i) Field of the Invention
This invention relates generally to an on-line method for determining lignin content and/or kappa number in wood pulp samples during the pulping and bleaching operations of a mill. The invention specifically relates to the application of visible-light Raman spectrometry for measuring the Raman-scattered light intensity of pulp samples containing variable amounts of lignin and cellulose.
ii) Description of Prior Art
In a chemical pulp manufacturing process, the production of pulp and/or paper products from wood chips is effected by either partially or entirely removing lignin from the wood prior to the manufacture of pulp/paper sheets. Lignin is a polymeric chemical compound that binds wood fibers together. The most common method of lignin removal is by chemical means, whereby wood chips and chemicals are combined and cooked together at controlled temperature and pressure in a vessel known as a digester. In the kraft process, lignin removal is performed by cooking wood chips in highly alkaline liquor called white liquor, which selectively dissolves lignin and releases the cellulosic fibers from their wooden matrix. The white liquor typically contains caustic soda, sodium sulphide and sodium carbonate. The extent of lignin removal is measured in terms of the blow-line Kappa number [xe2x80x9cG-18-Kappa Number of Pulpxe2x80x9d, Standard Methods of the Technical Section of the CPPA, Montreal; xe2x80x9cT-236-Kappa Number of Pulpxe2x80x9d, TAPPI Standard Methods, TAPPI PRESS, Atlanta]. This method is performed in the laboratory and takes approximately thirty minutes. The blow-line Kappa number is then used for controlling the pulping operation and for estimating the charge of chemicals used for bleaching without producing waste. Furthermore, mill personnel need reliable Kappa-number values to avoid excessive delignification and fibre degradation during the oxygen-delignification stage. Batch digesters control strategies use feedforward control, and rely on keeping the chip and white liquor feeds at levels predetermined by the overall production rate. Kappa number targets are calculated with the use of the H-factor [VROOM, K. E., Pulp Paper Mag. Can., 58(3):228-231 (1957)]. The temperature profile of the cook is adjusted approximately halfway through the cook after determining the blow-line kappa number with the laboratory method, which introduces a 30-minute delay, thereby producing significant process variability. Such a delay is incompatible with control strategies requiring timely analysis of pulp properties. Continuous digesters can be controlled more precisely by adding a feedback control loop around the lower cooking zone, but the control strategy must still allow for the dead time introduced by the laboratory method [WELLS, C. xe2x80x9cVII Chemical Pulping Areaxe2x80x9d, in Pulp and Paper Manufacture (3rd Ed.) Vol. 10 Mill-wide Process Control and Information Systems, TAPPI/CPPA, Atlanta/Montreal, 1983, pp. 79-123]. The ongoing development of modern chemical pulping and bleaching processes has thus underscored the need for a real-time Kappa number sensor which would provide the timely information towards better control of pulping and bleaching operations and a more efficient use of the chemicals involved in the process.
In order to fill this need, several automated analysers are available commercially. These analysers measure the optical properties of pulp suspensions by a variety of methods that use different regions of the electromagnetic spectrum. The current generation of analysers uses the strong absorption of lignin in the ultraviolet region of the spectrum as a basis for kappa-number measurement. For example, many current Kappa number analysers use UV light with a combination of reflectance, scattering and transmittance measurements [YEAGER, R., Pulp and Paper, September 1998, 87-88,91-92: BTG KNA 5100 (reflection); Kubulnieks et al., Tappi J. 70(11) 38-42 (1987): STFI OptiKappa(trademark) ABB Analyzer (absorption)]. Although the principle is simple, the actual measurement is complex for both of these methods because the lignin absorption cannot be measured accurately without accounting for interferences from light scattering and reflectance artifacts produced by variations in pulp consistency, as well as by the physical characteristics of the fibres. This problem can be addressed by building calibrations that are valid for a relatively narrow range of sampling conditions and furnishes. These calibrations invariably fail during process upsets and rapid changes in furnish. Calibration is done by characterising the relationship between the three types of measurement at a given consistency. These types of sensing devices are very sensitive to consistency variations. Reliable samples from the mid-digester and blow-line sampling points cannot be obtained because the consistency of the samples is then outside the range allowed for by the two-point UV calibration procedure. Although the calibration works well for bleach-plant samples [YEAGER, R., Pulp and Paper, September 1998, 87-88,91-92] and for single-species furnishes and mixed furnishes of constant composition, the sensors do not provide accurate results for furnishes of unknown or rapidly changing composition [BENTLEY, R. G., SPIE Proceedings, Vol. 665, p. 265-279 (1986)]. Moreover, maintenance of the two-point calibration procedure and of the sampling system requires constant attention from mill personnel. For example, the instrument has to be re-calibrated every time when either the source or the electronics are replaced, by using a wide variety of kappa-number pulp samples. This involves time-consuming trial and error and tweaking the process, during which period the mill have to rely on manual analysis. Furthermore, when the composition of chips is constantly changing, instruments have to be constantly re-calibrated to keep up with the changes in furnish, which is a considerably time-consuming exercise. Also, the complexity of the sampling system makes current analysers very sensitive to variations in water quality and variations in sample consistency. In addition, sample throughput is relatively low, achieving about two samples per hour for each location.
