MEASUREMENT
A spectroscopic method for monitoring bleaching chemicals in a ClO2 bleaching process By H. Yuan, T.M. Garver, G. Sedgwick and C. Hein Abstract: UV-Visible and Raman spectroscopic measurements at two kraft mills demonstrate that Raman spectroscopy provides direct measures of ClO2 and sulphate at different points in the bleaching process, and UV absorbance measurements provide measures ClO2 and dissolved lignin levels at different bleaching stages. Measurement and control strategies are discussed based on optimization of bleach addition with respect to input kappa number, ClO 2 concentration, and lignin removal are presented. ClO 2 and other species related to the ClO 2 bleaching process can be directly detected by Raman and UV-Visible spectroscopic methods. The characteristic Raman peaks and UV absorbance bands of dissolved ClO 2 and other chlorinated species are given in Table I [1-3]. Our laboratory work indicated that the dissolved ClO 2 has a peak at 944 cm–1 in the Raman spectrum and that the peak intensity has a linear relationship to the concentration of the ClO 2 dissolved in water. The UV-visible absorbance band at 355 nm also shows a linear relationship to the amount of dissolved ClO 2. These results indicated that the Raman and UV techniques have potential for monitoring chemical consumption and reaction mechanisms of the ClO2 bleaching processes. Since the UV absorbance values at 280 nm are also proportional to the amounts of lignin that dissolved in process water during the bleach process, the use of UV measurements for monitoring the lignin removal during the ClO 2 bleaching process is also equally important. Preliminary on-site Raman and UV-visible spectroscopic measurements were carried at two kraft mills. The results suggested that both the UV and the Raman spectroscopic methods provide a sensitive measure of the concentration of bleaching chemicals and dissolved lignin in bleach plant process streams. Specifically, the amounts of ClO2, sulphate and dissolved lignin can be directly measured at a rate sufficient for process control purposes. Based on these on-site measurements results, further work is required to expand this research to online measurement for bleach plant process control.
T
H. YUAN Alberta Research Council Edmonton, AB
[email protected]
T.M. GARVER Alberta Research Council Edmonton, AB
G. SEDGWICK Alberta Research Council Edmonton, AB
C. HEIN Alberta Research Council Edmonton, AB
38
❘❘❘ 105:2 (2004)
HE DISSOLVED
ON-SITE SPECTROSCOPIC MEASUREMENTS AND DATA ACQUISITION Various sampling points right after the ClO 2 addition in the process streams at D0, D1 and D2 stages have been chosen. Filtrates were separated from the fibre immediately. All the samples were filtered through a 0.2-micron filter and measured on Raman and UV-visible spectrometers. In order to compare the level of lignin removal at different stages, the samples were also taken from the process points right before the ClO 2 addition. Raman spectra were obtained using a Chromex R2000 spectrometer with ANDRO CCD
TE cooled detector. The excitation light source was a 785-nm diode laser. The power at the sample was 180 mw and the integration time was 120 seconds. The standard 10-mm quartz cuvette from Hellma was used as the liquid sample cell. UV-visible spectra were collected using an Ocean Optics USB2000 spectrometer. The D-Star light source used deuterium and tungsten lamps, respectively, as UV and visible light sources.
RESULTS, DISCUSSION Observations of ClO2 and Dissolved Lignin From Raman and UV On-Site Measurements: Typical Raman spectra obtained at D0 and D2 stages from mill A were shown in Fig. 1. The peaks appear at 944 cm–1 were from the un-reacted ClO2 dissolved in the sample filtrate and the peaks at 980 cm–1 are due to the SO2– 4 species converted from the sulphuric acid added during the process. The intensities of these peaks can be used to measure the concentration of each species. The concentration calibration of dissolved ClO2 and the Raman peak intensity at 944 cm–1 is given in Fig. 2. Based on the peak intensity and concentration calibration, the amount of un-reacted ClO2 dissolved in filtrate can be measured by Raman spectrum. The UV spectra of the filtrate samples collected from the process points right before and after ClO2 additions at D0 stage from mill A are given in Fig. 3. The absorbance band at 355 nm is contributed from the un-reacted ClO2 dissolved in filtrates. The absorbance band intensities or the peak areas are also shown the linear relationship to the dissolved ClO2 concentration. Therefore, the UV spectra could also be used to measure the unreacted ClO2 in process streams. Since the absorbance value at 280 nm in UV spectrum is directly related to the dissolved lignin in process filtrate, the measurement of the absorbance at this wavelength before and after the ClO2 addition gives an indication of the amount of lignin removed by the ClO2 during the bleach process. The new process variables may be provided through monitoring the dissolved ClO2 level as well as the dissolved lignin level at the same time by UV measurements. Measured ClO2 Levels and Correlations With ClO2 Dosages and Dosage per Kappa Number: Correlations between the extracted data and the mill operating parameters have been evaluated. Most sam-
T 40 Pulp & Paper Canada
MEASUREMENT
FIG. 1. Raman spectra of D0 (—) stage and D2 (---) stage filtrates from mill A.
