IMPROVED PLUG SCREW FEEDER

This invention relates to a method for improving the performance of compression type feeders for bulk materials such as wood chips. The pulp and paper industry utilizes compression devices (plug screw feeders) which are based on the principle of a screw rotating inside a cylindrical or conical cage, where the volume available at the inlet of the screw is greater than the volume available at the discharge. The cage can be equipped with holes, usually conically drilled, or slots or bars arranged in such a fashion as to provide drainage of liquor squeezed from material being compressed. The cage is usually equipped with anti-rotation devices such as bars, pins or slots to prevent the compressed material from turning with the screw.

United States Patent 2,943,012 entitled "Method and Apparatus for Fiberizing" discloses a plug screw feeder assembly in which wood chips are compressed to remove excess water and the wood chips are further compressed into a plug. Mechanical force is applied at an angle to said plug for the mechanical tearing off of layers from the plug. Pressure is built up in the press barrel by restricting the discharge at the opening by an adjustable choke device and by progressively increasing the diameter of the shaft while maintaining the barrel diameter uniform.

Some typical uses of such compression devices are as follows:

1) To remove pitch, wood extractives salts and other naturally occurring substances or substances absorbed by the bulk material during transportation or processing.

2) To improve the liquor pick up in chemical impregnation of bulk materials such as wood

chips or other fibrous materials when material is exposed to a liquor on expansion at discharge from the compression device by means of a liquor bath or shower. 3) To extract liquor absorbed in a previous stage or soaking stage. 4) To provide a pressure seal in passing from one pressure to another in the process operation.

Conventional plug screw feeders have a theoretical volumetric compression ratio in the range of 1.5:1 to 2:1. Recently feeders with compression ratios of 3.5:1 to 4.5:1 have been designed and built.

Another device called a pressafiner and a similar device called a French Oil Press have similar applications and usually have theoretical compression ratios of 6:1 or higher.

United States Patent 2,975,096 entitled "Impregnation of Wood Chips" discloses a plug screw feeder in which fluid is pressed from the wood chips in the first part of the plug screw feeder, impregnating liquor is added to the high consistency wood chips in the next part of the plug screw feeder. The apparatus also discloses the use of conical formations on the rotating shaft. The exterior of the conical formations is in close proximity to the inside of the chamber and the compressed material provides plug seals at the restricted points in the chamber. While Figure 1 of the patent discloses screw feeders, the screw feeders are not designed to force feed the wood chips into the chamber of the plug screw feeder.

In the process of chemical impregnation of chips, it is common knowledge that improper penetration of chips with chemical can result in a lower quality product and a lower yield. In view of the inefficiency of the plug screw feeders a number

of persons have resorted to shredding the chips before attempting impregnation of the chips in an effort to achieve a more uniform chemical application to individual fibres. When chemical treatments are applied to chips in high yield processes prior to defibration, the resulting fibres exhibit low wet web strengths and increased energy is required to reach a given freeness. When defibration occurs prior to chemical treatment, wet web properties are enhanced and energy required to achieve a given freeness is reduced.

The use of a high compression device on chips prior to chemical treatment of chips results in retained wet web properties and lower energy consumption to a given freeness. It is believed that the high compression of chips results in fibre dislocations along the S1/S2 fibre wall, giving increased specific surface which results in improved pulp properties. Additionally, fractures and fissures occur in the chips which allows full penetration of the chips with chemicals.

Although high yield processes such as chemical thermal mechanical pulp (CTMP) reduce the amount of effluent containing BOD, COD and toxic products, primarily due to higher yield, particularly when compared to the sulfite process, the effluent from CTMP processes is nevertheless highly toxic, and many mills have difficulty in meeting their effluent permits. One of the most toxic effluent streams is the pressate, from the plug screw feeders. By using high compression ratio plug screw feeders, it is possible to extract a very concentrated effluent stream which can then be treated independently of the general effluent system.

The various uses of plug screw feeders involve a number of mechanisms for creating pressure between

the chamber and the shaft bearing flights. The inner diameter of the chamber may be cylindrical, conical, or may contain restricted areas. All of these features together with variations in the diameter of the shaft or diameters of the flutes on the shaft can produce changes in the pressure exerted on the wood chips or other material being treated in the plug screw feeder. The chamber of the plug screw feeder may be comprised of bars, screens or be solid depending upon whether the plug screw feeder is being used to drive off excess water or being used to refine wood chips or both remove excess fluid and refine. In various applications the pressure and throughput is controlled by the voids if any in the chamber, the restrictions in the chamber, the shaping of the shaft or flutes and the torque applied to the screw feeder. The applicant has found that in many applications improved efficiency and throughput of a plug screw feeder can be achieved by feeding wood chips or other materials under pressure into the plug screw feeder.

Several companies have attempted to build high compression plug screw feeders with high performance but many problems have occurred. First of all, the true compression ratio was much less than the theoretical ratio and plugging of units occurred due to plug spinning.

