Method and system for producing micro- or nanofibril suspension

Abstract

The invention relates to a method for producing a micro or nanofibril suspension, in which fibre suspension is processed to generate micro or nanofibrils from fibres and a measurable quantity that describes micro and nanofibrils is determined from fibre suspension as measurement data, in which measurement data is determined as an online measurement in the following steps where a sample is taken from the fibre suspension, and fibre porosity is determined by determining the ratio of fibre-internal water to fibre-external bound water as an online measurement using an NMR spectrometer (10) in the following steps where a backward signal of the frequency pulse that returns from water molecules excited by frequency pulses (v) to the coil (22) is measured, - a proton' s relaxation time and an amplitude of the backward, signal is determined from the backward signal and fibre porosity of the fibre suspension is determined, based on the amplitude and the relaxation time of the exponential backward signal, and the method also includes a decision to end the processing of the fibre suspension made on the basis of a criterion selected on the basis of measurement data. The invention also relates to a corresponding system.

Description

[0001] METHOD AND SYSTEM FOR PRODUCING MICRO- OR NANOFIBRIL SUSPEN-

[0002] SION

[0003] The invention relates to a method for manufacturing micro- or nanofibril suspension, in which fibre suspension is processed for a set period to generate micro- or nanofibrils from fibres, and a measurable quantity that describes micro- and nanofibrils is determined from fibre suspension as measurement data, so that the determination of measurement data is performed as an online measurement where

[0004] - a sample is taken from the fibre suspension.

[0005] The invention also relates to a corresponding system.

[0006] Water retention value (WRV) is an important property in terms of the quality of paper and pulp. WRV can be used to estimate fibre fibrillation, the moisture control and water retention capacity of the material, which affect many paper properties. A high WRV can help paper retain its moisture level and reduce waviness and edge problems. WRV also affects paper' s mechanical properties, such as tensile and compressive strengths. A correctly balanced WRV may improve paper' s printability and absorption capacity. WRV may also affect paper' s appearance, such as smoothness and gloss. WRV can be used to optimise the paper manufacturing process and to achieve the desired properties, such as moisture control, strength and printability.

[0007] The water retention value (WRV) measures a fibre sample's ability to retain water, and it is commonly used in the pulp and paper industry to characterise the extent of fibre fibrillation. A fibre sample' s WRV increases as a result of refining as the swelling of fibre increases because of the internal fibrillation of fibre (Dekker 2003; Hartman 1985; Kerekes 2005b; Roffael and Kraft 2012) . Refining also increases the external area of fibre as a result of fibrillation (Hartman 1985, Kerekes 2005a) , which improves fibre's water retention capacity. There are several standards that pertain to WRV' s laboratory measurements, such as TAPPI UM256, SCAN— C 62:00 ja ISO 23714: 2012. The measurement is an empirical capillary test, which consists of adding fibrous suspension into a metal tube with a wire screen on the bottom. The screen allows water to pass through but retains fibres. The tube is centrifuged at a specific relative centrifuge force (RCF) for a specific time, and the formed pad is weighed before and after oven drying.

[0008] Microf ibrillated cellulose (MFC) and nanofibril lated cellulose (NFC) are raw materials formed by fibres separated during the refining of fibre, which improve the WRV of fibre suspension when added to standard fibres. In industry, significant efforts have been made to develop a production process that can be used to produce MFC or NFC or both in a controlled and cost-efficient manner. The challenge is to determine efficiently the generation of MFC or NFC during the production process to enable its efficient control.

[0009] The WRV and the fibre length of standard fibres can be determined but for MFC and NFC, the WRV and fibre length are difficult to determine. When conventional fibre analysers are used, MFC and NFC fractions are mixed with the fine aggregate and no reliable evaluation can be made.

[0010] Standardised test conditions exist, but they are not suited to the measurement of MFC or NFC because their water retention capacity is exceptionally high. In literature, modifica- ions to standard conditions to enable the assessment of

[0011] MFC's water content have been reported. However, various factors influence WRV results, such as RCF, centrifugation time, temperature, the dry weight of samples and the pore size of any filter paper or membranes used. For example, a higher RFC results in higher centrifugal pressures, which press more water out of the material. At the same time, a longer centrifugation time results in a lower WRV. The use of filter paper or membranes may also affect the results via two mechanisms. On the one hand, small pore sizes may create resistance to water flow because of the pore size itself and because of fine particles blocking the pores. On the other hand, larger pores may result in the loss of fibres, thereby causing measurement errors. Therefore, the conventional measurement methods cannot be used because of their lack of reliability .

[0012] Another problem with the traditional determination of the WRV value (or the WRV index) is its slowness and labour intensity. The total duration of several weighing processes and drying steps is several hours and the determination requires multiple manual work steps. Thus, the utilisation of measurement results for controlling the MFC / NFC production process is slow.

[0013] Publication FI 126688 B presents a method by which pulp fibres are broken down to monitor the quality of the manufactured nanofibril pulp. This is done by measuring optically in real time the turbidity of the nanof ibril-containing dispersion generated in the aforementioned dissolution process, and by determining the quality of the nanofibril pulp using the correlation between the measured turbidity and the quality of nanofibril pulp. Reduced turbidity indicates higher quality of the nanofibril pulp. This method uses the optical measure- ment of turbidity for the determination of nanofibril pulp quality for monitoring purposes. It s e n s i t i v e to di r t .

[0014] The purpose of the invention is to create a technical method and a system for the production of MFC or NFC, or both, that is more cost-ef f icient and reliable than current and previous methods and systems. The characteristic features of this invention regarding the method and the system are presented in the appended Claim 1 and Claim 11, respectively.

[0015] The purpose of the method according to the invention can be achieved with the method for producing micro- or nanofibril in which method fibre suspension processed for a set period to generate micro- or nanofibrils from fibres, and a measurable quantity that describes microfibrils is determined as measurement data from fibre suspension. The determination of measurement data is performed as an online measurement in the following steps in which a sample is taken from the fibre suspension, and fibre porosity is determined by determining the ratio of fibre-internal water to fibre-external bound water as an online measurement using an NMR spectrometer. The determination of fibre porosity is performed in the following steps, in which the backward signals of the frequency pulses returning to the coil from the water molecules excited by frequency pulses are measured, the proton relaxation time and the amplitudes of the backward signals are determined from the backward signals, and fibre porosity of the fibre suspension is determined based on the amplitudes of the exponential backward signals and the relaxation times. In addition, a decision to terminate the set period is made in the method using a criterion selected on the basis of the ratio between the amount of water bound inside and outside the fibre. The new method is based on low field NMR technology, where the relaxation times of the protons of water molecules, which depend on the water molecules'’ mobility status, are measured. The mobility status of the water molecules changes clearly depending on whether they are free or bound to fibre. This makes it possible to determine in which state water molecules are in the fibre in question. On the basis of the water molecule state, the features of the physical state of the MFC and NFC fibre-water suspension, such as WRV indeces, can be determined. In addition, the new method enables the performance of online measurements directly from the processes.

[0016] In the method according to the invention, the production of MFC or NFC can be realised cost-ef f iciently as the production process and the generated amount of the MFC or NFC or both can be monitored almost in real time via online measurements, on the basis of which a decision on the termination of the production process can be made. This enables the optimisation of the amount of energy used in production as the production process can be terminated in a timely manner on the basis of measurement results. With the determination based on NMR technology, the measurement can be performed rapidly and reliably as an online measurement. Tn the method according to the invention, processed fibre suspension with a total fines content of 50-99%, preferably 65-95%, of the total fibre mass can be regarded as MFC or NFC products. The amount and type of processing depend on the desired final product, in other words, on the desired proper- tion of rod- or flake-like objects in the primary fines of the final product. More infromation on the rod-like and flake-like objects is available in the article "Reinforcement effect of pulp fines and micro fibrillated cellulose in highly densified binder less paperboards" (Winter et al. , Journal of

[0017] Cleaner ProductionfVolume 281, 25 January 2021, 125258) .

