Improving Reaction Selectivity with NaOH Charges and Reaction Time in the Medium Consistency Oxygen Delignification Process

As more delignification is targeted, fiber degradation becomes a main issue in the pulping process. Sodium hydroxide (NaOH) is highly related to pulp quality in the medium consistency oxygen delignification process. Accordingly, the purpose of this research was to study the effect of NaOH charges and reaction time on reaction selectivity during the pulping process through medium consistency oxygen delignification. This research used Eucalyptus pelita and Acacia mangium pulp with a kappa number (KaNo) of 17–18. The medium consistency oxygen delignification process condition included a temperature of 80°C and oxygen pressure of 1 bar, while the reaction times were 20, 40, 60, 80, and 100 min. The NaOH charges were 5, 10, 15, 20, and 25 kg/t of pulp. The analysis parameters used were KaNo and viscosity. The variation in reaction time did not show a significant change in KaNo. The increase in reaction time and NaOH charge variations, meanwhile, can reduce pulp viscosity. Higher NaOH values significantly increased the delignification degree, and the higher NaOH charges and reaction times together lowered the degree of polymerization (DP). The variation in reaction time indicated that with a longer reaction time, the lower the DP, and with a higher NaOH charge and longer reaction time, the lower the reaction selectivity. 

Delignification degree, Fiber degradation, Medium consistency oxygen, Polymerization degree, Reaction selectivity

    The degradation of fiber and dissolving of lignin can be predominantly found in the intermediate cooking and bleaching process. The medium consistency oxygen delignification system as intermediate process completed by using chemical treatments, including acidic and basic treatments (Hermansyah et al., 2019). The alkaline treatment is an efficient method for delignification (Harahap et al., 2019), especially using sodium hydroxide (NaOH) to influence the physical properties of the fibers. This treatment removes the hemicellulose and lignin contained in the fiber (Fatra et al., 2016). This process is also in part considered a continuation of the pulping alkaline process and, somehow, the first step in the bleaching process. The oxygen delignification in the medium consistency oxygen stage decreases the kappa number (KaNo) prior to chlorination and provides the bleaching plant with a pulp that has a considerably reduced KaNo (Bajpai, 2012). This process also removes part of the residual lignins from kraft cooking through the reaction of pulp with oxygen and NaOH under high temperatures condition (Júnior and Gomes, 2018). 

    Oxygen delignification helps in part by substituting both chlorine and chlorine dioxide during bleaching and has additional technical and economic benefits (Akim et al., 2011). Carbohydrate degradation occurs in particular during the initial stage and continues toward the end of kraft cooking. The oxygen-alkali process (i.e., oxygen delignification) is known to be more selective (i.e., carbohydrate yield/delignification) than the final kraft cooking phase (Jafari et al., 2014a; Jafari et al., 2014b). The equipment used for medium consistency oxygen delignification consists of a steam medium pressure injection system, centrifugal pump, pressurized reactor, distribution reactor, and gas mixer with high turbulence (Hart and Rudie, 2012). The medium consistency oxygen with single reactor system can improve delignification selectivity and depolymerization as seen from change the KaNo and viscosity of the pulp produced. The variables involved in medium consistency oxygen delignification are reactor pressure, temperature process, pH, reaction time, and pulp consistency. The variation in this process’s conditions is shown in Table 1 (Júnior and Gomes, 2018).

Table 1 Typical conditions in industrial medium consistency oxygen delignification

The addition of oxygen gas to the pulp in an oxygen mixer produce maximum contact between the pulp and oxygen (Nasser, 2015). The results of this process in turn reduce the KaNo values, thus decreasing bleaching chemical consumption (Markus and Pearce, 2017). KaNo is the volume of 0.1 N potassium permanganate solution consumed by 1 g of moisture-free pulp under the acidic conditions, with the result corrected to 50% consumption of the added permanganate (TAPPI, 2006). The initial KaNo value influences the level of delignification in the same NaOH charge. The KaNo target for the cooking process are 25 to 30, and further delignification using the alkaline oxygen process recommends a KaNo of 15?20 (Jafari et al., 2014a). 

The delignification in the medium consistency oxygen stage reflects on KaNo reduction. The classification and species of wood also affect delignification in the medium consistency oxygen delignification process (Hart and Rudie, 2012). The reduction in KaNo can reach up to 75% for softwood species and 45?50% for hardwoods. 

