WO2000058676A1 - Procede de sechage de bois - Google Patents

Procede de sechage de bois Download PDF

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Publication number
WO2000058676A1
WO2000058676A1 PCT/SE2000/000636 SE0000636W WO0058676A1 WO 2000058676 A1 WO2000058676 A1 WO 2000058676A1 SE 0000636 W SE0000636 W SE 0000636W WO 0058676 A1 WO0058676 A1 WO 0058676A1
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WO
WIPO (PCT)
Prior art keywords
wood
drying
temperature
cooling
phase
Prior art date
Application number
PCT/SE2000/000636
Other languages
English (en)
Inventor
Björn Källander
Original Assignee
Assidomän AB
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Assidomän AB filed Critical Assidomän AB
Priority to AU41611/00A priority Critical patent/AU4161100A/en
Publication of WO2000058676A1 publication Critical patent/WO2000058676A1/fr

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B21/00—Arrangements or duct systems, e.g. in combination with pallet boxes, for supplying and controlling air or gases for drying solid materials or objects
    • F26B21/06—Controlling, e.g. regulating, parameters of gas supply
    • F26B21/10—Temperature; Pressure
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B2210/00—Drying processes and machines for solid objects characterised by the specific requirements of the drying good
    • F26B2210/16—Wood, e.g. lumber, timber

