EP1248059A1 - Method for drying sawn timber and device for implementing said method - Google Patents

Abstract

involves pressurising wood and injecting water vapour before pressure reduction and dehumidifying. The wood drying method includes the following stages: - a pressure stage in which a set pressure is maintained in a sealed enclosure (1) by injection of a saturated water vapour and maintenance of the pressure for a set period of time, and ensuring forced air circulation with saturated water vapour; - a heating stage at the heart of the wood and in the central zone of the pieces of wood to be dried, by transmission of micro-waves in a frequency range from 400 to 2450 MHz; - an evacuation stage for the water running into the bottom of the enclosure (1), followed by a progressive reduction of the pressure to atmospheric pressure after the micro-wave radiation has stopped. The pressure reduction stage is completed by a dehumidifying of the ambient air in the enclosure, by passage of an air flow from the enclosure onto a humidity absorbing and air cooling system.

Description

The present invention relates to a method of drying wood from sawing or wooden objects.

The invention is particularly useful for the treatment of "wood sawing ". By the term" sawing wood "is meant a wood which comes from directly from the first transformation (sawing).

It is known from patent application WO 82/01766 a method of wood drying, using microwaves with a frequency of 915 MHz applied to the elements of wood to be dried to raise their temperature interior and make them reject the water. The water thus discharged is evaporated at the wood surface by low speed air circulation obtained by fans. The air charged with humidity of the order of 80% passes over condensers to extract this moisture.

In document WO 82/01411, the same principle is used but in the latter, it is further specified that the air temperature must be always lower than the interior temperature of the wood. This document reports the disadvantage of heating the surface of the material before heating the internal part of the material. In this document, we therefore propose to control the energy conversion process electromagnetic in heat energy to concentrate waves on water in the material. In addition, it is suggested to act on the climate inside the room by maintaining a sufficient percentage of air humidity high so that the surface of the product is not dried before extraction moisture in the heart of the wood. To this end, during the initial phase of the water drying process, in atomized form is introduced into the room to maintain high humidity.

Similarly, the article published in the review "Holz als roh und werkstoff" in 1995, pages 333 to 338, Springer-Verlag Edition and entitled "Microwave drying of pine and spruce" by A. L ANTTI teaches to dry wood with microwaves operating in frequencies of 915 or 2450 MHz and a power density in the range of 25 to 78 kW / m ³ to raise the interior temperature of the wood to approximately 140 ° C and achieve an internal vapor pressure 25 KPa wood. The internal pressure thus produced is very high to allow very rapid evacuation of the water. The disadvantage of the process is that it develops fiber breaks. The drying process begins with rapid microwave drying at around 70 ° C, then intermittent microwave exposure during drying, and finally a drying operation by controlling the temperature of the wood below saturation. fibers, limited to a maximum temperature of 110 ° C.

In all cases, it is clear that air is used as vehicle to remove moisture from the wood. For these reasons, the air humidity must remain below the air saturation rate in water vapour. It is therefore necessary in known devices to dehumidify the air in order to dry the wood. In addition, it is necessary to have an air temperature lower than that of wood for allow evaporation. All of these devices have the disadvantage of generate large energy losses and not optimize consumption energy. The higher the wood temperatures, the more the power of microwave generators must be proportional and their high cost taking into account the duration of several hours of the operations of drying, the energy consumed is high. Indeed, we can see in the article cited above that the drying times are between 3 and 5 hours depending on the thickness of the wood and the power of the material. In addition, none known processes happen to drop below 30% humidity in wood after drying.

The object of the present invention is to propose a method which optimizes the energy and reduces the powers of the microwave while very quickly obtaining a complete drying of the wood, from the green state to a final humidity level of around 10%, or even lower depending on the conditions of implementation.

• une étape de mise en pression à une pression déterminée d'une enceinte (1) étanche au moins par injection ou création de vapeur d'eau saturante et de maintien de cette pression pendant un temps déterminé, tout en assurant dans l'enceinte une circulation forcée d'air et de vapeur d'eau saturante ;
• une étape de chauffage au coeur du bois et dans la zone centrale des pièces de bois à sécher par émission de micro-ondes dans des fréquences comprises entre 400 et 2450 MHz ;
• une étape d'évacuation des exsudats liquides sortant du bois et recueillis par ruissellement dans le fond de l'enceinte (1).

