FI58212C - TORKNING AV VIRKE - Google Patents

Description

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Sweden-Sweden (SE) 2189/72 (71) Hager Aktiebolag, Heleneborgsgatan 9, 117 31 Stockholm, Sweden-Sweden (SE) (72) Bror Olof Häger, Djursholm, Sweden-Sweden (SE) (7 ^) Berggren Oy Ab (5 * 0 Timber drying method - Torkning av virke

The present invention relates to a method for drying timber very quickly and gently.

Rapid drying requires not only sufficient heat input to evaporate the water, above all, that the wood is heated in all proportions so gently that the wood is not damaged. The issues involved have proved difficult to manage.

In conventional equipment, when air-drying, great care must be taken to avoid damaging the timber. It is especially important that the surface of the timber does not dry out too quickly compared to the interior of the timber, as this will create distortions, cracks and color defects. The temperature of the drying air must therefore be kept low and must also contain a certain amount of moisture. This makes drying slow. Even if these problems could be better managed in air drying - which does not seem likely - the question of heat import remains. The heat input required for rapid drying is difficult to achieve if the drying medium is air. The temperature differences required for this become far too great.

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Technical advantages can be achieved by drying with certain gases other than air. The gases replace part of the atmospheric pressure that affects the water vapor leaving the timber. It is as if a vacuum is formed, as a result of which the boiling point of the water to be evaporated decreases, so that the water leaves more quickly. Generally, gases have been used which can be recovered by condensation after drying.

In this case, drying can in principle be carried out faster and at a lower temperature than when air is used. Fruit. of course, the amount of heat required to evaporate the water in the goods does not decrease, the heat of evaporation being the same in both cases. Here, too, there is a risk of the surface drying out too quickly, with consequent damage to the timber. In addition, gas - like air - has a low heat capacity. It is therefore not an effective heat carrier. Drying with gas is more complicated than with air. As a whole, gas drying does not offer significant advantages.

According to U.S. Patent 3,205,589, plywood is dried to a moisture content of 2-7% at 121-1-9 ° C under normal pressure. Such a treatment would not be suitable for ordinary timber. U.S. Pat. No. 3,571,934 discloses a process for drying wood at 51-135 ° C, preferably under overpressure in the presence of a preservative-containing hydrocarbon, e.g. pentachlorophenol, after which the wood is treated under reduced pressure.

With certain electrical methods, it is possible to immediately heat the interior of the timber. In this way, rapid and fairly even drying can be achieved without causing damage.

However, these methods are difficult to perform for larger timber packages, and in addition, the operating costs of the methods are high.

Drying methods have also been proposed in which the wood is heated in different liquid media, but for various reasons - mainly due to its effect on the quality of the wood - they have not spread widely.

In connection with the pressure impregnation, drying and dyeing of timber according to the Hager process - Swedish patent 301 870 has in recent years proved that the drying of timber with high boiling oil can be carried out surprisingly quickly and gently. In this process, the timber absorbs some drying oil, which cannot be removed. The amount absorbed by the special measures 3 58212 is kept as low as possible. This amount can be reduced to such a level that it does not exceed what is desirable to achieve some of the specific effects desired in the Hager process. In normal drying, it is not desired to include these amounts of oil in the timber. A German method for pressure impregnation and drying of wood is known from German patent 1,492,511. The aim of this method is to make the surface of impregnated wood water-repellent and therefore it is desirable for a certain amount of oil to remain on the surface of the wood.

The method of the present invention has some similarities to the Hager process. Briefly, the me-t method is as follows.

Moist timber - which is to be dried - is taken to an iron cylinder, which is sealed. A petroleum product is also introduced into the cylinder, with which the drying takes place. It boils at about 200 ° C. The heat is introduced so that the petroleum product remains at 80 ° C. In addition, a large vacuum is created so that the water in the timber boils off.

The water vapor leaving the timber and the accompanying evaporated drying medium are led to a condenser for condensation. From there, the condensate is led to a collection vessel. The drying medium is separated from the collection vessel and recovered for reuse.

