CN1290234A - Method of final cooling of anhydrous alumina - Google Patents

Abstract

The invention relates to a method for final cooling of aluminium oxide produced from aluminium hydroxide in a circulating fluidized bed, wherein the final cooling is carried out in a fluidized bed reactor consisting of two cooling stages connected in series, each divided into several cooling chambers (A, B, C, D, E, F). In the first cooling stage (5a), the fluidizing gas supplied to the fluidized-bed reactor, which is introduced as the primary gas of the second cooling stage, is heated and the anhydrous alumina is then cooled by the countercurrent introduction of a catalyst-exchange medium. According to the process of the invention, the dispersion containing anhydrous alumina from the first cooling stage (5a) is passed through a cyclone (2), while anhydrous alumina from the lower part of the cyclone (20) is introduced directly into the second cooling stage.

Description

Method of final cooling of anhydrous alumina

The present invention relates to a process for the final cooling of alumina produced with aluminum hydroxide in a circulating fluidized bed, wherein the final cooling is carried out in fluidized bed coolers arranged in series by It consists of 2 cooling sections, and each cooling section is divided into several cooling chambers. Wherein, in the first cooling section, the fluidizing gas to be supplied to the fluidized bed reactor is heated, the fluidizing gas is introduced as primary gas into the first cooling section, and in the second cooling section, the non- Alumina hydrate is cooled by a countercurrently introduced liquid heat transfer medium.

Methods for final cooling of anhydrous alumina are known. DE-OS 195 42 309 discloses a method for producing anhydrous aluminum oxide from aluminum hydroxide in a circulating fluidized bed consisting of a fluidized bed reactor, a separator and a return line, wherein the aluminum hydroxide is Introduced into the second stage of the gas side of a two-stage suspension preheater (the preheater is operated on the exhaust gas in the fluidized reactor of the circulating fluidized bed), and then at least partially dehydrated, from the suspension preheater The dehydrated aluminum hydroxide of the second stage is introduced into the first stage on the gas side of the suspension preheater operating on the exhaust gas in the fluidized bed reactor of the circulating fluidized bed, whereupon it is further dehydrated and then supplied to To the circulating fluidized bed, the bed is operated with oxygen-containing fluidizing gas indirectly heated by the alumina produced and indirectly heated oxygen-containing secondary gas supplied at a higher level in the subsequent cooling stage , wherein the indirect heating of the fluidizing gas is carried out in a fluidized bed cooler. On leaving this last suspension cooler, the resulting anhydrous alumina is subjected to final cooling in a fluidized bed cooler provided with three cooling chambers. In its first chamber, the fluidizing gas to be supplied to the fluidized bed reactor is heated, and in the two latter chambers, the fluidizing gas is cooled by means of a heat transfer medium, preferably water, said medium is introduced countercurrently.

The disadvantage of this method is that the temperature of the product entering the water-cooled section is rather high, which results in considerable heat energy of the product entering the water-cooled circulation system instead of being recirculated to the process.

It is therefore an object of the present invention to provide a method for the final cooling of anhydrous alumina in which the heat energy obtained from the anhydrous alumina is recovered almost entirely so that it can be reused in the process. Furthermore, the process can be adapted in an existing plant in a fairly easy manner.

The object of the present invention is achieved in that the dispersion containing the anhydrous alumina drawn from the first cooling section is passed through a cyclone, and then the anhydrous alumina drawn from the lower part of the cyclone is directly introduced into the first cooling section. Second cooling section. Before entering the fluidized bed cooler, the anhydrous alumina has a temperature of 800-1200° C. and is technically anhydrous, ie, only has a water content of 0.1-1% by weight. The primary gas introduced into the first cooling section is utilized as a heat transfer medium and thus as a coolant for the first stage. The cooling stage is designed in such a way that it consists of a plurality of cooling chambers, each chamber being filled with secondary gas as fluidizing gas, so that a fluidized bed is formed in each chamber. There are generally 2-6 cooling chambers in each cooling section. As heat transfer medium for the second cooling stage, it can be particularly beneficial to use water. The dispersed phase in this dispersion is solid and consists of anhydrous alumina. The dispersion phase is gaseous and consists of air. The dispersion itself, consisting of anhydrous alumina and air, thus has a dusty character. The cyclone arranged between the two cooling sections advantageously acts as a cooling cyclone for the final cooling, where indirect heat exchange takes place. It has been surprisingly found that with this method of final cooling of the anhydrous alumina, the thermal energy from the anhydrous alumina in the fluidized bed cooler can be almost completely reused in the process, so that only the observed Little heat loss. These heat losses include only the heat energy transferred to the circulation system of the liquid heat transfer medium in the second cooling section. However, since this final cooling method reduces the inlet temperature of the anhydrous alumina at the inlet of the fluidized bed cooler to below 450°C, this heat loss is minimal. Such a temperature reduction at the inlet of the fluidized bed cooler is also possible when the first cooling section is designed correspondingly larger. However, it is unfavorable that a relatively large amount of primary gas corresponding to the design of the larger first cooling section must be passed through this section as a heat transfer medium. ), which would result in the generation of an excessive amount of heated primary gas. For example, in order to regulate the fluidized bed in a reactor of a circulating fluidized bed, excess fluidization gas, consisting of primary gas, is supplied. The final cooling method thus advantageously allows direct cooling of the anhydrous alumina at the inlet of the second cooling section of the fluidized bed cooler without the need for large quantities of primary gas as the heat transfer medium for the first cooling section. Since most of the thermal energy obtained from the anhydrous alumina is transferred to the gas, it may be relatively easy to reuse the resulting thermal energy in a process by supplying the superheated gas to the process. For example, the hot gas withdrawn from the cyclone is advantageously supplied to a process whereby the thermal energy in this gas is utilized. In terms of construction, the arrangement of the cyclone separator between the two cooling sections can be done relatively easily, so that existing installations can be retrofitted relatively easily.

