Effect of Defoamer on the Gas Content and Performance of O2 washers – Mill Case Study

Riku Kopra1, Jari Käyhkö,1 and Anna Pesonen2 1 South-Eastern Finland University of Applied Sciences, FiberLaboratory 2 Stora Enso Consumer Boards  

ABSTRACT 

In brownstock washers, the application of defoamer is required to improve mat drainage and to control foam to enable the washers to operate efficiently. Pulp air content is a very important factor that must be considered because increasing air content clearly decreases washer capacity. Air in the pulp retards its drainage. Free air which passes along with the fibers can create permanent foam that blocks channels, and when it breaks, it flocculates impurities, which are accumulated in the bubbles.

In this study, two different defoamers with various dosages and dosage points were tested in the oxygen stage of a hardwood line. In addition to the process, measurements and their changes were monitored using an on-line gas content measurement unit and on-line process refractometers (measuring total dissolved solid content, TDS). The bubble size of the O2 stage pre-washer’s feed pulp was measured using a bubble monitoring system (bms). The results showed that the defoamers and their point of dosage had a clear effect on the air content of the pulp suspension and thus on the operation of the washer both before and after the oxygen stage. Choosing suitable dosage points, defoamers and dosages can intensify the performance of the washers. This also has a great effect on the dewatering and thus the filtrate balances. 

INTRODUCTION 

The function of defoamers is to eliminate foam and entrained air attached to the fibers (1). In brownstock washers, the application of defoamer is required to improve mat drainage and control foam to enable overloaded washers to wash efficiently. Today, most pulp mill defoamers for brownstock washing are water-based silicone defoamers. Water-extended oil-based defoamers are also still widely used.

Air in the pulp retards its drainage. Hakamäki and Kovasin 1985 (2) found that the capacity of test bed washers decreased 25% when the pulp air content grew from 0 to 5 vol-%. According to Kurtz 1981 (3), free air which passes along with the fibers can create permanent foam that blocks channels and, when it breaks, flocculates impurities which are accumulated in the bubbles. In addition, very small air bubbles reduce drainage, because bubbles block the spaces between fibers and makes flow through a fiber net complicated. The dissolved air in wash filtrates can also cause bubbles, which complicates drainage even in ideal circumstances. Although the harmful effects of air/gas on the performance of washing are known, most research results are based on laboratory tests (2, 4), but only a few published studies have been based on continuous measurements (5). With the use of a bubble monitoring system (bms) (6-8), together with on-line gas content measurement, creates new opportunities to measure the amount of gas going into the washers, the size distributions of bubbles and thus the effect of concentration changes on the performance of washing. Figure 1 shows on-line bubble data in DD2 washer feed pulp flow measured by bms at two different time points.

Figure 1. Two different gas content situations in DD2 feed pulp flow measured by bms.

Washing before oxygen delignification is important, because washing loss consumes oxygen and alkali in oxygen delignification and at high incoming washing losses the anticipated kappa reductions are not achieved (9). Moreover, at high wash loss levels, residual alkali present in the black liquor and the exothermic oxidation reaction of black liquor’s sulfur compounds can increase the temperature in the oxygen delignification tower at the beginning of the reaction step and thus accelerate unselective reactions in the fiber (10).

MATERIALS AND METHODS 

The experiments for this study were carried out in a Scandinavian pulp mill’s hardwood (mainly Betula pendula and Betula pubescens) pulp fiber line, which consists of a continuous digester with a high-heat washer in its lower section, an atmospheric diffuser (AD), one DD washer before 1-stage oxygen delignification and two DD washers after the oxygen stage. Production was about 2000 a.d.metric tons/day during research, the kappa target after digester was 17 and after oxygen delignification 11 (the hexa part was about 6). The measurement arrangements of the hardwood pulp fiber line’s O2 delignification are shown in Figure 2.

Figure 2. Measurement arrangements of hardwood pulp fiber line (650,000 a.d.metric tons/year) O2 delignification.

In this work, a stepwise trial was conducted during one day and the longer-term effect of air content on the process was monitored. The quality and dosing points of the defoamer were changed in the stepwise trials. In normal operation, defoamer dosing points 1 and 2 are used (see fig 2). Dosing point 1 is the last wash water into DD1 and dosing point 2 is the vacuum filter from the DD1 washer. In normal operation, oil-based defoamer is used. In this trial run, dosing point 3 was tested with silicone-based defoamer. Gas content was monitored mainly using EchowiseTM gas content measurement in the DD2 feed pulp. The torque and rotation speed of the DD washers, consistencies and TDS contents were also monitored. Stepwise trials 1) Time 10:00, oil-based defoamer to dosing point 1 and 2 were stopped. Oil-based defoamer replaced with

silicone-based to dosing point 2. 2) Time 11:15, silicone defoamer was started to dosing point 3. 3) Time 14:10, dosing points 1 and 2 went back to normal operation (oil-based). 4) Time 14:45, dosing point 3 was stopped.