The decrease in lignin content during medium consistency oxygen delignification is represented by the degree of delignification. The degree of delignification is the percentage decrease in lignin content before and after the medium consistency oxygen process. Lignin content is calculated based on KaNo, where L (%) = 0.147×KaNo (Violette, 2003), while the formula delignification degree (%) is (Wistara et al., 2015):

where DD is the degree of delignification, Lo(%) is the lignin content before medium consistency oxygen delignification, and Lt(%) is the lignin content after medium consistency oxygen delignification

where the values 0.905 and 0.75 are constants characteristic of the polymer-solvent system and [?] is intrinsic viscosity (mL.g^-1); (Henrique et al., 2015).

                                                                                    (2)

where the values 0.905 and 0.75 are constants characteristic of the polymer-solvent system and [?] is intrinsic viscosity (mL.g^-1); (Henrique et al., 2015).

DP indicates the level of cellulosic degradation during the cooking process. The higher the DP, the stronger the cellulose (fiber) in cellulose degradation events. The solubility of cellulose in soda also decreases as DP increases (Yamane et al., 2015). DP is calculated from the intrinsic viscosity [?] of the pulp in mL g^-1 and the weight fractions of hemicelluloses (H) and cellulose (G) in the pulp according to Equation 3 (Jafari et al., 2014b):

                                                                   (3)

The selectivity of the medium consistency oxygen delignification stage improved by offering a greater effect on KaNo reduction, but induced more serious cellulose degradation (Chong et al., 2013). Reaction selectivity is commonly calculated as the ratio of the change in KaNo to the change in pulp viscosity during medium consistency oxygen delignification. The reaction selectivity coefficient (?) is defined as the ratio between the change in KaNo (?K) and (1/DPt-1/DP0) during the delignification process as long as the medium consistency oxygen process. The (1/DPt-1/DP0) value represents the number of chain scissions per polymer cellulose unit. Reaction selectivity is defined as the reduction in KaNo divided by the number of cellulose chain scissions (1/DPt-1/DP0) (Ji, 2007):

                                                 (4)

In medium consistency oxygen delignification, the most important analysis parameters are KaNo and viscosity. This study demonstrated that higher NaOH charges enhance the delignification rate. More lignin will dissolve with an increased NaOH charge. The longer reaction time also results in an increased delignification rate. The decrease of DP values ??in Eucalyptus pelita and Acacia mangium was due to higher NaOH charge and longer reaction times. Based on the variations in NaOH charge, reaction selectivity decreases according to increased NaOH charge. The 20-min reaction time variations show the highest reaction selectivity.

This article was prepared thanks to cooperation between several parties, including the Education Fund Management Institution of  Indonesian Finance Ministry as research founder; Prof. Dr. Ir. Danawati Hari Prajitno, M.Pd. as corresponding author; Prof. Dr. Ir. Achmad Roesyadi, D.E. as mentor; the head of Balai Besar Pulp dan Kertas (BBPK), Bandung; the Chairman of the Department of Chemical Engineering, Sepuluh Nopember Institute of Technology (ITS), Surabaya; and the friends of the research team.

Akim, L.G., Colodette, J.L., Argyropoulos, D.S., 2011. Factors Limiting Oxygen Delignification of Kraft Pulp. Canadian Journal of Chemistry, Volume 79(2), pp. 201–210

Bajpai, P., 2012. Oxygen Delignification. In: Environmentally Benign Approaches for Pulp Bleaching, Pulp and Paper Consultants, Patiala, India, pp. 19–57

Chong, Y.H., Daud, W.R.W., Leh, C.P., 2013. Effect of Hydrogen Peroxide and Anthraquinone on the Selectivity and Hexenuronic Acid Content of Mixed Tropical Hardwood Kraft Pulp during Oxygen Delignification. BioResources, Volume 8(2), pp. 2547–2557

Fatra, W., Rouhillahi, H., Helwani, Z., Zulfansyah, Asmura, J., 2016. Effect of Alkaline Treatment on the Properties of Oil Palm Empty Fruit Bunch Fiber-reinforced Polypropylene Composite. International Journal of Technology, Volume 7(6), pp. 1026–1034

Harahap, A., Rahman., Aditya, A., Sadrina., Nur, I., Misri, G., 2019. Optimization of Pretreatment Conditions for Microwave-Assisted Alkaline Delignification of Empty Fruit Bunch by Response Surface Methodology. International Journal of Technology, Volume 10(8), pp. 1479–1487

Hart, P.W., Rudie, A.W., 2012. The Bleaching of Pulp. 5th Edition. USA: TAPPI PRESS

Henrique, M.A., Neto, W.P.F., Silvério, H.A., Martins, D.F., Gurgel, L.V.A., Barud, H.S., Morais, L.C., Pasquini, D., 2015. Kinetic Study of the Thermal Decomposition of Cellulose Nanocrystals with Different Polymorphs, Cellulose I and II, Extracted from Different Sources and using Different Types of Acids. Industrial Crops and Products, Volume 76, pp. 128–140