[0018] The total fines may be a length-weighted value F (i) (or Fr(l) ) , 0-200 pm, measured with Valmet Fiber Image Analyzer measuring equipment (Valmet FS5) .

[0019] Herein, set period refers to the period of time that starts when the processing of fibre begins and ends when the processing of fibre is terminated and the fibre suspension is removed from the processing process.

[0020] The length of micro- and nanofibril fibres is 0.01-10 pm and their width is 0.1-10 pm. These are commonly used definitions for MFC and NFC in the field, and indicate that the processed fibre suspension includes a significant amount of micro- or nanofibril cellulose.

[0021] Preferably, the processing of the fibre suspension is also controlled with another criterion selected on the basis of measurement data. When controlled on the basis of measurements, the production process can be optimised to improve cost-efficiency. In addition, the determination based on NMR technology performed as online measurements can be performed sufficiently quickly to enable the active adjustment of fibre processing, in contrast to methods according to prior art, which are unsuitable for the active adjustment of processing because of their slowness.

[0022] Another criterion may be the measurable quantity Bext - me / (mi + me) , which depicts the ratio of water bound outside the fibre to the total amount of bound water. It is known empirically that the generation of MFC and NFC slows down significantly when the ratio exceeds 0.8. it d v a n t a g e o u s 1 y , f i b r e p o r o s i t y measured during the set period .

[0023] Adv a nt a g e o u s 1 y , fibre porosity is determined at a frequency of 0.01-1 Hz during the production process. As the determination is quick to perform, it can be repeated several times in order to select the optimal processing termination time.

[0024] Adv a nt a g e o u s 1 y , the determination of porosity is performed to 200 times (preferably 20 to 100 times) during the determination. With the quick and cost-efficient measurement, the determination can be repeated several times to improve the reliability of the measurement data.

[0025] According to an embodiment, before making a decision to terminate refining, measurement data is filtered with an index combined from 2 to 20, preferably 4 to 8, consecutive measurement events to eliminate any errors caused by an individual measurement. It has been noticed that the difference between samples may be greater than the measurement accuracy. When the refining has proceeded further, the samples are more homogenous, and such a high number of measurements is not required.

[0026] Advantageously, fibre processing consists of refining of the fibre. Refining is one of the best-known and most extensively studied fibre-processing methods for producing MFC or NFC

[0027] JT I? 3 C T.1 o n S e

[0028] Advantegously the criterion for terminating the refining is the proportion of external bound water, which is 80-95% of all bound water, determined as the measurement data. Unexpectedly, empirical studies showed that in the case of the most common fibre types., the continuation of refining after the achievement of this point does not significantly affect the result, at least not with regard to the porosity of the fibre suspension, and therefor? ' it is not necessary.

[0029] An alternative choice for the criterion is a WRV index calculated on the basis of measurement data, which is 3-14 g / g for MFC and 4-10 g / g for NFC, In this situation, the processing of fibre can be deduced to be sufficient for the fibre suspension on the basis of the WRV index.

[0030] Advantageously, a general model of the backward signal is as follows : where Exp is the backward signal, Axis the amplitude of the backward signal, Rxis the relaxation speed and t is the signal measurement time and C is an empirical constant. This calculation model has been found to effectively correlate with the WRV index determined as a laboratory measurement .

[0031] In the simplest model, S - A exp(-Rs*t) , where A is amplitude, R is relaxation speed and t is time. Samples1,relaxation speed R2 and WRV values can be determined using the following calibration formula:

[0032] WRV = a*Rz + b, where a is the slope and b is the constant. The parameters a and b depend on the wood species used for the cellulose. In this way, with a simple model, it is possible to perform the determination of the WRV index with the help of parameters obtained from the NMR measurement using a calculation model based on one exponential signal, which also works very well for lower consistencies. The model uses a relaxation speed, i.e. Rx, the ratio of which to the relaxation time T

[0033] Advantegously , the determination of empirical constants is performed by comparing the WRV values given by the NMR spectrometer to the WRV values measured pursuant to the standard from the reference fibre suspension., with 2 to 10, preferably 2 to 6 reference fibre suspensions, mixed by adding to unprocessed fibre suspension a known mass proportion of micro- or nanofibril suspension, with the mass proportions being between 0.5 m-% and 50.0 m-% and by extrapolating a linear model for mass proportions of >50-100 m-%. This enables the achievement of a reliable model for determining the prosify of a processed fibre suspension as an online measurement.

[0034] In more detail, the determination of empirical constants is performed in following steps where 2 to 10, preferably 2 to 6 reference fibre suspensions are generated by adding to unprocessed fibre suspension a known mass proportion of micro- or nanofibril suspension, with the mass proportions being between 0.5 m-% and 50.0 m-%, the WRV index is determined with a standard method for each reference fibre suspension, the determined WRV indeces and mass proportions are set against a point group according to a backward signal model and a linear model for mass proportions >50-100 m-% is extrapolated on the basis of the WRV indeces and mass proportions determined with the backward signal. In addition, an NMR spectrometer is used to measure a backward signal of the frequency pulse that returns from water molecules excited by frequency pulses to the coil from a sample of 100 mass proportion microfibril or nanofibril suspensions, the proton relaxation time and the backward signal amplitude are determined from the backward signal and empirical constants are determined by comparing the relaxation speed calculated as the reciprocal of relaxa- tion speed with the linear model determined from the reference fibre suspension.

[0035] Unexpectedly, studies nave shown that in the case of MFC and NFC, the WRV index cannot be determined reliably with the WRV index determination methods pursuant to the standard because of the physical properties of MFC and NFC. Because of this, the determination of WVR must be performed partly by interpolating it with partial proportions from the performed determinations. Even though such determination of references may result in partly erroneous data, because the ratio between NMR parameters and the WRV index may not be fully linear across the entire interpolated area, the accuracy is sufficient in terms of efficient control of the fibre-processing process .

[0036] According to an embodiment, for softwood fibre, the slope a value is 15-16 and the constant b is -13- (-14) in the singleexponential model .

[0037] According to an embodiment, for birch fibre, the slope a value is 6.5-7.5 and the constant b is -6- (-7) in the singleexponent i a 1 mode 1.

[0038] According to an embodiment, for eucalyptus fibre, the slope a value z and the constant b 2— (-3) in the single- exponential model.

[0039] Advantageously, fibre porosity of a fibre suspension is determined based on the amplitude and relaxation time of the backward signal using a single or a double exponential signal model or both for determining the relaxation time. The single and double exponential signal models for determining the relaxation time can be alternative relative to each other. but preferably complement each other when used simultaneously for determining fibre porosity.

[0040] Advantageously, fibre porosity is determined as a WRV value. The WRV value is a commonly used measurement for porosity, which depicts the weight of water in relation to the weight f fibre.

[0041] In addition to fibre porosity, the method preferably determines the consistency of the sample. In this way, the WRV value preferably used as a fibre porosity measure can be interpreted relative to consistency, thus improving the usability of the measurement. The determination of consistency can be performed using existing consistency analysers. Consistency measurement is typically included in fibreprocessing automation systems. In the measurement of consistency, equipment called Valmet Microwave Consistency Measurement (Valmet MCA) can be used, for example.

