CN102873730A - Preprocessing method for shortening wood drying time - Google Patents

Abstract

A pretreatment method for shortening the drying time of wood. The method comprises the following steps: 1. Selecting wood panels to be dried; Parameters adjust the distance between the upper and lower rollers of each group of roller presses and the speed of the rollers, so that the rollers have a set amount of compression on the wood sheet; 3. Wood sheet pretreatment: start the roller press, and place the wood sheet flat on the Between the upper and lower pressure rollers, relying on the friction or external force on the surface of the pressure rollers, the plate is passed from the input end of the roller press to the output end through one set of rolling or multiple sets of continuous rolling; the number of rolling is one or repeated Multiple times; 4. The wood board after rolling is dried. The invention is used for pretreatment before wood drying, can shorten the wood drying time for solid wood boards containing a certain amount of water, saves energy, has simple equipment structure, convenient operation, safe use, and can reduce production costs.

Description

A Pretreatment Method for Shortening Timber Drying Time

technical field

The invention relates to a pretreatment method for shortening wood drying time, and belongs to the technical field of solid wood processing in the wood industry.

Background technique

During the growth of trees, a certain amount of water is stored inside the wood, and it is necessary to reduce the water content to the required range through the drying process in wood processing. Wood drying is the core technology in the wood industry and a key production link in the production of solid wood products; conventional wood drying takes a long time, ranging from a few days, a dozen days, to dozens of days, and consumes a lot of energy. 50%-70% of the total energy consumption of the link. On the premise of ensuring the drying quality, the research work to shorten the drying time is very necessary and urgent.

In order to shorten the running time of wood drying, research results at home and abroad focus on microwave pretreatment and blasting pretreatment. Using microwave technology to pretreat wood before drying to improve the wood's ability to conduct moisture and shorten the drying time has long been a hot topic of research at home and abroad, and many research results have been formed. Blasting technology is also widely used in the research of wood drying pretreatment. The theory holds that under the condition that the shape and mechanical properties of the wood are not affected after micro-blasting, when there is high-pressure gas inside the wood and the external environmental pressure instantly returns to normal pressure, a pressure difference is formed inside and outside the wood, and the high-pressure gas inside the wood forms a high-speed gas. The airflow impacts from the inside out. When the high-speed airflow encounters parts blocked by sediments, occluded pits, plug edges with extremely small pores, etc., it will dredge, break and crush the obstacles, thereby improving the permeability of wood. The described wood microwave pretreatment and blasting pretreatment methods can both achieve corresponding effects, but the microwave pretreatment method needs to consume a large amount of energy, and the blasting pretreatment method requires special pressure-resistant equipment, resulting in an increase in the cost of the pretreatment process. It is difficult to popularize in actual production, and the market is looking forward to the emergence of wood drying pretreatment technology with simple method, convenient operation and low cost.

Contents of the invention

The purpose of the present invention is to provide a new pretreatment method for shortening the drying time of wood. This method can increase the flow and speed of wood moisture, shorten the drying time of wood, and the equipment has simple structure and convenient operation, which can save energy and reduce Production costs for the pretreatment process.

The technical scheme of the present invention is: a kind of pretreatment method that shortens wood drying time, it is characterized in that, this method comprises the steps:

(1) Select wood boards: select wood boards to be dried within the design size range, and the maximum moisture content of the boards can be saturated with water;

(2) Adjust the process parameters of the equipment: the equipment is a group or multiple groups of roller presses arranged in sequence. Each group of roller presses is composed of upper and lower pressure rollers. The upper and lower pressure rollers rotate in opposite directions. Adjust each group of rollers according to the set process parameters. The distance between the upper and lower pressure rollers of the press and the speed of the pressure rollers should be set to be smaller than the thickness of the wood board, so that the pressure rollers can compress the wood board to a certain extent;

(3) Wood sheet pretreatment: Start the roller press, place the wood sheet flat between the upper and lower pressure rollers, rely on the friction or external force on the surface of the pressure roller, and make the sheet pass through a set of rollers from the input end of the roller press. Rolling or multiple groups of continuous rolling reaches the output end, and the number of rolling is one time or repeated multiple times;

(4) Drying treatment: Drying treatment is carried out on the wood panels after pretreatment by rolling. The drying methods include atmospheric natural drying, conventional steam drying, microwave drying or high-frequency drying.

