US2435219A - Art of dehydrating and impregnating wood - Google Patents

Description

HOURS URYING TIME TO OBTAIN 30% RESIDUAL MOISTURE IN WOOD Feb. 3, 1948.

M. s. HUDSON 2,435,219

ART OF DEHYDRATING AND IMPREGNATING WOOD Filed Sept. 17. 1947 PER CENT BY WEIGHT OF ORGANIC VAPOR FOR DRYING CHAMBER ATMOSPHERE Inventor MON/E .s. HUDSON B award/$1 ,4 il'orn ega- Patented Feb. 3,1948

UNITED STATES PATENT OFFICE ART OF DEHYDRATING AND IMI'BEG NATING WOOD Monie S. Hudson, Spartanburg, S. 0. Application September 1'7, 1941', Serial No. 174,639 '1 Claims. (01. 117-57) This application is a continuation-in-part of my copending application Ser. No. 324,893, flied March 19, 1940, now U. S. Patent No. 2,273,039, granted February 1'7, 1942, and my copending application Ser. No. 431,176, filed February 16, 1942, and allowed April 4, 1947.

In my above Patent 2,273,039 I have disclosed and claimed a process for drying wood with heated organic vapors, the process being carried out in a closed chamber in which the atmosphere is substantially saturated with organic vapor; that is, as rapidly as the moisture is liberated from the wood during the drying process it is removed from the chamber so that the'atmosphere around the wood is substantially saturated with organic vapor. 'The temperature in the drying chamber is maintained above 212 F.

and up to about 450 F. That method of drying wood with an atmosphere composed substantially entirely of the organic medium enabled wood and wood products tobe dried commercially in a matter of hours instead of days as was required in prior conventional practice. One practical objection to that process is the relatively high cost of the fuel for vaporizing the organic compounds and the cost-of handling the relativeiy large amount of organic vapor condensate produced during the drying operation. This is especially true where concentrations of about 99%-400% organic vapor are attempted to be maintained.

In accordance with the present invention, I have discovered that a substantially saturated atmosphere of organic vapor in the drying chamber, as disclosed in my above patent, is not necessary and that, within certain limits, moisture vapor may be permitted in the drying chamber, and with substantially less fuel and handling costs. An important factor is to prevent too long a drying time which wouldcause hydrolysis of the wood in the presence of the moisture vapor and would also involve excessive costs of operation of the drying cylinder for such long period of time.

I have found that the extreme limit of water vapor is around 50% (the other 50% being organic vapor) and that operation at or above that percentage of water vapor is unreliable and protracts the drying operation much too long; and that a good reliable and economical percentag of water vapor is approximately 35%. At this percentage the organic vapor content of the atmosphere in the drying chamber would be approximately 65%.

This organic vapor percentage may be increased (with corresponding decrease in the water vapor content) up to 100%, theoretically. but I have found from actual experience in the commercial operation of the process that there is no real practical advantage in increasing the organic vapor content above about 90%; and that increase in the drying time. Accordingly a good commercially advantageous range is about to organic vapo with a corresponding moisture vapor range 01' about The single figure in the drawing is a graphic illustration of the relationship between the drying atmosphere concentration and varying time, characteristic of the drying process of this invention.

The curve shown in the drawing is based upon data from various tests that I have made but is to be understood as representative only generally of the relationship between drying vapor con--' centration and hours drying time for my process. It would naturally vary some with each particular type and size of timber or other wood products being dried. The very steep part oi the curve corresponding to about 50% and less of organic vapor shows that a slight decrease in percentage of organic vapor causes a very large it is not practical to operate under conditions that fall under that portion of the curve.

At the other extreme limit, that is, approximately organic vapor, the curve is flat and from the standpoint of time of operation, it is satisfactory, in fact, his optimum. However, practical experience has shown that the cost of operation rises extremely rapidly as a saturated organic vapor atmosphere is approached and that above 99% organic vapor the cost goes infinitely high.

It is therefore apparent "from the above facts and from the curve in the drawing that the range of about 65% to 90% is a good, safe, and commercially advantageous range in which to operate, and that the organic vapor percentage may be increased or decreasedto some extent on either side of thisrange but without any great benefit. As compared with the saturated atmosphere process of my above patent, the process of the present application, especially when operated in the approximate 65% to 90% range, results in a very substantial reduction in the fuel cost, for heating the organic medium, and a substantial reduction in the amount oflorganic condensate which must be handled and reclaimed during the drying operation.

