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%.
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.
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.
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.
This paper studies the correlation between the time-of-flight (TOF) of laser pulses and the talc content of pulp. Samples of thermomechanical pulp (TMP) and talc were made up with consistency values ranging from 0.0% to 3.0% and increasing by steps of 0.5%. The proportion of filler, in this case talc, in the TMP varied from 0% to 100%. Laser pulses were shot through the samples and changes in the attentation and TOF of the pulses were recorded. The results show that TOF increases as consistenscy increases. This indicates an increase in the number of scattering surfaces. An increase in the proportion of talc leads to a corresponding change in the TOF. This indicates that talc has a larger scattering surface than does TMP of equivalent mass. Varying consistencies and talc contents produced different points on the delay-attenuation axes. Consquently, the measurement of TOF and attenuation may be used to determine the consistency and proportion of talc.
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.
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