According to the research results of modern wood drying theory, wood drying is the process of water discharge in wood, and the premise of water discharge is the movement of water. According to the analysis of the ultra-microscopic structural characteristics of wood and the wall structure of wood molecular cell walls, the movement of water is mainly It is realized through the pits on the cell wall, and the water passes through the pits in a gaseous and liquid phase alternately, from one cell cavity to the cell cavity of other cells adjacent to it, and finally reaches the surface of the wood.

The pit on the wood cell wall refers to the part of the primary wall that is not thickened when the wood cell wall thickens to produce the secondary wall, that is, the depression on the secondary wall. In standing trees, pits are channels for water and nutrients between adjacent cells. A pit is generally composed of a pit ring, a pit cavity, a pit opening, a pit membrane, a pit chamber, and a pit channel; water flows from a cell lumen to an adjacent cell lumen through the pit membrane, and it can be seen that , the pit membrane is the key part of water movement; according to the wood ultrastructure theory, the pit membrane is composed of pit plugs and the surrounding plug edges, and there are many gaps on the plug edges, the size of which is between 0.1-1 μm, which is smaller than this Molecules of this size can permeate through the plug edge; water can pass through, but the flow rate and velocity are severely restricted by the dense structure of the plug edge.

The core content of the present invention is to apply lateral compression to the wood plate through the pressure roller in the special equipment, so that the pit membrane, the main part of the water movement in the wood, will be broken in microscopic form due to the action of external force, and the water will move from the deep layer of the wood to the surface cells. The increased number of paths and pore size increases the amount and speed of water flow and the drying time of the wood is significantly shortened.

Wood is a natural polymer composite material composed of hollow cells, most of which have a density between 0.40g/cm3 and 0.75 g/cm3, and have viscoelastic (elasticity and viscosity) characteristics; the cells in wood are absolutely Most of them are elongated, hollow, and fibrous, with a large aspect ratio, and the arrangement of cells is nearly parallel to the height direction of the trunk. Under normal temperature and pressure conditions, wood molecules are subjected to mechanical pressure perpendicular to the fiber direction, wood cells are flattened laterally, and the cell cavity becomes smaller or disappears. For single cells, based on the special shape of the pit membrane (approximately round shape or ellipse) and the ultra-microscopic structural characteristics of the thinnest part of the cell wall, making it the area with the least mechanical strength on the cell wall; under a certain compression rate, one side of the cell wall is pressed to the opposite side, and the deformation and stress are in the pit When the external force is greater than the internal stress between the microfibrils of the pit membrane, the microscopic rupture of the pit membrane occurs; after the wood leaves the pressure roller, the cells return to their original shape due to elasticity, but the grain The microscopic physical deformation formed at the porous membrane is preserved.

Except for a part of the cell wall material in the wood, the rest is the cell cavity and intercellular space occupied by moisture (water vapor), air, inorganic substances and inclusions, and the moisture in the wood is divided into free water and absorbed water. Wood rollers Compression pretreatment takes a short time and is completed in an instant. Macroscopically, the transverse dimension of the wood becomes smaller, while microscopically, the cell cavity and intercellular space in the wood are flattened, and part of the free water and air are squeezed out of the wood; After the treatment is completed and the external force is removed, the flattened cell lumen and intercellular spaces will return to their original shape and size (approximately complete elastic recovery), negative pressure will appear in the gaps in the wood, and the outside air will take the opportunity to enter the wood again. The outflow of moisture, the outflow and inflow of air during pretreatment, when the wood is dry, objectively promote the removal of free water and sorbed water from the wood. During the process of compressing wood, the pit membrane on the wood cell wall (the main channel for moisture to enter and exit the wood) may be damaged, and cracks may appear in individual parts of the cell wall. These microstructural variations will significantly improve the fluidity of water in the wood.

Above-mentioned explanation to the principle of the present invention is quite different from other wood drying pretreatment methods.

