Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B5/00—Drying solid materials or objects by processes not involving the application of heat
  • F26B5/005—Drying solid materials or objects by processes not involving the application of heat by dipping them into or mixing them with a chemical liquid, e.g. organic; chemical, e.g. organic, dewatering aids
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B5/00—Drying solid materials or objects by processes not involving the application of heat
  • F26B5/04—Drying solid materials or objects by processes not involving the application of heat by evaporation or sublimation of moisture under reduced pressure, e.g. in a vacuum
• • 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 wood may, after removal of the bath of medium from the container, be held for some time, under vacuum, in the container.
• a fast drying of wood has two seemingly conflicting criteria: (a) there is needed a good supply of heat for the evaporation of the water, but above all, (b) it is essential that the heating of the wood in different respects be effected so mildly that no damage occurs to the wood. These criteria have proved to be difficult to meet.
• the drying fan principally be fulfilled faster and at a lower temperature than when air is used.
• the amount of needed heat in order to evaporate the water in the wood is of course not reduced, the heat of vaporization in both cases being the same. Even here there is a risk of a too fast drying of the surface of the wood with consequent damages thereto.
• the gas just as in the case of air has a low heating capacity. It is therefore not an effective heat carrier. Drying with gas is more complicated than with air, and, as a whole, does not offer more evident advantages.
• the wood drying method according to the present invention has some resemblance to the Hager-process. Briefly described it is carried out in the following way.
• the moist wood to be dried is introduced into a metal cylinder adapted to be closed.
• a petroleum product drying medium
• Heat is supplied so that the petroleum product is kept at 80 C.
• a high vacuum is applied so that the water in the wood boils away.
• the water-steam that leaves the wood and the accompanying evaporated drying medium are led to a coolercondenser.
• the condensate so obtained is then led to a collecting vessel. From this latter the drying medium is separated and taken care of for re-use.
• the drying medium is carried away from the cylinder. This is done under a maintained vacuum (reduced pressure). The vacuum in the cylinder is maintained even some time after the moment the drying medium has been caried away from it. Drying medium remaining in the wood surface is evaporated from the hot wood during this period. The wood is then ready for further treatment or use.
• a suitable apparatus comprises an iron cylinder with an end cover which is possible to open and close.
• a heating mantle so placed that the cylinder and its contents can be heated.
• the heating can also be effected for example via heating tubes within the cylinder, etc.
• heat transfer there can for instance be used water, other liquid such as oil, or steam.
• the cylinder ought to be thermally insulated.
• a conduit communicates between the cylinder and a container for drying medium, through which conduit the medium can be transported to or from the cylinder. It must be possible to carry away the drying medium when the cylinder is under vacuum, for example, via a suitable liquid pump or by gravity flow to an underlying evacuated container.
• drying medium is taken care of. It is of small importance if a part of the water accompanies the medium on the contrary, separated water ought not to contain drying medium.
• the necessary vacuum pump which evacuates the drying cylinder, the cooler and the separating container is suitably connected with the cooler or the container. Further, suitable measuring and controlling devices shall be present. Details in this apparatus can be such an amount that it is when in foaming condition (i.e., when during the drying period it is mixed with evaporating steam bubbles) covers the wood but no more.
• the drying medium should thus not completely fill the cylinder but the medium forms a free foaming surface over the wood from which surface water and drying medium leave in the form of vapors into the conduit to the cooler-condenser. Successive fillings of drying medium during the drying period are as a rule necessary.
• drying media there may be used liquids which are not miscible with water and which boil within a certain interval (have a suitable steam pressure curve).
• the boiling point down is limited by the fact that it shall be possible to heat the wood under vacuum without causing the drying medium to boil or foam too vigorously, resulting in a fast evaporation. Undue vigor would cause an unnecessary charge to the process; drying would be delayed; permanent fillings of the drying medium would be necessary; the consumption of heat would increase; and there would be larger amount of drying medium to recover.
• Thedrying medium may for example be composed of petroleum products. Even different organic solvents can be used,- for example, such as contain chlorine. As a rule, the cheaper petroleum products are to be preferred for reason of economy. Further, such products ought to be selected which are odorless or in any case free from disagreeable smell. This is specially important if risks exist that small quantities of the high boiling parts of the media may be retained in the wood.
• the flash point of themedium should be as high as possible.
• chlorinated products have the advantage of being rather resistant to fire. They are definitely more expensive than the simple petroleum products. Therefore, it is also rather important to recover them as completely as possible. A little lower boiling point of the substances can be chosen in order to facilitate their recovery. Even if the drying is carried out in a closed container the health hazard possibly caused by chlorinated products should be considered.
