WO1996007070A2

WO1996007070A2 - Method for drying raw material for the manufacture of particleboard

Info

Publication number: WO1996007070A2
Application number: PCT/US1995/011164
Authority: WO
Inventors: William Benny Teal
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-09-01
Filing date: 1995-09-01
Publication date: 1996-03-07
Anticipated expiration: 1997-03-01
Also published as: AU3626695A; WO1996007070A3

Abstract

A method for drying raw material for the manufacture of particleboard is shown and described. In a preferred embodiment, the raw material is fed onto a conveyor (11) which then moves the raw material as a bed through a dryer housing (28). The housing (28) may be partitioned into various zones and levels, depending on the desired configuration. A volume of drying air is heated and then passed through the bed of raw material to reduce the moisture content of the raw material. In a preferred embodiment, the raw material is agitated to promote the even exposure of the raw material to the drying heat. The raw material is then conveyed through a conditioning zone (36) before being discharged from the dryer housing.

Description

METHOD FOR DRYING RAW MATERIAL FOR THE MANUFACTURE OF PARTICLEBOARD

Technical Field

This invention relates to a method for drying wood elements, and more particularly, to the drying of raw material for the manufacture of particleboard.

Background of the Invention

Particleboard is a wood based panel product, the raw material of which is various forms of wood, for example, planer shavings, chips or sawdust. Because the raw material may have a high moisture content, it must be dried before it is further processed into the final product. The raw material is therefore collected and fed into a dryer, after which it is discharged and refined, or ground up, and may be dried again. (The raw material may also be refined prior to being initially dried.) The refined and dried raw material is then blended with adhesive and formed into mats, for example, on caul plates or on a continuous conveyor. The mat of raw material is then separated into discrete lengths which are compressed and heated to produce panels. (This process may be done in reverse, namely, the continuous mat may be compressed and heated prior to being separated into discrete lengths.) The panels are then sawed, sanded, and otherwise processed into the finished product. The current method of drying raw material for particleboard manufacture is to feed the raw material into a rotary dryer. Such dryers tumble the raw material while exposing it to a flow of heated air, the inlet temperature of which is typically 800°F or higher. However, the current method has several disadvantages. One such disadvantage is the generation and emission of airborne pollutants. Such pollutants include paniculate matter entrained in the drying airstream and gaseous pollutants such as carbon monoxide. Although secondary and tertiary cleaning equipment may be used in an attempt to separate particulate and gaseous matter from the airstream prior to its discharge into the atmosphere, such systems are expensive and may still allow a single, typical plant to exhaust hundreds of tons of particulate and gaseous matter into the atmosphere. Another significant disadvantage of current methods is that a relatively large volume of drying air is exhausted to the atmosphere.

A need therefore exists for a method for drying raw material for the manufacture of particleboard wherein the volume of exhaust and the emission of airborne pollutants is reduced.

Summary of the Invention

It is therefore an object of this invention to provide an improved method for drying raw material for the manufacture of particleboard. It is another object of this invention to provide a method for drying raw material for the manufacture of particleboard that will reduce the levels of airborne pollutants emitted to the atmosphere.

It is another object of this invention to provide a method for drying raw material for the manufacture of particleboard that will minimize the volume of drying air exhausted to the atmosphere.

These and other objects of the invention, as will be apparent herein, are accomplished by providing a conveyor dryer. In accordance with a preferred embodiment of the present invention illustrated herein, wet raw material is fed onto a conveyor, thereby forming a loosely compacted bed of raw material. The conveyor, which includes means for allowing air to pass through it and therefore through the bed of raw material, moves the bed of raw material through a dryer housing. By controlling the volume of raw material fed onto the conveyor and the speed with which the conveyor moves through the dryer housing, it is possible to control the depth of the bed of raw material as well as the retention time, that is, the amount of time the raw material is retained in the dryer. To determine the proper retention time several methods may be used, although in a preferred embodiment it is determined by using a moisture meter or any other commercially available instrument to measure the moisture of the material, and periodically sampling the raw material to assess its moisture content.

A volume of drying air or other suitable gas is heated for use in the dryer. Although heating may be accomplished in a variety of ways and may occur either internally or externally to the dryer, in a preferred embodiment, heat exchangers having thermal oil or steam coils are used, the oil or steam being heated by the combustion of bark and other waste wood. The heated drying air is then forced or drawn through the bed of raw material and circulated through and around the conveyor to dry the raw material.

