Method for drying wood products in order to produce wood products with reduced VOC emissions

Abstract

The invention relates to a method for drying wood products with reduced discharge of VOC.

Description

[0001] Process for drying wood products to produce wood products with reduced VOC emissions

[0002] The invention relates to a method for drying wood products to produce wood products with reduced VOC emissions. In particular, the present invention relates to a method for producing wood products, which method allows for improved removal of VOCs from the wood product, such as in particular from the wood, through an improved drying step.

[0003] Emission problems are known for panels in the wood-based materials industry, particularly particleboard, OSB, and pallets, but also for the production of pellets from wood chips or lumber. These occur, for example, in the exhaust air after drying and / or in the finished product itself. For particleboard and OSB, for example, the so-called "blue haze" is a well-known phenomenon in the exhaust gas. These are aerosols of dust with condensing VOCs, particularly terpenes. It is also generally known that particleboard and OSB emit VOCs indoors.

[0004] Such emissions include primary emissions of substances contained directly in the products. Primary emissions can include, for example, ingredients of the wood, such as terpenes, e.g. alpha-pinene, delta-3-carene, but also formaldehyde, for example from the glue it contains, or organic acids, if, for example, finished boards are intended. In addition to primary emissions, however, secondary emissions also occur, i.e. emissions of reaction products of the substances contained in the product, for example through reactions in the finished, possibly already used product or through reactions in the indoor air between primary emissions and other airborne compounds, such as the reactive species ozone, hydroxyl radicals, nitrogen oxides or sulfur dioxide. One example is the oxidation reactions of terpenes with ozone. The reaction of the monoterpene limonene with ozone can, for example, produce formic and acetic acid.

[0005] It is also known that wood contains fats or fatty acids. These are gradually reduced to aldehydes. The main substances here are hexanoic acid and hexanal.

[0006] Secondary emissions can therefore consist of secondary components, such as aldehydes from the degradation of fats or fatty acids contained in wood, for example, hexanoic acid from triglycerides, which can react to form hexanal. Related substances include formaldehyde from formic acid, acetaldehyde from acetic acid, etc.

[0007] In the production of particleboard, OSB, pallets, lumber, or plywood, for example, the manufacturing process is essentially "dry." Unlike the production of MDF / HDF, for example, processing takes place without contact with water or steam. For particleboard and OSB, high temperatures must be used during drying due to the desired low residual moisture content or target moisture content. This results in high VOC emissions.

[0008] The production of wood products therefore still offers potential for improvement.

[0009] The object of the present invention is to provide a solution that can at least partially overcome at least one disadvantage of the prior art. In particular, the object of the present invention is to provide a solution that allows for particularly efficient removal of VOCs present in the wood.

[0010] The object is achieved according to the invention by a method for drying wood products having the features of claim 1. Preferred embodiments of the invention are specified in the subclaims and the following description, which may each individually or in combination represent an aspect of the invention.

[0011] A method for drying wood products is described, the method comprising the following process steps: a) providing a wood product, b) drying the wood product by thermal treatment; wherein

[0012] Process step b) is carried out in two stages and comprises at least the following steps: b1) optionally adding steam to the wood product and drying the wood product while removing a first amount of steam, in particular by means of partial steam distillation; and b2) drying the wood product while removing a second amount of steam up to a predeterminable target moisture content, wherein in process step b1) water steam is optionally added in a predefined first quantity range and removed in a predefined second quantity range such that a lower limit and an upper limit of at least one of the first quantity range and the second quantity range are selected depending on at least one specification of the wood product provided in process step a).

[0013] Such a process offers a particularly advantageous way to efficiently reduce environmentally and / or health-damaging VOC emissions during the production and / or application of wood products. This allows for low-emission further processing and application of the wood products, as well as for trouble-free utilization of the VOCs removed from the wood.

[0014] For the purposes of the present invention, the term VOC (Volatile Organic Compounds) refers in particular to those volatile compounds that are present in the wood that serves as the starting material for the process described here. In particular, the VOCs described in this process are terpenes, which occur in the wood as wood oil. Examples include the following substances, which may occur in the weight percentages stated in parentheses based on the VOCs contained: α-pinene (20-70%), β-pinene (5-20%), limonene (1-5%), camphene (1-5%), phenol (0.2-2%). Other components may include myrcene, α-, β-phellandrene, 3-carene, cymene / cymene, terpinol, ocimene. Furthermore, VOCs can be understood to mean organic acids, present as free acids or bound as fats, i.e. typically as triglycerides, such as formic acid, acetic acid, hexanoic acid, etc.For the purposes of the present invention, VOCs also include the corresponding subgroups, in particular VVOCs (very volatile organic compounds), SVOCs (semi-volatile organic compounds), and MVOCs (microbially volatile organic compounds). The process described here is used to produce dried wood-containing products, also referred to as wood products, and in particular to dry wood-containing products. A product can be understood to mean an end product or an intermediate product that still requires further processing into a final product. Furthermore, the term wood-containing product or wood product means that the product comprises wood at least in part or consists solely of wood. Examples include WPC products (wood plastic composite).