The description of the equipment used in the present invention: a pair of steel pressure rollers with sufficient mechanical strength are arranged up and down. It is a driven roller, or both are driving rollers, but the direction of rotation is opposite; the distance between the two rollers can be adjusted, and the distance between the rollers must be smaller than the thickness of the wood board, causing a certain pressure on the board, and the wood board passes between the rollers by friction or external force. The length of the pressing roller is greater than the width of the board, and the size of the roller spacing can be determined according to the thickness and compression of the board to be processed. Multiple pairs of pressure rollers can also be set as required, all of which are protected by patents.

The present invention has the following characteristics: ①The rolling pretreatment time is extremely short, completed instantaneously, with high efficiency and continuous production; ②There is no selectivity in the moisture content range of the wood to be pretreated, and it is applicable to a wide range of moisture content conditions; ③According to the wood The ultramicrostructure characteristics of the cell wall and the mechanism of water movement, scientifically adopt mechanical rollers to carry out lateral compression on the wood plate, the wood cells are instantly flattened, due to the stress concentration, the pit membrane on the cell wall ruptures, the permeability of the wood to water and The penetration is greatly improved, and the running time of wood drying is significantly shortened; ④Rolling pretreatment of wood drying can be completed once or multiple times with one set of pressing rollers, or can be completed at one time with multiple sets of pressing rollers. The compression rate can be the same or different; the diameter and speed of the pressing roller can be changed at any time according to the needs, and the adjustment of the rolling process is convenient and fast; ⑤The working environment is safe and can be carried out under normal pressure, saving expensive pressure resistance Processing equipment, energy consumption is significantly reduced, and the cost is significantly reduced. ⑥In terms of process, the inventive method can connect the rolling machine with the log lumber equipment, and the plate after lumber can be immediately pretreated by rolling, no additional work section is needed, and it is very convenient in terms of process and equipment arrangement ; ⑦ The theoretical basis of this rolling pretreatment method is to destroy the pit membrane on the wood cell wall, and the speed and flow of moisture in the wood will be significantly improved due to the destruction of the pit membrane. blasting method) has been confirmed.

For oak boards with a size of 2000 (length) × 100 (width) × 30 (thickness) mm, the moisture content is dried from about 70% to 8%-10%, and it takes 21-22 days in a conventional steam medium drying kiln; After pretreatment by different processes of roller pressing, the final moisture content can be reached in 18-19 days, the drying time is shortened by more than 2 days, and the production time can be saved by 5%-10%. According to the current material and energy price calculation cost, the energy consumption and equipment depreciation costs of drying ^1m3 oak wood plank for one day are about 10 yuan, and a 100 ^m3 drying kiln can save more than 2000 yuan in one drying cycle, which is economically beneficial. significantly.

The device of the invention has simple structure and convenient operation, can shorten the running time of the drying link, can save energy compared with the microwave pretreatment method, and can reduce equipment cost and production cost compared with the blasting pretreatment method.

Description of drawings

Figure 1 is a schematic diagram of the principle of wood drying and rolling pretreatment.

Detailed ways

The present invention is a pretreatment method implemented before the wood drying process, and the processing equipment used in the method is commissioned to manufacture according to the technical indicators and requirements proposed by the inventor. The structure of the equipment is simple, the production operation is convenient and safe, and it can directly preprocess solid wood panels with a wide range of moisture content.

The present invention adopts the following steps to complete, (1) select wood boards: the materials used in the present invention are boards of solid wood, the species of trees are not limited, and if the external dimensions of the boards are selected within the design size range, the width of the boards needs to be greater than the thickness. The moisture content should be above the saturation point of wood fiber. (2) Adjust the process parameters of the equipment: the equipment is a group or multiple groups of roller presses arranged in sequence. Each group of roller presses is composed of upper and lower rollers. In Figure 1, 1 is the upper roller, 2 is the lower roller, and 3 is wood. For plates, the upper and lower pressure rollers rotate in the opposite direction. Adjust the distance between the upper and lower pressure rollers of each group of roller presses and the speed of the pressure rollers according to the set process parameters. The process parameters are set according to the thickness of the plate, tree species and other factors. The spacing is smaller than the thickness of the wood plate, so that the pressure roller has a designed compression amount on the wood plate; (3) Wood plate pretreatment: start the roller press, place the wood plate between the upper and lower pressure rollers, and rely on the surface of the pressure roller The frictional force or external force makes the plate reach the output end from the input end of the roller press through one set of rolling or multiple sets of continuous rolling; Use a set of rollers for one or more rolls; for continuous rolls and multiple rolls, the next set of rolls is pretreated, and the distance between the rolls can be the same or different than that of the previous set of rolls . (4) Drying treatment: Drying treatment of wood panels after pretreatment by rolling, including natural air drying, conventional steam drying, microwave drying or high-frequency drying.

