The invention is related to an NMR measurement unit,which includes

- a flow channel comprising a first end and a second end for separating a sample from a fluid stream present in a process channel via the first end and for returning the sample to the fluid stream via the second end,

- a magnet arranged in relation to the flow channel for creating a magnetic field at least in a part of the flow channel,

- acoil arranged inrelationtotheflow channelforexciting NMR active nuclei of the sample moving in the flow channel and for receiving the frequency pulse that returns to the coil from the NRM active nuclei.

The invention is also related to an NMR measurement system.

NMR (Nuclear Magnetic Resonance) measurement can be used to determine properties of materials for several different purposes. Inthe processindustry,NMRmeasurement canbeused inthestructural analysis of molecules and thereby, in the measurement of materials contained in processes.

Publication WO 2017/220859 A1 representing prior art proposes a method and equipment for determining the beating rate of a fibre suspension based on an online NMR measurement. In the method, a sample is separated from a process stream into a separate sampling channel passing through an excitation coil and a magnet. Based on the TimeDomainNMR Spectroscopy,protons contained in the sample are excited and the magnitude of the frequency pulse returning from protons is measured for determining the characteristics of the sample.

However, a problem related to a method and equipment similar to the publication is that the equipment requires space outside the process pipe and is difficult to move from one place to another. In addition, collecting a representative sample from the process stream can be challenging.

The object of the invention is to provide an NMR measurement unit, which can be placed in the process in the same way as a measurement sensor so that the sample need not be taken out from the process channel. The characteristic features of this invention are set forth in the appended claim 1. Another object of the invention is to provide an NMR measuring system, where the measurement unit can be installed in the process in the same way as a measurement sensor. The characteristic features of this invention are set forth in the appended claim 14.

The object of the invention can be achieved with an NMR measurement unit, which includes a frame comprising a fastening flange for sealing the measurement unit to the process channel and a chamber closed at least relative to the fluid stream and arranged to be installed at least mainly inside the process channel. In addition, the measurement unit includes a flow channel comprising a first end and a second end for separating a sample from a fluid stream present in the process channel via the first end and for returning the sample to the fluid stream via the second end.The flow channel passes through the chamber and the frame can be installed in such away thatthe flow channelispositioned inside theprocesschannel. Themeasurement unit further includes amagnet arranged in relation to the flow channel for creating a magnetic field at least in a part of the flow channel and a coil arranged in relation to the flow channel for exciting NMR active nuclei of the sample moving in the flow channel and for receiving the frequency pulse that returns to the coil from the NRM active nuclei. The magnet and the coil are arranged inside the chamber.

Thanks to the frame comprising a fastening flange and a chamber, the NMR measurement unit can be placed inside the process channel in such a way that the fluid stream flowing in the process channel goes through the NMR measurement unit via the flow channel and the measurement can be taken without separating the sample into separate pipelines outside the process channel. Due to the frame, the NMR measurement unit is easy to install and move in the same way as any conventional sensor designed for process measurement, such as a temperature sensor, for example.

Henceforth,the NMRmeasurementunit isreferredto withasimplified designation 'measurement unit'.

Advantageously, the magnet encircles the flow channel. In this way, a magnetic field can be created with one magnet.

Alternatively, it is also possible to provide two or more magnets arranged around the flow channel. In this case, magnets need not be special magnets provided with an opening at the centre for the flow channel.

Advantageously, the coil encircles the flow channel. Thus, excitation of nuclei and the receipt of the frequency pulse can be implemented with one coil.

Alternatively, it is also possible to provide two or more coils arranged around the flow channel.Then, a large coil is not needed, for example,around a large-diameter process channel,but the coils can be smaller in size.

According to a first embodiment, the measurement unit includes a first valve and a second valve both arranged in the flow channel inside the chamber,onepreceding themagnet and the other following the magnet, for stopping the flow in the region of the magnetic field. With the first valve and the second valve, it is possible to stop and isolate a sample in the flow channel and keep it uniform andmainly inplaceduringthemeasurement .In thisway,each excited NMR active nucleus stays in the region of the magnetic field and the coil,untilthe pulse reflectingback from theNMR active nucleus can be received with the coil and thus included in the measurement. In this way, it is possible to avoid a situation in which part of protons become excited but can exit from the coil region prior to the discharge of excitation and formation of a reflecting pulse at the coil.In this context,the definition "preceding the magnet" means the location in the travel direction of the sample in the flow pipe upstream of the magnet or, in other words, the location between the first end of the flow channel and the magnet.

