subjected to a stripping process by introducing an air or gas jet to reduce the CO 2 and/or SO 2 or other impurities content in the condensed water.
4. The process as claimed in any of the preceding claims, characterized in that prior to the subsequent reuse of the condensed water in the process the water is subjected to a cleaning or purification process. -23 0 The process as claimed in any of the preceding claims, characterized in that at least a part of the condensed water is fed as a cooling agent in a heat exchanger, preferably in a fluidized bed heat exchanger (18). cl 5 6. The process as claimed in claim 4, characterized in that in the fluidized bed heat exchanger (18) metal oxide is cooled preferably to about 95° C, cI whereas the condensed water is heated to a temperature suitable for hydrate filtration, preferably to about
7. The process as claimed in any of the claims 1 to 5, characterized In that at least a part of the condensed water Is fed as a cooling agent in a heat ex- changer, preferably a countercurrent cooler in which the exhaust gas of the reactor is cooled.
8. The process as claimed in any of the preceding claims, characterized in that in a preheating or drying stage preferably a fluidized bed heat ex- changer being provided upstream of said reactor hydrate is heated and fluidized by introducing a gas or gas mixture in the preheating or drying stage (12) such that water is evaporated and discharged together with exhaust gas from the preheating or drying stage (12).
9. The process as claimed in claim 8, characterized in that the percentage of water in the exhaust gas of the preheating stage (12) is between about and about 100%. 26 The process as claimed in any of the claims 8 or 9, characterized in that the exhaust gas of the preheating or drying stage (12) is fed In a heat ex- changer, preferably a countercurrent cooler in which the exhaust gas of the preheating or drying stage (12) is cooled to a temperature below the water vapor dew point such that at least a major part of the water vapor Is condensed. 1 -24- 0 In
11. The process as claimed in any of the claims 8 to 10, characterized in that process water for the hydrate filtration is used as a cooling agent in the heat exchanger (15) and that the process water is heated in the heat exchanger c 5 to about 95" C. O S12. The process as claimed in any of the claims 8 to 11, characterized in that the exhaust gas of the preheating stage (12) is fed to an exhaust gas clean- ing stage, preferably an electrostatic filter or a bag filter.
13. The process as claimed in any of the claims 1 to 12, characterized in that after condensation of the exhaust gas the latter is (re-)heated to prevent a visible plume at the stack
14. The process as claimed in claim 13 characterized in that the exhaust gas is (re-)heated by a burner by adding dry air (20) and/or by adding pre- heated air (21). The process as claimed in claim 12, characterized in that as starting ma- terial aluminum hydroxide or magnesium hydroxide is supplied in the reactor (2) which is calcined in the reactor into aluminium oxide (alumina) and magne- slum oxide (magnesia) respectively.
16. A plant for performing a process as claimed in any of claims 1 to 15 com- prising at least one reactor and/or at least one preheating stage (12) which are configured such that exhaust gas containing water vapor is produced and further comprising at least one heat exchanger, characterized in that said reac- tor(s) and/or said preheating stage(s) (12) being provided upstream of said reactor has an exhaust gas conduit 14) connected to a heat exchanger 5, 11; 15) for cooling the exhaust gas to a temperature below the water vapor 0 c dew point, and that the heat exchanger 5, 11; 15) has a conduit (16) con- nected to the plant for reintroducing condensed water in the process.
17. The plant as claimed in claim 16, characterized in that the exhaust gas conduit of the reactor is connected to a first cooling stage, preferably a recuperator or regenerator, which is connected to a further cooling stage, Spreferably a spray cooler which is connected to the conduit for reintroducing condensed water in the process.
18. The plant as claimed in any of claims 16 to 17, characterized In that a conduit for condensed water leads from the heat exchanger to a fluidized bed heat exchanger (18) for cooling metal oxide and/or heating condensed wa- ter which heat exchanger (18) is provided upstream of a hydrate filtration stage and downstream of the reactor
19. The plant as claimed in claim 18, characterized in that a stripper (6) and/or a cleaning stage (31) is provided upstream of the fluidized bed heat ex- changer (18) and downstream of the heat exchanger for producing con- densed water. The plant as claimed in any of claims 16 to 19, characterized in that a further conduit for condensed water leads from the heat exchanger for pro- ducing condensed water via a pump and a cooling stage, preferably an air cooler to the cooling agent inlet of said heat exchanger
21. The plant as claimed in any of claims 16 to 20, characterized in that a mist collector (10) is provided in the heat exchanger for cooling the exhaust gas of the reactor -26- 0
22. The plant as claimed in any of claims 17 to 21, characterized in that an exhaust gas conduit leads from the spray cooler to the recuperator or re- generator using the exhaust gas from the spray cooler as a cooling agent in Sthe recuperator or regenerator.
23. The plant as claimed in claim 16, characterized in that the exhaust gas conduit of the reactor is connected to a first cooling stage, preferably a recuperator which is connected to a second cooling stage which is connected to a further cooling stage, preferably a spray cooler which is con- nected to a conduit reintroducing condensed water as cooling agent in the sec- ond cooling stage (11).
24. The plant as claimed in claim 23, characterized in that a conduit for con- densed water leads from the second cooling stage (11) to a hydrate filtration stage which is provided downstream of the reactor The plant as claimed in claim 16, characterized in that a fluidized bed heat exchanger (13) of a preheating stage (12) is provided upstream of said re- actor for heating hydrate, and that the exhaust pipe (14) of this heat ex- changer (13) leads to a further heat exchanger (15) using process water for the hydrate filtration as a cooling agent.
26. The plant as claimed in claim 25, characterized in that the exhaust gas pipe (17) of the heat exchanger (15) leads to an exhaust gas cleaning stage, preferably an electrostatic filter or a bag filter. 27
27. The plant as claimed in any of claims 16 to 26, characterized in that the O z stack is provided with a burner for (re-)heating the exhaust gas. SDated this 25th day of November 2005 SOUTOKUMPU TECHNOLOGY Oy t- cBy their Patent Attorneys SGRIFFITH HACK 0 10 Fellows Institute of Patent and Trade Mark Attorneys of Australia