[0042] According to a second embodiment, fibre porosity of a fibre suspension is determined using a model that consists of two exponential signals Exp = Ai*exp (Rzi*t) + As*exp (R2E*t ) + C for determining the relaxation time, where C is a third empirical constant. When using such a double exponential signal model, a so-called physical model is provided, which correlates particularly well with fibre porosity. In the physical model, one exponential signal is from fibre-internal water and the other one is from fibre-external water. However, the physical model alone can be sensitive to external errors, if the consistency of the sample is very low and the proportion of fibre-internal water is very small relative to that of external water. Fibre porosity is preferably determined as a WRV value in the double exponential signal model in successive steps of calculating, based on the amplitudes Ai and AB, the relative amount of fibre-internal water Ai__reiand the relative amount of external water AE__reiand calculating the amount of bound waterin fibre WTB based on the relative amount of fibre-internal water Ax_reiand the consistency c. The method additionally involves calculating the proportion of water in fibre bound inside fibre pis based on the relaxation speed of completely free water R2F, the relaxation speed of completely bound water F<2B and the relaxation speed of fibre-internal bound water R2I, calculating the proportion of water in fibre bound outside fibre DEB based on the relaxation speed of completely free water R2F, the relaxation speed of completely bound water R2Band the relaxation speed of fibre-external bound water R2B, and calculating the amount of bound water in fibre WiBbased on the relative amount of fibre-internal water Ai__rei, the proportion of water bound in fibre piBand the consistency c, and calculating the amount of bound water outside fibre WEB based on the relative amount of fibre-external water AB„rei, the proportion of water bound outside fibre PEE and the consistency c. Furthermore, the method involves calculating the WRV value as a sum of the amount of bound water in fibre WTB and the amount of bound water outside fibre WTB. Information obtained in intermediate steps regarding the amounts of fibre-internal bound water and external bound water can be utilised when assessing the performance of refining. The amount of fibre-internal water correlates to how refining has provided fibre-internal fibrillation, whereas the amount of external bound water correlates to how refining has provided fibre-external fibrillation.

[0043] Advantageously, the method also involves determining the ratio of bound to free water both inside and outside fibre. In the double exponential model, this can be determined directly from the values provided by the model, whereas in the single exponential model, measured relaxation times or relaxation speeds or both are compared to empirically defined reference values.

[0044] According to a third advantageous embodiment, fibre porosity is determined using both the single exponential signal model for determining a first porosity value and the double exponential signal model for determining a second porosity value, and said first porosity value and second porosity value are compared to each other for forming a comparison result, based on which fibre porosity is determined. Simultaneous use of both the single and the double exponential signal models improves the reliability of the method. While the computational single exponential signal model forecasts the actual situation slightly defectively, a physical situation of two exponential signals models the physical situation better. Instead, the double exponential model is more sensitive to external disturbances, which do not affect the single exponential model in a corresponding way. Thus, by comparing the results of these, it is possible to improve the reliability of the method.

[0045] The comparison result can be calculated as the remainder of the first porosity result and the second porosity result and by dividing said remainder by the first porosity result. If the comparison result is less than 5% of the first porosity result, fibre porosity is calculated as the mean value of the first porosity result and the second porosity result. This is one way of utilising the comparison of the results of the single and the double exponential signal models for improving the reliability of the method. On the other hand, if the result differs more than 5%, one of the first porosity value and the second porosity value can be considered as fibre porosity with a pre-selected criterion, where the preselected criterion can be, for example, the selected limit value of the sample consistency.

[0046] According to an embodiment, the calculated fibre porosity value is used in controlling the refiner after filtering, for example, by using a Kalman filter for reducing the fluctuate i o n o f c o n t r o 1.

[0047] Alternatively, instead of the model that uses one or two exponents of the general model, it can be contemplated that the model can include even three or four exponents, if such calculation method correlates well with the reality.

[0048] In the method, the sample is advantageously stopped for performing the NMR spectroscopy measurement. Thus, the measurement accuracy can be improved when the frequency pulse resent by all of the protons included in the sample can be measured.

[0049] The purpose of the system according to the invention can be achieved with the system for producing a micro- or nanofibril suspension, which includes processing equipment for pro- cessing fibre suspension to generate micro- or nanofibrils from fibres, and a measurement equipment for determining a measurable quantity from the fibre suspension that describes micro- and nanofibrils as measurement data. The measurement equipment includes a time-domain NMR spectrometer for deter- mining fibre porosity of a fibre suspension on the basis of a sample, comprising a sample channel, a computer and programmable means, as well as a means of connecting the equipment to processing equipment, either directly to a fibre suspension flow channel or to a side flow channel for determining fibre porosity with an online measurement, which side flow channel is arranged to lead part of the fibre suspension flow arriving from the refiner to form a separate sample. The NMR spectrometer includes at least one coil arranged around the sample channel to excite water protons of the fibre suspension in the sample with frequency pulses, a magnet arranged around the sample channel for generating a magnetic field in the sample channel, and a power source complete with controllers connected to the coil for forming frequency pulses. The NMR spectrometer also includes measuring equipment for measuring the intensity of current generated by the frequency pulse that returns to the coil from protons to generate a backward signal and a computer equipped with programmable means for determining the porosity of samples based on the backward signal. These programmable means are arranged to determine the proton relaxation time and the amplitude of the backward signal from the backward signal and to determine fibre porosity of the fibre suspension based on the amplitude and relaxation time of the exponential backward signal, and to make a decision to terminate the processing of the fibre suspension based on a criterion selected through measurement

[0050] With a system according to the invention, determination of fibre porosity can be performed as an online measurement by connecting a NMR spectrometer to the fibre suspension flow channel and by directing fibre suspension flowing in the flow channel through the NMR spectrometer. The determination is very fast to perform and does not require manual work steps from the operator during the measurement. Using time-domain spectroscopy, the determination can be quite simple and affordable to realise. With the help of a measurement of such accuracy, the fibre-processing equipment can be controlled precisely to process fibres in an optimal manner to produce

[0051] MFC or NFC fractions or both.

[0052] Advantageously, the criterion selected for terminating the processing by programmable means is the proportion of external bound water, which is 80-95% of all bound water, determined by a measurement. As stated previously, this is a costefficient way to control the process.

[0053] The connection means advantageously include the side flow channel, in which the measurement equipment is installed. Thus, the fibre suspension of the entire side flow channel can be led through the NMR spectrometer and it is not necessary to arrange sampling inside the flow channel, where it would be easily exposed to accumulation of fibre suspension and clogging.

[0054] According to an embodiment, the measurement device includes a pump located in the side flow channel after the NMR spectrometer for aspirating a sample from the flow channel to the side flow channel and the NMR spectrometer. By using a pump, the flow of the fibre suspension to the NMR spectrometer, and after the measurement, through it can be ensured even in situations where the stock consistency is higher than 1%.

[0055] According to an embodiment, the measuring equipment may be built as a separate measurement unit, which comprises a flow channel, a magnet and a coil arranged as a single unit, which can be installed through a maintenance hatch, for example, to the flow channel running from the processing equipment. The details of such measurement unit may correspond to the details of the applicant' s previous patent application WO

[0056] 2021 / 079027 Al with regard to the technical realisation. Advantageously, programmable means are arranged to determine the proton relaxation time and the amplitude of the backward signal from the backward signal and to determine fibre porosity of the fibre suspension based on the amplitude and relaxation time of the backward signal using both the single and the double exponential signal models for determining the relaxation time. A simultaneous parallel use of the single and double exponential models enables the comparison of the results and achievement of better reliability in determining porosity .