According to a second embodiment, the measurement unit includes a first valve and a pump both arranged in the flow channel inside the chamber, one preceding the magnet and the other following the magnet, for stopping the flow in the region of the magnetic field, wherein the pump is arranged to aspirate the sample to the flow channel and stop the sample together with the first valve. With such an implementation, it can be ensured that the sample is taken to the flow channel in the case that the fluid stream consists of a high viscosity fluid or other thick stream, for example.

According to a third embodiment, the diameter of the flow channel is continuous and uniform throughout its length. In other words, the measurement unit thus does not include valves arranged in the flow channel, but the flow channel is of a flow-through type and the measurement is taken from a moving fluid.Advantageously, this is suitable for fluids moving at a lower speed than 1 m/s, wherein the nuclei cannot escape through the magnetic field too fast and, on the other hand, have enough time to become sufficiently excited so that an adequately large frequency pulse can be generated at the coil. Advantageously, in a situation according to a third embodiment, wherein, in addition, the flow speed of the fluid exceeds 1 m/s, a pre-magnet placed in the process stream prior to the measurement unit is used in relation to the measurement unit, enabling partial excitation of nuclei already prior to the generation of a magnetic field by the magnet of the measurement unit, for providing sufficiently large frequency pulses at the coil regardless of the high flow speed of the fluid.

Advantageously, the flow channel includes a pipe provided with a coating that prevents soiling, preferably a Teflon pipe having two ends. In this way, the flow channel keeps clean in a better way in the region of the magnetic field, the measurement unit is more maintenance-free and soiling does not cause an error in the measurement.

The pipe preferably includes fastening means for fastening the pipe removably to the first valve and the second valve or to the first valve and the pump. Thus, the pipe is easy to maintain and clean in case of soiling, without removing the entire measurement unit.

Alternatively, themeasurementunit may alsoinclude anozzleplaced in relation to the first end of the flow channel for feeding a pressurized medium, preferably water or air, to the flow channel at intervals for keeping it clean.

The flow channel is advantageously composed of a pipe and a first valve and a second valve, or a pump instead of the second valve.

Advantageously, the chamber extends perpendicularly from the fastening flange. Such a construction is easy to manufacture. The chamber can have a wheel shape or preferably an elliptical shape seen in the perpendicular direction relative to the fastening flange for reducing the flow resistance caused by the measurement unit. Thus, the measurement unit does not notably increase pumping costs related to the fluid stream of the process channel.

The chambermay include a closable cover for closing themeasurement unit. In this way, the magnetic field generated by the magnet of the measurement unit cannot extend to the environment and, on the other hand,components of the measurement unit are protected inside a closed chamber.

Advantageously, the fastening flange includes a bolt rim for fastening themeasurementunit to an opening included in theprocess channel. In thisway,themeasurementunit iseasy to fastentightly, for example, to an inspection hatch type opening in the pipe that functions as the process channel, instead of an inspection hatch cover.

Advantageously, the measurement unit is a time domain NMR measurementunit .Timedomainmeasurement isa simple and relatively fast measurement method, thus enabling a fast measurement of the sample.

The diameter of the chamber may be in the range of 200 - 450 mm, preferably 300 - 350 mm. Thus, the flow resistance caused by the measurement unit remains moderate.

The NMR active nucleus is preferably a proton.

The fluid stream is preferably a liquid stream. A liquid stream has a sufficiently large amount of NMR active nuclei, preferably protons, sothatNMRmeasurement canbecompleted relativelyquickly without a long sampling time. The inner diameter of the flow channel may be in the range of 2 - 30 mm, preferably 10 - 20 mm. A flow channel with an inner diameter of 10 - 20mm isparticularly suitable for analysing liquid samples, since the flow channel is thus sufficiently large to prevent the sample from adheringtotheflow channelwallsbecause ofthe surface tension between the flow channel and the sample.

