BACKGROUND OF THE INVENTION For the purpose of describing the present invention, reference will be made to the removal of moisture from wood or wood products. However, it will be clear to the skilled addressee that the present invention also has application in the removal of moisture from other materials. It will therefore be understood that the invention extends to the treatment of such other materials.

Green timber contains a high degree of moisture, and to be a useful material in the manufacture of wood products, that moisture content must be reduced. Many methods exist to achieve this. U. S. Patent 4,343,095 teaches placing the timber into a hermetically sealed chamber and then applying steam heating to the timber for a predetermined degree of temperature, pressure and time. Some moisture is then removed from the timber by reducing the atmospheric pressure within the chamber and drawing off the resultant water vapor. The steps of alternately heating and drying the timber are then repeated until the desired moisture content of the timber is obtained. Other means of conveying heat and moisture to the timber in a kiln drying chamber are taught by U. S. Patent 4,194,296 in which hot air is utilized in the heating step instead of steam. Other examples of methods for reducing the moisture content of wood are disclosed in U. S. Patents 4,198,763 and 3,921,309.

The utilization of either air or an air/steam mixture as the heating means for the drying of timber is less than desirable because the oxygen in either of these fluids tends to discolor the timber. Moreover, as the timber becomes progressively drier, there is an increasing risk of fire within the drying chamber. In addition, where steam alone is used as the heating/ pressurizing means, the minimum possible operating temperature for the process is 100°C, whereas timber strength is degraded at temperatures above about 70°C.

We have developed an improved method for drying wood and wood products which reduces wood discolouration and weakening during drying and also reduces the risk of fire during the drying process. We also believe that the method of the present invention also has application in reducing the moisture content of other materials.

SUMMARY OF THE INVENTION In a first aspect, the present invention provides a process for reducing the moisture content in a material, the method including exposing the material to a substantially oxygen free heating fluid including H20 and a diluent gas.

The material treated may be any moisture containing material. The material may be of plant or animal origin or it may be synthetic material. The material may be a foodstuff, for example, a plant, cereal or grain. Preferably the material is a wood or wood-based product.

By use of the term"wood product"we include timber, lumber and the like.

The term"fluid"as use herein includes the gaseous and/or vapor state.

The diluent gas may be any gas or mixture of gases that is capable of preventing the temperature of the heating fluid from rising beyond a predetermined level during the method. Preferably the diluent gas prevents the temperature becoming so high that it results in a degradation of one of more properties of the material and/or increases the risk of combustion or fire. Preferably the diluent gas is substantially inert to the material being treated. The diluent gas may be a single gas or be two or more gases. The diluent gas may be a non-combustible gas such as N2 or CO2. The diluent gas may be noble gas e. g. argon. Preferably the diluent gas is a combination of CO2 and N2.

The term"substantially oxygen free"used herein is intended to indicate that the level of oxygen in the heating fluid is below a level that would lead to a degradation in a property of the material being treated, for example, in the case of a wood or wood product, below that which would cause discoloration.

The method of the present invention is particularly suitable for use in the heating phase of a vacuum drying process.

In a second aspect, the present invention provides a process for reducing the moisture content of a material, the process including the steps of: (i) introducing into a drying chamber in which the material has been placed, a substantially oxygen-free heating fluid comprising H2O and a diluent gas to heat the material and vaporize water therefrom; and (ii) withdrawing at least part of the gas from the chamber.

Steps (i) and (ii) may be repeated one or more times. Preferably, in the first cycle of the process of the second aspect, the drying chamber is evacuated to remove air prior to the introduction of the heating gas in step (i). In step (i), the heating fluid is preferably introduced into the chamber until a predetermined elevated pressure is reached. This predetermined elevated pressure may be in the range of about 150-300 kPa abs.

Discharge step (ii) preferably takes place once the core temperature of the material, e. g. wood or wood product reaches a temperature in the range of about 50-60°C.

Preferably, in this case of wood or a wood product, the temperature of the gas used in the process of the first and second aspects of the invention is below that which results in degradation of the strength of the wood or wood product being treated. The temperature of the gas may be in the range of about 50°C to about 70°C. A gas temperature that does not exceed about 70°C is preferred. More preferably the gas temperature is no greater that 65°C. A gas temperature not exceeding about 55"C is particularly preferred.

The gas used in the process of the invention may be derived from any suitable source. The gas may be a gas resulting from the combustion of a fuel.

The heating gas may be the exhaust from a combustion engine run on a fuel.

