Pulp and Paper Applications KRAFT PROCESS

Pulp and Paper Applications

Industry: Pulp and Paper Product: NR800

INFORMATION KRAFT PROCESS On-line measurements in the pulp and paper industry are Although there are a number of different processes for among the most difficult challenges of process analytical making pulp and paper from wood, the Kraft or “sulfate” chemistry. Pulp and paper production streams are typically process of chemical pulping is the most common. Wood very high in both dissolved and suspended solid material. chips are fed into a solution of Sodium Hydroxide and The samples are often optically opaque or nearly so. Sample Sodium Sulfide at high temperature in a digester which conditioning systems are prone to plugging and corrosion. breaks the lignin chemical bonds which bind the cellulose On-line titrators tend to be very troublesome due to the fibers together. After cooking, the fiber or “pulp” is then problems with handling the streams, dispensing measured separated by screening and/or filtering from the undigested amounts of sample, reagent consumption and disposal of the wood chips and washed. Additional steps to remove residual titrated wastes. Here we report on the application of the lignin are often employed in modern plants. The pulp is then InfraSpec NR800 FT-NIR analyzer to the determination of bleached to whiten the fibers, formed into a sheet, dried and alkali salts in various Kraft process streams. rolled.

Figure 1: Simplifies schematic of pulp and paper mill

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Chemical reprocessing in the Kraft process is illustrated in Fig. 2

Figure 2: Chemical reprocessing block flow diagram

MEASUREMENT of ALKALI The spectra of white liquor and green liquor are shown in Fig.3. Changes in the absorbance spectra are related to changes in the hydrogen bonding of water caused by the concentration of the various alkali salts dissolved in the water.

Figure 3: Spectrum of Green Liquor

Pulp and Paper Applications

PLS calibration models were created for NaOH, Na2S and Na2CO3 concentrations and are shown in Figures 4-6. Correlations for all 3 calibrations are 0.97 or greater and the Standard Error of the Prediction (SEP) is approximately 1 g/L for all 3 calibrations.

A key point to be noted is that these spectra and calibrations were made at a fixed temperature. One issue that must be addressed in this application of FTNIR is the change in the spectrum of water caused by temperature. Figure 7 shows the spectrum of water at various temperatures. The change in the spectrum due to temperature change is great compared to the change in spectrum caused by the alkali concentration.

Figure 5: Predicted vs. Measured Plot for Na2CO3 (g/L)

Figure 4: Predicted vs. Measured Plot for NaOH (g/L) Figure 6: Predicted vs. Measured Plot for Na2S (g/L)

There are basically three ways to deal with the effects of temperature on the spectra such that it does not affect the measurement. The first, and most obvious is to control the sample temperature using a sample conditioning system. Figure 8 shows such a system for white liquor, green liquor and black liquor. Note the provisions for flushing the system with water and with Hydrochloric Acid to remove deposits. This approach has the advantage of being the most precise, but is also usually the most expensive.

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Figure 8: System for Pulp and Paper Liquors

Another approach is to create the calibration at a constant effects that are not correlated with the measurement. To do and convenient temperature, then to measure the effect of this, the set of samples used to make the calibration model temperature on the output of the calibration model. (typically 30-50 samples) must have spectra taken over the Temperature changes in the sample often apply a linear bias entire range of temperatures likely to be seen. This need not to the output that can be compensated by a temperature be done systematically, that is, by varying the temperature of measurement and a factor applied to the analyzer output. each sample by set amounts. The spectral scans can be taken at random temperatures, provided the range of The third approach is to include the variations in temperature temperatures covers the range likely to be observed in the into the calibration model. One of the advantages of Partial process. This approach is referred to as making the model Least Squares (PLS) calibration models is that they separate “robust” to temperature effects.

Figure 9: Temperature compensated FT-NIR measurement

Figure 10: Robust Calibration Model

Figure 11 shows an on-line trend graph of FT-NIR measurements on green liquor. Figure 12 shows the measurement of effective alkali in black liquor.

Figure 11: On-Line Measurement of Alkali in Green Liquor

Figure 12: On-Line Measurement of Effective Alkali in Black Liquor

Pulp and Paper Applications

Other Applications of FT-NIR and Paper Process Measurements of kappa number in black liquor have been reportedi. Near infrared spectral measurements have been Clearly, we have only begun to scratch the surface of used to predict pulp yield ii. In-situ ATR probes have been applications for NIR in the pulp and paper industry. Given the used on spent liquorsiii. Andersson and Wilsoniv report capital-intensive nature of the industry, and the size of the excellent measurements of effective alkali (EA), lignin, Xylan, facilities, the economic justifications will be found for new total organic material, Cellulose and Glucomannan among and creative applications of NIR technology over the next other parameters in black liquor by NIR. several years.

i R. E. Hodges, “Applications of near infrared spectroscopy in the pulp and paper industry,” PhD thesis, Auburn University, Auburn Alabama, US, 1997. ii T. Lindgren and U. Edlun, “Prediction of lignin content and pulp yield,” Nor. Pulp Pap. Res. J., 13(1):78-80, 1998. iii V.M. Saucedo and G.A. Krishnagopalan, “Applications of in-situ near-infrared analysis for the measurement of cooking liquor components during kraft pulping,” J. pulp Paper Sci., 26(1):25-30, 2000. iv N. Andersson and D. I. Wilson, http://www.ee.kau.se/forskning/ModSim/nir_tappi.pdf

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