Since lignin also has well-defined infrared absorption bands, the use of the mid-infrared region has been proposed in the past by many investigators as a means to overcome this problem. For example, the kappa number of pulps was determined by using the lignin peak at 1510 cmxe2x88x921 and a cellulose peak as an internal standard [MARTON, J., SPARKS, H. E., Tappi J., 50 (7), 363-368 (1967)]. The lignin/cellulose peak-area ratio was found to be insensitive to variations in basis weight. Also, another method was developed with the use of DRIFTS for estimating lignin in unbleached pulp [BERBEN, S. A., RADEMACHER, J. P., SELL, L. O., EASTY, D. B., Tappi J.,70(11), 129-133 (1987)]. Lignin-free cotton linters were used as the reference material. A lignin spectrum is thus obtained after spectral subtraction of the cellulose contribution. A linear relationship is found between the area of the band at 1510 cmxe2x88x921 and kappa number for a wide variety of species. The relationship holds for a range of hardwood and softwood pulp having Kappa numbers ranging from 10 to 120. However, these methods used dry pulp samples. Mid-infrared methods are not amenable to on-line kappa number determination because of the presence of large and variable amounts of water in mill samples, which interferes with lignin determination.
The use of the near-infrared region has also been proposed as a means of eliminating this limitation. Advantages over previous techniques include: no sample preparation, short measurement times, relatively long optical paths and the possibility of using fiber-optic technology for real-time, in situ measurements. Water peaks in this region are smaller and do not affect kappa measurements. Multivariate calibration methods such as PCA or PLS are used to account for species variability. Also, temperature effects and interferences by other cations and anions can be modelled in this spectral region through the use of partial least-squares (PLS) multi-component calibration techniques. PLS is a multi-component calibration method that is well known in the art [HAALAND, D. M. and THOMAS, E. V., Anal. Chem., 60(10):1193-1202 (1988); Anal. Chem., 60(10):1202-1208 (1988)]. This method enables one to build a spectral model, which assumes that the absorbance produced by a species is linearly proportional to its concentration. However, this method only works well with dried pulp, and attempts to adapt it to dewatered pulp have failed because of baseline artefacts produced by variations in water content [YUSAK, E., LOHRKE, C., Proc. 1993 TAPPI Pulp. Conf., 663-671]. Commercial instruments based on NIR reflectance measurements are nevertheless available from Honeywell-Measurex (PulpStar(trademark)) [web site: www.iac.honeywell.com] and from Asea Brown Boveri (ABB) [Proceedings of the 10th Biennial ISWPC Vol. 3 266-69 Yokohama, June7-10 1999]. Jeffers et al [U.S. Pat. No. 5,486,915] describes a method for on-line measurement of lignin in wood pulp by color shift of fluorescence. Although the sampling system is simple, the method is sensitive to species variability and thus unsuitable for analysis of variable pulp furnishes.
