Using online bubble size and total dissolved solids (TDS) measurements to investigate the performance of oxygen delignification

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1 Using online bubble size and total dissolved solids (TDS) measurements to investigate the performance of oxygen delignification Riku Kopra 1, Heikki Mutikainen 2 and Olli Dahl 3 1 South-Eastern Finland University of Applied Sciences, FiberLaboratory 2 Andritz Ltd., Fiber Technologies Division 3 Aalto University, School of Chemical Engineering ABSTRACT The main target of oxygen delignification is to continue delignification that started in cooking, i.e. remove a substantial fraction of the residual lignin using oxygen and alkali at a moderate temperature. Delignification with oxygen is a gentler way of reducing the Kappa number than extended cooking. Furthermore, the lower amount of lignin that goes in the pulp to bleaching, the smaller the organic waste water load from bleaching to the waste water plant and the less bleaching chemicals are needed. Because oxygen is a weak oxidizing agent in its normal molecular state, it is necessary to promote its reaction by raising the temperature and/or providing alkaline conditions so the oxygen is more reactive in demolishing lignin structures. The organic substance dissolved in cooking creates side reactions during the oxygen stage. This causes additional oxygen consumption and degradation of cellulose chains. An increase in washing loss before the oxygen stage causes reductions in pulp viscosity, strength, and yield. Correspondingly, efficient post-oxygen washing is the key for low-cost bleaching. We studied the performance of oxygen delignification by installing bubble size imaging systems and refractometers which measure dissolved dry solids in the oxygen stage feed and the top of the reactor. Based on these measurements, we gathered information about gas dispersion (bubble size distribution) and the behavior of dissolved matter in the hardwood mill s oxygen delignification stage. We aimed to investigate the effect of different variables on the oxygen stage s gas dispersion, Kappa reduction, yield, and pulp quality. Gas dispersion improved (average bubble size decreased) when the chemical mixer speed (RPM) increased. Increasing both the mixer speed and the amount of oxygen indicated higher Kappa reduction and increased the amount of dissolved organic matter. INTRODUCTION In the late 1970s, medium-consistency mixing technology became available and the new high-shear mixing devices made it possible to efficiently disperse oxygen as very small bubbles in 10% 14% consistency pulp. Thus, most of the oxygen delignification systems installed today are medium-consistency ones. The main reasons for the interest in medium-consistency applications are lower capital costs, greater ease of stock handling with medium-consistency mixing and pumping technology, and improved selectivity in the presence of appreciable amounts of black liquor solids. (McDonough 1996) The process is very flexible, and is best viewed as a bridging strategy between cooking and final bleaching. The method and degree to which oxygen delignification is applied involves a comprehensive optimization of the fiber line process, considering total cost, pulp quality, and environmental impacts. Process variables such consistency of pulp, temperature, pressure, amount of alkali, amount of oxygen, and also wash loss and inhibitors such as magnesium salts have an effect on the performance of oxygen delignification. Agarwal et al stated that consistency has no major effect on Kappa reduction if mixing is sufficient. However, in an industrial environment, neither mixing nor mass transfer is perfect. Elton et al showed that higher consistency results in lower diffusion distance and higher alkali concentration at a given alkali charge. Alkali and temperature are the most important factors affecting delignification during the oxygen delignification process. Because oxygen is a weak oxidizing agent in its normal molecular state, it is necessary to promote its reaction by raising the temperature and/or providing alkaline conditions so that the oxygen is more reactive in demolishing lignin structures. The oxygen charge has a less important role than the alkali charge and temperature. Regarding the oxygen charge, it is important that a sufficient amount of oxygen is present during the process in order to achieve a decent degree of delignification. However, increasing the oxygen charge too much can, in turn, provoke the formation of larger gas bubbles and channeling through the tower, which again decreases the

