MXPA05006821A - Methods and apparatus for heat treatment in a fluidised bed - Google Patents

Abstract

The present invention relates to a method for the heat treatment of solids, in particular for the thermal decomposition of salts, in which the solids are heated to a temperature of 200 to 1400°C in a fluidized-bed reactor (1), and to a corresponding plant. To improve the utilization of energy and the decomposition of salts, it is proposed to introduce a first gas or gas mixture from below through a preferably central gas supply tube (3) into a mixing chamber (7) of the reactor (1), the gas supply tube (3) being at least partly surrounded by a stationary annular fluidized bed (10) which is fluidized by supplying fluidizing gas, and the gas velocities of the first gas or gas mixture and of the fluidizing gas for the annular fluidized bed (10) being adjusted such that the Particle Froude Numbers in the gas supply tube (3) are between 1 and 100, in the annular fluidized bed (10) between 0.02 and 2, and in the mixing chamber (7) between 0.3 and 30.

Description

METHOD AND PLANT FOR THE THERMAL TREATMENT OF SOLVENTLY GRANULATED SOLIDS Field of the Invention The present invention relates to a method for the thermal treatment of finely granulated solids, in particular for the thermal decomposition of salts, wherein the solids are heated to a temperature of 200 to 1,400 ° C in a bed reactor. fiuidized, as well as this invention relates to the corresponding plant.

BACKGROUND OF THE INVENTION Such methods and plants are known, for example, from DE 27 10 978 A1 for the thermal decomposition of salts that mainly contain iron sulphate to obtain iron oxide and sulfur dioxide in a circulating fluidized bed. of a reactor. In the lower part of the reactor, primary air is supplied to fluidize the solids, while secondary air is additionally introduced into the upper portion of the reactor. Fuel is introduced into the reactor between the primary and secondary air feed lines. Preferably, incomplete combustion of the fuel must always be effected first to produce a reducing atmosphere in the reactor, by means of which decomposition of the salts is improved. In a second stage, the secondary air supply provides a complete combustion of the fuel. For the fluidization of the solids, however, a large amount of primary air is necessary, which leads to an additional combustion of the fuel. On the other hand, a reduction in the amount of the primary air effects only an insufficient fluidization of the solids, which leads to a reduced heat transfer. Therefore, the amount of primary air can only be adjusted within narrow limits. In addition, the introduction of secondary air must be designed rather complex to achieve sufficient mixing of secondary air with fuel.

From DE 24 08 308 C2 a similar method is known for the production of magnesium oxide and sulfur dioxide from magnesium sulfate. For this purpose, magnesium sulfate is thermally decomposed during combustion of a fuel in a fluidized bed furnace.

Furthermore, reactors for the thermal treatment of solids are generally known, whose fluidized beds are formed either stationary or circulating. However, the energy utilization achieved when using a stationary fluidized bed needs improvement. This is particularly due to the fact that the transfer of mass and heat is rather moderate due to the comparatively low degree of fluidization. On the other hand, circulating fluidized beds have better mass and heat transfer conditions due to the higher degree of fluidization, but are restricted in terms of their solid retention times.