Raman spectroscopy is a technique that measures the intensity of light produced by the inelastic scattering of photons originating from a monochromatic light source such as a visible-light laser. This inelastic scattering occurs with a small frequency shift with respect to the frequency of the light source. The Raman effect generates a spectrum that is similar to that of an infrared spectrum, but where only the absorption bands that are produced by symmetric-mode vibrations are present. The mid-infrared water band that usually interferes with lignin measurements is thus no longer active. Unlike the ultraviolet region that is sensitive to light scattered and reflected by the pulp fibres, the Raman signal should not be affected by the physical characteristics of these fibres such as coarseness. Atalla et al. have discussed the challenges posed by early applications of conventional Raman spectroscopy to lignin-containing samples [ATALLA, R. H., AGARWAL, U. P., BOND, J. S., 4.6 Raman Spectroscopy, in LIN, S. Y., DENCE, C. W. eds., Methods in Lignin Chemistry, Springer-Verlag, Berlin, 162-176 (1992)]. It is well known in the art that lignin produces a strong fluorescence background, thereby obscuring its Raman spectrum. Very noisy Raman spectra are obtained, even after many hours of data acquisition, and these are thus unsuitable for on-line applications. The state of the art regarding pulp and paper applications of Raman spectrometry was reviewed a few years ago by Agarwal and Atalla [AGARWAL, U. P., ATALLA, R. H., 8xe2x80x94Raman Spectroscopy, in CONNERS, T. E., BANERJEE, S. eds., Surface Analysis of Paper, CRC Press, Inc., Boca Raton Fla., 152-181 (1995)], and more recently by Agarwal [AGARWAL, U. P., Chapter 9: An Overview of Raman Spectroscopy as Applied to Lignocellulosic Materials, in ARGYROPOULOS, D. S., Advances in Lignocellulosics Characterization, Tappi Press, Atlanta Ga., 201-225 (1999)]. According to Agarwal and Atalla, using red or near-infrared excitation and measuring the back-scattered light, one can analyse lignin-containing samples by minimizing sample laser-induced fluorescence and heating, but at the expense of the Raman signal. The much weaker Raman signal can still be enhanced by the use of a Fourier transform spectrometer, but a much longer acquisition time than that suitable for on-line analysis is needed. Increasing the laser power enhances the Raman signal, but increases the likelihood of sample bleaching, especially at longer acquisition times. Investigators have nevertheless used FT-NIR Raman to characterise wood samples [KENTON, R. C., RUBINOVITZ, R. L., Appl. Spectrosc. 44 (8), 1377-1380 (1990); EVANS, P. A., Spectrochemica Acta, 47A (9/10), 1441-1447 (1991); LEWIS, I. R., DANIEL, N. W., jr., CHAFFIN, N. C., GRIFFITHS, P. R., Spectrochemica Acta, 50A (11), 1943-1958 (1994); TAKAYAMA, M., JOHJIMA, T., YAMANAKA, T., WARIISHI, H., TANAKA, H., Spectrochemica Acta, 53A, 1621-1628 (1997); AGARWAL, U. P. RALPH, S. A., Appl. Spectrosc., 51 (11), 1648-1655 (1997)]. Even so, for wood samples, the fluorescence is very significant and still overcomes the Raman bands. FT-NIR Raman investigations have also been performed on pulp: samples were ground up, diluted with potassium bromide (KBr) and either made into KBr pellets [AGARWAL, U. P., WEINSTOCK, I. A., Proceedings of the 1996 International Pulp Bleaching Conference, 531-535], or immersed in alcohol prior to analysis [SUKHOV, D. A., EVSTIGNEYEV, E. I., DERKACHEVA, O. YU, NABIEV, I. R., KUPTSOV, A. H., Proceedings of the 7th International Symposium on Wood and Pulping Chemistry, Volume 2, 969-974 (1993)] so as to reduce the fluorescence, especially for samples with higher lignin content. Direct analysis of damp pulp samples of diverse western softwood species has also been performed by Ibrahim et al. [IBRAHIM, A., OLDHAM, P. B., CONNERS, T. E., SHULTZ, T. P., Microchemical Journal, 56, 393-402 (1997)]. Data acquisition for each sample took about 15 minutes, a time much too long for a realistic on-line application with multiple sample points. The acquisition time could be shortened if the data obtained was of high quality, with low scatter and no systematic bias. In the abstract of that paper, Ibrahim et. al. claim that a linear relationship is obtained over a range of Kappa numbers between 10 and 38. However, the data shown in FIG. 4 of Ibrahim et al. suffers from a xe2x80x9csigmoidal-like systematic errorxe2x80x9d, and is fact very far from a line, especially at Kappa numbers between 10 and 15. This prevents an accurate determination of lignin content below Kappa numbers of 15. Also, the Raman signal is affected by detector noise for pulps having Kappa numbers below 10, thereby preventing any meaningful application of this technique to either O2 delignification or bleach-plant