Figure 3. Timing of the defoamers at dosing points 1 and 3. Pulp samples were taken from the O2 delignification feed pulp and outlet pulp from DD washer 2. The filtrate sample was also extruded from the pulp through wire gauze around 30 min after sampling. Kappa from the pulp samples and the TDS by laboratory standard of all filtrate samples were determined afterwards in the laboratory. Because there were no significant changes in the kappa reduction, this work mainly focused on the effect of gas content on the performance of O2 delignification washers.

Analytical determinations

Analyses were conducted to define the chemical situation of the fiber line. The samples were analyzed using the following methods: • Determination of dry matter content (analytical), ISO 638 “Paper, board and pulps—determination of dry matter content—oven-drying method” • Determination of dry matter content (on-site), refractometer • Pulps, ISO 302 “Determination of kappa number”

RESULTS AND DISCUSSION

Stepwise trials, effect of defoamer changes on the performance of the DD1 washer The brown stock washing line was becoming dirtier and the TDS content of the DD1 washer’s feed pulp filtrate fraction was increasing. When the defoamer and dosing points were changed, the TDS content coming out of the DD1 washer decreased. DD1 was washing dissolved solids more effectively, see fig 4.

Figure 4. TDS contents into and out from DD1 washer under stepwise trial. Stepwise trials, effect of changes to DD2 When defoamer 1 dosing was stopped and pulp without defoamer came through the O2 reactor, the gas content of the DD2 washer’s feed pulp increased from 0.2% to 0.6%. When defoamer dosing 3 was started, the gas content decreased to a level of 0.16 to 0.17, which was lower than the reference level, see fig 5. Dosing point 3, which was closer to the DD2 washer than points 1 and 2, showed better results with the new silicon-based defoamer than in the earlier normal operation. Longer period trials are still needed for the silicone-based defoamer and new dosing points, so that the changes in gas content compared to normal operation can be documented and the operation of O2 delignification and O2 washers can be seen fully.

Figure 5. Gas content into DD2 washers during stepwise trial. When gas content into the DD2 washer decreased, the drum torque increased and the drum rotation speed decreased, see fig 6. Overall, the performance of the washer settled down and the operational parameters of the DD2 washers stayed at a better level. This resulted in more efficient washer operation.

Figure 6. DD2 washer’s drum torque and rotation speed during stepwise trial.

In addition, the DD2 washer outlet consistency increased, which could be seen by the feed valve angle control of the following screening process and also in the rising filtrate tank level.

Figure 7. Consistency control into knot screening after O2 delignification. The results showed that defoamer quality and dosing points have a clear effect on the performance of washing in the area of O2 delignification. In our previous studies, Käyhkö et al 2019 (11), we found that defoamer has a clear effect on the size of the oxygen bubbles in the feed of the O2 stage. If at this experimental mill, the defoamer dosing point is transferred to after the O2 reactor, the gas bubbles are smaller and thus more reaction surface area is obtained. By utilizing on-line gas content and TDS measurement, the performance of washing can be monitored. The measurement also makes it possible to build control system solutions. Long-term trials As the air content increases in the DD1 washer’s filter cycle, the washer starts to rotate faster and the torque decreases. The performance of the washer is then not optimal. With on-line gas content measurements, such operation situations can be detected and remedial action can be taken.

Figure 8. DD washer circulation filtrates gas content and drum rotation speed. Fig 9 shows the kappa reduction over the O2 reactor and the COD content into the O2 stage for a two-month period. It can be seen that if the washing does not work and there is a large amount of washing loss coming into O2 stage, the kappa reduction is negatively affected.

Figure 9. Incoming COD to O2 delignification and kappa reduction

CONCLUSIONS Air/gas in the pulp feed has a clear effect on the operation of the washing equipment, mainly on its efficiency and capacity. By using defoamers correctly, these side effects can be reduced. New on-line measurement techniques such as a gas bubble monitoring systems and air content measurements enable monitoring of the situation when the gas content is rising and responding to changes. In addition, the measurement arrangement disclosed herein can in real time calculate the efficiency of washers based on the TDS content measurements. Preliminary results will soon be published on these on-line efficiency calculations. However, more research will be needed to verify our findings. ACKNOWLEDGMENTS

The research was carried out within the “GasOpti” and “KuituMOD” projects, by South-Eastern Finland University of Applied Sciences / FiberLaboratory. The projects were funded by Business Finland (the Finnish Funding Agency for Technology and Innovation),Regional Council of the South-Eastern Finland, the European Union and the participating companies. The authors thank Liz Dexter for editing the manuscript and Anu Pihlajaniemi and Hanna Laukkanen for helping with the data analysis. The authors are grateful to all participants involved with this study.