When the timber has dried to the desired extent, the drying medium is removed from the cylinder. This is done while maintaining a vacuum. The vacuum in the cylinder is also maintained for some time after the drying medium has been removed. The drying medium then removes the drying medium left on the surface of the timber. The timber is now pre-processed.

According to the present invention, there is provided a method of drying wood and other fibrous products by heating under confinement at 50-90 ° C, preferably 80 ° C, under reduced pressure in a water-immiscible high temperature boiling drying medium which is removed after drying while maintaining reduced pressure, and the invention is characterized in that the boiling point of the medium at atmospheric pressure is 150-250 ° C, preferably about 200 ° C, and that the reduced pressure is adapted to exceed the vapor pressure of the drying medium a few times - e.g. 2 or M times - at the drying temperatures in question and that the reduced pressure is maintained for as long as the medium is removed so that substantially all of the medium on and on the products evaporates.

The apparatus is suitably an iron cylinder, the end of which is openable. A heating jacket is fitted to the cylinder so that the cylinder and its contents can be heated. Heating can also be performed in other ways, for example by heating coils inside the cylinder. Water, other liquid or steam, for example, is used as the heat transfer medium. The cylinder must be heat-insulated. It must be possible to remove the drying medium while the cylinder is under reduced pressure, for example with a suitable liquid pump, or by draining it into a lower vacuum tank. In addition, a pipeline for the exhaust gases leads from the cylinder to the condenser, where the gases condense into water and a liquid drying medium. From the condenser, the condensate is led to a tank where the water and the drying medium are separated. The medium is recovered - it does not harm even if some water accompanies it - on the other hand, the separated water must not contain a drying medium. The necessary vacuum pump, which develops a vacuum in the cylinder, the coolers and the separation tank, is suitably connected to the cooler or tank. In addition, suitable measuring and control equipment must be available. The details of this stock are subject to change and do not need to be discussed further.

Once the moist timber has been introduced into the cylinder, it is locked in place so that it cannot rise towards the top of the cylinder during the process. The drying medium is usually charged in such an amount that it foams, i. when it is mixed with steam bubbles escaping during drying, covers the timber, but no more. Thus, the drying medium must not fill the entire cylinder, but forms a free foaming surface above the timber, from which water and the drying medium escape in gaseous form into the conduit leading to the condenser. It is usually not necessary to add drying medium during drying.

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The drying medium used is water-immiscible liquids which boil in a certain range (i.e. have a suitable vapor pressure curve). The boiling point is limited from below by the fact that it must be possible to heat the wood under reduced pressure without boiling the drying medium, or without foaming too vigorously, so that it evaporates quickly, as this would place an unnecessary strain on the process. Drying would be slower, drying medium would have to be added all the time, heat consumption would increase and larger amounts of drying medium would have to be recovered.

Drying can be performed with fairly low boiling drying media. Media boiling between 150 and 160 ° C and even lower can be used. In practice, drying media only become more useful when the boiling point rises to 180 ° C. However, the drying process is easier to perform if the boiling point of the medium is high. The boiling point, on the other hand, must not be too high so that the medium remains in the wood after drying, but must be able to evaporate out of the wood. The upper limit of the boiling point appears to be at or immediately below 250 ° C. From various points of view, the most suitable are media boiling between 200 and 250 ° C, preferably between 200 and 220 ° C, i.e. at or slightly above about 200 ° C.

Petroleum products, for example, can be used as the drying medium. Various organic solvents can also be used, for example containing chlorine. In general, petroleum products are used primarily because they are so cheap. It is appropriate to choose those that are odorless or at least free of an unpleasant odor. This is essential if there is a risk that small amounts of high-boiling parts of the medium may remain in the timber. The flash point should be as high as possible. Chlorinated products from organic solvents have the advantage that they are difficult to burn. They are clearly more expensive than petroleum products, which is why their fullest possible recovery may also be more important. For these substances, a somewhat lower boiling point may be chosen to facilitate their recovery, even if the drying is carried out in a closed system, the environmental hazards which chlorinated products may pose must be taken into account. When petroleum products are used, chlorinated substances can be used as an addition to increase the flash point.

