WO1993024219A1

WO1993024219A1 - Fluidized bed reaction pipe with gas distributor, and its use

Info

Publication number: WO1993024219A1
Application number: PCT/EP1993/001389
Authority: WO
Inventors: Ulrich Vogt; Peter Werner; Albert Kerber; Franciscus Van Dijen; Rudolf Strittmatter; Heinz-Withold Schmitz
Original assignee: Lonza Ag
Priority date: 1992-05-26
Filing date: 1993-05-26
Publication date: 1993-12-09
Also published as: AU4321493A

Abstract

A fluidized bed reaction pipe (1) made of sintered silicium carbide has a fritted plate (2) made of silicium nitride or carbide. The fritted plate is preferably sintered together from individual, preferably spheric, sintered silicium nitride bodies. The reaction pipe is characterized by good thermal and thermal change resistances and is suitable for carrying out reactions at up to 1650 DEG C. Also disclosed is a process for high temperature treatment of corundum micro-grains in the fluidized bed which can be carried out without the admixture of coarse grains and which supplies an agglomerate-free product.

Description

Fluid bed reaction tube with gas distributor and its use

The present invention relates to a high-temperature-resistant fluidized-bed reaction tube with a gas distributor and its use, in particular for the high-temperature treatment of corundum microgranules.

In chemical reactions in which both gaseous and solid substances are involved, carrying out in a fluidized bed often brings considerable advantages over other methods. These advantages are partly due to the good heat and shock transfer in the fluidized bed, partly because a fluidized bed is well suited for continuous processes because it behaves similarly to a liquid.

In practice, the use of the fluidized bed process is limited by the limited temperature resistance and / or chemical resistance of the reactors available. Furthermore, a low thermal shock resistance often hinders efficient use, especially on the laboratory and pilot plant scale, where short heating and cooling times are desired. In this area, mainly reaction tubes made of quartz glass or quartz material have been used, which have good chemical resistance and resistance to temperature changes and are relatively easy to machine. Disadvantages are a low thermal conductivity, which limits the heat input through the pipe wall, and the limitation to working temperatures below 1000 ° C. Tubes made of aluminum oxide ceramic, depending on the Al 2 O 3 content, allow temperatures of up to approx. 1800 ° C. and have better thermal conductivity than quartz glass, but their resistance to temperature changes is poor. This requires very slow heating and cooling to avoid cracking due to temperature stresses. In addition, problems arise at the pipe ends if they are in a colder zone than the central part or if they are to be additionally cooled.

Similar material problems arise for the gas distributor required to produce the fluidized bed.

Frit plates are preferably used as the gas distributor in small fluidized-bed reaction tubes. Chemical resistance plays a special role here because of the large surface area. Furthermore, the pore structure must not deteriorate significantly under the influence of the operating temperature and the coefficient of thermal expansion should

REPLACEMENT LEAF deviate as little as possible from that of the pipe material in order to avoid tension and risk of breakage when the temperature changes.

The object of the present invention was therefore to provide a fluidized bed reaction tube which is resistant to high temperatures and resistant to changes in temperature and has good chemical resistance, in particular in non-oxidizing and oxidizing atmospheres, with a frit plate which is matched to these properties.

According to the invention, this object has been achieved by the fluidized bed reaction tube according to claim 1.

It has been found that the high temperature resistance and temperature change resistance of sintered silicon carbide as tube material in combination with a frit plate based on silicon nitride or recrystallized silicon carbide results in a fluidized bed reaction tube of high thermal and chemical resistance.

The frit plate based on silicon nitride advantageously consists of individual sintered silicon nitride bodies with an average diameter of 0.5 to 5 mm, preferably 0.5 to 2 mm, particularly preferably 0.8 to 1 mm. These individual silicon nitride bodies are expediently sintered together with silicon nitride powder with the addition of sintering aids to form a gas-permeable body. Some of the individual sintered silicon nitride bodies are commercially available (e.g. as grinding media), but can also be produced from silicon nitride powder and sintering aids by methods known per se. Y2O3, A-2O3, MgO, SiO2 and Fe2O3, as well as the oxides of rare earths, such as e.g., are suitable as sintering aids for this as well as for sintering together the silicon nitride bodies for frit. CeÜ2 or La2θ3.

