A METHOD AND DEVICE FOR MEASURING MOISTURE CONTENT DURING OPTIMISING OF A DRYING PROCESS

Abstract: A method for measuring the moisture content during drying of an organic material, by which the moisture content in the material is determined based on the dielectric properties of the material, and by which electromagnetic radiation is emitted towards a surface of the material, and the determination of the moisture content is based on the effect of the material on said electromagnetic radiation. A change of the effect of the material on the electromagnetic radiation versus time is detected, and data input to a mathematical model for determining the moisture content of the material during the drying is determined based on the amount of energy consumed for the evaporation of liquid in connection to the drying process during a given time period.

Description: A method and device for measuring moisture content during optimising of a drying process TECHNICAL FIELD The invention relates to a method of measuring the moisture content during drying of an organic material, in which the moisture content in the material is determined based on the dielectric properties of the material and in which electromagnetic radiation is emitted towards the surface of the material, and the determination of the moisture content is performed based on the effect that the material has on said electromagnetic radiation. The invention also relates to a device for measuring the moisture content during the drying of an organic material, said device being provided with means for determining the moisture content in the material based on the dielectric properties of the material, said means comprising means for emitting electromagnetic radiation towards a surface of the material, and the determination of the moisture content being executed based on the effect that the material has on said electromagnetic radiation. The invention is applicable to all types of drying by which the material that is to be dried has such dielectric properties that it will effect the electromagnetic radiation that is directed towards it. The invention is primarily intended for the drying of an organic material of biological origin, such as wood, peat or biomass. The invention primarily relates to measuring by means of non-contact methods in which the measurement is executed at one or more locations in a load of material. Preferably, the measurement comprises so-called bulk measurement for the purpose of determining one or more bulk parameters of the material in question. BACKGROUND OF THE INVENTION AND PRIOR ART In connection with the drying of organic material of biological origin, it is often of advantage to be able to determine or estimate the moisture content of the material continuously or continually during the drying process. As an example, drying of wood should be mentioned. Drying of wood is a process by which the process economy has to be optimised with regard to the wood quality. If the drying is too fast, the wood might crack and twist while, on the other hand, a slow drying is not desirable from an economical point of view. In order to optimise the drying process and to be able to produce wood of a high quality it is, therefore, desired that the moisture content and wood structure is determined. Wood structure is here referred to as the effect that growth has on the structure of wood, where a slow grow results in a compact wood structure and a rapid growth results in a more porous wood structure. The moisture content and the wood structure can be determined by using the effect that the dielectric properties of the wood have on the electromagnetic radiation, particularly a spectrum of radio frequency radiation. Moisture content and wood structure is often used for optimising the drying process. Either, a drying scheme is calculated with predetermined variations of moisture and temperature over time based on specific quality parameters (an important such parameter is the moisture content), or the determined quality parameters (moisture and wood structure) are used in order to predict a suitable moisture and temperature profile that is maintained during the drying process. The drying process is controlled in order to follow a predetermined profile while the moisture content in the wood is continually supervised or supervised by means of discrete measurement by means of radio frequency radiation, deviations between measured values and the predicted moisture content of the wood resulting in control signals used for controlling the process. Normally, the moisture and the wood structure are determined by means of measurements in the drying oven, the drying room or the autoclave, while an evaluating unit and a control unit may be comprised in a conventional process computer equipped with software that comprises those algorithms that are required for controlling the process. The evaluating unit may be adapted to process data input by means of multivariate analysis methods in order to produce a correlation between these data inputs and a quality parameter such as the moisture content of the material in question. The above-described way of measuring the moisture content by means of radio frequency radiation is described in the Swedish patent application No. 9700828-8 of the applicant, and the principles for the drying of wood that are disclosed in that application are directly applicable also to the present invention. From