Advantageously, the frame includes a lead-through for leading said communication cable from the measurement unit to the computing unit. In this way, measurement data and control commands can be reliably taken into a closed chamber, as the computing unit is located elsewhere.

Alternatively, if the sample is gas, the flow channel preferably has a diameter of 2 - 10 mm so that the NRM active nuclei over the magnet range can be collected closer to each other for the measurement. When analysing gas, instead of a second valve, it is particularly advantageous to use a pump to pressurize and more intensively compact the NMR active nuclei in the region of the magnetic field, the measurement thus being faster.

According to an embodiment, the measurement unit can include cleaning equipment arranged in relation to the first end of the flow channel, comprising a nozzle, a pressure medium channel connected to the nozzle and a pump connected to the pressure medium channel for feeding a pressure medium from the nozzle to the flow channel for cleaning it. For example, the pressure medium can be water or compressed air. With such cleaning equipment, the flow channel can be cleaned from the solid matter without removing the measurement unit from the process stream.

The object of the NMR measurement system according to the invention can be achieved with an NMR measurement system, which includes an NMR measurement unit for performing an NMR measurement on a sample separated from a process channel for forming a measurement signal and a computing unit connected to the measurement unit for controlling the operation of the measurement unit and computing selected properties based on the measurement signal. In the measurement system, the NMR measurement unit includes a frame comprising a fastening flange for sealing the measurement unit to the process channel and a chamber closed at least relative to the fluid stream and arranged to be installed at least mainly inside the process channel. In addition, the measurement unit includes a flow channel comprising a first end and a second end for separating a sample from a fluid stream in the process channel via the first end and for returning the sample to the fluid stream via the second end. The flow channel passes through the chamber and the frame can be installed in such a way that the flow channel is positioned inside the process channel. The measurement unit further includes a magnet arranged in relation to the flow channel for creating a magnetic field at least in a part of the flow channel and a coil arranged in relation to the flow channel for exciting NMR active nuclei of the sample moving in the flow channel and for receiving the frequency pulse that returns to the coil from the NRM active nuclei. The magnet and the coil are arranged inside the chamber.

In the NMR measurement system according to the invention, the measurement unit comprises a frame, which enables the positioning of the measurement unit at least partly inside the process pipe, thus avoiding the need of guiding the fluid stream out from the process pipe. On the other hand, the computing unit can be located in a freely selectable position, reachable via a connection. Henceforth, the NMR measurement system is referred to with a simplified designation 'measurement system'.

Themeasurement unit advantageously includes a communication cable for transferring control commands from the computing unit to the measurement unit and for transferring measurement data from the measurement unit to the computing unit, and the frame includes a lead-through for leading through said communication cable. In this way, measurement data and control commands can be reliably taken into a closed chamber, as the computing unit is located elsewhere.

According to an embodiment, the measurement system is arranged to determine the solid content of black liquor and the computing unit includes software means arranged to compute the solid content of black liquor using the general correlation function form SC = A * T2(R2TC) + B, where SC is the solid content of black liquor, T2(R2TC) is the spin-spin relaxation time with corrected temperature,Aisaconstant andB isaconstant.Mostadvantageously, thesolid contentofblackliquor isdeterminedusingthecorrelation function form SC = 14.211n(R2TC) + 3.05, where R2TC is the temperature-corrected relaxation time.Ithasbeen discovered that this kind of computing method has good accuracy.

With the measurement unit and measurement system according to the invention, it is possible to reduce costs related to the use of NMR measurement in the process environment and facilitate its introduction intouse.Ameasurement unitusinga frameispositioned inside a process channel, thus the sample obtained from the fluid stream being more representative and the measurement unit does not take space outside the process channel. On the other hand, a measurement unit according to the invention is easy to install in inspection hatches or service openings of an existing process channel without forming additional openings or channels. Furthermore, a measurement unit according to the invention is relatively simple and maintenance-free and enablesNMR measurement in the same way as conventional sensors. Traditionally, NMR measurements have been performed using separate, even offline systems, which at least guide the fluid stream out from the process channel. The NMR measurement unit and the NMR measurement system according to the invention are suitable for use for the analysis of both liquid and gaseous samples; however, samples are most preferably liquid, in which case there are more NMR active nuclei in the sample and the determination can be performed faster.

