Kappa number measurement

Figure 6.1 The acid group content of pulps expressed as an ion exchange capacity as a function of kappa number (measure of lignin content) for Kraft and sulfite pulps.

E Yuzak and C Lohrke. At-Line Kappa Number Measurement by Near-Infrared Spectroscopy. TAPPI Pulping Conference, Atlanta, 1993, pp. 663-672. 

Figure I. Mediator dosage study results of kappa number measurements of IMS bio-pretreatments using ABTS, VA, and HBT as mediators. The confidence level associated with this pulping condition is 1.5 kappa number units.

Technology has been introduced for on-line estimation of the kappa number based on absorption of ultraviolet light (35). This breakthrough ia optical sensor technology permits closed-loop feedback control of digesters from on-line measurement of the kappa number. 

There are many control challenges in this process. These include strong nonlinearity, distributed system, long deadtimes, and a feedstock that varies significantly because of its biological source. The key variable is kappa number (degree of delignifica-tion), which cannot be measured online, so it must be estimated from secondary measurements. 

The repeatability of the measurement is high for semi-bleached pulps but decreases with increasing kappa numbers and becomes fairly constant for kappa numbers above 20. With semi-bleached pulps with kappa numbers below 20, the method will have a minimum repeatability of 0.2 kappa number. Repeatability decreases systematically from 13% for kappa numbers below 2 to 2.3% for kappa numbers between 10 and 20. The average repeatability is 3.8% for kappa numbers between 2.5 and 20. With unbleached pulps having kappa numbers above 20, the repeatability varies unsystematically between 0.9 and 1.5%. The average repeatability is 1.2% for kappa numbers between 20 and 190. 

Semichemieal pulps produced by the sodium bisulphite and neutral sulphite-anthraquinone pulping processes at 75% yields have properties that are rather similar to a lower yield chemical pulp shown in Table 13.4. The pulp eharaeteristies from this proeess are of considerable interest, since it has been shown that ehemical pulps made by the so-ealled ASAM process, in which the wood is delignified with a liquor containing sodium sulphite, sodium carbonate or sodium hydroxide and methanol (c. 40% of the liquor by volume) with a trace of anthraquinone ean be delignified more rapidly and to a lower kappa number (this surrogate measure of the lignin content of pulp is explained in the appendix) than pulps made in the absenee... 

Analysis of reprecipitated xylan on fibres from kraft pulping of hardwoods by modern methods indicated that the model system of 20 years earlier had been very perceptive both hexeneuronic acid residues and p-L-iduronyl residues were found (The finding caused a stir in the pulp industry, since a KMn04 titre ( kappa number ) had been used traditionally as measure of the residual lignin in the pulp, but in hardwood pulps these titres were too high, by factors of <2, because of the reaction of permanganate with the hexeneuronic acids" ). 

FIGURE 10.3 A laboratory flow-through kraft cook of softwood with a lowered concentration of hydrosulfide ions present in the cooking liquor during the time interval of 60-105 min, after the start at 70°C, corresponding to a temperature interval of 130-170°C -H 5 min at 170°C. Pulping selectivity is measured as pulp viscosity as a function of kappa number. (Sjdblom, K., Mjdberg, J., and Hartler, N., Pap Puu, 65, 227-240, 1983.)... 

FIGURE 9.42 Simulated concentration profile of (a) lignin (w%), (b) the Kappa number, and (c) porosity inside a wood chip during the cook. The Kappa number is a measure of the lignin content. 

At the bottom of this zone, the outlet device is located. The most digester input variables are the wood chips flow ood,m, the liquor flow (pngour,m and the heat supply Qheatmg and the most important output variables are the Kappa number x which is a measure for the amount of lignin in the wood chips and the yield % which is a measure of the amount of wood recovered in comparison to the amount fed to the digester. 

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