In carrying out the process of my invention I use any suitable drying apparatus having a closed chamber and which is preferably maintained at substantially atmospheric pressure, although in some cases a reduced pressure or vacuum may be employed. An example of a suita- 35% to 10%. The higher the percentage oi organic vapor the raster of approximately 65% to which was filed concurrently with my above identified application Ser. No. 431,176, r which the present application is a continuation-in-part; the said application Ser. No. 431,177 having been allowed on May 7, 1947. For the operation or my process with that equipment, vapor-pressuretemperature relations or the organic material to be used in the process are determined on samples, and a graph made up by using Dalton's law calculations showing the relation of percentage of weight of organic material in the vapor phase, (water being the B component) to temperature. A temperature corresponding to a concentration oi about 65%, for example, of organic material in the vapor phase is selected from this chart. This temperature is then set on a thermostatic-recording controller whose actuating bulb is situated in the drying chamber of the drying apparatus. 1

This controlling device operates a suitable mechanism for varying the heat. input to the evaporator which might be part of or a separate vessel from the drying chamber. In this evaporator, the organic drying medium is maintained at its boiling point, and any impulse 'set up by the thermostatic controller increases or decreases the heat input, which modifies the boil-up of organic drying material into the cylinder, time maintaining the concentration or organicmaterial always within the desired critical limit.

The wood or wood products to be dried are placed in the drying chamber, and the temperature used is one which will assure in'the apparatus throughout the actual drying period (exelusive. oi the short start-up period), an organic vapor concentration within 1 the range of approximately 65% to 90%. Toinsure an accurate check on this, during drying, I employ suitable meters and a continuously separating device by which the proportion of organic material to water in the condensate may be measured and held at the desired percentage by controlling the. heat input to the system.

Suitable organic drying media for use in the present invention are listed below, and some of these are also excellent preserving materials for wood and wood products.- Furthermore, where impregnation with'preservativedrying media is desired, this may be accomplished substantially with the drying, so that drying and preserving is carried out as a single step, thus eliminating the subsequent pressure impregnation cycle which is necessary if a non-preserving drying agent is used.

Preferred examples of suitable organic drying media are:

Distillates of petroleum and coal tar and distillates of wood and wood tar.

Alcohols, such as butyl, isobutyl, amyl, diethylene glycol, or glycerol.

Aldehydes, such as butyric or furfural.

Amines, such as aniline or quinoline.

-Esters, such as glycol mono-ethyl-ether acetate orbutyl-or amylacetates.

Ethers, such as diamyl ether.

Halides, such as dichloro phenol, tetrachlorphenol, pentachlorphenol and paradichlorobenzene.

Nitro compounds, such as nitrobenzene or orthonitro-chlorobenzene.

Hydrocarbons. such as mesitylene, xylene, toluene, benzene, dipentene, or diphenyl.

Phenols. creosols, or xylenols.

For simultaneous numerous or the above compounds or mixtures thereof are satisfactory, e. g., tetrachlorphenol alone or in solution in a petroleum distillate. Compounds having boiling points at temperatures higher than can be used in contact with the wood may be utilized in-solution in solvents that ab-v normally raise their vapor pressures so that they can be distilled at lower temperatures. 1

Certain oi the high melting point compounds will be found to possess vapor pressures that are too low for satisfactory operation as drying and impregnating media at atmospheric pressure, that is, too high a temperature would be required. to volatilize appreciable amounts at atmospheric pressure even though a solvent which raises the vapor pressure of the solute abnormally, is used. This difficulty may be, easily overcome by employing a liquid solvent having a boiling point near that or the toxic solute and impressing vacuum on the system to lower the boiling point'of the mixture.

The term wood and wood products" as used herein is intended to comprehend such materials as green cross-ties, structural timbers, boards and other 7 acterized by commercial subjecting the wood to forms, pulps, wood flour of various kinds. wood molding powders, etc.

I claim: 1. A rapid, high temperature method 01' dehydrating wood in a closed space, which is chardrying oi' the wood in a matter of hours instead of days, comprising composition containing not substantially less than about 65% by weight oi. an inert organic vapor but less than a saturated atmosphere of said vapor, and an appreciable amount or water vapor not in excess of about; 35% by weight, said organic vapor being of a type suitable for permeatingthe wood and replacing the water in said wood and having no adverse effects on the wood, and said vapor composition being heated to and maintained during drying at a high temperature substantially above the vaporization temperature of the moisture in the wood so that the moisture is flashed ofl as vapor, progressively displacing the liberated moisture vapor so that the water vapor content of said drying vapor composition does not substantially exceed 35% by weight at any time during the drying treatment, and continuing this treatment until the desired degree 01' dryness of the wood is obtained.

2. A rapid, hi h temperature method or dehydrating wood in a closed space, which is characterized by commercial drying oi the wood in a matter of hours instead of days, comprising subjecting the wood to a highly heated vapor composition containing approximately 65% to 90% by weight of an inert organic drying vapor and approximately 10% to 35% by weight of water vapor, said organic vapor being of a type suitable for permeating the wood and replacing the water in said wood and having no adverse eflects on the wood, and said vapor composition being heated to and maintained during drying at a high temperature substantially above the 4 drying and impregnating,

or partially seasoned poles, pillngs,

a highly heated vapor desired degree of dryness or the wood is obtained.