Definitions

  • the present invention relates to a method for drying of wood, comprising the steps of drying the wood with hot air at a predetermined drying temperature in a drying chamber to a desired average moisture ratio, and subse- quently cooling the wood to a temperature below the drying temperature. After this cooling phase, the temperature can again be raised and lowered once more.
  • the temperature in drying plants has been gradually raised in the 20th century.
  • the temperature in drying plants is often about 80 °C or higher.
  • high-temperature drying in contrast to conventional drying which is now often referred to as low temperature drying.
  • low temperature drying the surface layer of the wood is dried by means of a hot current of air and the moisture is conveyed from the interior of the wood to the surface essentially by capillary forces at the beginning of the drying and after that mainly by diffu- sion.
  • Low temperature drying is relatively slow, and it is difficult to produce an even moisture ratio profile in the wood, which causes deformations and cracking.
  • High temperature drying is significantly quicker among other things because the high temperature makes the water evaporate inside the wood and leave the wood in the form of water vapour, under the action of the vapour pressure. At high temperatures, the wood becomes soft and, consequently, the drying stress decreases during drying. This results in reduced deformations as well as reduced cracking. In several aspects, high temperature drying is therefore preferable to low temperature drying. In both low temperature and high temperature drying it is often advantageous to let the wood cool down after termination of the drying phase. This is applicable, for example, if the wood is to be treated further in a so-called conditioning step to reduce deformations, cracking and stress. It is also applicable, for example, if the wood is to be additionally treated, such as by planing or sawing, or be used immediately after drying.
  • Cooling of wood has previously occurred by using outdoor air, simply by opening the doors of the drying plant, opening dampers in the walls and top of the drying chamber, or alternatively discharging the wood from the drying plant. Such cooling results in the surfaces of the wood being subjected to extreme drying since the air is cold and dry and, thus, cracks arise in the surfaces of the wood.
  • FR 2,685,247 discloses a device and a method for burning of charcoal.
  • the raw material is dried initially, which is necessary for the wood to be treated at the high temperatures as required. Otherwise, boiling/evaporation of the water would prevent the temperature from rising sufficiently, and at atmospheric pressure the temperature of the material would be close to 100 °C owing to boiling.
  • the charcoal is ready, it is cooled with water with a view to lowering the temperature below the ignition point so as to prevent ignition of the charcoal when discharging it from the furnace.
  • FR 2,720,969 discloses a method and a device for pyrolysis/hydrolysis of wood, i.e. high temperature treatment by partial carbonisation of at least the wood surfaces in order to provide improved moisture resistance and rot resistance. Also this patent specification thus does not relate in the first place to the drying of wood, but in the same way as in the former patent specifica- tion, the wood is initially dried to enable treatment at the high temperatures as required. In the drying phase the wood is repeatedly sprayed with water to cool the wood surfaces. This is done to accelerate the drying process by using the reverse temperature gradient which thus arises in the wood, i.e. that the interior of the wood is warmer than the surface, which thus accelerates the drying owing to the fact that moisture strives to pass from hot to colder areas.
  • drying therefore occurs in alternating heating phases during which thermal energy is conducted into the wood, and cooling phases during which the moisture in the wood is driven towards the surface.
  • the process is graphically illustrated in Fig. 3, and the described drying procedure occurs essentially within the marked areas a and bl along the x axis.
  • the spraying with water thus occurs repeatedly during the actual drying procedure and, as is evident form the curve in Fig. 3, no considerable cooling of the wood occurs except temporarily, but the average temperature of the wood increases practically continuously up to a tempe- rature between 180 °C and 280 °C.
  • Such a method for drying of wood should, if anything, be expensive since the consumption of energy must increase drastically.
  • the wood reaches the so-called ignition point and the pyrolysis/heat treatment of the wood occurs in the period which is marked b2.
  • the wood must be cooled before being dis- charged from the furnace so as to prevent ignition.
  • this second cooling phase which in the diagram is designated c, the wood is once more sprayed with water. As illustrated in the diagram, the cooling is, however, terminated at about 110 °C. The temperature of the wood has then been lowered below the critical ignition point and the wood can be discharged from the furnace. The object of this second cooling with water is to ensure that the wood can be discharged from the furnace with no risk of ignition.
  • the present invention aims at obviating the above- mentioned problems and drawbacks in drying of wood. More specifically, the invention aims at a method, which shortens the total time of the drying process, which reduces deformations and cracks in the wood, and which reduces inherent stress in the wood. At least these objects are achieved by a method according to claim 1.
  • the invention is thus based on the knowledge that the wood can be cooled quickly, efficiently and easily, at a reduced risk of deformations, cracks and remaining stress in the wood, by cooling the wood in a controlled climate by injecting water into the drying chamber and, thus, cooling both the atmosphere of the drying plant and the surface of the wood. Thus the cooling occurs on the one hand by water absorbing heat from the environment and, on the other hand, by water being evaporated.
  • the method gives maximum advantages in high temperature drying but also gives great advantages in low temperature drying.
  • the entire drying process can be shortened by about 6 hours or about 25% in high temperature drying.
  • the water will have a maximum cooling effect and be capable of penetrating everywhere in, for example, a pile of wood.
  • the water nozzles are capable of injecting at least 5 kg of water/h and m 3 of wood.
  • the wood is cooled with a quantity of water of at least 4 1/m 3 of wood.
  • a drying process which comprises five more or less delimited phases, viz. heating phase, drying phase, first cooling phase, conditioning phase and a second, terminat- ing cooling phase. It is preferred for both the first and the second cooling phase to be carried out according to the present invention.
  • the invention is not limited to precisely this.
  • the invention could comprise only three phases, viz. heating phase, drying phase and cooling phase, after which the wood could be used or treated further, e.g. be planed.
  • the inventive cooling process gives a "reverse" conditioning of the wood with levelling of stress and levelling of moisture ratio while lowering the temperature, so that the wood can be used immediately after drying and subsequent cooling.
  • a conditioning step be introduced after the first cooling phase, in which conditioning step the temperature is again raised under controlled moisture conditions, and that the wood then be cooled once more in a second cooling phase.
