Oxygen Delignification Process Research Articles

On the nature of the selectivity of oxygen delignification

Abstract This work has focused on oxygen’s role in the delignification process within the context of pulp production. We have investigated the role of oxygen in a complex set of chemical reactions taking place during this process, including both oxidative and non-oxidative reactions. This study explores the impact of pH changes during the oxygen delignification process and the characteristics of the resulting pulps. Additionally, this research examines the effect of oxygen, by comparing conventional oxygen delignification with trials using air and nitrogen. Industrial softwood kraft pulps with a kappa number of 35 were subjected to delignification for 20–120 min under alkaline conditions. The resulting pulps were assessed for kappa number, intrinsic viscosity, fiber charge, and ISO brightness. An important observation from this research is the reduction in lignin molecular weight upon exposure to oxygen and air, suggesting depolymerization reactions facilitated by oxygen species, whereas nitrogen exposure results in less pronounced changes. This finding underscores the impact of oxygen in altering lignin structure, thus informing the selectivity and effectiveness of the delignification process.

Model development for real oxygen delignification processes

Previous extensive work has been done on modeling the oxygen delignification process, based on how the basic parameters, i.e., temperature, kappa number, concentration of alkali, and concentration of oxygen, affect the delignification rate. However, these models are not used extensively to evaluate the performance of real processes, primarily because they have not been able to properly consider all the essential issues affecting delignification in practice. Such issues include the mass transfer and consumption of oxygen, which defines the concentration of dissolved oxygen in the process, and the effect of that concentration on the delignification rate. In this paper, a new way to model the oxygen delignification process is used in which these parameters, among other smaller matters, are taken into account. The basic model and its parameters were defined by the information obtained from the literature, delignification made in the laboratory tests, and mill processes and mill tests. An essential aspect of these studies was the information obtained from the oxygen concentration measured in the residual gas obtained from the top of the reactor. With the aid of this measurement, it was possible to define more accurately the consumption of oxygen and partial pressure of oxygen that define the concentration of dissolved oxygen in the reactor. Using mill experiments, a model was formed that predicts the operation of the oxygen delignification process. The model was used to show how much the process could be improved by optimizing the charge of the oxygen. The mill experiments also confirmed that mass transfer of oxygen is modeled correctly enough, except when the charge of oxygen is very low and/or the mixing is not efficient enough. In that case, there is variation in the concentration of oxygen in the process that should be taken into account in the modeling.

Gas dispersion in the oxygen delignification process

There has been very little knowledge about the state of gas dispersion in the oxygen delignification process, even though this has a major impact on the performance of the reactor. This paper presents a new continu-ous inline method for measuring oxygen bubble size distribution in the reactor, as well as results from studies con-ducted in softwood and hardwood lines. This new measurement worked well, and new information about oxygen bubble size, as well as how different reactor conditions affected the distribution, was obtained. For example: • In the softwood line, the mean volume-weighted bubble size was about 0.1 mm, whereas in the hardwood line, this size was almost 10 times higher. For both lines, there was considerable variation in the measured bubble size over the long term. • For both lines, an increase in mixer rotation speed caused a discernible decrease in the bubble size, and an increase in oxygen charge caused a discernible increase in the bubble size. • In the softwood line, no coalescence of the bubbles in the reactor was observed, but in the hardwood line, some coalescence of the larger bubbles occurred. • In the test conducted in the hardwood line, the use of brownstock washer defoamer caused a discernible increase in oxygen bubble size. • In the hardwood line, reactor pressure had a noticeable effect on the amount of delignification, which indicated that improving mass transfer of oxygen (e.g., by decreasing the oxygen bubble size, in this case) should also have an increasing effect on the delignification.