an economical and process technical point of view, it is desirable to be able to dry for as long as possible with an effect as large as possible in order to move over to drying with a lower effect just at the correct moment before the moisture content in the material has gone below a specific minimum value. In the case of drying of wood, the time for obtaining a so-called fibre saturation in the material and the required final moisture content plays a vital role as to how different drying parameters (primarily added heat) are to be controlled during the drying process. An optimisation of the control is desirable, and it is of substantial importance that the moisture content determinations that are continually performed and that form the basis of the control are correct and reflect the true conditions in the material well. According to prior art, another way of measuring the moisture content of wood is to measure the moisture content in the air in the drying room while, at the same time, the energy used for the drying is registered. However, this technique results in a pretty inexact indication of the moisture content in the material. Prior art that comprises contacting the material during the measurement has the disadvantage of only resulting in local measurement of less accuracy, while, at the same time, it requires further work operations in order to apply and remove the measuring units from the material. THE OBJECT OF THE INVENTION The object of the present invention is to provide a reliable method and an uncomplicated device for continuously or continually performing a determination of the moisture content of a material that is subjected to a drying operation in a drying process, and to optimise the control of the drying process based on such a determination. The measuring method must be a non-contact method and permit determination of the bulk moisture content of the material. SUMMARY OF THE INVENTION The object of the invention is achieved by means of the initially denned method, characterised in that a change of the effect of the electromagnetic radiation of the material versus time is detected and that data input to a mathematical model for the determination of the moisture content of the material during the drying is determined from the amount of energy consumed for evaporation of liquid in connection to the drying process during a given time period. By connecting the moisture content measured by means of the electromagnetic radiation to the amount of energy that has been used for the evaporation of a certain amount of liquid, a more accurate determination of the actual moisture content in the material is permitted. By taking into consideration the energy that has been used, better determination of the material properties can be made, which is utilised for the adjustment of the model that predicts the moisture content in the material during the remaining part of the process. Preferably, a plurality of measuring antennas is used. These antennas are positioned or directed for the measurement of differ- ent parts of the load of material in order to deliver a plurality of local deliveries and give an indication of the moisture condition in different parts of the load of material. Preferably, said data input comprises an adjustment parameter JP obtained in accordance with: JP = f(spectral data, Fkin, energy (ΔE) where FKm is the initial moisture ratio of the drying load. FKin is measured in a laboratory or is determined by means of a model built from a plurality (m) of spectral data measurements on a plurality of drying loads, and based on data input for moisture ratio established in laboratory measurements. The model for determining Fkin preferably has the following form: Fkin = f( spectral data (1-m), further data input) where further data input are related to at least one of wood type, drying load volume and wood dimension. The model into which the adjustment parameter JP is introduced as data input is a model of the type: Moisture ratio = f(spectral data, Fkin, JP, other input) where other input are related to at least any one of wood type, drying load volume, wood dimension, drying temperature and air flow in the drying chamber. Accordingly, this model gives the moisture ratio of the drying load, said moisture ratio forming data output that can then form data input in already existing control models for the control of one or more process parameters. By means of this model, that takes into consideration also the energy that has been used for a certain evaporation, the moisture ratio can be determined with several times better accuracy. Without taking the adjustment parameter JP into consideration, a relative accuracy of approximately +/-20% is typically achieved, while, by means of the above-mentioned model for the determination of JP and fine adjustment of this model, a relative accuracy of approximately +/-5% can be achieved. The proportion of free and bound water, respectively, changes as time passes. According to the invention, a model that is adjusted to the case of free water in the material is built. For this model, only the effect of the material on a certain part of the emitted frequency spectra needs to be taken into consideration, that is, the effect of the material on a specific, predetermined part of the frequency spectrum is used as data input in the model. Another model is built for the case of only, or nearly only, bound water in the material. Data