[0015] According to process step a), the process described here comprises providing a wood product. As described above, the wood products within the meaning of the invention can be intermediate products or end products. Examples include OSB boards, particle boards, pallets, flexboard products, i.e., insulation mats and insulation rolls, or even pellets or sawn timber. Fiberboards produced using a dry process, for example, with a bulk density of less than 650 kg / m 3 , which is available as lightweight MDF (LDF) or with a density of less than 550 kg / m 3can be used. In principle, the wood products can include products produced using a dry process. In contrast to a wet process, which is used to produce classic MDF boards, for example, a dry process can be understood as one in which no water is added during production. A dry process within the meaning of the present invention is therefore to be understood in particular as a process in which the wood product, after being provided in the form of lumpy parts or particles which are smaller than the log or sections of the log, for example wood chips, shavings, strands or sawn wood, is not further treated with liquid or vaporous water, as is typically the case in the production of MDF / HDF when cooking at elevated temperatures before the refiner.However, the addition of glue or paraffin emulsion in aqueous suspensions or emulsions is excluded from this treatment. However, a dry process does not involve suspending wood chips in liquid water or heating lumps with water or steam.

[0016] The wood used is not fundamentally limited; for example, wood selected from pine, spruce, larch, birch, beech, dead oak, alder, etc. can be used, but is not limited to these. Although the wood product was produced in a drying process and thus generally has a comparatively low dry matter content, a drying step is usually provided due to the desired very low final moisture content. Such a drying step is implemented in the process described here according to process step b). In particular, process step b) comprises drying the wood product by thermal treatment, i.e., by subjecting it to temperatures elevated above room temperature. For this purpose, the wood product can generally be treated in a dryer.

[0017] More specifically, the drying process, or process step b), according to the present invention, is carried out in two stages and comprises process steps b1) and b2). Preferably, process steps b1) and b2) can be carried out in two consecutive, for example, directly consecutive, drying devices or separate drying areas. For example, known drum dryers, for example with built-in coiled tubing, can be used as drying devices.

[0018] When using chipboard, for example, a partial steam distillation can be carried out as step bl) in a knife ring chipper, during pneumatic conveying to the dryer, in a screw apparatus, in a belt dryer, in a preferably small drum dryer or similar.

[0019] According to process step bl), a first drying step optionally comprises adding steam to the wood product and drying the wood product while removing a first amount of steam. In principle, the first amount of steam can be applied with substantial exclusion of air by evaporating the partial amount of steam based on the water contained in the wood product. In other words, the water used for process step bl) can originate entirely from the wood to be dried, which, as described in more detail below, can be determined depending on at least one specification of the wood product provided in process step a). Thus, it can be advantageous that no steam is added in process step bl) or that the wood is not moistened before or during process step bl).

[0020] In this step, steam distillation or partial steam distillation (WTD) can be carried out by removing a defined amount of steam. This process step can enable the effective removal of VOCs, such as terpenes, from the wood product. This is because it can be exploited that VOCs form a heteroazeotrope with water and can therefore be removed from the wood relatively easily and at low temperatures. For example, the boiling point of alpha-pinene, an important terpene, drops from 155°C to approx. 95°C when mixed with water at a pressure of p=1 bar, forming a heteroazeotrope. The same is also possible with hexanoic acid (caprylic acid) and all high-boiling, non-polar substances such as hexanal, oils / fats or fatty acids. The lead substance here is hexanoic acid, which is found in linseed oil, for example.

[0021] It is provided that in process step bl) water vapor is optionally added in a predetermined first quantity range and removed in a predetermined second quantity range such that a lower limit and an upper limit of at least one of the first quantity range and the second quantity range are selected depending on at least one specification of the wood product provided in process step a).

[0022] As described above, the invention is based in particular on the fact that by separating steam from the process according to process step bl), VOCs can be efficiently separated from the production stream, since these accumulate in the steam from step bl). Thus, appropriate steam separation reduces the emission of VOCs, for example, as exhaust gases or as vapors from the manufactured product.

[0023] The process according to the invention allows significant advantages to be achieved, particularly for the separation of VOCs from wood, as will be explained in the following considerations.

[0024] According to the current state of the art, the entire wood is usually brought to boiling temperature for drying to remove the water from the wood. Examples of boiling conditions include approximately 1 bar at 100°C. According to the current state of the art, this is done down to a specified target moisture content without considering the VOCs contained. Assuming an exemplary water content of 50 wt.% of the wood and heating from 20°C to 100°C, a heat quantity of approximately 1,406,000 kJ must be applied, for example using a contact dryer or a convection dryer. However, the VOCs expelled in this way are very diluted, making it economically difficult to isolate them or use them for other purposes.