The role of gas dispersion in the oxygen delignification process

Oxygen delignification is an essential part of the pulp production process. Delignification occurs with the aid of alkali and dissolved oxygen. Dissolved oxygen is obtained by dispersing oxygen gas into the pulp suspension by using efficient mixers. Little is known about the state of oxygen gas dispersion and its effect on oxy-gen delignification kinetics and efficiency. This paper will present the results for the effect of gas bubble size on the performance of oxygen delignification. The results are mainly based on detailed studies made in a Finnish hardwood mill where the oxygen bubble size distribution could be altered at the feed of the reactor. An essential aspect of these studies was the use of a new continuous inline gas bubble size measurement system to simultaneously determine the bubble size distribution at the feed and top of the reactor. Information about oxygen consumption in the reactor could also be obtained through the bubble size measurements. Accordingly, these studies quantify the effect of oxygen bubble size on the kappa reduction of the pulp. The effect of different chemical factors on the oxygen bubble size is also studied. Finally, the relationship between the gas bubble size and the liquid phase oxygen mass transfer coefficient (kLa) is presented. This connects the bubble size to the kappa reduction rate. Based on the presented modeling approach and the evaluation of practical factors that are not taken into account in the modeling, it was concluded that the volumetric average oxygen bubble size should preferably be smaller than 0.2 mm in practice. The information obtained with the new gas bubble size measurement system and the presented modeling approach give a very new basis for understanding, monitoring, adjusting, and designing oxygen delignification processes.

The Dual Effect of Ionic Liquid Pretreatment on the Eucalyptus Kraft Pulp during Oxygen Delignification Process.

Oxygen delignification presents high efficiency but causes damage to cellulose, therefore leading to an undesired loss in pulp strength. The effect of ionic liquid pretreatment of [BMIM][HSO4] and [TEA][HSO4] on oxygen delignification of the eucalyptus kraft pulp was investigated at 10% IL loading and 10% pulp consistency, after which composition analysis, pulp and fiber characterizations, and the mechanism of lignin degradation were carried out. A possible dual effect of enhancing delignification and protecting fibers from oxidation damage occurred simultaneously. The proposed [TEA][HSO4] pretreatment facilitated lignin removal in oxygen delignification and provided fibers with improved DP, fiber length and width, and curl index, resulting in the enhanced physical strength of pulp. Particularly, its folding endurance improved by 110%. An unusual brightness reduction was identified, followed by detailed characterization on the pulps and extracted lignin with FTIR, UV, XPS, and HSQC. It was proposed that [TEA][HSO4] catalyzed the cleavage of β-O-4 bonds in lignin during the oxygen delignification, with the formation of Hibbert’s ketones and quinonoid compounds. The decomposed lignin dissolved and migrated to the fiber surface, where they facilitated the access of the oxidation agent and protected the fiber framework from oxidation damage. Therefore, it was concluded that ionic liquid pretreatment has a dual effect on oxygen delignification.

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

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

Modeling of oxygen delignification process using a Kriging-based algorithm

A phenomenological model of cellulose production processes presents limitations due to the presence of species and chemical reactions of complex computational representation. Modeling based on machine learning techniques is an alternative to overcome this drawback. This paper addresses the Gaussian process regressor (Kriging) method to model the oxygen delignification process in one of the largest pulp production plants of the world. Different correlation models were used to evaluate this method; furthermore, an optimization routine, based on the constrained optimization by linear approximation method, was coupled to model to minimize the objective function, which is based on the input cost. Results have shown the good performance of using a combined Kriging method with optimization routines in the non-linear industrial processes to obtain a representative model capable of providing optimized operating scenarios. A reduction of 36.5% in consumption of NaOH was obtained, while required restrictions are obeyed.

Improvement of Reaction Selectivity Performance with Alkali Charge and Reaction Time on Oxygen Delignification Process

The chemicals of NaOH is highly related to the quality of the pulp in the medium consistency oxygen process. This research used Acacia mangium and Eucalyptus pulp with KaNo 18. The condition of the medium consistency oxygen process, the temperature of 80 o C, oxygen pressure of 1 bar. Reaction time was 20 to 100 minutes at intervals of 20 minutes. NaOH charges 5 to 25 kg /t at intervals of 5 kg/t. The analysis parameters used are KaNo and viscosity. The higher value of NaOH charge and reaction time, the higher value of the delignification degree. The higher value of NaOH charge and the reaction time, the higher the value of chain scission number. The variation of NaOH charge in the same reaction time indicated that the higher NaOH charge value, the lower value of the polymerization degree. The variation in reaction time indicated the longer the reaction time, the lower the polymerization degree. The reaction selectivity value was not affected by the high value of NaOH charge. The longer the reaction time used, the lower the reaction selectivity.

