FI20185221A1

FI20185221A1 - Method and measurement apparatus for measuring suspension

Info

Publication number: FI20185221A1
Application number: FI20185221A
Authority: FI
Inventors: Pasi Kärki; Matti Törmänen; Mikko Haapalainen
Original assignee: Valmet Automation Oy
Priority date: 2018-03-09
Filing date: 2018-03-09
Publication date: 2019-09-10
Also published as: SE1950243A1; CN110243774A; AT521003A1; CN110243774B; AT521003B1; DE102019105668B4; FI128736B; SE542895C2; CA3035947C; DE102019105668A1; CA3035947A1

Abstract

The invention relates to method of measuring a suspension which contains wood fibres. The consistency of the suspension is changed (100) in a consistency range. Optical radiation using a first optical wavelength and a second optical wavelength is directed (102) at the suspension. A first intensity value of the optical radiation related to the first optical wavelength and a second intensity value related to the second optical wavelength on at least one given consistency value is determined (104). The ratio of the first and second intensity values is determined (106). Kappa number of the suspension is determined (108). A raw value for hexenuronic acid, HexA is obtained (110) by applying predetermined factors to the ratio of the first and second intensity values. The content of HexA in the suspension is determined (112) by multiplying the determined ratio with the kappa number.

Description

Method and measurement apparatus for measuring suspension

Technical Field

The exemplary and non-limiting embodiments of the invention relate generally to measurement of a wood fibre suspension.

Background

The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some of such contributions of the invention may be 10 specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.

In paper and pulp manufacturing the purpose is to obtain end product having a good and uniform quality. To ensure the quality measurements are performed during the manufacturing process. For example, lignin content of the 15 pulp is measured. The lignin content of a suspension such as pulp is usually denoted with a kappa number. In standard SCAN-C 1:77, which is known in the field of pulp manufacturing, the kappa number is defined as the amount of potassium permanganate solution with a concentration of 20 mmol/1 in millilitres which one gram of dry pulp consumes in the conditions defined in the standard.

Another substance, which content in the pulp is has an effect on the process and end product, is hexenuronic acid, often denoted as HexA.

The content of HexA from the pulp can be measured in laboratory environment with known methods. However, laboratory measurements are problematic as they typically take time (from 30 minutes to hours) as in 25 manufacturing environments results should be obtained quickly in the different process stages to enable control of the manufacturing process based on the measurements. Thus there is a need for a solution which enables monitoring HexA content during manufacturing phase.

Brief description

An object of the invention is to provide an improved method and an apparatus implementing the method to reduce or avoid the above-mentioned problems.

20185221 PRH 09 -03- 2018

The objects of the invention are achieved by method as claimed in claim 1 and by apparatus as claimed in claim 10. Some embodiments of the invention are disclosed in the dependent claims.

Brief description of the drawings

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

Figure 1 is a flowchart illustrating an example of an embodiment of the invention;

Figure 2 illustrates an example of a measurement arrangement according to an embodiment;

Figure 3 illustrate an example of measurement arrangement;

Figures 4A, 4B and 4C illustrate examples of measurement arrangement;

Figures 5A and 5B illustrate examples of measurement results;

Figure 6 illustrates the calibration of measurement apparatus;

Figure 7 illustrates an example of an apparatus configured to act as a measurement controller and

Figure 8 illustrates an example of a measurement arrangement 20 according to an embodiment.

Detailed description of some embodiments

The solution according to the invention is particularly suitable for measuring suspension which contains wood fibres, but it is by no means limited 25 to this.

In this application 'optical radiation’ means electromagnetic radiation with a wavelength of approximately 40 nm to 1 mm, and 'ultraviolet radiation’ means electromagnetic radiation with a wavelength of approximately 40 nm to 400 nm.

In the proposed solution, a suspension which contains wood fibres, is exposed to optical radiation and interaction of the radiation with the suspension is measured while the consistency of the suspension is changed during the measurement process.

Figure 1 is a flowchart illustrating an example of an embodiment of the 35 invention, where suspension which contains wood fibres is measured.