Objective and Compendium of the Invention Therefore, it is the object of the present invention to improve the conditions of mass and heat transfer, as well as the decomposition of the salts during the thermal treatment of finely granulated solids. According to the invention, this objective is achieved by a method as mentioned above, in which a first gas or mixture of gases is introduced from below through a gas supply pipe (central pipe) preferably arranged centrally. inside a reactor mixing chamber, the central tube is at least partially surrounded by an annular and stationary fluidized bed, which bed is fluidized by the supply of fluidizing gas, and in which the gas velocities of the first gas or The gas mixture, as well as the fluidisation gas for the annular fluidized bed are adjusted in such a way that the Particle Fraud Numbers in the central tube are between 1 and 100, in the annular fluidized bed between 0.02 and 2., and in the mixing chamber between 0.3 and 30. In the method of the invention, the advantages of a stationary fluidized bed, such as a sufficiently long retention time of solids, and the advantages of a circulating fluidized bed, such as a good transfer of mass and heat, can be combined surprisingly between themselves during the heat treatment, in particular the thermal decomposition of the salts, while avoiding the disadvantages of both systems. When passing through the upper region of the central tube, the first gas or mixture of gases entrains solids from the annular and stationary fluidized bed, which is referred to as an annular fluidized bed, inside the mixing chamber, so that due to the high sliding speeds between the solids and the first gas form an intensively mixed suspension and an optimal transfer of mass and heat between the two phases is achieved. By corresponding adjustment of the bed height in the annular fluidized bed as well as the gas velocities of the first gas or gas mixture and the fluidizing gas, the solids loading of the suspension above the orifice region of the The central tube can be varied within wide intervals, so that the loss of pressure of the first gas between the region of the orifice of the central tube and the upper outlet of the mixing chamber can be between 1 mbar and 100 mbar. In the case of a high solids loading of the suspension in the mixing chamber, a large part of the solids will separate from the suspension and fall back into the annular fluidized bed. This recirculation is called internal recirculation of solids, the stream of solids circulating in this internal circulation is usually significantly greater than the amount of solids supplied to the reactor from the outside. The (smaller) amount of unprecipitated solids is discharged from the mixing chamber together with the first gas or gas mixture. The retention time of the solids in the reactor can be modified within wide limits by selecting the height and area (cross section) of the annular fluidized bed and adapted to the desired heat treatment. Due to the high charge of solids, on the one hand, and the good suspension of the solids in the gas stream, on the other hand, excellent conditions are obtained for a good transfer of mass and heat above the orifice region of the central tube. The quantity of solids entrained from the reactor with the gas stream is separated from the gas stream and completely or at least partially recirculated to the reactor, with the recirculation being rapidly fed into the stationary fluidized bed. The stream of solid matter recirculated in this way to the annular fluidized bed normally falls in the same order of magnitude as the stream of solid matter supplied to the reactor from outside. Apart from the excellent use of energy, another advantage of the method according to the invention consists in the possibility of quickly, easily and reliably adjusting the energy transfer of the method and mass transfer to the requirements by changing the speeds of flow of the first gas or mixture of gases and the fluidization gas. Therefore, the quantity of fluidisation gas (primary gas) supplied can be metered so that, for example, a reducing atmosphere is obtained in the annular fluidized bed, while an intensive transfer of liquid takes place in the mixing chamber. hot. To ensure a particularly effective heat transfer in the mixing chamber and a sufficient retention time in the reactor, the gas velocities of the first gas and the fluidization gas are preferably adjusted for the fluidized bed such that the Froude Numbers of particle, without dimensions, (Frp) are from 1.15 to 20 in the central tube, in particular between approximately 7 and 8, 0.115 to 1.15 in the annular fluidized bed, in particular between approximately 0.4 and 0.5, and / or 0.37 to 3.7 in the mixing chamber, in particular between approximately 1.5 and 1.8. Each of the Particle Froude Numbers are defined by the following equation: with u = effective gas flow velocity, in m / s. ps = density of a solid particle, in kg / m3. Pf = effective density of the fluidizing gas, in kg / m3 dp = average diameter in m of the particles in the reactor inventory (or of the particles that form) during the operation of the reactor. g = gravitational constant, in m / s.

When this equation is used, it should be considered that dp does not indicate the average diameter (d50) of the material used, but the average diameter of the reactor inventory formed during the operation of the reactor, which can differ significantly from the average diameter of the material used. (primary particles). Even from very finely grained material with an average diameter of, for example, 3 to 10 μm, the particles (secondary particles) with an average diameter of 20 to 30 μm can for example be formed during the heat treatment. On the other hand, some materials, for example, ores, are calcined during the heat treatment. According to a development of the invention, it is proposed to adjust the bed height of the solids in the reactor such that the annular fluidized bed extends, for example, at least partially beyond the end of the upper orifice of the central tube in a few centimeters and, in this way, solids are constantly introduced into the first gas or mixture of gases and carried by the gas stream into the mixing chamber located above the orifice region of the central tube. In this form, a particularly high load of solids in the suspension is achieved above the orifice region of the central tube. By means of the method according to the invention, in particular sulphate-containing solids, such as iron sulphate or magnesium sulfate, can be exposed to an effective heat treatment, in particular to obtain oxides from salts. The solids can be introduced into the reactor when preheated to, for example, about 350 ° C. The generation of the amount of heat necessary for the operation of the reactor can be effected in any manner known to those skilled in the art for this purpose, for example also by internal combustion of fuel in the reactor. It is advantageous to operate the reactor at a pressure of 0.8 to 10 bar, and particularly preferably at atmospheric pressure. Through the central tube, hot gas can be supplied to the reactor, for example air preheated to about 300 to 500 ° C. Preheated air at 300 to 500 ° C can also be supplied to the reactor as a fluidizing gas. Preferably, the fuel is only incompletely burned in the annular fluidized bed under a reducing atmosphere and will only burn completely in the mixing chamber. Upstream of the reactor, one or more preheating steps can be provided, in which the solids are suspended, dried and / or preheated in a preheating stage before the thermal treatment that takes place in the reactor, where at least part of the content Moisture of solids can be eliminated.