control. Sun et al. [SUN, Z., IBRAHIM, A., OLDHAM, P. B., J. Agric. Food Chem. 45,3088-91 (1997)] obtained similar results for dry hardwood samples over a range of Kappa numbers between 4 and 20, whereby the relative standard deviation was found to be about 10% for samples having Kappa numbers below 10. Finally, the data is very scattered for softwoods above a Kappa number of 15 probably because of significant fluorescence interference and/or sample bleaching, thereby rendering the FT-NIR Raman technique useless for digester control. Therefore, the presence of even more intense fluorescence produced by lignin chromophores when using visible-light excitation seemingly creates an insurmountable obstacle: a large, species-dependent fluorescence background would then swamp the Raman signal, thereby making it unusable for on-line application on mixed furnishes. Furthermore, acquisition times requiring several hours are required for reducing the noise of the Raman portion of the spectrum. No indication was given by any of the aforementioned investigators as to how to reduce the fluorescence background or to perform adequate measurements of lignin content at low or moderate Kappa numbers. Agarwal and Atalla have stated [AGARWAL, U. P., ATALLA, R. H., 8xe2x80x94Raman Spectroscopy, in CONNERS, T. E., BANERJEE, S. eds., Surface Analysis of Paper, CRC Press, Inc., Boca Raton Fla., 152-181 (1995)] that one cannot use visible-light excitation to perform quantitative lignin content measurements with Raman spectroscopy because of laser-induced fluorescence (LIF) in pulp samples, as well as pre-resonance effects produced by a small minority of lignin chromophores that distort the lignin signal by enhancing the contribution of only part of the lignin available. Agarwal has discussed these effects in detail [AGARWAL, U. P., Chapter 9: An Overview of Raman Spectroscopy as Applied to Lignocellulosic Materials, in ARGYROPOULOS, D. S., Advances in Lignocellulosics Characterization, Tappi Press, Atlanta Ga., 201-225 (1999)]. Therefore, a person skilled in the art would be discouraged from analysing pulp samples for lignin content with visible-light excitation, because of the presence of non-resonance effects and a much stronger fluorescence background than that found in the near-infrared region.
Therefore, none of the methods cited in the prior art constitutes a simple reliable on-line method capable of accurately measuring the residual lignin in wood pulps irrespective of wood species or consistency variations, at Kappa numbers below 15. Such a method would feature a stable calibration which would not drift or be sensitive to wood species or variation in pulp consistency. In the following, we disclose such a method.
It is an object of this invention to provide a method for determining lignin and/or kappa number in wood pulp during pulping and bleaching operations of a pulp manufacturing process.
It is a specific object of the invention to provide such a method employing the Ramon spectrum of the pulp.
In accordance with the invention there is provided a method for determining lignin content and/or kappa number in a wood pulp sample during pulping and bleaching operations of a chemical pulp manufacturing process, comprising the steps of: a) withdrawing a plurality of high consistency samples of wood pulp from a chemical pulp manufacturing process; b) subjecting said plurality of samples to a monochromatic visible-light source, and allowing the samples to scatter said visible light; c) determining the Raman spectrum of the scattered visible light over a predetermined range of wave numbers to produce Raman-scattered light intensity measurements; and d) comparing the Raman-scattered light intensity measurements of said plurality of samples with the Raman-scattered light intensity measurements shown by known combinations of lignin content, cellulose content and consistency and evaluating the lignin content and/or kappa number of said wood pulp therefrom.
The present invention provides a rapid method for determining lignin or kappa number in wood pulps. This method overcomes the disadvantages previously discussed. The method enables one to measure lignin content and/or kappa number independently of species variations and pulp consistency.
The method has application for determining kappa number of wood pulp in a range of 5 to 1 10 kappa, preferably 5 to 15 kappa.
The method is suitably employed with pulp suspensions having a consistency of 15 to 30%.
The comparison step d) suitably comprises evaluating by univariate or multivariate calibration, relationships between the Raman-scattered light intensity measurements of the plurality of samples and the Raman-scattered light intensity measurements shown by the known combinations of lignin content, cellulose content and consistency.
