• International Journal of Technology (IJTech)
  • Vol 15, No 2 (2024)

Differences of Chemical and Physical Properties on Fractionated Dissolving Pulp During Alkalizing and Aging Process

Differences of Chemical and Physical Properties on Fractionated Dissolving Pulp During Alkalizing and Aging Process

Title: Differences of Chemical and Physical Properties on Fractionated Dissolving Pulp During Alkalizing and Aging Process
Hadi Ramansah Chaidir, Yin Ying Hng, Jaroslav Stavik, Rochmadi

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Cite this article as:
Chaidir, H.R., Hng, Y.Y., Stavik, J., Rochmadi, 2024. Differences of Chemical and Physical Properties on Fractionated Dissolving Pulp During Alkalizing and Aging Process. International Journal of Technology. Volume 15(2), pp. 279-288

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Hadi Ramansah Chaidir 1. Department of Chemical Engineering, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia. 2. PT. Riau Andalan Pulp and Paper (RAPP), Asia Pacific Resources International Lim
Yin Ying Hng PT. Riau Andalan Pulp and Paper (RAPP), Asia Pacific Resources International Limited (APRIL) Group, Pangkalan Kerinci 28300, Riau, Indonesia
Jaroslav Stavik PT. Riau Andalan Pulp and Paper (RAPP), Asia Pacific Resources International Limited (APRIL) Group, Pangkalan Kerinci 28300, Riau, Indonesia
Rochmadi Department of Chemical Engineering, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
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Abstract
Differences of Chemical and Physical Properties on Fractionated Dissolving Pulp During Alkalizing and Aging Process

In recent years, there has been a consistent increase in viscose staple fiber (VSF) production, leading to an increased demand for dissolving pulp among producers. The properties of the pulp play a crucial role in influencing the efficiency of viscose operation, necessitating the selection of efficient raw materials. Therefore, this study aimed to evaluate the differences in chemical properties, fiber morphology, and viscose performance of fractionated pulp during alkalizing and aging processes. Alkalizing and aging were carried out under similar conditions for all fractions tested. The pulp samples consisted of reference (REF), long-fiber (LF), medium fiber-long (MFL), medium fiber-short (MFS), and short-fiber fractions (SF). The chemical analysis results showed that SF had the lowest viscosity, alpha-cellulose, and molecular weight. Physical properties of SF also had the lowest cell wall thickness (CWT), cross-section area (CSA), and coarseness, but it had the highest fiber population and water retention value (WRV) compared to LF. These characteristics significantly affected the quality of alkaline cellulose and the degree of polymerization rate during the aging process. SF showed the fastest depolymerization time (145 minutes) to achieve a target viscosity of 240 mL/g compared to others (192–194 minutes). However, aging rate was constant for all the samples examined. Based on the results, MFL was considered the ideal fraction for the viscose process due to its suitable fiber characteristic, leading to good behavior in alkalizing and aging. The samples also had an acceptable initial viscosity (478 mL/g) and a predominant fraction yield (30%), showing enhanced productivity.

Aging; Alkalizing; Dissolving pulp; Fractionation; Viscose

Introduction

Dissolving pulp, also referred to as dissolving cellulose, is often crafted from bleached wood pulp or cotton linter, and possesses a high cellulose content (more than 90%). Furthermore, it is popularly known for lower levels of hemicellulose, lignin, extractives, and minerals compared to paper pulp, offering enhanced brightness and a uniform molecular weight distribution (Chen et al., 2016; Li et al., 2012). The surge in the production of regenerated cellulose in recent years has intensified interest in its usage (Hammerle, 2011).
     The production of dissolving pulp is often carried out through the viscose process, which begins with the preparation of a slurry and sodium hydroxide.  This stage, known by various terms, including steeping, alkalizing, and mercerizing, typically uses 17–19% sodium hydroxide (Woodings, 2001). The process aims to produce an alkaline cellulose slurry, induce swelling, and remove impurities.