[0057] Advantageously, the system includes the computing unit arranged to calculate fibre porosity of the samples taken before and after the processing equipment and the related comparison value, and to adjust the processing equipment based on the comparison value. Thus, changes can be made very quickly to the operation of the processing equipment.

[0058] According to an embodiment, the system includes a first valve and a third valve arranged in side flow channels for stopping the sample for the duration of the measurement, in this way, the measurement is more reliable when it is known that each excited proton also releases its energy back to the coil so that it can be measured.

[0059] The term "sample" as used throughout this document refers to the portion of the fibre suspension diverted from the process flow to the NMR spectrometer when the NMR spectrometer is located in connection with the flow channel and when the porosity of the fibre suspension is determined as an online measurement almost in real time. method and a system according to the invention solve the problem of slowness related to the production of MFC and NFC fractions and porosity measurement and offer a solution for the rapid adjustment of fibre processing to optimise the production process .

[0060] The invention is described below in detail with reference to the accompanying drawings that illustrate some of the embodi- merits of the invention., in which

[0061] Figure 1 is a basic view of the location and movements of water relative to fibre.

[0062] Figure 2a depicts the process position according to a first embodiment of the system according to the invention,

[0063] Figure 2b depicts the process position according to a second embodiment of the system according to the invention,

[0064] Figure 3 is a basic overview of a system according to the invention,

[0065] Figure 4 is a cross-sectional view of a device according to the invention,

[0066] Figure 5 is a block diagram illustrating the steps of the method according to the invention,

[0067] Figure 6 is a basic view of the design of a control system according to the invention,

[0068] Figure 7 is a graphic representation of the double exponential model,

[0069] Figure 8 is a graphic representation of models defined for different fibre types,

[0070] Figure 9 is a representation of the WRV values measured from the reference fibre suspension as a function of the values determined with the help of the models depicted in Figure 8, Figure 10 is a representation of the proportion of water bound outside the fibre as a function of the refining time,

[0071] Figure 11 is a representation of the proportion of water bound outside the fibre as a function of the refining time, as defined with a method according to the invention and with a commercial measurement device.

[0072] Figure 1 depicts a basic view of how water is located relative to fibre. It is known that fibre includes internally bound water pBiand f ibre-internally free water pri . Water molecules can change place f ibre-internally binding to water bound from free water or releasing from bound water into free water quite quickly due to physical phenomena or chemical reactions. Correspondingly, outside fibre, water PBE as well as free water PFE has bound to the fibre surface. Outside fibre, the transfer from bound water to free water and vice versa can also take place quickly. Instead, the transfer from fibre-internal water to external water has been found to be slow, due to the secondary wall structure of fibre. The sum of the proportions of fibre-internal free and bound water pBi+ pri = 1 and the total sum of the proportions of external free and bound water PBE + PBE = 1.

[0073] Figures 2a and 2b depict the two different embodiments of the system according to the invention. In these embodiments, the NMR spectrometer 10, which functions as measurement equipment, is arranged to be used in connection with the refiner 100 serving as the processing equipment 20. According to Figure 2a, the NMR spectrometer 10 can be located in such a way that a small side flow is deviated from the flow of the fibre suspension flow channel 101 going to the refiner 100 into a side flow channel 102, which is positioned, as shown in Figure 2a, after the refiner 100 and which leads part of the fibre suspension flow to the NMR spectrometer 10 according to the invention. The NMR spectrometer 10 is advantageously connected to the side flow channel 102, which can be an already existing flow channel associated with the refiner or a part of a system according to the invention installed for this purpose.

[0074] The side flow channel 102 advantageously has a first valve 16 for sampling, with which it is possible to adjust the volume and flow rate of fibre suspension entering the NMR spectrometer 10, and a second valve 17, with which the sample can be stopped in the NMR spectrometer 10 for the duration of the measurement. According to Figure 2b, fibre suspension flow can also be led to the NMR spectrometer 10 via two side flow channels 102. One side flow channel 102 is preferably a flow channel, which is used to lead the fibre suspension flow before the refiner 100 through the one first valve 16 to the device 10. The other side flow channel 102 is advantageously a flow channel, which is used to lead the fibre suspension flow after the refiner 100 through the second valve 15 to the device 10. in this way, it is possible to perform comparative measurements on both the fibre suspension that enters the refiner and the fibre suspension, already refined, that exits the refiner. The size of the sample led from the fibre suspension to the device can be as small as 1-10 cm3, in which case the equipment is also relatively small-scale. Nevertheless, such a sample is sufficient for determining fibre porosity .

[0075] According to Figures 2a - 4, in the method according to the invention, part of the fibre suspension flow can be led to a side route for an online measurement, wherein the measurement is performed using the NMR spectrometer 10. The first valve 16 is used to control sampling from the fibre suspension flow 14, The NMR spectrometer 10, which comprises connection devices 13 for connecting the NMR spectrometer 10 to the side flow channel 102, is installed in connection with the side flow channel 102. The connection means 13 can consist of connections, with which the sample channel 12 of the NMR spectrometer 10 is connected to the side flow channel 102 so that the sample channel 12 and the side flow channel 102 form a continuous route for the fibre suspension up to and through the NMR spectrometer 10.

[0076] More specifically, the NMR spectrometer 10 includes a sample channel 12, at least one coil 22 for exciting protons p contained in free and bound water of fibres in the fibre suspension flow, arranged around the sample channel 12 as shown in Figure 3. The NMR spectrometer 10 also includes a magnet 24 arranged around the sample channel 12 for generating a magnetic field E in the sample channel 12. Advantageously, the magnet 24 is also arranged around the coil 22 in the radial direction relative to the sample channel 12 above the coil 22. The magnetic field E generated by the magnet 24 is advantageously a magnetic field as homogeneous and static as possible, through which the fibre suspension flow 14 passes inside the sample channel 12. The magnetic field E is depicted in the figure with lines in the transverse direction relative to the sample channel. The direction of the magnetic field is advantageously transverse relative to the longitudinal direction of the sample channel . The magnet is advantageously a permanent magnet, which can be implemented without separate driving power in order to operate. 21 permanent magnet generates a static permanent magnetic field in itself. Alternatively, the magnet can also be an electromagnet, the magnetic field of which is provided by electric current. In addition, the NMR spectrometer 10 includes, as shown in Figure 3, a power source 26 connected to a coil 22 for generating frequency pulses, measuring equipment 28 for measuring the intensity of voltage generated by the frequency pulse returning to the coil 22 from protons p for generating a backward signal, and programmable means 30 for determining fibre porosity of samples based on the backward signal and for controlling a first valve 16 for taking samples. With the power source 26, a frequency pulse is delivered to the coil 22 to excite protons p contained in bound and free water travelling inside the coil 22 into a higher energy state (spin) as the protons absorb the frequency pulse. This energy state discharges rapidly (in milliseconds) , the proton p thereby delivering or emitting energy to its surrounding, which again generates a voltage in the coil 22, i.e. , a backward signal, the amplitude of which can be measured with the measuring equipment 28.

[0077] In addition, the NMR spectrometer 10 includes a computer 25 equipped with the programmable means for determining fibre porosity of samples based on the backward signal by determining the proton relaxation time and the amplitude of the backward signal from the backward signal and fibre porosity of a fibre suspension based on the relaxation time and the amplitude .