The Ramon-scattered light intensity measurements of the known combinations of lignin content, cellulose content and consistency may suitably be determined as an uncorrected, baseline corrected or integrated Raman-scattered light intensity over a predetermined range of wave numbers.
Suitably the Raman-scattered light intensity measurements are carried out within a range of wave number shift situated from 1000 to 2000 cmxe2x88x921, with respect to the excitation frequency.
The laser excitation for the measurements is suitably carried out over the visible wavelength range situated between 700 and 850 nm.
The spectrophotometry for the measurements is suitably performed with visible-light scattering measurements; and a fiber-optic excitation/collection probe is used in the spectrophotometry. The spectrophotometry may be performed in a flow-through cell for continuous measurements.
The relationships between the Raman-scattering measurements of the samples under investigation and the corresponding measurements of the known samples employed in the comparison of step d) of the method can conveniently be obtained with the use of partial least-squares (PLS) multivariate calibrations.
The method can be performed using a laser power setting that is low enough to prevent sample bleaching. A simple washing cycle with either water or preferably a mildly alkaline water solution may be provided. Since data acquisition only takes a few seconds, a high sample throughput will allow many process streams to be multiplexed to a single analyzer through either the use of fiber optics or a multiple-stream sampling system.
The analysis method described below uses Raman-scattered light intensity measurements obtained from the Raman spectra of pulp samples illuminated by the monochromatic light emitted by a visible-light laser. These measurements are generally free from water interference. The integrated Raman-scattered light intensity of the pulp is measured along predetermined spectral regions around 1600 cmxe2x88x921 for lignin and around 1100 cmxe2x88x921 for cellulose. Alternatively, the Raman-scattered light-intensity spectrum can be normalized to the value of the Raman-scattered light intensity around 1100 cmxe2x88x921 so as to mimic the lignin/cellulose integrated intensity ratio. With the aid of a partial least squares (PLS) calibration, the normalized lignin/cellulose intensity ratio for each sample is made to correlate directly with the lignin concentration obtained from the standard-method laboratory analysis described previously. This correlation is generated by supplying spectra of known pulp samples to training software, which then develops a model for the spectral region being used. Although not necessary, it is generally preferable that the concentration of all pulp components be accounted for within a PLS calibration so that the lignin measurements are accurate and without bias, thereby creating a noise-free model that can be characterised with a small number of basis vectors. The model then uses these basis vectors for characterising components in unknown samples. The lignin content and/or Kappa number of the pulp sample is then calculated with the PLS model. Selected pulp process samples are also analyzed with standard analytical methods (CPPA G. 18) so as to establish a more reliable calibration set with the mill data obtained by Raman spectrophotometry. Visible-light Raman lignin measurements could then be used for reducing the process variability of pulping and bleaching operations. The application of this invention to pulp and paper liquors provides a method for determining lignin content and/or Kappa number that is faster, more reliable, and requires less maintenance than existing methods. In summary, this new method replaces currently used UV or NIR sensors, and addresses the previously discussed shortcomings of these devices.
In one particular embodiment the present invention provides a method for the on-line spectroscopic determination of lignin content and/or kappa number in wood pulps. Unlike currently available commercial instrumentation, the method enables one to measure lignin content independently of species variations and pulp consistency. The method includes the steps of: (1) withdrawing high-consistency samples of wood pulp from a chemical pulp manufacturing process; (2) subjecting these samples to a monochromatic visible-light source; (3) recording the resulting scattered light and its Raman spectrum over a predetermined range of wave numbers so as to produce Raman-scattered light intensity measurements; (4) determining either the uncorrected, baseline-corrected and/or integrated Raman-scattered light intensity of the samples over a predetermined range of wavenumbers shown by different combinations of lignin content, cellulose content, and consistency; (5) correlating by either univariate or multivariate calibration the relationships between the Raman-scattered light intensity measurements of unknown samples and the Raman-scattered light intensity shown by known combinations of lignin content, cellulose content, and consistency so that the lignin content and/or kappa number in wood pulps can be accurately determined over a Kappa range of 5 to 110, preferably 5 to 20 and more preferably 5 to 15, for any levels of sample cellulose content or pulp consistency.
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