Patent Description:
A process of high-temperature heat treatment of wood is already known from <CIT>. A cooling process with injection of a shield gas is disclosed in <CIT>. In this paragraph, the current state of the art is described. In this regard, it is worth stating that the heat treatment processes of wood currently used in the industry substantially consist in the sequence of the following three steps:.

Currently, three technologies are known to obtain thermal-treated wood, which are described herein below while pointing out the features thereof.

The stack, composed of the wood pre-arranged in battens and dried in other devices, is loaded within a cell that is suitably insulated and made impermeable to vapour, similar to a conventional wood drying cell, provided with internal fans to generate the circulation of the heating fluid through the stack, in the present case, vapour superheated at a temperature of <NUM>-<NUM>, provided by an external vapour generator and rigorously kept, for obvious safety reasons, at atmospheric pressure, by means of a duct and/or safety chimney open to the atmosphere, which the cell has necessarily to be provided with to prevent dangerous internal overpressures. The wood is protected against triggering of combustion, since it is heated at the heat treatment temperature of <NUM>-<NUM> in an environment that is inert, since the air initially existing therein (therefore, the oxygen contained therein) has been replaced by the superheated vapour.

Therefore, the first two steps, i.e., pre-heating and treatment, are carried out with the same method using a superheated vapour circulation.

The third step, i.e., cooling, is carried out by interrupting the superheated vapour inlet and injecting demineralised water into the cell by means of special sprayers that provide to "atomize" the water jet, which, being instantly converted into vapour, subtracts heat to the treatment cell, in a ratio of about <NUM> Kcal per litre of evaporated water, thus carrying out the cooling of the wood contained therein; the so-generated vapour is discarded into the atmosphere via the above-mentioned "chimney".

Of course, there are some variants in the method for producing the heating vapour, which will be omitted herein for sake of brevity, but which anyway always find their origin in the use of superheated vapour as a heating and inertization medium of the cell.

The limits of this technique consist in the following points:.

The process is similar to the previous one, except for the variation that the interior of the cell is saturated with nitrogen, which, being a inert gas, prevents the triggering of the combustion of the wood material.

Such process completely differs from the two previous processes, and consists in the feeding of the wood into a suitably insulated autoclave and suitable to resist an internal pressure up to <NUM> Bar.

The heating is carried out by directly injecting superheated vapour into the autoclave, therefore it does not provide for the arrangement of ventilation devices and/or internal exchangers.

The process takes place according to the following steps:.

The process that is the object of the present invention is defined in claim <NUM>. The present invention moreover concerns an apparatus as defined in claim <NUM>. The above process has as its aim to overcome most of or all the limitations described above with reference to the prior art, and it is characterised in that the treatment of wood in order to prevent the combustion thereof is carried out by totally eliminating the oxygen contained in the cell, by first dramatically reducing the initial amount thanks to the partial vacuum created in the cell that makes the air extremely rarefied, since it is brought to an absolute pressure of <NUM>-<NUM> mBar, and subsequently by totally consuming the small residual quantity of oxygen, by inducing a driven micro-combustion of an infinitesimal portion of the wood mass subjected to the treatment. This phenomenon is defined by the Author as "self-inertization of wood", since it is the wood itself to generate its own inertization, by sacrificing a negligible part of its matter.

Such thermal-chemical reactions induce the following macroscopic modifications in the wood characteristics:.

It is necessary to point out that the wood, even before being subjected to the heat treatment temperatures, must be however dried to final moisture values near to zero, to prevent that the residual water contained in the cell walls of the wood material, brought to temperatures of <NUM>-<NUM>, generates such pressures as to produce a collapse thereof, resulting in irreparable damages.

Further characteristics and advantages of the invention will be apparent from the following detailed description, given by way of non-limiting example only, with reference to the annexed drawings, in which:.

The process described herein below aims to overcome all, or most of, the negative implications of the currently existing processes, while maintaining a far superior quality of the final product, and more precisely:.

In order to understand the dynamics of the self-inertization phenomenon, fist consider that the interior of the autoclave-cell (insulated and perfectly watertight) is a virtually adiabatic system, that is, it does not allow energy exchanges with the outside.

This means that, once the wood has been loaded into the cell, the system appears to be completely insulated and contains only the following elements: the wood to be heat-treated and the air that fills the remaining internal free volume.

When the wood reaches the pyrolysis temperature (<NUM>-<NUM>) at a pressure of <NUM>-<NUM> mBar, the internal atmosphere of the cell is formed by air that is made very rarefied, as seen, by the combined effect of vacuum and temperature, yet having an oxygen content that, while now being very low, could still induce a triggering of combustion of the wood.