The dryer housing may be configured into one zone or it may be partitioned into several zones, such that the direction and temperature of the drying air in each zone may be controlled separately. For example, warm air may be forced upward through the bed of raw material in a first zone, downward through the bed in a second zone and then upward again in a third zone, the temperature in each zone gradually decreasing. The direction of the air is accomplished in an embodiment illustrated herein by positioning and regulating fans and dampers. In a preferred embodiment, a volume of the heating air is recycled, thereby more efficiently using the thermal energy present in the drying air and thereby reducing the volume of air expelled to the atmosphere as exhaust. In one embodiment, the dried raw material is then discharged from the dryer housing. In an alternative embodiment, the dried raw material is passed through a conditioning zone before being discharged.

The configuration of the dryer may be altered for particular applications. For example, the conveyor may be configured in a "single pass" whereby the raw material is conveyed in a substantially straight line path through the dryer. In an alternative embodiment, the conveyor may be partitioned into sections, each section being positioned at different elevations such that the raw material is forced to fall from one conveyor section to the next at a lower elevation. Such a "multi-pass" system occupies less floor space than a single- pass system, and also serves to agitate the raw material, thereby promoting even exposure to the drying air. Agitation in either a single- or multi-pass configuration may be accomplished by exposing the raw material to a suitable turnover device.

Brief Description of the Drawings

Figure 1 is a flow chart illustrating the steps of a preferred embodiment of the present invention.

Figure 2 is a diagram illustrating a dryer in accordance with a preferred embodiment of the present invention.

Figure 3 is a diagram illustrating a dryer in accordance with an alternative embodiment of the present invention. Figure 4a is a sectional end view taken through a zone of the dryer of Figure 2. Figure 4b is a sectional end view taken through a zone of the dryer of Figure 2.

Detailed Description of the Invention Particleboard is composed of wood "waste" material, for example, planer shavings, chips or sawdust. This raw material is refined, dried, possibly refined again, coated with adhesive, pressed, and otherwise formed to create a finished board or panel product. Under current methods, raw material for the manufacture of particleboard is dried in a rotary dryer. However, there are several disadvantages to this method, including the generation and emission of a high volume of exhaust and pollutants. These problems, among others, are reduced by drying raw material for the manufacture of particleboard in accordance with the present invention.

Figure 1 illustrates the steps comprising a preferred embodiment of the present invention, whereby raw material for the manufacture of particleboard is dried in a conveyor dryer. (As noted above, the raw material may be dried more than once during the manufacturing process both before and after being screened and refined. The present invention is applicable to every stage of the process in which the raw material is to be dried.) Commercially available conveyor dryers such as those manufactured by Proctor & Schwarz of Horsham, Pennsylvania, are suitable for raw material drying in accordance with the present invention. As illustrated in Figures 1-3, the raw material is fed onto a conveyor 26 or 11, step 10. Although this may be accomplished by a variety of means, in a preferred embodiment, the raw material is fed onto the conveyor by an oscillating belt or screw conveyor (not shown).

The raw material is conveyed through a dryer housing 28, step 12, and will follow different paths depending on the configuration of the system. A single pass configuration 25 is illustrated in Figure 2, wherein the raw material is conveyed through the dryer housing 28 in a straight line path. An alternative embodiment is illustrated in Figure 3, wherein a multipass configuration 27 conveys the raw material along a first level, after which the raw material drops to a second level or section of the conveyor before being discharged from the dryer housing, step 20.

As further illustrated in Figures 1, 4a and 4b, a volume of drying air is heated, step 14. Although this may be accomplished in a variety of ways, in a preferred embodiment, heat exchangers having thermal oil or steam coils 35 are used, the oil or steam being heated by the combustion of bark and other waste wood. Although the temperature of the drying air may be set at any level in the preferred embodiment illustrated herein, it is set at a temperature between ambient and 600°F. As illustrated in Figure 3 by arrows 38 and 40, the drying air is passed through the bed of raw material, step 16, in either an upward or a downward direction. In a preferred embodiment, the direction of the drying air is controlled by the particular arrangement of fans, partitions and dampers, as illustrated in Figures 4a and 4b. Figure 4a is a sectional end view taken through drying zone 19 of Figure 2 wherein the drying air is forced downward through the bed of raw material 46 on conveyor 11. The relative positioning of intake 47a, fan 48a and partitions 49a, 50a and 51a serve to direct the airflow. More specifically, a volume of drying air is drawn into the drying zone 19 from a supply duct (not shown) at 47a by a fan 48a which then exhausts the drying air, forcing it down through the heat exchanger coils 35a and bed of raw material, thereby evaporating moisture from the raw material. As the air circulates around back up towards the top of zone 19, a portion of the moisture-laden air is exhausted at 52a while the remainder is drawn back through fan 48a along with additional drying air. As a result, a relatively large volume of the drying air is recirculated, thereby making efficient use of the thermal energy in the drying air and minimizing the amount of air that exits the dryer housing 28 as exhaust.