This object is achieved by the fact that the wood drying process comprises:

• a step of pressurizing a sealed enclosure (1) at a determined pressure at least by injection or creation of saturated water vapor and maintaining this pressure for a determined time, while ensuring circulation within the enclosure forced air and saturated water vapor;
• a step of heating in the heart of the wood and in the central zone of the pieces of wood to be dried by emission of microwaves in frequencies between 400 and 2450 MHz;
• a step of evacuating the liquid exudates leaving the wood and collected by runoff in the bottom of the enclosure (1).

According to another particularity, the liquid exudates are evacuated in permanently.

According to another particularity, the exudates are evacuated by intermittently.

According to another particular feature, the evacuation step is followed by a step of gradually decreasing the pressure to the pressure atmospheric after microwave shutdown.

According to another particular feature, the evacuation step comprises a physico-chemical treatment step of the exudates to make them compatible with discharge to the wastewater circuit.

According to another particularity, the stage of evacuation of exudates liquids is followed by a recovery step in a container with a view to chemical reprocessing.

According to another particular feature, the step of reducing the pressure is supplemented by a dehumidification step of the ambient air of the enclosure by passage of the air flow coming from the enclosure over an absorption device humidity and cooling of the enclosure air.

According to another particular feature, the microwave emission powers applied are decreasing from the center of the pieces of wood towards outside.

In another feature, the saturated water vapor pressure is between 2 bars and 15 bars.

In another feature, the vapor pressure will be less than 10 bars to obtain a humidity level of the treated wood greater than 6%.

According to another particularity, the vapor pressure will be understood for at least a determined drying time between 10 and 15 bars and the temperature produced will reach a value within a range of 200 to 220 ° C to obtain a naturally polymerized dry wood with a humidity level close to 0%.

In another feature, the power of the microwave generator is calculated so that the internal heat of the wood is greater than the saturated water vapor temperature.

Another object of the invention is to propose a device allowing the implementation of the process.

This object is achieved by the fact that the device consists of a waterproof pressure-resistant enclosure communicating through quartz or any other material suitable for microwaves with a guide wave connected by impedance adapters to a microwave generator, said windows being arranged transversely to the stack of wood, the enclosure communicating with a pressurized air recirculation path sucking air from one side of the wood stack through grates and pushing it out the other side of the wood stack by diffusion grids, a device pressurized steam generator connected to the enclosure.

According to another particularity, the device comprises a circuit steam condenser connected, in parallel on the recirculation circuit air and selectively by valves.

According to another particularity, the device comprises in its part the lower an outlet for gravity drainage of runoff water controlled by a valve.

• une extrémité obturable par une porte automatique assurant les conditions d'étanchéité à la pression et aux micro-ondes ;
• des moyens de transport des charges de bois vert à dessécher désolidarisés électriquement des moyens de transport situés de l'autre côté du sas automatique par rapport à l'enceinte.

According to another particularity, the device comprises:

• an end which can be closed off by an automatic door ensuring the sealing conditions under pressure and in the microwave;
• means of transporting loads of green wood to be dried electrically separated from the means of transport situated on the other side of the automatic airlock with respect to the enclosure.

According to another particular feature, the entire enclosure and the area of preloading are wrapped in a second enclosure of protection against radiation leaks, this enclosure being accessible from the outside by flexible doors.

In another feature, the microwave generator is buried into the ground and communicates with the drying chamber through a guide wave.

According to another particular feature, the enclosure comprises a valve security.

In another feature, the valve is opened intermittently.

According to another particular feature, the valve is permanently open.

According to another particular feature, the orifice is connected to a device for physico-chemical treatment making the exudates compatible with sewage disposal standards.

• à traiter un seul type d'essence de bois vert par application de micro-ondes dans une ambiance de vapeur d'eau saturante dans des conditions de pression et de température définies,
• à récupérer l'exsudat liquide produit par l'opération de traitement de l'essence unique,
• et éventuellement à retraiter cet exsudat par des procédés physico-chimiques pour en extraire différents composants chimiques utilisables dans l'industrie cosmétique, de parfumerie, agro-alimentaire, pharmaceutique, et chimique.

