GB2028988A - Method and apparatus for drying granulated dielectic materials - Google Patents

Abstract

The method for drying granulated dielectric materials consists in superhigh frequency heating of granules which are moved in the heating space by a strong hot air flow. The granules are moved at a variable speed, and a negative temperature gradient is produced therein. The humid air is removed from the heating space. The apparatus for drying granulated dielectric materials comprises a superhigh frequency generator 1 and a drying chamber which, in turn, comprises an input waveguide 2 and an output waveguide 3 interconnected by a waveguide elbow 4. An air inlet 8 and a granules inlet 9 are connected to an input of the input waveguide. The output waveguide has an air separator 5 arranged at its input and a resonator bin 6 arranged at its output. Air introduced at 11 slows down the granules in the waveguide 3, and granules may be recycled periodically (Fig. 2, not shown). A continuous process is described (Fig. 3, not shown). <IMAGE>

Description

SPECIFICATION Method and apparatus for drying granulated dielectric materials The present invention relates to superhigh frequency heating of dielectric materials and, more particularly, to a method and apparatus for drying granulated dielectric materials.

The invention is chiefly applicable to processes aimed at complete drying, including complete dehydration, of different granulated dielectric materials; it can also be used to dry materials after storage and immediately before use in order to ensure a high quality of cast plastic articles. The apparatus of this invention is applicable to drying cereal and oil crops and to eliminating infestants in agriculture.

The invention essentially consists in providing a method for drying granulated dielectric materials by superhigh frequency heating of granules moved in the heating space by a strong hot air flow, which method is characterized, according to the invention, in that the granules are moved at a variable speed, humid air is continuously removed.

from the heating space, while fresh air is continuously supplied, and in that a negative temperature gradient At = i n 1 i n 100C is produced in the granules, where i is the heat and mass transfer vector and n is a positive number, for which purpose the air temperature is set at a point 20 to 500C below the maximum temperature to which the granules are heated.

Preferably, a variable speed of motion of granules is provided for by dividing the heating space into alternating zones of intensified motion of granules in the air flow and granules accumulation zones where the motion of granules is slowed down.

The invention further consists in providing an apparatus for drying granulated materials, comprising a heating section having a superhigh frequency generator, a drying chamber, an air inlet, a granules inlet, and a superhigh frequency input, which apparatus is characterized, according to the invention, in that the drying chamber is composed of a vertical input waveguide and a vertical output waveguide interconnected by a waveguide elbow, the air inlet and the granules inlet being connected to the input of the input waveguide, whereas the output waveguide has an air separator arranged at its input and a resonator bin arranged at its output.

In order to produce a closed granules circulation loop, it is necessary that the resonator bin should have an outlet communicating through a hose and a valve with the granules inlet of the heating section, and that the valve should include a granules charging inlet.

In order to provide for a continuous drying process, it is expedient that the apparatus should additionally include n-i heating sections, each having an individual supply of superhigh frequency energy, all the sections being successively interconnected by pipes to produce a single pneumatic conveyor for a continuous flow of granules through then sections.

A better understanding of the present invention will be had from a consideration of the following detailed description of prefered embodiments thereof, taken in conjunction with the accompanying drawings, wherein: FIG. 1 is a general view of an apparatus for drying granulated materials, in accordance with the invention; FIG. 2 is a view of an apparatus for drying granulated materials, constructed as a closed granules circulation loop, in accordance with the invention; FIG. 3 is a general view of an apparatus for continuous drying of granulated dielectric materials, in accordance with the invention.

The method according to the invention is as follows. A granulated material is dried while the granules move at a variable speed in a superhigh frequency field. The granules are moved at a variable speed due to the fact that the heating space is divided into alternating zones of intensified motion of granules in an air flow used for granules transportation, and granules accumulation zones where granules move slower.

Air forced into the heating space has a temperature 20 to 50"C below the maximum temperature to which the granules are heated, and a negative temperature gradient A t = i n .1 1 00C is produced in the granules. The negative temperature gradient A t means that the temperature inside a granule is higher than the temperature of its air-cooled surface. Spent humid air is continuously removed from the heating space and replaced with an equivalent supply of fresh air; gases released in the course of drying are also removed from the heating space.

