DE102007041586B4 - Process and plant for the heat treatment of granular solids - Google Patents

Abstract

Process for the heat treatment of granular solids, in particular for the production of aluminum oxide from aluminum hydroxide, the solids are heated after preheating in at least one preheating in a fluidized bed reactor and then fed to at least one fluidized bed cooler, in which the heat-treated solids are cooled by means of fluidizing air the fluidizing air is withdrawn from the condenser and passed as secondary gas into the fluidized bed reactor and split the secondary gas flow and a bypass flow is passed to the fluidized bed reactor in a feed line for the solids, characterized in that the size of the bypass flow is variable and in that the size of the bypass stream Depending on the feed rate of the solids is regulated in the fluidized bed reactor.

Description

The invention relates to the heat treatment of granular solids, in particular the production of aluminum oxide from aluminum hydroxide, the solids are heated after preheating in at least one preheating in a fluidized bed reactor, in particular calcined and then fed to at least one fluidized bed cooler, in which the heat-treated solids with the aid of fluidizing gas are cooled, wherein the fluidizing gas is withdrawn from the condenser and passes as a secondary gas in the fluidized bed reactor.

Such a process for producing anhydrous alumina (Al ₂ O ₃ ) from aluminum hydroxide (Al (OH) ₃ ) is known from EP 0 861 208 B1 known. Here, the aluminum hydroxide is calcined after passing through two preheating stages in a circulating fluidized bed. The preheating of the aluminum hydroxide is carried out with the aid of exhaust gas from a downstream of the fluidized bed reactor separator. From the return line of the separator anhydrous hot alumina is branched off and cooled directly and indirectly in a fluidized bed cooler with air. The thereby indirectly heated air is passed as fluidizing air in the fluidized bed reactor, while the introduced for direct cooling as fluidizing air into the fluidized bed cooler air is withdrawn from the cooler and also passed as secondary air in the fluidized bed reactor.

From the generic DE 25 24 540 C2 For example, a method of heat treatment of granular solids is known in which the solids are heated after passing a preheat stage in a fluidized bed reactor and then passed into a fluidized bed cooler in which the solids are cooled by means of fluidizing air. The fluidizing air withdrawn from the cooler is partially recycled as a secondary gas into the fluidized bed reactor, wherein as a possible embodiment feature an additional, uncontrolled bypass flow is described which directs parts of the secondary gas into the conveying line for solids.

The DE 1 767 628 A1 also describes a method for carrying out endothermic processes according to the fluidized bed principle, wherein a portion of the withdrawn from the cooler gas stream is introduced directly into a second drying stage.

Also from the DE 102 60 741 A1 a device for heat treatment of granular solid is known, which includes in addition to preheating, fluidized bed reactor, discharge line and secondary gas line branching off from the secondary gas line bypass line back into the conveying line of the solids.

It has been found that at partial load operation of such a calcining plant the specific energy consumption (kJ / t Al ₂ O ₃ ) is higher than in full load operation. This is due to the fact that in partial load operation a strong superstoichiometric combustion takes place (combustion air / fuel ratio λ >> 1), which leads to higher exhaust gas temperatures than in full load operation. In partial load operation, the combustion air can not be arbitrarily reduced in order to adjust the λ ratio, since the combustion air is also used as a delivery gas in the delivery lines and thus must not fall below a minimum speed or minimum delivery air quantity.

The object of the invention is to reduce the specific energy consumption of a system even at partial load operation.

This object is achieved with the invention in a method of the type mentioned above essentially by the features of claim 1. The secondary gas stream is split at one or more locations of the stream, and one bypass stream or multiple bypass streams is passed past the fluidized bed reactor into a solids delivery line. The transport and combustion tasks of the air are thus decoupled. The introduced as combustion air in the fluidized bed reactor fraction of the secondary gas stream can be adjusted according to the sufficient for the calcination of the supplied solids fuel. The remaining part of the secondary gas stream is passed as a bypass flow past the fluidized bed reactor directly into a feed line for the solids, so that there is a sufficient amount of conveying gas is ensured. The size of the bypass stream is variable, and it is preferably controlled together with the fuel supply as a function of the feed rate of the solids in the fluidized bed reactor. As a result, the λ ratio in the fluidized bed reactor can be optimally adjusted for different load states of the reactor.

It has been found that even with a very low system load, in which up to 70% of the secondary gas flow are bypassed as a bypass flow to the fluidized bed reactor, a significant reduction in specific energy consumption can be achieved.

