Technical Field

[0001] The present invention relates to an apparatus and a method for modification and preservation treatment of wood, in particular for preserving wood for building purposes.

Background Art

[0002] DK 179238 and DK 179094 describe methods by which wood is preserved by treating it in an inert atmosphere, such as nitrogen under high pressure and high temperature for a prolonged time.

[0003] It has however, turned out that the wood treated by these methods sometimes are treated unevenly. It is therefore a need to improve the method and devise an apparatus by which an even treatment of the wood can be achieved.

[0004] US 20080263891 discloses a process for treating lignocellulosic material, such as wood for example, the process including the steps of: a) providing lignocellulosic material; b) evaluating parameters of the lignocellulosic material; c) preparing the lignocellulosic material; d) heating the lignocellulosic material in a treatment chamber following a given profile based on parameters of the lignocellulosic material; e) stabilizing the lignocellulosic material; and f) cooling down the lignocellulosic material. The process enables to improve the quality of the wood being treated, while ensuring a greater repeatability and uniformity of coloring results. The known method attempts to achieve a balanced circulation through the pile of wood. However, it also describes the use of air as medium and a subsequent step of re-humidifying by water.

[0005] US 20080263890 discloses a wood heat-treating method, a plant for carrying out the method, and the heat-treated wood. The heat-treating method consists in bringing each wood piece of a treatable lot into contact with a temperature controlled conductive press, whose temperature is accurately controllable in time and in intensity. The pieces are heated to or held at a desired temperature by any heat control for treatment and the wood is conductively heated to maintain the temperature and to cool the wood. [0006] This publication describes a similar process as in US 20080263891 by using air. It is an attempt to achieve a better circulation of the air.

[0007] CA1159643 discloses a method of dimensionally stabilizing wood products prepared by a compression molding step comprising heating the wood material at 160 to 220°C in a gaseous atmosphere of 4 to 15 bar pressure for a time sufficient to stabilize the dimensions thereof.

[0008] A circulation of inert gas is achieved by the use of a fan or blower. Little details of how the gas is circulated is explained.

[0009] EP2196295 describes a process by which the wood is treated at a temperature in the range of 20 to 150 degree Celsius at a pressure in the range of 860 to 1090 hectopascals in an atmosphere of inert gas for 5 to 40 hours. The wood is further treated at a temperature in the range of more than 150 degree Celsius at a pressure in the range of 860 to 1090 hectopascals in the inert gas atmosphere. The inert gas is selected from one of nitrogen, argon, neon, helium, carbon dioxide or nitrous oxide, and exhibits temperature higher and/or lower than inner temperature at a time point of a pulse.

[0010] The wood to be treated is placed in a treatment room and heated. Inert gas is circulated around the wood.

[0011] US4377040 describes a process for the modification of wood and wood products by heat treatment in a closed, heatable vessel, wherein the water content of the starting material is controlled to be no higher than 10% by weight. An autoclave is used for this and the publication describes circulation of the gas. However, little details are shown.

[0012] EP3406994 and WC00/67970 discloses inventions for drying and/or curing impregnated wood with gas in one flow direction across an arranged set of wood.

[0013] SE507206 describes another invention for drying wood utilizing a heat exchanger and gas, but it does not disclose how the gas circulates through the prepared wood.

[0014] Other less relevant prior art examples are shown in FR2854831 , CN103659973, CN106003321 , JP6707572 and US200440148795. [0015] None of the above prior art solutions ensure proper circulation of the gas around the wood so that each and every piece and part of wood are sufficiently treated.

Summary of invention

[0016] The present invention therefore has as its main objective to ensure that an inert gas is circulated properly around the wood so that each piece thereof and each part of each piece of wood are sufficiently treated. The purpose is to modify and stabilize the wood through homogeneous treatment and thereby to extend the durability properties of the wood.

[0017] A further objective of the invention is to achieve proper treatment within a prescribed time and to ensure that a pile of wood can be treated within said time.