[0025] In contrast, in accordance with process step bl), in a first step only the VOCs can be removed without significantly drying the wood, which in turn can be achieved by heating the wood to 100 °C while evaporating minute amounts of water, approximately 1% of the total water content of the wood. An exemplary heat quantity of 22,000 kJ under comparable conditions is sufficient to remove essentially all VOCs. This exploits the fact that step bl) can be completed after the VOCs have been removed regardless of the residual moisture content of the wood. This demonstrates that even with a very low-energy dryer that also requires very little installation space, VOCs can be removed from the wood, which also produces a highly concentrated amount of steam with regard to VOCs.

[0026] Heating the wood to remove VOCs according to process step bl) can be carried out in various ways, such as applying heat through contact, preferably with substantial exclusion of air. Wood oil can then be extracted particularly preferably using steam. The latter is also possible by heating it with the addition of steam. Superheated steam, for example at temperatures of 140°C, can be used, although avoiding condensation may be preferable. Heating is also possible by adding warm air, for example at temperatures of 160°C. Condensation can also be used to extract wood oil.

[0027] Surprisingly, it has been shown that it is not necessary to constantly separate large quantities of steam from the process in order to achieve a significant reduction in VOC emissions. In fact, it has been found that almost the entire amount of VOCs, particularly terpenes, can be removed by separating comparatively small quantities of steam. This can significantly reduce the amount of steam removed and therefore, for example, the amount of steam that needs to be further processed. This can reduce the effort and costs of the overall process. The same applies to the amount of steam fed in, as this should be sufficient to effectively remove the VOCs, but as low as possible in order to limit the size of the dryer and the energy requirement for step bl) and bl).when condensing at least part of the water vapor supplied in step b1), the energy expenditure in step b2) is also kept as low as possible.

[0028] The process described here further exploits the fact that although terpenes, the most important VOC in this process, have a boiling point above 150°C, it has nevertheless been found that even evaporation streams or, in general, steam streams with temperatures below 100°C can contain very significant amounts of volatile organic substances, particularly terpenes. This is due to the formation of heteroazeotropes as described above. Therefore, it is advantageous in the process described here to focus on the total amount of steam separated, regardless of its origin or the local separation point.

[0029] The separation of steam streams can generally be carried out using state-of-the-art methods, and it is advantageous for the steam to be treated to collect the VOCs and not released into the environment along with the VOCs. For example, the steam can be separated using positive or negative pressure.

[0030] The fact that in process step bl) water vapor is optionally added in a predetermined first quantity range and removed in a predetermined second quantity range such that a lower limit and an upper limit of at least one of the first quantity range and the second quantity range are selected depending on at least one specification of the wood product provided in process step a) can be implemented in a variety of ways, as described in more detail below.

[0031] After the first drying step, i.e., process step b1), in which the VOCs were essentially removed from the wood of the wood products, a further drying step follows according to process step b2), namely the drying of the wood product with the removal of a second amount of steam until a predefined target moisture content is reached. The target moisture content is the moisture content that can or should be achieved in the wood product through drying.

[0032] This second drying step can be carried out in the manner generally known for such wood products to bring the wood products to the desired residual moisture content. In particular, this step can also be carried out using heat or a conventional technical drying device.

[0033] Through this two-stage drying process, consisting of steps b1) and b2), the VOC content can be significantly reduced, initially through partial steam distillation. This, of course, makes it possible to significantly reduce emissions from the dried wood products, both indoors and in subsequent process steps. Process step b1) can be tailored to the removal of VOCs, thus combining low energy consumption with efficient VOC removal.

[0034] In step b2), drying can then be carried out in the usual way in order to achieve the desired final moisture content.

[0035] By designing the drying process as a two-stage process, a particularly simple implementation into existing processes is also enabled, since the partial steam distillation according to step b1) can be easily inserted before the usual step b2). This can be achieved, for example, by inserting an additional dryer before the actual dryer in conventional processes. The additional dryer can be very small in size, for example in the range of less than or equal to 5 vol.% drying volume compared to a conventional dryer. This is because the residence time of the wood in the dryer required for step b1) is extremely short, so that a large amount of VOCs can be removed very quickly. Furthermore, even a very small amount of energy is sufficient to remove the VOCs.

[0036] Furthermore, by performing step b1) which is spatially separated from process b2), the VOCs can be collected particularly easily, with the collected vapor quantity being very small, particularly in comparison to the possibility of collecting the total vapor quantity generated during the entire drying process. Efficient collection of VOCs from the vapor stream of a conventional dryer is practically impossible. According to the invention, however, it is easily possible to collect the discharged VOCs and then reuse them. Furthermore, the concentration of VOCs in the vapor quantity separated in step b1) is comparatively high, thus simplifying isolation. However, within the meaning of the present invention, it is not excluded that steps b1) and b2) are carried out successively in a dryer.Finally, the process can be carried out with particularly low energy consumption and thus in a resource-saving manner, since the formation of a heteroazeotrope as described above requires only a small amount of energy to evaporate the VOCs and remove them from the wood.