Therefore, to assess the extent, therefore the potential hazard of the above-mentioned combustion phenomenon, it is desirable to know the amount of wood mass that could be induced to burn by the oxygen existing in the residual air contained within the autoclave-cell, once the operative point of the treatment has been reached.

Due to constructive causes of geometry, mechanics, and aerodynamics, whatever the internal dimensions of the autoclave-cell are, the ratio between the free volume of internal air and the net volume of the stowed wood is about <NUM>:<NUM> = <NUM>. This means that per each m3 actually stowed wood, an amount of oxygen approximately equal to that contained in <NUM> m3 air at a room temperature of <NUM> ° C and at an atmospheric pressure of <NUM> mbar is available.

Physics teaches that air density varies when the temperature and pressure vary, and, in particular, it decreases when temperature increases and pressure decreases. Some significant values for the present discussion are set forth in the following Table <NUM>:.

From the Table, it follows that the air density reduction during the process is drastic and ranges between <NUM>% and <NUM>% compared to the density at normal temperature and pressure.

Applying such observation to the ratio of volume of internal air to volume of stowed wood (which initially was <NUM>:<NUM> = <NUM>), it is as if the amount of available air would be reduced by a value equal to the coefficient of density reduction, i.e., according to the Table <NUM>:.

From the analysis of the different values of the ratio air volume/wood volume in various operative points, it is seen that by increasing the temperature and reducing the pressure, the amount of available air is dramatically reduced together with the amount of available oxygen for a possible combustion.

Chemistry teaches that the amount of oxygen required to generate the combustion of <NUM> wood mass under standard conditions (i.e., atmospheric pressure and a temperature of <NUM>) is approximately equal to that contained in <NUM> m3 air, from which the amount of "burnable" wood mass per m3 of stowed wood within the autoclave-cell depending on the parameters of temperature and pressure used during the treatment can be easily obtained:.

Given that the Specific Gravity of the dry wood varies, for the European species, from <NUM> to <NUM>/m3, it is evident that the amount of wood mass possibly sacrificed in the combustion phenomenon due to the residual oxygen is infinitesimal, and it may be calculated as follows:.

In order to assess the dangers inherent in the combustion of the aforementioned tiny wood masses, it is necessary to know the amount of thermal energy released by the exothermic chemical process during the combustion thereof.

Given that the calorific value of wood is, as a maximum, equal to <NUM> Kcal/kg, one can easily calculate the thermal energy released by the combustion process:.

This energy, even if it were released instantaneously, is not capable of producing any significant temperature variation of the internal environment of the cell, and thereby it cannot create any danger either to the mechanical structure, or the wood, nor to the control staff.

In reality, the oxygen OR has not "disappeared", but it is combined with the carbon C of the wood during the exothermic chemical reaction of the combustion:.

being converted into carbon dioxide, which, being an inert gas, prevents each additional combustion: from this the initial definition derives according to which the described process is a process of "SELF-INERTIZATION" of the wood.

It is clear that, this percentage of burnable wood being absolutely insignificant, the redox phenomenon which is the basis of the combustion of this small amount makes the internal atmosphere of the cell totally inert, since the small percentage of oxygen still present in the residual air before the combustion, combining with the carbon contained in the wood, is converted into carbon dioxide CO2, totally eliminating the number of oxygen molecules present in the cell, and making the final mixture of gases contained in the autoclave-cell totally inert.

It is worth analyzing the different compositions of the gas mixture within the cell before reaching the pyrolysis temperature and during the wood self-inertization process caused by the process described:.

This solution provides the great advantage of preserving the wood material, keeping it under the current conditions, so as to allow the apparatus to automatically restart the heat treatment cycle from the interruption point, without losing any information about the current state of the apparatus and the wood, and with the maximum safety. To overcome the problem of a possible failure of one of the electrical components of the apparatus (motors, valves, sensors, etc.), in accordance with an embodiment, each component is supervised by a device (for example: magneto-thermal protections, auxiliary probes, etc.) that immediately alerts the PLC controller of the occurred failure, so that the PLC can take the appropriate measures:.

In this case again, the system immediately reacts, ensuring the preservation of the wood and allowing the technicians to intervene in order to obviate the drawback and restart the system.

In accordance with an embodiment, in any case the apparatus can be provided with an emergency system against the possibility of fire of the wood contained in the cell: such system provides for the inertization of the interior of the cell by means of an input of nitrogen or water.