A final object of the invention is to propose a process for extracting chemical components by using the process and the device of the invention consisting of:

• to treat a single type of green wood essence by applying microwaves in an atmosphere of saturated water vapor under defined pressure and temperature conditions,
• recovering the liquid exudate produced by the single essence treatment operation,
• and optionally to reprocess this exudate by physicochemical processes in order to extract different chemical components which can be used in the cosmetic, perfumery, agro-food, pharmaceutical and chemical industries.

According to another particular feature, the processing of a pine essence provides an exudate with insecticidal properties.

• la figure 1A représente une vue en coupe transversale du dispositif selon l'invention ;
• la figure 1B représente une vue de dessus en coupe longitudinale du dispositif selon l'invention ;
• la figure 2 représente une vue de côté du dispositif implanté.

Other features and advantages of the present invention will appear more clearly on reading the description below made with reference to the accompanying drawings in which:

• Figure 1A shows a cross-sectional view of the device according to the invention;
• FIG. 1B represents a top view in longitudinal section of the device according to the invention;
• Figure 2 shows a side view of the implanted device.

As shown in Figure 1A, the device consists of a enclosure (1), preferably cylindrical, of metallic material ensuring on the one hand, both good thermal insulation and watertightness air pressure and secondly, sealing against waves. This enclosure is open at one end by a door (16, Figure 1B) or two. of the openings (14) are made in the enclosure to constitute windows made of airtight material but allowing microwave radiation to pass through. These windows (14) under pressure are made of material allowing to emit the waves towards the interior of the enclosure and say emitting windows. The waves are brought by a waveguide (40) to a plurality of windows arranged longitudinally and on each side at intervals regular or not along the stack of wood (3) to obtain a distribution the the most homogeneous waves possible. The waveguide (40) communicates to through an impedance adapter (41) and a 3-decibel divider (42). with an isolator (43) and the microwave generator (44). Between each window (14) emitting or between the emitting windows of the ends and each enclosure bottom, are preferably arranged a plurality of pipes (12) of forced air circulation by a fan V. These pipes (12) communicate over a height corresponding approximately to that of the stack of wood through grids (13) with the internal area of the enclosure containing the stack of wood (3) transported on a conveyor means such as, for example, a carriage consisting of wheels (32) mounted on a tray support (31). The wood stack is preferably made of pieces (30) in the form of beams or planks or planks of thickness and width any, from sawing and arranged contiguously across their width in a longitudinal direction to form a layer. Each layer of wood is spaced from the bottom layer by battens or rods (33) arranged perpendicularly in a non-contiguous manner to spare between the adjoining layers of pieces of wood in the circulation passages of air, waves, and water. The air circulation circuit is also made in material favoring the reflection of waves towards the interior of the enclosure and the wood. The enclosure is placed in communication by a pipe (15) with a steam generator system (2) and possibly a compressor air (20). Moisture arrives from the steam generator through grids (13) of diffusion, which makes it possible to diffuse it homogeneously in the enclosure without risk of creating frontal attacks on the wood. The air compressor (20) is used to produce compressed air intended to accelerate the water circulation in the wood and, when the steam generator system (2) cannot generate steam under sufficient pressure to rise at the desired temperature or to support the rise in temperature and accelerate the water circulation of the wood. However, when using a pressurized steam generator system sufficient to reach temperatures and pressures desired, the air compressor can be deleted. The wheels of the carriage rest on rails (10A, 10B) which are integral from the bottom of the tank (1) these are provided with a device for eliminating electric arcs. A grid (19) prevents the propagation of waves to liquid exudates or runoff collected in the bottom of tank. This runoff is discharged through a pipe (18) controlled by a valve (17). This pipe (18) opens into a removable or drainable container for recovering liquid exudates resulting from the drying process. Alternatively, the pipeline is open continuously or intermittently. In another variant, the pipeline leads into a physico-chemical treatment device making the exudates compatible with the standards in force for waters waste. Finally, the upper part of the tank has a safety (11) which makes it possible to maintain the tank at the desired pressure, relieve pressure if it is too high and finally put the tank to the atmosphere after the drying process is complete.