The invention is described below in detail by making reference to the appended drawings that illustrate some of the embodiments of the invention, in which

Figure la is a basic view of a measurement system according to the invention, with the measurement unit according to the invention split,

Figure lb is an enlarged view, with the measurement unit according to the invention split,

Figure 2 is a lateral basic view of the magnet of the measurement unit,

Figure 3 is an axonometric view of a measurement unit according to the invention, separated and partly cut,

Figure 4 is a bottom view of a measurement unit according to the invention without the bottom,

Figure 5 is a top view of a measurement unit according to the invention,

Figures 6a and 6b illustrate the magnet of a measurement unit according tothe invention,separated,indifferent directions,

Figures 7a and 7e illustrate the frame of a measurement unit according tothe inventionindifferentdirections,

Figure 8 is a block diagram illustrating the operation of a measurement system according to the invention.

In the embodiment of the invention illustrated in Figures la - 7b, the sample is liquid and the NMR active nuclei contained therein are protons. In this context, it is obvious to those skilled in the art that the invention can also be implemented when the fluid stream is gas and the NMR active nuclei are generally known NMR active nuclei other than protons, such as oxygen or phosphor.

According to Figures la and lb, a measurement unit 10 according to the invention is arranged to be used installed in a process channel 18 as part of a measurement system 100 according to the invention in the same way as a measurement sensor. The measurement unit 10 can be positioned, as shown in Figure la, at least partly inside the process channel 18 in such a way that at least part of the process stream flowing in the process channel 18 meets the measurement unit 10 and flows through the measurement unit 10. The measurement unit 10 is advantageously connected to an existing process channel 18, such as a process channel related to the processing of black liquor. In other words, a separate side flow channel separated from the process channel is not needed for the measurement unit.

The measurement system 100 according to the invention includes, as the main components, a measurement unit 10 and a computing unit 50, quite the same way as in prior art measurement systems. The measurement unit 10 and the computing unit 50 are preferably placed separated from each other, thus avoiding the need to protect the computing unit 50 from surrounding conditions. It is advantageous to place the computing unit in the control room of a process plant, for example, or another similar room where the conditions are favourable in regard to the durability of electronics, contrary to what is often the case with process channels. In this way, it is possible to increase the lifecycle of the computing unit and extend itsmaintenanceinterval, asthecomputingunit isnotexposed to heat, vibration or dust. According to the invention, the measurement unit 10 is integrated as part of the process channel 18,thus enablingthemeasurement without leadingtheprocessstream out from the process channel for the measurement. This results in that the measurement system can be relatively small in size and easily installable in the process channel.

The measurement unit 10 includes a frame 24 comprising a fastening flange 26 for sealing and fastening the measurement unit 10 to aprocess channel 18and a chamber 28that isclosed at leastrelative tothe fluidstream andconnectedtothefastening flange26,arranged to be installed at least mainly inside the process channel 18. In addition to the frame 24, the measurement unit 10 includes a flow channel 12 comprising a first end 14 and a second end 16 for separating a sample from a fluid stream present in the process channel 18 via the first end 14 and for returning the sample to the fluid stream via the second end 16. More precisely, the flow channel passes through the chamber 28. Thanks to the fastening flange 26,the frame 24 is installable in such a way that the chamber 28 and the flow channel 12 passing through it are located inside theprocess channel 18.Thus,the fluid stream flowing in theprocess channel 18 meets the chamber 28 of the measurement unit 10 and can be carried to the flow channel 12 via the first end 14 of the flow channel 12 and discharged from the second end 16 of the flow channel 12 back to the process channel 18 after the measurement.