FIG. 2. The calibration curve for ClO2 measurement by Raman.
FIG. 3. UV spectra of D0 (---) stage filtrates before and after ClO2 (—) additions from mill A.
FIG. 4. The correlation of D0 stage filtrate pH values and the intensities of sulphate band measured by Raman from mill A.
ples for ClO2 measurements from both mills were taken at the points very close to the downstream of ClO2 addition. Thus, the samples were drawn from a zone of high chemical activity. As a consequence, the spectral measurements give the ClO2 concentration at the moment the sample filtrate was collected rather than a residual value. Similar on-site Raman and UV measurements were also conducted at mill B over a one-week period. The correlation of D0 stage filtrate pH values and the intensities of sulphate band measured by Raman peak at 980 cm–1 from mill A samples taken on May 30th were given in Fig. 4. The sample pH values have the linear relationship with the concentration of sulphate in sample filtrate measured by Raman peak intensity at 980 cm–1. Measuring the relative intensities of ClO2 and sulphate peaks may create a new possibility to monitoring and optimizing the bleach reaction pH and bleach chemicals usage. The correlation of ClO2 dosage and the un-reacted ClO2 level, as measured by Raman peak intensity, at the D0 stage is given in Fig. 5. The ClO2 level was increasing
Pulp & Paper Canada T 41
while the ClO2 dosage remained constant. The reason is that the kappa number at this stage dominates the ClO2 consumption. In other words, the higher the kappa number, the higher the ClO2 consumption. Therefore, the correlation of ClO2 dosage per kappa number and the Raman measurement becomes a more useful factor. The correlations of ClO2 level with dosage and dosage per kappa number at the D1 stage are given in Fig. 6. The results of the Raman measurements indicate that the kappa number still plays a strong role at the D1 stage but little at D2 stage according to the results shown in Fig. 7. Dissolved Lignin Level Measured by UV Spectra: As mentioned previously, in UV measurement, the absorbance at 355 nm is the indication of the ClO2 level in the filtrate sample, while the absorbance at 280 nm is related to the amount of dissolved lignin in filtrate during the bleaching process. In Fig. 8, the dissolved lignin in D0 stage filtrates indicated by UV absorbance at 280 nm has linear relationship with kappa number and the ClO2 dosage. The higher kappa number is, the more dissolved lignin
TABLE I. Raman and UV-visible spectral peaks of chlorinated species. Species ClO2 ClO– ClO2– ClO3–
Raman (cm–1)
UV-Visible (nm)
944 712 799 934
355 289 257
in filtrates at this bleaching stage. The amount of dissolved lignin was also increasing with an increasing ClO2 dosage. The correlations of dissolved lignin level and ClO2 level measured by UV absorbance at D0, D1 and D2 stages were plotted in Fig. 9. The amount of dissolved lignin increases when the level of ClO2 in the sample increases at D0 stage. While the correlation at D1 stage shown that the dissolved lignin decreases instead when the ClO2 level increases. This difference could be explained as the ClO2 mainly function as a lignin-removal reagent at the D0 stage, but at the D1 stage, part of ClO2 was used to destroy or bleach the lignin, which 105:2 (2004) ❘❘❘
39
MEASUREMENT
FIG. 5. ClO2 (●) level measured by Raman and the correlations with ClO2 added dosage and dosage per Kappa number (▲) at D0 stage from mill B.
FIG. 6. The ClO2 level measured by Raman and the correlations with ClO2 added dosage (●) and dosage per Kappa number (▲) at D1 stage from mill B.
FIG. 7. The ClO2 level measured by Raman and the correlations with ClO2 added dosage (◆) and dosage per Kappa number (▲) at D2 stage from mill B.
FIG. 8. The correlations of dissolved lignin measured by UV at 280 nm with the Kappa number (O) and ClO2 dosage (▲) of D0 stages filtrates from mill B.
information about the on-going bleach reaction at different stages, and also about the room to trim the dosage of ClO 2 at D1 and D2 stage.
