Enzyme Applications in Conventional Kraft Pulping - American

Chapter 5

Enzyme Applications in Conventional Kraft Pulping 1

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C. J. Jacobs-Young , Richard A. Venditti , and T. W. Joyce 1

Division of Paper Science and Engineering, University of Washington, Seattle, WA 98195 Department of Wood and Paper Science, North Carolina State University, Raleigh, NC 27695 Department of Paper and Printing Science and Engineering, Western Michigan University, Kalamazoo, MI 49008-5060

Downloaded by PENNSYLVANIA STATE UNIV on September 24, 2013 | http://pubs.acs.org Publication Date: March 31, 1998 | doi: 10.1021/bk-1998-0687.ch005

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A cellulase/hemicellulase mixture was used to increase the diffusion of alkali in sapwood and enhance the efficiency of conventional kraft pulping. Diffusion was enhanced by pretreating sycamore (Platanus occidentalis) sapwood with enzymes. Tangential diffusion was increased by 26%, while longitudinal diffusion was increased by approximately 110% in the enzyme treated samples of sycamore sapwood. Sycamore chips were pretreated using cellulase/hemicellulase/pectinase mixtures prior to conventional kraft pulping. After enzyme pretreatment followed by kraft pulping, the kappa number of the control was as much as 10% higher than the treated chips. In addition, enzyme treated pulps required less (approximately 9 kg ClO / ton pulp bleached) than the control. Pulp strength properties for the pretreated chips were comparable to kraft pulps receiving no enzyme pretreatments. Pulp viscosity was lower in pulps treated with cellulase/hemicellulase alone. 2

Increasing environmental pressures have made it necessary to investigate methods for reducing the amount of energy, sulfur, and chlorine containing compounds utilized during pulping and bleaching. These pressures have also lead to the design of many technologies aimed at lowering the lignin content of the pulp entering the bleach plant. Lowering the kappa number of the pulp by increasing the efficiency of the pulping process has lead to a reduction in chemicals necessary for pulp bleaching and a concomitant reduction in pollutants dischargedfromthe bleach plant. The objective of this study is to determine the effect of enzyme pretreatment on conventional kraft pulping. Improving Transport Processes in Wood Processes which improve liquid transport in wood are of great interest to the pulp and paper industry. Impregnation of cooking chemicals during pulping is a crucial step for

© 1998 American Chemical Society

In Enzyme Applications in Fiber Processing; Eriksson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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56 producing uniform pulps. Incomplete impregnation results in non-uniform pulps which are characterized by high reject levels, non-uniform delignification, and lower strength properties. The size, nature, and number of intercommunicating structures in wood have a major effect upon movement of water and other materials through wood (1). Enzymatic pretreatments can be utilized to alter these structures, thus improving the penetration of chemicals. Successful implementation of enzymatic pretreatments can provide substantial increases in diffusion, which can possibly be attributed to various factors including the dissolution of pit membranes and the removal of carbohydrates from the lignincarbohydrate matrix. Numerous studies on the treatment of wood with biological agents have revealed the attack of these substances on pit membranes (2-4). In hardwoods, the primary pathways for liquid transport are vessels which are not occluded by tyloses, while fiber tracheids are responsible for conducting fluid in softwoods. In both hardwoods and softwoods, fluid is communicated from one cell to the next through interconnecting pits. The membranes of these pits which offer resistance to flow are composed of primarily lignin, pectin and hemicellulose. For conventional kraft pulping systems, it is imperative that research be conducted that can assist mills in meeting environmental regulations Enzymes and Kraft Pulping Delignification in Conventional Kraft Pulping. Conventional kraft pulping is one of several processes used to solubilize and remove ligninfromwood used for papermaking. Sodium hydroxide and sodium sulfide are applied to wood and the temperature is raised slowly to a maximum temperature of « 170C. Kraft delignification occurs in three phases; initial, bulk and residual. The initial phase is characterized by rapid delignification, while a major portion of the lignin is removed during the subsequent bulk phase. The residual lignin removal phase, which occurs at about 90% delignification, is characterized by slow delignification coupled with rapid carbohydrate degradation reactions. The low rate of delignification is believed to be caused by the presence of alkali-stable lignin carbohydrate bonds (5,6). In kraft pulping, initial delignification occurs preferentially in the secondary wall. At about 50% delignification, the lignin in the middle lamella and cell corner areas dissolve rapidly, leaving the residual lignin in the secondary wall. This topochemical effect is the result of physical and chemical factors (7,8). Experimental evidence suggests that the topochemical variations in delignification can be partially attributed to the difference in the structure of lignin from the secondary wall and middle lamella regions (9,10). In addition, important physical factors such as the differential penetration of reagents into various morphological regions, the accessibility of lignin, and the diffusibility of degraded lignin may lead to topochemical effects. The dissolution of SW lignin has been related to the pore size of the cell wall and thus the removal of hemicellulose (11,12). The location of residual lignin has a significant bearing on the bleachability and papermaking properties of pulp fibers.