Effect of the particle size of magnesium hydroxide on the cellulose polymerization during the oxygen delignification of radiata pine kraft pulp

Magnesium salt is a known cellulose protector during the oxygen delignification process and it has been widely used in commercial practice, in particular, for feedstocks with a low magnesium presence. Minimizing the negative impact of transition metals by magnesium is one of the main mechanisms. A key parameter affecting the magnesium salt performance for this purpose is its particle size. In this study, different particle sizes of magnesium hydroxide were used as additives to the oxygen delignification system, and various transition metal ions (Cu2+, Mn2+ and Fe3+) were added to the radiata pine Kraft pulp. The results show that the magnesium hydroxide particle sizes had negligible effects on the pulp yield, Kappa number and brightness; however, when large size magnesium hydroxide was used, handsheet physical properties suffered a great loss, which was accompanied with much lowered viscosity and degree of polymerization in the resultant pulp. The explanation is that small sized magnesium hydroxide particles have large specific surface area and can adsorb more transition metal ions than the magnesium hydroxide with larger particle size, consequently, the transition metal contents of resultant pulp in the former are significantly lowered, which then decreases the negative impact of transition metals on the cellulose degradation in the process. It is thus concluded that smaller particle size of magnesium hydroxide is favorable as a cellulose protector during the oxygen delignification process.

Biomass pretreatment by cooking with active oxygen and solid alkali (CAOSA): Selectively oxidation of CAOSA wastewater to formic and acetic acids

Cooking with active oxygen and solid alkali (CAOSA), a oxygen delignification process for lignocellulose fractionation was developed by our group. In this study, a treatment process for yellow liquor, which is the aqueous solution of the CAOSA process, was proposed whilst producing formic and acetic acids with hydrothermal deep oxidation. A series of experiments were performed at reaction temperature of 160–280 °C and residence time of 30 s to 30 min. The highest yield of the acids defined as the total mass percentage of formic and acetic acids to biomass feedstock is 21.47%. The highest yield which defined as the percentage of carbon content relative to the total organics in the yellow liquor is 57.01%. Our results show that the process could be a universal and effective to other wastewater from the traditional pulping processes.

Fundamental understanding of distracted oxygen delignification efficiency by dissolved lignin during biorefinery process of eucalyptus

In this work, a fundamental understanding of oxygen delignification distracted by dissolved lignin was investigated. In the new biorefinery model of shortening kraft pulping integrated with extended oxygen delignification process, increasing content of residual lignin in the original pulp could result in enhanced delignification efficiency, higher pulp viscosity and less carbonyl groups. However, the invalid oxygen consumption by dissolved lignin could be increased with the increase of process temperature and alkali dosage. The normalized ultraviolet absorbance (divided by absorbance at 280 nm) also showed that the content of chromophoric group in dissolved lignin decreased with oxygen delignification proceeded, both of which indicated that dissolved lignin could enhance the invalid oxygen consumption. Therefore, a conclusion that replacement of the liquor at the initial phase of oxygen delignification process would balance the enhancement of delignification efficiency and invalid oxygen consumption was achieved.

