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Online Monitoring of Alkali, Sulfide, and Dissolved Lignin during Wood Pulping by Attenuated Total Reflection-Ultraviolet Spectroscopy and Flow Injection Techniques X. S. Chai, Q. Hou, J. Y. Zhu,* and W. Ban Institute of Paper Science and Technology, 500 10th Street NW, Atlanta, Georgia 30318
This study demonstrated online monitoring of hydroxide, or effective alkali (EA), sulfide, and dissolved lignin in kraft wood pulping processes using flow injection analysis (FIA) and attenuated total reflection (ATR)-ultraviolet (UV) spectroscopy. The FIA technique was found to be effective in the resolution of ATR probe fouling in alkaline pulping liquors. Good spectral repeatability with a maximum relative standard deviation of the spectral intensity of less than 15% was obtained over a period of 5 days. Successful online monitoring of EA, sulfide, and dissolved lignin during laboratory pulping processes was demonstrated. The species concentrations measured by the ATR-UV sensor were found to be in excellent agreement with those obtained by traditional titration and conventional UV absorption measurements. Introduction Kraft wood pulping is the dominant process for the production of chemical pulps in the pulp and paper industry because of its versatility and insensitivity to variations of the wood supply. Wood chips are subjected to digestion in pulping liquor at an elevated pressure and temperature of around 170 °C in a pressure vessel (digester) to release cellulosic fibers. The pulping liquor mainly consists of sodium hydroxide (NaOH) and sodium sulfide (Na2S) to remove wood lignin. The concentrations of the active pulping chemicals, i.e., hydroxide and sulfide, as well as of the spent species, e.g., dissolved wood lignin in the pulping liquor, significantly affect the rate of delignification and the mechanical and chemical properties of the pulp. Reliable, rapid, and accurate online monitoring of the composition of the pulping liquor, mainly the concentrations of hydroxide, sulfide, and dissolved lignin, can provide in situ data to allow process control in kraft pulping to improve the quality and productivity (fiber uniformity, strength, and yield) in chemical pulp manufacturing. Unfortunately, such an online monitoring capability is not available in the pulp and paper industry, which prevents the realization of process control and results in the loss of productivity and compromised quality. An offline titration method1 has been widely used in industry practice for the determination of effective alkali (EA; defined as the sum of the concentrations of NaOH + 0.5Na2S, where the concentrations were expressed as of Na2O) and sulfide. Although the traditional ABC titration method1 can determine the alkali and sulfide concentrations, it is unable to measure dissolved lignin in the cooking liquors. Furthermore, it cannot be used for process control because the ABC method needs to be performed offline and takes 30 min to produce results. Because of the high maintenance requirements, the commercial online automatic titrator has not been widely accepted in mill applications. Therefore, there * To whom correspondence should be addressed. Present address: USDA Forest Product Laboratory, One Gifford Pinchot Drive, Madison, WI 53726-2398. E-mail: JYZhu@ fs.fed.us.
is a great need to develop nontitration-based sensors for rapid kraft liquor analysis. Conductivity sensors2-4 and conventional ultraviolet (UV) spectroscopy5 were developed for the analysis of hydroxide and sulfide in white and green liquors, respectively. These two techniques were also used for the analysis of hydroxide, sulfide, and lignin in black liquors.6 However, these sensors cannot provide a complete analysis of all of the key species of interest. Multiple sensors have to be used. Furthermore, the interference from other inorganic ions such as sulfide affects accurate analysis of hydroxide through conductivity determination. Physical dilution by a factor of 1000 is required when using a conventional UV spectroscopic method6 to detect the absorption of sulfide in unfiltered pulping liquors due to spectral intensity saturation and plugging of the flow cell of the UV system. A high dilution ratio not only compromises the measurement accuracy but also makes sulfide determination impossible unless a high degree of dissolved oxygen removal in the water used for dilution is achieved to avoid sulfide oxidation. The equilibrium concentration of dissolved oxygen at room temperature in water is of the same order of magnitude as the concentration of sulfide. Recently, near-infrared (NIR) spectroscopy has been developed for simultaneous analyses of hydroxide, sulfide, and lignin. Because sulfide, hydroxide, and lignin do not absorb in the NIR region, the measurements of these species using the NIR technique7,8 were relying on the change of the concentrations of the species that have NIR absorption, such as water, due to the changes of hydroxide, sulfide, and lignin concentrations. The choice of the first overtone in the literature7,8 for good accuracy requires the use of short-path-length flow cells, which cause frequent cell plugging problems in a mill environment;9 frequent maintenance to clean the cell is required. Attenuated total reflection (ATR) spectroscopy was developed in the 1960s by Fahrenfort10 and Harrick11 based on Newton’s discovery that an evanescent wave extends into the rear medium beyond the interface between two media of different refractive indices. One
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Figure 1. Schematic diagram of the FIA/ATR-UV system used in the present study. Shown in the sampling mode.