A use of the exhaust gas obtained from the reactor, rich in sulfur dioxide, can be achieved when a plant for the production of sulfuric acid downstream of the reactor is provided. A plant according to the invention, which is particularly suitable for the embodiment of the method described above, has a reactor that constitutes a fluidized bed reactor for the thermal treatment of finely granulated solids, in particular salts, the reactor has a gas supply system that is formed in such a way that the gas flowing through the gas supply system it draws solids from an annular and stationary fluidized bed, which at least partially surrounds the gas supply system, into the mixing chamber. Preferably, this gas supply system extends into the mixing chamber. It is also possible, however, to let the gas supply system terminate below the surface of the annular fluidized bed. The gas is then introduced into the annular fluidized bed, for example, via lateral openings, by drawing solids from the annular fluidized bed into the mixing chamber due to its flow rate. According to the invention, the gas supply system preferably has a central tube that extends upwards in substantially vertical form from the lower region of the reactor, preferably inside the mixing chamber, central tube which is surrounded by a chamber that at least partially extends around the central tube and in which the annular and stationary fluidized bed is formed. The central tube can constitute a nozzle in its outlet opening and / or has one or more openings distributed around its housing surface, so that during the operation of the reactor solids constantly enter the central tube through the openings and are entrained by the first gas or mixture of gases through the central tube inside the mixing chamber. Of course, two or more central tubes with different or identical dimensions and shapes can also be provided in the reactor. Preferably, however, at least one of the central tubes is disposed approximately centrally with reference to the cross-sectional area of the reactor. According to a preferred embodiment, a separator, in particular a cyclone which is used to separate solids, is provided downstream of the reactor, separator which has a solids circuit leading to the annular fluidized bed of the reactor and / or to a stage of treatment downstream. According to the invention, the finished product can also be extracted directly from the reactor through a solids circuit leading outside the annular fluidized bed of the reactor. To provide a reliable fluidization of the solids and the formation of a stationary fluidized bed, a gas distributor is provided in an annular chamber of the reactor, which divides the chamber into an upper annular fluidized bed and a lower gas distributor chamber, the gas distributor chamber being connected to a supply conduit for fluidizing gas. Instead of the gas distribution chamber, a gas distributor composed of tubes and / or nozzles or an investment box can also be provided. For the use of the exhaust gas obtained in the reactor, which has a sulfur dioxide content, a plant is provided for producing sulfuric acid downstream of the reactor according to an embodiment of the invention. Means may be provided for the deflection of the solids and / or fluid flows in the annular fluidized bed and / or the mixing chamber according to the invention. It is possible, for example, to place an annular weir, whose diameter falls between that of the central tube and that of the reactor wall, in the annular fluidized bed such that the upper edge of the weir protrudes beyond the level of the solids obtained during the operation, while the lower edge of the weir is disposed at a distance from the gas distributor or the like. In this way, the solids separated out of the mixing chamber in the vicinity of the reactor wall must first pass through the landfill at the lower end thereof, before they can be carried away by the gas flow of the central pipe of the reactor. return inside the mixing chamber. In this form, an exchange of solids is forced into the annular fluidized bed, so that a more uniform retention time of the solids in the annular fluidized bed is obtained. Developments, advantages and possible applications of the invention can also be taken from the following description of a modality, as well as the drawing. All the features described and / or illustrated form the subject matter of the invention per se or in any combination, regardless of their inclusion in the claims or their references.