The drying process may be continuous or periodical.

For continuous drying, an air flow is used to carry granules first to a zone of intensified granules motion and then to a granules accumulation zone. As this takes place, a superhigh frequency field is produced in the heating space. The spent air is removed from the first zone.

Hot air is forced into the granules accumulation zone and, as it absorbs moisture, is removed therefrom. When the granules accumulation zone is full the dried granules are discharged therefrom to be replaced by an equivalent amount of moist granules, the granules flow rate at the inlet being equal to that at the outlet.

When the drying time has to be increased, use is made of a plurality of alternating zones of intensified motion of granules and granules accumulation zones. From the outlet of the first accumulation zone, granules proceed to the next intensified motion zone, wherefrom they follow to the second accumulation zone to be acted upon by a superhigh frequency field. Spent air is removed from each zone. As the second accumulation zone is filled, the granules proceed therefrom to the next zone of intensified motion, etc.

Dried granules are pneumatically discharged from the outlet of the last zone. The discharge is carried out continuously at a rate to the flow rate of granules introduced into the first zone. As mentioned above, the air temperature is set at a point 20 to 500C below the maximum temperature to which the granules are heated, which accounts for a negative temperature gradient in the granules.

The periodical drying process is carried out as follows. Granules are driven through the intensified motion zone into the accumulation zone until the latter is full. The outlet of the accumulation zone is then connected to the inlet of the intensified motion zone, whereupon granules from the accumulation zone are pneumatically forced into the intensified motion zone to circulate through the closed loop thus produced. In both zones the granules are dried by an air flow and a superhigh frequency field. Spent humid air is continuously removed and replaced by an equivalent supply of fresh air. As in the former case, the air temperature is 20 to 500C below the maximum temperature to which the granules are heated. The drying is carried out during a specified period of time.When this period expires dried granules are discharged, the accumulation zone is again filled.with moist granules, and the foregoing sequence of events is repeated.

The method according to the invention will be explained in greater detail with reference to the following examples.

EXAMPLE 1 The product to be dried is acrylonitrilebutadienestyrene plastic with a mean-; moisture content of 0.5 percent.

The basic process parameters and equipment are as follows: the drying process: continuous; eight 2.5-kw superhigh frequency generators; full power output: 20 kw; oscillation frequency: 2,450 megs; airflow rate: 6 m3/min of which 2 m3/min is supplied at t = 45 to 500C; throughput; 350 kg per hour with a final humidity of the product of 0.05 to 0.07 percent; throughput; 400 kg per hour with initial humidity of 0.3 percent; 450 kg per hour with initial humidity of 0.2 percent; floorspace occupied by the drying apparatus: 11 m2.

EXAMPLE 2 The product to be dried as acrylonitrilebutadienestyrene plastic with a mean moisture content of 0.5 percent.

The basic process parameters and equipment are as follows: The drying process: periodical; two 2.5-kw superhigh frequency generators; full power output: 5 kw; oscillation frequency: 2,450 megs; air flow rate: 40 m3 per hour of which 12 m3 per hour is supplied at t = 45 to 500C; drying time for continuous circulation of granules in the closed loop: 40 minutes final humidity: 0.05 to 0.07 percent; throughput: 80 kg per hour; throughput: 90 and 1 00 kg per hour for initial humidity of 0.3 and 0.2 percent, respectively; amount of material charged into the accumulation zone: 60 + 4 kg; floorspace occupied by the drying apparatus: 2.7 m2.

The drying time is 30 to 50 minutes for most thermoplastic materials. For example, it takes 30 minutes to reduce an original humidity of 0.5 to 0.7 percent of styrene with methylomethacrylate and of styrene with methylomethacrylate and acrylonitrile to a 0.1 percent level by circulating granules in a closed loop; it takes 45 minutes to reduce the above-mentioned original humidity to a level of 0.05 to 0.07 percent.

The only exception is caprolactam-based materials which take 80 to 90 minutes to dry.