20185221 PRH 09 -03- 2018

In step 100, consistency of the suspension is changed in a consistency range. In an embodiment, the consistency range extends from an initial consistency to a final consistency.

In step 102, optical radiation using a first optical wavelength λΐ and a 5 second optical wavelength Ä2 is directed at the suspension. In an embodiment, the first optical wavelength is 235 nm ± 50 nm and the second optical wavelength is 280 nm ± 50 nm.

In step 104, a first intensity value of the optical radiation within the consistency range related to the first optical wavelength and a second intensity 10 value related to the second optical wavelength is measured on at least one given consistency value.

In step 106, the ratio of the first and second intensity values is determined. Thus, values Ιλί and Ιλ2 are obtained

Thus in an embodiment, intensity values are measured using two 15 different wavelengths on a given consistency value. A ratio of these intensities is determined.

In another embodiment, consistency of the suspension is changed so that the consistency continuously goes through all consistencies in the consistency range.

The intensity of optical radiation interacted with the suspension is measured at different consistencies in the consistency range. The maximum intensity of the optical radiation related to the first optical wavelength and the second optical wavelength is determined and the ratio of the maximum intensity of the optical radiation related to the first optical wavelength to the maximum 25 intensity of the optical radiation related to the second optical wavelength is determined. Thus, values lÄlmax and IÄ2max are obtained.

Thus as the consistency of the suspension is changed from the initial consistency to the final consistency the measurement is repeated at given intervals using both first and second wavelength. The interval may be a 30 measurement parameter. As a result a value for intensity Ιλί for the first optical wavelength λΐ and Ιλ2 for the second optical wavelength Ιλ2 are obtained.

In an embodiment, the optical radiation is directed to the suspension using one or more optical power sources. There may be a power source for each wavelength, or the wavelength of the radiation outputted by the source may be 35 changed or the wavelength if the radiation is selected using filters, for example.

20185221 PRH 09 -03- 2018

The intensity of optical radiation interacted with the suspension is measured with one or more optical measurement sensors having a given surface area and distance from the one or more optical power sources.

In an embodiment, the given surface area and distance are selected on 5 the basis of the consistency range and desired amount of intensity.

In an embodiment, the first optical wavelength and the second optical wavelength are within the ultraviolet radiation wavelength range.

In step 108, kappa number of the suspension is determined.

There are various ways of determining the kappa number K. In an 10 embodiment, the kappa number of the suspension is determined based on one or both of the determined maximum intensity values lÄlmax, lÄ2max. However, any prior art method for determining the kappa number of the suspension may be utilised here as well.

In step 110, a raw value for hexenuronic acid, HexARaw, is obtained by 15 applying predetermined factors to the determined ratio ΙΆ1/ΙΆ2 or lÄlmax/lÄ2max. The predetermined values calibrate the measurement results. An example of obtaining the predetermined values is explained below in connection with Figure 6.

In step 112, the content of hexenuronic acid, HexA, in the suspension is 20 determined by multiplying the raw value with the kappa number. Thus,

HexA = K * HexARaw or HexA = K * HexARaw.

HexA content in pulp may have an effect in kappa measurements. HexA and lignin have different properties and cause different effects in bleaching of the manufacturing process. Thus knowledge of the HexA content is important. 25 The oxidation phase of the manufacturing process HexA content is not reduced as the lignin content. Using CIO2 in the manufacturing process reduces both HexA and lignin, but due to the high cost of CIO2 it is not a good choice for HexA removal as there are cheaper substances for removing HexA.

Next, an example of a measurement arrangement of an embodiment 30 will be described with reference to Figure 2, which shows application of the invention in the pulp and paper industry.

Figure 2 shows a pipe 200 where a suspension 202 containing wood fibres, i.e. wood fibre pulp, is flowing. A sample of the suspension is taken with a sampler 204 from the pipe 200. The sampler 202 may be a solution known per se, 35 e.g. based on a piston and a cylinder. The sample is conveyed using a pipe 206 to a measurement chamber 208, valve 210 being closed.

20185221 PRH 09 -03- 2018

The suspension in the measurement chamber may be processed prior measurement. For example, liquid may be filtered by using pressured air. Valve 212 may be opened and the air coming through the valve presses the sample against the wire 214 and liquid flows through valve 216.

