Apparatus For Treating Wood Chips With Electrons

Abstract

Apparatus for irradiating wood chips with electrons includes electron generator with means to inject electrons into conduit through which chips are pneumatically conveyed.

Parent Case Text

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No. 93,060, filed Nov. 27, 1970, now abandoned.

Description

BACKGROUND OF THE INVENTION

In the making of pulp from wood a conventional procedure involves the cutting of the wood into oblong chips about three-quarters of an inch square and one-eighth of an inch thick. A pulp mill may chip wood on its own premises but frequently purchases chips that are manufactured at other sites such as at sawmills from scrap pieces, and other residue material accumulating at the mill. It is not generally possible to place the chips into processing immediately as they are produced or received, and at most pulp mills large storage piles of chips are maintained. During storage, chips usually undergo deterioration in one form or another. In some instances chips develop fungi growth that causes discoloration of the chips and the resulting pulp and pulp product, and other cellular breakdowns occur that decrease the ultimate yield of pulp and thus paper or other products from the chips. Depending upon the length of storage this can result in from 5 to 20 per cent loss by weight.

Efforts have been made to devise methods of sterilizing chips so as to kill bacteria and fungi which cause chip deterioration, but economically feasible and practically acceptable systems of chip treatment have not heretofore been developed.

SUMMARY OF THE INVENTION

In accordance with the subject invention, wood chips are pneumatically conveyed through a specially shaped conduit through which electrons are passed transversely so as to impinge upon the chips as they move through the conduit. The chips are conveyed through the conduit under conditions assuring that essentially all surfaces of the chips are exposed to the electrons to effect irradiation and thus sterilization of all surface and some subsurface portions. By sterilizing the chips in such manner the sterilization can be carried out as chips are conventionally pneumatically conveyed from the chip producing mechanism or from their point of receipt to the chip storage pile, but assuring complete exposure of all surface portions to the electrons in an economically feasible manner.

In addition to enhancing the storage life of the chips and minimizing breakdown of the chips from bacteria, fungi, and like micro-organisma, it has been further discovered that the electron bombardment of the chips, if carried out at certain levels, enables the processing, that is, cooking time, of the chips to be substantially reduced and increase the amount of pulp producible from a given quantity of chips with betterment of its physical properties.

DRAWINGS

FIG. 1 is a side elevation of an apparatus for carrying out the invention;

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1; and,

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 1.

Referring to the drawings, indicated at 10 is a pipeline or conduit forming a portion of a pneumatic system through which wood chips are being conveyed. Conventionally, such pipelines are circular in cross-section being on average 6 to 24 inches in diameter, and wood chips are conventionally carried in an air stream having a velocity of about 6,000 feet per minute. In such conveying systems the wood chips are evenly dispersed in the conveying air stream as opposed to being layered as in belt conveyors and the like. In accordance with the illustrated embodiment of the present invention, the conveying system is provided with a conduit section 12 that is essentially of trapezoidal cross-section but which has a cross-sectional area substantially equal to that of the pipeline 10. A suitable transition 14 is provided at either end of the section 12 so as to permit its connection within the line 10. For purposes to be made apparent, section 12 preferably has a length of 6 to 12 feet.

Means are provided to effect exposure of the wood chips in the section 12 to high energy electrons. The illustrated means comprises a suitable source of high energy electrons 18 capable of producing a narrow beam of high speed electrons, the energy of which should be in the order of several hundred thousand volts. The generator may be of the VAN de GRAFF type or any other suitable type.

The electron beam generated by the source 18 in the scanner system illustrated is projected downwardly into a scanner 20 which is provided with means to cause the beam to reciprocate in a linear path having its axis coincident to the axis of the conduit section 12. The cycling frequency of the electron beam is preferably several hundreds to thousands of cycles per second. The deflection of the beam may be obtained by any conventional means such as by magnetic means or by electrostatically operating deflecting means, all of which are well known to the art. The scanner 20 comprises an evacuated chamber defined by vertical parallel sidewalls 24, 26 and inclined end walls 28, 30. The bottom wall 32 of the scanner is preferably formed of titanium or other material providing low resistance to the passage of electrons therethrough but resistant to temperature effects caused by the electron bombardment. Likewise, the top wall 34 of the pipe section 12 is formed of titanium or other easily electron penetrable material. The space between walls 32, 34 is washed with air in order to provide cooling of wall 32. The double effect of walls 32, 34 and the intervening space provide additional protection to the evacuated area of the scanner 20. An alarm or shut-down device (not shown) may be provided to indicate any rupture in the wall 34, which will be indicated by increase in pressure in the chamber 36, and to prevent damage to wall 32 and contamination of the scanner chamber.