2 effectiveness of the oxygen delignification. Higher pressure is seen to help the diffusion of the oxygen, and only the dissolved oxygen can react with the phenol groups of the lignin. An increase in washing loss before the oxygen stage causes reductions in pulp viscosity, strength, and yield. Magnesium salts in the softwood process improve the selectivity of the oxygen delignification and result in higher viscosity at a given Kappa number. The effectiveness and potential of a powerful oxygen stage is obvious when comparing it with cooking and bleaching using a yield vs. Kappa number curve, as can be seen from Figure 1 (Pikka et al. 2000). According to Gullichsen et al. 1992, removing one Kappa unit in cooking, decreases the yield by 0.16%, whereas removing one Kappa unit in oxygen delignification decreases the yield by 0.12%. Figure 1. Comparison of yield selectivity between extended cooking and oxygen delignification (Pikka et al. 2000) The purpose of this work was to study the effects of process conditions on the dissolved solids, Kappa, and pulp quality, and quantify the state of gas dispersion in the process as well as the effect on this of different factors. MATERIALS AND METHODS The research was carried out by installing two continuous refractometers that measured the concentration levels, i.e. total dissolved solids (TDS), of filtrates in the oxygen stage feed and at the top of the reactor in a Finnish hardwood kraft pulp mill (Fig 2). Simultaneously, gas dispersion was monitored in real time using a continuous Pixact Bubble Monitoring (PBM) system. The principle behind the PBM system was presented in detail in an earlier study (Mutikainen et al. 2017). Measurement devices were installed to allow the evaluation of the performance of the oxygen delignification. Figure 2. Installation sites of the refractometers and PBM systems in the pulp mill s oxygen delignification. A complete two-week mill trial was performed. The alkaline used in test week one was oxidized white liquor and during the second week was NaOH (14%). The oxygen dosage was ( kg/adt), the mixer rotation speed

3 ( rpm), the operation pressure ( kpa) and the reactor temperature ( C). Consistency was quite stable, at about 11%, and the alkaline dosages were controlled so that the ph was about 10.5 in the discharge of the reactor. In test week one, we had 10 test points and in week two, we had 15 points. Pulp samples were taken from the oxygen stage feed pulp (sample 1) and after the oxygen reactor (sample 2), see Fig 1. The Kappa and viscosity were measured from the pulp samples. The filtrate sample was also extruded from the pulp through wire gauze after sampling. The ph, conductivity and COD were measured from the filtrates. The samples were analyzed using the following methods: Determination of dry matter content (analytical), (ISO 638) Determination of dry matter content (on-site), refractometer Conductivity (on-site), Mettler Toledo Conductometer ph (on-site), Mettler Toledo Conductometer COD liquor, samples filtrated using 1.6 µm GF/A paper and then analyzed in a COD analyzer (ISO 15705) Pulps, determination of Kappa number (ISO 302) Pulps, determination of viscosity (ISO ) RESULTS AND DISCUSSION Continuous Pixact Bubble Monitoring (PBM) system An image analysis algorithm developed by Pixact Ltd was utilized to detect gas bubbles and measure their diameters. An original image of the bubble flow is presented in Fig. 3 a) and the detected bubbles in Fig. 3 b). The bubble size data produced by PBM is averaged over 60 seconds and reported every 30 seconds to the mill s data collection and as a row in an Excel file. The data includes numerical, Sauter and volumetric distributions of bubble size and other relevant statistical values, such as mean and standard deviation values, percentiles of D10, D50 and D90 of the distributions and number of bubbles per image. This makes it possible to calculate and monitor different bubble size fractions under different process conditions. Figure 3. a) Original image of bubble flow illuminated with ring light. b) The detected bubbles are circled with red outlines on top of the image (Mutikainen et al. 2017). Fig. 4 shows the volumetric bubble size measured over the 1.5-month period. The measurement and obtained pictures (see Fig. 3) indicated that gas distribution in this hardwood pulp suspension was not as homogeneous as in the softwood pulp suspension observed earlier (Mutikainen et al. 2014); there happened to be several very large bubbles, which were also irregularly shaped and could not be measured accurately. However, the average arithmetic bubble diameter was 48 µm under typical process conditions and the average volumetric bubble diameter was around 500 µm; the arithmetic bubble diameter is approximately equal but the volumetric average diameter is ten times higher compared to the measurements made on the softwood pulp line (Mutikainen et al. 2014). Fig. 4 also displays the short- and long-term variation in the average volumetric bubble size, ranging from 0.4 to 1 mm. This means that the surface area of the bubbles enabling the dissolution of oxygen changes in the same ratio. The existence of large bubbles may also promote the coalescence and uneven distribution of oxygen