input to this model primarily includes the effect of the material on another part of the emitted frequency spectrum and in the case of free water. The different parts of the spectrum may overlap each other, but the most important parts during the measurements of free and bound water are not identical. The parameters within parentheses in the above models form examples of preferred data input to each model, respectively, in the case of drying of timber. It should be realised that they can be supplemented with further material parameters if required. The time period mentioned is preferably a period during which generally all energy consumed for drying is also consumed for the evaporation of liquid present in the material. In normal cases in which the air humidity during that same period is generally constant, said time period is a period during which the temperature in the material is generally constant by a constant delivery of energy/ time unit. Said time period is before the point at which fibre saturation appears in the material. Preferably, said time period is within an initial portion of the time period within which generally all energy consumed for the drying is also consumed for the evaporation of liquid present in the material and/or during which the temperature in the material is generally constant when added energy/ time is constant. Preferably, the electromagnetic radiation comprises a known first frequency spectrum that is continuously or continually compared to a second frequency spectrum formed by the effect of the material on the first frequency spectrum. The first frequency spectrum essentially comprises wave lengths within the frequency range of 1-1000 mHz, preferably within the range of 1-500 MHz, and most preferably within the frenquency range of 10-250 MHz. According to one embodiment, the radiation reflected from the material is detected and compared to the radiation emitted towards the material in order to identify the effect of the material on the radiation or, more precisely, the absorption of the electromagnetic field by the material. Alternatively, or as a supplement, the radiation or electromagnetic field transmitted through the material is detected and compared to the emitted radiation in order to establish the effect of the material on the radiation. The material is of biological origin and preferably comprises wood, preferably intended to be used as building timber. The invention also relates to the initially defined device, which is characterised in that it comprises - means for detecting a change of the effect of the material on the electromagnetic radiation during time, and - means for establishing data input into a mathematical model for determining the moisture content of the material during the drying based on the amount of energy consumed for evaporation of liquid under a given time period during the drying process. Said data input corresponds to the adjustment parameter JP that has been discussed above and which is used for trimming the mathematical model for the determination of the moisture ratio. The means for establishing the adjustment parameter JP comprise relevant program sequences in a computer program stored on a memory and connected to a processor in which they are executed. Data output from the mathematical model described above are sent as data input to any device for the control of process parameters based on this data input. The means for emitting radiation towards the material comprises one or more antennas, each arranged to emit radiation of a known frequency spectrum towards the material. Preferably, the antennas are wide band antennas. Preferably, the antennas are arranged in order to enable measurement of the whole or parts of the drying volume. The means for determining the moisture content preferably comprises means for detecting the radiation reflected by or transmitted through the material, and a unit for continuous or continual comparison of differences between spectral data for each emitted frequency spectrum and spectral data for the corresponding reflected or transmitted frequency spectrum, and evaluation of these differences for the purpose of determining the moisture content of the material. These means preferably comprise electronic means for dielectric spectroscopy. The detecting means may be the same antennas as those that emitted the radiation, or separate antennas arranged for receiving, primarily, the transmitted radiation. Preferably, the antennas used have higher natural frequency than those frequencies that are within the used frequency spectrum. It should be realised that the term antenna should be regarded in a wide sense, and that it can comprise all types of elements, electrodes or the like from which electromagnetic radiation can be emitted and/ or received. It is commonly known that a plurality of parameters can be simultaneously determined by correlating a plurality of measured values to each other by means of algorithms, statistical models and data analysis methods such as Partial Least Square regression (PLS), principle component analysis (PCA), multi linear regression analysis (MLR) and neural networks. Normally, the models and algorithms that are used for the data analysis are created and calibrated by means of calibration and reference measurements that are performed for a plurality of reference samples with known quality-determining properties, quality parameters. These data