AU3252499A

AU3252499A - Method for final cooling anhydrous aluminum oxide

Info

Publication number: AU3252499A
Application number: AU32524/99A
Authority: AU
Inventors: Klaus Janssen; Martin Rahn; Hans-Werner Schmidt
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-02-13
Filing date: 1999-02-02
Publication date: 1999-08-30
Anticipated expiration: 2019-02-02
Also published as: EP1053211A1; CN1290234A; BR9907869A; WO1999041199A1; DE19805897C1; AU747716B2

Description

Translation of PCT/EP99/00670 Method for the Final Cooling of Anhydrous Alumina Description This invention relates to a method for the final cooling of anhydrous alumina, which was prepared from aluminum hydroxide in a circulating fluidized bed, where the final cooling is effected in a fluidized-bed cooler which consists of two cooling stages arranged in series, which are each divided into several cooling chambers, where in the first cooling stage the fluidizing gas to be supplied to the fluidized-bed reactor is heated, which fluidizing gas is introduced as pri mary gas into the first cooling stage, and in the second cooling stage the anhydrous alumina is cooled against a liq uid heat-transfer medium, which is guided in a countercurrent flow. Methods for the final cooling of anhydrous alumina are known. The DE-OS 195 42 309 discloses a method of producing anhy drous alumina from aluminum hydroxide in a circulating fluid ized bed formed of a fluidize~d-bed reactor, a separator and a return line, where the aluminum hydroxide is introduced into the second stage on the gas side of a two-stage suspension preheater operated with the exhaust gases of the fluidized bed reactor of the circulating fluidized bed and is at least -2 partly dehydrated, dehydrated aluminum hydroxide from the second stage of the suspension preheater is introduced into the first stage on the gas side of a suspension preheater op erated with the exhaust gases of the fluidized-bed reactor of the circulating fluidized bed and is furthermore dehydrated and subsequently supplied to the circulating fluidized bed, which is operated with oxygen-containing fluidizing gas indi rectly heated in a subsequent cooling stage by the alumina produced and with indirectly heated, oxygen-containing secon dary gas supplied at a higher level, where the indirect heat ing of the fluidizing gas is effected in a fluidized-bed cooler. Upon leaving the last suspension cooler, the anhy drous alumina obtained undergoes a final cooling in a fluid ized-bed cooler equipped with three cooling chambers. In the first chamber thereof, the fluidizing gas supplied to the fluidized-bed reactor is heated, in the two subsequent cham bers the same is cooled against a heat-transfer medium, pref erably water, which is guided in a countercurrent flow. The disadvantage of this method consists in that the tempera ture at which the product enters the water-cooled portion is relatively high, which leads to the fact that a relatively large amount of thermal energy of the product gets into the cooling circuit of the water and cannot be recirculated to the process. It is therefore the object underlying the invention to create a method for' the final cooling of anhydrous alumina, where the thermal energy withdrawn from the anhydrous alumina is recovered almost complete-ly and can be reused in technical processes. Retrofitting the process into already existing plants should in addition be possible in a relatively easy way. The object underlying the- invention is solved in that the dispersion containing anhydrous alumina, which has been with- - 3 drawn from the first cooling stage, is passed through a cy clone, and that the anhydrous alumina withdrawn in the lower portion of the cyclone is subsequently directly introduced into the second cooling stage. Before entering the fluidized bed cooler, the anhydrous alumina has a temperature of 800 to 1200 0 C and is technically anhydrous, i.e. it has a water con tent of merely 0.1 to 1 wt-%. The primary gas introduced into the first cooling stage is used as heat-transfer medium and thus as coolant of the first stage. The cooling stages are designed such that they consist of several cooling chambers, each of which is charged with secondary gas as fluidizing gas, so that in each cooling chamber a fluidized bed is formed. In general, 2 to 6 cooling chambers are provided in each cooling stage. As liquid heat-transfer medium of the second cooling stage water can be used to a particular advan tage. The disperse phase of the dispersion is solid and is formed by the anhydrous alumina. The dispersion phase is gaseous and is formed by air. The dispersion itself, which is formed by the anhydrous alumina and air, thus has a dust-like character. The cyclone disposed between the two cooling stages advantageously acts as cooling cyclone for the final cooling, where an indirect heat exchange is effected. Sur prisingly, it turned out that with the method for the final cooling of anhydrous alumina it is possible to almost com pletely reuse the thermal energy withdrawn from the anhydrous alumina in the fluidized-bed cooler in technical processes, so that only very small thermal losses are observed. These thermal losses merely include that thermal energy which in the second cooling stage is transferred into the circuit of the liquid heat-transfer medium. However, these thermal losses are small, as the method for the final cooling pro vides for reducing the inlet temperature of the anhydrous alumina at the inlet of the fluidized-bed cooler below 450 0 C. Such reduction of the temperature at the inlet of the fluid ized-bed cooler would also be possible when the first cooling stage would accordingly be designed larger. However, it would