The fuel may be any fuel suitable for running a combustion engine, the combustion gas of or which is substantially oxygen free and contains H2O and a diluent. The fuel may be a solid, liquid, gas or vapour.

Preferably the gas resulting from combustion of producer gas is cleaned and/or cooled prior to being used as the heater gas. For example, the gas may pass through a scrubbing unit prior to being introduced into the chamber.

A preferred fuel is producer gas, although any other suitable fuels may be used e. g, gas produced by digestion of biomass. The process of the invention will now be described in reference to the use of producer gas as the

fuel source, and wood or wood product as the material. However, as noted above, the invention is not limited to this particular embodiment.

A particularly preferred means for combusting the producer gas is an internal combustion. An internal combustion engine running on producer gas produces an exhaust gas consisting predominantly of CO2, N2 and H2O and substantially freeof 02 and is thus an ideal source of heated gas for the process of the present invention. A further advantage of using a combustion engine to provide a source of heating gas for the process of the invention is that the engine may be used to drive an electrical generator to generate electricity which may be used to supply part or all of the power requirements of the drying process. Moreover heat generated by the combustion engine may be recovered and used to provide any supplemental heat required in the drying process.

The producer gas may be supplied from any suitable source. The producer gas may be produced on site using a gas producer plant. Many sawmills have as a by-product of their operations, relatively large quantities of sawdust and small pieces of timber having little commercial value. In a particularly preferred embodiment of the process of the present invention, sawdust and/or small pieces of timber are used as the primary fuel source in a producer gas apparatus to generate producer gas. The products of combustion of such gas have a low content of oxygen and therefore have the property required to obtain the minimum discoloration of the wood or wood product.

The present invention also provides an apparatus for decreasing the moisture content of a material. The material may be any material described above.

Accordingly, in a third aspect, the present invention provides an apparatus for vacuum drying a material, the apparatus including: at least one drying chamber; means for creating a reduced pressure in the at least one chamber; means for introducing a heating fluid into the at least one chamber; and a combustion engine, the exhaust of which provides at least part of the heating fluid.

The apparatus of the invention may further include a source of fuel for the combustion engine.

As indicated above, the source of fuel may be any fuel the combustion of which results in a fluid containing H20 and a diluent gas and which is substantially oxygen free. In one preferred embodiment, the fuel is a producer gas. The source of producer gas is preferably a producer gas plant.

The drying chamber may be formed from any suitable material, for example, a metal or metal alloy. Because the heating step of the process of the invention can be carried out at a relatively low temperature compared to that used in conventional drying methods, the drying chamber may be formed from a material that need not withstand very hight temperatures. For example, the drying chamber may be made from a rigid plastic material that is capable of withstanding the temperatures and pressures involved in the process of the invention. An avantage in the use of a plastic material is that it reduces the prospect of corrosion.

The drying chamber of the apparatus of the invention may be hermetically sealed by covers at the ends thereof. The chamber may include means for supporting a plurality of material, e. g. wood or wood products therein.

The drying chamber may have secondary heating means, for example, one or more heat exchangers. It may include means for circulation the heating gas within the chamber, for example, one or more fans. It may further include means for adding moisture to surface of the material. For example the chamber may include one of more water sprays appropriately positioned in the chamber.

The apparatus of the invention may include condensing means for cooling gas evacuated from the chamber to condense water vapour therein.

The condensing means may be one or more water sprays.

The method and apparatus of the invention may include a step or means respectively to extract component (s) from the moisture extracted from the material.

The means for creating a vacuum in the chamber may be a pump.

The means for introducing heating gas into the chamber may be a pump. The gas introducing means may be separate from the vacuum creating means. Preferably the vacuum creating means also serves as the gas introducing means.

Preferably the waste heat generated by the combustion engine is recovered and used as a supplemental heating source in the apparatus of the

invention. For example, combustion engine coolant may be used to provide secondary heat to the chamber by circulating the coolant to one or more heat exchangers associated with the chamber. Preferably the combustion engine provides electric power to the apparatus. For instance, the engine may drive an electricity generator for providing electricity for running various components of the apparatus, for example, circulating fans in the chamber.

Preferably the combustion engine is an internal combustion engine.

The apparatus of the invention may include means for cleaning and/or cooling the exhaust gas from the combustion engine prior to use of that gas as the heating gas. The means for cleaning and/or cooling may be a scrubbing unit.