The sample may be washed using water and air by opening valves 212 and 218, the waste water flows through the valve 216.

When the sample has been washed measurement process may start by mixing the sample using pressured air through valve 220 and by adding water through valve 222. When sample has been mixed air valve 220 is closed. Water 10 valve 222 is left open. Water comping through the valve changes the consistency of the sample and at the same time mixes the sample. The consistency of the suspension is changed in a consistency range. In an embodiment, the consistency range extends from an initial consistency to a final consistency.

Measuring may be performed during the chancing of the consistency of 15 the sample using measurement arrangement 224, 226 which may be controlled by a measurement controller 228. In an embodiment, the measurement arrangement comprises a source and detector part 226 and optical fibre and measurement head part 226.

Figures 3 and 4A to 4C illustrate examples of measurement 20 arrangement 224, 226. In an embodiment, the arrangement comprises one or more optical power sources 300. For simplicity, only one source is shown in Figure 3. Measurements are usually made in the ultraviolet light, for which reason the optical power source may typically emit at least ultraviolet light. The source 300 may be a Xenon lamp or a LED (light emitting diode), for example. The optical 25 power source direct may be configured to direct optical radiation at the suspension. In an embodiment, the radiation is directed to the suspension using first optical fibre 306. The first optical fibre 306 may be configured to direct the optical radiation at the suspension, the first end of the fibre being connected to the optical light source 300 and the second end of the fibre, located at a 30 measurement head and being inserted in the measurement chamber 208.

In an embodiment, the arrangement further comprises one or more detectors 302, 304 arranged to measure the intensity of optical radiation interacted with the suspension. In an embodiment, each detector is connected to a set of optical fibres 308, 310, the ends of the optical fibres being positioned next 35 to the second end of the first optical fibre 302.

20185221 PRH 09 -03- 2018

Figure 4A to 4C illustrate examples of the fibre arrangement in the measurement head 312 which may be inserted into the measurement chamber 208.

Figure 4A illustrates an embodiment, where the measurement 5 arrangement comprises the optical power source 300 connected to first optical fibre 308 and detector 302 connected to optical fibre 308. At the measurement head the first optical fibre 306 and the optical fibre 308 are located side by side with a given distance 400 from each other.

Figure 4B illustrates another embodiment, where the measurement 10 arrangement comprises the optical power source 300 connected to first optical fibre 308 and detector 302 connected to a set of optical fibres 308. At the measurement head the ends of the optical fibres 308 are positioned next to the end of the first optical fibre 306 at a same given distance 402 from the first optical fibre.

Figure 4C illustrates another embodiment, where the measurement arrangement comprises the optical power source 300 connected to first optical fibre 308 and detectors 302, 304 connected to a set of optical fibres 308, 310. At the measurement head the ends of the optical fibres 308 are positioned next to the end of the first optical fibre 306 at a same given distance 404 from the first 20 optical fibre and the ends of the optical fibres 310 are positioned next to the end of the first optical fibre 306 at a same given distance 406 from the first optical fibre.

In an embodiment, the measurement chamber 208 comprises a window 230 in the wall of the measurement chamber. The optical power source 25 300 or the first optical fibre 306 connected to the source may be placed outside the measurement chamber behind the window for directing optical radiation at the suspension.

Likewise one or more detectors 302, 304 or optical fibres 308, 310 connected to the detectors may be placed outside the measurement chamber 30 behind the window 230 in the measurement chamber wall.

The use of optical fibres described above is merely an example. The measurement may be realised also without optical fibres, in an embodiment, the optical radiation is led to the measurement chamber using a radiation conductor such as a lens, a wave guide or any suitable medium. For example, the optical 35 source and detectors may be placed behind the window 230 without the use of any optical fibres.

20185221 PRH 09 -03- 2018

Figures 5A and 5B illustrate examples of measurement results when the intensity of optical radiation interacted with suspension at different consistencies is measured using above described measurement arrangement using first optical wavelength and second optical wavelength. In the nonlimiting 5 examples of Figure 5A and 5B the first optical wavelength is 235 nm and the second optical wavelength is 280 nm. Depending on the embodiment the wavelength may vary, for example by ± 50 nm.