The alkaline treatment converts cellulose to Na cells and removes short-chain cellulose and hemicellulose (Fechter, Brelid, and Fischer, 2020; Fatra et al., 2016). This effect can be attributed to the passage of NaOH through slightly swollen cellulose fibers. Sodium hydroxide influence on this diffusion is often affected by the amount of water in the system (Forsberg, Stridh, and Sundman, 2019). The pulp obtained in the process is typically alkaline and steeping occurs at 45–55oC (Woodings, 2001). According to Tatevosyan et al., a temperature of approximately 50oC is required to obtain optimal alkali cellulose quality, and under lower temperatures, the treatment period on an industrial scale can span up to 30 minutes (Mozdyniewicz, Nieminen, and Sixta, 2013). Furthermore, the consistency (the weight percentage of wood fiber) of the slurry is typically less than 6% (Sixta, 2006), and the material is pressed out after alkalizing. This process helps to improve the cellulose/NaOH/H2O ratio for the next production step, namely aging. The pressed Na-Cell has a cellulose content of 30–36 % and a NaOH content of 13–17% (Woodings, 2001). The alkali cellulose is then shredded after pressing, thereby enhancing oxygen availability and reducing pulp density. The shredding process is often followed by aging to reduce the degree of polymerization (DP) of the molecular chains, achieve the desired degree of polymerization, and produce an ideal viscose solution. Degree of polymerization (DP) of cellulose is lowered from 750–850 to 270–350 at 40–60oC for 0.5–5 hours (Woodings, 2001), leading to a reduction in the solution viscosity to target levels (Sharma et al., 2019). Aging time and temperature are critical factors in this process, contributing to viscose viscosity, where the longer the time, the lesser the viscosity (Shaikh, Chaudhari, and Varma 2012). Cellulose degradation, which shows a change in the initial cellulose chains, can be calculated from DP. The correlation between DP and intrinsic viscosity using the relationship proposed by Immergut (Irawan et al., 2020; Duan et al., 2017) is expressed in Equation (1).


Where DP is degree of polymerization,  is intrinsic viscosity in mL/gram, 0.905 and 0.75 are constants characteristic of the polymer-solvent system (Irawan et al., 2020; Duan et al., 2017). Intrinsic viscosity of pulp samples was measured using copper ethylenediamine (CED) solution as solvent. Furthermore, the linear Ekenstam relationship is often used to track the kinetics of cellulose degradation according to a law of first-order reaction in Equations (2). Cellulose chains can be calculated from DP, as shown in Equation (3) (Calvini, Gorassini, and Merlani, 2008).


Where DPt is degree of polymerization at time t, DPo is degree of polymerization at initial, k is the rate reaction in (min-1), and t is the time for cellulose degradation in (min). Chain scission number (CSN) is expressed in terms of scissions per initial chain.

Several studies showed that the properties of fiber affect the quality of the viscose dope produced. Fiber fractionation separates fiber based on length, coarseness, or specific surface, thereby providing various benefits, such as enhancing the homogenous quality of the pulp. Previous results showed that fractionated pulp had not yet been used on an industrial viscose scale. However, it had the potential to optimize fiber potential in the viscose process during alkalizing and aging.

According to a previous study, fractionation treatment has been used in pulp and paper manufacturing to reduce size-induced differentiation. Abubakr, Scott, and Klungness (1995) stated that fractionation could increase paper strength by separating and improving long fiber fraction. Fiber type has also been identified as a major factor affecting the strength of paper to create a network structure (Plazonic et al., 2020). Long fiber has better strength to hold and build the internal bonding compared to short fiber. Therefore, fractionation can be used to upgrade paper strength with the combination of short and long fiber to achieve a specific grade.