key characteristic of ATR spectroscopy is the short optical path length of each pass of the evanescent wave, which makes it possible to apply it to the analysis of a very concentrated process solution such as black liquor without dilution. ATR-IR spectroscopy was demonstrated to provide hydroxide, carbonate, and sulfate analysis in kraft liquors12 but not lignin and sulfide analysis, the two key species for kraft pulping process control. The present authors have demonstrated ATRUV spectroscopy for simultaneous EA, sulfide, and carbonate analysis in kraft white liquor (regenerated process pulping liquor; named from its color) and green liquor (named from its color; an aqueous solution of smelt generated from combustion of concentrated spent pulping liquor in chemical recovery boilers).13-15 We have also demonstrated the feasibility for simultaneous analysis of hydroxide, sulfide, and dissolved lignin in pulping liquors using the ATR-UV technique together with multivariate calibration.16 However, the study16 indicates that the fouling of the ATR probe can occur after the probe is in contact with alkaline pulping liquor for a prolonged period of time. The fouling of the probe significantly distorts the absorption spectra and makes the analysis of pulping impossible. The objective of the present study is to demonstrate that a flow injection analysis (FIA) technique17 can be integrated with ATR-UV spectroscopy for the resolution of ATR probe fouling to achieve online and simultaneous analysis of hydroxide, sulfide, and dissolved lignin in process pulping liquors. The FIA/ ATR-UV system can be applied to online monitoring in practical operations for process control. Experimental Section Apparatus. Figure 1 shows the schematic diagram of the FIA/ATR-UV system. The system consists of a spectrophotometer (HP-8452; Hewlett-Packard, now Agilent Technologies, Palo Alto, CA) equipped with an ATR flow cell (model TNL-120H23-3; Axiom, Irvine, CA), an injection valve system (model LabPro700; Rheodyne, Cotati, CA), a peristaltic pump (RP-1; Rainin, Emeryville, CA), and a metering pump (model A141155; LMI Milton Roy, Acton, MA). The system operates in two modes that represent the sampling and injection status, respectively, alternatively throughout an experiment. When the injection valve positions 1 and 2, 3 and 4, and 5 and 6 are connected (as shown in Figure 1), the process stream flows into the ATR flow cell from the sampling loop to measure the absorption spectra of the process stream. When the injection valve is switched to the injection status, valve positions 2 and 3, 4 and 5, and 6 and 1 are connected and freshwater is injected into the sampling loop to clean the ATR cell. To avoid the potential risk of clogging in the sampling channel,
a cross-flow net filter (200 mesh; made of a stainless steel screen) is placed inside a union tee located at the sample intake point of the sampling loop. The two-status injection valve is controlled by a computer program. The absorption spectrum of the incoming flow stream is recorded every 1.4 min by the spectrometer through Lab-View software. Pulping. A pulping experiment was conducted in the labortory-scale MK digester (M/K System Inc.). Wood chips of loblolly pine were used. A total of 2 kg of wood chips containing 50% moisture was used and mixed with 2 L of green liquor obtained from a kraft mill for pretreatment in the digester. The temperature was ramped from a room temperature of 23 to 120 °C in 60 min (or at a rate of 1.62 °C/min) and then kept at this temperature for another 60 min. The liquor used for pretreatment was discharged. The pretreated wood chips were then cooked with fresh pulping liquor. Conventional kraft batch pulping was conducted with active alkali (AA; defined as the sum of the concentrations of NaOH + Na2S, where the concentrations are expressed as of Na2O), and the charge on wood and sulfidity [defined as the ratio of the concentrations of Na2S and (Na2S + NaOH), where the concentrations are expressed as of Na2O] were 15 and 30%, respectively. The cooking liquor-to-wood ratio was 4:1. The cooking temperature was ramped from a room temperature of 23 to 170 °C in 60 min or at a rate of 2.45 °C/min and then kept at this temperature for another 100 min. Pulping Liquor Sampling. The pulping liquor was drawn from the sampling point at the recirculation line, which flows into the injector’s sample loop of the FIA/ ATR-UV system after passing the cooling coil, and then was pumped back to the digester. The dead volume of the sample loop was 100 µL as measured. Therefore, the pulping liquor was diluted by about 7 times based on the total volume of the FIA system. The washing water flow rate was 4 mL/min. The process liquor was cooled by running tap water as the liquor flowed through a cooling coil submerged in the running water. It was found that the temperature of the liquor was close to constant when the liquor reached the ATR flow cell. According to Chai,18 when the temperature of the sample varies within 5 °C, the effect of the temperature on the ATR spectrum is negligible. Calibration. A set of nine black liquor samples were collected during each pulping process for calibration. Two reference methods were used to calibrate the ATRUV system. The ABC titration method1 was used to determine EA and sulfide in the calibration samples. Conventional UV absorption at 280 nm was used to obtain dissolved lignin in the samples after dilution. ATR-UV spectroscopic measurements of the calibration samples were also conducted. A commercial chemometrical software (SIMCA, 6.0 Demo version, Sweden) was used to conduct partial least-squares (PLS) analysis19,20 of the ATR-UV absorption spectra of the calibration samples. On the basis of the known concentrations obtained by the titration and conventional UV methods, a calibration coefficient matrix (or predictive model) for species concentration predictions can be obtained from PLS analysis. A detailed description of the calibration procedures can be found in our previous study.13 Results and Discussion Performance of the Online FIA System. The performance of the FIA system was tested using a