In the layout diagram of Figure 2, the tank (1) is enclosed in an enclosure (5), which communicates through the airlock of the door (16) automatically controlled at the end and at the start by a system electronic control. A preloading area (50) makes it possible to bring the carriages on a pair of rails (10C, 10D) which are not related electric with the rails (10A, 10B) of the enclosure (1). A device for spraying (52) allows water to be sprayed during the use phase microwaves to prevent radiation from leaking out. A reserve tank not shown, removable and drainable, is connected to the enclosure (1) by a pipe (18) and makes it possible to collect the exudates liquids resulting from the drying of wood. To ensure reduced leakage the microwave generator (44) is buried like the reserve tank (6) and communicates with the drying chamber (1) through the waveguide (40).

The drying process includes the following operations: introduction by means of transfer of a load of green wood into the enclosure; automatic closing of the enclosure door, this preferably to avoid all errors and shocks due to handling; pressurization of the enclosure and diffusion of saturated vapor in the enclosure until the pressure corresponding to the temperature of desired operation in saturated steam. We can use a pressure of 2 bars for a temperature of 120 ° Celsius of steam saturated and 2.7 bars for a temperature of 130 ° Celsius of steam saturation. We can, if the need arises, rise to temperatures of saturated vapor greater than 180 ° Celsius, 200 ° Celsius, or even 220 ° Celsius by increasing the pressure up to 10 bars or respectively 15 bars. The temperature and pressure increases of the process can be done in successive steps, or according to ramps or again, according to cycles allowing the optimization of the desired result, 0% complete drying, drying to a certain humidity level or production of chemically exploitable liquid exudates. This phase of pre-drying under saturated steam pressure and temperature determined is maintained for the time necessary to pass from one moisture content of green wood at least 65% depending on the species at a so-called "saturation" rate of 30%. During this pre-drying phase, it is possible to trigger the microwaves to speed up the passage of the rate of green wood at the saturation rate. When the desired saturation rate is reached, the residual moisture contained in the wood is prevented from leaving the wood. At this time, the microwave heating phase of the wood takes all its importance. During the implementation of the heating phase by microwave, the microwave power emitted by the central window (14c) can be higher than the power emitted by the windows (14l) located on either side of the central window, is used so as to obtain a temperature differential in wood which corresponds to a differential of vapor pressure in wood. This pressure differential will be oriented by in order to favor the evacuation of water towards the outside of the wood and in the direction fibers when the determined operating temperature has been reached. The power of microwave generators is calculated so to reach a wood temperature higher than that of saturated vapor which can be close to 120 ° Celsius or higher and produce the effect sought in the context of drying from the inside to the outside of the wood.

Given the presence of pressure and air saturated with water, the liquid chemical components and the water evacuated by the wood cannot no case to be evaporated and run off by gravity to be recovered below of the grid (19) by the siphon (18). The siphon (18) is triggered at regular intervals by the control system as soon as the level closer to the grid. The enclosure includes a level detector device allowing the automatic opening of the valve (17). Each cycle water evacuation is followed by a vapor pressure reset cycle saturated water from the enclosure. This last phase brings the wood moisture content of 30% at the desired final rate which can be 20%, 10%, 6% or 0%. To obtain complete drying of the wood with a rate close to 0%, the process will include at least one phase of fixed duration during which the temperature will be kept within a range around 200 to 220 ° C and in a vapor atmosphere pressure saturating greater than 10 bars. By the use of a vapor atmosphere saturated water and higher microwave temperature, but despite all below temperatures generally used in processes so-called "crosslinking" in an atmosphere not saturated with water vapor, we obtain wood drying with a humidity level close to 0% at the same time that a natural polymerization phenomenon giving wood moisture resistance, dimensional stabilization and great ease of machining. This result is obtained in less time than known processes and especially with a conservation of the natural color of the wood. Indeed, the method of the invention does not produce the phenomenon of known browning, resulting from roasting obtained by temperatures between 240 and 300 ° C.