Li et al. (2015) investigated a sequential treatment consisting of pulp fractionation followed by purification with caustic treatment to remove hemicelluloses from softwood sulfite pulp to be dissolved. Fiber fractions were purified using a cold caustic extraction (CCE) or a hot caustic extraction (HCE). The results showed that hemicellulose removal was more apparent in long fiber fraction compared to short fiber in both CCE and HCE procedures. Duan et al. (2017) examined a combined process comprising pulp fractionation and cellulase treatment to improve molecular weight distribution (MWD) and pulp reactivity (fock reactivity). The results showed that short fibers with low viscosity had the best accessibility compared to long variants. Therefore, this study aims to evaluate the chemical properties, fiber morphology, and performance of fractionated dissolving pulp during the viscose process (alkalizing and aging). The results are expected to provide recommendations for the best fraction of dissolving pulp in the viscose industry.

Experimental Methods

    All analysis had been evaluated in R&D Laboratory Asia Pacific Resources International Limited (APRIL) at Royal Golden Eagle Technology Center (RGE-TC), Kerinci, Riau, Indonesia. The flow chart of this research is shown in Figure 1.


Figure 1 Flow chart of research

2.1. Raw Material

Raw materials were collected from pulp and paper producer, namely Riau Andalan Pulp and Paper (RAPP). Pulp samples were hardwood dissolving pulp from the pre-hydrolysis kraft-based (55% of moisture content). An industrial grade of 32% NaOH from the chemical plant of APRIL was diluted to obtain 18% concentrated.

2.2. Pulp Fractionation

Pulp samples were fractionated using Bauer-McNett fiber classifier (Frank-PTI GmbH type S40180004) based on TAPPI T233 cm-95, which had the principle of fiber separation using length. The process was often performed by passing fiber through vertical screens possessing different sieve sizes, with a flow rate of 11.355 L/min for 20 minutes. Raw material was prepared from reference (REF) as without fractionation, long-fiber (LF, > 0.595 mm screen), medium fiber-long (MFL, 0.595–0.297 mm), medium fiber-short (MFS, 0.297–0.149 mm) and short-fiber fractions (SF, 0.149–0.074 mm). After fractionation, pulp was collected to analyze the chemical composition, fiber morphology (FS5 Valmet), SEM (FESEM JEOL JSM-6340F), and processing in the viscose pilot plant (Ing. A. Maurer S.A).

2.3. Alkalizing and Aging in Viscose Pilot Plant

The viscose process (alkalizing and aging) was carried out using a viscose pilot plant scale (Ing. A. Maurer S.A), and the reactor used for alkalizing was a stirred batch reactor (Ing. A. Maurer S.A type 10595) at 53oC for 38 minutes. Furthermore, the temperature was kept constant by setting it on the control panel (isothermal and isobaric). Each batch of pulp was approximately 1,500 grams oven-dry (OD) at a consistency slurry of 4% with 18% NaOH. After alkalizing process was completed, the slurry was pressed using AC press (Ing. A. Maurer S.A type 10596) to get 34–35% cellulose and 15–16% residual alkali. Shredded alkaline cellulose was aged in a stirred tank (Ing. A. Maurer S.A type 10571) at 50oC for 2.5 hours and 30oC for 1.5 hours.

2.4. Analysis

Chemical and fiber morphology analysis of fractionated dissolving pulp were tested after fractionation. Slurry of alkali cellulose was tested during alkalizing. Furthermore, the shredded alkali cellulose was evaluated before and after aging. The chemicals used were lab-grade, and standards analysis were shown in Table 1.

Table 1 Parameters and methods of analysis

Results and Discussion

3.1. Raw Material Characteristics

3.1.1.  Fractionation distribution

The dominant fiber fractions were from MFL and MFS, accounting for 29.5% and 29.8% of the samples, respectively. Meanwhile, LF (15.3%) and SF had a minor distribution of approximately 6.1%. Fiber loss of 19.3% (was not used in this study) was obtained due to the smaller mesh size of the screen plate (< 0.074 mm).

Based on the result, MFL fraction was selected as an ideal sample for the viscose process. Combining MFL and MFS was also an alternative to improve fiber performance during alkalizing and aging process. Fraction distribution in raw material of dissolving pulp could be optimized by adjusting cooking and bleaching conditions.

