RU2847701C1 - Microwave installation for drying bulk materials - Google Patents

Abstract

FIELD: mining, chemical, food, pharmaceutical industries; processing of agricultural products.

SUBSTANCE: invention relates to the field of microwave drying of bulk materials. In a microwave installation for bulk materials, containing a housing with a frame in the form of a fixed base with a frame installed on it with a mechanism for changing its position relative to the horizontal plane, a working heating chamber 1 rigidly fixed on the frame, connected to a drive system, the working chamber is equipped with at least three waveguide-slot emitters, a loading hopper and a discharge sluice for bulk material, located diametrically opposite each other on the upper and lower walls of the working chamber. In the upper part of the working chamber, along the longitudinal axis, there are at least three windows for connecting, via radio-transparent fluoroplastic inserts, at least three waveguide-slot emitters, each of which is connected to two microwave generators; on both sides between the waveguide-slot emitters, there are boundary waveguide tubes designed to supply heated air and remove moisture vapour through an exhaust pipe using an exhaust fan; the unloading gate is designed as boundary waveguide tubes, inside the working chamber along its longitudinal axis there is a shaft to which feed/unloading screws and blades are connected, designed to mix the bulk material; the shaft is connected to an electric drive for shaft rotation; the working chamber housing.

EFFECT: increased efficiency of microwave drying of bulk materials.

6 cl, 6 dwg

Description

Field of technology to which the invention relates

The invention relates to the field of microwave drying of bulk materials and can be used in the mining, chemical, food, pharmaceutical industries, as well as in the processing of agricultural products.

State of the art

A device for microwave drying of bulk materials is known (see patent RU 2141180, IPC H05B 6/64, published 10.11.1999), which comprises a microwave generator connected to a rectangular waveguide, and loading and unloading units connected to a heating chamber made of a material transparent to microwave energy, connected to the wide wall of the rectangular waveguide, at the point of connection coaxially to the chamber outside it, two circular waveguides are installed, the diameters of which do not exceed the size of the wide wall of the rectangular waveguide, and the length of each circular waveguide is not less than one wavelength in the waveguide. To obtain an extended high-performance installation, several sections, each of which contains at least one microwave generator connected to a rectangular waveguide connected to two circular waveguides, are connected to each other via circular waveguides.

A disadvantage of this device is the use of a circular waveguide coupled to a rectangular waveguide, which has poor matching, as well as the use of a matched absorbing load, which reduces the device's efficiency. Furthermore, the lack of drying process control and microwave power regulation prevents the drying process from adapting to changes in material moisture content.

A microwave drying unit for bulk materials is known (see patent RU 2255434. IPC H05B 6/64, published 27.06.2005), which comprises a housing, a drying chamber, a microwave generator, an emitter in the form of two meander lines mounted so as to be able to rotate about an axis, a cover, matchers, a drive, a cylindrical screen with openings, and an air supply unit connected to the cavity between the housing and the screen. The screen is designed as a ribbed reflector with triangular ribs formed along the circumference of the screen, having a depth of λ/8 and a distance between the ridges of λ/4, and the cavity between the housing and the screen is divided into an even number of alternating channels for the supply and exhaust of air.

The disadvantage is the lack of forced mixing of the bulk material, which reduces the drying efficiency, as well as the lack of means for controlling temperature, humidity, and microwave power regulation, which does not allow maintaining the required performance of the microwave unit when the moisture content of the materials changes.

A known installation for microwave processing of bulk products or materials (see patent RU 2479954, IPC H05B 6/64, published 20.04.2013) comprises a working chamber connected to a microwave generator, loading and unloading devices located diametrically opposite on the upper and lower walls of the working chamber, and a transport device made of a radio-transparent material and mounted horizontally with the ability to change the angle of inclination. The working chamber is equipped with another microwave generator and is rigidly attached to a frame. The frame is equipped with connections to a fixed base on two opposite sides in the horizontal plane. Moreover, the connection connections are movable to ensure the possibility of swinging the frame together with the working chamber about their horizontal axis. Swinging is achieved using a drive system, which in turn is connected to the frame. The working chamber's transport device is designed as trays, rigidly attached to the front and rear walls of the working chamber in three tiers relative to each other along the vertical axis. The upper and lower trays are installed at an angle different from the middle tray's relative to the horizontal axis of rotation. The horizontal width of each tray, transverse to the axis of rotation, is equal to the width of the working chamber.