4 in the pulp. The mill has been in a normal production state including typical process variations: production, chemical dosage, pulp consistency, etc. Figure 4. Volumetric bubble size after the oxygen mixer on a Finnish hardwood pulp fiber line during a measuring period of 1.5 months (Mutikainen et al. 2017). The results from the factorial analysis confirmed that all the trial factors had a statistically significant effect on the bubble size. The effect of the used alkali (EVO = Oxidized white liquor and ENA = NaOH) was the most significant, but the rotor speed of the chemical mixer also had a remarkable effect, even with a small increase of 1,000 rpm to 1,175 rpm. The results from the factorial analysis also confirm the assumption that the measuring method is able to measure the impact of mixing efficiency and other process parameters on the state of dispersion. Figure 5. Results from the factorial analysis, main effects plot for bubble size (Mutikainen et al. 2017). Total dissolved solids (TDS) measurement by process refractometer The measurements allowed us to calculate the change in the dissolved solids within the oxygen reactor. As the figures illustrate, this correlated clearly with the oxygen delignification feed Kappa (6a) and changes in Kappa (6b). Changes in temperature also clearly affected the behavior of dissolved solids, Fig 7.

5 Figure 6. a) ΔTDS and kappa in to O 2 reactor (test week 1). b) ΔTDS and Δkappa in O 2 reactor (week 2), (Kopra et al. 2017) Figure 7. Temperature and ΔTDS to O 2 reactor (test week 1), (Kopra et al. 2017) As expected, the MC mass flows in the oxygen reactor in a plug-flow like manner. Gas flows, however, may find an easier passage for themselves, which leads to their channeling. Solids, which have dissolved from the mass to the liquid, will also be seen to pass in a plug-flow like way. Measuring the TDS before and after the oxygen stage and aligning these two measurements allows us to calculate the reactor s delay. Fig. 8 shows how the delay calculation was carried out using TDS measurements. The average delay between measurements was 36 minutes. The larger differences in the graphs represent changes in production levels and times when the delay deviated from the average delay of 36 minutes. The results were consistent with the delays calculated from the oxygen stage feed. Figure 8. Calculation of the oxygen stage time delay (Kopra et al. 2017) The measurements, alongside additional measurements and calculation, allow us to calculate the oxygen stage yield. Fig. 9 shows the yield as a function of temperature. The yield seems to decrease when the temperature increases. We did not observe an increased Kappa drop as a function of temperature. A lower temperature leads to an increased yield in oxygen delignification, which leads to lower wood consumption, if the higher yield in the oxygen delignification can be maintained through the bleaching. The level of the yield is lower, which would be expected based on the literature.