3. A rapid, high temperature method of dehydrating'wood in a,closed space, which is characterized by commercial drying of the wood in a matter of hours instead of, days, comprising subjecting the wood to a highly heated vapor composition maintained at a temperature substantially within the range of about 212 F. to 450 F. and containing not substantially less than about 65% by weight of an inert organic vapor but less than a saturated atmosphere of said vapor, and an appreciable amount of water vapor not in excess of about 35% by weight, said organic vapor being of a type suitable for permeating the wood and replacing the water in said wood and having no adverse eflects on the wood. and said vapor composition being heated to and maintained during drying at a temperature substantially above the vaporization temperature of the moisture in the wood so that the moisture is flashed of! as vapor, progressively displacing the liberated moisture vapor so that the water vapor content of said drying vapor composition does not substantially exceed 35% at any time during the drying. treatment, and continuing this treatment until the desired degree of dryness of the wood is obtained.

4. A rapid, hightemperature method of dehydrating wood in a closed space, which is characterized by commercial drying of the wood in a matter of hours instead of days, comprising subjecting the wood to a highly heated vapor composition containing not substantially less than about 65% by weight of an inert vapor of a wood preservative organic compound but less than a saturated atmosphere of said vapor, and an appreciable amount of water vapor not in excess of about 35% by weight, said organic vapor being of a type suitable for permeating the wood and replacing the water in said wood and having no adverse efiects on the wood, and said vapor composition being heated to and maintained during drying at a high temperature substantially above the vaporization temperature of the moisture in the wood so that the moisture is flashed oil as vapor, progressively displacing the liberated moisture vapor so that the water vapor content of said drying vapor composition does not substantially exceed 35% at any time during the drying treatment, and continuing this treatment until the desired degree of dryness or the wood is obtained and the wood is impregnated after the drying treatment with said wood preservative organic compound.

5. A rapid, high temperature method of dehydrating wood in a closed space, which is characterized by commercial drying of the wood in a matter of hours instead of days, comprising, maintaining a reduced pressure in said closed space, subjecting the wood to a highly heated vapor composition containing not substantially less than about 65% by weight of an inert organic vapor but less than a saturated atmosphere of said vapor, and an appreciable amount oi water vapor not in excess of about 35% by weight, said organic vapor being of a type suitable for permeating the wood and replacing the water in said wood and having no adverse eflects on the wood, and said vapor composition being heated to and maintained during drying at a high temperature substantially above the vaporization temperature of the moisture in the wood so that the moisture is flashed of! as vapor, progressively displacing the liberated moisture vapor so, that the water vapor content of said drying vapor composition does not substantially exceed by weight at any time during the drying treatment, and continuing this treatment until the desired degree of dryness oi the wood is obtained.

6. A rapid, high temperature method of de- I hydrating wood in a closed space, which is char,- acterized by commercial drying of the wood in av matter of hours instead of days, comprising subjecting the wood to a highly heated vapor composition containing not substantially less than about 65% by weight of an inert organic vapor but less than a saturated atmosphere of said vapor, and an appreciable amount of water vapor not in excess of about 35% by weight, said organic vapor containing a toxic preservative I compound, and being of a type suitable for permeating the wood and replacing the water in said wood and having no adverse effects on the wood, and said vapor composition being heated to and maintained during drying at a high temerature substantially above the vaporization temperature of the moisture in the wood so that the moisture is flashed oi! as vapor, progressively displacing the liberated moisture vapor so that the water vapor content of said drying vapor composition does not substantially exceed 35% by weight at any time during the drying treatment, and continuing this treatment until the desired degree of dryness of the wood is obtained.

'7. A rapid, high temperature method of dehydrating wood in a closed space, which is characterized by commercial drying of the wood in a matter of hours instead or days, comprising subjecting the wood to a highly heated vapor composition containing not substantially less than about 65% by weight of an inert organic vapor but less than a saturated atmosphere of said vapor, and an appreciable amount of .water vapor not in excess of about 35% by weight, said organic vapor being obtained by heating an organic compound selected from the group consisting of distillates of petroleum, coal tar, wood and wood tar, the said-organic vapor being of a type suitable for permeating the wood and replacing the water in said wood and having no adverse effects on the wood, and said vapor composition being heated to and maintained during drying at a high temperature substantially above the vaporization temperature of the moisture in the wood so that the moisture is flashed ofl the wood is obtained.

' MONTE B. HUDSON.

Certificate of Correction Patent No. 2,435,219. r Februery3,1948.

MONIE S. HUDSON It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Column 3, line 51, for the word substantially read simultaneously; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 6th day of April, A. D. 1948.

THOMAS F. MURPHY,

Assistant Uommissioner of Patents.

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