According to the invention, the apparatus for drying granulated dielectric materials is a heating section comprising a superhigh frequency generator 1 (FIG. 1) and a drying chamber which, in turn, comprises a vertical input waveguide 2 and a vertical output waveguide 3 interconnected by a waveguide elbow 4. The waveguide 3 has an air separator 5 arranged at its input and a resonator bin 6 arranged at its output. Connected to the input of the waveguide 2 are an injectortype pneumatic conveyor 7 having an air inlet 8 and a granules inlet 9, and an SHF oscillation energy input 10. The resonator bin 6 has air inlets 11, a branch pipe 12 for removal of spent air and a branch pipe 13 for discharge of granules.

Inserted into the input waveguide 2 and the elbow 4 is a pipe 14 of a dielectric material featuring low SHF oscillation energy absorption.

The purpose of the pipe 14 is to prevent clogging of the input 10 with dust and granules, narrow down the channel through which granules are carried by the air flow and thus reduce the air consumption.

The air separator 5 is connected to an exhaust blower by a branch pipe 15 and has branch pipes 1 6 for removal of spent air.

The apparatus according to the invention can be modified so that the drying process can be carried out over an unlimited period of time. For this purpose, the resonator bin 6 (FIG. 2) is provided withan outlet 17 communicating by means of a hose 18 and through a valve 19 with the granules inlet 9 of the pneumatic conveyor 7, whereby a closed granules circulating loop is produced. The valve 1 9 has a granules charging inlet 20; the resonator bin 6 is further provided with a gate means 21.

According to the invention, the apparatus for continuous drying of granulated dielectric materials comprises n (FIG. 3) heating sections, each with an individual superhigh oscillation energy supply. The n sections are successively interconected by pipes 22 to produce a single pneumatic conveyor for a continuous flow of granules through the n sections.

The apparatus according to the invention operates as follows.

As air is forced through the inlet 8 (FIG. 1) into the injector-type pneumatic conveyor 7, granules are sucked through the inlet 9 into the vertical input waveguide 2 and carried by the strong air flow through the smoothly curved waveguide elbow 4. While in motion, the granules are acted upon by the air flow and a superhigh frequency field. The granules then get into the air separator 5 where the air is separated from the flow of granules and directed through the branch pipes 15 and 1 6 to the exhaust blower. As the granules continue to move through the output waveguide 3, their speed is somewhat reduced by a counter flow forced into the resonator bin 6 through the inlets 11.The force of the counter flow can be adjusted with the aid of a restrictor means installed in the branch pipe 1 2. When in the waveguide 3, the granules are acted on by superhigh frequency oscillation; as a result, they are heated, while their outer surface is cooled. The granules accumulate in the resonator bin 6 where they are heated by superhigh frequency oscillation and hot air coming through the inlets 11. The spent air and moisture are removed through the branch pipe 12. When the resonator bin 6 is full, an electrocontact level gauge (not shown) is actuated and brings into play a pneumatic conveyor connected to the outlet pipe 1 5 to discharge the dry granules. The discharge rate corresponds to the feed rate, and the amount of granules discharged from the apparatus is equal to the amount of granules fed to the drying chamber.

Thus the drying process is carried out continuously over a predetermined period of time.

The heating temperature is adjusted with the aid of a thermoelement by switching on and off the superhigh frequency generator 1 in the course of the drying process.

Periodic operation by circulating a granulated dielectric material through a closed loop is effected as follows. The outlet 1 7 is connected by means of the hose 18 (FIG. 2) and through the valve 1 9 to the inlet 9 of the pneumatic conveyor 7, whereby a closed granules circulation loop is provided for.

The resonator bin 6 is charged through the inlet 20 of the valve 19, the charging being done by the pneumatic conveyor 7. The charging being over, the valve 1 9 is set in the circulation position, the superhigh frequency generator 1 is switched on, and granules are fed from the resonator bin 6 to the vertical input waveguide 2 where they are exposed to the effects of the air flow and the superhigh frequency field. The granules proceed through the waveguide elbow 4 and the output waveguide 3 to the resonator bin 6, wherefrom the spent air and moisture are directed through the air separator 5 and the branch pipe 1 5 to the exhaust blower.When in the bin 6, the granules continue to be dried by superhigh frequency oscillation and air heated to a temperature 20 to 500C below the maximum permissible temperature to which granules may be heated.