[0018] It is further a goal by the invention to remove hemicellulose, and preserve the content of cellulose and lignin, to maximize increase of durability and minimize loss of strength of the treated wood.

[0019] The invention enables the use of the hemicellulose exotherm as process control parameter.

[0020] These and other objectives are achieved by the features of the hereinafter appended claims.

Brief description of drawings

[0021]

Figure 1 shows a treatment chamber according to the invention,

Figure 2 shows the treatment chamber with a pile of wood arranged therein, as well as some of the internal structure of the chamber,

Figure 3 shows a longitudinal section through the chamber with arrows illustrating the flow of gas in the longitudinal direction,

Figure 4 shows a cross section through the chamber with arrows illustrating flow of gas in the vertical plane,

Figure 4a a cross section through the chamber with arrows illustrating flow of gas in the vertical plane in an alternative embodiment of the chamber, and Figure 5 shows graphs of temperature and pressure etc. in the chamber during treatment of a batch.

Detailed description of the invention

[0022] Figure 1 illustrates a treatment chamber 1 for a pile of wood. In the figure a part of the chamber wall has been cut away to reveal the inside. The chamber is preferably a long circular cylinder having endcaps 2, 3 at each end. One of the endcaps 3 is designed to work as a door that can be opened to give access to the interior of the chamber 1 and closed tightly to allow for high pressure and high temperature treatment.

[0023] At the end of the chamber 1 opposite of the door 3 are couplings 4, 5 for supplying a heating medium, such as hot oil to a heat exchanger 6 (see figure 3) within the chamber 1 , and an electric motor 7 coupled to a fan 8 (see figure 3). The heat exchanger can be used both for heating and cooling of the gas. Conveniently, the chamber 1 also comprises a drain (not shown) for draining liquid (predominantly water) that is emitted by the wood during the treatment.

[0024] The heat exchanger is preferably supplied by hot oil that is heated or cooled in an external heating/cooling system (not shown).

[0025] In figure 2 a pile 9 of wood has been placed in the chamber 1 . In practice there is one or several separate piles of wood that have been arranged end to end. However, this has no significance to the functioning of the chamber, so in the following the piles will be referred to as a single pile.

[0026] The pile of wood 9 is arranged on a cart 10 (actually one cart for each pile), that runs on rails 11 , so that the pile 10 can be easily moved into and out of the chamber 1 .

[0027] In figure 2 some of the main interior panels in the chamber 1 are shown. There is a top panel 12, perforated side panels 13 and deflector panels 14 and 15, all of which will be explained in detail in the following.

[0028] The cart 10 may have a closed bottom 17 that may temporarily attach to the cart, but will be left in the chamber after the pile has been placed therein and the cart 10 has been removed from the chamber 1 (see figure 4). After the treatment has been completed, the cart 10 will be rolled into the chamber 1 again and the pile 9 with the bottom 17 will be removed from the chamber 1. The removable bottom 17 allows for easy access to the bottom of the chamber for cleaning. Most of the debris that arises from the process will collect at the bottom of the chamber 1 .

[0029] Figure 3 shows a longitudinal section through the chamber 1 . Here the heat exchanger 6 and the fan 8 with motor 7 are shown. Also illustrated is an inert gas port (not shown) to let in inert gas, such as nitrogen. The fan 8 can be arranged through the top of the chamber, as shown in figure 3, through the end cap 2 or any other place that will ensure circulation as explained below.

[0030] Figure 3 also illustrate the substantially longitudinal circulation of inert gas in the chamber. The inert gas is let into the chamber via the inert gas port while the air in the chamber is allowed to escape out through another port (not shown) of the chamber.

[0031] When the air has been replaced with inert gas, the fan 8 is started. The fan 8 may also be running while the gas is being replaced. The fan draws the gas vertically, or alternatively horizontally, through the heat exchanger 6 and pushes the gas along the upper side of the top panel 12 where a top channel 45 is defined, extending along the whole length of the chamber 1 . As will be explained in detail below, parts of the gas flows downward to one side of the chamber as the gas is pushed along the top panel. The gas emerges at the bottom of the chamber 1 and flows back to the heat exchanger 6 between the bottom plate 17 and the chamber wall though a bottom channel 28.

