Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process of carrying out
high-temperature reactions between a hot gas and previously
heated solids, which when treated at a high temperature, are
no longer free flowable, in a substantially vertical
conveyor passage, whereafter the gas-solids suspension is
cooled and solids are separated from the cooled suspensionO
In high-temperature reactions, solids are heated to a
temperature above that at which said solids are no longer
free flowable, so that the individual particles tend to
stick or adhere to each other and/or form crusts on the
inside surface of the reactor or in pipelines. The loss of
free flowability may be due to the reaction of the solids
with the gas phase or with other components. The problems
mentioned above may be encountered in the burning or
sintering, e.g., of alumina, lime, dolomite, magnesite or a
yround mixture of raw materials for making hydraulic cement.
Numerous processes and apparatus have been provided in an
effort to avoid the difficulties which are due to said
process technology.
For instance, in the process disclosed in German Patent
Publication No. 23 50 768, solids are preheated in a first
zone and are subsequently passed into a second zone through
a flame by which they are heated to a final temperature. An
air stream for surrounding the flame is supplied in the
second zone in order to prevent a formation of crusts on the
inside surface of the flame-containing space. That measure
has the disadvantage that the air stream supplied to the
second zone will inevitably be mixed with the gas-solids
suspension which has initially been heated to a sufficiently
high temperature and which is thus cooled by the air stream
before the desired high-temperature reaction has been
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completed. As a result, it is not ensured that the solids
will be maintained at the high temperature for the residence
time required to complete the desired reaction.
German Patent Specification No. 28 46 584 discloses a
process and apparatus for heat-treating fine-grained solids
in a preheating zone, a calcining zone, a sintering zone,
which consists of a suspension-reacting zone, and a cooling
zone. In that process at least part of the substantially
calcined material is supplied to the sintering zone after a
separate heat treatment, by which molten components forming
constituents are volatilized. It will be understood that
said prior art can be used only when the solids can be
maintained in a free flowable state by volatilisation of
said constituents. In other cases, that technology cannot
avoid a formation of crusts during the sintering in the
suspension-reacting zone as a result of a change of the
direction of flow of the gas-solids suspension. Moreover,
an additional reaction zone is undesirable.
It is an object of the invention to provide for high-
temperature reactions between a hot gas and preheated solids
a process which is free of the known disadvantages, particu-
larly those described hereinbefore, and which permits a
satisfactory processing, is universally applicable and can
be conveniently carried out.
In a process of the kind described first hereinbefore that
object is accomplished in accordance with the present
invention in that the preheated solids are supplied from
below in the direction of conveyance of a conveyor passage,
through a burner flame, which is disposed in the lower
portion of the conveyor passage, and are subsequently caused
to flow throughout the length of a sufficiently long
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reaction zone; and when the desired reaction has been
completed, the gas-solids suspension still flowing in the
same direction and are contacted behind the reaction zone
with a separately introduced coolant and are thus cooled to
a temperature at which the solids are free flowable.
Because the solids are supplied from below and in the
direction of conveyance and coolant is contacted with the
gas-solids suspension after the reaction has been completed
and when the suspension still flows in the same direction,
the direction of flow of the gas-solids suspension will not
be changed as it passes through the critical reaction zone
between the heating of the suspension to a high temperature
and the cooling of the suspension to a temperature at which
the gas-solids suspension can be handled without difficulty.
Because the direction of flow of the suspension is not
changed, no crusts can be formed. By the subsequent contact
with a separately introduced coolant, the residence time for
which the suspension is to be maintained at the required
high temperature in a given case can be adjusted exactly.
The mean gas velocity to be maintained in the coveyor
passage should be so selected that the relative velocities
between the solids and the wall will be high. The mean gas
velocity will usually lie in the range from 2 to 10 m/sec
(stated as an empty-pipe velocity).
The preheated solids are preferably contained in a gas-
solids suspension as they enter the conveyor passage and
said suspension is passed through the center of an annular
burner, which is supplied with fuel, e.g., with a fuel gas.
That mode of supplying the solids ensures a virtually
instantaneous heating of the solids to the desired
temperature, which depending on the feedstock and the
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desired result lies approximately in the range from 1300 to
1700C, preferably between 1400 and 1500 C.
The length of the conveyor passage may be selected depending
on the residence time required for the desired reaction. As
a few seconds are sufficient, as a rule, the conveyor line
will usually have a length not in excess of 20 meters and
generally a length between 5 and 15 meters.