REFERENCES

1. Wilson, R.E., The role of defoamer in brown stock washing, TAPPI 2016 Pulping, Engineering, Environmental, Recycling and Sustainability (PEERS) Conference, Tappi Press, Atlanta, Session 7-3 (2016).

2. Hakamäki H., and Kovasin K., The effect of some parameters on brown stock washing: A study made with a pulp tester. Pulp & Paper Canada 86(9):243-249 (1985).

3. Kurtz K.D., Effect of air on the pulp washer. Proc. of Tappi 1981 Engineering conference, Atlanta, GA,

USA, 191-196 (1981). 4. Wang, J., Pelton, R, Hrymak, A.N., and Kwon, Y., New insights into dispersed air effects in

brownstock washing, TAPPI J. 84(1):8 (2001). 5. Dougherty, S.J., On-line entrained air measurement for brownstock-washer defoamer control, Tappi J.

72(1): 50-54 (1989). 6. Mutikainen, H., Strikina, N., Eerola, T., Lensu, L. Kälviäinen, H., and Käyhkö J., Online measurement

of the bubble size distribution in medium-consistency oxygen delignification, Appita Journal, 68(2): 159-164 (2015).

7. Mutikainen, H., Kopra, R., Pesonen, A., Hakala, M., Honkanen, M., Peltonen, K., and Käyhkö, J., Measurement, state and effect of gas dispersion on oxygen delignification, Proc. Of the International Pulp Bleaching Conference, Porto Seguro, BA, Brazil, p.57-61, (2017).

8. Käyhkö, J., Mutikainen, H., Peltonen, K., Kopra, R., Hakala, M., and Honkanen, M., Gas dispersion in the oxygen delignification process, Proc. of the TAPPI PEERS Conference, October 28-31, 2018, Portland, OR, USA.

9. Andbacka S., The importance of washing in oxygen delignification and TCF bleaching. Pulp & Paper

Canada 99(3):57-60 (1998).

10. Vuorenvirta K., Fuhrmann A., Gullichsen J., Effect of black liquor carry-over on selectivity of oxygen

delignification. Proc. of International Pulp Bleaching Conference, Halifax, Canada, 65-70 (2000). 11. Käyhkö, J., Mutikainen, H., Peltonen, K., Kopra, R., Hakala, M., and Honkanen, M., The state and role

of gas dispersion in the oxygen delignification process, Proc of the Paper week 2019, February 4-7, 2019, Montreal, QC, Canada.

Gateway to the Future

Effect of Defoamer on the Gas Content and Performance of O2 washers – Mill Case Study

Riku Kopra1, Jari Käyhkö,1 and Anna Pesonen2

Contents

• FiberLaboratory• Background and objective of the work• Importance of O2 washing and the effect of gas on the performance of the washers.

• Measurement principles, measurement arrangements, tests plant• Results of the study• Conclusion

FiberLaboratoryAndritz and FiberLaboratory have had successful co‐operation in development of chemical mixing technology since 2005. One important tool in these studies has been the mill size (6000 t/day) MC‐loop.

Our latest studies mainly considering BSW and O2 delignification. For example: Kopra et al. ABTCP and TAPPI PEERS 2018 (BSW)Käyhkö et al. ABTCP and TAPPI PEERS 2018 (O2)

BackgroundSeries of investigations to find solutions that minimize washing loss and consume less energy, water and chemicals. Addition, investigation to find solutions that maximize Kappa reduction with minimal viscosity loss using reasonable amount of alkali and energy in the oxygen stage.

Oxygen delignification is a more selective and gentler process remove lignin than extended cooking.

Brown stock washing is notable sub‐process in chemical pulping, because it has effects on the followed treatments of pulp and other hand it’s first step in chemical recovery cycle.

• Investigate the effect of the defoamer to the performance of O2

delignification. Target was get smaller bubbles to O2 reactor. • Investigate the effect of defoamers to gas content and then to the performance of washers.

• Use real‐time measurements and data for monitoring process and calculating efficiencies.

• Understanding better defoamers feed places and effect of different defoamers.

Objectives of the work

Importance of O2 washing

• Washing before oxygen delignification is important, because washing loss consumes oxygen and alkali in oxygen delignification and at high incoming washing losses the anticipated kappa reductions are not achieved.