Figure 4b is a cross-sectional end view taken through drying zone 21 of Figure 2 wherein the drying air is directed upward through the bed of raw material 46 on the conveyor 13. A volume of drying air is drawn into zone 21 at 47b by fan 48b which then exhausts the drying air into a plenum created between the partition 51b and the dryer housing 28. The drying air is then forced through heat exchanger coils 35b and beneath and up through the conveyor 13 and bed of raw material 46, thereby evaporating moisture from the raw material. While a portion of the moisture-laden air is exhausted at 52b, the remainder is recirculated in a manner similar to the pattern illustrated in Figure 4a. In one embodiment, dampers are used at an entrance 47 and an exit 52 of a drying zone, and between partitions 49 and 50, to further regulate airflow. Alternatively, the fans, partitions and dampers may be configured in drying zone 21 as illustrated in Figure 4a. As illustrated in Figures 2 and 3, the dryer housing may be partitioned into multiple zones, step 22, which zones may be drying zones 19, 21, 23, 30, 32, 34 and/or conditioning zones 36. As illustrated in Figure 2, the drying zones 19, 21, 23 are separated by buffer zones 31, 33 defined by partitions within the dryer housing 28. The buffer zones allow access to the dryer at various points along the dryer path for inspection and maintenance. In a conditioning zone, step 18, the raw material may be either heated or cooled, depending on whether it is desirable for the raw material to exit the dryer at a temperature that is higher or lower than the temperature it would be at if it was discharged from the dryer immediately following the completion of the drying process. For example, it may be desirable for the raw material to exit the dryer at a relatively high temperature, thereby potentially increasing the thermal efficiency of the pressing process. The raw material would therefore be heated in a conditioning zone. The heating or cooling of the raw material in a conditioning zone is accomplished in accordance with the present invention by controlling the temperature of the air in the conditioning zone, similar to controlling the temperature of the drying air.

If the dryer housing 28 is partitioned into several zones, as illustrated in Figures 2 and 3, the temperature in a given zone may be controlled independently of the other zones. By also controlling the volume of raw material fed into the conveyor and the rate with which the conveyor moves through the housing, a depth of a bed of raw material and the retention time, or amount of time the raw material remains in the housing, may be controlled. As a result, the drying process may be accurately controlled to gradually and evenly dry the raw material. The required retention time will depend on the moisture content of the wood, which may vary with the species of wood and time of season, among other factors. The moisture content of the wood may be monitored, however, by periodically measuring the moisture content of the raw material and/or periodically sampling the raw material. (A commercially available moisture meter, such as those manufactured by Moisture Systems, Inc., is suitable for measuring the moisture content of the raw material in accordance with the present invention.) In this manner, the raw material is dried evenly and efficiently.

By drying raw material in accordance with the present invention, it is possible to achieve good results at temperatures below 400°F. as opposed to temperatures ranging from 800°F to 1600°F in prior art rotary dryers. In one embodiment, for purposes of illustration, raw material for the manufacture of particle board may be dried by passing it through a single pass, single zone wherein the temperature is 350°. Because fewer pollutants such as carbon monoxide are produced at lower temperatures, this reduction in operating temperature should significantly reduce the level of pollutants emitted to the atmosphere. This ability to achieve good results at lower temperatures is due in part to the fact that unlike prior art rotary dryers, it is possible to accurately control and lengthen the retention time when drying raw material in accordance with the present invention. In a rotary dryer, the raw material is showered into an entrance of a large, rotating drum. As the drum rotates, the raw material is lifted toward the top of the dryer to fall through the drying air stream. The rotational movement of the drum and the motion of the drying airstream advance the raw material through the dryer housing. When the raw material reaches the end of the drum, it falls into a collection bin. Because a minimum quantity of air flow is required to remove the moisture from the system and a minimum speed of rotation of the drum is required to adequately expose the raw material to the airstream, it is very difficult to slow down the progression of the raw material through the dryer to accurately control the retention time. As a result, if the temperature is reduced in a prior art rotary dryer, the raw material is not adequately dried before being discharged. In contrast, the present invention allows complete control of the retention time, such that the temperature may be lowered and the retention time increased sufficiently to adequately dry the raw material before discharging it from the dryer.

In addition, by drying raw material in accordance with the present invention, the velocity of drying air passing through and around the raw material is considerably less than the velocity of air in a rotary dryer, which is believed to also significantly reduce the emission of pollutants by reducing the volume of fines that are entrained in the airstream.