Should the remote event of a fire ignition within the cell occur, for a cause such as, for example, a loss of watertight seal that makes the environment air to enter the cell, resulting in an increased concentration of oxygen, therefore the initiation of a combustion, special pressure and temperature sensors alert the PLC controller, which provides to activate the RAPID INERTIZATION process, substantially consisting in the following operations:.

From the foregoing, it shall be apparent that the described apparatus is capable of implementing a process suitable to carry out in sequence and/or separately the two physical processes:.

With reference to the annexed Figures, in accordance with an embodiment, the apparatus <NUM> comprises the following elements:.

After anticipating the three possible configurations, let us consider the descriptive analysis in more detail for each individual intended application:.

With reference to <FIG>, in the Example illustrated the apparatus <NUM> comprises the following elements:.

The description of the operation of the apparatus <NUM> as a vacuum drier will be omitted, since it is already known, in that it has been the object of a number of patents granted in the past to the Applicant, and being a part of the know-how licensed by the Applicant to a number of companies, both in Italy and abroad (for example, <CIT>; <CIT>).

After loading the wood stack <NUM>, i.e., the wood mass to be treated, on the special carriage <NUM> and feeding the carriage into the treatment cell (autoclave) <NUM> through the door <NUM>, it is hermetically sealed by means of special devices.

At this point, the actual treatment cycle can be started, that consists, as seen in the introduction, in the sequence of the following <NUM> steps:.

During this step, the fans 6A and 6B (according to the teachings of the <CIT> by the Applicant regarding the longitudinal-lateral "LO-LA" type ventilation system) provide for the circulation of the internal air through the heating electrical batteries 5A and 5B and the wood stack, so as to transfer the thermal energy produced by them to the wood itself (of course, it is possible to arrange differently the batteries and fans, provided that the fluid circulation is equally efficient).

Preferably, a system of at least two temperature probes <NUM> immersed in the air flow allows measuring the temperatures upstream and downstream the wood stack <NUM>, and consequently carrying out the adjustment thereof according to the program set by the operator.

At the same time, the vacuum pump system provides to suck the air from within the cell <NUM> until the desired absolute pressure value is reached and maintained.

According to the need, such value can range between <NUM> mBar and the atmospheric pressure (<NUM> mBar) until when the internal temperature of the treatment chamber is less than <NUM>, that is, the temperature at which a beginning of combustion can be triggered in the wood; subsequently, the apparatus <NUM> proceeds to decrease the pressure to a value ranging between <NUM>-<NUM> mBar so as to ensure that, once the pyrolysis beginning temperature has been reached, the internal atmosphere is already extremely poor in oxygen, so that, as seen, the amount of "burnable" wood material due to redox phenomena between the residual oxygen and the carbon in the wood is infinitesimal and produces the desired "self-inertization" phenomenon. A temperature probe <NUM>, located in a hole drilled in the middle of a wood plank that has been selected as a sample, allows measuring and adjusting the thermal-chemical treatment temperature, generally ranging between <NUM> and <NUM>, which can be reached in a number of successive steps and/or with a desired slope according to the type of wood, the thickness thereof, etc..

Once it has been assessed, by means of a special wood core probe <NUM>, that the wood stack <NUM> has reached the desired treatment temperature, the device provides to maintain such temperature constant for the time period set by the operator, the duration of which depends on the result of changes in the characteristics of the wood to be obtained (colour, durability, hygroscopicity, etc.), on the wood thickness, its type, etc..

In this step the fans, and, when needed, also the heating batteries, as well as the vacuum pump unit that provides to maintain the pressure in the cell at the operative value, also ranging according to the needs between <NUM> and <NUM> absolute mBar, are active.

The vapours and gases possibly produced by the wood stack <NUM> during the treatment are sucked by the vacuum pump <NUM>, which provides to convey them, via the tube <NUM>, through the condenser <NUM>, where they are cooled to the dew temperature, and then converted into the liquid phase, collected in the storage reservoir <NUM>, from where they can be extracted at the end of the cycle and sent, if needed, to the disposal according to the requirements of law.

At the completion of the treatment step, the wood is at a temperature of <NUM>-<NUM>, thereby it cannot be extracted from the cell if not after being cooled in an inert atmosphere until it falls below <NUM>, to avoid its exposure to the environment air when it is still at a temperature that is dangerous for the combustion, to not subject it to harmful thermal shocks, and, finally, to allow the handling thereof without risk of burns for the operator.