A drawback of this setup is the lack of a forced moisture vapor removal system from the working chamber, which reduces drying efficiency. Furthermore, when placing radio-transparent trays containing bulk material one above the other, the material on the top tray, located closer to the microwave emitters, will absorb microwave energy more intensely, resulting in uneven heating and drying throughout the material.

A multifunctional microwave conveyor unit for drying and microwave processing of bulk materials is known (see patent RU 139803, IPC F26B 17/04, F26B 3/347 published 20.04.2014), which comprises a supporting frame, a dosing hopper, a conveyor, microwave generators, an exhaust system, a fan, and humidity sensors. The conveyor is stretched with a Teflon-coated mesh. The unit contains two or more working chambers divided into microwave heating, holding, and cooling zones; a drying agent uniform distribution system consisting of air ducts, an inlet diffuser, a housing, guides, and two rows of outlet diffusers with rectangular adjustable openings positioned one above the other at a 90° angle; temperature sensors; a material height leveling device; a gear motor; a conveyor mesh tension adjustment device; wave traps designed as quarter-wave transducers; load absorbers; an absorber-lined discharge channel; and a control unit. The unit operates at a frequency of 2.45 GHz.

The disadvantage of this device is the uneven drying of bulk materials.

The closest to the claimed device is a microwave drying unit for highly dispersed products (see patent RU 200970, IPC H05B 6/64, F24C 7/02, F26B 3/32, published 20.11.2020), which comprises a unit body with a frame in the form of a fixed base with a frame mounted on it with a mechanism for changing its position relative to the horizontal plane. A working heating chamber, connected to a drive system, is rigidly fixed to the frame. Moreover, the working chamber is equipped with two microwave radiation sources, as well as a loading hopper and an unloading airlock, located diametrically opposite each other on the upper and lower walls of the working chamber. The unit is provided with starting and drive mechanisms, control and regulation devices. The working chamber of the unit is designed as a rectangular elongated parallelepiped, inside which a dielectric drum is located with the ability to rotate along the longitudinal axis. The latter is designed as a hollow cylinder with open ends, secured to the axis of rotation by radial spokes. The inner surface of the dielectric drum has longitudinally arranged baffle blades. The loading hopper is equipped with feed chutes for feeding the dispersed product directly into the drum. The discharge sluice consists of openings located at the bottom of the working chamber, connected to sections of transextreme waveguides. The working chamber is equipped with a forced ventilation system and another microwave radiation source. All three microwave radiation sources are mounted on different walls of the working chamber and are designed as three magnetron generators, each equipped with its own transextreme waveguide with mutual polarization isolation. The system operates at a frequency of 2.45 GHz.

A disadvantage of the presented setup is the introduction of microwave energy using transverse waveguides, which have low efficiency and create an uneven distribution of electric field strength near the emitters, leading to uneven heating and drying throughout the material. Furthermore, the use of this setup is limited to drying highly dispersed products.

Disclosure of the essence of the invention

The technical problem is to develop a microwave drying system for bulk materials that ensures uniform distribution of microwave power density, temperature field in the bulk material, and acceleration of the drying process.

The technical result consists in increasing the efficiency of microwave drying of bulk materials due to the intensification of heat and mass transfer under the influence of microwave energy, convective surface heating and mixing of bulk material in the working chamber.