Petroleum products, which are typical, high-aromatic and high-solvent solvents, as well as other more specific solvents, tend to dissolve substances from timber. The media then often turn strongly yellow. After treatment of a few batches of wood, equilibrium (saturation) is usually formed in these cases, so that the medium releases as much wood as i.nkru :: t <· and as the wood soaks. Despite the fact that this is therefore not as alarming as one might think at first glance, these media should not be chosen as a priority. Petroleum products with low aromatics have so far proved to be more suitable and, moreover, do not generally smell disturbing.

Some petroleum products of this species are sensitive to oxidation in air. Upon oxidation, higher boiling products and precipitates may be formed. However, oxidation in air has not been disadvantageous in the process. This is probably due to the fact that the drying is performed under reduced pressure, i.e. the oxidizing air does not come into contact with the desiccant at all.

A petroleum product boiling between 187 and 212 ° C has proven to be very suitable. It was colorless, odorless, and remarkably it had no odor. The flash point was 55 ° C. With this substance, the wood has been dried quickly, retaining its fresh color and obtaining excellent recovery of the medium. The trade name of the product is Shellsol T. Another similar product had a boiling range of 191 to 257 ° C. The final boiling point here looks a bit high and the boiling range wide. The flash point was 66 ° C. The trade name of this substance was Shellsol K.

Other drying media tested - which have not been as suitable for use - have been dichlorobenzene, dichloropentane, tetralin and the like.

Substances that affect the timber can be added to the medium. These include, for example, wax which makes the surface water repellent, bleaching agents which lighten the wood, substances which make the surface hard such as resin and linseed oil, preservatives against rot, bluish or fire, and color can also be added. These additives or their effects remain on the surface of the timber.

In order to reduce the formation of foam during drying, anti-foaming agents, one of which is good octyl alcohol, can be added to the medium, but it does not seem necessary.

The drying temperature mentioned in the above example is 80 ° C. This temperature is probably the most suitable in most cases. Of course, drying can be performed at other temperatures. For sensitive timber, temperatures of 50-60 ° C or even lower can be used. Drying then requires considerably more time. However, it is still quite fast, especially when compared to the very limited possibilities of drying in the traditional way in these cases. It does not seem necessary to use temperatures above 90 ° C.

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The vacuum must be so high that the water boils, but not so high that the drying medium evaporates too quickly. Drying works best if the vacuum is as high as it can be for the drying medium. The higher the boiling point of the medium, the higher the vacuum can and should be. The pressure difference between the vacuum used and the vacuum corresponding to the boiling point of water is quite decisive. This pressure difference must overcome many obstacles. It must not only contribute to the evaporation of water and the removal of steam. It must also affect some distance inside the timber. After all, the vacuum inside the timber does not become as high as outside it in the cylinder. The timber must not be impermeable. 'Even the pressure inside the timber must be sufficient to boil the water and the pressure differences must be large enough to allow the steam to escape. The importance of a large vacuum is further emphasized by the fact that the temperature gradually decreases inside the timber. It follows that the coarser the timber, the greater the vacuum and the higher the temperature desired.

Another detail should be touched on. In the case of timber which is some distance below the surface of the drying medium - in particular that which is at the bottom of the cylinder - the vacuum must overcome the pressure of the drying medium. However, this pressure is quite low due to the presence of considerable amounts of steam bubbles in the drying medium. The bulk density of this liquid-vapor mixture is thus relatively low. Thus, although the drying conditions at the bottom of the cylinder are somewhat worse, the difference is so small that it is usually of no practical significance.

The limits of the most suitable boiling point, drying temperature and vacuum of the drying medium thus depend on the conditions. They may also be affected by other specific factors. In the case of a cheap drying medium, a certain inconvenience associated with over-distillation may be allowed if the medium is easily volatile, or a certain loss of medium remaining in the wood if it is difficult to evaporate. However, this is an exception.

In general, however, it is relatively easy to determine the framework for the most rational method with a particular drying medium. To illustrate this and related questions, as well as the above, the following table may be provided as a guide.

