Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With testing – sampling or analyzing
Reexamination Certificate
2000-12-20
2003-04-22
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With testing, sampling or analyzing
C356S317000, C356S318000, C356S326000, C356S417000
Reexamination Certificate
active
06551451
ABSTRACT:
BACKGROUND OF THE INVENTION
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 [“G-18-Kappa Number of Pulp”, Standard Methods of the Technical Section of the CPPA, Montreal; “T-236-Kappa Number of Pulp”, 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. “VII Chemical Pulping Area”, in Pulp and Paper Manufacture (3rd Ed.) Vol. 10 Mill-wide Process Control & 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™ 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 cm
−1
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 cm
−1
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. Al
Leclerc Denys
Trung Thanh P.
(Ogilvy Renault)
Alvo Steve
Pulp and Paper Research Institute of Canada
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