• Moreover, at high wash loss levels, residual alkali present in the black liquor and the exothermic oxidation reaction of black liquor’s sulfur compounds can increase the temperature in the oxygen delignification tower at the beginning of the reaction step and thus accelerate unselective reactions in the fiber.

• Post O2 washing is important, because carry‐over have to remove before the pulp enters bleaching. Diffusion plays an important role especially at the end of the line when the impurities inside the fibers often account for almost of the carry‐over.

Effect of gas content to washers performance• Free air which passes along with the fibers can create permanent foam that blocks channels and, when it breaks, flocculates impurities which are accumulated in the bubbles.

• In addition, very small air bubbles reduce drainage, because bubbles block the spaces between fibers and makes flow through a fiber net complicated. 

• The dissolved air in wash filtrates can also cause bubbles, which complicates drainage even in ideal circumstances. 

Feed pulp into DD2 washer, consistency about 9 %, left side normal situation, right side more gas 

Measurements

• 10 refractometers which measures TDS

• 2 gas content measurements• 2 bubble size measurements

Measurement arrangements

‐Defoamer feed point 1 is the last wash water into DD1 and dosing point 2 is the vacuum filter from the DD1 washer. Point 3 is dilution water to pulp after O2 reactor.

Test arrangements, Scandinavian HW pulp millStepwise trials‐ Normally oil based defoamer is used, now water based silicone defoamer was tested‐ Point 1 is the last wash water into DD1 and dosing point 2 is the vacuum filter from the DD1 washer. Point 3 is dilution water to pulp after O2 reactor.

1) Time 10:00, oil‐based defoamer to dosing point 1 and 2 were stopped. Oil‐based defoamer replaced with silicone‐based to dosing point 2.

2) Time 11:15, silicone defoamer was started to dosing point 3.3) Time 14:10, dosing points 1 and 2 went back to normal operation (oil‐based).4) Time 14:45, dosing point 3 was stopped.

Results, TDS by refractometer

• Can be monitored TDS levels• Can be done wash yield or effectiveness calculations.

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Results, Gas content

• When defoamer 1 dosing was stopped and pulp without defoamer came through the O2reactor, the gas content of the DD2 washer’s feed pulp increased from 0.2% to 0.6%.

• When defoamer dosing 3 was started, the gas content decreased to a level of 0.16 to 0.17, which was lower than the reference level.

→ Enables real‐time monitoring of gas content. → Changes, error situations in the process are clearly seen in the measurement→ Gives you the ability to react or adjust.

Results, Effect on the performance of washers

• When gas content into the DD2 washer decreased, the drum torque increased and the drum rotation speed decreased 

• Overall, the performance of the washer settled down and the operational parameters of the DD2 washers stayed at a better level. 

Results, Consistency into knot separation

• In addition, the DD2 washer outlet consistency increased, which could beseen by the feed valve angle control of the following screening process andalso in the rising filtrate tank level.

Results, Long‐term trials, Gas content

• Long‐term monitoring supports our findings, if there is gas in the washers, starts washer rotate faster.

• By monitoring, you can see in real time when problems start and you can react faster.

Results, Long‐term trials, COD level

• The kappa reduction over the O2 reactor and the COD content into the O2 stage for a two‐month period. 

• It can be seen that if the washing does not work and there is a large amount of washing loss coming into O2 stage, the kappa reduction is negatively affected.

Results, Long‐term trials, effect of wash loss to T

• At high wash loss levels, residual alkali present in the black liquor and the exothermic oxidation reactions of black liquor’s sulfur compounds can increase the temperature in the oxygen delignification tower at the beginning of the reaction step and thus accelerate unselective reactions in the fiber (Vuorenvirta et al. 2000).

Conclusion

• The results showed that defoamer quality and dosing points have a clear effect on the performance of washing in the area of O2 delignification.

• Defoamer has a clear effect on the size of the oxygen bubbles in the feed of the O2 stage. If at this experimental mill, the defoamer dosing point is transferred to after the O2 reactor, the gas bubbles are smaller and thus more reaction surface area is obtained.

• By utilizing on‐line gas content and TDS measurement, the performance of washing can be monitored. The measurement also makes it possible to build control system solutions.

Thank you for your attention!Questions?

The research was carried out within the “GasOpti” and “KuituMOD” projects, by South‐Eastern Finland University of Applied Sciences / FiberLaboratory. The projects were funded by Business Finland (the Finnish Funding Agency for Technology and Innovation),Regional Council of the South‐Eastern Finland, the European Union and the participating companies.

Contact: Riku KopraPhone +358 50 443 6411E‐mail. Riku.kopra@xamk.fi