In a particularly preferred form, the apparatus of the invention includes two drying units. The apparatus may be such that, in operation, one of the chambers is evacuated while the other chamber performs a heating stage. In a preferred form, the two chambers are connecte by means of a conduit including means to open and shut the conduit so that when the conduit is open, the pressure of the two chambers may be equalised.

Throughout this specification the word"comprise", or variations such as"comprises"or"comprising", will be understood to imply the inclusion of a stated element, integer of step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The invention will hereinafter be described by way of the following non-limiting examples.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of a vacuum drying plant according to the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 there is shown a producer gas plant 5. What is known as producer gas consists essentially of the products derived from the passage of air and steam through a bed of incandescent fuel. In this embodiment, the fuel comprises sawdust and small pieces of wood, bark etc. such fuel being derived from the normal operation of a sawmill. The resultant gaseous fuel may be utilized for power production in an internal

combustion engine. It is to be understood that the term"internal combustion engine"is used for illustrative purposes only and that any type of combustion engine may be used for the purpose of the invention.

Producer gas plant 5 has a connecting pipe 6 for the purpose of conveying producer gas to the fuel intake of internal combustion engine 3.

Internal combustion engine 3 provides the motive power to drive an electrical generator 4. All of the electrical power requirements of the vacuum drying plant may be provided by electrical generator 4, electrical wiring 46 representing a typical electrical connection to fan motor 9. The product of combustion of internal combustion engine 3 is a fluid comprising mainly carbon dioxide, nitrogen and water. This fluid is utilized in the present invention to convey heat and moisture to and from the timber undergoing treatment.

One phenomenon of known combustion engines is the production of heat. Generally this heat has been unwanted and has been dissipated to the atmosphere by means of a circulating fluid transferring heat from the engine to a heat exchanging device such as an air cooled radiator. The heat thus lost to the atmosphere represents a loss of efficiency. In the present embodiment the normally wasted engine heat is put to a useful purpose in providing the heat required for the timber drying process.

Referring to FIG 1. There are shown two vacuum drying kilns 1 and 2 comprising cylindrical chambers hermetically sealed by covers at the ends (not shown) and having support means (also not shown) to carry the timber 7 and 8 to be dried in the kilns. The timber within each drying chamber is stacked in such manner as to permit free circulation of the heating gas.

Within each drying chamber are located an array of electrically driven fans 9 and 10 (only two of which are shown in each chamber as illustrated by FIG.

1) The direction of rotation of the fans is such that the flow of drying gas through each fan is assisted by the flow through the fan located on the opposite side of the chamber.

Also provided within each drying chamber is an array of water sprays 38 and 39 (only two of which are shown in each drying chamber as illustrated in FIG. 1) for the purpose of adding moisture to the surface of the timber as it is known that when moisture is removed too rapidly from the surface of timber in a drying process, stresses are set up within the timber which lead to distortion and cracking.

Also provided within each drying chamber is an array of heat exchangers 17 and 18 (only four of which are shown in each drying chamber as illustrated in FIG. 1).

A pump 11 takes a primary fluid (which may be engine coolant) heated by the engine 3 and passes the fluid through a circuit comprising pipe 12 and heat exchanger 13 and returning to the engine 3 by means of pipe 14. Pump 15 takes a secondary heated fluid stored in heat exchanger 13 and circulates the fluid by means of pipes 16 through the array of heat exchangers 17 and 18 located within drying chambers 1 and 2, returning said fluid to heat exchanger 13 through pipes 45. The flow of secondary heating fluid to heat exchangers 17 and 18 is controlled by valves 19 and 20.

The engine exhaust manifold 21 is put into communication with exhaust gas cooling and scrubbing unit 22 by means of pipe 23. Within said scrubbing unit Z2 there is provided an array of water sprays 40 for the purpose of cooling the engine exhaust gas and for removing any solid particulate therein. Also provided with scrubbing unit 22 is exhaust port 44 which discharges to the atmosphere any gas with a pressure above 1 Atm.

Engine exhaust manifold 21 is also put into communication with the outlet port 43 of pump 24 by means of pipe 41 and valve 42.

In a preferred embodiment the pump 24 is of the liquid ring type and may be utilized to produce a condition of low atmospheric pressure within the drying chambers during a drying cycle, and a condition of elevated atmospheric pressure in the drying chambers during a heating cycle. The pump 24 takes heating gas from scrubbing unit 22 by means of pipework 25 and 2G through an open valve 27 and transfers said heating gas to drying chambers 1 or 2.'I'lie flow of the heating gas may be controlled by valves 28 and 29.