A preferred form of the invention consists in recirculating the gas withdrawn from the central tube of the cyclones to at least one of the pre-cooled cooling cyclones. This ensures that a particularly large amount of thermal energy drawn from the anhydrous alumina in the cyclone, for example, can advantageously be recycled into the process with a circulating fluidized bed as process.

According to another form of the invention, the gas is mixed with the secondary gas leading from the fluidized bed cooler before being introduced into the fluidized bed reactor. Thereby the heat energy in the secondary gas can likewise advantageously be recycled to the process in the circulating fluidized bed, where, since the co-transfer of the secondary gas and the gas introduced from the central tube of the cyclone separator can be retained, the This recirculation of thermal energy is at the same time ensured compared to separate transfers.

According to another preferred form of the invention, anhydrous alumina is passed through 3 cooling chambers in each cooling section. The layout of each of the three cooling chambers allows for uniform cooling of the anhydrous alumina while optimizing the space required for the fluidized bed cooler. This arrangement of the three cooling chambers each ensures a particularly beneficial release of heat energy from the anhydrous aluminum oxide.

A further preferred form of the invention is based on passing the secondary gas passing through the second cooling section through a cyclone separator together with the anhydrous alumina-containing dispersion withdrawn from the first cooling section. It is particularly advantageous that the heat energy in the secondary gas of the second cooling section can be provided particularly simply for direct heat exchange in the cyclone separator.

The subject matter of the invention will be described in detail below by way of example with reference to the accompanying drawings ( FIGS. 1-3 ).

Figure 1 shows a simplified schematic flow diagram of a method for final cooling of anhydrous alumina.

Figure 2 shows a simplified schematic flow diagram of a variation of the method for final cooling of anhydrous alumina.

Figure 3 presents a simplified schematic flow diagram of a known, prior art process for final cooling of anhydrous alumina.

Figure 1 shows a simplified schematic flow diagram for the final cooling of anhydrous alumina. The anhydrous alumina withdrawn from the fluidized bed reactor is fed in the form of a dispersion via line 1 to a cooling cyclone 2 where the actual cooling is carried out. The cooling cyclone 2 may consist of several cooling cyclones arranged in series. The gas drawn from the central tube of the cooling cyclone 2 is discharged via line 3 and then possibly led to the reactor of the circulating fluidized bed. The cooled anhydrous alumina is transported through line 4 to the first cooling section 5a of the fluidized bed cooler, which is composed of three cooling chambers A, B, C. Each cooling chamber A, B, C is filled with a secondary gas via line 29 by means of a Roots vacuum pump 11, 12, 13, which is used to form the respective fluidized bed. The secondary gas leaves the first cooling section 5a via line 14 and can then be fed to the fluidized bed reactor again. However, it is also possible to feed the secondary gas in line 14 to the cooling cyclone 2 again via line 1 . Primary gas is supplied via line 16 to the first cooling section 5a by means of Roots vacuum pumps 6, 7, which primary gas passes through separate cooling chambers C, B, A and acts as a heat transfer medium. The primary gas leaves the first cooling section 5a via line 15 and is then advantageously used as fluidization gas for a fluidized bed reactor of a circulating fluidized bed (not shown). Anhydrous alumina from the first cooling section 5a is introduced into cyclone separator 20 via lines 17 and 18 . The line 18 is gassed by means of Roots vacuum pumps 8 , 9 , 10 so that the anhydrous alumina is reintroduced into the cyclone 20 in the form of a dispersion. The gas withdrawn from the central tube of the cyclone 20 is withdrawn via line 21 and then advantageously supplied to the fluidized bed reactor or cooling cyclone 2 . The anhydrous alumina separated in the cyclone separator 20 is introduced via line 22 into the second cooling section 5b of the fluidized bed cooler. The second cooling section 5b is likewise divided into three cooling chambers D, E, F. In contrast to the first cooling section 5a, these cooling chambers are filled with a liquid heat transfer medium via line 23, where water is advantageously used as heat transfer medium. The liquid heat transfer medium passes through the three cooling chambers F, E, D in sequence, and then leaves the second cooling section 5b through the line 24 . By means of Roots vacuum pumps 11 , 12 , 13 , the second cooling section 5 b is also filled with secondary gas via line 29 , so that the respective fluidized beds are also formed. The secondary gas passing through the second cooling section 5 is introduced into the first cooling section 5a via line 25 and finally extracted from the first cooling section 5a via line 14 . Anhydrous alumina is withdrawn from the second cooling section 5b via line 26 , outlet sluice 27 and line 28 .