The following is an example of oak wood plank roll pretreatment and wood drying

This example examines the comparison between the moisture content drop rate of rolling pretreated wood and the material (without rolling pretreated board) at different stages of wood drying, to illustrate the effect of rolling pretreatment on shortening wood drying time.

Material: Oak (Quercus mongolica Fisch.) board, from the same log, with similar wood properties, no defects, moisture content of 60% to 80%, made into standard quarter-sawn boards and string-sawn boards, 900 (length) × 100 (width) × 30 (thickness) mm.

Equipment: roller press machine, the upper and lower rollers are active rollers, the rotation speed is 30r/min, the length of the roller is 350mm, and the diameter of the roller is 500mm.

Roller pretreatment process: carry out chord compression on quarter-sawn slabs, and radial compression on chord-sawn slabs, the compression ratios are 10%, 20% and 30%, and the compression times under different compression ratios are 1, 4 and 9 respectively Second-rate.

Wood Drying Benchmarks:

In the table: MC—moisture content, T—dry bulb temperature, Δt—dry and wet bulb temperature difference.

Preheat treatment: implemented before the wood drying starts, that is, before the drying benchmark starts.

t=65℃, Δt=0-1℃, time=6h.

Intermediate treatment 1: Occurs in the middle of the second stage, that is, when the average moisture content of the wood is 35%.

t=75℃, Δt=0-1℃, time=6h.

Intermediate treatment 2: Occurs in the late stage of the third stage, that is, when the average moisture content of the wood is 25%.

t=75℃, Δt=2-3℃, time=8h.

Final treatment: It occurs in the late stage of the sixth stage, that is, when the average moisture content of the wood reaches a moisture content of 8% to 10%.

t=80℃, Δt=7-8℃, time=8h.

To determine the moisture content, use the moisture content test board and dry box method to measure together.

Stacking method

The number of layers: 10 layers, the number of plates in each layer: 6 pieces, the position of the inspection board: the 5th layer, the thickness of the spacer: 15mm.

Oak quarter-sawn boards and string-sawn boards were pretreated by chordwise and radial rolling with different compression ratios and compression times, and then put into a wood dryer for drying together with plain boards. Six stages were selected for different The water content drop rate of the process-treated board is measured and studied.

Declining speed of moisture content of test material under 10% compression rate

Table 1 Under 10% compression rate, compared with the plain board, the moisture content of the pre-rolled pretreated material decreases at each stage

In Table 1: Drying stage 1 is expressed as the rate of decline of moisture content during the period from the beginning of drying to 46 hours;

Drying stage 2 is expressed as the rate of decline of moisture content during the drying process of 96h to 102h;

Drying stage 3 is expressed as the rate of decline of moisture content during the drying process of 108h to 129h;

Drying stage 4 is expressed as the rate of decline of moisture content during the drying process of 165h to 201h;

Drying stage 5 is expressed as the rate of decline of moisture content during the drying process of 201h to 223h;

Drying stage 6 is expressed as the rate of decline of moisture content during the drying process of 223h to 252h.

The drying phase times in the following tables are the same as in this table.

Through the analysis of the moisture content decline rate of the plates in different compression directions and compression times under 10% compression rate in Table 1, it can be found that the drying moisture content of the plates pretreated by chord or radial roll pressing The rate of decline Significantly faster than the corresponding plain board without pre-rolling. For example, in the drying stage 1, the water content of the board pressed once in the chord direction is 0.038%/h faster than that of the corresponding material, that is, from the beginning of drying to 46 hours after drying, the board pressed once in the chord direction under 10% compression rate The moisture content of the plate is 0.038%×46h=1.79% lower than that of the corresponding plain plate; the moisture content of the plate rolled once in the radial direction is 0.084%/h faster than that of the corresponding plain plate, that is, from the beginning of drying to the drying At 46h, the moisture content of the sheet radially rolled once at 10% compression rate is 0.084%×46h=3.84% lower than that of the corresponding plain sheet.