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Water and organic liquids boiling at 120 and 220 ° C

vapor pressures at 30 and 60 ° C Boiling point 80 ° C 60 ° C

100 ° C 355 mm Hg 149 mm Hg (water) 120 200 90 160 50 20 180 20 5 200 10 3 220 5 2

First of all, it may be noted that the purpose of the table is to provide an overview. The values given for organic substances are schematic. After all, the vapor pressure curves of different substances with the same boiling point are, of course, not the same, although the differences are usually small.

The vapor pressure values of Shellsol T are as follows Temperature Pressure 25 ° C 1.4 mm Hg 50 4.2 75 10.6 100 46.0

It is apparent from the above1 that substances boiling at about 120 ° C have relatively high vapor pressures at the drying temperatures shown. The vapor pressure is substantially lower for substances boiling at 160 ° C. The vapor pressure is still considerably lower with substances boiling at 180 ° C, which can be used for drying at 60 ° C, perhaps even at 80 ° C. In the latter case, however, it is better to use higher-boiling media. For substances boiling at 200 ° C, the vapor pressure is low enough for drying at 80 ° C.

At 80 ° C the water vapor pressure is 355 mm. In order for the water to boil, the pressure must therefore be lower than this value. The corresponding value at 60 ° C is 149 mm. Thus, at 80 ° C there is a substantially higher chance than at 60 ° C of creating a sufficient pressure difference by means of a vacuum so that the steam overcomes the internal resistance of the wood, the medium pressure and the pressure losses to the radiator, etc. This means that the drying conditions are decisively better. ° C than 60 ° C. It was mentioned above that the vacuum must be as high as it can be considering the drying medium. How high is the drying pressure tolerated by the drying medium? It is partly a question of how much it is allowed to distill over. It seems rational that the pressure (vacuum) is not kept lower than 2 times the vapor pressure of the medium at the temperature in question. Usually operated at 80-93% vacuum, i.e. 15-150 mn under reduced pressure, at higher vacuum with high boiling media and at lower lower boiling. At 60 ° C a high vacuum is more desirable - because the margins are smaller - than at 8 ° C. When using Shellsol T, a pressure of 20-120 mm has generally been used at a drying temperature of 80 ° C.

In order to be able to maintain good vacuum and obtain good drying, it is necessary to maintain efficient cooling. As cooling deteriorates, losses of heating medium occur through the vacuum pump, and this becomes heavily loaded in the removal of vapors. If the cooling is good, no losses occur and the load on the vacuum pump remains very small (basically zero).

One detail in this connection is that the vacuum may be lower at the beginning of the treatment and then gradually rise to the desired value. In the beginning, large amounts of water on the surface are removed from the timber and strong foaming occurs. This foaming can be reduced by initially using a lower vacuum. Vigorous foaming usually significantly increases the removal of the drying medium.

Instead, there seems to be no reason to keep the temperature low at the beginning. Here is the difference compared to traditional drying. If the temperature is kept high from the beginning, as much heat as possible is brought into the wood, which heat then becomes useful as the vacuum increases. Another thing is that the available heat capacity is usually insufficient to maintain a higher temperature from the beginning. The initial temperature is then lower for this reason. In this case, this lower temperature contributes to the reduction of foaming at the start of the process.

After drying, the drying medium is removed from the cylinder while maintaining a vacuum, as mentioned above. If the vacuum is interrupted while the drying medium is left in the cylinder, external (air) pressure compresses large amounts of drying medium into the vacuumed timber. Once the medium has been removed, the timber may remain in the cylinder, under reduced or, if possible, increased vacuum. In this case, the remaining drying medium evaporates, while the temperature of the wood decreases. When the treatment is carried out in this way, only small amounts of medium remain in the timber. This is located in the surface layer of the timber. During the drying period, a considerable amount of heat has been introduced into the timber. This is sufficient to substantially or completely destroy the drying medium. In this context, it should be noted that the large amount of heat contained in the timber is well insulated. On the one hand, the cylinder is thermally insulated, on the other hand, there is a thermally insulating vacuum. Besides, the wood is poorly thermally conductive (good insulation) which is why the heat in the wood at a certain point is only slowly removed. The large amount of heat contained in the wood can thus be practically entirely utilized to evaporate the drying medium remaining in the wood.