The technical result is achieved in that in a microwave installation for drying bulk materials, comprising an installation body with a frame in the form of a fixed base with a frame mounted on it with a mechanism for changing its position relative to a horizontal plane, a working heating chamber rigidly fixed to the frame, connected to a drive system, wherein the working chamber is provided with at least three waveguide-slot emitters, as well as a loading bin, a loading lock and an unloading lock for bulk material, located diametrically opposite on the upper and lower walls of the working chamber, according to the solution, the working chamber of the installation has a cylindrical shape, in its upper part, along the longitudinal axis, at least three windows are made for connecting through radio-transparent fluoroplastic inserts, at least three waveguide-slot emitters, each of which is connected to two microwave generators; On both sides between the waveguide-slot emitters there are mounted cut-off waveguide tubes, designed with the possibility of supplying heated air and removing moisture vapor through an exhaust pipe using an exhaust fan; on opposite sides of the working chamber, in the upper and lower parts thereof, there are dispensers with an electric drive; the loading and unloading airlocks are designed in the form of cut-off waveguide tubes; a loading hopper is connected to the loading airlock; inside the working chamber, along its longitudinal axis, there is a shaft to which a feed auger and blades designed with the possibility of mixing bulk material are connected; the shaft is connected to an electric drive for shaft rotation; moisture meters are mounted in the body of the working chamber, above the loading airlock and the bulk material unloading airlock, and a contactless pyrometer is mounted above the unloading airlock; wherein the microwave generators, the dispensers with an electric drive, the electric drive for rotating the shaft of the working chamber and the tilt mechanism are electrically connected to the control unit.

An electric heater is used to supply heated air.

On one side, the frame is attached to a fixed base via a hinge. The centers of the middle waveguide slot emitters are positioned at the center of the vertical axis of symmetry of the working chamber, while the remaining waveguide slot emitters are positioned on either side of the vertical axis, offset by 60°.

Waveguide slot radiators are made from a waveguide tube, one of the wide walls of which is designed as a slot structure.

Each pair of blades is secured to the shaft by means of two radial spokes located on one common axis passing through the axis of rotation of the shaft, while the relative position of each pair of radial spokes with the blades is offset in the cross-section of the working chamber by an angle of 90°, and in the axial direction along the shaft by a distance of L = d, where d is the width of the mixing blade.

Brief description of the drawings

The invention is explained by means of drawings, where Fig. 1 shows a structural diagram of the claimed installation; Fig. 2 shows a cross-section of the working chamber; Fig. 3 shows a general view of the claimed installation without connecting microwave generators, a control system and supply and exhaust ventilation; Fig. 4 shows a graph of the change in temperature over time at the control point of the material at the outlet of the working chamber ( a ) and a graph of the change in microwave power during its regulation over time ( b ); Fig. 5 shows the calculated distribution of the electric field strength in the longitudinal section of the microwave chamber; Fig. 6 shows the calculated isotherms in the longitudinal section of the bulk material at the time of 30 minutes.

The following positions are indicated on the drawings:

1 – working chamber;

2 – waveguide slot radiators;

3 – radio-transparent fluoroplastic insert;

4 – extremal waveguide tube;

5 – electric heater;

6 – exhaust pipe;

7 – exhaust fan;

8 – dispenser with electric drive;

9 – loading gate;

10 – unloading gateway;

11 – loading hopper;

12 – shaft;

13 – feed auger;

14 – shoulder blade;

15 – spoke;

16 – electric shaft rotation drive;

17 – frame;

18 – hinge;

19 – base;

20 – tilt mechanism;

21 – microwave generator;

22 – control unit;

23 – bulk material;

24 – humidity meter;

25 – non-contact thermometer.

Implementation of the invention

The claimed installation contains a working chamber 1 of cylindrical shape, in the upper part of which there are rectangular windows in the amount of n, for connecting n waveguide-slot emitters 2 through radio-transparent fluoroplastic inserts 3, where n is at least equal to three (Fig. 1, Fig. 2).