[0032] Figure 4 is a cross-section through the chamber 1 . This also shows the fan 8 with the motor 7 and the couplings 4, 5. Inside the chamber 1 is the pile of wood 9. The individual pieces of wood are separated in the vertical direction by spacers, such as wooden lists, in a per se known manner.

[0033] The top panel 12 and the upper deflector panels 15 are joined at the corners where they meet. The bottom panel 17 is in the same way joined with the lower deflector panels 14. This way two continuous enclosures are made along the upper part of the pile and along the lower part of the pile, respectively. The two enclosures are separated by a slit 18 at each side of the chamber 1 , the purpose of which will be explained hereinafter. [0034] The perforated panels 13 are arranged on the inside of the deflector panels 14, 15 at each side of the pile 9, extending along the whole length and height of the pile 9.

[0035] A narrow passage 19 is formed at one longitudinal side of the top panel 12, where the top panel 12 joins the upper deflector panel 15. At the opposite side 20 there is no passage. This forces the flow of inert gas through the passage 19 and downwards along the upper deflector panel 15. There is no passage at 21 where the lower deflector panel 14 joins the bottom panel 17, so there will be a pressure build up within the void 23 created by the deflector panels 14, 15 and the wall of the chamber 1 . The inert gas has only one way that it can flow, and that is through the slit 18. Between the deflector panels 14, 15 and the perforated plate 13 is a second void 24.

[0036] The narrow passages 19, 22 can be equipped with an adjustable damper (not shown) to adjust the opening of the slits 19, 22 in order to regulate the flow trough the slits 19, 22. By having adjustable dampers also at the corners 21 , 22, the direction of the flow through the pile of wood can be reversed, so that the flow will go from left to right instead of the direction shown in the figure. Thereby a more dynamic circulation can be achieved and pockets of stand still gas can be avoided.

[0037] The top channel 45 may be divided longitudinally by a plate (not shown). The longitudinal plate extends at an angle relative to the longitudinal axis of the channel 45, so that one part of the channel 45, closest to the endcap 2, is wide and the part closest to the door 3 is narrow. This reduction of cross section of the flow channel will ensure that substantially the same amount of gas flows through the slit 19 at any given place along the channel 45.

[0038] The longitudinal plate may be equipped with a dampener at the end closest to the door 3 to open for the gas to flow from one part of the channel 45 to the other. This way gas may flow first in the direction from the end cap 2 towards the door 3 and then back again from the door 3 towards the end cap 2. At the same time dampers (not shown) will close the slit 19 and dampers (not shown) will open the slits 20, so that the gas can flow in the opposite direction through the pile 9 of wood.

[0039] The top channel 45 may also be equally divided longitudinally by a plate (not shown). The fan 8 may be located on one side of the divided top channel 45, so that fan 8 draws inert gas from the heat exchanger 6 deriving from the opposite side of the divided top channel 45, from the bottom channel 28 or a combination of both. The fan 8 then pushes the said drawn inert gas longitudinally along one side of the divided top channel 45 towards the door/end cap 3. The narrow passages 19, 20 at the top panel 12 are open for allowing the inert gas to pass through. The narrow passages 22, 21 at the bottom panel 17 are open for allowing the inert gas to pass in towards the bottom channel 28, or the narrow passages 22, 21 can be closed to prevent any inert gas flow in to or from the bottom channel 28. In this configuration the inert gas flows longitudinally along one side of the divided top channel 45, through a narrow passage 19,20 at the top panel 12 and flow horizontally through the pile 9 of wood towards the other side within the chamber and finally the inert gas is provided in the opposite side of the divided top channel 45. The flow direction of the inert gas can be reversed by reversing the drawing/pushing of the fan 8.

[0040] The bottom channel 28 can also be equally divided longitudinally by a plate (not shown), and alternatively the fan 8 can be provided in the bottom, mirroring the configuration of the top channel 45 being equally divided longitudinally.