In a further development of the invention, the bypass stream is passed before a preheating stage for the solids in the delivery line to ensure there adequate fluidization. As a result, at the same time the heat energy contained in the secondary gas can be used to preheat the solids.

If the solids are fed to a preheater, in particular a suspension preheater, in which they are preheated with exhaust gas from a separator arranged downstream of the fluidized bed reactor, the gas / solids mixture being fed from the suspension preheater via a second delivery line to a second separator, according to one particular preferred embodiment of the invention, the bypass flow fed into the second delivery line. In the suspension preheater fresh hydrate is mixed in the calcination of aluminum hydroxide, which should react directly with the withdrawn from the first separator hot furnace gases.

According to another preferred embodiment of the invention, the bypass flow is fed into a first delivery line, via which exhaust gas from the fluidized bed reactor downstream first separator is introduced into the suspension preheater.

In a further preferred embodiment of the invention, the gas / solids mixture from the suspension preheater is fed to a second separator, the exhaust gas for preheating and conveying fresh solids via a third delivery line of a first preheating is fed, wherein the bypass flow is fed directly into the third delivery line.

In principle, the secondary gas stream can be divided at any point of the process with one or more bypass streams. In the preferred embodiment of the invention, however, the secondary gas stream is divided after passing through the preheating stages of the secondary gas stream (cooling stages of the material from the reactor).

A plant for heat treatment of granular solids, which is suitable for carrying out the method according to the invention is described by the features of claim 7. In the bypass line, a control valve, slide or the like. Provided including measuring device for the flow rate. Furthermore, in a development of the invention in the bypass line and / or in the remaining secondary air line, a device for controlling the pressure and / or the pressure loss can be provided.

In order to optimally utilize the heat contained in the secondary gas flow, the bypass line is connected to a conveying line leading to the at least one preheating stage.

In one embodiment of the invention, the fluidized bed reactor is followed by a first separator, the exhaust gas via a first feed line in a preheater, in particular a suspension preheater is passed, the suspension preheater is connected via a second delivery line with a second separator and the bypass line in the second delivery line empties.

In another embodiment of the invention, the bypass line is connected to the first delivery line.

In a still further embodiment, the second separator is connected via a third delivery line to a first preheating stage for fresh solids, and the bypass line opens into the third delivery line.

The above-described feed points for the bypass lines can be provided alternatively or cumulatively, depending on the conditions of the installation, wherein the respective proportions are controlled individually via the control valves provided in the bypass lines.

The process control according to the invention can be used for all processes that require a promotion of solids z. As calcination of magnesium carbonate, cleavage of magnesium sulfate, roasting ores or Eisenerzvorwärmung.

Further developments, advantages and applications of the invention will become apparent from the following description of exemplary embodiments and the drawings. All described and / or illustrated features alone or in any combination form the subject matter of the invention, regardless of their combination in the claims or their dependency.

Show it:

1 1 schematically a system for carrying out the method according to the invention according to a first embodiment of the invention,

2 1 schematically a system for carrying out the method according to the invention according to a second embodiment,

3 1 schematically a system for carrying out the method according to the invention according to a third embodiment,

4 1 schematically a system for carrying out the method according to the invention according to a fourth embodiment,

5 1 schematically a system for carrying out the method according to the invention according to a fifth embodiment,

6 1 schematically a system for carrying out the method according to the invention according to a sixth embodiment,

7 1 schematically a system for carrying out the method according to the invention according to a seventh embodiment,

8th 1 schematically a system for carrying out the method according to the invention according to an eighth embodiment,

9 a diagram illustrating the reduction of specific energy consumption in dependence on the bypass flow past the fluidized bed reactor.

Each of the figures shows only the preferred embodiment, in each case a bypass line branches off after the last preheating of the secondary air from the secondary air line.

At the in 1 shown plant according to the first embodiment of the invention is filter-moist aluminum hydroxide (Al (OH) ₃ ) at a loading station 1 in a first flash reactor 2 or a suspension preheater (first preheating stage) is entered, in which it is detected by an exhaust gas stream coming from the plant and a separation device 3 is supplied. That from the separator 3 exiting exhaust gas is used for dedusting an example. Electrostatic gas cleaning 4 and finally fed to a fireplace, not shown.