[0078] Advantageously, the magnet 24, the coil 22 and the sample channel 12 are encased using a box construction 32 according to Figure 3. Advantageously, the box construction is made of metal, thus preventing expansion of the magnetic field to the environment and, on the other hand, access of disturbances external to the device to the magnetic field. In this way, the NMR spectrometer according to the invention can easily provide a closed magnetic field and is thus easily applicable in mill conditions. In principle, the aforementioned components of the NMR spectrometer 10 can be placed within the same box construction; however, there are preferably two box constructions. One box construction includes measuring equipment 28, a computer 25 and a power source 26, whereas the other box construction encloses a magnet 24 and a coil 22. In this way, damage to sensitive electronic components is avoided in cases of leaking of a water-containing fibre suspen-

[0079] The relaxation time correlates with the ratio of free water to bound water contained in fibres in the fibre suspension, which ratio will change during fibre processing as fines detach on the surface of fibres and fibres fibrillate. With increasing fibrillation, the relaxation time T2 decreases. The so-called CPMG (Carr-Parcell-Meiboom-Gill ) pulse sequence, which contains one 90° pulse and several 180° pulses, can be used to determine the spin-spin relaxation time T2. Amplitudes of echoes of the pulse sequence attenuate according to the following equation: where ao is the amplitude at the time t -= 0s and T2 ==- spinspin relaxation time. Parameters aoand T2 can be defined by placing the equation in an experimental signal.

[0080] The diameter of the sample channel can be at least 10 mm, preferably 10-20 mm, to allow for the fibre suspension to flow in the sample channel without problems. The dry solids content of the fibre suspension can generally range between 0.5% and 4.0% by weight, at which it remains pumpable. Fibre suspensions at a higher dry content may require a higher pressure to move in the sample channel, but when placed after the refiner, the sample is taken from the fibre suspension flow where the pressure is generally sufficient. Advantageously, a separate pump is also used in the side flow channel for moving the fibre suspension forward. A small diameter of the sample channel proposed above also enables the use of a smaller coil. In this case, the centre hole of the magnet placed on the coil, advantageously above the sample channel, can have a smaller diameter, approximately as small as between 30 mm and 40 mm. The manufacturing costs of the magnet are generally the lower, the smaller is the hole that needs to be produced in the magnet .

[0081] The NMR spectrometer according to the invention can be realised using one coil or with two coils. When one coil is used, the same coil both delivers and receives the frequency pulse. When two coils are used, one coil can deliver the frequency pulse and the other one receives it. The use of one coil is possible, if the sample flows so slowly that the same protons that are exposed to the frequency pulse will also have time to deliver the backward signal in the coil area. Alternatively, the NMR spectrometer may include two valves, which are used to stop the sample momentarily at the coil and the magnet. In turn, the use of two coils enables the determination of porosity from a moving flow when correctly adjusted. The coil, also called a bobbin, used in the device is electrically dimensioned in such a way that, with a selected power source, it can produce the desired frequency pulse, or excitation pulse, in a selected magnetic field. For example, when the strength of the magnetic field E is 0.5 T, the frequency pulse applied is in the frequency range of 25 MHz - 26 MHz. Generally, the frequency pulse used is in the range of 50 kHz - 150 MHz. When one coil is used for the measurement, the length of the coil used may be approximately 10-20 cm, whereby protons in the fibre suspension flow will have time to get excited and deliver energy across the coil. The coil may have 100-200 turns

[0082] Energy released by the proton p excited according to Figure 3 provides a backward frequency in the coil 22, which can be measured as a backward signal. The backward signal to be measured can be measured with extremely sensitive measuring equipment 28, for example, with a receiver whose measuring accuracy can be in the class of 1 uV. The backward signal to be measured is only an average signal; that is, momentary values are measured for the backward signal in a certain period and, based on these values, an average is calculated for this period. In other words, the entire spectrum is not measured, as is usually the case in spectroscopy. For example, the duration of the period may be between 0.5 s and 2.0 s. Based on the strength of the backward signal, the relaxation times T1 and T2 of the proton can be calculated. The relaxation time can be calculated with the following formula:

[0083] T2 = -t / { In [a (t ) / ao}

[0084] Programmable means 30 have been implemented in a computer 25, which can be used for presenting results as well as for controlling the device. The computer can be a normal PC or equivalent. The material of the flow channel is preferably glass, Teflon or other equivalent non-magnetic material, which does not disturb the generation of the magnetic field within the flow channel. In turn, the power source is an AC power source, in relation to which a frequency converter can be used to achieve the correct frequency.

[0085] The control of NMR spectrometer operations can take place with the same computer, equipped with programmable means f determining fibre porosity using an empirical formula based on measured relaxation times. To control the system, it is possible to use separate control software that provides electric controls via a field bus, for example, for valve actuators, which open the valve of the flow channel for taking the sample either periodically or continuously.

[0086] Figure 5 shows steps 40--- 62 of an embodiment of the method according to the invention in a block diagram. The method according the invention begins with the procesing of fibre with processing equipment at step 40.

[0087] Various methods, which are homogenising, microfluidisation, refining, grinding, cryocrushing, ball milling, ultrasonication, extrusion and cavitation, can be used for the mechanical production of cellulose micro- and nanof ibr il s . Currently, the methods in the most common use are based on homogenising, microfluidisation and refining at high shearing forces. Homogenisation, which was described as early as the 1980s, is the conventional method for producing cellulose nanofibrils. In homogenisation, a dilute fibre-water suspension is run through a small nozzle at a high pressure, and large pressure differences cause f ibrillisat ion . In homogenisation, the material has to be run through the nozzle several times to generate gel-like cellulose nanofibrils, and therefore the specific energy consumption of the process is extremely high unless the process can be terminated at a correct time.

[0088] Homogenising can be performed with a microf luidi ser . In this case, the fibre suspension is usually run through the micro- fluidiser several times to increase the fibres' fibrillisa- tion degree. With an increase in the number of stock' s homogenisation runs, the energy consumption of the process also increases . In addition to homogenisation, refining, depicted in Figures 1-7, is a commonly used mechanical method. PFI refiners or, preferably, Masuko refiners installed parallel to increase capacity, may be used as the refiners. Refining may also be combined with homogenisation in the production of cellulose micro- or nanofibrils. In refining, fibre suspension is first forced through a gap in the refiner plate, with the mechanical stress causing the fibre walls to peel, revealing the internal layers (external fibrillation) , and the fibre structure to loosen (internal fibrillation) . The structural changes to fibres caused by refining facilitate the generation of micro- and nano-sized fibres during homogenisation.

[0089] In the comparison of cellulose nanofibril production on a laboratory scale with various mechanical processing methods, it has been noticed that microfluidisation and refining with Masuko refiners generate fibrils, which can be used to produce films with better mechanical properties, at lower energy consumption levels than homogenisation. Cellulose micro- and nanofibrils generated by different mechanical means differ from one another in terms of their appearance, macrostructure and micro / nanostructure .

[0090] Different fibre-processing methods are presented in the following publications:

[0091] 1. SUBRAMANIAN, R. , KONONOV, A. , KANG, T. , PALTAKARI, J. , and PAULAPURO, H. , Structure and Properties of Some Natural Cellulosic Fibrils. Bi oResources, 2008. 3 (1) : p. 192-203.

[0092] 2. LIU, H. and HSIEH, Y . , Ultrafine fibrous cellulose membranes from electrospinning of cellulose acetate. Journal of Polymer Science, 2002. 40: p. 2119-2129. 3. TOKOH, ( TAKABE, FUJITA, M., and SAIKI, H., Cellulose synthesised by Acetobacter xylinum in the presence of acetyl glucomannan. Cellulose, 1998. 5: p. 249-261.