6 Figure 9. Oxygen stage yield calculation as a function of temperature (Kopra et al. 2017) CONCLUSIONS The Pixact Bubble Monitoring (PBM) system can be used for the continuous monitoring of gas dispersion in oxygen delignification. In addition to the improvement of general knowledge related to the process, this method would form the basis for adjusting the oxygen charge and gas dispersion in delignification processes. TDS measurements at the oxygen stage feed and at the oxygen reactor outlet allow us to calculate the changes in dissolved solids within the oxygen reactor (Delta concentration), and provides a new way of calculating the delay in the oxygen reactor, and the yield. Measurements at the oxygen stage feed also indicate the amount of washable dissolved material coming through to the oxygen stage. These TDS measurements, together with other measurements, allow for both the monitoring and controlling of the oxygen process in the future. ACKNOWLEDGMENTS This study was initiated under a contract between the South-Eastern Finland University of Applied Sciences, K- Patents OY, Andritz Oyj and Stora Enso Oyj. This work was financed partially by the South Savo Center for Economic Development, Transport and the Environment (ELY Center). Thanks to Liz Dexter for revising the English of the manuscript and thanks also to Anu Pihlajaniemi for helping me with my analysis. The authors are grateful to all participants. References 1. T. J. McDonough, Oxygen Delignification, in Pulp Bleaching - Principles and Practice, Institute of Paper Science and Technology, Inc., Atlanta, Georgia, TAPPI Press, 1996, pp Agarwal, S., Kinetics of Oxygen Delignification, Journal of Pulp and Paper Science, vol. 25, no. 10, 1999, pp Elton, E., Magnotta, V., Markham, L and Courchene, C., New Technology for Medium-Consistency Oxygen Bleaching, TAPPI, vol. 63, no. 11, 1980, pp Gullichsen, J., Kolehmainen, H. and Sundqvist, H., On the Nonuniformity of the Kraft Cook, Paperi ja Puu, vol. 74, no. 6, 1992, pp Pikka, O., Vesala R., Vilpponen, A., Dahllöf, H., Germgård, U., Norden, S., Bokström, M., Steffes, F. and Gullichsen, J., Bleaching Applications in Chemical Pulping, Published in cooperation with the Finnish Paper Engineers Association and TAPPI, Jyväskylä, Finland, 2000, pp Mutikainen, H., Kopra, R., Pesonen, A., Hakala, M., Honkanen, M., Peltonen, K. and Käyhkö, J., Measurement, State and Effect of the Gas Dispersion on the Oxygen Delignification, Proc. of International Pulp Bleaching Conference, Aug , 2017, Porto Seguro, BA, Brazil. 7. Mutikainen, H., Peltonen, K., Pikka, O., Käyhkö, J., Characterization of Oxygen Dispersion in Delignification Process. Proc. of International Pulp Bleaching Conference, October 29-31, 2014, Grenoble, France. 8. Kopra, R., Mutikainen, H., Pesonen, A., Käyhkö, J. and Tervola, P. Using Online Total Dissolved Solids (TDS) Measurements for Investigating the Performance of Oxygen Delignification, Proc. of Int. Pulp Bleaching Conf., Aug , 2017, Porto Seguro, BA, Brazil.

7 Using Online Bubble Size and Total Dissolved Solids (TDS) Measurements for Investigating the Performance of Oxygen Delignification TAPPI PEERS Conference, November 5 8, 2017 Norfolk, Virginia USA Riku Kopra, Heikki Mutikainen and Olli Dahl TAPPI PEERS Conference Norfolk, Virginia, November 5-8,2017

8 Content Background of the study Why to control/monitor O2 stage Measurement principle of the refractometer and bubble monitoring system Measurement arrangements, test plants Results of the study Conclusion TAPPI PEERS Conference Norfolk, Virginia, November 5-8,2017

9 Background An extensive mill study was carried out to determine the performance of hardwood pulp mill s oxygen delignification. TDS measurements were used as measurements units by side bubble monitoring system (state of gas dispersion). Investigation to find solutions that maximize Kappa reduction with minimal viscosity loss using reasonable amount of alkali and energy.

10 Why to control/monitor O2 stage? The motivation behind boosting O2 stage is economic and environmentally driven. Raw materials are increased in price (wood and chemicals) In Finland, Forest industry target is reduce mills COD load 10 percent per produced ton by 2020*. In addition, they committed to reduce solid and nutrient load on the water courses. * Environmental and responsibility commitment of the Finnish Forest Industry, temporary report 07/2015 (reference 2012). The targets of the boosting O2 stage are: Improve the yield of the fiber line (oxygen delignification and bleaching) a significant cost benefit Effec vely remove lignin save bleaching chemicals Keep pulp quality, above all strength of pulp. Reduce waste water load (good brown stock washing) all dissolved material (both those recovered in brown stock washing and those dissolved in the oxygen delignification) are kept to recovery boiler.

11 Optimization concept Key points 1) An optimal alkali dosage, oxidized white liquor or NaOH. (ph>10) 2) An optimal temperature and pressure. 3) An optimal, sufficient oxygen dosage and good mixing. (MC pulp: small bubbles by fluidizing mixer, need contact between oxygen and fibers) 4) A right and stable consistency 5) Time delay, reaction time Oxygen delignification occurs in a three phase system (liquid gas solid). Oxygen must move through the interface between the gas and liquid, diffuse through the static liquid layer around the fiber and finally diffuse through the fiber wall. In order for the reaction to be homogenous and rapid the distances d2 and d3 must be made as small as possible and/or the diffusion rate as high as possible.