Preferably a suitable screen is provided beneath the wall 34 to protect such wall from chip impingement and air may be injected into the space between such screen and wall 34 by nozzles positioned along conduit section 12 to wash the screen of chips and provide additional cooling to wall 34.

In place of a scanner type beam source any other suitable electron source may be utilized which will provide a flux of electrons transversely of the conduit section 12 so as to enable complete exposure of chip surfaces at sufficiently high dosages. For example, a source which will provide a steady or pulsed curtain of electrons extending the length of the conduit section 12 may be utilized.

After entrance into the conduit the electron beam of whichever type used will spread latterly by reason of the repulsive force between electrons and the scattering resulting from impact with air molecules to form a diverging beam. The trapezoidal configuration of the pipe section 12 is selected so that the side walls 38, 40 are substantially coincident with the effective side edges of the electron beam so as to obtain substantially uniform coverage across the entire width of the conduit section 12. As the wood chips pass along the conduit section 12, due to the "magnus" effect on the flat chip surfaces they will continually tumble and change elevation across the pipe section. In a six-foot length of travel the wood chip will ordinarily rotate and change elevation about six times when conveyed in an air stream having a velocity of about 6,000 ft. per minute. As will be apparent at the high oscillation frequency of a scanned electron beam a chip will be exposed to the beam a multitude of times during the course of travel through the section 12 to assure that all surfaces of the chip will be exposed to electron bombardment. The radiation intensity is controlled such that with the velocity and attitude variations of the chips, the chips accumulate sufficient irradiation over all surfaces to attain the desired sterilization or other effect, which in most instances is thought to be between 0.1 and 0.5 megarads.

The energy level of irradiation required for treatment of wood chips in accordance with the invention necessitates that the irradiation be applied over a relatively large area. One requirement is imposed by limitations of the window through which the irradiation must pass. The energy must be spread out over a large area of the window to prevent overheating. More importantly the irradiation must be applied over an elongated area in order to permit the wood chip which is being irradiated to sufficiently change in attitude and position so as to expose all surfaces of the chip to the desired amount of irradiation as it travels through the conduit section. Thus, as a practical matter the irradiation must be supplied to the conduit in an area of application that is rectangular in configuration when viewed in a direction normal to the axis of the conduit. The length of application should be a minimum of about six feet to permit complete exposure of all surfaces of the chip where complete exposure of chips is desired on all surfaces. Experience has indicated that a length of travel of 12 feet is adequate where the air velocity is 100 feet per second in a conduit section having a cross-sectional area slightly greater than 1 square foot. If another type of source is utilized, the source intensity and conveying conditions are adjusted so as to attain the desired uniform exposure level over the entire chip.

In order to obtain the required dosage on all surfaces of a chip without disrupting the pneumatic conveying characteristics of a system or reducing the conveying air velocity, it is necessary to convey the chips through an irradiating conduit section which does not depart too much in configuration from that of the conveying pipe.

The pipe section 12 is preferably surrounded by a housing 44 defining a chamber 46 through which water or other coolant fluid may be circulated to prevent overheating of the section 12 as a result of the electron bombardment.

Because of the fact that essentially all surfaces of the chips are exposed to radiation by their dispersion and tumbling in their passage through the section 12, the source of electrons 18 can be of much lesser intensity than if attempts were made to treat the chips by bombardment only from one side, such as by treating them on a belt conveyor or the like. In the latter case the intensity of the source would have to be such as to permit penetration of the layered chips by the electrons.

It has been found that treatment of chips by electron bombardment substantially increases the storage life of the chips or, putting it conversely, the deterioration of chips over a given length of time is materially reduced.

Perhaps more importantly, it has been discovered that irradiation of wood chips improves the pulp obtained from the chips in quality and yield. It has been found, for example, that the pulp freeness is substantially increased but that the strength of the pulp is maintained or increased. It has also been found that the lignin content of the pulp may be higher. This can reduce the necessary pulping time and, of course, reduces the amount of lignin that has to be disposed of, while at the same time increasing the strength properties and the yield of pulp obtained from a given amount of wood chips.

EXAMPLE

The effectiveness of a system such as has been described herein for substantially achieving uniform irradiation of wood chips is shown in the following example.