Figure 5A illustrates measurements made using the first optical wavelength 235 nm. In the graph consistency is on the x-axis 500 and measured 10 intensity is on the y-axis 502. Figure 5B illustrates measurements made using the second optical wavelength 280 nm. In the graph consistency is on the x-axis 504 and measured intensity is on the y-axis 506. The consistency of the suspension sample is changed as a function of consistency. Typically, in the beginning the suspension is large and as more water is mixed with the sample the suspension 15 gets lower.

The consistency of the sample of the suspension is changed during measurement process. Figures 5A and 5B show consistency on x-axis, where the small consistency value is on the left and higher consistency value on the right. In the actual measurements process the consistency is large in the beginning and as 20 water is added the consistency diminishes.

As optical radiation from the optical power source is directed to the sample of the suspension, part of the radiation scatters from the wood fibres to the detector, part scatters elsewhere and part absorbs in lignin. At some point, as the consistency changes, there is a maximum value 508, 510, for the measured 25 intensity. The measurement arrangement may be configured to detect the maximum value 508, 510 of the intensity detected by the detector.

The consistency with which the maximum intensity is reached depends on absorption. The greater the absorption the smaller the consistency with which the maximum intensity occurs.

In an embodiment, the initial consistency of the consistency range measurement depends on the properties of the suspension. The measurement continues until the maximum intensity has been detected and is terminated when the measured intensity is getting smaller after the maximum value.

In an embodiment, the measurement arrangement is calibrated to 35 function correctly by performing calibration measurements. These measurements may be performed using a normalizing reference plate placed in front of the

20185221 PRH 09 -03- 2018 measurement arrangement. In an embodiment, the calibration is performed using reference pulp. Calibration is necessary before the measurement apparatus is actually used and needs to be performed from time to time because the route of optical radiation, for example, may change or the detector responses may change 5 in the course of time. The reference pulp is wood fibre pulp whose properties have been measured in the laboratory and stabilized with respect to time. There is reference pulp commercially available for calibration of the measurement apparatus, e.g. Paprican standard reference pulp 5-96 from a Canadian manufacturer.

In an embodiment, the surface areas and numerical apertures of the optical source and the detectors are selected on the basis of the consistency range of the suspension and desired amount of intensity.

In an embodiment, the distances 400, 402, 404, 406 and the surface area of the cross sections and numerical apertures of optical for fibres or sets of 15 optical fibres 306, 308 and 310 are selected on the basis of the consistency range of the suspension and desired amount of intensity.

The distances 400, 402, 404, 406 and the surface area of the cross sections of optical for fibres or sets of optical fibres 306, 308 are denoted in following as measurement geometry. Measurement geometry relates to the 20 consistency range. When measurements are made, the consistency of the suspension must be such that sample processing (washing of sample and changing the consistency) are possible. If the consistency of the suspension is too large the sample processing may not succeed. On the other hand, if the consistency is too low dynamics of the measurement suffers. Also available 25 intensity of light from the optical light source has an effect on the measurements. When kappa number is measured, the large the kappa is the more the lignin in the sample absorbs light.

In an embodiment, the purpose is to detect the maximum intensity of the optical radiation interacted with the suspension within the consistency range. 30 The consistency at which the maximum intensity is reached may depend on following issues:

The distance 400, 402, 404, 406 between the optical power source and the measurement point, i.e the distance between the end of the first optical fibre 306 and the ends of other optical fibres 306, 308. The 35 larger the distance the smaller is the consistency when maximum intensity occurs.

20185221 PRH 09 -03- 2018

The surface areas of the optical power source and measurement points. The larger the surface areas the smaller is the consistency when maximum intensity occurs.

The kappa number of the sample. The larger the kappa number the 5 smaller is the consistency when maximum intensity occurs.

Wavelength of the radiation outputted by the optical power source. Absorption of the radiation in the suspension depends on the wavelength. The larger the absorption the smaller is the consistency when maximum intensity occurs.