[0037] Depending on the type of heating, internal heat recovery can of course be achieved, e.g. by cooling exhaust air or exhaust steam.

[0038] The separation of the steam, and thereby the process steps b1) and b2), can preferably be carried out continuously. Continuous separation of the steam includes, for example, uninterrupted separation or continuous periodic separation, i.e., comprehensively definable periodically recurring pauses.

[0039] The present process also offers advantages over prior art processes. For example, a process known as UTWS drying or eco dry is known, in which unwanted substances are removed using high energy. However, the entire drying process is carried out using superheated steam. In this case, the condensation process is impeded. Retrofitting existing plants is rarely technically possible and practically impractical from a commercial perspective. In contrast, the process according to the invention allows significantly shorter residence times in the dryer for step b1), lower separated gas quantities during VOC separation, and improved condensation conditions. Drying with superheated steam is known from the production of pellets, for example.However, similar to the UWTS process, the entire drying process is carried out with superheated steam, which also results in the above-described advantages compared to this process.

[0040] With regard to the at least one specification of the wood products, it should be mentioned that only one specification can serve as the basis for determining the amount of steam to be added and / or separated, or that preferably a plurality of specifications can serve as the basis for determining the amount of steam to be added and / or separated.

[0041] For example, one specification or a plurality of specifications can be selected from the following specifications. Preferably, a lower limit and an upper limit of the quantity range of at least one of the steam optionally added in process step b1) and the steam separated in process step b1), and thus a lower limit and an upper limit of at least one of the first and second quantity ranges, can be determined depending on the amount of wood provided in process step a) in the wood product. The amount of both the wood and the steam can be the absolute amount, for example in a batch process, or the amount of both the wood and the steam can be the amount per unit of time in a continuous process.It is understandable that, regardless of the specific design and components of the wood product, the amount of wood has a significant influence on the VOCs introduced into the process by the wood and thus equally on the VOCs to be removed, so that the amount of wood in the wood product should preferably be taken into account when determining the amount of vapor to be separated.

[0042] The amount of wood can be equally important for the amount of water vapor introduced and the amount of water vapor removed. This ensures that, on the one hand, the VOCs are removed as completely as possible, while, on the other hand, avoiding the need to remove excessive amounts of unused water vapor introduced in step b2) and b1).

[0043] It may further be preferred that a lower limit and an upper limit of the quantity range of at least one of the water vapor added in process step bl) and the water vapor separated in process step bl), and thus a lower limit and an upper limit of at least one of the first and the second quantity range, are determined depending on the amount of VOCs, in particular terpenes and / or fatty acids or fats, contained in the wood product provided in process step a).

[0044] In this embodiment, it is possible, in particular, to determine or estimate the amount of VOCs, such as terpenes and / or fatty acids or fats, contained in the wood product per quantity of wood product. In other words, the amount of VOCs present in the wood product can be considered in weight percent, based on the amount of wood product. This specification can be particularly advantageous since it has been shown that different wood species also contain different amounts of VOCs. Accordingly, the amount of VOCs present in a given amount of wood can depend on the specific wood species used.

[0045] By selecting such specifications, the amount of steam that may need to be introduced and / or separated can be reduced particularly reliably, as it ensures that fluctuations in the components present during steam separation do not result in too little steam being separated, thus resulting in an undesirably high VOC content. Furthermore, the amount of steam that needs to be separated and possibly produced can still be reduced reliably and without the aforementioned risks.

[0046] With regard to the determination of the amount of VOCs contained in the wood product, it may further be preferred that the amount of VOCs contained in the wood product provided in process step a) is determined by examining the wood product used or is estimated based on the type of wood product used, in particular the wood contained.

[0047] Determining the amount of VOCs by examining the wood product can enable a particularly precise determination of the VOCs contained in the wood product, so that the amount of vapor to be introduced and / or removed can also be determined very precisely. The respective amount can be determined in a conventional manner by analyzing the components of the wood product. This can be advantageous, for example, because the VOC content can simply be reduced due to evaporation during storage, or because fluctuations in the VOC content can occur within the same wood species.

[0048] Estimating the VOC components contained in the wood product based on the type of wood product used, particularly considering the type of wood used in the wood product, can allow for particularly simple determination of the components, while keeping the effort required to a minimum. This approach can be based, in particular, on the fact that different types of wood, such as birch or spruce, often contain different amounts of VOCs, but the amount contained is characteristic of the type of wood. Thus, with knowledge of the type of wood used, the amount of VOCs can be estimated in advance without the need for analytical testing.

[0049] To ensure that potentially occurring inaccuracies in the amount of steam in the respective wood are not critical, the amount of steam added and / or removed can be determined with a definable safety factor, i.e., a definably larger amount of steam can be added and / or removed than required according to the data used for the quantity ranges. This also allows for a particularly safe and reliable reduction in the amount of VOCs and / or fatty acids released from the process.