The cooling required after the high-temperature reaction can
be effected with gaseous, liquid or solid coolants. Said
coolants should be supplied in such a manner that the
coolant is rapidly dispersed in the gas-solids suspension
and a contact of the solids with the boundary wall of the
conveyor line will be avoided. It is particularly desirable
to supply the coolant at high velocity in a tangential
direction, at right angles or at an angle up to 60 opposite
to or in the direction of flow.
After the cooling below the critical temperature, the gas
and solids are separated in conventional manner, e.g., in a
cyclone separator.
The solids to be subjected to the high-temperature reaction
may be preheated in any conventional manner and such
preheating may generally be combined with a chemical
reaction. It will be particularly desirable to preheat the
solids in a so-called circulating fluidized bed.
Whereas an "orthodox" fluidized bed constitutes a dense
phase, which is separated by a distinct density step from an
overlying gas space, a circulating fluidized bed has states
of distribution without a defined interface and involves no
density step between a dense phase and an overlying gas
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space but the solids concentration in the reactor gradually
decreases from bottom to top. Details relating to the
operation of circulating fluidized beds have been described
by L. Reh at al. in "Wirbelschichtprocesse f~r die Chemie
und H~ttenindustrie, die Energieumwandlung und den
Umweltschutz", published in Chem. Ing. Techn. 55 (1983), No.
2, pages 87 to 93, and have also be described in German
Patent Specification No. 17 67 628 and U.S. Patent No.
3,579,616.
A circulating fluidized bed has the advantage that it can be
operated at a high throughput rate per unit of cross
sectional area of the reactor and in that such a long
residence time can be selected for the solids to be heated
that the chemical reaction combined with the preheating will
virtually be completed. In such a case, only the high-
temperature reaction proper must be carried out in the
process in accordance with the invention so that said
process will involve virtually no reactions which can be
effected also at lower temperatures.
When the solids leaving the conveyor passage have been
separated they are usually cooled further. Said further
cooling may be accomplished by conventional coolers,
preferably by fluidi~ed bed coolers.
In a preferred embodiment of the invention the high-
temperature treatment carried out in accordance with the
invention is so integrated in the overall process comprising
the preheating and the final cooling of the solids that the
several gas streams may be used interchangeably. For
instance, oxygen-containing gas may be preheated in the
cooler and may then be supplied to the solids-preheating
stage and/or the high-temperature reaction stage. The
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exhaust gas from the conveyor passage may be supplied to the
solids-preheating stage.
In an optimum overall process, the feedstock is preheated in
a circulating fluidized bed, which is preceded by preheaters
supplied with the exhaust gases, and the final cooling is
carried out in a fluidized bed cooler, which comprises a
plurality of successive cooling chambers, for through flow.
The solids may be directly and/or indirectly cooled with
oxygen-containing gases, which are subsequently supplied as
an entraining gas to the conveyor passage and/or as a
fluidizing gas to the circulating fluidized bed. The
fluidizing gases which have been used in the fluidized bed
cooler may finally be used as a coolant in the pneumatic
conveyor and the exhaust gases from the pneumatic conveyor
may be used as secondary gas in the circulating fluidized
bed.
The invention will be explained more in detail and by way of
example with reference to the drawing and to the Examples.
The drawing is a flow scheme of a combined system of the
kind described hereinbefore.
The solids to be treated are supplied by metering means l to
a venturi heat exchanger 2, which is the last of a plurality
of such exchangers in the gas flow path. In said last heat
exchanger 2 the solids are heated by the available heat of
the exhaust gas. The solids are separated from the gas in a
cyclone separator 3 and are then supplied through a conveyor
4 to another preheating system, which consists of a venturi
heat exchanger 5 and an associated cyclone separator 6 and
of a venturi heat exchanger 7 and an associated cyclone
separator 8. A by-pass line 9 can be used to supply
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conveyed solids directly to the venturi heat exchanger 7
without passing through the preceding preheating stage.
From the cyclone separator 8, the solids are supplied to a
circulating system, which consists of a fluidized bed
reactor 10, a recycling cyclone 11 and a return line 12.
The fluidized bed reactor 10 is supplied with fuel through
line 13, with fluidizing gas through line 14 and with
secondary gas through line 15.