3.1.2.  Chemical properties

The results of chemical analysis are presented in Table 2. Each pulp fraction had different characteristics and was expected to behave differently. Furthermore, viscosity, freeness, alpha-cellulose content, and molecular weight value decreased in SF compared to LF fraction. Other parameters, such as S10, S18, WRV, and fock reactivity were found to increase in SF compared to LF.

The solubility of pulp in alkali provided information on the cellulose degradation and loss or retention during the pulping. This showed the amount of degraded cellulose or short-chain glucan and hemicellulose in pulp. S10 and S18 alkali solubilities showed the proportions of low molecular weight carbohydrates soluble in 10% and 18% sodium hydroxide, respectively. SF had the highest S10 and S18 values compared to the other fractions, with 5.354% and 2.75%, respectively. The result showed that it also had the lowest alpha-cellulose (95.09%). REF and LF did not pass the Chinese filterability analysis due to low surface area.

Table 2 Chemical properties of raw material

Parameter

Unit

REF

LF

MFL

MFS

SF

Brightness

ISO

90.8

90.4

90.7

90.5

90.6

Viscosity

mL/g

444

508

478

456

434

Alpha cellulose

%

95.52

96.11

95.89

95.59

95.09

DCM extractive

%

0.030

0.018

0.042

0.029

0.021

S10

%

5.267

4.701

4.740

5.188

5.354

S18

%

2.819

2.505

2.527

2.704

2.750

Pentosan

%

2.614

2.802

2.280

2.856

3.090

Ash content

%

0.059

0.019

0.025

0.027

0.037

Total lignin

%

0.250

0.340

0.280

0.180

0.290

The acid in soluble ash

ppm

31

34

28

27

65

Chinese filterability

Not Passed

Not Passed

Passed

Passed

Passed

Fock reactivity

%

45.50

41.02

43.55

46.16

50.21

Copper number

0.2529

0.2402

0.2000

0.2564

0.2552

Carbonyl group content (100 gr)

Mmol/g OD

0.3048

0.2837

0.2104

0.3107

0.3087

Weight average mol. weight (Mw)

g/mol

241,749

271,928

NA

252,109

239,159

        In this study, the viscosity of pulp sample provided an estimate of the cellulose chain DP. Viscosity determination was one of the most informative procedures to characterize cellulose. This analysis showed that degree of degradation decreased in the molecular weight. Furthermore, SF had the lowest viscosity (434 mL/g) and average molecular weight (239,159 g/mol), with the highest silica content (65 ppm) compared to others. Meanwhile, MFL had low silica (28 ppm) and carbonyl group content (0.2104 mmol/g OD), leading to its selection as the ideal fraction for the viscose process.

3.1.3.  Fiber morphology and images

Fiber morphology analysis was a crucial test to identify the characteristics of fiber before it was processed in viscose rayon. The results of fiber morphology analysis of raw materials to be used in this study are presented in Table 3.

After fractionation, fiber length was different for each fraction and had a downward trend from LF to SF. Meanwhile, each fraction cell wall density and fiber width were similar.

Figure 2 showed the differences in fiber cross-section for each fraction. Cell wall thickness (CWT) determined the penetration, accessibility, and diffusion rate of chemicals. In this study, CWT for each sample decreased with the shorter fiber, which had the lowest CWT and coarseness of 2.58 µm and 0.04 mg/m, respectively.

Table 3 Fiber morphology of raw material

Parameter

Unit

REF

LF

MFL

MFS

SF

Freeness

CSF mL

585

655

636

615

570

WRV

g/g

1.3547

1.2694

1.2829

1.3154

1.3828

Length

mm

0.72

0.86

0.79

0.71

0.59

Curl

%

15.65

15.61

15.13

14.76

13.28

Coarseness

mg/m

0.049

0.054

0.049

0.048

0.040

Cross Section Area

µm2

98.3

107.8

95.5

95.3

79.7

Cell Wall Density

kg/m3

499

501

500

504

502

Fibrillation

%

1.22

1.22

1.18

1.21

1.27

Population

pcs/mg

21,671

19,776

23,676

25,158

34,957