RAQL

-4 be disadvantageous that corresponding to the larger design of the first cooling stage larger amounts of primary gas would have to be passed through the same as heat-transfer medium, which would lead to the fact that for the technical proc esses, to which the thermal energy withdrawn should be sup plied advantageously, too large an amount of heated primary gas would be available. For instance, for adjusting the flu idized bed in the reactor of the circulating fluidized bed, much too much fluidizing gas would be present, which is formed by the primary gas. The method for the final cooling thus provides for the advantageous reduction of the tempera ture of the anhydrous alumina directly at the inlet of the second cooling stage of the fluidized-bed cooler, without having to use major amounts of primary gas as heat-transfer medium of the first cooling stage. Since most of the thermal energy withdrawn from the anhydrous alumina is transferred into gases, it is relatively easily possible to reuse the thermal energy withdrawn in technical processes, by supplying the heated gases to the technical processes. It is, for in stance, advantageous to supply the hot gases withdrawn from the cyclone to a technical process and thus utilize the ther mal energy of such gas. In terms of construction, the ar rangement of the cyclone between the two cooling stages can be effected relatively easily, so that already existing plants can be retrofitted relatively easily. A preferred aspect of the invention consists in that the gases withdrawn from the central tube of the cyclone are re circulated to at least one cooling cyclone of the precooling. This ensures that a particularly large amount of thermal en ergy, which is withdrawn from the anhydrous alumina in the cyclone, can fo? instance advantageously be recirculated to the process in the circulating fluidized bed as technical process. In accordance with a further aspect of the invention the gases are mixed with secondary gas withdrawn from the fluidized-bed cooler before being introduced into the fluidized bed reactor. Thus, the thermal energy of the secondary gas can likewise advantageously be recirculated to the process in the circulating fluidized bed, where it is ensured at the same time that due to the common transport of secondary gas and the gases withdrawn from the central tube of the cyclone tubes can be saved as compared to a separate transport. In accordance with a further preferred aspect of the inven tion the anhydrous alumina is passed through three cooling chambers in each cooling stage. The arrangement of three cooling chambers each provides for a homogeneous cooling of the anhydrous alumina while at the same time optimizing the space required for the fluidized-bed cooler. The arrangement of three cooling chambers each ensures a particularly advan tageous release of thermal energy from the anhydrous alumina. A further preferred aspect of the invention consists in that the secondary gas passed through the second cooling stage to gether with the dispersion containing anhydrous alumina, which has been withdrawn from the first cooling stage, is passed through the cyclone. It is particularly advantageous that the thermal energy of the secondary gas of the second cooling stage can be supplied particularly easily to the di rect heat exchange in the cyclone. The subject-matter of the invention will subsequently be de scribed in detail and by way of example with reference to the drawing (Fig. 1 to Fig. 3). Fig. 1 shows a simplified schematic process flow diagram of the method for the final cooling of anhydrous alumina. Fig. 2 shows a simplified schematic process flow diagram of a variant of the method for the final cooling of anhydrous alu mina.