Effect of shortening kraft pulping integrated with extended oxygen delignification on biorefinery process performance of eucalyptus

The aim of this work was to study the impact of shortening kraft pulping (KP) process integrated with extended oxygen delignification (OD) on the biorefinery process performance of eucalyptus. Data showed that using kraft pulps with high kappa number could improve the delignification efficiency of OD, reduce hexenuronic acid formation in kraft pulps. Pulp viscosity for a target kappa number of ∼10 was comparable to that obtained from conventional KP and OD process. The energy and alkali consumption in the integrated biorefinery process could be optimized when using a KP pulp with kappa number of ∼27. The process could minimize the overall methanol formation, but greater amounts of carbonate and oxalate were formed. The information from this study will be helpful to the future implementation of short-time KP integrated with extended OD process in actual pulp mill applications for biorefinery, aiming at further improvement in the biorefinery effectiveness of hardwood.

Effect of Hydroxyl Radical on the Selectivity of Delignification during Oxygen Delignification of Bamboo Pulp

The main challenge in the oxygen delignification process is the improvement of selectivity. To further understand the effect of the amount of hydroxyl radicals on the selectivity of oxygen delignification, in this work, the hydroxyl radicals were generated by the improved Fenton reaction system and were quantified by UV spectrophotometry. Antioxidants such as ascorbic acid were applied for the scavenging of hydroxyl radicals so that the amount of hydroxyl radicals could be regulated. The bamboo kraft pulps were treated with the Fenton system or the Fenton system combined with ascorbic acid. The results indicated that the improved Fenton reaction system could be used for the determination of hydroxyl radicals by UV spectrophotometry. The amount of hydroxyl radicals could be regulated to a suitable extent by the hydroxyl radical scavengers during oxygen delignification so that the delignification selectivity improvement was achieved.

СВОЙСТВА ПЕРОКСИДНОЙ ЦЕЛЛЮЛОЗЫ ИЗ ОДНОЛЕТНИХ РАСТЕНИЙ. 2. ДИНАМИКА ПРЕВРАЩЕНИЯ ПОЛИСАХАРИДОВ В ПРОЦЕССЕ ДЕЛИГНИФИКАЦИИ ПШЕНИЧНОЙ СОЛОМЫ

Изучена динамика растворения полисахаридов пшеничной соломы в процессе делигнификации реакционной смесью «Н2О2–Н2О–АсОН–АсООН-катализатор». Основные компоненты, переходящие в раствор в первом периоде пероксидной варки пшеничной соломы (до достижения выхода твердого остатка 80–82%), – лигнин и арабиноксилан. Дальнейшее снижение выхода при продолжении варки происходит за счет растворения продуктов окислительной и гидролитической деструкции всех полисахаридов твердого остатка.

Research on Kraft Pulping Delignification of Bamboo (<i>Neosinocalamus</i>)

Bamboo, as a kind of non-wood raw material, has a long, thin fiber with wide distribution and rich abundance in China. Its pulping properties are between softwood and hardwood. Adapting to the Chinese situation, developing bamboo pulp industrialization, according to the local conditions, is an available way to solve the shortage of fiber material for paper-making. Delignification of bamboo (Neosinocalamus) was carried out by conventional kraft and soda pulping under varying conditions to determine the relationships between selected cooking parameters (EA 14~20%, sulfidity 0~40%, maximum temperature 160 and 165°C, and time at maximum temperature 60~110 min) and pulp properties (kappa number and yield). Results indicated that in order to obtain relatively low kappa numbers(17~27), high sulfidity(20~40%) at lower EA(14~16%) increased pulp yield compared to the case of low sulfidity(0~10%) at higher EA(16~18%). Pulp with lower kappa numbers (13~15) and acceptable yield can be obtained at a sulfidity level of 20~30% with 18% EA or at a sulfidity level of 10~30% with 20% EA. Meanwhile, bamboo pulp in this condition not only helps to decrease kappa number in the subsequent oxygen delignification process, but also reduces the cost of chemicals in the bleaching process.