[0093] 4. DUFRESNE, A. , DUPEYRE, D., and VIGNON, M.R., Cellulose microfibrils from potato tuber cells: Processing and chrac- terisation of starch-cellulose microfibril composites. Journal of Applied Polymer Science, 2000. 76: p. 2080- 2092.

[0094] 5. MATSUDA, Y. , HIROSE, M., and UENO, K. , Super microfibril- lated cellulose, process for producing the same and coated paper and tinted paper using the same, PATENT, U.S. , Editor. 2001, Tokusnu Paper Mfg. Co. , Ltd. , : US.

[0095] 6. TANIGUCHI, T. and OKAMURA, K. , New films produced from microf ibrillated natural fibres. Polymer International, 1998. 47 (3) : p. 291-294.

[0096] 7. PAAKKO, M., ANKERFORS, M., KOSONEN, H. , NYKANEN, A., AHOLA, S. , OSTERBERG, M. , RUOKOLA1NEN, J. , LAINE, J. , LARSSON, P.T. , IKKALA, 0. , and LINDSTROM, T. , Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenisation for nanoscale cellulose fibrils and strong gels. Biomacromolecules, 2007. 8: p. 1934-1941.

[0097] Advantageously, Masuko refiners are used in a parallel installation to increase capacity, but refiners may also be installed in a series to enable the achievement of the correct processing degree. The amount of processing can be adjusted by changing the amount of specific energy used for processing or the duration of processing. Fibre suspension can also be recycled from the refiner' s outlet to its infeed to increase processing.

[0098] Advantageously, each processing round is followed by a measurement step 41, in which the porosity of the fibre suspension is determined with an NMR spectrometer to indicate the generation of MFC or NFC fractions. After processing, a sample is taken from the fibre suspension leaving the processing equipment, in this case the refiner 100, either after the refiner 100 according to Figure la, or both before and after the refiner 100 according to Figure lb. Advantageously, the sample is taken, according to Figure lb, both before the refiner 100 and after the refiner 100 by leading the fibre suspension to a separate side flow channel 102 as a sample, according to step 41, which allows the determination of the fibre porosity of the sample. Connected to the flow channel 101 that enters the refiner 100 or exits the refiner 100, there is a side flow channel 102 having a first valve 16 and a second valve 17. By opening the first valve 16, part of the fibre suspension is led to the side flow channel 102 as a sample either periodically or continuously. Advantageously, the flow is led to the side flow channel 102 periodically, since then the sample flow can be stopped within the magnet of the device 10 for the duration of the measurement by means of the first valve 16 and the second valve 17. Periodically repeated, sampling can be repeated at intervals of 1 to 2 minutes, for example.

[0099] The first valve 16 and the second valve 17 are controlled preferably with a computer 25 and computer-operated programmable means 30, in which the sampling interval or the necessary volumetric flow per period has been defined. Based on the control software, the computer 25 sends a control command via a field bus, for example, advantageously to a relay 36 of Figure 2, via which the power supply is connected to the actuators of the first valve 16 and the second valve 17. Advantageously, the first valve 16 and the second valve 17 are solenoid valves, since solenoid valves are not as sensitive to environmental disturbances as other valve types. When the power supply to the actuators of the valves 16 and 17 is disconnected with the relay 36, the valves 16 and 17 will close, while when under voltage, the valves 16 ja 17 are in their open positions enabling the fibre suspension flow in the side flow channel 102.

[0100] Advantageously, the side flow channel 102 also includes a pump 34, with which a fibre suspension that is difficult to move can be reliably transferred along the side flow channel 102 to the equipment 18 for determining fibre porosity. For example, the pump can be a hose pump. Advantageously, power is supplied to the pump 34 via the same relay 36 so that the entire sampling process can be managed by controlling one relay 36. The sample is aspirated to the side flow channel 102, until the sample is conveyed into the magnet 24, at which time the power supply to the first valve 16 and the second valve 17 is disconnected with the relay 36, at which time these will close. At the same time, the power supply to the pump 34 is disconnected. The control of the relay 36 can be implemented by time control, for example.

[0101] At the same time, a magnetic field has been generated in the device preferably using a permanent magnet applied as the magnet 24 in the device, according to step 42 of Figure 4. The purpose of the magnetic field is to enable excitation of protons with frequency pulses generated by the coil 22. When generated by a permanent magnet, the magnetic field is permanent and does not require any specific control. The computer can also be associated with an electronic control unit controlled by the control means, while the control unit, in turn, controls the power source of the device to generate frequency pulses for the coil, according to step 44 of Figure 4. Frequency pulses are preferably generated at the frequency indicated above while the sample is in the magnetic field. Advantageously, the frequency pulse used is the so-called CPMG frequency pulse, which includes one 90° pulse and sever- al 180° pulses. Pulses are delivered one after the other and they excite the protons in the magnetic field, according to step 46 of Figure 4. The excitation is very rapidly discharged and the energy delivered by the proton arrives at the coil providing a low voltage in the coil, which is measured with the measuring equipment according to step 48. From the measuring equipment, the voltage data can be transferred in the analog form to an A / Q converter or as a digital signal directly to the computer 25, where it will be stored in a memory 35 with the programmable means 30 for further processing .

[0102] The amplitude of voltage is advantageously measured continuously and momentary measuring results of voltage are stored in the memory. Advantageously., the sample in the magnetic field is exposed to four different frequency pulses generated with the coil generating thereby four different attenuating signals, the amplitudes of which are measured with the measuring equipment. Based on the amplitudes measured, an average value can be calculated with the programmable means. In addition, an average value can be calculated over successive samples, since variations between individual samples are notably greater than variations between the successive signals of the same sample.

[0103] The proton relaxation time T1 or T2 calculated based on the measured amplitude of the backward signal is used together with an empirically defined calculation model to determine fibre porosity with the programmable means 30 in step 50 of Figure 5. The general form of the calculation model is as follows : The model uses a relaxation speed or Rx, the ratio of which to the relaxation time T is T === 1 / R.

[0104] The number of summable exponential factors used in the model can be between one and four, preferably one or two, most preferably two. The value of the exponential signal is directly obtained in the NMR measurement by the measuring equipment, measured at the coil and, based on it, it is possible to calculate the relaxation time and the amplitude using general calculation methods.

[0105] The experimental signal S measured with the low-field NMR is of the form

[0106] S = (when a single-exponential arrangement is used) where A is amplitude, R is relaxation time and t is time. Samples?relaxation speeds R2 and WRV indeces can be determined with the formula WRV = a*R2 + b, where a is the slope and b is the constant. The parameters a and b depend on the wood species used for the cellulose.

[0107] Alternatively, for the single-exponential factor model, it is possible to use the double-exponential factor model, or the so-called physical model, in the calculation, where the experimental signal Exp consists, as shown in Figure 7, of two exponential parts, of which part 66 is from fibreinternal water and part 68 is from fibre-external water. This is because the exchange between fibre-external and fibreinternal water is slow. Tn this case, the signal can be arranged using two exponential terms S - Aiexp (-R2i*t ) + Aeexp ( -R2e*t ) , where subscript i refers to water inside fibre and e refers to water outside fibre. The form of the exponential signal in the case of the double exponential model is shown in Figure 7, but it should be understood that the single exponential model also follows quite well the shape of the exp curve of Figure 7.

[0108] With relaxation speeds R21 and Rae, , the proportions of bound and free water inside and outside fibre can be calculated with the following formulas R2=PBRB +PFRF and pB I PF:::1, wherein pB and PF are the proportions of bound and free water and RB and RF are the relaxation speeds of bound and free water. Further, the proportion of bound water inside and outside fibre can be deduced from the formulas pB = ( R-2 ~ RF) / (RB ~ RF) .