12 Measurement principle of the process refractometer We have done several mill studies using refractometers as wash loss measurement units Refractometer indicates total dissolved solids (TDS) in solution, both organic and inorganic

13 Bubble monitoring system in operation Installation valve Installation tube Imaging probe On site control and monitoring Pressurized air feeding system for cooling Synchronization unit PC including software's and remote access, enabling control and data collection through internet

14 Oxygen is fed just before mixer. Kappa before cooking is about and before O2 stage about 11 13, consistency is normally about 11%, T = 90 C and pressure 280 kpa. Illustrations of our mill case

15 Analyzing methods: Determination of dry matter content (analytical), (ISO 638) Determination of dry matter content (on site), refractometer Conductivity (on site), Mettler Toledo Conductometer ph (on site), Mettler Toledo Conductometer COD liquor, samples filtrated using 1.6 µm GF/A paper and then analyzed in a COD analyzer (ISO 15705) Pulps, determination of Kappa number (ISO 302) Pulps, determination of Viscosity (ISO )

16 Test series I (Oxidized white liquor) and II (NaOH) Day mixer speed, rpm Oxygen dosage, kg/adt Temperature, C Pressure, kpa Used alkali oxidized white liquor, ph target after reactor 10.5 ( ) after that constant alkaline dosage (16 kg/adt) Day mixer speed, rpm Oxygen dosage, kg/adt Temperature, C Pressure, kpa Used alkali 14% NaOH. ph target after reactor 10.5

17 Refractometer results

18 Calculation of the oxygen stage time delay Plug flow: allows us to calculate the reactor s delay

19 Calculation of the oxygen stage yield On line yield calculation possible using TDS measurements The yield seems to decrease when the temperature increases

20 TDS vs Kappa The measurements allowed us to calculate the change in the dissolved solids within the oxygen reactor This correlated clearly with the oxygen delignification's feed Kappa and changes in Kappa. TDS, % by weight TDS, % by weight ΔTDS Kappa to oxygen reactor Change in the TDS and O2 feed kappa(oxidized white liquor) Kappa in Kappa ΔTDS Delta kappa Change in the TDS and change in the Kappa in the =2 reactor (NaOH)

21 TDS vs Temperature Changes in temperature also clearly affected the behavior of dissolved solids Temperature, C Total dissolved solids, % by weight Temperature to reactor ΔTDS

22 Bubble monitoring results

23 Original image and detected bubbles Example image of gas dispersion after oxygen mixer in Finnish hardwood pulp fiber line. The detected bubbles are circled with red outlines on top of the image.

24 Average volumetric bubble size trend line measured from oxygen reactor feed during 1,5 month trial period in Finnish hardwood pulp fiberline.

25 Results from factorial analysis Results from factorial analysis revealed that all trial factors had statistically significant effect on measured bubble size. Most effect had used alkali, mixer rotor speed and oxygen dosage. Used mixing power range might have been too narrow to establish correlation between bubble size and kappa reduction. Increase in mixing power increased yield and hemi yield in oxygen delignification. Main effects plot for Bubble size from factorial analysis results.

26 Conclusions TDS measurements allow us to calculate the changes in dissolved solids within the oxygen reactor (Delta concentration). The Delta concentration correlates moderately with the change in Kappa. TDS measurements at the oxygen stage feed and at the oxygen reactor outlet provides a new way of calculating the delay in the oxygen reactor and also allows the yield calculation, but yield calculation does however require additional measurements and calculations. There seem to be moderate variation in gas bubble size of oxygen delignification between fiber lines. Bubble Monitoring System can be used to continuous monitoring of gas dispersion in oxygen delignification. The effect of bubble size on the dissolution of oxygen and delignification rate should be quantified eg. through mill measurements and with theoretical calculations. Using both refractometers and bubble monitoring systems it is possible to get new information about the performance of oxygen delignification and it gives new tools for process control.

27 Thank you for your attention! Questions? Contact: Riku Kopra Phone E mail. Riku.kopra@xamk.fi