Matti Törmänen

Measuring pulp consistency and fines content with a streak camera

The problem of determining the fines content of pulp in cases where consistency is not known remains to be solved. We explored the hypothesis that this problem may be solved by studying shape changes in a laser pulse after it has travelled through the pulp.A matrix was constructed of pulp samples with consistencies varying from 0 to 1.5% by increments of 0.2% and fines contents varying from 0 to 50% by 10% increments. A streak camera was used to record three pulses simultaneously. The first was a reference pulse, which was used to calibrate the measurement pulses. The second was a pulse measured at an angle of 90° to the straight light path. The third was the straight path pulse.Different fines contents form their own lines on consistency–maximum power graphs and consistency–time of 50% power fall graphs. When transmitted power is plotted against time of 50% power fall the lines representing different fines contents cross each other. These results indicate that the fines content and consistency can be measured in some cases with a single measurement. Also, if water is added in a controlled manner, measurement of the lowering in consistency allows the original consistency and fines content to be determined.

Pulp consistency determined by a combination of optical and acoustical measurement techniques

In this study, methods based on ultrasonic attenuation and optical time-of-flight measurements are used simultaneously in determining both the fibres and fines mass fractions, respectively, of a cellulose pulp fibre suspension. The optical measurements are done by a laser radar and the acoustical measurements are based on ultrasonic attenuation measurements in a pulse-echo set-up. Two kinds of long-fibre fractions are studied, thermo-mechanical pulp and chemical softwood pulp. Fibre and fines mass fraction ranges are 0.25–1.0% and 0–0.75%, respectively. The results show that the fibres are the predominant source for absorption and scattering of ultrasonic waves and are thus mainly contributing to the attenuation of ultrasound in the pulp. It is also found that the fines are the predominant source for optical scattering and fines are thus mainly contributing to the propagation delay of the light pulse in the laser radar set-up. By combining the ultrasonic attenuation and the optical time-of-flight measurements, it is shown that the mass fraction of fines and the mass fraction of fibres in a pulp sample could be determined, respectively.

A preliminary measurement of fibres and fines in pulp suspensions by the scattering photoacoustic technique

The consistency of fibres and concentration of fines need to be controlled during the production process in the paper industry. In paper pulp, fibre lengths range from less than a millimetre to several millimetres whereas fines particles have sizes of a few tens of micrometres. Therefore, the two fractions have different properties of optical scattering and acoustic attenuation, i.e., fibres produce more forward optical scattering and acoustic attenuation, while fines produce larger and more homogeneous scattering but less acoustic attenuation. Based on these facts, we specifically develop a new method, a scattering photoacoustic technique, to measure the consistency of fibres and concentration of fines simultaneously. It employs near-infrared light (1064 nm wavelength) to produce three acoustic waves with MHz frequencies. One piezoelectric transducer detects these waves, which are used to measure optical scattering and acoustic attenuation of pulp samples. The results indicate that our current apparatus successfully discerned the pure fibre and fines samples. It also proved capable of extracting the consistencies of fines and fibres in the studied samples that consisted of mixtures of fibre and fines. Finally, the scattering photoacoustic technique has a potential ability in online measurement of fibre and fines consistencies in pulp suspensions.

Determination of optical parameters of pulp suspensions by time-resolved detection of photoacoustic signals and total diffuse reflectance measurements

Time-resolved photoacoustics were used to measure the optical parameters of pulp suspensions for the first time. Reconstructing stress distribution along the direction of the incident laser light allows the effective attenuation coefficient of these suspensions to be determined. Simultaneously, the total diffuse reflectance of the suspensions was measured by the same laser source. Based on the effective attenuation coefficient and total diffuse reflectance, the absorption and reduced scattering coefficients of pulp suspensions can be calculated. In this study, three kinds of pulp suspensions with different kappa number (2, 13, and 16), a measure of lignin content in pulp fibers, were diluted with water to make samples with a consistency range from 1% to 5%, and studied at 355 nm wavelength. The results showed that the optical coefficients were approximately proportional to pulp consistency; on the other hand, the absorption coefficient was linearly correlated with kappa number, but the reduced scattering coefficient was not. Therefore, by determining its optical parameters, it is possible to extract the consistency and kappa number of an unknown pulp suspension.