The silicon nitride bodies from which the frit plate is composed are preferably essentially spherical. They differ from each other in their diameter advantageously by no more than a factor of 2-3 in order to ensure a uniform pore distribution in the frit. A broad particle size distribution leads to a reduced permeability and thus to a higher pressure drop at the frit during operation.

The frit plates according to the invention have a pressure drop of the order of 100 Pa when air or nitrogen flows through them at a flow rate (calculated as plug flow in the empty tube) of 0.1 m / s.

REPLACEMENT LEAF The frit plate is preferably fastened in the fluidized bed reaction tube according to the invention using mortar which is resistant to high temperatures. Mortars based on silicon carbide are particularly preferred for this purpose. Mortars of this type, which contain a hydraulic binder, are sold, for example, by Höganäs AB, Sweden, under the names SICTO refractory concrete or SICTO refractory cement. They have good adhesion to both the silicon carbide tube and the frit plate, high temperature resistance and good chemical resistance to both reducing and oxidizing atmospheres.

A particularly preferred embodiment of the fluidized-bed reaction tube according to the invention additionally contains at least one tube made of a high-temperature-resistant material, the axis of which runs essentially parallel to the axis of the reaction tube and is felt through the frit plate from the upstream side, so that it Area of the fluidized bed ends. This means that the pipe opening is either flush with the surface of the frit plate or during operation lies within the fluidized bed. The other end of the additional tube is expediently located outside the fluidized bed reaction tube and is equipped with at least one connection for a suction and / or gas supply device.

Aluminum oxide, silicon nitride, silicon carbide or zirconium oxide is preferably used as the material for the additional tube. The additional tube can advantageously be used for emptying and / or feeding the fluidized bed by suctioning off or introducing the layer material by means of a gas stream. The side of the frit plate facing the fluidized bed is advantageously designed such that its surface is not flat, but rather forms a trough or a flat funnel, and the additional tube is attached to the deepest point of the trough in order to enable complete emptying ¬ lichen.

Reaction gases can also be introduced into the fluidized bed. This is particularly advantageous if several different gases are used in the fluidized bed process and these cannot or should not be mixed beforehand. This is also advantageous if thermally unstable gases such as ammonia have to be added because the cross-section of the additional tube can be selected to be significantly smaller than that of the reaction tube, so that there is a high flow rate and thus a short residence time until penetration into the fluidized bed . An inert gas can then optionally also be used to produce the fluidized bed.

REPLACEMENT LEAF The fluidized bed reaction tube according to the invention is basically suitable for carrying out all processes in which solid substances interact with one or more gaseous substances with the formation of a fluidized bed with the exchange of materials, heat exchange and / or the course of chemical reactions. These include, for example, drying processes, calcining processes, annealing and tempering processes and chemical reactions in which gaseous reactants have to be added and / or gaseous reaction products have to be removed. The working temperature can be up to 1650 ° C and both inert and reducing or oxidizing gases can be used. The only exceptions are gases that react with silicon carbide and or silicon nitride, such as hydrogen fluoride or fluorine.

A particularly preferred use of the fluidized-bed reaction tube according to the invention is the high-temperature treatment of corundum abrasive grain, in particular of such with average grain sizes of less than 100 μm (micro-grain). The high-temperature treatment of corundum abrasive grain is known per se and is usually carried out in rotary kilns. However, this method is unsuitable for the treatment of micrograins for several reasons. On the one hand, particles below a certain size are not transported correctly in a conventional rotary kiln with a slightly inclined axis, so that it is not possible to control the dwell time to the desired extent by a suitable choice of inclination and rotational speed. On the other hand, micro-grains made of corundum tend to agglomerate by sintering together at the required temperatures of, for example, 1200 ° C. While the transport can be forced by adding coarser grain ("drag grain"), the sintering together cannot be completely avoided. In any case, the addition of trailing grain is unfavorable because it has to be separated after passing through the oven. The entrained grain is then appropriately recycled and must be reheated each time the furnace is passed, since it is not practical to remove it when it is hot (for example at 1200 ° C.). This significantly increases energy consumption. For example, in the case of P-360 grains with an average grain size of approximately 40 μm, tests have shown that approximately twice the amount of drag grain had to be added in order to obtain reasonably useful results.