In one embodiment of the present invention, the raw material is agitated as it passes through the dryer housing 28, step 24, thereby promoting the even exposure of the raw material to the drying air. This agitation may be accomplished in a variety of ways. For example, the raw material may be agitated as it falls from one level of the conveyor to a second level of the conveyor, as illustrated in Figure 3. In an alternative embodiment illustrated in Figure 2, the raw material is conveyed through the dryer housing 28 by three independently driven conveyors 1 1, 13, 15. At the end of the first drying zone 30, the raw material is received by a turnover device 42 comprising an inclined conveyor, which may be independently driven or driven by takeoff from either conveyor 11 or 13. The raw material then moves upwards and fall from turnover device 42 onto conveyor 13, thereby being agitated. A similar step is repeated between drying sections 21 and 23 by turnover device 44. If desired, the raw material may be conveyed through a conditioning zone, as discussed above, step 18, before being discharged from the dryer housing, step 20.

A method for drying raw material for the manufacture of particleboard and the like has been shown and described. From the foregoing, it will be appreciated that, although embodiments of the invention have been described herein for purposes of illustration, numerous modifications may be made without deviating from the spirit and scope of the invention. Thus, the present invention is not limited to the embodiments described herein, but rather is defined by the claims which follow.

Claims

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1. A method for drying raw material for the manufacture of particleboard, comprising the steps of: feeding raw material onto a conveyor to form a bed of raw material; conveying the raw material on the conveyor through a dryer housing; heating a quantity of drying air to a desired temperature; passing the drying air through the bed of raw material in the dryer housing; and after passing the drying air through the bed of raw material, discharging the raw material from the dryer housing.

2. The method according to claim 1, further comprising the step of: controlling the temperature of the drying air as the raw material moves through the dryer housing to regulate a rate of moisture removal from the raw material.

3. The method according to claim 2 wherein an inlet temperature of the drying air is maintained between ambient and 600°F.

4. The method according to claim 1, further comprising the step of: burning waste wood to heat a volume of fluid that is passed through coils of a heat exchanger, the heat exchanger being used to heat the drying air.

5. A method for drying raw material for the manufacture of particleboard, comprising the steps of: feeding a quantity of raw material onto a conveyor provided with means for allowing air to pass through it, to form a bed of raw material; conveying the raw material through a dryer housing; passing a volume of heated drying air through the bed of raw material; and discharging the raw material from the dryer housing.

6. The method according to claim 5, further comprising the step of: oscillating a belt to feed the raw material onto the conveyor.

7. The method according to claim 5, further comprising the step of: varying a speed with which the conveyor moves through the dryer housing thereby controlling a retention time for the raw material.

8. The method according to claim 7, further comprising the steps of: measuring the moisture content of the raw material periodically; and sampling raw material periodically, thereby monitoring the moisture content of the raw material as it passes through the dryer housing.

9. The method according to claim 5, further comprising the step of: feeding the raw material onto the conveyor at a pre-selected rate, thereby controlling a depth of the bed of raw material.

10. The method according to claim 5, further comprising the step of: recirculating a volume of the drying air thereby minimizing a loss of thermal energy.

11. The method according to claim 5, further comprising the step of: agitating the raw material as it passes through the dryer housing thereby promoting even exposure to the drying air.

12. The method according to claim 11 wherein the agitation is achieved by dividing the conveyor into different sections having varying elevations and forcing the raw material to fall from one conveyor section at a first elevation to a second conveyor section at a second, lower elevation.

13. A method for drying raw material for the manufacture of particleboard, comprising the steps of: feeding raw material onto a conveyor to form a bed of raw material; conveying the raw material on the conveyor through a dryer housing; heating a quantity of drying air to a desired temperature; passing the drying air through the bed of raw material in the dryer housing; conveying the raw material through a conditioning zone; and discharging the raw material from the dryer housing.

14. The method according to claim 13, further comprising the step of: varying the speed with which the conveyor moves through the dryer housing thereby controlling a retention time for the raw material in the dryer housing.

15. The method according to claim 13, further comprising the step of: recirculating a volume of the drying air thereby reducing a loss of thermal energy and a quantity of air that exits the dryer housing as exhaust.

16. The method according to claim 13, further comprising the step of: agitating the raw material as it passes through the conveyor housing thereby promoting even exposure to the drying air.

17. A method for drying raw material for the manufacture of particleboard, comprising the steps of: conveying a quantity of raw material through a dryer housing; partitioning the dryer housing to create a plurality of zones; circulating a volume of heated drying air through the raw material; and discharging the raw material from the dryer housing.

18. The method according to claim 17, further comprising the steps of: regulating a direction of flow and a temperature of the drying air; and regulating a speed with which the raw material moves through the plurality of zones thereby encouraging a controlled and gradual drying of the raw material.

19. The method according to claim 18 wherein the direction of the drying air is controlled by positioning fans and dampers.