BRIEF DESCRIPTION OF THE DRAWINGS The only Figure shows a process diagram of a method and a plant according to one embodiment of the present invention.

Detailed Description of the Preferred Modes In the method shown in Figure 1, which is particularly suitable for thermal treatment of sulfate-containing salts or similar solids, a solid is charged into a reactor 1 through a supply conduit 2. The, for example, cylindrical reactor 1 has a central tube 3 arranged approximately coaxial with its longitudinal axis, the central tube which extends upwards from the bottom of the reactor 1 in a substantially vertical form. In the vicinity of the bottom of the reactor 1, an annular gas distribution chamber 4 is provided, which is terminated at the top by a gas distributor 5 having through openings. A supply duct 6 is opened inside the gas distribution chamber 4. In the vertically upper region of the reactor 1, which forms a mixing chamber 7, a discharge duct 8 is arranged, which is opened inside a separator 9 constituting a cyclone. When the solids are now introduced into the reactor 1 via the supply conduit 2, a layer annularly surrounding the central tube 3 is formed in the gas distributor 5, which layer is referred to as an annular fluidized bed 10. Fluidizing gas introduced inside the gas distributor chamber 4 through the supply conduit 6 it flows through the gas distributor 5 and fluidizes the annular fluidized bed 10, so that a stationary fluidized bed is formed. The velocity of the gases supplied to the reactor 1 is adjusted such that the particle Froude Number in the annular fluidized bed 10 is between approximately 0.4 and 0.5. Due to the supply of more solids inside the annular fluidized bed 10, the solids level in the reactor 1 rises to such a point that the solids penetrate into the orifice of the central tube 3. Through the central tube 3, it is introduced at the same time within of reactor 1 heated air. The velocity of the gas supplied to the reactor 1 is preferably adjusted such that the Number of Froude of Particle in the central tube 3 is approximately 7 to 8 and in the mixing chamber approximately 1.5 to 1.8. Due to these high gas velocities, the gas flowing through the central tube 3 draws solids from the annular fluidized bed 10 into the mixing chamber 7 as it passes through the region of the upper orifice. Since the level of the annular fluidized bed 10 is raised above the upper edge of the central tube 3, the solids flow on this edge inside the central tube 3, by means of which a widely mixed suspension is formed. central tube 3 can be straight, corrugated or it can have some other configuration, for example it can be toothed. It may also have side intake openings in the carcass region. As a result of the decrease in the flow velocity due to the expansion of the gas jet upon leaving the central tube and / or the impact on one of the walls of the reactor, the entrained solids rapidly lose speed and fall partially back into the reactor. annular fluidized bed 10. The quantity of non-precipitated solids is discharged from the reactor 1 together with the gas stream via the conduit 8. Between the reactor regions of the stationary annular fluidized bed 10 and the mixing chamber 7 a circulation of solids, by means of which a good heat transfer is ensured. Prior to further processing, the solids discharged through the conduit 8 are separated from the gases or gas mixture in the cyclone 9. In the method shown in the Figure, preheated and finely grained salt with a grain size of less than 3 mm it is, for example, charged to reactor 1 via a screw conveyor and fluidized by preheated (primary) air in the annular fluidized bed 10. At the same time, for example, gaseous fuel together with primary air or solids or liquid fuel is introduced into the annular fluidized bed 10 via a separate supply conduit 11. In the annular fluidized bed, the fuel is incompletely burned under reducing conditions. The amount of primary air supplied for the fluidization can be varied within wide ranges, so that, for example, a strongly reducing atmosphere is obtained in the annular fluidized bed 10, which promotes the decomposition of the salts. A large part of the heat transfer takes place in the mixing chamber 7 and is therefore not greatly influenced by the change in the supply of fluidizing gas within the annular fluidized bed 10. The reduction in the amount of primary air supplied in the addition prevents excessive cooling of the solids, so that at the same time they remain hot. Through the central tube 3, preheated air (secondary air) is supplied to the reactor 1, so that an intensive mixing of the pre-burned solids with the secondary air is possible in the mixing chamber. Under these oxidation conditions, a substantially complete combustion of the fuel is achieved in the mixing chamber 7, while at the same time an intense heat transfer takes place. Through the central tube 3, additional fuel can be supplied, so that the energy input inside the reactor 1 can be controlled without changing the reducing conditions in the annular fluidized bed 10. Constructively, the introduction of secondary air through the central tube 3 is effected in a simple manner, so that the manufacturing costs of reactor 1 can be decreased. The amount of solids that due to the high velocities of the gas flowing through the central tube 3 are entrained when they pass there and are discharged through the conduit 8 inside the cyclone 9, can either be dosed and recirculated to the bed fluidized ring 10 via conduit 12, in order to control the bed height of the solids in reactor 1, or to be supplied to a subsequent treatment in conjunction with the stream of solids extracted from annular fluidized bed 10 via conduit 13. Exhaust gas from the cyclone 9 downstream of the reactor 1 can be supplied to a plant not illustrated for the production of sulfuric acid via a conduit 14. In this form, the sulfur dioxide contained in the exhaust gas can be used. By changing the supply of air through the duct 6 (primary air) and the amount of fuel in the annular fluidized bed 10, the nitrates or chlorates can also be specifically decomposed under both reducing and oxidizing conditions, by supplying the fuel not to the annular fluidized bed 10, but first to the mixing chamber 7. A decomposition of other sulfates such as magnesium sulfate, a waste product from the production of fertilizers, is also possible. In the following examples, the invention will be explained with reference to two examples that demonstrate the invention, but do not restrict it.