Furthermore, the measurement unit 10 includes amagnet 20,arranged inside the chamber 28, which preferably encircles the flow channel 12 for creating a magnetic field E at least in part of the flow channel 12, and a coil 22, which preferably encircles the flow channel 12 for exciting the protons of the sample moving in the flow channel 12 and receiving the frequency pulse that returns from the protons back to the coil 22. The magnet 20 and the coil 22 can be formed as one easily connectable and removable package, which is illustrated in Figures 4 and 6a and 6b. In this way, the magnet 20 and the coil 22 can be easily removed from the measurement unit for maintenance and cleaning without removing the entire measurement unit 10.

Alternatively, according to an embodiment, the measurement unit 10 can include cleaning equipment shown in Figure la, arranged in relation to the first end 14 of the flow channel 12, comprising a nozzle 59, a pressure medium channel 63 connected to the nozzle 59, and a pump 65 connected to the pressure medium channel 63 for feeding a pressure medium from the nozzle to the flow channel 14 for cleaning it. For example, the pressure medium can be water or compressed air.With such cleaning equipment, the flow channel canbe cleanedfrom thesolidmatterwithout removingthemeasurement unit from the process stream. Instead of what is illustrated in the figure, the nozzle can also be connected to a different point in the flow channel.

The frame 24 can be made of acid-proof steel, for example, with a thickness of 4 - 8 mm, thus enduring the conditions prevailing in the process channel, such as a high pH or alternatively a low pH of liquid that serves as the fluid stream. A metal frame 24 also prevents the magnetic field from extending to the environment and, on the other hand, disturbances external to the measurement unit from entering the magnetic field of the measurement unit. In this way, the measurement unit according to the invention can easilyprovide a closed magnetic field and isthuseasily applicable in plant conditions. Advantageously, the fastening flange 26 has acircular shapeand isdimensionedtocorrespond withthe inspection hatches provided in the process channel of the application. This enablesthe fasteningofthe measurementunitdirectly toanexisting inspection hatch, thus avoiding the need to provide the process channel with new openings or lead-throughs, which are susceptible to leaks. The fastening flange 26 includes, in relation thereto, a bolt rim 46 shown in Figure 3, with the bolts 58 installed therethrough enabling the locking and sealing of the measuring unit 10 to the opening 48 of the process channel 18 and to the inspection hatch of the counter bolt ring 56. A seal 61 is advantageously provided between the bolt rim 46 and the counter bolt ring 56 to seal the connection.

In turn, the chamber 28 also advantageously has a circular cross-section cut in the direction of the plane of the fastening flange 26,asinFigures5,7a and 7b,or alternatively,anelliptical or oval shape, so that the pressure loss caused by the measurement unit in the process channel is as small as possible. Most advantageously, the shape of the cross-section is elliptical, in which case the smallest cross-section of the ellipse is setparallel to the flow direction for minimizing pressure losses in the fluid stream.According toFigures laand lb,thechamber 28isdimensioned in such a way that the chamber 28 extends inside the walls of the process channel 18sothatthe flow channel 12ispositioneddirectly in the pathway of the fluid stream. In some cases, the chamber 28 can also be dimensioned even so that the flow channel 12 is positioned in the centre line of the process channel 18 deviating from it by a maximum of 10% of the diameter of the process channel. Thanks to the flow channel placed in the centre line of the process channel or near it,sampling isreliable and arepresentative sample can be taken from the fluid stream, as the fluid stream is uniform at the centre of the process channel.

In an advantageous embodiment, the frame 24 is so dimensioned that the height of the frame 24 is in the range of 250 - 330 mm, the diameter of the fastening flange in the range of 400 - 600 mm and the chamber diameter in the range of 250 - 400mm.The inner diameter of the flow channel may be in the range of 5 - 30 mm, preferably 10 - 20 mm and the diameter of the centre hole in the range of 30 - 50 mm. Since the weight of such a measurement unit is at most 30 kg, it is easy to install and move to the application site. In an advantageousembodiment,themeasurement unit 10additionally includes at least a first valve 32, with which the sample guided tothe flow channel 12 isstopped during themeasurement.Inaddition to the first valve 32, the measurement unit 10 then includes either a second valve 34 or, alternatively, a pump. In this case, the purpose of the first valve 34 or the pump is to stop the sample, together with the first valve 32, in the flow channel 12 by momentarily closing the flow channel 12 at both the first end 14 and the second end 16. The purpose of stopping the sample is to ensure that protons p of the sample that are excited thanks to the frequency pulse provided by the coil 22 also release their energy in the region of the magnetic field E thus enabling the signal that returns to the coil from the protons p to be received again at the coil 22 for the measurement, as shown in Figure 2.