Pulsed laser photoacoustic monitoring of paper pulp consistency

This study involves measurements of pulp consistency in cuvette and by an online apparatus, by innovatively scattering photoacoustic (SPA) method. The theoretical aspects were described at first. Then, a few kinds of wood fiber suspensions with consistencies from 0.5% to 5% were studied in cuvette. After that, a pilot of online apparatus was built to measure suspensions with fiber consistency lower than 1% and filler content up to 3%. The results showed that although there were many fiber flocks in cuvette which strongly affected the measurement accuracy of samples consistencies, the apparatus can sense fiber types with different optical and acoustic properties. The measurement accuracy can be greatly improved in the online style apparatus, by pumping suspension fluids in a circulating system to improve the suspension homogeneity. The results demonstrated that wood fibers cause larger attenuation of acoustic waves but fillers do not. On the other hand, fillers cause stronger scattering of incident light. Therefore, our SPA apparatus has a potential ability to simultaneously determine fiber and filler fractions in pulp suspensions with consistency up to 5%.

Laser radar and laser photoacoustics in paper pulp measurements

The size distribution and total amount of the particles in paper pulp is vital information for the paper manufacturer in optimising process control and maintaining a high product quality. There is a further need for improving the on-line measurement methods to measure these parameters. It is known that fibre and fines fractions in the pulp have different optical and acoustic properties. In this study, we simultaneously use laser radar and laser generation of acoustic waves to further study optical and acoustic parameters, such as optical time delay, acoustic speed and attenuation. A near infrared pulse laser is used to illuminate the pulp suspensions and the time-of-flight of scattering photons is recorded; and at the same time, a high energy pulsed laser is applied to produce an acoustic wave. The acoustic wave propagates through the pulp suspensions and an acoustic transducer is used to detect the signals from which the attenuation and acoustic speed are determined. The results show that these combined optical techniques can potentially determine the content of fibres and fine particles simultaneously.

Scattering photoacoustic method in measurement of weakly absorbing turbid suspensions

Conventional photoacoustic techniques in composition determination and biomedical diagnose and imaging are based on the optical absorption in target substance or objects from which the photons to be scattered are not concerned. It is obvious that the intensities of scattered lights closely relate to the property of the interrogated substance, therefore measuring the signals produced by them can give rise to more information of the substance. Based on this idea, a novel method entitled scattering photoacoustic (SPA) method is put forward to study weak absorption suspensions with highly scattering. In this method, a near infrared pulse laser irradiates the studied object which contacts with external absorbers, resulting the generation of a few photoacoustic signals; one is produced in the studied object as conventional case, others are in external absorbers which are produced by the scattered photons. All these signals are successively received by a piezoelectric detector with short damping period. Analyzing these signals is capable of determining reduced scattered coefficient and absorption coefficient, as well as acoustic attenuation of studied suspensions. Some measurement results in intralipid and fibre (paper pulp) suspensions are given rise to in the end.

Two approaches for assessing photon path length distribution in pulp

Pulp consists of distinguishable particles, the refractive indices of which differ from the index of the employed medium, water. We are thus dealing with an optically scattering material. Particle distribution in pulp is nowadays a focus of interest. This parameter is related to the photon path length distribution (PPLD) determined by the inhomogeneity of the scatterer. In order to assess PPLD, we use two methods. In the first, the particle properties in pulp are estimated by means of a microscope. A model for Monte Carlo simulation is then built to obtain PPLD. In the second, the signal generated by a laser pulse passing the cuvette filled with water or pulp is detected with a streak camera and the assessment of PPLD accomplished by a deconvolution procedure. To obtain the particle distribution, the two methods may be used together, so that the streak-camera measurements give PPLD, and in the following simulation process the particle distribution is found, which corresponds to the determined PPLD. The number and diversity of the sample sets currently used do not fulfill the statistical requirements of the industry. Nevertheless, the results achieved encourage us to develop the methods further.

Changes in time of flight in thermomechanical pulp (TMP) measured with a streak camera

The aim of this study was to determine the best setting for measuring time-of-flight in TMP with a streak-camera. In the first setting, referred to as straight setting, the laser, TMP sample and streak-camera were in a line. In the second setting, referred to as diagonal setting, the streak- camera was placed at a 90-degree angle to the light source. The results indicate that the diagonal setting allows an easier time-of-flight measurement than the straight one in low-consistency TMP. In medium-consistency TMP, however, there are no performance differences between the settings. Nevertheless, if drift in the laser or streak-camera necessitates the use of a reference pulse, the straight setting provides an easier measurement. Neither setting allows the measurement of high-consistency TMP.