CONCLUSIONS
FIG. 9. The correlations of dissolved lignin level and ClO2 level measured by UV at D0 (▲), D1 (O) and D2 (■) stages from mill B samples.
already dissolved in bleaching pressate. The plot from the D2 stage correlation shown that the dissolved lignin level remain unchanged or slightly drop by increases ClO2 absorbance from 0.5 to 1.4. These differences shown in Fig. 9 might be providing
40
❘❘❘ 105:2 (2004)
• Both the UV and the Raman spectroscopic methods provide a sensitive measure of the concentration of bleaching chemicals and dissolved lignin in bleach plant process streams. Specifically, the amounts of ClO2, sulphate and dissolved lignin can be directly measured at a rate sufficient for process control purposes. The UV observations show that the concentrations of dissolved lignin and ClO2 can be measured at the same time. This provides a tool for optimizing the lignin removal. The Raman measurements are sensitive to the concentrations of ClO2 and sulphuric acid. Consequently, a Raman sensor is useful for optimizing the ClO2 and the sulphuric acid dosages in the bleach plant. • The Raman measurements, specifically the ClO2 peak at 944 cm–1 and –1 SO2– 4 peak at 980 cm , can be used to control and optimize ClO2 and sulphuric acid addition. The UV measurements, the ClO2 absorbance at 355 nm and the dissolved lignin absorbance at 280 nm, can be used to control and optimize the ClO2 dosage and lignin removal. • Both UV and Raman signals show fairly strong correlation with the ClO2 dosage, as well as kappa number. These results suggest that online sensors based on the UV and Raman methods have potential to monitor ClO2 addition, sulphuric acid addition and the concentration of dissolved lignin.
T 42 Pulp & Paper Canada
MEASUREMENT • The ClO 2 dosage can be optimized by making measurements at the different stages. The direct measurements of ClO2 and dissolved lignin levels can provide fast control parameters to complement measurements of kappa number and final pulp brightness. In summary, a large number of exploratory spectral measurements were completed across all stages of the bleach plant. Observation and analysis of the UV and Raman spectra show that both methods are sensitive to process variation, that signal intensities are sufficient for online measurement and that they provide complementary information for monitoring the bleaching process. Some preliminary assessment was made on the correlation of the spectral measurements with process and product quality variables.
ACKNOWLEDGEMENTS The authors are gratefully acknowledging the funding support from PRECARN Inc., Alberta Pacific Forest Industries Inc. and Daishowa-Marubeni International Ltd., Peace River Pulp Division. Most particularly we wish to acknowledge the support from mill operational and laboratory teams.
LITERATURE
1. NAKAMOTO, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds. 5th Ed. New York: John Wiley & Sons, Inc. (1997). 2. HOLGER, S. P. MULLER, H. WILLNER. Vibrational and Electronic Spectra of Chlorine Dioxide, OclO, and Chlorine Superoxide, ClOO, Isolated in Cryogenic Matrices. Journal of Chemical Physics 97(41): 10,589-10,598 (1993). 3. ALVIN II., N. , WOLTZ, P.J.H. The Infrared Spectrum of Chlorine Dioxide. Journal of Chemical Physics 20 (12): 1878-1883 (1952).
Résumé: Les mesures prises à l’aide de la spectrophotométrie UV et visible et de la spectrométrie Raman dans deux usines kraft démontrent que la spectrométrie Raman fournir des mesures directes du ClO 2 et du sulfate à divers points du procédé de blanchiment et que les mesures d’absorption dans les UV fournissent une mesure du ClO 2 et du niveau de lignine dissoute à diverses étapes du blanchiment. Des statégies de mesure et de contrôle font l’objet de discussion en se basant sur l’optimisation des produits de blanchiment ajoutés par rapport à l’indice Kappa, la teneur en ClO 2, et l’extraction de la lignine.
Reference: YUAN, H., GARVER, T.M., SEDGWICK, G., HEIN, C. A spectroscopic method for monitoring bleaching chemicals in a ClO2 bleaching process. Pulp & Paper Canada 105(2):T40-43 (February, 2004). Paper presented at the 2002 International Pulp Bleaching Conference in Portland, OR, on May 19, 2002. Not to be reproduced without permission of PAPTAC. Manuscript received on June 16, 2003. Revised manuscript approved for publication by the Review Panel on October 1, 2003. Keywords: RAMAN SPECTROSCOPY, MEASUREMENT, KRAFT PULPS, CHLORINE DIOXIDE, SULFATES, BLEACHING, OPTIMIZATION, PROCESS CONTROL, ULTRAVIOLET SPECTROSCOPY, LIGNINS.
Pulp & Paper Canada T 43
105:2 (2004) ❘❘❘
41