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Figure 2. Time-dependent absorption spectral signals from a black liquor sample at wavelengths 200, 230, and 300 nm during five cycles of sample injection operation.
pulping spent liquor, or black liquor. The black liquor was sampled by the FIA/ATR-UV system intermittently for several hours. Time-dependent absorption spectra were recorded. It is known that dissolved lignin absorbs in the entire UV range. Sulfide ion (HS-) has an absorption peak at 230 nm, and hydroxide ion (OH-) has a broad absorption band of around 200 nm.13 To assess the stability of the FIA system, time-dependent absorption signals at three wavelengths of 200, 230, and 300 nm that correspond to the absorptions of OH-, HS-, and dissolved lignin, respectively, are plotted. As shown in Figure 2, a sharp peak was observed for each wavelength examined for a given system operation cycle (sample and injection) of 1.4 min. When the injection valve of the FIA system was turned to the sampling status, the absorption signal increased rapidly with time. When the injection valve was switched to the injection status after 0.7 min, water started to flow into the ATR flow cell, which diluted the sample stream and reduced the absorption signal. The signal peak tails down to zero gradually, as shown in Figure 2. The results also indicate that the time-dependent absorption signals at the three wavelengths were repeatable and stable. The time-dependent profiles of the signal at each wavelength from different operating cycles are close to identical, indicating excellent stability of the FIA system. To further examine the FIA system, we plotted the peak value of the black liquor ATR absorption signals of all of the operation cycles at the three wavelengths recorded over a time period of 200 min. As shown in Figure 3, the peak values are very stable with only a couple of outliers. It was found that the maximum relative standard deviation of the peak values of signals of the three wavelengths was less than 2.0% as calculated from the data shown in Figure 3. ATR Probe Fouling Test. The effect of the corrosion of the quartz probe in alkaline solution on the ATR signal was found to be not significant in our previous study.12 However, resolution of ATR-UV probe fouling caused by the cooking liquor is critical to the application of the ATR-UV technique to mill process monitoring. Our previous study16 found that probe fouling could occur when the quartz ATR probe was in contact with pulping liquor for a prolonged time period of more than 8 h. However, it was found that the fouling formed on
Figure 3. Time-dependent peak levels of the absorption spectral signals from a black liquor at wavelengths 200, 230, and 300 nm measured during each operation cycle over a period of 200 min.
Figure 4. Spectral repeatability test of the FIA/ATR-UV system over a period of 5 days.
the ATR probe can be easily removed by flushing the probe with water.16 To demonstrate that the FIA system can effectively resolve the ATR probe fouling, we extended the FIA performance test described in the previous section for several days. Figure 4 shows five recorded black liquor absorption spectra over a period of 5 days. The results indicate that the five spectra are essentially identical; severe distortion of the black liquor absorption spectrum along with baseline shift such as that described in our previous study16 was not observed. The maximum relative standard deviation of the five spectra of about 15% was observed in the near-UV and visible region of the spectrum where the absorbance is very low, which may be attributed to the slight thermal oxidation of dissolved lignin due to rising of the black liquor temperature when the liquor was pumped continuously through a 2-mm-i.d. tube using a peristaltic pump over a period of several days for the test. Our laboratory study indicates that lignin oxidation at very low temperature for a prolonged time period can take place. When a black liquor sample was exposed to air at a temperature of 60 °C for a period of 4 h, the color of the liquor was changed from black to light brown because of oxidation. The spectral intensities of the black liquor ATR absorption signals at wavelengths of 200, 230, and 300 nm recorded over a period of 12 days were found to be unchanged. The maximum relative standard deviation of the signals at a peak intensity of
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Figure 5. Online measured concentrations of EA, sulfide, and dissolved lignin in a wood chip pretreatment process using green liquor.