The pump is used to stop the sample particularly in the case that the measurement unit is used to take a sample from a fluid stream that has a high viscosity. Thus, the pump can be used to aspirate the sample from the process channel to the flow channel and thereby ensure that the sample can be guided to the flow channel that has a notably smaller diameter compared to that of the process channel. This also enables the use of a relatively small diameter for the flow channel, since, assisted by the partial vacuum provided by the pump, the sample can be led to the flow channel in spite of the surface tension between the flow channel and the sample. If the viscosity of the fluid stream is low, a second valve can be used. For example, the pump can be a hose pump.

Advantageously, there is a pipe 36 functioning as a flow channel 12 between the first valve 32 and the second valve 34 or between the first valve and the pump. The pipe 36 advantageously includes fastening means 42 at both ends 40 enabling disconnection of the pipe 36 from the measurement unit 10 for cleaning. The diameter of the flow channel can be in the range of 2 - 30 mm, preferably 10 - 20 mm, thus allowing for the liquid sample to flow in the flow channel without problems. If the sample is gas, the diameter of the flow channel can be in the range of 2 - 10 mm. The solid content of the sample can generally range between 0.5% and 4.0% by weight remaining thus pumpable. A small diameter of the flow 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 flow channel,canhave a smaller diameter, approximately as small asbetween 30mm and 40mm.The manufacturing costs of the magnet are generally the lower, the smaller is the hole that needs to be produced in the magnet.

Instead of the straight pipe shown in Figures la and lb, the flow channel can also be a pipe that forms a curve and has sections running toward the cover of the chamber. In this way, the magnet can be placed in the sections that run toward the cover of the chamber,upper from the chamberbottom,and theheight ofthe chamber can be made lower. The measurement unit can also include a bypass pipe, which passes by the first and the second valve and the pipe between these allowing for the fluid stream that enters from the first end of the flow channel to pass the magnet during the stopping of the sample present therein. This reduces the flow resistance caused by the measurement unit in the process channel.

The size of the sample conveyed from the fluid stream to the flow channel of the measurement unit can be as small as 1 - 10 cm ³ , in which case the measurement unit is also relatively small-scale. However, such a sample is sufficient for determining a selected property of the fluid stream using the NMR measurement technique.

Figure 2 is an enlarged basic view of an advantageous design of the magnet 20 and the coil of the measurement unit, placed around thepipe 36included in the flow channel 12.The coil 22 ispreferably arranged around the pipe 36 for exciting the protons p contained in the sample. The magnet 2 is also preferably arranged around the pipe 36 for creating a magnetic field E in the flow channel 12. Advantageously, the magnet 22 is also arranged around the coil 22 in the radial direction relative to the flow channel 12 above the coil 22. The magnetic field E generated by the magnet 22 is advantageously a magnetic field as homogeneous and static as possible, through which the sample travels within the flow channel 12. The magnetic field E is depicted in the figure with lines in thetransverse directionrelative tothe flow channel.Thedirection of the magnetic field is advantageously transverse relative to the longitudinal direction of the flow channel. The magnet is advantageously apermanentmagnet,which canbe implemented without separate driving power in order to operate. A permanent magnet generates a static permanent magnetic field in itself. Alternatively, themagnetcan alsobeanelectromagnet,themagnetic field of which is provided by electric current.