On the other hand, it was found that with high-temperature treatment in the fluidized bed, in particular in the reaction tube according to the invention, even much finer grains down to, for example, F-800 (grain size d5Q <6.5 μm) can be processed without problems and without agglomeration or sintering together. A change in the grain size distribution observed with very fine grains is essentially due to the discharge of very fine material

REPLACEMENT LEAF from the fluidized bed and not due to agglomeration.

Air is preferably used as the carrier gas for the fluidized bed. The treatment temperatures are of the same order of magnitude as those used in the conventional rotary tube processes, namely in the range from 900 to 1300 ° C. An advantage of the fluidized bed is the better temperature control compared to the rotary kiln. It results from the good heat transfer in the fluidized bed and the low thermal inertia. It has also been shown that, especially with very fine corundum grains, slightly lower temperatures are required than is the case with coarse grains and in a rotary kiln. On the basis of his experience, the person skilled in the art can recognize the change in the grinding properties from the color change of the corundum during high-temperature treatment and accordingly determine the required treatment temperature and duration.

Further preferred uses of the reaction tube according to the invention are, for example, the production of silicon nitride from silicon dioxide, carbon and nitrogen or silicon dioxide and ammonia, in which case the unreacted carbon can also be oxidized in the same reaction tube after the nitride formation has ended.

The heat is preferably supplied indirectly by means of a suitable tube furnace, but because of the electrical conductivity of the silicon carbide it is also possible to supply heat by passing current directly through the fluidized bed reaction tube, if appropriate in combination with indirect heating. All conventional tube furnaces which reach the desired temperatures are suitable as tube furnaces, for example those with heating elements made of silicon carbide.

The fluidized bed reaction tube according to the invention can be equipped in a manner known per se with conventional additional parts such as gas heaters, expansion vessels, sight glasses, cyclones, etc.

For the connection of such parts to the upper end of the reaction tube, a cooled flange is advantageously used, which can be attached, for example, with an adhesive or putty.

The figure is only intended to illustrate a preferred embodiment of the reaction tube according to the invention, without restricting the subject matter of the invention to this embodiment. This embodiment is composed of the actual

REPLACEMENT LEAF lend reaction tube 1 made of sintered silicon carbide, the silicon nitride frit plate 2 with inserted inner tube 3, which is guided through the plug 4 to the outside. Also through the stopper 4 is an inlet pipe 5 for the gas flowing through the frit plate 2 and with the filled solid (not shown) forming the fluidized bed. At the upper end, the reaction tube 1 is provided with a flange 6, on which an attachment 7 with a gas discharge tube 8 and a sight glass 9 is attached.

The following examples are intended to illustrate the production of the fluidized-bed reaction tube according to the invention and its uses, without restricting the subject matter of the invention to the embodiments described therein.

REPLACEMENT LEAF Example 1 :

Manufacture of a silicon nitride frit

A powdery mixture of silicon nitride (100 parts by weight) aluminum oxide (5 parts by weight), yttrium oxide (5 parts by weight) and polyethylene glycol (1 part by weight) was mixed with five times the amount of silicon nitride balls (0.8 - 1 mm 0 /) first dry and then mixed with the addition of approx. 8 ml water until a sticky but not flowing mass had formed. This mass (approx. 200 g) was placed in a mold treated with a release agent (silicone oil), consisting of a flat, conical core of approx. 3 cm in height and an annular jacket of 70 mm in inner diameter, smoothed and in the mold for 2 hours dried at 80 ° C. The green body was then carefully removed from the mold, completely dried in 24 h at 120 ° C. and finally sintered in a powder bed at 1750 ° C. for 2 h. A mixture of 40% by weight of silicon nitride and boron nitride and 10% by weight of yttrium oxide and aluminum oxide was used as the powder bed.

By inserting an appropriately shaped core, a frit plate with an opening for attaching an additional tube can be produced by the same method.