57 Materials and Methods


The sycamore (Platanus occidentalis) utilized in this study was obtained from an NC State University experimental forest near Raleigh, NC. Enzyme Pretreatment. Prior to pulping, 650 OD grams of chips were treated with Pergalase A40 (CIBA, Greensboro, NC) which has cellulase and xylanase activity. Xylanase activity was estimated at 407 U/ml using birch glucuronoxylan as substrate according to the method of Poutanen and Puis (75). Protein concentration was estimated at 175 mg/ml of enzyme solution according to the method of Lowry et al. (14). Cellulase activity was determined using a filter paper assay described by Mandels (15). Cellulase activity was approximately 70filterpaper units (mmols of reduced sugar) per milliliter. Thefilterpaper activity was determined from liberated reducing sugar as measured by the dinitrosalisylic acid method using glucose as a standard. Pergalase was applied based on the cellulase activity at 5 FPU per oven dry gram of wood. SP 249 (Novo Nordisk Biochem North America, Inc.), an enzyme with pectolytic, cellulolytic and hemicellulolytic activities was also utilized. The supplier reports the activity of pectinase at 1,600 k PU/g determined using citric pectin, arabanase activity at 98 units/g using sugar beet araban, and a-galactosidase activity as 63 units/g using p-Nitro-phenylgalactoside as the substrate. SP 249 was charged at 1% on OD chip weight. The enzyme preparations were used with a citrate buffer solution (pH « 5) at 50°C. The chips were placed in stainless steel bombs along with the enzyme preparation, citrate buffer (pH 5), and water. Enzyme impregnation was aided by applying 67.73 kPa of vacuum to each bomb. The chips were reacted in a constant temperature device for 24 h at 50°C. The control chips were treated as described above, but without enzyme addition. Pulping Conditions. Sycamore sapwood chips were pulped in a 7L M&K digester vessel using the following conditions: 15% AA, 30% Sulfidity, 4:1 L/W ratio, and 1000 H Factor. The chips were screened and sorted to remove bark and knots. 650 OD grams of chips were used for all cooks. At the completion of the cook, the spent black liquor was blown into a holding vessel. The chips were not blown under pressure, but removed manually, washed, disintegrated and screened. The screened pulp was centrifuged to approximately 30% consistency and fluffed. Pulp Testing. Control and enzyme treated pulps were beaten in a PFI mill at 10% consistency with an applied load of 1.8 kg/cm. Samples were collected at predetermined intervals between 0 and 6000 revolutions. Pulpfiberlength, coarseness, kink and curl were determined using the Fiber Quality Analyzer (Optest Inc.). Handsheet properties, pulp viscosity,finesanalysis and kappa number were determined for each sample using TAPPI standard methods. Sugar Analysis. Pulp sugars were measured by anion exchange liquid chromatography of their borate complexes, followed by a post column reaction to produce a blue copper complex. The absorbency of the copper complex is measured by a detector set at 560


58 nm. Quantification was done by comparing sample peak heights to those produced by a standard (7 6).