The granules circulation speed is varied by adjusting the air flow rate through the pneumatic conveyor 7. The heating temperature is adjusted with the aid of a thermoelement. Upon completing the drying process the granules are discharged either through the pneumatic conveyor 7 and branch pipe 1 3 or through the gate means 21, whereupon the charging and drying operations are repeated.

The periodic process with the circulation of granules through a closed loop is advisable for drying limited amounts of a variety of products.

The process permits of simultaneous drying of a number of different materials.

The embodiment shown in FIG. 3 is intended for continuous operation and comprises n heating sections, each having an individual SHF energy supply. All the n sections are successively interconnected by the pipes 22 to produce a single pneumatic conveyor. The drying process is carried out as in the case of the single-section apparatus.

As the pneumatic conveyor 7 fills the resonator bin 6 of the first drying chamber, a level gauge (not shown) is actuated to simultaneously switch on the superhigh frequency generator 1 and pneumatic conveyor 2 of the second heating section. When the charging of the resonator bin 6 of the second heating section is completed the level gauge is situated to switch on the superhigh frequency generator 1 of the third heating section and the pneumatic conveyor 7 of the next heating section; this sequence of events goes on until the resonator bin 6 of the last, n-th, section is filled with granules, which is followed by pneumatically unloading the resonator bin 6 of the last section.

The use of a plurality of heating section is advantageous for drying materials with a high bound moisture content. In each section the drying process takes 3 to 8 minutes.

The apparatus according to the invention makes it possible to achieve a complete dehydration of granulated dielectric materials.

Naturally, completed dehydration takes more time than normal drying. The apparatus is quite econimical with an average power consumption of 50 to 60 watt-hours per kilogramme of a granulated polymer or copolymer material. The apparatus may be of a small size. The drying is highly uniform so that the apparatus according to the invention can be used to advantage for drying cereal and oil crops and eliminating certain pests of these crops. The invention is also applicable to pre-seeding operations.

Claims (7)

1. A method for drying granulated dielectric materials, comprising superhigh frequency heating of granules moved at a variable speed in the heating space by a strong hot air flow, removing spent humid air from the heating space and continuously supplying fresh hot air, as well as producing a negative temperature gradient A t = i n ODC in the granules, whereas the heat and mass transfer vector and n is a positive number, for which purpose the air temperature is set 20 to 50"C below the maximum temperature to which the granules are heated.

2. A method for drying granulated dielectric materials, whereby granules are moved at a variable speed by producing in the heating space alternating zones of intensified motion of granules in the air flow and granules accumulation zones where the motion of granules is slowed down.

3. An apparatus for carrying out the method set forth in claims 1 and 2, comprising a heating section having a superhigh frequency generator, for air inlet, a granules inlet, a superhigh frequency input, a drying chamber composed of a vertical input waveguide and a vertical output waveguide interconnected by a waveguide elbow, the air inlet and granules inlet being connected to the input of the input waveguide, whereas the output waveguide has an air separator arranged at its input and a resonator bin arranged at its output.

4. An apparatus as claimed in claim 3, wherein the resonator bin has an outlet communicating by means of a hose and through a valve with the granules inlet of the heating section to produce a closed granules circulation loop, the valve being provided with a granules charging inlet.

5. An apparatus for continuous drying of granulated dielectric materials as claimed in claim 3, which additionally comprises n-1 heating sections, each having an individual SH energy supply, all the sections being successively interconnected by pipes to produce a single pneumatic conveyor for a continuous flow of granules through the n sections.

6. A method for drying granulated dielectric materials, substantially as hereinbefore described with reference to the accompanying drawings.

7. An apparatus for drying granulated dielectric materials, substantially as hereinbefore described with reference to the accompanying drawings.