Figure 6. Online measured concentrations of EA, sulfide, and dissolved lignin in kraft batch pulping of wood chips pretreated using green liquor.
the three wavelengths was less than 2.0%. The results shown in Figure 4 clearly indicate that the ATR probe fouling problem observed in our previous study16 was effectively eliminated using the FIA system. The results further prove that the performance of the FIA system is excellent even over a time period of 12 days. Online Measurements in Pulping Processes. Online measurements of the concentrations of hydroxide ions or EA, sulfide ions, and dissolved lignin in pulping process liquors during two pulping processes described earlier were conducted using the present FIA/ATR-UV system. Figures 5 and 6 show the measured concentration profiles of EA, sulfide, and dissolved lignin by the FIA/ATR-UV system in a wood chip pretreatment process with green liquor and in a conventional kraft batch pulping process, respectively. The solid data points are measured from a set of samples collected during the two processes using the reference methods, i.e., titration for EA and sulfide and conventional UV absorption for dissolved lignin after the dilution of the black liquor. The solid symbol data points were also used for calibration to obtain the predictive model for the ATR-UV method using a PLS fit procedure. The ATR-UV method provided detailed species concentration profiles in the process liquors that agree in trend
Figure 7. Comparisons between the reference measured and ATR-UV predicted EA (a), sulfide (b), and dissolved lignin (c) concentrations in 18 different black liquor samples.
with those obtained by the reference methods. To quantitatively demonstrate excellent agreement between the ATR-UV model predicted species concentrations and those measured by the reference methods,
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direct comparisons of the species concentrations of the calibration samples measured by the ATR-UV and reference methods were made as shown in Figure 7. Excellent linear correlations with proportional coefficients of 0.975, 1.021, and 1.000 were obtained for measured concentrations of EA, sulfide, and dissolved lignin in the pulping liquor, respectively, indicating near perfect agreement between the ATR-UV measurements and those by the reference methods. To implement the system in mills, a large pipeline may need to be used in the system to avoid unnecessary flow clogging by wood chips. In addition to regular system calibration (once per month), a new blank spectrum (in water) should be taken during the injection of water into the sample loop every 10-30 min to replace the previous blank spectrum. The new blank spectrum should be used in data processing to alleviate spectral baseline shift due to any instability of the optical system. With this online monitoring capability, kraft batch pulping does not need to terminate at a preset pulping time (a common practice in the industry and laboratory) but rather is based on the amount of dissolved lignin measured. Furthermore, the time-dependent pulping chemicals concentration provides a better understanding about the reaction process inside the digester, making process adjustment possible. Multiple sensors can be used to obtain spatial resolution of the pulping process inside the digester to investigate the mixing uniformity of the process materials with chemicals and the status of pulping at various physical locations of a digester. Conclusions A laboratory system that integrates FIA and ATRUV spectroscopy was developed for simultaneous online monitoring of EA, sulfide, and dissolved lignin in kraft pulping processes. It was found that the FIA technique was effective at eliminating ATR probe fouling in kraft pulping liquors. Spectral distortion was minimal over a period of 12 days using FIA to flush water through the ATR flow cell intermittently at a cycle time of 1.4 min. The FIA/ATR-UV system overcomes the shortcoming of a clogging problem when a short-path-length flow cell has to be used in the NIR technique to avoid saturation in analyzing kraft liquors. Because optical dilution is achieved using the ATR technique, physical dilution of the pulping liquor is avoided. Successful online monitoring of EA, sulfide, and dissolved lignin during laboratory pulping processes was demonstrated. The species concentrations measured by the ATR-UV sensor were found to be in excellent agreement with those obtained by traditional titration and conventional UV absorption measurements. Acknowledgment This research was supported by the State of Georgia, USA, through the Traditional Industry Program in Pulp and Paper (TIP3; Grant PP02-MP-03).
