DE102011016066B4 - Process for the continuous microwave vacuum drying of honeycomb ceramic bodies and apparatus for carrying out the same - Google Patents

Abstract

A method of continuous microwave vacuum drying of honeycomb ceramic bodies (26) comprising continuously moving honeycomb ceramic bodies (26) arranged in a straight line through a microwave oven (12) from an input (16) to an output (22) for microwave irradiation, the gap ( 28) has a width b between directly successive honeycomb ceramic bodies (26), with 0 <b? 1/4 of the wavelength of the microwaves used, wherein the microwave oven (12) has a cylindrical wall (14) and the microwaves in at least two different radial planes via at least two over the circumference of the microwave oven (12) arranged at equal angular intervals microwave radiator (32, 34, 36) are irradiated, wherein the microwave radiators (32, 34, 36) from different radial planes to each other have an angular offset about the longitudinal axis (30) of the microwave oven (12).

Description

The present invention relates to a method and apparatus for continuous microwave vacuum drying of honeycomb ceramic bodies.

Extruders today are able to extrude 9 to 15 inches and larger honeycomb ceramic cross sections than green compacts. The larger the cross section and the smaller the extruded channels within the honeycomb ceramic bodies, the more sensitive the green compacts are to mechanical deformation in handling as well as in the subsequent drying. Predrying must be done quickly to mechanically stabilize the honeycomb ceramic bodies without causing deformation. For this purpose, in particular microwave systems are used, such as in the DE 10 2007 012 912 B3 and in the DE 10 2007 000 573 A1 disclosed.

Due to the low penetration depths, the microwave energy does not sufficiently reach the core of the large-volume honeycomb ceramic bodies in order to promptly initiate the diffusion of water vapor volumetrically in the atmospheric drying process. The unevenness of the energy input and the staggered diffusion processes create tensions and cracks in the honeycomb ceramic bodies, especially when it comes to dry quickly. Thus, an outer layer of a honeycomb ceramic body is first heated and dried until the heat reaches the core of the honeycomb ceramic body by heat conduction or by low residual microwave radiation. To compensate for the uneven drying, the drying times are increased in order to control tensions sensibly without cracking. However, even and rapid drying is very important in order to dry the honeycomb ceramic bodies evenly and economically without tension and without deformation. In addition, used high-absorbency ceramics, such as SiC in principle exacerbate this problem, while weakly absorbent. ceramic materials, such as cordierite, this problem is not as pronounced as in the highly absorbent SiC ceramic.

To solve this problem, drying in vacuum instead of atmospheric drying has been proposed. Due to the reduced evaporation temperature in vacuo, the conditions for drying in the core of honeycomb ceramic bodies can be generated, as long as the heating by microwave input in the core is still slightly above the evaporation temperature. It results in a more uniform drying, because the time for the start of drying for all locations in the volume of the honeycomb ceramic body begins at the same time. In addition, the honeycomb structure provides a large surface area and good diffusion properties to quickly release the free water vapor. In addition, a powerful vacuum system quickly removes free water vapor. Consequently, vacuum drying, more specifically microwave vacuum drying, offers outstanding advantages over microwave drying under atmospheric pressure.

From the WO 2009/080155 A1 For example, a method is known of drying a ceramic honeycomb body in vacuum under microwave irradiation in a continuous process.

The US 2008/0 023 886 A1 relates to a method for microwave drying ceramic bodies, wherein the ceramic bodies are arranged one behind the other at a distance which is equal to a quarter of the wavelength of the microwaves or smaller.

Furthermore, the disclosure WO 02/054829 A2 a method of processing ceramics using electromagnetic energy. The distance from adjacent ceramic bodies therein is not more than about half the wavelength of the microwaves.

• 1. Multimode-Mikrowellenverteilungen innerhalb eines Mikrowellenofens (Vakuumkammer),
• 2. die Tatsache, dass die eingetragene Energie an der Oberfläche der wabenkeramischen Körper größer als im Kern ist und für eine vorzeitige Austrocknung der Endbereiche (Stirnflächen) der wabenkeramischen Körper sorgt. Fokussiereffekte unterstützen diesen Effekt noch, was als Edge-Overheating bezeichnet wird.

In the microwave vacuum drying of individual honeycomb ceramic bodies, however, irregularities still occur during the course of the drying, which are caused by:

• 1. multimode microwave distributions within a microwave oven (vacuum chamber),
• 2. the fact that the registered energy at the surface of the honeycomb ceramic body is greater than in the core and ensures premature drying of the end regions (end faces) of the honeycomb ceramic body. Focusing effects still support this effect, which is called edge overheating.

The present invention is therefore based on the object to enable a more uniform and rapid continuous microwave vacuum drying of honeycomb ceramic bodies.

According to the invention, this object is achieved by a method for continuous microwave vacuum drying of honeycomb ceramic bodies, comprising continuously moving honeycomb ceramic bodies arranged in a straight line through a microwave oven from an input to an output for irradiation with microwaves, the gap between directly successive honeycomb ceramic bodies having a width b with 0 <b ≤ 1/4 of the wavelength of the microwaves used, the microwave oven having a cylindrical wall and the microwaves in At least two different radial planes are irradiated via in each case at least two arranged over the circumference of the microwave oven at equal angular intervals microwave radiator, wherein the microwave radiators from different radial planes to each other have an angular offset about the longitudinal axis of the microwave oven.