The drying process can also be used in the device of the invention for producing a liquid exudate incorporating molecules chemicals constituting a wood essence, such as pine, eucalyptus, oak, beech, spruce, etc., or a determined mixture of species. This exudate is recovered and possibly reprocessed by physicochemical processes to obtain exploitable chemical components in the cosmetic industry, in the pharmaceutical industry, in the perfumes, in the food industry, chemistry or the insecticide industry. Thus, by treating only pine, the exudate obtained has properties insecticides.

At the end of the time necessary to obtain this humidity level final, when the wood is dried, the circulation of saturated water vapor is stopped, the steam generator circuit (2) is closed if necessary feel. Valves (191, 192) for communication with the condenser (19) are open to allow the enclosure vapor to condense and lower the temperature of the enclosure. After a while, the microwave generator is also stopped and the pressure decreased to gradually get to atmospheric pressure.

By saturating the surrounding environment around the wood and judicious use of microwave power with much lower energy consumption than generally used in the prior art, we manage to speed up the evacuation process the internal humidity of the wood and obtain faster drying with a lower energy consumption. City water can be used in the sprinkler system.

Other modifications within the reach of those skilled in the art make also part of the spirit of the invention. So any transfer device can be used in place of the rail-mounted cart. Likewise, devices control and regulation will initiate the phases successive steps of the process in combination with more or less automation less push. Similarly, the enclosure has a safety valve (11) allowing the enclosure to be vented, either at the end of the process or excess pressure detected by the control system.

Claims (13)

Wood drying process, characterized in that it comprises: a step of pressurizing a sealed enclosure (1) at a given pressure at least by injection or creation of saturated steam and maintaining this pressure for a determined time, while ensuring in the enclosure (1 ) forced air circulation and saturated water vapor pressure conditions; a step of heating the wood by emitting microwaves through several windows (14) so as to create a temperature differential creating a vapor pressure differential in the oriented wood to promote the evacuation of fluids to the outside; a step of modifying the temperature and pressure increases according to cycles optimizing the process as a function of the desired result; a step of stopping the microwave generator (44) and reducing the pressure of the enclosure (1). Method according to claim 1, characterized in that it comprises a step of evacuation of exudates leaving the wood. Method according to claim 2, characterized in that each exudate evacuation cycle is followed by a re-pressure cycle of saturated water vapor pressure from the enclosure (1). Method according to one of claims 1 to 3, characterized in that the step of reducing the pressure is completed by a step of dehumidifying the ambient air of the enclosure (1) by passing the air flow coming from the enclosure (1) on a device for absorbing humidity and cooling the air in the enclosure (1). Method according to one of claims 1 to 4, characterized in that the microwave emission powers applied to the different windows are decreasing from the center of the pieces of wood to the outside. Method according to one of claims 1 to 5, characterized in that the saturated water vapor pressure is between 2 bars and 15 bars. Method according to claim 6, characterized in that the saturated water vapor pressure is between 2 bars and 2.7 bars. Method according to one of claims 1 to 7, characterized in that the power of the microwave generator (44) is calculated and controlled so that the internal heat of the wood is higher than the saturated water vapor temperature. Wood drying device for carrying out the method according to any one of claims 1 to 8, characterized in that it consists of a sealed pressure-resistant enclosure (1) communicating through windows (14) allowing microwave radiation supplied by at least one microwave generator (44) to pass, said windows (14) being arranged transversely to the stack of wood (3), the enclosure (1) comprising a circulation system forced air and a homogeneous vapor diffusion system in the enclosure. Device according to claim 9, characterized in that it comprises a system preventing the propagation of waves towards the liquid exudates collected in a part of the enclosure. Device according to one of claims 9 or 10, characterized in that it includes a condenser circuit (19). Device according to one of claims 9 to 11, characterized in that it includes a system for evacuating liquid exudates leaving the wood. Device according to one of Claims 9 to 12, characterized in that it comprises: an end closable by an automatic door (16) ensuring the conditions of tightness of the enclosure under pressure and microwaves; means for transferring loads of green wood to be dried from the outside of the enclosure to the inside of the enclosure by the closable end.

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