In the upper part of the working chamber 1 there are installed overpressure waveguide tubes 4, to which an electric heater 5, an exhaust pipe 6 with an exhaust fan 7 are connected. On the opposite sides of the working chamber, in its upper and lower parts, there are dispensers with an electric drive 8, made in the form of a rotary valve. The loading sluices 9 and unloading 10 are made in the form of overpressure waveguide tubes; a loading hopper 11 is connected to the loading sluice 9. Inside the working chamber, along its longitudinal axis, there is a shaft 12, to which a feed auger 13 and blades 14 made with the possibility of mixing the bulk material are connected; the shaft 12 is connected to an electric drive for rotating the shaft 16. In the body of the working chamber 1, above the loading sluice 9 and the unloading sluice 10 of the bulk material, moisture meters 24 are installed, above the unloading sluice 10 - a contactless pyrometer 25 is installed; wherein the microwave generators 21, the dispensers with electric drive 8, the electric drive for rotating the shaft 16 of the working chamber and the tilt mechanism 20 are electrically connected to the control unit 22.

The tilt mechanism 20 is mounted on a fixed base 19.

Each of the waveguide-slot emitters 2 is connected to two microwave generators 21. The microwave generators 21, dispensers 8, electric drive for rotating the shaft 16 of the working chamber and the tilt mechanism 20 are controlled by a control unit 22. The waveguide-slot emitters 2 are made of waveguide

a pipe, one of the wide walls of which is made in the form of a slot structure, ensuring a uniform distribution of the microwave energy density in the bulk material being processed 23. The design of a waveguide with a slot structure is described, for example, in Russian patent No. 2454786.

For uniform distribution of microwave power density and temperature field in bulk material 23 during its movement in working chamber 1, waveguide-slot emitters 2 are placed in the upper part of the working chamber along its longitudinal axis in accordance with Figs. 1-3. The centers of the middle waveguide-slot emitters 2 are placed in the center of the vertical axis of symmetry of the working chamber 1, the remaining waveguide-slot emitters 2 are placed on both sides relative to the vertical axis with an offset of 60° (Fig. 2).

To improve the mixing of bulk material 23 and to intensify the drying process, each pair of mixing blades 14 is secured to shaft 12 by means of two radial spokes 15 located on one common axis passing through the axis of rotation of shaft 12 (Fig. 1-2). In this case, the relative position of each pair of radial spokes 15 with blades 14 is offset in the cross-section of the working chamber by an angle of 90° (Fig. 2), and in the axial direction along the shaft - offset by a distance L = d, where d is the width of the mixing blade (Fig. 1).

Radio-transparent fluoroplastic inserts 3 (Fig. 1-2) provide input of microwave energy into the working chamber 1 and protection of waveguide-slot emitters 2 and microwave generators 20 from moisture and particles of bulk material getting into them.

For automatic regulation of the heating temperature, the relative humidity of the bulk material 23 at the outlet of the installation and the productivity of the installation, a microprocessor control unit 22 is used, with the help of which, according to signals from the humidity meters 24 at the inlet and outlet of the installation and the non-contact pyrometer 25 at the outlet of the installation, the power of the microwave generators 21 is regulated, the speed of movement of the bulk material is regulated using the tilt mechanism 20, the position of the dispensers is regulated using the electric drive of the dispensers 8, the speed of rotation of the shaft 12 is regulated due to the electric drive of rotation of the shaft 16, the heating temperature and the speed of air supply to the working chamber 1 are regulated using the electric heater 5, the productivity of removing moisture vapor is regulated using the exhaust fan 7.

Humidity meters 24 and a non-contact pyrometer 25 are built into the working chamber 1.

The installation works as follows.