It can also be found that at a compression rate of 10%, the decline rate of the moisture content of the board in each drying stage in the same compression direction increases with the increase of the number of compressions. For example, in the drying stage 4, the water content of the 4 times of chord rolling is 0.014%/h faster than that of 1 time of chord rolling, and the 4 times of radial rolling is 0.01 faster than that of 1 time of radial rolling. %/h; in the 5th stage of drying, the water content of the 9 times of chord rolling is 0.027%/h faster than that of 1 time of chord rolling, and the 9 times of radial rolling is faster than 1 time of radial rolling The speed is 0.028%/h.

Under the condition of 10% compression rate and the same number of rolling times, the moisture content of the radially compressed sheet decreases faster than that of the chordally compressed sheet, and the corresponding materials also have the same characteristics. For example, in the drying stage 3, the moisture content of the string-sawn board material decreases 0.023%/h faster than that of the quarter-sawn board material; in the drying stage 1, the moisture content of the radial rolling 1 time decreases faster than the chord rolling 1 time 0.058%/h faster for one time; in the drying stage 5, the water content decrease rate of 4 times of radial rolling is 0.012%/h faster than that of 4 times of chord rolling; in the 3rd stage of drying, 9 times of radial rolling The water content drop rate is 0.043%/h faster than that of chord rolling 9 times.

Declining speed of moisture content of test material under 20% compression ratio

Table 2 Under 20% compression rate, compared with the plain board, the moisture content of the pre-rolled pretreated material decreases at each stage

Through the analysis of the moisture content decline rate of the boards with different compression directions and compression times under 20% compression ratio in Table 2, it is found that the moisture content decline speed of the boards pretreated by chord or radial rolling is significantly faster For the corresponding plain plate that has not been pretreated by rolling. For example, in the drying stage 3, the water content of the plate rolled once in the chord direction is 0.049%/h faster than that of the corresponding plain plate, and the moisture content of the plate rolled once in the radial direction is 0.051% faster than that of the corresponding plain plate. /h; In the drying stage 2, the water content of the plate rolled 4 times in the chord direction is faster than the corresponding plain plate by 0.025%/h, and the water content of the plate rolled 4 times in the radial direction is faster than the corresponding plain plate. 0.043%/h.

It can also be found that at a compression rate of 20%, the rate of decline in the moisture content of the board in each drying stage in the same compression direction increases with the increase in the number of compressions; The decrease rate of water content in chord rolling is 0.016%/h faster for 1 time, 0.059%/h faster for 4 times of radial rolling than 1 time of radial rolling; in drying stage 3, 9 times of chord rolling It is 0.01%/h faster than the water content of 4 times of chord rolling, and 0.027%/h faster of 9 times of radial rolling than 4 times of radial rolling.

Under the condition of 20% compression rate and the same number of rolling times, the water content of the radially compressed sheet decreases faster than that of the chordally compressed sheet. For example, in drying stage 1, the water content decrease rate of radial rolling once is 0.027%/h faster than that of chord rolling once; in drying stage 3, the moisture content decrease rate of radial rolling 4 times is faster than The speed of rolling 4 times in the chord direction is 0.027%/h faster; in the drying stage 5, the moisture content of the 9 times of radial rolling is 0.007%/h faster than that of 9 times of chord rolling.

Declining speed of moisture content of test material under 30% compression ratio

Table 3 Under 30% compression ratio, compared with the plain board, the moisture content of the rolling pretreated material decreases at each stage

Due to the macroscopic damage on the surface of the oak wood board after rolling 4 times and 9 times under 30% compression rate during the pre-rolling process, no drying treatment and testing were carried out.