[0050] It may further be preferred that the total amount of steam separated in process step bl) is in a range of 0.5 to 100 times the mass, preferably 0.5 to 50 times the mass, particularly preferably 0.5 to 10 times the mass, based on the amount of VOC in the wood product provided.

[0051] The amount of steam to be added can be within a similar range to the amount of steam removed. However, this depends on the process and the type of heat input. For example, it is conceivable to heat the wood product with extensive or (almost) complete exclusion of air, converting only the water contained in the wood into a vapor state. Alternatively, heating can be achieved using saturated or superheated steam. If the latter is the case, the amount of steam to be added is correspondingly higher. It follows that the amount of steam to be added is in a range of 0-300 times the mass, preferably 0-150 times the mass, particularly preferably 0-30 times the mass, based on the VOC content of the wood product provided.

[0052] This amount is significantly lower than the amount of steam that would be separated if the wood product were dried to its final moisture content in a single drying step, but is surprisingly sufficient to remove essentially the entire amount of VOCs or fatty acids from the wood product and thus significantly reduce VOC emissions in the process described here. Thus, it has been shown that, for example, if the process described here or the amount of steam separated in the process is adjusted accordingly, a surprisingly small amount of steam can be separated, which is sufficient to achieve the object of the invention.

[0053] Alternatively or additionally, it may be preferred that the total amount of steam separated in process step bl) is in a range of 0.001 to 0.2 times the mass, preferably 0.001 to 0.1 times the mass, particularly preferably 0.001 to 0.02 times the mass, based on the dry mass of the wood product provided.

[0054] The quantity of steam introduced is in a range of 0-0.6 times the mass, preferably 0-0.3 times the mass, particularly preferably 0-0.1 times the mass, based on the dry mass of the wood product provided.

[0055] This design is based on the fact that the dry mass of the wood product can also be a good indicator for determining the amount of steam to be introduced and / or separated. Even with such a correlation, the amount of steam to be separated is significantly lower than the amount used in state-of-the-art solutions for single-step drying, but is surprisingly sufficient to remove almost the entire amount of VOCs from the process and thus significantly reduce VOC emissions in the process for manufacturing wood products described here.

[0056] The dry mass of the wood product or wood refers in particular to absolutely dry wood (dry dry), as is common in wood processing. The dry mass of the wood used can be determined analytically or estimated based on known data for the type of wood used. Furthermore, the mass can be easily determined in continuous or batch processes, either as a quantity per unit of time or as an absolute quantity, as described above.

[0057] Purely by way of example, such small quantities in process step bl) enable residence times in a dryer of a few seconds, for example less than 10 seconds. This is possible because, for example, drying by only a few percent, for example by 3%, is sufficient. This also allows the use of corresponding dryers with a small volume. This demonstrates the problem-free implementation in existing processes with small installation sizes and thus correspondingly low investment costs. It may further be preferred for process step bl) to take place at least partially under exclusion of air. In this embodiment, it can be exploited accordingly that the amount of water present in the wood can be sufficient as a carrier substance for the VOCs.

[0058] With regard to the separation of water vapor in process step b1), it may further be preferred that water vapor discharged in process step b1) be continuously separated from the process at at least one vapor emission point. This allows for a particularly efficient process and can be carried out under steady-state conditions without significant control effort.

[0059] It may further be preferred that the VOC-containing water vapor removed according to process step b1) be collected and, if appropriate, one or more components be further treated. In this embodiment, the process can thus not only serve to reduce VOC emissions, but the process can also be carried out significantly more economically due to the possibility of collecting separated vapor streams and, if appropriate, further treating them. This is because the materials contained in the vapor stream or other properties of the vapor stream, such as its heat, can be used in the process or other processes, thus saving costs and resources.

[0060] For example, it may be advantageous to isolate a mixture of terpenes or turpentine oil as a further treatment step. Additionally or alternatively, it may be advantageous to isolate a mixture of fatty acids or other organic substances as a further treatment step. While such substances should be reduced as process emissions to prevent release into the environment, these substances can be valuable products for other processes or applications. The same applies, of course, if the individual substances are further isolated. This embodiment can therefore be particularly advantageous with regard to the economics of the process described here and with regard to the added value of the wood used. The same applies if, for example, a hydrolate is isolated as a further treatment step.For the purposes of the invention, hydrolate is generally understood to mean the aqueous phase obtained after condensation of the vapor and separation of water-insoluble substances, which may contain correspondingly water-soluble components, such as formaldehyde or short-chain organic acids or the like. It may also be advantageous for the separated vapor or one or more components to be further treated by combustion or exposure to high temperatures, adsorption, absorption, membrane technology methods, condensation, crystallization or other suitable process engineering methods.

[0061] Combustion or exposure to high temperatures, for example, enables thermal afterburning and, in doing so, the energetic utilization of the VOCs contained in the separated vapor stream. The other methods mentioned can all relate to the isolation or separation of individual substances.