Examples Example 1 Thermal decomposition of iron sulphate. In a plant according to the Figure, 240 Kg / h of pre-heated iron sulphate with a temperature of 350 ° C were introduced via line 2 into the annular fluidized bed in reactor 1. At the same time, a mixture of 20 Nm3 / h of preheated air with a temperature of 350 ° C and 17 kg / h of fuel (40,000 KJ / Kg) was introduced into the gas distributor chamber 4 via the conduit 6 and burned in the annular fluidized bed 10 under a reducing atmosphere. Through the central tube 3, another 180 Nm3 / h of preheated air with a temperature of 350 ° C were introduced into the reactor 1, which was mixed with the solids coming from the annular fluidized bed 10, the fuel burned incompletely and the primary air. In the mixing chamber 7, the fuel was completely burned. 310 Nm3 / h of exhaust gas with a temperature of 950 ° C and a sulfur dioxide content of 9.5% were obtained, which was discharged from the reactor through conduit 8. In this form, 120 Kg / h of oxide of iron could be extracted from the annular fluidized bed 10 via conduit 13.

EXAMPLE 2 Thermal decomposition of magnesium sulfate Through pipe 2, 2.39 t / h of anhydrous magnesium sulfate preheated at a temperature of 350 ° C was charged into the annular fluidized bed 10 of the reactor, where the solids were fluidized with 200 g. Nm3 / h of preheated air with a temperature of 400 ° C via the gas distribution chamber 4. In addition, they were introduced 450 kg / h of fuel (40,000 kJ / kg) inside the reactor through the gas distributor chamber. The fuel was incompletely burned in the annular fluidized bed 10 under a reducing atmosphere. Through the central tube 3, at the same time 3,800 Nm3 / h of preheated air with a temperature of 400 ° C were supplied to the reactor 1, air which was mixed are the solids coming from the annular fluidized bed 10, the fuel burned in a manner incomplete and the primary air supplied via conduit 6. In the mixing chamber 7, the fuel was then completely burned, so that 4,500 Nm3 / h of exhaust gas with a temperature of 1,130 ° C and a dioxide content were obtained. of sulfur of 10.5%, which was discharged from reactor 1 via conduit 8. At the same time, 1.07 t / h of magnesium oxide could be extracted from the annular fluidized bed 10 via conduit 13.