analysis methods and other methods for multi variate data analysis by which the measured values are correlated against parameters in order to determine a plurality of independent parameter values from a relatively delimited number of values is common technique that is known per se and used within the area of chemistry, and in particular within the area of chemical analysis. Hereinafter in this application these methods will be referred to as "multivariate correlation", and a calibration based on such a method will be referred to as "multivariate calibration". The unit for establishing the mathematical model or models for controlling the drying process preferably comprises a neural network and/ or an electronic unit equipped with a software for, for example, Partial Least Square regression, PLS. The input to the multivariate correlation comprises the detected change of the moisture content in the material that is dried, and the amount of energy that has been consumed for the drying during the time period for which the difference in moisture content has been established. By means of this technique, a model is created for the moisture content and the change thereof in the material during the drying process. In the case of building timber, the time when fibre saturation can be expected to be obtained is determined as well as the final content of moisture in the wood. The process is then controlled with regard to this result. The models that the device is arranged to establish and to comprise have already been described above in connection with the disclosure of the inventive method. EXAMPLE Hereinafter, the invention will be described with reference to a preferred embodiment focused on the drying of building timber. In order to optimise the timber drying process it must be possible to measure the moisture content on line and with great accuracy, in particular in the case when the timber approaches fibre saturation. One must also be able to determine the final moisture content in the material, the final content. The measuring of the moisture content/ ratio is performed by means of dielectric spectroscopy in accordance with what has been described above and within the radio frequency range of 10-250 MHz. Absorption as well as reflection is measured when the electromagnetic field has been emitted by means of an antenna mounted inside the dryer. By the reflection measurement the reflected signal is measured by the emitting antenna, and by absorption measurement one or more receiving antennas are mounted inside the dryer. The antennas have a natural frequency that is above the frequencies of the primary measuring spectrum. The received frequency spectrum is further processed in accordance with the models described above, said models transforming spectral data to moisture content on line. A plurality of frequencies in the spectrum are used for this purpose. A plurality of RF measurements are performed separately during a certain time period in order to obtain one or more measurements of a change of moisture content in the material during a given time period. Also the amount of energy/ amount of heat that has been applied to the material or to the dryer device during a corresponding time period is registered. The measurement results are automatically introduced as data input to a computer unit arranged to, that is, equipped with software in order to establish the models for determining the moisture content and its change in the material. The computer unit is arranged to perform the moisture content measurement with regard to the amount of energy that actually has been added in order to evaporate a certain amount of liquid in the material. The obtained moisture content/ ratio, or more precisely, the change thereof, is then used in a control system for optimising the drying processing, with special regard to the moisture ratio upon fibre saturation and the final moisture ratio in the material. The term optimisation includes the optimising of any type of process parameters that are used. It should be realised that the invention only has been described as to its principles and with reference to the proposed embodiments. Alternative embodiments will therefore be obvious for a man skilled in the art without thereby departing from the scope of the invention as defined in the annexed claims supported by the description. For example, data input that has been given as an example for the respective models can be supplemented with further input data, for example the air humidity inside the drying chamber. In advantageous cases also some of the data input mentioned above can be eliminated and, for example, the model for determining FKι n may be included in the model for determining the moisture ratio. The adjustment parameter that has been elaborated by means of the model intended for this purpose will, however, always be used. Further, it should also be emphasised that the terms moisture ratio and moisture content should be regarded as terms of equal value for describing the amount of liquid present in the material, although, per definition, there is a difference between them where: moisture ratio = (weight of damp wood/weight of dry wood) /(weight of dry wood) and moisture ratio = (weight of damp wood/ weight of dry wood). Accordingly, it shall not be possible to go beside the claimed invention only by claiming that moisture ratio is measured instead of moisture content or vice versa.