The from the separator 3 escaping solid passes through a pipe 5 in a second preheater, in particular as a suspension preheater 6 is formed (second preheating stage), in which the solid of the via a first conveying line 7 from a recycle cyclone (first separator) 8th detected exhaust gas exiting a circulating fluidized bed and further dehydrated or dehydrated. The gas / solids mixture from the suspension preheater 6 is via a second delivery line 9 a separation cyclone (second separator) 10 fed, in which the solids are separated from the gas. The gas is delivered via a third delivery line 11 as a delivery gas in the flash reactor 2 introduced and transported the fresh aluminum hydroxide to the separator 3 while preheating the solids.

The in the separation cyclone 10 separated solids are passed through a solids feed line 12 in a fluidized bed reactor 13a enlisted in which they with the help of over a fuel line 14 supplied fuel at a temperature in the range of 850 to 1000 ° C calcined. The oxygen-containing gas streams required for combustion, for example air or oxygen-enriched air, are conveyed via a primary gas line 15 as fluidizing gas and via a secondary gas line 16 fed as secondary gas.

The gas-solid suspension passes through a connecting line 17 in the return cyclone 8th the circulating fluidized bed, in which a further separation of solid and gas takes place. Via a discharge line 18 becomes the one from the recycling cyclone 8th escaping solid a first, from riser 19 and cyclone separators 20 fed suspension cooler supplied. The exhaust gas of the cyclone separator 20 passes through the secondary gas line 16 in the fluidized bed reactor 13a , the solid in a riser 21 and cyclone separators 22 formed second suspension cooler. The exhaust gas of the second suspension cooler is via a line 23 as conveying gas into the riser 19 passed the first suspension cooler. After leaving the last suspension cooler, the alumina produced undergoes final cooling in a fluidized-bed cooler equipped with four cooling chambers 24 , In the first chamber there is a heating of the Wirbeischichtreaktor 13a supplied fluidizing gas (primary gas), in the downstream chambers cooling against a heat transfer medium, preferably water, which is conducted in countercurrent. The alumina eventually passes over the line 25 out.

From the secondary gas line 16 branches in front of the fluidized bed reactor 13a a bypass line 26a starting in the second delivery line 9 empties. In the bypass line 26a is a control valve 27a or a slider or the like provided to the proportion of the secondary gas line 16 Set branched bypass current.

At full load operation of the system becomes the control valve 27a normally be closed, leaving the entire over the secondary gas line 16 and the associated upstream coolers and lines brought secondary gas into the fluidized bed reactor 13a is fed and available for combustion. In contrast, the system only operates at partial load, ie only a smaller amount of aluminum hydroxide is supplied via the feed station 1 abandoned, so the proportion of over the line 16 in the fluidized bed reactor 13a be fed reduced secondary gas to adjust the λ ratio to the required for the calcination of lower amounts of aluminum hydroxide smaller fuel supply to avoid an increase in the combustion and thus the exhaust gas temperatures. The remaining part of the secondary gas flow is via the bypass line 26a directly into the second delivery line 9 fed and supports the promotion of solids from the suspension preheater 6 through the other parts of the plant. hereby it is ensured that there is always a sufficient amount of conveying gas available and settling and caking of particles on the walls of the delivery lines or the separator is avoided. At the same time, the heat energy contained in the bypass flow is used to preheat the solids.

Alternatively, the bypass flow can also be taken from the line, for example 23 or the first cooling stage 29 the fluidized bed cooler are diverted.

In the 2 shown second embodiment of the present invention is substantially similar in structure to the system according to 1 , However, at the in 2 shown installation of the fluidized bed cooler in two separate cooling stages 29 . 30 built, the first cooling stage 29 the first chamber of the fluidized bed cooler 24 according to the first embodiment and the heating of the fluidized bed reactor 13a supplied fluidizing gas (primary gas) is used. In the second cooling stage 30 the aluminum oxide produced is finally cooled in three cooling chambers against a heat transfer medium, preferably water, which is conducted in countercurrent.

Incidentally, the structure and functioning of the in 2 shown installation of the system of the first embodiment according to 1 so that reference may be made to the above description.

At the in 3 shown third embodiment of a system according to the invention is a partial stream of moderately warm aluminum hydroxide before feeding into the suspension preheater 6 branched off and via a hydrate bypass 31 a mixing chamber 32 supplied in which it in the fluidized bed reactor 13a produced hot alumina is mixed. This procedure is in the EP 0 861 208 B1 described in detail.

In the third embodiment, the leads from the secondary gas line 16 branching bypass line 26b in the first funding line 7 over which the hot exhaust gas from the recycle cyclone 8th in the suspension preheater 6 is introduced. This becomes the one for fluidization in the suspension preheater 6 increased available gas flow. The size of the bypass flow is via the control valve 27b regulated.