  • the wood temperature is increased in the conditioning phase to a temperature exceeding 75 °C during simultaneous remoistening .
  • the inventive method makes it possible to control the cooling speed and the relative air moisture in the cooling phase.
  • the drying of the wood surface decreases while at the same time compressive and tensile stress can be balanced against each other, which prevents the wood from cracking while at the same time temperature distribution and moisture ratio profile in the wood can be affected.
  • cold water which gives a maximum cooling effect.
  • heated water for controlling the ratio of cooling speed to air moisture, it may however in certain cases be advantageous to use heated water.
  • the average temperature of the wood is lowered to a temperature between 40 °C and 75 °C.
  • the inventive cooling process is carried out in the same chamber as the drying.
  • the drying chamber be equipped with amply dimensioned ventilation holes designed in such manner that they protect the drying chamber against negative pressure. Otherwise there is a great risk that the drying chamber is deformed since at the end of the drying phase it contains practically pure vapour at a high temperature. As water is injected, a large amount of the vapour will be rapidly condensed and then the pressure in the drying chamber drops quickly.
  • the free flow area of the air valves amounts to at least 0.04 m 2 /m 3 of wood.
  • the invention is applied to a so-called batched drier where a pile of wood is introduced in a drying chamber and removed after drying, cooling and optionally conditioning.
  • the invention could, however, also be used in so-called progressive dryers where piles of wood are introduced at one end of the dryer and undergo all phases of the drying process while being slowly moved through the dryer so as to be discharged at the other end of the dryer after termina- tion of the drying.
  • the different drying phases can then be delimited by different sections in the dryer which are separated by movable partitions or the like.
  • Such a solution would be feasible by the drying time in high temperature drying in combination with a cooling process according to the present invention being so short that the "running belt” would have a reasonable length.
  • the drying process comprises the following five phases: 1. heating phase 2. drying phase
  • the wood is heated by supplying heat from the heating batteries or the heating batteries in combination with injection of hot water vapour into the dryer at the same time as fans in the drying chamber circulate the mixture of vapour and air. Dampers in the walls and top of the drying chamber are closed to prevent moisture from leaving the drying chamber. Water is supplied to the atmosphere in the drying chamber, if needed, to control the relative humidity. It is important to prevent drying of the wood surfaces in the heating phase so as to prevent cracking. Since the wood is cold, it is relatively brittle.
  • the drying phase the vapour is turned off and the air in the dryer is heated essentially by means of heating batteries. If needed, also superheated vapour can be supplied.
  • the dryer is operated accord- ing to a time-controlled schedule with preselected values of temperature and relative humidity. At the beginning of the drying phase the humidity is high to prevent too rapid drying of the wood surfaces before the interior of the wood begins to dry, so as to become dryer and dryer later on in the drying, to allow expelling of moisture from the dryer wood.
  • the temperature is about 100-120°C, and the relative humidity at the end of the drying phase is about 30%.
  • the first cooling phase is carried out .
  • the temperature of the wood is then lowered in a time-controlled manner and regulated with predetermined values of temperature and relative humidity by injecting water into the dryer in the form of droplets.
  • both the heating batteries and the dampers can be used according to the same principle as in the drying phase, and the cooling water pumps are switched off if the cooling proceeds too quickly.
  • Temperature, flow, pressure and size of droplets of the water can, if required, be regulated to control the effect on the climate in the drying chamber. The important thing is that the surface of the wood is cooled according to the predetermined schedule, i.e. the surface of the wood is to be kept moist and the relative humidity is to be kept at the correct level.
  • the wood surfaces are cold and moist .
  • the heat from the interior of the wood results, however, in the surfaces partly drying after the cooling phase.
  • the interior of the wood should be cooled to a level considerably below the boiling point and preferably about 70 °C. Moisture from the interior of the wood has been driven in the form of vapour closer to the surfaces but has to a great extent stopped a distance therefrom, where it has condensed.
  • the temperature in the dryer is usually about 60 °C and the relative humidity about 95 °C.
  • the wood undergoes a conditioning phase .
  • the surfaces of the wood are now cooled to about 60 °C and the wood can be remoistened and heated again.
  • Vapour is injected into the dryer and the fans are operated to distribute the vapour in the pile of wood.
  • the vapour condenses on the surfaces of the wood and moistens and heats them simultaneously.
  • the wood is dried to an average moisture ratio of about 4 percentage units below the desired moisture ratio so as to be once more moistened in the conditioning phase. Wood that is to be delivered with 16% moisture ratio is thus dried to 12% so as then to be remoistened to 16% in the conditioning phase. This is done to reduce the distribution of moisture ratio between different pieces of wood in the pile.
  • the temperature is raised during the conditioning from 60 °C to about 100°C.
  • the wood is subjected to a second, final cooling phase.
  • the surface temperature of the wood is now again about 100 °C and its outermost surfaces have been moistened to practically fibre saturation, i.e. 25-30%.
  • the final cooling phase resembles the first cooling phase by water being injected as droplets and the wood being cooled in a time-controlled manner according to a predetermined schedule for temperature and relative humi- dity. Heating batteries and dampers can be used for this control of climate in the same way as in the first cooling phase.
  • the temperature will be lowered at a controlled speed to a sufficient extent for the water to be able to condense on the wood surfaces.
  • the climate is controlled to keep the humidity in the dryer at a maximum theoretical level during cooling.
  • the wood After the cooling phase, the wood will be able to efficiently absorb moisture from the vapour or the water that is injected to condition the wood while at the same time the wood has a sufficiently low temperature to pro- vide space for a sufficient increase in temperature in the conditioning phase.
  • a double conditioning effect is achieved; first through the great decrease in temperature of the wood surfaces, which results in relaxation as well as a quick decrease in the moisture gradient by the great temperature gradient driving moisture from the interior of the wood to the surfaces, and then by a conventional conditioning by an increase in temperature together with moistening of the surfaces.
  • the effect will be that the moisture ratio gradient in the wood as well as residual stress are minimised.