• Petroleum products having high aromatic contents, and other more specific solvents will as a rule leach out incrustations (resins) from the wood.
• the media will, in these cases, often be strongly yellow colored.
• an equilibrium (saturation) is as a rule formed so that the medium leaves on the wood as much wood incrustation as is leached out from the wood. In spite of the fact that this leaching may not be as serious as it may seem, these media ought not to be chosen in the first place.
• Petroleum products with a low content of aromatic compounds have up to now been most suitable; besides, they have (as a rule) no bad smell.
• Some petroleum products of this kind are sensitive to oxidation by air. By oxidation more high boiling products and precipitates can be formed. Any trouble with oxidation by air has, however, not been encountered by present drying process. Probably this is explainable by the fact that the drying is carried out under vacuum, oxidizing air does not on the whole come in touch with the drying medium.
• a petroleum product boiling between 187 and 212 C. has proved to be very suitable. It was uncolored, free from aromatic compounds and remarkably enough it had no odor. The flash point was 55 C. With this product it hasproved possible to dry wood fast and with maintained fresh color and with excellent recovery of the medium.
• the trade name of the product is Shellsol T.. Another similar product boils at between 19 1 and 257 C. The end boiling point of this distillate seems a little too high, and the boiling interval a little too wide.
• the flash point was 66C.
• lts trade name is Shellsol K3.
• Other tested drying media which, however, have not been equally suitable to use have included dichlorobenzene; dichloropentane; tetralin; and many more.
• other substances can be added for giving the wood different properties.
• wax which gives a water-repellent surface
• bleaching substances which give a lighter color to the wood
• substances which give a hard surface such as resins and linseed oil
• substances for protection against decay, blue stain or fire also, color may be added.
• antifoaming agents In order to reduce foaming of the medium during the drying period one can but it seems not necessary add antifoaming agents to the medium; a good antifoaming agent is octylalcohol.
• the drying temperature has in the example above been stated to be C. This temperature seems in most cases to be the most suitable. Of course the drying can be efiected at other temperatures. Thus, in the case of an easily damaged wood a drying temperature of 50 or 60 C. or even lower temperature can be used. The drying will then be much more time consuming. It is still, however, very fast in contrast to the very limited possibilities that in these cases prevail for drying in the conventional way. It seems un-necessary to use temperatures over C.
• the vacuum ought to be sufficiently high that the water boils energetically, but not so high that the drying medium evaporates too fast.
• the drying proceeds best if the vacuum is as high as it can be with respect to the particular drying medium in use.
• the pressure difference between applied vacuum and the vacuum corresponding to the boiling point of the water is a very important factor. This pressure difference has more resistances to overcome. It has not only to assist in the evaporation of the water and the removal of the steam:
• the pressure of the water-steam is 355 mm.
• the pressure has to be below the pressure value stated, a value which at 60 C. sinks to 149 mm.
• the steam needs these differences in order to overcome the inner resistance of the wood, the pressure of the medium, and the loss of pressure on the way to the cooler, etc. This means that the drying conditions are definitely much better at 80 C. than at 60 C.
• the vacuum has to be as high as it can be with respect to the selected drying medium. How high a vacuum the drying medium resists is partly a question about how much medium that is allowed to distil over. It seems to be preferable that the pressure (the vacuum) is not kept lower than 2 to 4 times that of the steam pressure of the medium at drying temperature. Usually one employs an 80 to 98 per cent vacuum, i.e. under a pressure between 15 and 150 mm; a higher vacuum is selected for higher boiling media, and a lower for lower boiling media. At 60 C. it is more desirable with a high vacuum since the margins are less than at 80 C. When working with Shellsol T at a drying temperature of 80 C. a pressure between 20 and mm Hg has been used.
• This foaming can be reduced by an initial lower vacuum.
• An intense foaming usually increases the evaporation of the drying medium very much.
• the drying medium After the drying the drying the drying medium is carried away from the cylinder with maintained vacuum as above stated. if the vacuum is broken while the drying medium is left in the cylinder the outer air pressure will press in large quantities of the drying medium into the evacuated wood.
• the wood After the removal of the medium the wood is allowed to remain in the cylinder with maintained or if possible increased vacuum. During this period the drying medium left in the wood evaporates at the same time as the temperature of the wood decreases. In the way the drying has been carried out there is only a small quantity of the medium left in the wood, the same being present in the surface layers of the wood. During the drying period the wood has received a considerable amount of heat. This is sufficient for the purpose of evaporating the drying medium mainly or completely. It is here to be observed that the large heat amount in the wood is well insulated. The cylinder is heat insulated and furthermore there exists a terminal insulating vacuum. in addition the wood has low thermal conductivity (good insulation) whereby heat in one place in the wood is carried away only slowly. The large amount of heat in the wood can thus practically completely be utilized for evaporation of the drying medium left in the wood.