- Particle size of the sample of the suspension. The smaller the particles, the smaller is the consistency when maximum intensity occurs.

Thus in an embodiment, measurement parameters may comprise the measurement geometry, the wavelength of the optical radiation and the consistency range used in the measurements.

Further, the consistency range may depend on the properties of the suspension. For example, when measuring pine suspension consistency range may be 0.3-0.1% and when measuring birch suspension consistency range may be 0.4-0.2%. These numerical values are only non-limiting examples.

Typical values for optical fibre diameters are around few hundred pm, 20 but also other values may be used depending on the property to be measured.

In general, the above discussion applies also when optical fibres are not used but the optical source and detectors are connected to the measurement chamber using some other suitable medium.

In an embodiment, as mentioned above in connection with Figure 1, 25 the ratio of the maximum intensity of the optical radiation related to the first optical wavelength to the maximum intensity of the optical radiation related to the second optical wavelength IÄlmax/lÄ2max is determined.

Figure 8 illustrates an embodiment of a measurement arrangement. In this example intensity values are measured in a measurement chamber. The 30 measurement arrangement comprises a measurement chamber 800 having suspension with a given consistency. The arrangement comprises one or more light sources 802, 804. In an embodiment, a light source may transmit light with multiple wavelengths, such a Xenon light source, for example. In an embodiment, there may be a light source for each wavelength. An example of a single 35 wavelength light source is a led. The arrangement further comprises one or more detectors 806, 808. In an embodiment, a detector may comprise a filter passing

20185221 PRH 09 -03- 2018 through only a given wavelength. The filter may be changeable. This is suitable especially when the light source transmit multiple wavelengths. In an embodiment, where the light source transmits only one wavelength a filter is not required.

Further, kappa number of the suspension is determined. In an embodiment, the kappa number of the suspension is determined based on one or both of the determined maximum intensity values Rlmax, IÄ2max. However, any prior art method for determining the kappa number of the suspension may be utilised here as well.

When the ratio of the first and second intensity values and kappa number has been determined, a value for HexA may be determined. To calibrate the measurement results predetermined factors are applied to the ratio and a so called raw HexA value is obtained. The HexA value in umol/g units is obtained by multiplying the raw HexA value with the kappa number.

Figure 6 illustrates an example of determining the predetermined factors. In determining the factors, the consistency of the suspension is changed and the intensity is measured at two wavelengths, in this example 235 and 280 nm. The samples from which the intensity values are measured are taken also to laboratory premises where kappa number and HexA value is determined using laboratory procedures. Thus, for each intensity value ratio there exists a Laboratory HexA and kappa values, which may be denoted as HexALAB and KappaLAB. Figure 6 illustrates the relationship of ratio HexALAB/KappaLAB as a function of ratio of intensity values. As can be seen, in this example, the relationship follows a power function.

In a general form, the power function may denoted as y = ax^b where y equals HexALAB/KappaLAB and x equals ΙΆ1/ΙΆ2 and where variables a and b are the predetermined factors.

In the specific example of Figure 6, the power function is y =0.6561 χ1.402.

Thus, when the relationship follows the above power function, the

RawHexA value may be obtained from the measured ratio of intensity values as RawHexA = a * (Ιλ1/Ιλ2)^b or RawHexA = a * (IÄlmax/IÄ2max)^b.

The power function is here used as an example only. Depending on the situation, the relationship may also be a linear function, or a polynomial function 35 or some other function which maps the ratio of intensity values to the ratio HexALAB/KappaLAB.

20185221 PRH 09 -03- 2018

In general, for each measurement apparatus, the determination of the predetermined factors needs to be done only once if the configuration of the apparatus or the suspension type (from one tree type to another, for example) does not change. In an embodiment, the correctness of the factors may be checked 5 from time to time using measurements.

Figure 7 illustrates an embodiment. The figure illustrates a simplified example of an apparatus configured to act as a measurement controller 228.

It should be understood that the apparatus is depicted herein as an example illustrating some embodiments. It is apparent to a person skilled in the 10 art that the apparatus may also comprise other functions and/or structures and not all described functions and structures are required. Although the apparatus has been depicted as one entity, different modules and memory may be implemented in one or more physical or logical entities.