In addition, according to Figure 2, the measurement system 100 includes a power source 62 connected to the coil 22 for generating frequency pulses and measuring equipment 70 for measuring the intensity of voltage generated by the frequency pulse that returns to the coil 22 from protons p, for forming a backward signal. Furthermore, the measurement system 100 includes software means 64 for determining a selected property of samples based on the backward signal. Software means 64 are also arranged to control the first valve and the second valve or the first valve and the pump for sampling and stopping the sample. With the power source 62, a frequency pulse is delivered to the coil 22 to excite protons p travelling through the coil 22 inside the pipe 36 into a higher energy state (spin) while the protons absorb the frequency pulse. This energy state discharges quickly (within milliseconds), the proton p thus releasing or emitting energy to its environment. Energy emitted by the proton generates a voltage in the coil 22, i.e., a backward signal, the amplitude of which can be measured with the measuring equipment 70.

Properties of the sample can be determined based on NMR measurement by measuring the relaxation time between the excitation and discharge of protons.The relaxation time correlates with physical properties of the sample.When examining black liquor,for example, the relaxation time correlates with the dissolved solid content ofblack liquor in such away that an increased solid content changes the relaxation time so that the relaxation time T2 shortens as the solid content increases. 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: a(t) = a ₀ exp(-t/T2), where ao is the amplitude at the time t = 0s and T2 = spin-spin relaxation time. Parameters a ₀ and T2 can be defined by placing the equation in an experimental signal.

The measurement unit according to the invention can be implemented 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. However, use of one coil is also possible, when the sample is stopped in the flow channel, thus the same protons that are exposed to the frequency pulse also having time to deliver the backward signal in the coil region. When implemented with one coil, the measurement unit is simpler in design compared to the use of two coils.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 in the measurement, the length of the coil used may be in the range of approximately 10 - 20 cm, thus protons in the sample having time to become excited and release energy over the coil length. The coil may have 100 - 200 turns.

Energyreleased bytheproton pexcitedaccordingto Figure2provides 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 70, for example,with a receiver whose measuring accuracy can be in the class of 1 pV. 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 forthis period.In otherwords,the entire spectrum isnotmeasured, as is usually the case in spectroscopy. For example, the length 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 canbe calculated.Forexample, therelaxationtime canbecalculated with the following formula:

T2 = -t/{in [a(t)/a ₀ }

Advantageously, softwaremeans 64 can be implemented in a computing unit 50,which can be used todisplay results and controlthe device. The computing unit can be a normal PC or equivalent. The material of the flow channel ispreferably 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. The functions of the measurement unit can be controlled with the samecomputing unit,whichhas softwaremeans fordetermining sample properties based on the relaxation times measured.The measurement unit can be controlled with separate control software, which gives electrical controls along a field bus, for example,to the actuator 30 of the second valve 34, this actuator opening the second valve 34of the flow channel 12for taking a sampleperiodically.According to Figure 3, the field bus or the communication cable 52 can be led into the chamber through a lead-through 54 provided in the cover 44 of the chamber 28. Correspondingly, according to Figure la, electricity can be supplied to the magnet and the coil via the lead-throughs 55 using electric wires and a pressure medium for operating the valves via the lead-through 57.

Figure 8 shows steps 110 - 126 of a measurement system according tothe invention in ablockdiagram.The operation ofthemeasurement system according to the invention starts from taking a sample. Advantageously, a sample is taken from the process channel 18 according to Figure la by leading a fluid stream to the flow channel 12 from its first end 14 according to step 110. By opening the first valve 32 in step 112,part of the fluid stream is periodically conveyed to the flow channel 12 as a sample. The sample conveyed to the flow channel 12 to the region of the magnetic field generated by the magnet 20 is stopped in step 114 with the second valve 34 and, in step 116, the first valve 32 is closed, the sample thus remaining in the flow channel 12 between the first valve 32 and the secondvalve34.Periodically repeated,sampling canberepeated at intervals of 1 to 2 minutes, for example.

The first valve 32 and the second valve 34 are advantageously controlled with the computing unit 50 and software means 64 used in the computer in the computing unit 50, in which the sampling interval or the volume flow per a period of time for the necessary sample stream has been specified. Based on the control software, the computing unit sends a control command along a field bus, for example, preferably to the relay 68 of Figure la, which controls the turn off power supply to the actuators of the first valve 32 and the second valve 34. Advantageously, the first valve 32 and the second valve 34 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 32 and 34 is turned off with the relay 68, the valves 32 and 34 close, and when energized, the valves 32 and 34 are in their open position enabling the flow of the sample in the flow channel 12.