Example 2:

Reassembly tube assembly

A suitable clamp flange made of V4 A steel with a cooling jacket was pushed over one end (in operation above) of a tube made of sintered silicon carbide (inside diameter 70 mm, wall thickness 8 mm, length 1600 mm) and fastened with a two-component adhesive. This was a commercially available clamp flange (available e.g. from Balzers Vacuum Technology), which was equipped with a welded-on cooling jacket with hose connections for water cooling. The inside of the tube has been cleaned and degreased.

An aluminum oxide tube (outer diameter 10 mm, inner diameter 6 mm, length 800 mm) was inserted into a frit plate with a central opening of approximately 10 mm in diameter (produced according to Example 1), so that its end was flush with the frit surface Stand clamps fixed and cemented in from the other side of the frit plate (in operation below) with SiC mortar (SICTO 523 refractory concrete, Höganäs). After the SiC mortar had hardened (approx. 12 h), the frit plate with the cemented tube was introduced into the silicon carbide tube from the other end, fixed in the desired position with clips and from both sides with SiC mortar with the silicon carbide tube glued. After the mortar had hardened, the assembly was

REPLACEMENT LEAF Branded at 1500 ° C. The lower opening of the silicon carbide tube during operation was sealed and sealed with a suitable aluminum stopper with a central opening through which the inner tube was guided and a further hole for introducing the gas flowing through the frit.

Example 3:

Use of the reaction tube for the production of silicon nitride

The fluidized bed reaction tube according to Examples 1 and 2 was used in a vertical, electrically heated tube furnace (Carbolite ^ - STF 16/75, maximum temperature 1600 ° C., length of the temperature-constant zone approx. 190 mm) in such a way that the surface of the Frit plate was about 8 cm below the middle of the oven. The reaction tube was filled with 350 g of granules (grain size 60-400 μm) made of highly disperse silicon dioxide

U) in a molar ratio of SiO 2: C = 1: 3 and 10% by weight (based on SiO 2) of silicon nitride (as a nucleating agent). The granules were heated in a nitrogen stream (gas velocity in the reaction tube 23 cm / s) to 1500 ° C. for 4 h and then treated with air for 100 minutes at 700 ° C. to remove the excess carbon. An α-silicon nitride powder with a specific surface area (according to BET) of 12 m-^ / g and a free carbon content of 0.04% was obtained.

Example 4 (comparative example)

Corundum micro-grain annealing in a rotary kiln

In a rotary tube furnace (Al2O3 ceramic tube, inner diameter 350 mm, heated length 3 m), semi-precious corundum with the grain size P-360 was annealed at 1200 ° C and an average residence time of 20 min. The temperature was kept constant over the entire heated length.

Results: a) without drag grain:

The rotary tube grew from the beginning of the heating zone, initially in the radial, later also in the axial direction.

Within 1 h, caking of approx. 100 mm thickness built up in the radial direction, which after 3 h had grown approx. 30 cm in the product flow direction. There under

REPLACEMENT LEAF under these conditions, reasonable operation was not possible, the experiment was stopped after 3 hours

b) with drag grain:

Coarse-grained corundum with a grain size of 3 - 5 mm was used as the drag grain. Due to local caking in the rotary tube, the quantity ratio of dragging grain / grain to be treated had to be increased from 1: 1 to 2: 1 to ensure trouble-free operation.

The grain size distribution (determined with a US sedimentometer) was as follows (all values in μm

l. Com 3% value 50% value 95% value before annealing <87.0 <60.3 40.5 ± 1.5> 29.6 (specification) after annealing 93.9 81.6 64.6 50, 5

The grain coarsening (agglomeration) by sintering together can be clearly seen. The annealed material is in all values outside the specification for the P-360 grit.

Examples 5-10 (according to the invention)

The following corundum grains were treated in the fluidized bed reaction tube according to Example 2:

Semi-precious corundum: P-240, P-320, P-400, P-800 normal corundum: F-280, F-800,

Carrier gas was air, the annealing time was 20 min.

The starting material, the annealing temperature and the measurement method for the grain size are given.