The apparatus 228 of the example includes a control circuitry 700 15 configured to control at least part of the operation of the apparatus.

The apparatus may comprise a memory 702 for storing data. Furthermore the memory may store software 704 executable by the control circuitry 700. The memory may be integrated in the control circuitry.

The apparatus may further comprise an interface circuitry 706 20 configured to connect the apparatus to other devices. The interface may provide a wired or wireless connection. The interface may connect the apparatus to the measurement arrangement 224, 226. In an embodiment, the apparatus may be connected to an automatic process control computer used in the manufacture of pulp.

The apparatus may further comprise user interface 708 such as a display, a keyboard and a mouse, for example. In an embodiment, the apparatus does not comprise user interface but is connected to other devices providing access to the apparatus.

In some embodiments, the apparatus may be realised with a mini- or 30 microcomputer, a personal computer or a laptop or any suitable computing device.

In an embodiment, intensity measurements and kappa measurements may be performed in the same measurement chamber using different measuring geometry. For example, in the solution of Figure 4C one detector may measure 35 kappa number and other intensity.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

Claims

1. A method of measuring a suspension which contains wood fibres, the method comprising:

changing (100) consistency of the suspension in a consistency range;

5 directing (102) optical radiation using a first optical wavelength and a second optical wavelength at the suspension;

measuring and determining (104) a first intensity value of the optical radiation within the consistency range related to the first optical wavelength and a second intensity value related to the second optical wavelength on at least one

10 given consistency value; and determining (106) the ratio of the first and second intensity values;

determining (108) kappa number of the suspension;

obtaining (110) a raw value hexenuronic acid, HexA, by applying predetermined factors to the ratio of the first and second intensity values; and

15 determining (112) the content of hexenuronic acid, HexA, in the suspension by multiplying the raw value with the kappa number.
2. The method as claimed in claim 1, further comprising:

directing the optical radiation to the suspension using an optical 20 power source; and measuring the intensity of optical radiation interacted with the suspension with one or more optical measurement sensors having a given surface area, numerical aperture and distance from the optical power source.

25
3. The method as claimed in claim 1 or 2, wherein the first optical wavelength and the second optical wavelength are within the ultraviolet radiation wavelength range.
4. The method as claimed in any previous claim 1 to 3, wherein the 30 first optical wavelength is 235 nm ± 50 nm and the second optical wavelength is

280 nm ± 50 nm.
5. The method as claimed in any preceding claim 1 to 4, further comprising: changing the consistency of the suspension so that the consistency

35 continuously goes through all consistencies in the consistency range,

20185221 PRH 09 -03- 2018 measuring the intensity of optical radiation interacted with the suspension at different consistencies in the consistency range;

determining the maximum intensity of the optical radiation related to the first optical wavelength and the second optical wavelength; and

5 determining the ratio of the maximum intensity of the optical radiation related to the first optical wavelength to the maximum intensity of the optical radiation related to the second optical wavelength.
6. The method as claimed in any preceding claim 1 to 5, further 10 comprising:

taking a sample of suspension to be measured to an unpressurised measurement chamber.
7. The method as claimed in any previous claim 1 to 6, further 15 comprising:

directing the optical radiation at the suspension using a first optical fibre having a given diameter and numerical aperture and measuring the intensity of optical radiation interacted with the suspension with a detector connected to a set of optical fibres, each optical fibre 20 having a given diameter, and the ends of the optical fibres being positioned next to the end of the first optical fibre at a same given distance from the first optical fibre.
8. The method as claimed in any previous claim 1 to 7, further 25 comprising:

directing the optical radiation at the suspension using one or more light sources placed outside the measurement chamber behind a window in a measurement chamber wall; and measuring the intensity of optical radiation interacted with the 30 suspension with a detector placed outside the measurement chamber behind a window in a measurement chamber, the detector having a given diameter, and located a given distance from the one or more light sources.
9. The method as claimed in any previous claim 1 to 8, further 35 comprising:

20185221 PRH 09 -03- 2018 measuring the intensity of optical radiation interacted with the suspension at different consistencies in the consistency range using the first optical wavelength and the second optical wavelength;

obtaining HexALab and KappaLab which denote the HexA value and

5 kappa number of the suspension at the same consistencies determined at laboratory;

determining a function which maps the ratio of the measured first and second intensity values to the relation of HexALab and KappaLab;

determining the predetermined factors on the basis of the function.
10. A measurement apparatus for measuring a suspension which contains wood fibres, the measurement apparatus comprising one or more optical power sources (300) for directing optical radiation at the suspension and at least one optical measurement sensor (302) for measuring optical radiation 15 interacted with the suspension, the measurement apparatus being arranged to change (100) consistency of the suspension in a consistency range;

direct (102) optical radiation using a first optical wavelength and a second optical wavelength at the suspension ;

measure and determine (104) a first intensity value of the optical 20 radiation within the consistency range related to the first optical wavelength and a second intensity value related to the second optical wavelength on at least one given consistency value;

determine (106) the ratio of the first and second intensity values;

determine (108) kappa number of the suspension;

25 obtain (110) a raw value hexenuronic acid, HexA, by applying predetermined factors to the ratio of the first and second intensity values; and determine (112) the content of hexenuronic acid, HexA, in the suspension by multiplying the determined ratio with the kappa number.

30
11. The apparatus as claimed in claim 10, wherein:

at least one measurement sensor has a given surface area, numerical aperture and distance from the one or more optical power sources, the given surface area and distance being selected on the basis of the consistency range and desired amount of intensity.

20185221 PRH 09 -03- 2018
12. The apparatus as claimed in any previous claim 10 to 11, wherein the one or more optical power sources are configured to output the first optical wavelength and the second optical wavelength which are within the ultraviolet radiation wavelength range.
13. The apparatus as claimed in any previous claim 10 to 12, wherein the one or more optical power sources are configured to output first optical wavelength having the value of 235 nm ± 20 nm and the second optical wavelength having the value of 280 nm ±20 nm.
14. The apparatus as claimed in any previous claim 10 to 13, further configured to change the consistency of the suspension so that the consistency continuously goes through all consistencies in the consistency range
15 measure the intensity of optical radiation interacted with the suspension at different consistencies in the consistency range;

determine the maximum intensity of the optical radiation related to the first optical wavelength and the second optical wavelength; and determine the ratio of the maximum intensity of the optical radiation

20 related to the first optical wavelength to the maximum intensity of the optical radiation related to the second optical wavelength.

15. The apparatus as claimed in any previous claim lOto 14, further comprising:

25 a first optical fibre (306) configured to direct the optical radiation at the suspension, the first end of the fibre being connected to the optical light source (300) and the second end of the fibre being in the measurement chamber; and one or more detectors (302) for measuring the intensity of optical

30 radiation interacted with the suspension, each detector being connected to a set of optical fibres (308), each optical fibre having a given diameter, and the ends of the optical fibres being positioned next to the second end of the first optical fibre at a same given distance from the first optical fibre (306), the given diameter and distance being selected on the basis of the consistency range and desired amount

35 of intensity.
16. The apparatus as claimed in any previous claim 10 to 14, further comprising a window (230) in a measurement chamber wall, the optical power source being placed outside the measurement chamber (208) behind the window 5 in wall for directing the optical radiation at the suspension; and one or more detectors (302) for measuring the intensity of optical radiation interacted with the suspension, the detectors being placed outside the measurement chamber behind the window (230) in the measurement chamber wall, each detector having a given diameter, and located a given distance from the 10 optical power source (300), the given diameter and distance being selected on the basis of the consistency range and desired amount of intensity.
17. The apparatus as claimed in any previous claim 10 to 16, further comprising measuring the intensity of optical radiation interacted with the

15 suspension at different consistencies in the consistency range using the first optical wavelength and the second optical wavelength;

obtaining HexALab and KappaLab which denote the HexA value and kappa number of the suspension at the same consistencies determined at laboratory;
20 determining a function which maps the ratio of the measured first and second intensity values to the relation of HexALab and KappaLab;

determining the predetermined factors on the basis of the function.