The method used to cool the wood is new and original, and consists in subjecting the outer surface of the autoclave-cell <NUM> to a forced flow of fresh environment air, so as to create, through the wall of the cell itself, a large temperature differential between the interior (<NUM> ° -<NUM>) and the exterior (<NUM>), thus generating a corresponding thermal energy flow according to the known laws of thermodynamics, where the amount of heat transferred to the external environment is equal to: <MAT> where:.

from which we understand the high efficiency of the autoclave-cell when considered as a total surface air-air heat exchanger.

The cooling operations consist in opening the air shut-off port <NUM>, activating the fan <NUM>, which, through the upper manifold <NUM>, provides for the withdrawal of the fresh environment air from the lower hole <NUM> and to pass it into the interspace <NUM> with such a speed as to generate a turbulent flow, which, by touching externally the wall of the treatment cell <NUM>, produces a rapid cooling thereof, thus establishing, as seen, a highly efficient thermal exchange between the interior of the cell (with its entire wood content) and the environment air, without mutual direct contact: thanks to this thermodynamic measure, the cell wall is converted into a high efficiency total surface gas-air heat exchanger.

The internal fans 6A and 6B, which produce, as seen, the circulation of the internal air, provide to subtract the stored heat of the wood and to send it to the inner surface of the treatment cell wall, through which the thermal exchange and the transfer of thermal energy to the circulating fresh air current into the interspace <NUM> take place. All of this takes place in an absolute respect for the environment, since there is no contact and/or mixing between the internal fluid (mixture of air, vapours, and wood gases) and the cooling air. Of course, the hot air exiting the fan <NUM> may be, during the winter months, recovered and used to heat industrial buildings.

Once the wood has been cooled to the desired temperature, it is possible to open the watertight door <NUM> and withdraw the carriage.

The process is substantially equal to that previously described with reference to the <FIG>, with the following technological variants (see <FIG>) :.

In the Example illustrated, a pump <NUM> is provided for the forced circulation of diathermic oil.

For the drying and the actual treatment, all the measures described with reference to the <FIG> may apply.

The physical process is substantially similar to that described with reference 1a-1c, with the following technological variations (see <FIG>):.

For the drying and the actual treatment, see what has been already described with reference to <FIG>.

Claim 1:
A process for the thermal-chemical modification treatment of a wood stack (<NUM>), at temperatures in the range <NUM>-<NUM>, in an apparatus (<NUM>) comprising a vacuum autoclave-cell (<NUM>) comprising a cylinder (<NUM>) defining a treatment chamber suitable to house the wood stack (<NUM>), wherein the process always maintains the internal pressure of the treatment chamber at an absolute pressure of <NUM>-<NUM> mBar, and consists in the steps of pre-heating, actual heat treatment, and cooling of the wood stack (<NUM>), wherein the heating of the wood stack (<NUM>), in order to prevent its combustion, is carried out by totally eliminating the oxygen contained in the vacuum autoclave-cell (<NUM>) by virtue of measures that consist in first "reducing" an initial quantity of oxygen thanks to the partial vacuum created in the vacuum autoclave-cell (<NUM>) that makes the air extremely rarefied, and, subsequently, in totally "consuming" the small residual quantity of oxygen, inducing a driven micro-combustion of an infinitesimal portion of the wood stack (<NUM>) subjected to the treatment;
wherein:
- the process comprises a method for cooling the wood stack (<NUM>) at the end of the heat treatment, which is obtained by means of heat exchange systems between the internal atmosphere of the vacuum autoclave-cell (<NUM>) and the external atmosphere, without these ever coming into direct contact with each other;
- said cooling method is performed by subjecting the outer surface of the autoclave-cell (<NUM>) to a forced flow of fresh environment air;
said process being characterized in that:
- said heat exchange system comprises an outer cylinder (<NUM>) concentric to the vacuum autoclave cell (<NUM>);
- an interspace (<NUM>) in the shape of a cylindrical shell is defined between said outer cylinder (<NUM>) and said vacuum autoclave cell (<NUM>);
- said outer cylinder (<NUM>) is provided at its inferior side with a hole (<NUM>) extending through the entire length of said interspace (<NUM>);
- said outer cylinder (<NUM>) is provided ats its superior side with a suction manifold (<NUM>), an air shut-off valve (<NUM>) and a fan (<NUM>);
- said interspace (<NUM>) communicates with the atmosphere inferiorly via a hole (<NUM>) and superiorly through said suction manifold (<NUM>), said air shut-off valve (<NUM>) and said fan (<NUM>).