[0062] More preferably, at least one of the heat of a material stream occurring in the process and the heat of a separated vapor stream can be reused energetically in the process. In this embodiment, the energy inherent in the material stream can thus be reused in the form of heat, in particular to heat other material streams or to carry out the drying process. This step can also improve economic aspects of the process according to the invention and thus conserve costs and resources.

[0063] Further preferably, it can be provided that the drying in the first drying stage takes place at a temperature of > 100°C, preferably > 125°C, more preferably > 150°C. Surprisingly, it has been found that at this temperature, the VOCs to be removed escape from the wood material to a greater extent. This makes it possible to keep the residence time in the first drying stage short and to remove the vast majority of the VOCs in the process. The residence time in the first drying stage can, for example, be in a range between preferably > 0.5% to < 30% of the total drying time, more preferably between > 1.5% to < 25%, even more preferably between > 3% to < 20% of the total drying time.

[0064] Further preferably, it can be provided that the drying in the first drying stage takes place in recirculation mode, in which the released vapors essentially remain in the drying device. This makes it possible to obtain a drying atmosphere with a high water vapor content, in which the VOCs can be expelled from the wood material together with water in a heteroazeotrope. Further preferably, it can be provided that the proportion of fresh air in recirculation mode is <20%, preferably <10%, based on the total volume of the drying device in the first drying stage.

[0065] In a further embodiment of the process, it can be provided that the supply and / or removal of the wood products into and from the first drying stage takes place via a substantially airtight lock. This advantageously ensures that the drying atmosphere in the first drying stage optimally supports VOC release from the wood-based material.

[0066] The invention will now be explained by way of example with reference to the accompanying drawings using preferred embodiments, wherein the features presented below may represent an aspect of the invention both individually and in combination. It shows:

[0067] Fig. 1 is a schematic representation of a method according to the present invention.

[0068] Figure 1 schematically shows a method according to an embodiment of the present invention.

[0069] Arrow 10 illustrates that a wood product is being prepared. The wood product is formed, in particular, by a drying process and is to be further dried. Two dryers 12, 14 are provided for this purpose. Drying thus takes place in two stages, with water vapor optionally being added to the wood product in a first quantity in the dryer 12, which is to be represented by arrow 16, and with the wood product being dried while a second quantity of water vapor is removed. The latter is to be represented by arrow 18. As a result, heteroazeotropes are formed from the VOCs present in the wood product and the water, which are carried away with the water vapor.In this case, water vapor is optionally added in a predetermined first quantity range and removed in a predetermined second quantity range such that a lower limit and an upper limit of at least one of the first quantity range and the second quantity range are selected depending on at least one specification of the wood product provided.

[0070] From the first dryer 12, the partially dried and VOC-depleted wood product is conveyed to the second dryer 14, as indicated by arrow 20. The second dryer 14 dries the wood product while removing a second amount of steam until it reaches a predeterminable target moisture content, which is indicated by arrow 22. After the dryer 14, the wood product, brought to its final moisture content, is provided, as indicated by arrow 24.

[0071] After the wood product has been prepared with final moisture content, the wood product can be finished or further processed.

[0072] Furthermore, the discharged water vapor containing VOCs can be collected and, if necessary, one or more components can be further treated.

[0073] Examples 1 :

[0074] 1 t of pinewood chips (dry matter, dry) with a wood moisture content (load, u) of u=85% is dried in two stages. In the first drying stage, 20% of the initial moisture content is removed, starting from a load of u=85% as the specification to be considered, so that the load after the first drying stage is u=65%. In a further drying stage, the load of u=65% achieved in the first drying stage is dried to the target moisture content of u=3%. The vapors from the first drying stage are fed to an incinerator. At the exhaust stack of the second drying stage, a VOC concentration of 132 mg / m3 measured, whereas a direct one-stage drying to a target moisture content of u=3% resulted in a VOC concentration of 406 mg / m 3 was measured at the exhaust chimney of the drying stage.

[0075] Example 2:

[0076] 1 t of pinewood chips (dry matter, dry) with a wood moisture content (load, u) of u=82% is dried in two stages. In the first drying stage, 0.15 times the dry matter content of the pinewood chips is removed in moisture (water). In a second drying stage, the dry matter achieved in the first drying stage is dried to the target moisture content. The untreated chips have a terpene content of 3.2 kg / t dry. After the first drying stage, a terpene content of 1.36 kg / t dry is measured.

[0077] Example 3:

[0078] One ton of pinewood chips (dry matter, dry) with a wood moisture content (load, u) of u=82% is dried in two stages. The untreated chips have a terpene content of 3.08 kg / t dry. In a first drying stage, 49 times the terpene content of the pinewood chips, i.e., 3.08*49 = 150.9 kg / t water, is removed. Drying then takes place to the target moisture content. After the first drying stage, a terpene content of 1.36 kg / t dry is measured, which corresponds to a reduction in the terpene content of approximately 56 wt.% relative to the total terpene content of the untreated chips.