• the medium has low vaporization heat: as a rule, it is only one fifth of the same heat of the water.
• steam can be introduced.
• a suitable measure in such a case is to introduce a quantity of hot water in the bottom of the cylinder, the heat supply being maintained.
• the water evaporates successively and exposes the wood to steam treatment.
• Such a steam treatment means a water-steam distillation of the medium. Treatment of this kind easily evaporates substances with comparatively high boiling points. The small quantitiesof the medium that eventually remain in the wood are comparatively high boiling.
• the moist wood is as a rule heavier than the mixture of drying medium and steam bubbles.
• the wood dries and becomes lighter but the amount of steam bubbles in the drying medium decreases,- with the consequence that the mixture of medium-bubbles becomes heavier and heavier.
• the rather equal weights by unit of volume for wood and mixture of mediumbubbles thus meetleach other and are identical during a considerable part of the drying period.
• crossers i.e., cross pieces
• crossers i.e., cross pieces
• different actions could be made in order to obtain better and better results. No obstacle is at hand, however, to the use ofcrossers. If so they can be made thin and be used sparesely; they do not demand much space.
• the amount is, to a high degree, dependent of the boiling point of the medium, the vacuum used and the drying time. By rational working only a little medium is distilled as the water evaporates. If, however, a medium with a low boiling. point is used, or if too high a vacuum is applied, the amount of evaporated medium increases very fast. The amount can be tenfold. Even if the medium is relatively easy to recover by the separation the amount of overdistilled medium ought to be kept low for reasons given above.
• the amount of drying medium remaining in the wood also varies depending on the boiling point of the medium and measures taken. Remaining amount is of course more connected with the surface of the wood than with the volume of the wood. when the amount a of heat mainly is connected with the volume of the wood it is thus easier to remove the remainder of the drying medium from wood of heavy dimensions.
• Re maining amount of drying medium is as a rule less than 5 kilograms per cubic meter if drying media boiling at about 200C and a drying temperature around C are used. The amounts have been difficult to estimate with higher accuracy. It seems that the losses of drying medium need not exceed 2 kilograms per cubic meter of wood. The cost for this is very small.
• the drying of wood 25 millimeters in thickness can when using rational process drying medium boiling around 200C. to 220C. and a drying temperature of 80C be fulfilled in 4 to 6 hours. If the thickness of the wood is doubled 50 mm the drying time is incrased to 6 to 8 hours. This means 25 to 50 times faster than drying in conventional apparatus with air as drying medium.
• the duration for the evaporation of the drying medium may be stated as being one or a couple of hours but (as said above) it is an advantage if it can be prolonged.
• the actual drying advantageously is carried out in daytime and the wood then is allowed to stay in the cylinder during night until next day in order to give off vaporized drying medium and to become cool.
• a drying medium is used in the form of a liquid at the drying temperature.
• drying medium has sufficient heat capacity.
• the weight per unit of volume of a liquid medium is around 1,000 times higher than that of a gas at the temperatures here in question.
• the heat capacity is to a high heat-extent connected with the mass of the medium. With this high ing capacity it is possible to work easily even in vacuum, which is impossible when working with air.
• Process for drying products of wood and other fibrous products in the piece which comprises heating green wood at a temperature between 50 and C. in a closed container under reduced pressure in a water-immiscible drying medium having a boiling point of between and 250 C., and after the drying has been effected, removing the used medium from the closed container under reduced pressure.

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Citations (4)

• 1972
  • 1972-02-22 SE SE7202189A patent/SE393676B/sv unknown
• 1973
  • 1973-02-15 FI FI459/73A patent/FI58212C/fi active
  • 1973-02-20 DE DE2308314A patent/DE2308314C3/de not_active Expired
  • 1973-02-21 AT AT152073A patent/AT344616B/de not_active IP Right Cessation
  • 1973-02-21 CA CA164,302A patent/CA992307A/en not_active Expired
  • 1973-02-21 NO NO715/73A patent/NO136057C/no unknown
  • 1973-02-21 GB GB852173A patent/GB1402191A/en not_active Expired
  • 1973-02-22 CH CH257273A patent/CH558923A/xx not_active IP Right Cessation
  • 1973-02-22 US US00334938A patent/US3811200A/en not_active Expired - Lifetime

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