[0109] Amplitudes depict the amount of water in the state in question, and the factors of double exponential term model can be used to calculate the proportions of water inside and outside of fibre in the following manner

[0110] We can deduce the proportion m of bound water inside and outside fibre as follows and

[0111] Because WRV depicts the amount of bound water in a sample, the above formulas can be used to calculate WRV indeces for proportions of inside and outside fibre and

[0112] In addition, it must be noted that the measurement can be performed as an online measurement, in which case no separate laboratory measurement is required. value, relative to which all values are corrected computationally. The reference consistency can be 1-2.5%, adavntegously 1.2-1.9%.

[0113] Below is a description of parameters used in the NMR spectrometer, the use of which has provided particularly good accuracy with the empirical constant value. A resonance frequency of 21.73 MHz was used and the echo time in the CPMG pulse sequence is 2 ms. The time between the CPMG pulse sequences (same sample) is 6, 000 ms, when more than one pulse sequence is summed up, after which the sample is changed in the NMR spectrometer with a pump. If the number of samples is one, then the time between pulse sequences is 100 ms (sample changed in-between) . The width of a 90 degrees pulse is 16 or 35 microseconds, while the width of a 180 degrees pulse is 36 or 70 microseconds depending on the magnet and the sample unit .

[0114] The experimental signal can be represented by the following f o rmu 1 a where amplitudes Ai and AEindicate now much there is water in fibre and outside of it. Relaxation speeds calculated from the strength of the exponential signal in step 56, R21

[0115] PIB*RXB+PIF*RXF and RBE = PEB*REB+PEF*REF, indicate the average movement state of water molecules inside and outside fibre. Water internal and external to fibre takes two different states: bound (pB) and free (pr) • In addition, it is known that PiB + piF = 1 and PEB T PEF = 1. The relative amplitude AT_rei indicates the proportion of water inside fibre and can be determined in step 58 with the following formula: whereas the relative amplitude Ap. indicates the proportion of water outside fibre and can be determined with the fl 1 low- ing formula: on the outside. Both portions include both bound and free water .

[0116] The proportions of bound water p in fibre and outside t ibre can be calculated in step 60 as follows: where R.2F is the relaxation time of completely free water, which can be measured, and R2B is the relaxation time of completely bound water, which can be assessed. The determination can be based on the technique described by Zimmerman et al, particularly formulas 48 and 50 (Zimmerman, J. , & Brittle, W. (1957) . Nuclear Magnetic Resonance Studies In Multiple Phase Systems: Lifetime Of A Water Molecule In An Adsorbing Phase On Silica Gel. J. Phys. Chem. , 61 (10) , 1328-1333. doi : 10.1021 j 150556a015 ) . The assessment can be done using a calibration constant; i.e. , by defining an experimental WRV value for the sample, based on which the correct value is calculated for the relaxation speed of completely bound water R2B. Alternatively, R2B can be defined experimentally with the NMR equipment from a sample from which free water has been removed by centrifugation, for example, before the NMR measurement .

[0117] Tne product Ai_rei* piB= mi indicates how large a portion from the total water amount is bound inside fibre and the product ARrPi★ PEB -= meindicates how large a portion of the total amount of water is bound outside fibre. indicates how / many grams of water is bound per each gram of fibre inside fibre and outside fibre (the unit is g / g) . c is the sample consistency

[0118] Finally, it is possible to calculate, in step 62, the WRV value determined with the NMR technique using the following f o rmu 1 a :

[0119] This value corresponds to the WRV value measured in laborato- ry (unit g / g) .

[0120] According to an advantageous embodiment, the fibre porosity WRV value calculated in step 54 using the single exponential signal model and the fibre porosity WRV value calculated in step 62 using the double exponential signal model are compared to each other 64 for evaluating the reliability of calculation. A reliability metric can be, for example, the percent deviation of these two calculated WRV values relative to each other or relative to the previously calculated value. The final porosity measurement result can be an average of these WRV values or a filtered average.

[0121] On the basis of the determination of the measurement result, an assessment can be performed at step 66 to decide whether the criterion for terminating the processing is met and if the criterion is met, a decision to terminate the processing can be made at step 68. In this case, the selected criterion may be a certain target WRV index but alternatively it may also be an absolute value of the NMR parameter determined with an NMR spectrometer. The use of an absolute value reduces the need for calculations but on the other hand, it depicts the end result of processing less well. If the criterion is not met, a switch to a deviation review can be made at step 70 for reviewing the deviation, and at step 72, the required control commands are created with the calculation unit for the performance of adjustments at processing step 40. in other words, the required control parameters for fibre processing can be calculated at step 72 on the basis of the measurement to enable the achievement of an optimal processing result, on the basis of which the processing can be adjusted using adjustment parameters at step 74. For example, if the processing is performed with a refiner, and the first refining round gives a WRV index that is 60% of the target, the refiner' s blade interval or the refining duration can be adjusted so that the target can be achieved with minimal e n e r g y c o n s u mp t a o n .

[0122] Figure 6 snows an example of an embodiment of the system 11 according to the invention. The system 11 includes advantageously the NMR spectrometer 10 for measuring the porosity of individual fibres of a fibre suspension exiting the refiner 100, a computing unit 112 for calculating control parameters based on fibre porosity measured with the device 10 and a selected target, and data transfer equipment 114 for transferring control parameters from the computing unit 112 to the refiner 100. Information on fibre porosity can be utilised to control refining or improve the runnability of a fibre web machine 110 or both. Fibre porosity is measured at least from the fibre suspension that exits the refiner 100, but the measurement can be preferably performed both before and after the refiners 100 as in Figure 6. In this way, it is possible to obtain accurate information about the effect of refining on the properties of fibres of the fibre suspension.

[0123] In the control system, the computing unit 112 receives the fibre porosity information relating to the sample, calculated in near real time by the device 10, using the data transfer equipment 114, along a field bus, for example. A target value, which is the desired fibre porosity value, has preferably been entered in the computing unit 112. The remainder between the measured value and the target is calculated and, based on the remainder, the refiner is controlled, for example, by changing the specific energy consumption of refining or the blade angles of the refiner. The control performed based on the porosity value can also consist of controlling the fibre web machine, for example, by changing the ratio of fibre suspension to fillers in paper production, steam use in the dryer section of a fibre web machine or press pressures in the cress section.

[0124] The computing unit can be a separate computer, but preferably it is integrated into the computer of the NMR spectrometer. Advantageously, the system 11 includes, according to Figure 6, two side flow channels 102 for taking a sample both before and after the refiner 100. In addition, the system preferably also includes the first valve 16 and the third valve 15, which can be used to stop the sample at the device 10. If the sample is taken before the refiner 100, the sample is stopped by using a second valve 17 and the third valve 15. If the sample is taken after the refiner 100, the sample is stopped by using the second valve 17 and the first valve 16. The computing unit 112 is arranged to calculate fibre porosity from the samples taken before and after the refiner 100 and the related comparison value and to control the refiner 100 or the fibre web machine 110 based on the comparison value. The control of the refiner can be feedback control and it is possible to use prior art PID controllers associated with the control for accelerat inci the control.

[0125] According to an embodiment, programmable means of the NMR spectrometer are arranged to use previous memorised porosity measurement values for calculating the control in such a way that two or more latest porosity measurement values are used to calculate an average or other statistical value, with which the impact of an individual measurement deviation on the control is reduced and the control is stabilised.