Figure 2 shows a simplified schematic flow diagram of a variant of the method for final cooling of anhydrous alumina. An outlet chute 27 is provided between the cooling sections 5a, 5b. Can be by Roots vacuum pump 8,9,10 when needed, outlet chute 27 is filled with gas through branch pipe 31. Via line 17 , outlet chute 27 and line 17 ′, the anhydrous alumina from the first cooling section 5 a is introduced into line 18 , where it is fed to cyclone separator 20 . The secondary gas withdrawn from the second cooling section 5b via line 25 is likewise introduced into line 18 and thus supplied to cyclone separator 20 . Figure 3 shows a prior art flow diagram of a known method for final cooling of anhydrous alumina. Different from the method of the present invention, it only uses a pipeline 30 to connect the two cooling sections 5a, 5b. Through the pipeline 30, the alumina drawn from the first cooling section 5a is introduced into the second cooling section 5b, and passes through the second cooling section 5b. The secondary gas of the cooling section 5b is introduced into the first cooling section 5a. However, the disadvantage is that the temperature at the inlet of the second cooling section 5b is quite high, and since a large amount of heat energy is released in the liquid heat transfer medium of the second cooling section, such as water, it cannot be recovered, so it is impossible to recover from the second cooling section The thermal energy drawn from the anhydrous alumina in 5b is recycled to the process. However, cooling with a liquid heat transfer medium in the second cooling section cannot be omitted, because for example, a large amount of air is required for air cooling instead, and such a large amount of air is inconvenient to use in the process, such as inconvenient in the above-mentioned used in circulating fluidized beds. The three cooling chambers D, E, F of the second cooling section 5b are filled with secondary gas by means of Roots vacuum pumps 11', 11, 12, thereby forming a fluidized bed.

Claims (5)

1. Process for the final cooling of anhydrous alumina made with aluminum hydroxide in a circulating fluidized bed, wherein the final cooling is performed in two series arranged, each subdivided into several cooling Chamber (A, B, C, D, E, F) of the cooling section of the fluidized bed cooler, wherein the fluidization gas will be supplied to the fluidized bed reactor in the first cooling section (5a) is heated, the fluidizing gas is introduced as primary gas into the first cooling section (5a), and in the second cooling section (5b), the anhydrous alumina is cooled by a countercurrently introduced liquid heat transfer medium, The method is characterized in that the anhydrous alumina-containing dispersion drawn from the first cooling section (5a) is passed through a cyclone separator (20) and in that the anhydrous alumina-containing dispersion drawn from the lower part of the cyclone separator (20) is Alumina is then directly introduced into the second cooling section (5b). 2. 2. The method according to claim 1, characterized in that the gas withdrawn from the center tube of the cyclone (20) is recycled to the precooled at least one cooling cyclone (2). 3. 2. A method as claimed in claim 2, characterized in that the gas is mixed with a secondary gas leading from a fluidized bed cooler before being introduced into the fluidized bed reactor. 4. 3. Process according to any one of claims 1-3, characterized in that in each cooling section (5a, 5b) the anhydrous alumina is passed through three cooling chambers. 5. A method according to any one of claims 1-4, characterized in that the secondary gas passing through the second cooling section (5b) is passed together with the anhydrous alumina-containing dispersion introduced from the first cooling section (5a) Cyclone separator (20).

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