List of reference numerals 1 Process 2 Supply duct 3 Central pipe 4 Gas distribution chamber 5 Gas distributor 6 Supply duct 7 Mixing chamber 8 Duct 9 Separator 10 Annular fluidized bed 11 Fuel supply 12 Solid return duct 13 Conduit of solids 14 Conduit of gas of escape

Claims (17)

1. Claims 1. A method for the treatment of finely granulated solids, in particular for the thermal decomposition of salts, in which the solids are heated to a temperature of 200 to 1,400 ° C in a fluidized bed reactor (1), characterized in that a first gas or mixture of gases is introduced from below through a supply pipe (3) of preferably central gas into a mixing chamber (7) of the reactor (1), the supply pipe (3) being when less partially surrounded by an annular and stationary fluidized bed (10), which bed is fluidized by the supply of fluidizing gas, and in which the gas velocities of the first gas or gas mixture, as well as the fluidization gas for the annular fluidized bed (10) are adjusted in such a way that the Particle Froude Numbers in the supply tube (3) are between 1 and 100, in the annular fluidized bed (10) between 0.02 and 2, and in the cam The mixing method (7) between 0.3 and 30.
2. 2. The method according to claim 1, characterized in that the Froude Part Number in the gas supply tube (30) is between 1.15 and 20, in particular between approximately 7 and 8.
3. The method according to claim 1 or 2, characterized in that the Froude particle number in the annular fluidized bed (10) is between 0.115 and 1.15, in particular between approximately 0.
4. 4 and 0.5. The method according to any of the preceding claims, characterized in that the Froude Part Number in the mixing chamber (7) is between 0.37 and 3.7, in particular between approximately 1.5 and 1.8.
5. 5. The method according to any of the preceding claims, characterized in that the height of the bed of solids in the reactor (1) is adjusted in such a way that the annular fluidized bed (10) extends beyond the end of the upper orifice of the gas supply tube (3), and that the solids are constantly introduced into the first gas or gas mixture and are carried by the gas stream into the mixing chamber (7) located above the orifice region of the gas. tube (3) of gas supply.
6. 6. The method according to any of the preceding claims, characterized in that the sulphate-containing solids such as iron sulfate or magnesium sulfate, preheated, for example at about 350 ° C, are supplied as starting material.
7. The method according to any of the preceding claims, characterized in that preheated air with a temperature of about 300 to 500 ° C is supplied to the reactor (1) through the gas supply tube (3).
8. The method according to any of the preceding claims, characterized in that preheated air with a temperature of about 300 to 500 ° C is supplied to the reactor (1) as a fluidizing gas.
9. The method according to any of the preceding claims, characterized in that fuel is introduced into the annular fluidized bed (10) and / or the mixing chamber (7) of the reactor (1) and that the reactor pressure (1) ) is between 0.8 and 10 bar.
10. The method according to claim 9, characterized in that the fuel is incompletely burned under a reducing atmosphere in the annular fluidized bed (10) and burns completely in the mixing chamber (7).
11. The method according to any of the preceding claims, characterized in that the exhaust gas from the heat treatment (1) is supplied to a downstream plant for the production of sulfuric acid.
12. 12. A plant for the treatment of finely granulated solids, in particular for carrying out a method as claimed in any of claims 1 to 10, which comprises a reactor (1) that constitutes a fluidized bed reactor for the treatment thermal, characterized in that the reactor (1) has a gas supply system that is formed in such a way that the gas flowing through the gas supply system draws solids from an annular and stationary fluidized bed (10), which at least partially surrounds the gas supply system, inside the mixing chamber (7). The plant according to claim 12, characterized in that the gas supply system has a gas supply pipe (3) extending upwards substantially vertically from the lower region of the reactor (1) inside the gas supply system. the mixing chamber (7) of the reactor (1), the gas supply pipe (3) being surrounded by a chamber which at least partially extends around the gas supply pipe and in which a fluidized bed is formed annular and stationary (10). The plant according to claim 12 or 13, characterized in that the gas supply pipe (3) is disposed approximately centrally with reference to the cross-sectional area of the reactor (1). The plant according to any of claims 12 to 14, characterized in that a separator (9) for separating solids is provided downstream of the reactor (1), and that the separator (9) has a solids conduit (12). ) leading to the annular fluidized bed (10) of the reactor (1), and / or a solids conduit (13) leading to a downstream treatment step. The plant according to any one of claims 12 to 15, characterized in that in the annular chamber of the reactor (1) a gas distributor (5) is provided, which divides the chamber into an upper annular fluidized bed ( 10) and a lower gas distribution chamber (4) which is connected to a supply conduit (6) for fluidizing gas. The plant according to any of claims 12 to 18, characterized in that a plant is provided for the production of sulfuric acid downstream of the reactor (1), which plant is connected to the exhaust gas conduit of the reactor (1) and / or the separator (9) downstream.