Hermansyah, H., Putri, D.N., Prasetyanto, A., Chairuddin, Z.B., Perdani, M.S., Sahlan, M., Yohda, M., 2019. Delignificaton of Oil Palm Empty Fruit Bunch using Peracetic Acid and Alkaline Peroxide Combined with the Ultrasound. International Journal of Technology, Volume 10(8), pp. 1523–1532

Jablonsky, M., Majova, V., Skulcova, A., Haz, A., 2018. Delignification of Pulp using deep Eutectic Solvents. Journal of Hygienic Engineering and Design, pp. 76–81

Jafari, V., Labafzadeh, S.R., King, A., Ainen, I.K., Sixtaa, H., Heiningenab, A., 2014. Oxygen Delignification of Conventional and High Alkali Cooked Softwood Kraft Pulps, and Study of the Residual Lignin Structure. RSC Advances, Volume 4(34), pp. 17469–17477

Jafari, V., Sixta, H., Heiningen, V.A., 2014a. Kinetics of Oxygen Delignification of High-kappa Pulp in a Continuous Flow-through Reactor. Industrial and Engineering Chemistry Research, Volume 53(20), pp. 8385–8394

Jafari, V., Sixta, H., Heiningen, V.A., 2014b. Multistage Oxygen Delignification of High-kappa Pine Kraft Pulp with Peroxymonosulfuric Acid (Px). Holzforschung, Volume 68(5), pp. 497–504

Ji, Y., 2007. Kinetics and Mechanism of Oxygen Delignification. Master’s Thesis, Graduate Program, The University of Maine. Orono, Maine, USA

Júnior, E.A.B., Gomes, F., 2018. The Effects of Temperature, Alkali Charge and Additives in the Oxygen Delignification in High Kappa Number Eucalyptus Pulp Kraft. Scientia Forestalis, Volume 46(118), pp. 217–227

Markus, J., Pearce, C., 2017. Howe Sound: Oxygen Delignification and Optimization Analysis. Report Waterloo cases in Design Engineering, Canada

Nasser, A., 2015. The Effect of Oxidized and Unoxidized Filtrate on Oxygen Delignification. Karlstads University Press, Karlstads, Swedia

Rahmati, H., Ebrahimi, P., Sedghi, M., 2010. Effect of Cooking Conditions and Oxygen-Delignification on Bambusa Tulda Kraft Pulping. Indian Journal of Chemical Technology, Volume 17(1), pp. 74–77

Septia, E., Supriadi., Suwinarti, W., Amirta, R., 2018. Characterization and Ethanol Potential from Giant Cassava (Manihot esculenta) Stem Waste Biomass. In: IOP Conference Series: Earth and Environmental Science, Volume 144(1), pp. 1–8

TAPPI, 1999.  Viscosity of Pulp (Capillary Viscometer Method) TAPPI T 230 om-99. TAPPI Test Method, Don Guay, TAPPI Press, Atlanta, Georgia USA, pp. 1–9

TAPPI, 2006. Kappa Number of Pulp T 236 om-99. TAPPI Test Method, Don Guay, TAPPI Press, Atlanta, Georgia USA, pp. 7–11

Tunc, M.S., 2003. Relationship between Alkaline Pulp Yield and the Mass Fraction and Degree of Polymerization of Cellulose in the Pulp. Maine University, Orono, Maine, USA

Violette, S.M., 2003. Oxygen Delignification Kinetics and Selectivity Improvement, Chemical Engineering. The University of Maine, Orono, Maine, USA

Wetterling, J., 2012. Modelling of Hemicellulose Degradation during Softwood Kraft Pulping. Chalmers University of Technology, Gothenburg, Sweden

Wistara, N.J., Carolina, A., Pulungan, W.S., Emil, N., Lee, S.H., Kim, N.H., 2015. Effect of Tree Age and Active Alkali on Kraft Pulping of White Jabon. Journal of the Korean Wood Science and Technology, Volume 43(5), pp. 566–577

Yamane, C., Abe, K., Satho, M., Miyamoto, H., 2015. Dissolution of Cellulose Nanofibers in Aqueous Sodium Hydroxide Solution. Nordic Pulp & Paper Research Journal, Volume 30(1), pp. 92–98

Zhao, H., Li, J., Zhang, X., 2018. Fundamental Understanding of Distracted Oxygen Delignification Efficiency by Dissolved Lignin during Biorefinery Process of Eucalyptus. Bioresource Technology, Volume 258(January), pp. 1–4