[0079] Example 4:

[0080] II Pinewood chips (dry matter, dry) with a moisture content of 79% and a terpene content of 3.08 kg / t (dry matter) are dried in a drum dryer in the absence of fresh air at a temperature of 124°C to a dry matter content of 61%. The drum dryer is a pure contact dryer. The resulting vapors are condensed. After drying to a dry matter content of 61%, the chips have a reduced terpene content of 0.56 kg / t (dry matter). This corresponds to a terpene reduction of approximately 82% by weight. Example 5:

[0081] 1 1 Pinewood chips (dry mass abs.) with a moisture content u=79% and a terpene content of 3.08 kg / t abs. are dried in a drum dryer with the exclusion of fresh air at a temperature of 124°C to a moisture content of u=61%. The drum dryer is a pure contact dryer. The resulting vapors are condensed. After drying, the chips have a reduced terpene content of 0.56 kg / t abs. Due to the induction of "false air" (fresh air), the vapors contain 18.5 wt.% air and 91.5 wt.% water, respectively, and thus a dew point of 96°C. By cooling these vapors to 50°C, a water content in the air of 8 wt.% is adjusted. The organic components contained in the vapors are also condensed. A large part of this can be separated out as the organic light phase.A subsequent analysis of the organic phase shows that it contains the terpenes alpha-pinene, beta-pinene, delta-3-carene, and limonene, as well as hexanoic acid (caproic acid), caproaldehyde, and linolenic acid. A corresponding analysis of the aqueous phase (after separation of the organic phase) showed that it contains alpha-terpineol, beta-terpineol, vanillin, coniferylaldehyde, and C4, C6, C8, and C16 fatty acid methyl esters. The vapors are cooled in countercurrent with fresh air. The used fresh air can be used as supply air for a second drying stage to utilize the thermal energy content of the vapors to preheat the fresh air.

[0082] Example 6:

[0083] An additional dryer is being retrofitted to an existing chip drying plant. The additional dryer is installed upstream of the existing plant. In it, chips are dried from a moisture content of u = 91% to a moisture content of u = 81% and then dried in the second drying stage (main dryer) to a moisture content of u = 3%. The heat requirement for heating and evaporating the corresponding first portion amounts to 25% of the total heat requirement. Since the second stage primarily removes capillary moisture and the material being dried is hygroscopic, the first drying stage (bl) requires a residence time of only 5% compared to the second drying stage (b.2). While the dryer in stage 2 (main dryer) operates with a residence time of 1225 seconds (approximately 20 minutes), a residence time of 58 seconds is sufficient for the first drying stage. Accordingly, the required size is also correspondingly smaller.

[0084] Example 7:

[0085] An additional dryer is being retrofitted to an existing chip drying plant. The retrofitted dryer will be installed upstream of the existing dryer. This retrofitted dryer will be designed as a directly heated drum dryer with a gas inlet temperature of 298°C. Its size is approximately 5.1 vol% of the main dryer (existing dryer). The vapors discharged from the retrofitted dryer will be fed into the combustion process.

[0086] Example 8:

[0087] An additional dryer is being retrofitted to an existing chip drying plant. The retrofitted dryer will be installed upstream of the existing dryer. This retrofitted dryer will be designed as an indirectly heated drum dryer with a heating temperature of 190°C as a contact dryer. Its size is approximately 6.2 vol% of the main dryer (existing dryer). The vapors discharged from the retrofitted dryer will be fed into the combustion plant.

[0088] Example 9:

[0089] An additional dryer is being retrofitted to an existing chip drying plant. The retrofitted dryer is installed upstream of the existing dryer. The retrofitted dryer is designed as an indirectly heated drum dryer with a heating temperature of 185°C as a contact dryer. The vapors discharged from this dryer are returned to a mixing chamber. The water content of the vapors is 65.8%. Thus, the dew point of these vapors is 92°C. The size of the dryer is approximately 5.9 vol.% of the main dryer (existing dryer). The vapors discharged from the retrofitted dryer are condensed at 50°C. The resulting condensate is used as fuel to heat the air supplied to the main dryer (existing dryer).

[0090] Example 10:

[0091] An additional dryer is being retrofitted to an existing chip drying plant. The retrofitted dryer will be installed upstream of the existing dryer as an indirectly heated belt dryer with a heating temperature of 120°C and a convection dryer. The vapors discharged from this retrofitted dryer are returned to a mixing chamber. The water content of the vapors is 65.8%. The dew point of these vapors is therefore 92°C. The vapors are condensed at 50°C.

[0092] Example 11 :

[0093] An additional dryer is being retrofitted to an existing plant for drying OSB strands (coarse chips). The retrofitted dryer will be installed upstream of the existing dryer and will be an indirectly heated belt dryer with a heating temperature of 90°C as a convection dryer. The vapors discharged from this dryer are returned to a mixing chamber. The water content of the vapors is 45.3%. Thus, the dew point of these vapors is 85°C. The vapors removed from the process are incinerated.