The theoretical compression ratio of a plug screw feeder is the volume of the first enclosed pocket or flight spacing in the inlet zone, divided by the volume of the last enclosed pocket or flight spacing in the discharge zone. Reasons for the actual compression ratio not being equal to the theoretical are thought to be as follows: 1) Inadequate filling of the inlet zone.

2) Back flow of the chips from the compression cone to the inlet area.

The above plus rotation of the chip plug results in an actual capacity being much lower than the theoretical.

Wood chips being a non-homogeneous substance have a bulk packing density approximately 1/2 that of solid wood. For spruce, for example, the bulk density of chips is approximately 12 lbs/ft. Production tests show that a typical plug screw feeder will have 60% of its theoretical capacity when a packing density of 12 lbs/ft is used in the calculation.

A number of attempts have been made to improve the filling and feeding of plug screw feeders. Various types of inlet chute designs have been tried, including the use of vibrators and rotating paddles and agitators. All of these devices, however, rely principally on gravity to fill the inlet zone of the screw.

In attempting to reduce the problem of back flow and increase transportation efficiency, anti- rotation devices similar to that described in U.S. Patent 4,475,452 have been tried. Although these did improve the performance, their effect was marginal in that actual vs theoretical was improved by less than 10%.

Moreover, since the packing density is an uncontrolled variable affected by chip size classification and uniformity thereof, it is virtually impossible to predict or control the effective compression ratio in a given situation. The compression device for bulk material such as wood chips of this invention overcomes the limitations of inadequate and/or unpredictable inlet filling associated with such compression devices now in use. The varied force feed screw to the inlet of the compression device works like a "supercharger" by forcing material into the inlet section of the

compression device, thus providing an increased and uniform packing density in the inlet. The fully packed inlet also prevents back flow of chips from the compression cone. The full compression of the screw is then available to work on the material thus increasing the effective compression ratio.

One embodiment of the invention relates to a compression device for bulk material such as wood chips, comprising an inlet, a screw operating within a cylindrical or conical cage and an outlet wherein the volume of space available at the inlet of the compression device is greater than the volume of space available at the outlet, and the improvement wherein means is provided to feed bulk materials under pressure into the inlet of the compression device.

In another embodiment of the invention the means for feeding the bulk material under pressure into the inlet of the invention is variable and the feeding may be at variable pressures.

In a further embodiment of the invention a variable speed force feed screw is used to control the inlet pressure.

Another embodiment of the invention relates to a method of compressing bulk material such as wood chips comprising feeding the materials to be compressed under pressure into the inlet of the compression device, operating a screw within a cylindrical or conical cage, having a greater volume at the inlet than at the outlet.

In one embodiment of the method the means for feeding the compressible material is variable to provide variable feed pressures to the compression device.

In a further embodiment of the method a variable speed control feed screw is used to control the inlet pressure of the compressible material to the compressor. The force feed screw has a variable speed drive. By adjusting the torque applied to the force feed screw, the packing density can be affected, giving an _. ability to adjust the effective compression ratio of the compression feeder to meet the requirements of the process.

It has been observed that by increasing the torque on the force feed screw, the following will occur:

1) the power demand on the compression device increases;

2) the quantity of liquor extracted increases;

3) the dryness of material exiting the compression device increases;

4) the degree of maceration of the material increases.

In the drawings:

Figure 1 is a schematic drawing of a plug screw feeder.

As seen in Figure 1, the plug screw feeder 1 is comprised of a power source 2, power drive shaft 3, transmission 4, screw drive shaft 5 and screw 6 which are all interconnected. The screw 6 is enclosed within compression cone or cylinder 7. Compression cone or cylinder 7 includes screen plates 8 through which liquid is drained during compression and thickening of the bulk material. The plug screw feeder 1 has an inlet housing 9 and an outlet housing 10. The inlet housing 9 is comprised of a force feed screw 11 inside force feed cylinder 12. Force feed cylinder 12 rests below surge bin 13. Force feed screw 11 is attached by drive shaft 14 to variable feed screw power source

15. The outlet housing 10 includes discharge chamber 16, discharge outlet 17 and fluid or blowback damper 18. The plug screw 6 is comprised of a series of flights or pockets commencing at 19 proximate the inlet housing 9 and decreasing in volume until the flight or pocket 20 of least volume proximate the outlet housing 10. A series of antirotation pins 21 are mounted in the wall of compression cone or cylinder 7. In operation the compressible material is loaded in the surge bin 13 and the force feed cylinder 12. The power source 2 is activated and the plug screw 1 begins rotating and compressing the compressible material in compression cone or cylinder 7. The variable feed screw power source is turned on and forces feed screw 11 continues to force the compressible material into the largest volume flight pocket 19. By controlling power source 2 of screw plug 1 and variable feed screw power source 15 the compression of the compressible feed to the large volume flight 19 and subsequent flights of the plug screw feeder 1 may be controlled.