Here are some differences from the situation with ordinary air drying. In the latter, it is important that there is good space between the timber units and that there is strong ventilation covering all the slums. On the contrary, in the present method there is a certain similarity with the conditions of steam heating.

The steam finds itself in a cold place, giving up its heat content by condensing to exactly where heat is needed. In addition, the condensation creates a vacuum, new steam accumulates on the site as long as heat is needed, even if the passageways are narrow. The fact that the local amount of heat evaporates the drying medium indicates that an overpressure with respect to the environment is formed. The steam therefore escapes even if its passageways are narrow.

Other advantages of the process are that the heat of vaporization of the medium in question is low. Usually it is only one-fifth of the heat of vaporization of water. Due to the decrease in evaporation from the timber during this period, the vacuum easily increases gradually, which increases the chances of the medium leaving the timber. Despite the fact that the treatment is therefore quite effective, this period should not be made too short. It is an advantage -mm. in terms of rain, moisture, etc. - if the timber is allowed to cool before it is taken out of the drying cylinder.

The present invention also differs from the aforementioned patents. The Swedish patent 301 870 and the German patent 1 492 511 relate to drying with high boiling oil, a certain amount of oil always remains in the wood and the purpose is to give the wood surface protection and color. In these methods, the oil is removed under reduced pressure so that the oil is not compressed to a large extent into the interior of the timber by external pressure. The vacuum is maintained until the oil is pumped out, but is not maintained thereafter to remove the drying medium as in the method of the present invention.

Various measures can be taken to further improve the relatively good evaporation conditions of the remaining medium in the timber batch. In addition to the vacuum, the temperature can also be raised towards the end of the drying cycle. Furthermore, a limited amount of warm air can be introduced. This momentarily reduces the vacuum to some extent. The risks involved are small at this stage. The amounts of drying medium present are now so small that an external pressure rise cannot take them inside the timber. They are absorbed into the cell walls and are not mechanically mobile. Instead of air, steam can be fed in. A suitable measure in this case is to supply some amount of warm water to the bottom of the cylinder and continue to import heat. This water gradually evaporates and puts the wood into steam treatment. Such steam treatment (roasting) means the steam distillation of a medium in which substances with a relatively high boiling point are easily caused to evaporate. The small amounts of medium that may remain in the wood must be boiling relatively high. They hardly have any disturbing effect.

As little information, it can be reported that one ml of Shellsol T on a watch glass evaporates at 80 ° C in 80 minutes, at 60 ° C in 210 minutes, and at 20 ° C in just under 1 day. The corresponding figures for Shellsol K are 200 minutes, 12 hours and 10 days. The high-boiling parts of the latter product considerably delay complete evaporation. No vacuum was used.

Generally, there is no need to strip the timber. During drying, the timber is surrounded by a medium. The medium has a high heat capacity, it is subjected to a violent agitation movement due to the steam bubbles leaving the wood, which, moreover, have a very large volume due to the high vacuum. In addition, the volume weights of the timber and the drying medium are approximately the same, as a result of which there is no pressure or only very low pressure between the individual timber pieces, but instead the pieces receive a certain movement in the medium.

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At the beginning of the drying process, moist wood is usually heavier than a mixture of drying medium and steam bubbles. During the process, the wood dries and becomes lighter, and at the same time the number of steam bubbles in the drying medium decreases, with the result that the mixture of medium and bubbles becomes heavier. The even closer bulk weights of the timber and the medium-bubble mixture thus meet. They remain equal for a significant portion of the drying cycle.

Possible stripping is usually of more interest for the final evaporation of the drying medium from the timber than for the actual drying. But, as previously mentioned, there are a number of beneficial factors for the final evaporation of the drying medium. In addition, various measures can be taken to achieve ever better results.

However, there is nothing to prevent the use of slats. If so, they can be made thin and placed sparsely so that they require little space.

It is difficult to give exact numbers of the amounts of medium over-distilled during drying. They largely depend on the boiling point of the medium, the vacuum used, and the drying time. In rational use, the medium evaporates approximately as much or somewhat more than water. However, if a medium with a low boiling point or too high a vacuum is used, the amount of medium evaporated increases quite rapidly. The number can easily increase tenfold. Even when the medium is relatively easily recoverable in the separation step, the amount of overdistilled medium must be kept small for reasons previously discussed.