Furthermore, this object is achieved by a device for continuous microwave vacuum drying of honeycomb ceramic bodies, in particular for carrying out a method according to one of claims 1 to 5, comprising a microwave oven with a cylindrical wall and at least two microwave radiators, wherein the microwave radiator in at least one radial plane over the circumference the microwave oven are arranged at equal angular intervals and the microwave radiators from different radial planes to each other have an angular offset about the longitudinal axis of the microwave oven.

Conveniently, the width b is ≥ 0.5 cm. This allows the steam to escape better from the end faces of the honeycomb ceramic body.

Advantageously, the vacuum pressure in the microwave oven is set so that the temperature in the core of the honeycomb ceramic body is above the evaporation temperature of water based on the vacuum vapor pressure.

According to a particular embodiment of the invention, a honeycomb ceramic body is cyclically introduced into the microwave oven and at the same time a honeycomb ceramic body is ejected from the microwave oven.

According to a further particular embodiment of the invention, the honeycomb ceramic bodies are frozen in an entry lock before entering the microwave oven. This is in honeycomb ceramic bodies with a particularly large cross-section or with a particularly large. Length particularly advantageous. In fact, extruded honeycomb ceramic bodies (components) are becoming ever larger, so that mechanical stability is a limiting parameter. Deformations due to the influence of gravity on the net mass of the soft honeycomb ceramic destroy the calibration of the channels of the honeycomb. In particularly soft honeycomb ceramics, which may also be so large in diameter that the core is hardly heated, it may be useful to freeze them in advance and bring in the frozen state in such a deep vacuum below the triple point, bypassing the solid phase the liquid phase is vaporized directly into the gas phase (freeze-drying). In addition, freeze-drying has the advantage that water in the frozen state has very low dielectric losses and thus the penetration depths are determined only by the dielectric properties of the ceramic. Thus, the microwave energy in the frozen state can penetrate even deeper into the honeycomb ceramic body, in particular in ceramic materials with weakly absorbing dielectric properties, such as cordierite.

According to a particular embodiment, at least one temperature measuring device is provided for measuring the temperature on the surface of a honeycomb ceramic body.

In this case, in particular, a temperature control device may be provided for regulating the temperature on the surface of a honeycomb ceramic body.

Finally, the temperature control is conveniently designed so that the temperature is controlled by appropriate control of the microwave radiator.

The invention is based on the finding that a premature dehydration of the end regions is prevented or reduced by a specific choice of the gap between successive honeycomb ceramic bodies and thus a more uniform and rapid drying of honeycomb ceramic bodies is realized.

Surprisingly, more than 80 to 90% of the microwave energy can be coupled directly into the honeycomb ceramic body by virtue of the microwave coupling uniformly distributed over the circumference of the microwave oven according to the invention. This prevents further spreading of the microwaves into the microwave oven and also crosstalk to the adjacent microwave couplings. This prevents that standing waves can propagate and that a plasma forms.

Further features and advantages of the invention will become apparent from the appended claims and the following description in which several embodiments are explained in detail with reference to the schematic drawings. Showing:

1 a longitudinal sectional view of a device for continuous microwave vacuum drying of honeycomb ceramic bodies according to a particular embodiment of the invention, and

2 a radial sectional view of a device for continuous microwave vacuum drying of honeycomb ceramic bodies according to another particular embodiment of the invention.

At the in 1 shown device 10 Microwave radiators for microwave irradiation are omitted for better illustration. Said device 10 has a microwave oven 12 with a cylindrical wall 14 on. On the side of an entrance 16 is an entry lock 18 upstream. This has at least one Einfrierstutzen 20 for freezing honeycomb ceramic bodies 26 on.

An exit 22 The microwave oven is a discharge lock 24 downstream.

At least inside the microwave oven 12 are the honeycomb ceramic bodies 26 arranged in a straight line one behind the other, with a gap 28 (only one of which is marked) has a width b between directly consecutive honeycomb ceramic bodies, with 0 <b ≤ 1/4 of the wavelength in the microwave oven 12 used microwaves. The gaps can be the same width in a tolerance range, for different reasons but also different widths.

The honeycomb ceramic body 26 be continuously through the microwave oven 12 from the entrance 16 to the exit 22 moved rectilinearly, as indicated by the arrow F. The longitudinal axis L, which, as in the embodiment shown, preferably with the longitudinal axis 30 of the microwave oven 12 is identical, the successively arranged honeycomb ceramic body 26 runs parallel to the conveying direction F. More specifically, in the present case, the honeycomb ceramic body 26 transported horizontally.

Each time a honeycomb ceramic body from the entry lock 18 in the microwave oven 12 introduced and at the same time a honeycomb ceramic body 26 from the microwave oven 12 in the discharge lock 24 applied.