If a pump is located in the flow channel instead of a second valve, the power supply of the pump advantageously takes place via the same relay, the entire sampling event being thus manageable by controlling one relay. Thus, a sample is aspirated into the flow channel until the sample is conveyed into the magnet, at which time the power supply to the first valve and the pump is turned off with the relay, at which time these close. At the same time, power supply to the pump is turned off. The control of the relay can be implemented with time control, for example.

Atthe sametime,amagneticfield hasbeencreated inthemeasurement unit 10 advantageously with a permanent magnet used as the magnet 20 according to step 116 of Figure 8. 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. In relation to the computing unit, there can also be an electronic control unit, controlled by control means, which in turn controls the power source of the measurement unit to generate frequency pulses at the coil, according to step 118. Frequency pulses are advantageously generated at the frequency indicated previously 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 several 180° pulses. Pulses are delivered in succession and they excite the protons in themagnetic field in step 120.The excitation discharges very rapidly and the energy released by the proton arrives at the coil providing a low voltage in the coil, which is measured with the measuring equipment in step 122.From the measuring equipment, the voltage information can be transferred in the analogue form to an A/D converter or as a digital signaldirectly to the computing unit 50, where it is stored in a memory 60 with the software means 64 for further processing.

The amplitude of voltage is advantageously measured continuously andmomentary measuring results ofvoltage are stored in thememory. Advantageously, the sample in the magnetic field is exposed to 1 - 20, preferably 4-8 different frequency pulses generated with the coil; thus, attenuating signals are formed in a number corresponding to that of the frequency pulses and their amplitudes are measured with the measuring equipment. The greater the number ofmolecules inthe sample,the smallercanbethenumberoffrequency pulses with which a sufficient signal/noise value is achieved; it can be greater than 30, preferably greater than 50. An average can be calculated of the amplitudes measured with the software means 64ofFigures laand 2.Inaddition,anaverage canbecalculated over successive samples, since variations between individual samples are notably greater than variations between the successive signals of the same sample.

The relaxation time T1 or T2 of the proton calculated based on the measured amplitude of the backward signal is used together with the empirically defined correlation function to determine a selected property of the sample with software means 64 in step 124. Advantageously, the correlation function is defined with empirical tests. Based on a determination, for determining the solid content (SC) of black liquor, the following correlation function was obtained: SC = 14,211n(R2 _TC ) + 3.05, where R2 _TC is the temperature-corrected relaxation time. More generally, the correlation function has the form SC = A * T2(R2 _TC ) + B, where SC is the solid content of black liquor , T2(R2 _TC ) is the spin-spin relaxation time with corrected temperature, A is a constant and B is a constant. In the actual measurement, the solid content of a black liquor sample, for example, is determined by placing the defined relaxation time in the correlation function according to step 126. The SC index can be presented as a time sequence and an averaging process (moving averaging), for example, can be performed to this to be able to eliminate differences between the samples. Finally, the result of the determined solid content can be transferred to the plant information system, for example. Finally, the sample can be conveyed back to the process channel 18 from the flow channel of the measurement unit 12 according to step 128.

In this context, the determination of the dissolved solid content of black liquor is presented as one example of the applications of the measurement system and measurement unit according to the invention. However, it is to be understood that suitable applications can be, for example, determinations of water content of black liquor and any determination performed on a liquid-based sample, where the sample contains water and organic matter, thus the relaxation time correlating with the organic matter contained in the sample.Applications can thus be found in studies for mining wastewaters, which contain paramagnetic ions, or biofuels, which do not contain water but organic compounds.In the case of biofuels, the relaxation time correlates, for example, with biofuel properties, such as the cetane number or the carbon chain length.

Equipment according to the invention, excluding the measurement unit, can consist of a commercially available prior art time domain NMR spectroscope.One such useful spectroscope is the device known by the trade name "TD-NMR Analyzer Spin Track" manufactured by Resonance Systems Ltd. Instead of Time Domain NMR spectroscopy, the measurement unit and the measurement system according to the invention can also be used in low-field spectroscopy.