Literature Cited (1) The Pulping of Wood. In Pulp and Paper Manufacture, 2nd ed.; MacDonald, R. G., Ed.; McGraw-Hill: New York, 1969; Vol. 1, p 563. (2) Bertelsen, P. M.; Svensson, J.-O. Sensor-Based Causticizing Control. TAPPI J. 1987, 69 (8), 72. (3) Dorris, G. M.; Allen, L. H. Conductivity Sensors for Slaker Control: Part IsLaboratory Results. J. Pulp Pap. Sci. 1989, 15 (4), J122. (4) Paulonis, M. A.; Krishnagopalan, G. A. Kraft Liquor Alkali Analysis Using In Situ Conductivity Sensor. TAPPI J. 1990, 73 (1), 205. (5) Paulonis, M. A.; Krishnagopalan, G. A. Kraft White and Green Liquor Composition Analysis; Discrete Sample Analyzer. J. Pulp Pap. Sci. 1994, 20 (9), J254. (6) Liaw, S.-J.; Krishnagopalan, G. A. On-line Measurement of Sulfide and Alkali Concentrations During Kraft Pulping. TAPPI J. 1992, 75 (9), 219. (7) Saucedo, V. M.; Krishnagopalan, G. A. Application of In Situ Near Infrared Analysis for the Measurement of Cooking Liquor Components During Kraft Pulping. J. Pulp Pap. Sci. 2000, 26 (1), 25. (8) Hodges, R. E.; Krishnagopalan, G. A. Near-infrared Spectroscopy for On-line White and Black Liquor Analysis. Proceedings of the 1999 TAPPI Pulping Conference, 1999; TAPPI: Atlanta, GA, 1999; p 1097. (9) Goyal, G. C. Research and Development Center, Potlatch Corp. (now SAPPI), Cloquet, MN, private communication, 2002. (10) Fahrenfort, J. Attenuated Total Reflection: A New Principle for the Production of Useful Infrared Spectra of Organic Compounds. Spectrochim. Acta 1961, 17, 698. (11) Harrick, N. J. Internal Reflection Spectroscopy; John Wiley & Sons: New York, 1967. (12) Leclerc, D. F.; Hogikyan, R. M. Rapid Determination of Effective Alkali and Dead-Load Concentrations in Kraft Liquors by Attenuated Total Reflection Infrared Spectrometry. J. Pulp Pap. Sci. 1995, 21 (7), J231. (13) Chai, X.-S.; Li, J.; Zhu, J. Y. Simultaneous and Rapid Analysis of Hydroxide, Sulfide, and Carbonate in Kraft Liquors by UV-Attenuated Total Reflection Spectroscopy. J. Pulp Pap. Sci. 2002, 28 (4), 105. (14) Chai, X.-S.; Li, J.; Zhu, J. Y. An ATR-UV Sensor for Simultaneous Online Monitoring of Sulfide, Hydroxide, and Carbonate in Kraft Mill Recausticizing Operations. J. Pulp Pap. Sci. 2002, 28 (4), 110. (15) Chai, X.-S.; Zhu, J. Y. Simultaneous Analysis of Sulfide, Hydroxide, and Carbonate by ATR-UV Spectroscopy during Borate Autocausticizing Mill Trials. Process Control Qual. 2001, 11 (6), 531. (16) Chai, X.-S.; Luo, Q.; Zhu, J. Y.; Li, J. Attenuated Total Reflection UV Spectroscopy for Simultaneous Analysis of Hydroxide, Sulfide, and Dissolved Lignin in Pulping Liquors. Proceedings of 2001 TAPPI Pulping Conference, Seattle, WA, 2001 (submitted to J. Pulp Pap. Sci.) (17) Ruzcka, J.; Hansen, E. H. Flow Injection Analysis; John Wiley & Sons: New York, 1988. (18) Chai, X.-S. Process Analytical Chemistry Applied to Liquors in the Pulping Industry: Monitoring of Sulfur Species and Lignin. Ph.D. Dissertation, Royal Institute of Technology (KTH), Stockholm, Sweden, 1996; ISBN 91-7170-653-4. (19) Geladi, P.; Kowalski, B. R. Partial Least-Square Regression: A Tutorial. Anal. Chim. Acta 1986, 185, 1. (20) Hoskuldsson, A. PLS Regression Methods. J. Chemom. 1998, 2, 211.
Received for review June 19, 2002 Revised manuscript received November 7, 2002 Accepted November 9, 2002 IE0204585