Claims: Claims 1. Method for measuring the moisture content during drying of an organic material, by which the moisture content in the material is determined based on the dielectric properties of the material, and by which electromagnetic radiation is emitted towards a surface of the material, and where the determination of the moisture content is based on the effect of the material on the electromagnetic radiation, characterised in that a change of the effect of the material on the electromagnetic radiation versus time is detected, and that data input to a mathematical model for determining the moisture content of the material during the drying is determined from the amount of energy consumed for evaporation of liquid in connection to the drying process during a given time period. 2. A method according to claim 1, characterised in that said data input comprises an adjustment parameter JP obtained in accordance with: JP = f(spectral data, FKj n , energy (ΔE) where FKin is the initial moisture ratio of the drying load. 3. A method according to claim 1 or 2, characterised in that the model for determining the moisture content is a model of the type: Moisture ratio = f(spectral data, Fkin, JP, further input) where further input relates to at least one of type of wood, drying volume, wood dimension, drying temperature and air flow in the drying chamber. 4. A method according to any one of claims 1-3, characterised in that said time period comprises a period during which generally all energy consumed for the drying is also consumed for the evaporation of liquid present in the material. 5. A method according to any one of claims 1-4, characterised in that said time period is a period during which the temperature in the material is generally constant when the applied energy/ time unit is constant. 6. A method according to claim 5, characterised in that said time period is before the point at which fibre saturation appears in the material. 7. A method according to any one of claims 5-6, characterised in that said time period is within an initial portion of the time period during which generally all energy consumed for the drying is also consumed for evaporation of liquid present in the material and/ or during which the temperature in the material is generally constant when applied energy/ time unit is constant. 8. A method according to any one of claims 1-7, characterised in that the electromagnetic radiation comprises a known first frequency spectrum, and that this spectrum is continuously or continually compared to a second frequency spectrum, formed by the effect of the material on the first frequency spectrum, in order to obtain spectral data. 9. A method according to claim 8, characterised in that the frequency spectrum mainly comprises wave lengths within the frequency range of 1-1000 MHz, preferably in the range of 1-500 MHz, and most preferably 10-250 MHz. 10. A method according to any one of claims 1-9, characterised in that radiation reflected by the material is detected and compared to the radiation that has been emitted towards the material in order to determine the effects of the material on the radiation. 11. A method according to any one of claims 1-10, characterised in that radiation transmitted through the material is detected and compared to the emitted radiation in order to determine the effect of the material on the radiation. 12. A method according to any one of claims 1-11, characterised in that the material comprises wood, preferably wood intended to be used as building timber. 13. A method according to any one of claims 1-12, characterised in that the moisture content that is determined for the material is introduced as data input into one or more existing models or devices for controlling the drying process. 14. A method according to any one of claims 1-13, characterised in that said model or models is/are used for controlling one or more process parameters in a drying process. 15. A device for measuring the moisture content during drying of an organic material, said device being equipped with means for determining the moisture content in the material based on the dielectric properties of the material, said means comprising means for emitting electromagnetic radiation towards the surface of the material, and the determination of the moisture content being based on the effect of the material on said electromagnetic radiation, characterised in that it comprises means for detecting a change of the effect of the material on the electromagnetic radiation versus time, and means for establishing data input to a mathematical model for determining the moisture content of the material during the drying based on the amount of energy consumed for the evaporation of liquid in connection to the drying process during a given time period. 16. A device according to claim 15, characterised in that the means for emitting radiation towards the material comprises one or more antennas, each arranged to emit radiation with a known frequency spectrum towards the material. 17. A device according to claim 15 or 16, characterised in that it comprises means for detecting radiation that has been reflected by or transmitted through the material, and a unit for continuous or continual comparison of differences between spectral data for each emitted frequency spectrum and spectral data for the corresponding reflected or transmitted frequency spectrum, and evaluation of these differences for the determination of the moisture content of the material. 18. A device according to claim 17, characterised in that the unit for comparison and evaluation comprises a neural network and/ or an electronic unit with a software for Partial Least Square regression, PLS. 19. A device according to any one of claims 15-18, characterised in that the means for establishing data input to the mathematical model comprises an electronic unit with a software adapted to compare changes during time of the moisture content value that is determined and the amount of liquid that, during a corresponding time, has been forced to evaporate in connection to the drying of the material.

Inventors: BACKA STEFAN, CLAESSON BRITH, GAEFVERT UNO, BACKA, STEFAN, CLAESSON, BRITH, GAEFVERT, UNO
Applicants: ABB AB, BACKA STEFAN, CLAESSON BRITH, GAEFVERT UNO, BACKA, STEFAN, CLAESSON, BRITH, GAEFVERT, UNO
Application year: 2001
Main classification: G01N22
IPC codes: G01N22