For a given initial and final moisture content of bulk material 23 at the input and output of the installation and the required capacity of the installation, the initial values of the power of the microwave generators 21, the angle of inclination of the working chamber 1 using the tilt mechanism 20, the position of the dispensers using the electric drive of the dispensers 8, the speed of rotation of the shaft 12 due to the electric drive of rotation of the shaft 16, the heating temperature and the speed of air supply to the working chamber 1 using the electric heater 5, the capacity of removing moisture vapor using the exhaust fan 7 are set by means of the control unit 22. After this, the installation is started, wherein the bulk material 23 from the external feeder enters the loading hopper 11, from where, under the action of gravity, it moves through the loading lock 9, which is an ultrasonic waveguide cylindrical tube for protecting personnel from microwave electromagnetic radiation, and through the dispenser with the electric drive 8 enters the working chamber. When the unit is started, the electric drive of shaft 16 rotates shaft 12, feed auger 13, and blades 14. The electric drive of the tilt mechanism 20 moves frame 17 with the working chamber 1, ensuring a predetermined tilt angle of the working chamber relative to the horizontal plane. As a result, when shaft 12 with feed augers 13 and blades 14 rotates, bulk material 23 is mixed in the working chamber 1, moving along its axis from the loading zone to the unloading zone.

Microwave energy from microwave generators 21 is fed into the working chamber 1 through waveguide-slot emitters 2 and radio-transparent fluoroplastic inserts 3. The design of waveguide-slot emitters 2 with double-sided power supply and their location on the surface of the working chamber 1 ensures uniform distribution of the microwave energy density in the bulk material 23 being processed during its movement in the working chamber 1. In this case, volumetric heating of the particles of the bulk material 23 is carried out due to dielectric losses during microwave exposure, and moisture diffusion to the outer surface of the particles of the bulk material occurs. At the same time, moisture is removed from the outer surface of the particles of bulk material 23 due to forced convective drying, which is ensured by feeding heated air from the electric heater 5 into the working chamber 1 through the overflow waveguide tubes 4 and removing air with moisture vapor through the overflow waveguide tubes 4 and the exhaust pipe 6 using the exhaust fan 7. Mixing the bulk material due to the rotation of the blades 14 relative to the longitudinal axis of the working chamber 1 promotes the intensification of heat and mass transfer from the outer surface of the particles of bulk material 23 and increases the efficiency of drying.

When the relative humidity of bulk material 23 deviates from the set values at the input or output of the installation according to signals from humidity meters 24, and the temperature deviates from the set value at the output of the installation according to signals from non-contact pyrometer 25, with the help of microprocessor control unit 22, the power of microwave generators 21 is regulated, the speed of movement of bulk material 23 is regulated using tilt mechanism 20, the position of dispensers is regulated using electric drive of dispensers 8, the speed of rotation of shaft 12 is regulated due to electric drive of rotation of shaft 16, the heating temperature and the speed of air supply to working chamber 1 are regulated using electric heater 5, the capacity of moisture vapor removal is regulated using exhaust fan 7.

Thus, the bulk material is dried in the working chamber 1 through the combined action of microwave energy and convective surface heating. The dried bulk material is discharged from the working chamber via feed auger 13, through dispenser 8, and discharge sluice 10.

To ensure the required technical result of drying bulk materials in a microwave installation, it is necessary to determine, on the basis of computational modeling, the number of waveguide-slot emitters, the power of microwave generators, the mode of additional convective heating for the given physical properties of the processed material and the technological requirements for microwave drying modes.

The applicant carried out computational modeling using the example of a microwave installation operating at a frequency of 2.45 GHz with a working chamber with an internal diameter of 300 mm and a length of 1440 mm for drying coal with the following physical properties: relative permittivity ε' = 3.5, dielectric loss tangent tg δ = 0.15, thermal conductivity coefficient λ = 0.171 W/(m⋅K), specific heat capacity c = 1200 J/(kg⋅K), density ρ = 1315 kg/m3, initial and final humidity from 15% to 0%. The volume of bulk material during its movement in the working chamber is 30% of the total volume of the working chamber.

As a result of the modeling it was established that for drying coal with a capacity of 100 kg/h it is necessary to use six waveguide-slot emitters with a double-sided power supply from twelve microwave generators with a total power of 18 kW, 1.5 kW each, with a step-by-step power regulation and the use of additional convective heating with air at a temperature of 70 °C and a speed of 2 m/s. Step-by-step regulation of the microwave power was performed based on the temperature of the bulk material at the outlet of the unit: from the beginning of heating until the moment the drying temperature reached 100 °C, the microwave power of the first stage was 18 kW (100% of the total power), then the microwave generators were switched off, and they were switched on with a second stage power of 9 kW (50%) when the temperature dropped to 97 °C (Fig. 4).