Incidentally, the structure and functioning of the in 3 shown installation of the system of the first embodiment according to 1 so that reference may be made to the above description.

In the 4 shown fourth embodiment of a system according to the invention differs from the system according to 3 only to the effect that the branch point for the Hydratbypass 33 was moved. Instead of like

3 in front of the suspension preheater 6 branch off, the Hydratbypass branches 33 at the plant according to 4 after the second separator 10 before the introduction of the solids in the fluidized bed reactor 13a from. As a result, a precalcination of the bypassed aluminum hydroxide is already achieved, so that the final calcination of these solids in the mixing chamber 32 is accelerated.

In the 5 shown fifth embodiment of the present invention is essentially similar to the system according to 3 , However, only the feed point for the bypass line 26c laid. In this embodiment, the leads from the secondary gas line 16 branching bypass line 26c to the third funding line 11 that the separation cyclone 10 with the flash reactor 2 combines. As a result, a stronger preheating of the fresh, on the Aufgabestation 1 added aluminum hydroxide in the first preheating reached. In addition, similar to the embodiment according to 2 the fluidized bed cooler with two separate cooling stages 29 . 30 designed.

At the in 6 shown sixth embodiment of the present invention has been compared to the system according to 5 only the branch point for the Hydratbypass 33 laid in the same manner as in the fourth embodiment according to 4 ,

The 7 and 8th show alternative embodiments of the fluidized bed reactor 13a , b, c. While in the embodiments according to the 1 to 6 a circulating fluidized bed with a fluidized bed reactor 13a and a return line 13a ' is provided is in the seventh embodiment of the invention according to 7 a flash reactor 13b intended. In the eighth embodiment of the invention according to 8th on the other hand is a ring fluidized bed reactor 13c provided, as he eg. in the DE 102 60 741 A1 is described in more detail. Incidentally, the function and mode of operation of the systems comply with the 7 and 8th the first to sixth embodiments, so that reference is made to the above description. However, it is always possible to use other reactors than fluidized bed or flash reactors with separators, for example cyclones, rotary kilns or similar industrial furnaces.

It is understood that the alternative feed points of the bypass line 26a , b, c in the second delivery line 9 , first funding line 7 or third funding line 11 and the design of the fluidized bed reactor 13a , b, c or the fluidized bed cooler 24 . 29 . 30 , as described in detail in individual plants, can be used in any combination in the systems shown in the other figures. It is also possible to use all three feed points of the bypass lines 26a , b, c cumulatively. In this case, the distribution of the bypass current is carried out by appropriate control of the control valves 27a , b, c according to the requirements of the system. Furthermore, it is of course possible to place the branch points at any point of the secondary gas line or the entire upstream supply lines for secondary gas.

With the inventive division of the secondary gas flow in a secondary gas in the fluidized bed reactor 13a , b, c and one fed to the fluidized bed reactor 13a , b, c over, directly into the delivery lines 7 . 9 . 11 fed bypass stream, which serves the promotion and heat recovery, the specific energy consumption of a calcination plant can be significantly reduced during partial load operation.

In 9 is in a plant according to 1 the specific energy consumption is shown as a function of the partial load from 1250 t / d to 2500 t / d. The upper, solid curve corresponds to the course of the specific energy consumption when no bypass current is used. The lower, dashed curve corresponds to the course of the specific energy consumption with bypass flow and the condition that the calcination is operated with an excess air lambda = 1.2. If no bypass current is provided, a partial load of 1250 t / d results in a specific additional consumption of 20% (573 kJ / kg) compared to full-load operation of 3300 t / d. If, on the other hand, a bypass flow of 70% is provided, the specific additional consumption only increases by about 7 (195 kJ / kg).