The amount of drying medium remaining in the wood also varies depending on the boiling point of the medium and the measures taken. The remaining amount is, of course, more tied to the surface area of the timber than to its volume. As the amount of heat is mainly bound to the volume of the timber, it thus becomes easier to remove the residues of the drying medium from the coarser timber. The amount of drying medium remaining is generally less than 5 kg per m 2 of wood if drying media boiling at 200 ° C and a drying temperature of about 80 ° C are used. Quantities have been difficult to quantify accurately. However, the results suggest that the drying medium losses do not exceed 7 per kg of timber. The cost of this is insignificant.

Drying of 25 mm thick wood can be carried out by a suitable method - a drying medium boiling at about 200-220 ° C and a drying temperature of 80 ° C - in 4-6 hours. If the thickness of the timber doubles to -50 mm - the drying time is valid for 6-8 hours. This means many ten times faster drying than is achieved in conventional equipment when air is used as the drying medium. The time taken for the drying medium to evaporate is an hour or two, but as stated above, it is advantageous if this time can be made longer. Drying is preferably performed so that the batch is dried during the day and the timber is allowed to remain in the cylinder overnight to release the drying medium and cool.

According to the invention, a liquid drying medium is used at the drying temperature in question. This has significant benefits. When the timber is surrounded by a liquid medium during drying, the risk of various surface damage remains very small.

It may be that the medium partially replaces the water on the surface of the timber during the drying process and thus prevents hardening and various damage. The drying medium can also have a certain - relatively - inhibiting "effect on the removal of water from the surface itself, so that the wood - despite the rapid removal of water - dries more evenly. In other words, the difference in moisture content between drying and drying does not become as large as when treated with air. Another advantage is that the drying medium has a sufficient heat capacity, the bulk density of the liquid medium being about 1000 times that of the gas, at the temperatures in question, and the heat capacity is largely bound to the mass of the medium. question with air.

Undoubtedly, the fastest way to dry the timber is to raise the temperature so much that the water boils away. On the other side there is - a medium protective effect despite the - use the lowest possible temperature to avoid uneven drying which would result in corruption. Both of these aspects are taken into account by performing drying under reduced pressure. According to the invention, such drying with a liquid medium can be carried out with good efficiency without disadvantages due to the fact that special measures are carried out. In this case, e.g. concern that the drying medium during the process does not penetrate into the timber but only touches the surface of the timber, and that the medium is removed from the surface after the actual drying has been completed.

Claims (8)

5821 2 li » A method for drying wood and other fibrous products by heating in a closed space at 50-90 ° C, preferably 80 ° C, under reduced pressure in a water-immiscible high-temperature boiling drying medium which, after drying, is removed while maintaining reduced pressure, characterized in that the boiling point of the medium at a pressure of 150-250 ° C, preferably about 200 ° C, and that the reduced pressure is adapted to exceed the vapor pressure of the drying medium a few times - e.g. 2 or H times - at the drying temperatures in question and that the reduced pressure is maintained for as long as the medium is removed that essentially all the medium on and on the surface of the products evaporates. Process according to Claim 1, characterized in that the drying is carried out under a vacuum of at least 80%. Process according to Claim 1, characterized in that the vacuum is limited at the beginning of the drying cycle in order to reduce the amount of overdistilled medium. Method according to Claim 1, characterized in that the temperature is raised towards the end of the drying cycle. Method according to Claim 1, characterized in that the vacuum is increased during or after the removal of the medium from the products. Process according to Claim 1, characterized in that wax, bleach, resin, linseed oil, color pigments or anti-rot, anti-blueing or flame retardants are added to the drying medium. 1. Purification for the treatment of three or more fiber products with a temperature of 50 ° C to 90 ° C , kännetecknat av, att mediet har kokpont vid atmosfärstryck mellan 150 och 250 ° C »företrädesvis omkring 200 ° C> och att det reducerade trycket är ä avpassat att detververiger The temperature and the reduction of the tricycle are then reduced to a medium temperature of the medium to the product.