The vacuum pressure in the microwave oven 12 is set so that the temperature in the core of the honeycomb ceramic body 26 above the evaporation temperature of water based on the vacuum vapor pressure. Normally, the vacuum pressure in normal microwave drying at 20 to 40 mbar and in the microwave freezing technique in the range of 1 to 2 mbar.

Furthermore, it follows from the 1 that the honeycomb ceramic body 26 before entering the microwave oven 12 that is in the entry lock 18 be frozen.

In the 2 a radial plane is shown. In this are about the circumference of the microwave oven 12 that is, over the cylindrical wall 14 at the same distance from the center, that is, the longitudinal axis 30 of the microwave oven 12 three microwave radiators 32 . 34 and 36 or microwave coupling devices arranged at equal angular intervals φ = 120 °. In a further, not shown, radial plane, for example, again three microwave radiators, also at a distance of 120 ° to each other, but with an offset of, for example 60 ° to the microwave radiators 32 . 34 and 36 arranged offset in the other radial plane. Alternatively, for example, only two microwave radiators at equal angular intervals (180 °) or four microwave radiators at equal angular intervals (90 °) can be arranged in a radial plane. The arrangement of the microwave radiator should be chosen so that both the circumference of a sufficient number of microwave radiators irradiated the honeycomb ceramic body as well as axially the microwave radiators are offset so that sets a sufficient uniformity of the energy input for the transport or conveying speed. The arrangement of the microwave radiators should advantageously take place uniformly over the circumference of the microwave oven, in order to enable a uniform field distribution over the time average for the honeycomb ceramic body clocked at least virtually continuously through the microwave oven. Typically, the microwave powers are in the range of 0.1 to 1 kW per kilogram of load.

To monitor the surface temperatures of honeycomb ceramic bodies 26 on the central axis of the respective Mikrowelleneinkopplungen are temperature measuring devices 42 . 44 and 46 , such as pyrometers or infrared temperature measuring devices provided. About this temperature measurement, the temperature is monitored at the surface and controlled by a temperature control device (not shown), which controls the microwave radiator according to performance. It must not be exceeded certain maximum temperatures. Since the transport speed is very low, it is recommended to measure on the center axis.

The drying time is typically about 30 minutes to about 3 hours.

The features of the invention disclosed in the foregoing description, in the drawings and in the claims may be essential both individually and in any combination for the realization of the invention in its various embodiments.

Claims (9)

Process for the continuous microwave vacuum drying of honeycomb ceramic bodies ( 26 ), comprising continuously moving honeycomb ceramic bodies arranged in a straight line one behind the other ( 26 ) through a microwave oven ( 12 ) from an entrance ( 16 ) to an output ( 22 ) for irradiation with microwaves, wherein the gap ( 28 ) between directly successive honeycomb ceramic bodies ( 26 ) has a width b, where 0 <b ≦ 1/4 of the wavelength of the microwaves used, wherein the microwave oven ( 12 ) a cylindrical wall ( 14 ) and the microwaves in at least two different radial planes over at least two over the circumference of the microwave oven ( 12 ) arranged at equal angular intervals microwave radiator ( 32 . 34 . 36 ), wherein the microwave radiators ( 32 . 34 . 36 ) of different radial planes to each other an angular offset about the longitudinal axis ( 30 ) of the microwave oven ( 12 ) exhibit. A method according to claim 1, characterized in that the width b ≥ 0.5 cm. Method according to claim 1 or 2, characterized in that the vacuum pressure in the microwave oven ( 12 ) is set so that the temperature in the core of the honeycomb ceramic body ( 26 ) is above the evaporation temperature of water based on the vacuum vapor pressure. Method according to one of claims 1 to 3, characterized in that cyclically each one honeycomb ceramic body ( 26 ) in the microwave oven ( 12 ) and at the same time a honeycomb ceramic body ( 26 ) from the microwave oven ( 12 ) is applied. Method according to one of claims 1 to 4, characterized in that the honeycomb ceramic body ( 26 ) before entering the microwave oven ( 12 ) in an entry lock ( 18 ) are frozen. Contraption ( 10 ) for the continuous microwave vacuum drying of honeycomb ceramic bodies ( 26 ), in particular for carrying out a method according to one of the preceding claims, comprising a microwave oven ( 12 ) with a cylindrical wall ( 14 ) and at least two microwave radiators ( 32 . 34 . 36 ), wherein the microwave radiators ( 32 . 34 . 36 ) in at least one radial plane about the circumference of the microwave oven ( 12 ) are arranged at equal angular intervals and the microwave radiators from different radial planes to each other an angular displacement about the longitudinal axis ( 30 ) of the microwave oven ( 12 ) exhibit. Contraption ( 10 ) according to claim 6, characterized in that at least one temperature measuring device ( 42 . 44 . 46 ) for measuring the temperature at the surface of a honeycomb ceramic body ( 26 ) is provided. Contraption ( 10 ) according to claim 7, characterized in that a temperature control device for regulating the temperature on the surface of a honeycomb ceramic body ( 26 ) is provided. Contraption ( 10 ) according to claim 8, characterized in that the temperature control device is designed to the temperature by appropriate control of the microwave radiator ( 32 . 34 . 36 ).

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