After a sufficiently long residence time the preheated
suspension is supplied through line 16 to the lower portion
of the conveyor passage 17 and is supplied from below into
the burner flame, which is produced by means of fuel (line
18) and oxygen-containing gas (line 19). As the gas-solids
suspension rises in the lower portion of the conveyor
passage 17, the suspension is subjected to the high-
temperature reaction and when said reaction has been
completed the suspension is cooled by means of gases which
are supplied through line 20. When the gas-solids
suspension still flowing in the same direction has been
sufficiently cooled, the suspension is discharged through
line 21 and is separated in the cyclone separator 22 into
solids supplied to the fluidized bed cooler 23 and gas
supplied through line 15 as secondary gas to the fluidized
bed reactor 10.
The fluidized bed cooler 23 is divided into a plurality of
cooling chambers, which the solids flow through in
succession. The cooler 23 comprises three cooling stages.
In the hottest stage, which is the first in the solids flow
path, oxygen-containing gas is heated; that gas is
subsequently supplied through line 19 to the conveyor
passage 17. The second stage is used to heat the oxygen-
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containing gas which is subsequently supplied through line14 to the fluidized bed reactor lO. The third cooling stage
is used for the final cooling of the solids by means of
cooling water, which is supplied through a line 24 and
discharged through line 25. The cooled product is
discharged through a device 26. The fluidizing gas streams
which have been used in the fluidized bed cooler 23 are
collected and supplied as a coolant through line 20 to the
conveyor passage 17.
Example
Aluminum hydroxide which is moist after having been filtered
is to be converted to highly burnt alumina.
Aluminum hydroxide having a moisture of 12% by weight is
supplied at a temperature of 60 C and at a rate of 8690 kg/h
through the metering means 1 to the venturi heat exchanger 2
and is subjected in said heat exchanger 2 to a heat exchange
with the gases at 390 C which are supplied from the cyclone
separator 6. As a result, the aluminum hydroxide is heated
to 160 C and the gas is cooled approximately to the same
temperature.
By the conve~or 4, the preheated aluminum hydroxide is
contacted in the venturi heat exchanger 5 with the exhaust
gases at 510C from the cyclone separator 8. This results
in a heating of the solids and a cooling of the gas to about
390C. When the gas and solids have been separated in the
cyclone separator 6, the solids are supplied to the venturi
heat exchanger 7, which is supplied from the circulating
fluidized bed with exhaust gases at 1150C. The thorough
mixing action results in a gas-solids suspension at a
temperature of 510C. After another separation of gas and
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solids effected in the cyclone separator 8, the solids are
supplied to the circulating fluidized bed.
The fluidized bed reactor 10 of the circulating fluidized
bed is supplied through line 14 from the second stage of the
fluidized bed cooler 23 with fluidizing air at 580C, at a
rate of 2000 sm3/h, through line 15 from the cyclone
separator 22 with secondary air at 1020 C, at a rate of 4800
sm3/h, and through line 13 with natural gas at a rate of 390
sm3/h. This results in a temperature of 1150 C, which is
virtually constant throughout the circulating system
consisting of the fluidized bed reactor 10, the recycling
cyclone 11 and the return line 12.
lhe aluminum oxide is completely calcined within an average
residence time of about 20 minutes. Thereafter, solids
corresponding to the feeding rate are supplied through line
16 to the conveyor passage 17 and are heated to 1400C by
the burner flame and by the flue gases from the burner. The
burner is supplied with natural gas at a rate of 110 sm3/h
and from the first stage of the fluidized bed cooler 23 with
air at 650C, at a rate of 1200 sm3/h.
~ hen the high-temperature reaction has been completed after
about 4 seconds, the gas-solids suspension is cooled by a
supply of air, which is at a temperature of 470C and is
supplied from the fluidized bed cooler 23 at a rate of 3500
sm3/h. The suspension is thus cooled to a temperature of
1020 C, at which the solids are sufficiently free flowable.
The gas-solids suspension are subsequently separated in the
cyclone separator 22 into a gas, which is supplied at a rate
of 4800 sm3/h as secondary gas to the fluidized bed reactor
10, and solids, which are supplied to the fluidized bed
cooler 23.
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In the fluidized bed cooler 23 the sollds are cooled in a
plurality of successive stages to a final temperature of
80C. This is effected in the first stage in the di.rection
of flow of the solids by means of air which is supplied at a
rate of 1200sm3/h and is thus heated to 650C, in the second
stage by means of air which is supplied at a rate of 2000
sm /h and is thus heated to 580 C, and in the third stage
with water which is supplied at a rate of 20 m3/h and is
thus heated from 35 to 65 C. The gas streams are recycled
to the process as described hereinbefore. Alumina having a
B.E.T. surface area of 3 m /g is produced at a rate of 5000
kg/h.
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