Furthermore, the permissible grain size distribution according to the specification is that of the

Starting material, as well as that given after annealing (all values in μm)

REPLACEMENT LEAF Example 5

Normal corundum F-280, 1200 ° C (measurements Sedigraph)

The raw material was already slightly outside the specification on the rough side)

Example 6

Normal corundum F-800, 1100 ° C (measurements Sedigraph)

The fine material is probably discharged from the fluidized bed, so that a steeper grain size distribution is obtained and the dg-j-SrVeτt is outside the specification. This effect can be compensated for by using a somewhat finer starting material.

Example 7

Semi-precious corundum P-240, 1200 ° C (measurements US sedimentometer, also examples 8-10)

l. Grain 3% value 50% value 95% value

Specification <110 <81.7 58.5 ± 2.0> 44.0 before annealing 98.9 74.5 59.2 50.6 after annealing 96.7 82.1 60.4 51.0

REPLACEMENT LEAF

Example 9

Semi-precious corundum, P-400, 1180 ° C

l. Com 3% value 50% value 95% value

Specification <81.0 <53.9 35.0 ± 1.5> 25.2 before annealing 73.7 47.0 36.3 28.9 after annealing 68.1 49.7 36.6 29.0

Example 10

Semi-precious corundum, P-800, 1150 ° C

l. Com 3% value 50% value 95% value

Specification <67.0 <38.1 21.8 ± 1.0 -> 15.1 before annealing 59.4 33.5 22.6 15.1 after annealing 55.7 37.8 23.5 15.7

In Examples 9 and 10, the 50% values after annealing are somewhat too high, but this is due to the fact that the material before the annealing was already close to the upper tolerance limit.

REPLACEMENT LEAF

Claims

1. Fluidized bed reaction tube with gas distributor, characterized in that the reaction tube consists of sintered silicon carbide and contains a frit plate based on silicon nitride or recrystallized silicon carbide as gas distributor.

2. Fluidized bed reaction tube according to claim 1, characterized in that the frit plate based on silicon nitride from individual sintered silicon nitride bodies with an average diameter of 0.5 to 5 mm, preferably 0.5 to 2 mm, which with silicon nitride powder with the addition of sintering aids gas permeable body are sintered together, is built up.

3. Fluidized bed reaction tube according to claim 2, characterized in that the individual sintered silicon nitride body of the frit is substantially spherical.

4. Fluidized bed reaction tube according to one of claims 1 to 3, characterized in that the frit plate is fastened in the tube with high-temperature-resistant mortar.

5. Fluidized bed reaction tube according to claim 4, characterized in that the high temperature resistant mortar is a mortar based on silicon carbide.

6. Fluidized bed reaction tube according to one or more of claims 1 to 5, characterized in that within the reaction tube at least one tube made of high temperature-resistant material is attached, the axis of which runs essentially parallel to the axis of the reaction tube, which flows from the upstream side through the Frit plate is passed through and ends in the area of the fluidized bed and the other end is equipped with at least one connection for a suction and / or gas supply device.

REPLACEMENT LEAF

7. Fluidized bed reaction tube according to claim 6, characterized in that the inner tube consists of aluminum oxide, silicon nitride, silicon carbide or zirconium oxide.

8. Frit plates based on silicon nitride, consisting of individual sintered silicon nitride bodies with an average diameter of 0.5 to 5 mm, preferably 0.5 to 2 mm, which are sintered together with silicon nitride powder with the addition of sintering aids to form a gas-permeable body.

9. frit plates according to claim 8, characterized in that the individual sintered silicon nitride body are substantially spherical.

10. Use of the fluidized bed reaction tube according to one or more of claims 1 to 7 for carrying out methods of mass and / or heat exchange and chemical reactions involving solids in a gas stream to form a fluidized bed.

11. Use according to claim 10 for carrying out reactions between gaseous and solid reactants at temperatures up to 1650 ° C.

12. Process for high-temperature treatment of corundum grains, characterized in that they are brought into a fluidized bed state at 900 to 1300 ° C. with a carrier gas.

13. The method according to claim 12, characterized in that corundum grains with average grain sizes of less than 100 microns are used.

14. The method according to claim 12 or 13, characterized in that air is used as a carrier gas.

15. The method according to one or more of claims 12 to 14, characterized gekenn¬ characterized in that the high temperature treatment is carried out in a fluidized bed reaction tube according to claims 1 to 7.

REPLACEMENT LEAF