Thermo Treatment Process for Wood

Abstract

Thermo treatment process for wood comprising the following steps: a. Placing the wood batch to be treated in a treatment chamber; b. Exchanging the atmosphere inside the treatment chamber by evacuating the air, replacing the evacuated air by an inert gas atmosphere in gas form, at 8 to 12 bar pressure; c. Heating the inert gas atmosphere up to 165 to 175 C., d. increasing the pressure in the inert gas atmosphere to 14-16 bar; e. maintaining the temperature in step c. and the pressure in step d. for from 90 to 150 minutes; f. cooling the inert gas atmosphere to a temperature of 20 to 35 C. g. retrieving the treated wood batch.

Claims

1. Thermo treatment process for wood comprising the following steps: a. Placing the wood batch to be treated in a treatment chamber; b. Exchanging the atmosphere inside the treatment chamber by evacuating the air, replacing the evacuated air by an inert gas atmosphere in gas form, at 8 to 12 bar pressure; c. Heating the inert gas atmosphere up to 165 to 175 C., d. increasing the pressure in the inert gas atmosphere to 14-16 bar; e. maintaining the temperature in step c. and the pressure in step d. for from 90 to 150 minutes; f. cooling the inert gas atmosphere to a temperature of 20 to 35 C. g. retrieving the treated wood batch.

2. Thermo treatment process for wood according to claim 1, wherein the inert gas is Nitrogen.

3. Thermo treatment process for wood according to claim 1 wherein the process in steps c. and d. takes between 90 to 110 minutes.

4. Thermo treatment process for wood according to claim 1 wherein in step d. or e. a mineral or organic oil may be applied to the batch of wood.

5. Thermo treatment process for wood according to claim 1 wherein an impregnating agent is applied to the batch of wood in step d. or e.

Description

DESCRIPTION OF THE DRAWING

[0037] The invention will now be described with reference to the accompanying drawing in which

[0038] FIG. 1 illustrates how pressure builds up very slowly with steam at temperatures below 140 C.

[0039] FIG. 2 use of an inert gas as compared to steam

[0040] FIG. 3a-3d illustrate readouts from the inventive method at different stages through the method.

DETAILED DESCRIPTION OF THE INVENTION

[0041] The invention as already discussed above has two main goals, firstly to reduce the cycle time, i.e. the time that is necessary in order to thoroughly treat a batch of wood and secondly to improve the quality of such treatment, such that the batch of wood received an improved treatment and with less risk of damaging the wood structure during the treatment process.

[0042] By replacing the traditional water based atmosphere, i.e. steam inside the treatment chamber by an inert gas, it is possible to separate pressure and temperature in the heating and cooling phase. In prior art methods a pressure is created by producing steam by heating up water. This process is time consuming since the increase in steam pressure lacks behind the temperature increase. A requirement in the treatment chamber is that the relative humidity must be kept above 85% RH in order to avoid or minimize damage to the wood. This delay causes a very slow increase in pressure as a function of temperature, particularly at low temperatures. At the same time requiring relative high energy consumption.

[0043] In FIG. 1 is illustrated how pressure builds up very slowly with steam at temperatures below 140 C. From 30 C. to 140/170 C., which is the temperature range where most of the heating and cooling takes place for the inventive method and as such it can be seen that there is a distinctive difference in the inert gas' ability to heat exchange with the wood as compared to steam (at least for the particular temperature range). As the temperature and pressure building is not connected with an inert gas it is possible to heat and cool the gas as fast as the system allows and control the pressure inside the treatment chamber separately.

[0044] The use of an inert gas as compared to steam also increases the heat exchange with the wood such that it heats up faster. This is illustrated in FIG. 2 where it is clear that the rate of energy transfer between steam and wood as compared to nitrogen and wood is distinctively better for nitrogen and as such it is possible to transfer/exchange heat at a much higher rate using nitrogen (or an inert gas) than when using steam.

[0045] As discussed above one of the main drawbacks with prior art methods is the high risk of creating cracks in the treated wood.

[0046] These cracks emerge in any situation where the difference between the partial pressure inside the wood cells and the outside atmosphere is large enough to cause the cracks to develop. In the prior art heat treatment methods, it must be remembered that there is water present inside the wood, typically 10-14%. As the steam atmosphere and the wood is heated up, steam pressure builds up both inside and outside of the wood. Cracks typically develop in the following situations: [0047] In the heating phase, if the relative humidity (RH) of the steam atmosphere outside the wood becomes too low when heating up the atmosphere. In this situation, the partial pressure inside the wood may become larger than that outside the wood. Depending on the size of the relative overpressure inside the wood and other parameters such as wood species, cracks may result. [0048] In the modification phase, when the hydrolysis of the hemicelluloses becomes exothermic. Depending on wood species, thickness of the boards being treated, moisture content and other parameters, temperature in the core of the wood quickly increases, typically 15 to 25 C. above the temperature of the surrounding steam atmosphere. This can lead to significant differences in relative pressure, illustrated in FIG. 1. In FIG. 1, the pressure of steam in a closed system is shown as a function of temperature. Modification in prior art methods typically runs at 180 C., which corresponds to a pressure of 8.5 Bar at 85% RH. At 200 C., the pressure is 13.2 Bar. Since the exotherm develops in the center of the wood, in this case a relative overpressure in the center of the wood of (13.2-8.5) 4.7 Bar develops very quickly. These thermodynamics created by the hemicelluloses exotherm represent a major cause for potential cracks and quality problems in prior art heat treatment methods. [0049] In the cooling phase, if the temperature gradient in the wood becomes too steep. As illustrated in FIG. 1, if the steam atmosphere is cooled too fast, especially in the beginning of the cooling phase when temperature is still high, the relative pressure in the steam will drop quickly relative to the still hot center of the wood. In this case a relative overpressure may build in the wood, leading to cracks. [0050] Beside cracks, the presence of steam has also been reported to create other quality problems such as water stains and discoloring from condensates.

[0051] All of the above mentioned dysfunctional partial pressure thermodynamics of prior art methods are effectively eliminated by the invention, in two ways: [0052] In the initial vacuum and pressure phase, atmospheric air with its content of oxygen is removed from the wood cells and replaced by a condensed Nitrogen atmosphere at 10 Bar. At 10 Bar, the boiling point of water is approximately 180 C., so that the water in the wood is far below its boiling point. At 180 C., the pressure of Nitrogen has increased to approximately 15 Bar, so that the water in the wood is still kept below its boiling point. Thus the water present in the wood is far below its boiling point during the entire process, so that no significant partial steam pressure can build as temperature is increased. [0053] In the hemicelluloses exotherm, Nitrogen will not build significantly higher partial pressure inside the wood, as the temperature in the center increases. FIG. Y below clearly illustrates that while steam pressure increases exponentially in the high temperature range, Nitrogen pressure only increases moderately in a linear manner. An increase in wood core temperature from 180 to 200 C. will lead to an overpressure of (16.1-15.4) 0.7 Bar for Nitrogen, compared to 4.7 Bar for steam.

[0054] In FIG. 3a-3d, illustrating readouts from the inventive method at different stages through the method, it is clear to recognize the effects of the present invention.