-6 Fig. 3 shows a simplified schematic process flow diagram of a known method for the final cooling of anhydrous alumina in accordance with the prior art. Fig. 1 represents a simplified schematic process flow diagram of the method for the final cooling of anhydrous alumina. The anhydrous alumina withdrawn from the fluidized-bed reactor is supplied in the form of a dispersion via line 1 into a cool ing cyclone 2 for the actual cooling. The cooling cyclone 2 may consist of several cooling cyclones arranged in series. The gas withdrawn from the central tube of the cooling cy clone 2 is discharged via line 3 and possibly introduced into the reactor of the circulating fluidized bed. The cooled an hydrous alumina is delivered via line 4 into the first cool ing stage 5a of a fluidized-bed cooler, which consists of .three cooling chambers A, B, C. By means of Roots vacuum pumps 11, 12, 13, the individual cooling chambers A, B, C are charged with secondary gas via line 29, which secondary gas is used for the respective formation of a fluidized bed. The secondary gas leaves the first cooling stage 5a via line 14 and can then again be supplied to the fluidized-bed reactor. It is, however, also possible to again supply the secondary gases carried in line 14 to the cooling cyclone 2 via line 1. By means of the Roots vacuum pumps 6, 7, primary gas is sup plied to the first cooling stage 5a via line 16, which pri mary gas is passed through the individual cooling chambers C, B, A and serves as heat-transfer medium. The primary gas leaves the first cooling stage 5a via line 15 and is advanta geously used as fluidizing gas for the fluidized-bed reactor of the circulating fluidized bed (not represented). The anhy drous alumina withdrawn from the first cooling stage 5a is introduced via line 17 and line 18 into the cyclone 20. By means of the Roots vacuum pumps 8, 9, 10, line 18 is charged with gases, so that the anhydrous alumina is again introduced into the cyclone 20 in the form of a dispersion. The gases withdrawn from the central tube of the cyclone 20 are with- -7 drawn via line 21 and advantageously supplied to the fluid ized-bed reactor or the cooling cyclone 2. The anhydrous alu mina separated in the cyclone 20 is introduced into the sec ond cooling stage 5b of the fluidized-bed cooler via line 22. The second cooling stage 5b is likewise divided into three cooling chambers D, E, F. In contrast to the first cooling stage 5a, the cooling chambers are charged with a liquid heat-transfer medium via line 23, where water may advanta geously be used as heat-transfer medium. The liquid heat transfer medium is passed through the three cooling chambers F, E, D one after the other and leaves the second cooling stage 5b via line 24. By means of the Roots vacuum pumps 11, 12, 13, the second cooling stage 5b is also charged with sec ondary gas via line 29, so that there is also formed a re spective fluidized bed. The secondary gas passed through the second cooling stage 5b is introduced into the first cooling stage 5a via line 25 and finally withdrawn from the first cooling stage 5a via line 14. The anhydrous alumina is with drawn from the second cooling stage 5b via line 26, an outlet sluice 27, and a line 28. Fig. 2 represents a simplified schematic process flow diagram of a variant of the method for the final cooling of anhydrous alumina. The outlet sluice 27 is disposed between the cooling stages 5a, 5b. As desired, the outlet sluice 27 may be charged with gases by means of the Roots vacuum pumps 8, 9, 10 through a bypass line 31. Via line 17, the outlet sluice 27 and line 17' the anhydrous alumina from the first cooling stage 5a is introduced into line 18, in which it is supplied to the cyclone 20. The secondary gases withdrawn from the second cooling stage 5b via line 25 are likewise introduced into line 18 and thus supplied to the cyclone 20. Fig. 3 represents a flow diagram of the known method for the final cooling of anhydrous alumina in accordance with the prior art. In contrast to the inventive method it is provided to connect the two cooling stages 5a, 5b merely by one line -oQ -8 30, through which the alumina withdrawn from the first cool ing stage 5a is introduced into the second cooling stage 5b, and the secondary gas passed through the second cooling stage 5b is introduced into the first cooling stage 5a. It is, how ever, disadvantageous that the temperature at the inlet of the second cooling stage 5b is relatively high, so that it is not possible to recover the thermal energy withdrawn from the anhydrous alumina in the second cooling stage 5b and recircu late the same to technical processes, as a relatively large amount of thermal energy is released to the liquid heat transfer medium of the second cooling stage, for instance wa ter, and cannot be recovered. Cooling with liquid heat transfer medium can, however, not be omitted in the second stage, as for instance an alternative air cooling would re quire a relatively large amount of air, which cannot be util ized expediently in technical processes, as for instance in the preceding circulating fluidized bed. The three cooling chambers D, E, F of the second cooling stage 5b are each charged with secondary gas by means of the Roots vacuum pumps 11', 11, 12, so as to form a fluidized bed.

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Claims

1. A method for the final cooling of anhydrus alumina, which was prepared from aluminum hydroxide in a circulat ing fluidized bed, where the final cooling is effected in a fluidized-bed cooler which consists of two cooling stages arranged in series, which are each divided into several cooling chambers (A,B,C,D,E,F), where in the first cooling stage (5a) the fluidizing gas to be sup plied to the fluidized-bed reactor is heated, which-flu idizing gas is introduced as primary gas into the first cooling stage (5a), and in the second cooling stage (5b) the anhydrous alumina is cooled against a liqu d heat transfer medium, which is guided in a-countercurrent flow, characterized in that the dispersion containing an hydrous alumina, which has been withdrawn from the first cooling stage (5a), is passed through a cyclone (20), and that the anhydrous alumina withdrawn in the lower portion of the cyclone (20) is subsequently directly introduced into the second cooling stage e(5b).

2. The method as claimed in cla m 1, characterized in that the gases withdrawn from thA central tube of the cyclone (20) are recirculated to at least one cooling cyclone (2) of the precooling.

3. The method as claimed in claim 2, characterized in that before being introduced into the fluidized-bed reactor, the gases are mixed with secondary gas withdrawn from the fluidized-bed cooler.

4. The method as claimed in any of claims 1 to 3, character ized in that in each cooling stage (5a, 5b) the anhydrous alumina is passed through three cooling chambers. - 10

5. The method as claimed in any of claims 1 to 4, character ized in that the secondary gas passed through the second cooling stage (5b) is passed through the cyclone (20) to gether with the dispersion containing anhydrous alumina, which has been withdrawn from the first cooling stage (Sa).