[0126] In accordance with Figure 10, it has been noted that unexpectedly the ratio of bound water outside the fibre to the total bound water does not change significantly after a certain ratio value has been achieved even if the processing of fibre with refining is continued. In the example shown in Figure 10, refining can be terminated after 90 minutes when the ratio value of 0.85 has been achieved, indicating that the fibre suspension contains a sufficient quantity of the desired MFC or NFC fractions.

[0127] Figure 11 presents the relative proportion of water bound outside fibre as a function of refining time on the left-hand side vertical axis measured with a method according to the invention and with a commercial Valmet FS5 measuring device for comparison. On the basis of an optical determination, the FS5 measuring device provides information on the porosity of a fibre sample as a laboratory measurement for the separated samples. Figure 11 shows that the development of porosity given by the laboratory measurements corresponds well to the results gained with the online measurement according to the invention. The table in Figure 11 was created by refining eucalyptus fibre with the Masuko MKCA6-5 refiner. Refiners that use a similar technique to this refiner can be used in a parallel installation to increase capacity for production purposes. The horizontal axis shows the chronological ID number of the measurements.

Claims

CLAIMS1. A method for manufacturing a micro- or nanofibril suspension, in which fibre suspension is processed for a set period to generate micro- or nanofibrils from fibres, and a measurable quantity that describes micro- and nanofibrils is determined from fibre suspension as measurement data, so that the determination of measurement data is performed as an online measurement where a sample is taken from the fibre characterised in that in the determination of measurement data, fibre porosity is determined by determining the ratio of fibreinternal water to fibre-external bound water as an online measurement using an NMR spectrometer (10) in the following s t e p s w n e r e backward signals of the frequency pulses that return from water molecules excited by frequency pulses (v) to the coil (22) are measured,- the relaxation time of a proton and an amplitude of the backward signal from each backward signal are determined, fibre porosity of the fibre suspension is determined based on the amplitudes and the relaxation times of the exponential backward signal, and a decision to terminate the set period is made using a criterion selected on the basis of the ratio between the amount of water bound inside and outside the fibre.

2. A method according to Claim 1, characterised in that the said criterion for terminating the processing is theproportion of external bound water, which is 80-95% of all bound water, determined the measurement3. A method according to Claims 1 or 2, characterised in that fibre processing is refining.

4. A method according any of Claims 1 to 3, characterised in that additionally the processing of the fibre suspension is adjusted in the method on the basis of another criterion selected on the basis of the measurement data before a decision is made to end the processing.

5. A method according to any of Claims 1 to 4, characterised in that fibre porosity is determined 2-200, advantageously 20-100, times during the production process.

6. A method according to any of Claims 1 to 5, characterised in that the fibre porosity of a fibre suspension is determined using a single exponential signal model Exp = A*exp(R2*t) for determining the relaxation time and a linear formula WRV - a*R? + b for determining fibre porosity as a WRV value, where a is the slope of the linear formula and b is an e mp i r .1 c a 1 c o n s t a n t .

7. A method according to any of Claims 1 to 6, characterised in that the fibre porosity of a fibre suspension is determined using a double exponential signal model Exp = Ar*exp (R2i*t) + AE*exp (R2E*t) + C for determining the relaxation times, where C is a second empirical constant .

8. A method according to Claim 7, characterised in that the fibre porosity is determined as a WRV value in successivecalculating, based on amplitudes AT and As, a relative amount of fibre-internal water Ai__reiand a relative amount of external water AE„rei,- calculating a proportion of water in fibre bound inside fibre pis based on a relaxation speed of completely free water R2F, a relaxation speed of completely bound water RZB and a relaxation speed of fibre-internal bound water R2I, calculating a proportion of water in fibre bound outside fibre PEB based on a relaxation speed of completely free water R2F, a relaxation speed of completely bound water R?B and a relaxation speed of fibre-external bound water R?E, calculating an amount of bound water in fibre WIB based on the relative amount of fibre-internal water Ai__rei, the proportion of water bound inside fibre pis and consistency calculating an amount of bound water outside fibre WEB based on the relative amount of fibre-external water AE_rei, the proportion of water bound outside fibre DEB and the consistency c, calculating the WRV value as a sum of the amount of bound water in fibre WIB and the amount of bound water outside rib re WIB.

9. A method according to any of Claims 1 to 8, characterised in that fibre porosity is determined using both the single exponential signal model and the double exponential signal model by forming a first porosity value and a second porosity value and comparing the first porosity value and the second porosity value with each other for forming a comparison result, based on which fibre porosity is determined.A method according to Claim characterised in that the determination of empirical constants performed by comparing the WRV values given by the NMR spectrometer to theWRV values measured pursuant to the standard from the reference fibre suspension, which are generated by mixing 2 to 10, advantageously 2 to 6 reference fibre suspensions by adding a known mass proportion of micro- or nanofibril suspension to unprocessed fibre suspension, with the mass proportions being between 0.5 m-% and 50.0 m-% and by extrapolating a linear model for mass proportions of >50-100 m-%.

11. A system for producing a micro- or nanofibril suspension, which includes processing equipment (20) for processing fibre suspension by refining to generate micro- or nanofibrils from fibres, and- measurement equipment for determining a measurable quantity from fibre suspension that describes micro-- and nanofibrils as measurement data, characterised in that the measurement equipment comprises- a time-domain NMR spectrometer (10) for determining fibre porosity of the processed fibre suspension based on a sample, comprising a sample channel (12) , a computer (25) and programmable means (30) ,- connection means (13) for connecting the equipment (18) to processing equipment, either directly to a fibre suspension flow channel (101) or to a side flow channel (102) for determining fibre porosity as an online measurement, which side flow channel is arranged to lead part of the fibre suspension flow (14) arriving from the refiner (100) to form a separate sample, and which NMR spectrometer (10) comprises: at least one coil (22) arranged around the sample channel (12) to excite water protons (p) of the fibre suspension contained in the sample by means of frequency pulses,- a magnet (24) arranged around the sample channel (12) for generating a magnetic field (E) in the sample channel ( 12 ) ,- a power source (26) with controllers connected to the coil (22) for generating frequency pulses,- measuring equipment (28) for measuring an intensity of current generated by the frequency pulses returning to the coil (22) from protons (p) , for generating backward signals,- the computer (25) equipped with the programmable means (30) for determining fibre porosity of samples based on the backward signals, which programmable means (30) are arranged to- determine the relaxation time of a proton and an amplitude of the backward signal from the backward signals , determine the fibre porosity of the fibre suspension based on the amplitudes and the relaxation times of the exponential backward signal, and- make a decision to terminate the processing of the fibre suspension using a criterion selected on the basis of the ratio between the amount of water bound inside and outside fibre.

12. A system according to Claim 11, characterised in that the criterion selected for terminating the processing by programmable means (30) is the proportion of external bound water, which is 80-95% of all bound water, determined as the measurement data .

13. A system according to Claim 11 or 12, characterised in that the device (10) comprises a pump (34) for aspirating a sample from the flow channel (101) to the side flow channel (102) and to the NMR spectrometer (10) located in the side flow channel (102) after the NMR spectrometer (10) .14, A system according to any of Claims 11 to 13, characterised in that programmable means (30) are arranged to determine fibre porosity of a fibre suspension based on the amplitude and the relaxation time of the backward signal using both the single and the double exponential signal models for determining the relaxation time.

15. A system according to any of Claims 11 to 14, char- acterised in that a computing unit (112) for calculating control parameters based on fibre porosity measured with the device (10) and a selected target, and- data transfer equipment (114) for transferring control parameters from the computing unit (112) to the refiner (100) or the fibre web machine (110) or both.

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