Through the analysis of the moisture content decline rate of the boards with different compression directions and compression times under 30% compression ratio in Table 3, it can be found that the moisture content of the boards that have been pretreated by chord or radial rolling has a more obvious decline rate. Faster than the corresponding plain board without pre-rolling. For example, in the drying stage 1, the water content of the plate rolled once in the chord direction is faster than that of the corresponding plain plate by 0.087%/h, and the moisture content of the plate rolled once in the radial direction is faster than that of the corresponding plain plate. 0.114%/h; in drying stage 3, the water content of the plate rolled once in the chord direction is 0.079%/h faster than that of the corresponding plain plate, and the water content of the plate rolled once in the radial direction is faster than that of the corresponding plate. The speed of plain board is 0.058%/h.

It can also be found that at a compression rate of 30%, the decline rate of moisture content in each drying stage of the chordwise compressed board increases with the increase of compression times. For example, in the drying stage 1, the water content of the chord rolling 4 times is 0.011%/h faster than that of the 1 rolling; in the drying stage 2, the chord rolling 9 times is faster than the 4 rolls 0.017%/h; in the drying stage 6, the water content of the chord rolling 9 times is 0.087%/h faster than that of rolling 1 time.

Under the condition of 30% compression rate and the same number of rolling times, the water content of the radially compressed sheet decreases faster than that of the chordally compressed sheet. For example, in the drying stage 4, the water content decrease rate of radial rolling once is 0.023%/h faster than that of chord rolling once.

Conclusion: In summary, compared with plain boards, the moisture content of oak boards under different rolling pretreatment processes has the following characteristics in the drying process: (1) Oak boards after rolling pretreatment During the drying process, the rate of decrease of moisture content is faster than that of the corresponding plain board. It shows that after the wood is pretreated by rolling, it can indeed improve the ability of the wood to conduct water when it is dry, thereby speeding up the drying speed of the wood; (2) During the drying process, the moisture content of the oak wood board under the same compression rate decreases. With the increase of the number of rolling times, it will be accelerated; under the same compression times, the moisture content of the oak board will decrease with the increase of the rolling compression rate. It shows that with the increase of compression rate and compression times, the internal structure of wood will be different. The greater the compression rate and the more compression times, the greater the degree of cracking of the pit membrane in the wood, the greater the number of cracks, and the more cracks appear on the cell wall, thus creating more new ways for water to move and speeding up the process. The moisture movement speed of wood during the drying process; (3) Because oak wood has wide wood rays, it increases the moisture drainage capacity of oak wood in the radial direction, so the drying speed of string-sawn plain board is faster than that of quarter-sawn plain board ; and because oak wood is broad-leaved ring porous wood, radial compression is carried out on chord-sawn lumber, and the compression curve is a three-stage type, which causes more pressure loss to early wood, while tangential compression is performed on diametrically-sawn lumber, and the slab is compressed in the chord direction. The microscopic damage is not as great as the radial compression damage of the string-sheared board, so the radial compression of the string-sheared board can create more water movement channels inside the wood, which is more conducive to the movement of water during the drying process, so the radial compression The moisture content of the panel decreases faster than that of the panel compressed in the chord direction.

Claims (1)

1. a pretreatment method that shortens the drying time of wood, is characterized in that, the method comprises the steps: (1) Select wood boards: select wood boards to be dried within the design size range, and the maximum moisture content of the boards can be saturated with water; (2) Adjust the process parameters of the equipment: the equipment is a group or multiple groups of roller presses arranged in sequence. Each group of roller presses is composed of upper and lower pressure rollers. The upper and lower pressure rollers rotate in opposite directions. Adjust each group of rollers according to the set process parameters. The distance between the upper and lower pressure rollers of the press and the speed of the pressure rollers should be set to be smaller than the thickness of the wood board, so that the pressure rollers can compress the wood board to a certain extent; (3) Wood sheet pretreatment: Start the roller press, place the wood sheet flat between the upper and lower pressure rollers, rely on the friction or external force on the surface of the pressure roller, and make the sheet pass through a set of rollers from the input end of the roller press. Rolling or multiple groups of continuous rolling reaches the output end, and the number of rolling is one time or repeated multiple times; (4) Drying treatment: Dry the wood panels after pretreatment by rolling, including natural air drying, conventional steam drying, microwave drying or high-frequency drying.

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