A pneumatic conveying system having a fourteen inch diameter pipe supplied with air at a rate to cause air to pass through the system at 100 feet per second and convey wood chips therethrough at a velocity of about eighty feet per second was utilized. Mill run Douglas fir chips were conveyed through such a system and through a pipe section having a trapezoidal cross-section similar to that shown in FIG. 3 of the application and of 12 feet in length. An electron source was provided for irradiating wood chips passing through such section, the source having a potential of 500 kilovolts and a beam current of 200 milliamps which was swept along the length of the pipe section at 100 cycles per second. To measure the dosimetry amberperspex dosimeters were inserted in the pipeline at a point forward of the irradiation section so as to pass therethrough together with the wood chips which were being conveyed in the line. These dosimeters were 3/4 .times. 1/4 .times. 1/8 inch or approximately the size of wood chips although they were of slightly greater density. After passing through the equipment, the dosimeters were collected and the dosage received by each dosimeter analyzed and recorded. The dosage distribution is shown in the table below.

TABLE

Dosage Number (Megarads) Dosimeters 0 1 .05 0 .10 3 .11 0 .12 8 .13 7 .14 23 .15 24 .16 13 .17 4 .18 1 .19 2 .20 0 .21 3 .22 0 .23 1 .24 2 .31 1 .34 1

If chips were to be conveyed through a line with relatively uniform distribution across the cross-section, but without the tumbling and migration that is a part of the process of the present invention, the distribution of dosage would not be substantially uniform around the medium level in the manner obtained herein but a majority of the particles would receive little (i.e. less than 0.1 Megarads dosage) or no irradiation, while only a small number would acquire irradiation at medium and high levels because of the isodose profile that occurs within the pipeline relative to its cross section. A relatively high energy level of irradiation is present at the entrance of the beam to the pipeline, but the energy level rapidly falls off as the beam spreads out within the pipeline.

Having illustrated and described a preferred embodiment of the invention it should be apparent to those skilled in the art that it permits of modification in arrangement and detail.

Claims

I claim:

1. Apparatus for treating wood chips with electrons comprising

a pneumatic conveying system for conveying said wood chips including a conduit of substantially uniform cross-section and means for propelling air and wood chips therethrough, at a velocity such as to cause chips to tumble and change elevation in said conduit as they are carried therethrough,

and means for injecting high energy electrons into a straight section of said conduit free of obstruction over a substantially length sufficient to enable chips to tumble and migrate substantially throughout the cross-section of said conduit section whereby all surfaces of said chips will be exposed to irradiation by said electrons and accumulate the required dosage from the various energy levels of the injected electrons in said conduit substantially uniformly over all the chips and over each chip.

2. Apparatus for treating wood chips with electrons comprising:

means for creating a beam of high energy electrons,

cycling means for causing said beam to reciprocate in a linear path,

a pneumatic conveying system for conveying said wood chips including a conduit section free of obstruction having its axis coincident to said linear path and an electron transparent window in said path and means for propelling air and wood chips through said conduit section at a velocity such as to cause chips to tumble and migrate throughout the cross-section of said conduit section so that all surfaces of said chips will be substantially uniformly exposed to said electron beam.

3. Apparatus as set forth in claim 2 wherein said conduit section has side walls substantially coincident with the effective side edges of said beam.

4. Apparatus as set forth in claim 2 wherein said conduit section is trapezoidal in cross-section.

5. Apparatus as set forth in claim 4 wherein said conduit section is of uniform cross-sectional area throughout its length, and said pneumatic system comprises a conduit means of cross-sectional area substantially equal to the area of said conduit section connected to the inlet end of said section whereby the chips maintain a substantial uniform velocity through said section.

6. The apparatus of claim 2 wherein said conduit section is of a length of about 12 feet and said cycling means is adapted to cause said beam to sweep through substantially the entire length of said conduit section.

7. In apparatus for treating wood chips with electrons the combination comprising:

means defining an evacuated chamber,

means for creating a beam of high energy electrons at one end of said chamber and directing said beam toward the opposite end,

means to cause said beam to reciprocate in a linear path at said opposite end,

electron permeable window means in said opposite end of said chamber to permit emergence of electrons therefrom,

a conduit section adjacent said chamber opposite end having its axis coincident with the axis of said path,

said conduit section being trapezoidal in cross-section with the side walls of said section substantially coincident with the effective side edges of said electron beam,

the top wall of said conduit section comprising electron permeable window means so as to permit entry of said beam into said conduit section,

and means for pneumatically conveying wood chips through said conduit section at a high velocity whereby the wood chips will tumble and migrate throughout the cross-section of said conduit section exposing all surfaces of said chips to bombardment by said electrons and to electrons of various energy levels whereby said chips will receive substantially uniform dosage.