Selectivity of oxygen delignification for southern softwood kraft pulps with high lignin content

The selectivity of kraft pulping versus the oxygen delignification processes over the range of kappa nos. 25–90 was compared. Kraft pulping was found to be more selective than oxygen delignification for removing lignin from southern softwood kraft pulps. The greater selectivity is thought to be related to hydroxyl radicals that form in the oxygen delignification process that are not present in the kraft process. The hydroxyl radicals attack the carbohydrates and randomly cleave the polymeric chains, causing a significant decrease in the degree of carbohydrate polymerization and thus a loss of viscosity. Kraft pulping generates hydrosulfide ions that are highly selective and attack the lignin. Carbohydrate degradation occurs mainly from peeling reactions, which do not appreciably reduce the degree of polymerization of the cellulose and thus there is less viscosity loss. At low lignin content (i.e., low kappa number), the remaining lignin is likely bound covalently to the carbohydrate portion in both processes. Therefore, removal of the lignin results in significant degradation of the carbohydrates.

Lignin oxidation mechanisms under oxygen delignification conditions. Part 1. Results from direct analyses

AbstractOxidation of softwood and hardwood kraft lignins was observed under conditions of oxygen delignification (90°C and 110°C; 0.6 and 0.9 MPa) as a function of time by means of a number of analysis techniques and quantitative information was obtained on the degradation and formation of various compounds and structures. The decrease in reactor pressure was monitored during a 4-h reaction period. During the first 60 min, lignin reactivity was high, while a very intense stage took place during the first 20 min. The reactions decelerated after the first 60 min and after 120 min the reactions did not significantly advance. The oxygen consumption after 4-h reaction was 1.3–1.5 mole O2per 1 mole lignin depending on the conditions. In the first 20 min, 50%–60% of the oxygen was consumed and the consumption increased only slightly after 60 min. At 90°C, the changes in all observed quantities were smaller throughout the whole 4-h reaction period than at 110°C. Under the studied conditions, increasing the reaction temperature, rather than the pressure, had primary significance in the increasing rate of lignin degradation. Hardwood kraft lignin was more reactive than softwood kraft lignin. The results obtained in this study are the basis for the development of a mechanistic model for the oxygen delignification process of pulps to be published in subsequent papers.

The Kinetics of Oxygen Delignification of Reed Kraft Pulp (I)-Kinetics of Delignification

The kinetics of reed kraft pulp oxygen delignification process is studied, suitable kinetics model determined is -dk/dt=Aexp(-E/RT) [OH-]b[PO2]cKa, and the parameters in the model is calculated. The function for estimation of the kappa number at different reaction time is established for the reed kraft pulp oxygen delignification process. The reaction order fitted is 6.72 for delignification (a), 0.87 for alkali concentration (b), and 0.62 for oxygen pressure(c), respectively. The activation energy E is 80.96KJ/mol and frequency factor A 1.5×104.

Influence of plum gum and sodium perborate addition on spruce kraft pulp properties during oxygen delignification

The aim of this study was to investigate the effects of plum gum liquor and sodium perborate addition to oxygen delignification process on spruce kraft pulp properties. Yields and viscosity of pulp were protected against oxidative attack, when kappa number of pulp was not changed by adding plum gum to oxygen delignification process. Delignification ratio improved by 7.4% without any viscosity loss by adding only 0.25% sodium perborate as active oxygen with plum gum liquor to the oxygen delignification process. It was found that crystallinity of pulp was improved by adding plum gum and sodium perborate to oxygen delignification. It was also determined that sodium perborate had an affirmative effect on delignification ratio, when plum gum affected pulp viscosity positively.

Formation of vanillin and syringaldehyde in an oxygen delignification process

The formation of vanillin and syringaldehyde during an oxygen delignification process was evaluated. The cleavage of conjugated Cα-Cβ bonds in phenolic lignin caused by the attack of oxygen leads to the formation of aromatic aldehydes and ketones. The formation mechanism was confirmed by the organic compounds identified in the spent liquor by GC/MS. Additionally, the formation of aromatic aldehydes and ketones of hardwood, softwood, and grass was investigated. The results showed that the formed aromatic aldehydes and ketones were related to the units of lignin structure. Among the aromatic aldehydes and ketones present in the spent liquor from oxygen delignification, vanillin and syringaldehyde are of high content, making the spent liquor a potential resource for vanillin and syringaldehyde production.