[0041] The possibility of bidirectional flow through the pile 9 of wood makes it possible to reverse the flow at a point in the treatment process to even out any differences in temperature between the two sides of the pile 9 of wood. The switch of flow direction may take place about halfway through the treatment process, or the change may be done several times during the treatment.

[0042] Immediately on the inside of the slit 18 is a small diverter 25, which diverts the flow upwards and downwards. Hence the inter gas will fill the void 24. Gas will eventually seep through the perforated plate 13 along the full height and length thereof. This combination of features creates an even flow of inert gas through the pile 9 of wood, substantially horizontally from one side of the pile to the other.

[0043] At the opposite side of the pile, the gas meets the opposite perforated plate 13, flows therethrough and into another void 26. A small diverter 27 ensures that the gas does not flow directly through the slit 18 but is diverted up and down to fill the void 26.

[0044] Eventually the gas will flow through the slit 18 and into yet another void 28 created between the deflector plate 14, 15 and the wall of the chamber 1 . A small passage 22 is formed where the lower deflector panel 14 is joined with the bottom panel 17, which allows the inert gas to flow into a bottom compartment 28. The bottom compartment 28 extends along the whole length of the chamber 1 and brings the flow of inert gas back to the heat exchanger 6. At each end of the pile 9, end plates (not shown) are arranged to prevent gas from flowing into the voids at the side of the pile in a longitudinal direction. Moreover, the gas is prevented from flowing beyond the end of the pile near the door 3. This will ensure that all of the gas has to flow horizontally and transversely through the pile of wood.

[0045] The fan 8 need only ensure a sufficient flow of inert gas through the heat exchanger 6 and through the pile 9 of wood.

[0046] Figure 4a shows an alternative embodiment, where the deflector plates have been replaced by a continuous plate 50. The plate 50 has several openings, such as horizontal slits to let inert gas into the void 24. The small diverter can be dispensed off in this embodiment. In other respects, the embodiment of figure 4a is similar to the embodiment of figure 4.

[0047] Figure 5 shows graphs representing temperature and pressure during a typical treatment of a pile of wood. Temperature and pressure sensors are conveniently arranged at suitable locations in the system. The graphs of figure 5 shows only overall pressure and temperature curves, but it is to be understood that the temperature and pressure may be measures at different places within the system.

[0048] Curve 40 is the temperature measured in °C and curve 41 is the pressure measured in bar. Curve 42 is the boiling point in °C at the pressure indicated by curve 41 . It is vital that the temperature 40 does not go above the boiling point at any time during the treatment. If the boiling point is exceeded the wood will start to boil and cracks may form that will weaken the wood structure. To ensure that no boiling occurs, the pressure in the chamber is elevated as the temperature rises. This will lift the boiling point. Keeping the boiling point above the actual temperature is important throughout the process, especially during the cool down, as it will take time for the temperature of the wood to become sufficiently low at at the existing pressure. This is especially pronounced for thicker pieces of wood, which can retain a high internal temperature even after the gas in the chamber has cooled down to room temperature.

[0049] During the cool down, cold oil will be circulated through the heat exchanger, and the inert gas may be forced to circulate through the wood pile to rapidly cool down the wood.

[0050] In the treatment chamber of the present invention, the temperature and pressure can be controlled separately. This minimizes the cycle time with fast heating and cooling due to fast energy transfer to/from wood within a dense atmosphere. It also avoids stress and internal cracks from elevated water steam pressure internally in the wood, especially during cooling phase. It optimizes the modification process as pressure affects the presence of key catalysts such as water and acetic acid. Further it minimizes energy consumption and lowers treatment temperature as pressure acts as substitute for temperature.

[0051] With the present invention it has proven possible to reduce the cycle time (time from the start of treatment of one batch of wood until treatment of the next batch of wood can start) from about 24 hours to between 8 and 10 hours. This will more than double the capacity of a treatment facility.