LIST OF REFERENCE NUMBERS

1

feeding station

2

Flash reactor (first preheating stage)

3

separating

4

gas cleaning

5

management

6

preheater

7

first support line

8th

Recycle cyclone (first separator)

9

second funding line

10

Separation cyclone (second separator)

11

third funding line

12

Solid supply line

13a

Fluidised bed reactor (circulating fluidized bed)

13a '

Return line

13b

Flash reactor

13c

Annular fluidized bed reactor

14

fuel line

15

Primary gas line

16

Secondary gas line

17

connecting line

18

discharge line

19

riser

20

cyclone

21

riser

22

cyclone

23

management

24

Fluidized bed cooler

25

management

26a, b, c

bypass line

27a, b, c

control valve

29

first cooling stage of the fluidized bed cooler

30

second cooling stage of the fluidized bed cooler

31

hydrate bypass

32

mixing chamber

33

hydrate bypass

Claims (10)

Process for the heat treatment of granular solids, in particular for the production of aluminum oxide from aluminum hydroxide, wherein the solids are preheated in at least one preheating stage in a fluidized bed reactor and then fed to at least one fluidized bed cooler, in which the heat-treated solids are cooled by means of fluidizing air the fluidizing air is withdrawn from the condenser and passed as secondary gas into the fluidized bed reactor and split the secondary gas flow and a bypass flow is passed to the fluidized bed reactor in a conveying line for the solids, characterized in that the size of the bypass flow is variable and in that the size of the bypass flow in Depending on the feed rate of the solids is regulated in the fluidized bed reactor. A method according to claim 1, characterized in that up to 70% of the secondary gas stream are bypassed as a bypass stream to the fluidized bed reactor. Method according to one of the preceding claims, characterized in that the bypass flow is passed before a preheating stage for the solids in the feed line. Method according to one of the preceding claims, characterized in that the Solids are fed to a preheater, which is supplied via a first feed line exhaust gas from a downstream of the fluidized bed reactor first separator, that the gas / solid mixture from the preheater via a second feed line to a second separator is fed, and that the bypass flow is fed into the second feed line becomes. Method according to one of claims 1 to 3, characterized in that the solids are fed to a preheater, the exhaust gas from a downstream of the fluidized bed reactor first separator is fed via a first delivery line, and that the bypass flow is fed into the first delivery line. Method according to one of claims 1 to 3, characterized in that the solids are fed to a preheater, which is supplied via a first feed line exhaust gas from a downstream of the fluidized bed reactor first separator, that the gas / solid mixture from the preheater via a second delivery line a second separator is supplied, in which the exhaust gas is separated from the solids, that the exhaust gas of the second separator for preheating and conveying fresh solids via a third delivery line to a first preheating stage is fed, and that the bypass flow is fed to the third delivery line. Plant for heat treatment of granular solids, in particular for carrying out a process according to one of the preceding claims, with a fluidized bed reactor ( 13a . 13b . 13c ), in which the solids are heated, in particular calcined, at least one preheating stage ( 2 . 6 ) for preheating the solids prior to introduction into the fluidized bed reactor ( 13a . 13b . 13c ) and at least one cooler ( 20 . 21 . 22 . 23 . 24 ), in which the from the fluidized bed reactor ( 13a . 13b . 13c ) via a discharge line ( 18 ) solids are cooled by means of fluidizing gas, wherein the fluidizing gas from the cooler ( 20 ) and via a secondary gas line ( 16 ) in the fluidized bed reactor ( 13a . 13b . 13c ), wherein from the secondary gas line ( 16 ) and / or the line ( 23 ) and / or the radiator ( 29 ) a bypass line ( 26a . 26b . 26c ) branches off, which at the fluidized bed reactor ( 13a . 13b . 13c ) and with a delivery line ( 7 . 9 . 1 1) for the solids, characterized in that in the bypass line ( 26a . 26b . 26c ) a control valve ( 27a . 27b . 27c ) is arranged. Installation according to claim 7, characterized in that the fluidized bed reactor ( 13a . 13b . 13c ) a first separator ( 8th ) whose exhaust gas via a first delivery line ( 7 ) in a preheater ( 6 ), that the preheater ( 6 ) via a second pipeline ( 9 ) with a second separator ( 10 ) and that the bypass line ( 26a ) with the second delivery line ( 9 ) connected is. Installation according to claim 7, characterized in that the fluidized bed reactor ( 13a . 13b . 13c ) a first separator ( 8th ) whose exhaust gas via a first delivery line ( 7 ) in a preheater ( 6 ) and that the bypass line ( 26b ) with the first pipeline ( 7 ) connected is. Installation according to claim 7, characterized in that the fluidized bed reactor ( 13a . 13b . 13c ) a first separator ( 8th ) whose exhaust gas via a first delivery line ( 7 ) in a preheater ( 6 ), that the preheater ( 6 ) via a second pipeline ( 9 ) with a second separator ( 10 ), that the second separator ( 10 ) via a third funding line ( 11 ) with a first preheating stage ( 2 ) for fresh solids, and that the bypass line ( 26c ) with the third funding line ( 11 ) connected is.

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