For the above parameters of the microwave installation, the results of modeling the distribution of the electric field strength in the longitudinal section of the microwave chamber are shown in Fig. 5, and the calculated isotherms in the longitudinal section of the bulk material at a time of 30 min are shown in Fig. 6. From the presented results it is evident that the use of waveguide-slot emitters and their location in the microwave installation makes it possible to uniformly distribute the maximum electric field strength at a level of 7 - 9 kV/m in the bulk material along the working chamber (Fig. 5), which leads to the equalization of the microwave power density and the temperature field in the bulk material. Taking into account the influence of additional convective heating, the movement of bulk material and the regulation of microwave power, the calculated temperature in the bulk material from the time of 200 s reaches the drying temperature of 100 °C (Fig. 4) and is distributed evenly in the longitudinal section of the working chamber within the range from 95 °C in the middle of the working chamber to 109 °C at the exit from the working chamber (Fig. 6).

Thus, the claimed installation allows for achieving a uniform distribution of microwave power density in the bulk material volume, achieving uniform heating, intensifying the heat and mass transfer process, thereby accelerating the drying process and increasing the productivity of the installation.

Claims (6)

1. A microwave installation for drying bulk materials, comprising an installation body with a frame in the form of a fixed base with a frame mounted on it with a mechanism for changing its position relative to a horizontal plane, a working heating chamber rigidly fixed to the frame, connected to a drive system, wherein the working chamber is equipped with at least three waveguide-slot emitters, as well as a loading hopper, a loading lock and an unloading lock for bulk material, located diametrically opposite on the upper and lower walls of the working chamber, characterized in that the working chamber of the installation has a cylindrical shape, in its upper part, along the longitudinal axis, there are at least three windows for connecting through radio-transparent fluoroplastic inserts at least three waveguide-slot emitters, each of which is connected to two microwave generators; on both sides between the waveguide-slot emitters there are mounted cutoff waveguide tubes, designed with the possibility of supplying heated air and removing moisture vapor through an exhaust pipe using an exhaust fan; on opposite sides of the working chamber in its upper and lower parts there are dispensers with an electric drive; the loading and unloading airlocks are designed in the form of cutoff waveguide tubes; a loading hopper is connected to the loading airlock; inside the working chamber along its longitudinal axis there is a shaft, to which a feed auger and blades are connected, designed with the possibility of mixing bulk material; the shaft is connected to an electric drive for shaft rotation; Moisture meters are installed in the working chamber body above the loading and unloading airlocks for bulk material, and a non-contact pyrometer is installed above the unloading airlock; moreover, microwave generators, electric-driven dispensers, an electric drive for rotating the working chamber shaft, and a tilt mechanism are electrically connected to the control unit. 2. A microwave installation according to paragraph 1, characterized in that An electric heater is used to supply heated air. 3. A microwave installation according to paragraph 1, characterized in that On one side, the frame is attached to a fixed base using a hinge. 4. A microwave installation according to paragraph 1, characterized in that The centers of the middle waveguide slot emitters are located in the center of the vertical axis of symmetry of the working chamber, and the remaining waveguide slot emitters are placed on both sides relative to the vertical axis with an offset of 60°. 5. A microwave installation according to paragraph 1, characterized in that Waveguide slot radiators are made from a waveguide tube, one of the wide walls of which is designed as a slot structure. 6. A microwave installation according to paragraph 1, characterized in that each pair of blades is secured to the shaft by means of two radial spokes located on one common axis passing through the axis of rotation of the shaft, while the relative position of each pair of radial spokes with the blades is offset in the cross-section of the working chamber by an angle of 90°, and in the axial direction along the shaft - by a distanceL=d,Where d- width of the mixing blade.