The condensing unit 30 has communication to the intake of pump 24 by means of pipework 31 and 26 when valve 32 is in the open position. In addition, condensing unit 30 has communication with both drying chambers 1 and 2 by means of pipes 33 and 34. Drying chambers 1 and 2 may be isolated from condensing unit 30 by means of valves 35 and 36. Also shown provided within condensing unit 30 is an array of water sprays 37 for the purpose of lowering the temperature and condensing the vapor within the condensing unit.

EXAMPLE The vacuum drying plant as illustrated in FIG. 1 may operate as follows : Step 1. The drying chambers 1 and 2 are first stacked with timber 7 and 8 in a manner that permits at least two sides of each board exposure to the action of the heating gas, and allows a free circulation of said heating gas. The drying chambers 1 and 2 are then hermetically sealed by means of the end covers (not shown in FIG. 1) Producer gas plant 5, engine 3 and electric generator 4 are then put into operation. Primary heating fluid pump 11 (which may be either electrically driven or mechanically driven by engine 3) is then put into operation.

Step 2. The electrically driven fans 9 in drying chamber 1 are switched on and the water sprays 38 may be activated. Water vapor can then be circulated until the surface of the timber is saturated. Water sprays 38 may then be turned off.

Step 3. With valves 35,32 and 42 open, and with all other shown valves closed, electrically driven pump 24 is activated and a vacuum is drawn in drying chamber 1. The effect of this step is to evacuate air from the drying chamber without excessive removal of moisture from the previously saturated timber surface.

Step 4. Valve 19 is opened and secondary heating fluid pump 15 is activated.

Valves 27 and 28 are opened and with all other shown valves closed, heating gas is pumped by pump 24 from scrubbing unit 22 into drying chamber 1.

The transfer of heating gas and the pressurization of drying chamber 1 continues until a maximum pressure in the order of 300 kPa abs. is attained.

Valve 28 is then closed and valve 42 opened so that the drying gas drawn from scrubbing unit 22 by pump 24 is returned to scrubbing unit 22 by way of pump 24 outlet 43 and pipes 41 and 23. The fans 9 continue to circulate the heating gas through the stack of timber 7 and the array of heat exchangers 17 until the core temperature of the timber boards is in the order of 50°C.

Throughout this procedure the temperature of the drying gas does not exceed 65°C.

Step 5. Once pressurization of drying chamber 1 is completed and the heating of the timber therein proceeding, the same sequence of events occurring in Step 2. with drying chamber 1 are applied to drying chamber 2.

The electrically driven fans 10 are switched on and the water sprays 39 activated. Circulation of the water vapor continues until saturation of the timber surface has occurred. Water sprays 39 may then be turned off.

Step 6. Evacuation of drying chamber 2 commences. With valves 36,32 and 42 open, and valves and 35 remaining closed, pump 24 evacuates air and moisture from drying chamber 2 as with Step 3. and drying chamber 1.

Step 7. With evacuation of drying chamber 2 completed valves 36 and 32 are closed and valve 42 and 27 opened. Secondary heating fluid is then admitted to the array of heat exchangers 18 by closing valve 19 and opening valve 20.

Step 8. The pressure in drying chamber 1 is now at a pressure in the order of 300 kPa abs. and the pressure in drying chamber 2 is at a pressure in the order of 5 kPa abs. The differential pressure existing between drying chamber 1 and drying chamber 2 is now equalized by first shutting down pump 24 then closing valves 42 and 27. By the simultaneous opening of valves 28 and 29 the heating gas in drying chamber 1 is transferred to drying chamber 2 until the pressure between the two chambers has equalized. The sudden reduction of pressure in drying chamber 1 has the effect of vaporizing some of the surface moisture from the timber 7 and also causing moisture in the core of the timber 7 to commence migration towards the surface.

Step 9. Evacuation of drying chamber 1 commences again, the saturated heating gas being drawn by pump 24 through condensing unit 30 where the array of water sprays 37 cools said gas and condenses any water therein. As the pressure within drying chamber 1 decreases, more of the surface moisture on the timber 7 evaporates with the timber becoming progressively dryer during successive drying cycles. The heating gas thus removed during a

vacuum drying cycle in drying chamber 1 may be transferred directly to chamber 2 during a pressurization/heating cycle in drying chamber 2.

The cycle of alternate pressurization/heating, and vacuum cooling/ drying in drying chamber 1 and drying chamber 2 is continued until the required moisture content of the timber is obtained.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.