Landscapes

  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Drying Of Solid Materials (AREA)
  • Chemical And Physical Treatments For Wood And The Like (AREA)

Abstract

L'invention concerne un procédé de séchage de bois. Ce procédé consiste à sécher le bois par de l'air chaud à une température de séchage prédéterminée dans une chambre de séchage afin d'obtenir une teneur moyenne en eau souhaitée puis ensuite à refroidir le bois en l'amenant à une température inférieure à la température de séchage. Selon le procédé, le refroidissement est effectué par pulvérisation d'eau sur le bois.
PCT/SE2000/000636 1999-03-31 2000-03-31 Procede de sechage de bois WO2000058676A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU41611/00A AU4161100A (en) 1999-03-31 2000-03-31 Method for drying of wood

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE9901175A SE519702C2 (sv) 1999-03-31 1999-03-31 Förfarande för torkning av virke
SE9901175-1 1999-03-31

Publications (1)

Publication Number Publication Date
WO2000058676A1 true WO2000058676A1 (fr) 2000-10-05

Family

ID=20415074

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/SE2000/000636 WO2000058676A1 (fr) 1999-03-31 2000-03-31 Procede de sechage de bois

Country Status (3)

Country Link
AU (1) AU4161100A (fr)
SE (1) SE519702C2 (fr)
WO (1) WO2000058676A1 (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2856330A1 (fr) * 2003-06-17 2004-12-24 Perrachon Et Fils Procede de vieillissement du chene
FR2861009A1 (fr) * 2003-10-15 2005-04-22 Alain Dulac Procede et dispositif de sechage a haute temperature de materiau ligno-cellulosique
EP1759822A1 (fr) * 2005-09-02 2007-03-07 Stefano Grosso Procédé pour vieillir artificiellement des materiaux en bois ou similaires
DK201500134A1 (en) * 2015-03-05 2016-05-23 Danish Wood Technology As Method for Treatment of Wood

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2685247A1 (fr) * 1991-12-18 1993-06-25 Hartmann Entreprise Dispositif et procede de torrefaction de pieces de bois de grandes dimensions.
FR2720969A1 (fr) * 1994-06-14 1995-12-15 Herve Montornes Procédé de traitement haute température d'un matériau ligno-cellulosique.

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2685247A1 (fr) * 1991-12-18 1993-06-25 Hartmann Entreprise Dispositif et procede de torrefaction de pieces de bois de grandes dimensions.
FR2720969A1 (fr) * 1994-06-14 1995-12-15 Herve Montornes Procédé de traitement haute température d'un matériau ligno-cellulosique.

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2856330A1 (fr) * 2003-06-17 2004-12-24 Perrachon Et Fils Procede de vieillissement du chene
FR2861009A1 (fr) * 2003-10-15 2005-04-22 Alain Dulac Procede et dispositif de sechage a haute temperature de materiau ligno-cellulosique
EP1759822A1 (fr) * 2005-09-02 2007-03-07 Stefano Grosso Procédé pour vieillir artificiellement des materiaux en bois ou similaires
DK201500134A1 (en) * 2015-03-05 2016-05-23 Danish Wood Technology As Method for Treatment of Wood
DK178526B1 (en) * 2015-03-05 2016-05-23 Danish Wood Technology As Method for Treatment of Wood
US10596719B2 (en) 2015-03-05 2020-03-24 Danish Wood Technology A/S Treatment of wood

Also Published As

Publication number Publication date
AU4161100A (en) 2000-10-16
SE9901175L (sv) 2000-10-01
SE9901175D0 (sv) 1999-03-31
SE519702C2 (sv) 2003-04-01

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