[0094] Example 12: A discontinuous chamber dryer for pine lumber is operated for a period of 36 hours. Drying takes place in a circulating air drying chamber at a maximum temperature of 85°C. An additional drying stage is added at the beginning of the drying plan. In this stage, the lumber is first heated to 85°C with largely recirculated air. So little vapor is removed that the relative humidity is maintained at almost 100%. Thus, the vapors have a water content of 45.9%. During this time, the wood moisture content drops from u=85% to u=71%. The removed vapors are further treated and either condensed or incinerated. The terpene content of the sawn wood decreases from 3.1 kg / t dry to 0.5 kg / t dry. The duration of the first (newly planned) drying stage is 190 minutes, or 8.8% of the total drying time (drying schedule).After the newly introduced drying stage, the drying plan is completed as usual. The resulting vapors are discharged untreated into the environment. They have a VOC content reduced by 72% by weight.

[0095] List of reference symbols Arrow Dryer Dryer Arrow Arrow Arrow Arrow Arrow

Claims

Patent claims 1. A method for drying wood products, the method comprising the following steps: a) providing a wood product, b) drying the wood product by thermal treatment; wherein Process step b) is carried out in two stages and comprises at least the following steps: b1) optionally adding steam to the wood product and drying the wood product while removing a first amount of steam; and b2) drying the wood product while removing a second amount of steam up to a predeterminable target moisture content, wherein in process step b1) water steam is optionally added in a predefined first quantity range and removed in a predefined second quantity range such that a lower limit and an upper limit of at least one of the first quantity range and the second quantity range are selected depending on at least one specification of the wood product provided in process step a).

2. Method according to claim 1, characterized in that a lower limit and an upper limit of at least one of the first and second quantity ranges are determined as a function of the amount of wood provided in method step a) in the wood products.

3. Method according to claim 1 or 2, characterized in that a lower limit and an upper limit of at least one of the first and second quantity ranges are determined as a function of the amount of VOCs contained in the wood product provided in method step a).

4. The method according to claim 3, characterized in that the amount of VOCs contained in the wood product provided in process step a) is determined by examining the wood product used or is estimated based on the type of wood product used, in particular the wood contained.

5. Process according to one of claims 1 to 4, characterized in that the total amount of steam removed in process step bl) is in a range of 0.5 to 100 times the mass, based on the amount of VOC in the wood product provided.

6. Process according to one of claims 1 to 5, characterized in that the total amount of steam separated in process step bl) is in a quantity range of 0.001 to 0.2 times the mass, based on the dry mass of the wood product provided.

7. Process according to one of process steps 1 to 6, characterized in that no steam is added in process step bl).

8. A process according to any one of claims 3 to 7, characterized in that at least one of terpenes and fatty acids is considered as VOCs.

9. Process according to one of claims 1 to 8, characterized in that process step bl) is carried out at least partially under exclusion of air.

10. Process according to one of claims 1 to 9, characterized in that in process step bl) water vapor discharged is continuously separated from the process at at least one vapor emission point.

11. Process according to one of claims 1 to 10, characterized in that the VOC-containing water vapor removed according to process step bl) is collected and, if appropriate, one or more components are further treated.

12. Process according to claim 11, characterized in that a mixture of terpenes or turpentine oil is isolated as a further treatment.

13. Process according to one of claims 11 or 12, characterized in that a mixture of fatty acids or other organic substances is isolated as a further treatment.

14. Process according to one of claims 11 to 13, characterized in that a hydrolate is isolated as a further treatment.

15. A process according to any one of claims 11 to 14, characterized in that the separated vapor or one or more components are further treated by combustion or exposure to high temperatures, adsorption, absorption, membrane technology methods, condensation or crystallization.

16. A process according to any one of claims 1 to 15, characterized in that at least one of the heat of a material stream occurring in the process and the heat of a separated steam stream is further used energetically in the process.

17. Process according to one of claims 1 to 16, characterized in that process steps b1) and b2) are carried out in two successive drying devices.

18. A method according to any one of claims 1 to 17, characterized in that the wood product is a wood-containing product produced by a dry process.

19. Process according to one of the preceding claims, wherein the drying in the first drying stage takes place at a temperature > 100°C, preferably > 125°, more preferably > 150°C.

20. A method according to any one of the preceding claims, wherein the drying in the first drying stage is carried out in recirculation mode, in which the released vapors remain substantially in the drying device.

21. The method according to claim 20, wherein the fresh air proportion in recirculation mode is < 20%, preferably < 10%, based on the total volume of the drying device of the first drying stage.

22. The method according to one of the preceding claims, wherein the supply and / or The wood products are exported to and from the first drying stage via an essentially airtight lock.

Images