Results and Discussion Effect of Enzyme Pretreatment on Diffusion. In a previous study it was determined that a cellulase/hemicellulase (CH) mixture was capable of increasing the diffusion of sodium hydroxide in sycamore sapwood in both the tangential (along the rays) and longitudinal directions (77). The diffusion was determined according to a procedure outlined by Talton (18) in 1986. Diffusion into the tangential direction was increased from 7.26 χ 10" cm /sec to 9.17 χ 10* cm /sec. Longitudinal diffusion was 110% higher, with an increase in diffusion from 7.06 χ 10" cm /sec to 14.9 χ 10" cm /sec. The increase in diffusion was attributed to the degradation of pit membranes which served as a impedance to flow in the sapwood. 6

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Effect of Enzyme Pretreatment on Lignin Removal. Effects of the 24 h enzyme pretreatments on pulp delignification, viscosity and yield are listed in Table I. When compared to the control kraft pulp, both enzyme treatments resulted in higher levels of delignification. The CH mixture resulted in an approximately 1.5 point kappa difference, while maintaining similar unscreened yield values. The level of delignification was increased further by the addition of pectinase to the enzyme mixture. Screened yield values were comparable for all cooks. Effect of Enzyme Pretreatment on Viscosity and Pulp Strength Properties. Cellulase/herm^eUulase/rjectinase (CHP) was capable of lowering the kappa number of the pulp, while maintaining similar viscosity values. Pretreatment with CH alone resulted in lowerfinalpulp viscosities than the control and chips treated with CHP. The main differences in the carbohydrate compositions of the pulps were in the xylose and glucose compositions. (Table II) Arabinose and mannose contents were low in each pulp. More xylose was detected in the enzyme pretreated pulps than the pulp receiving no pretreatment. The reprecipitation of xylan during kraft pulping occurs when almost all of the xylan side chains have been cleaved off (19). This may indicate that the additional hydrolysis of xylan side chains during pretreatment could have resulted in increased xylan reprecipitation during pulping. As would be expected, hydrolysis of carbohydrates by the cellulase/hemicellulase pretreatments resulted in lower levels of glucose in the final pulps. Filtrate obtained directly after enzyme pretreatment was analyzed using a method outlined by Mandels (75). Effluent following enzyme pretreatment had significantly higher levels of reducing sugars than the control. Using the HPLC method, traces of galactose, arabinose and glucose were identified in filtrate following pretreatment. Material also eluted in the tri- and disaccharide region near cellobiose for each sample, but the peaks were too small to quantify. The physical properties of each pulp were tested before and after refining. In Figure 1, it can be seen that, although the tensile index is not significantly different for the three pulps, the pulp receiving pectinase treatment had a higher tensile index at each


59 Table I. Pulping Data for Enzyme Pretreated and Conventional Kraft Cooks. Cellulase/Hemicellulase (C/H); Cellulase/Hemicellulase/Pectinase (C/H +P) Control

C/H

C/H+P

(Cook 1, Cook 2)

(Cook 1, Cook 2)

(Cook l . C o o k 2)

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Pergalase Dosage (Filter Paper Units/g)


Pectinase Solution Dosage

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(% on OD basis)

Kappa Number

17.6,17.5 50.9

Average Viscosity, mPa-s Unscreened Yield, % Screened Yield, %

49.1,49.8 43.1,44.9

16.3,16

15.6,15.9

46.7 48.6,49.9 43.4,45.3

50.1 46.2,48.2 43.7,44.9

Table II. Sugar Analysis of Enzyme Pretreated and Conventional Kraft Pulps. Cellulase/Hemicellulase (C/H); Cellulase/Hemicellulase/Pectinase (C/H +P) Composition of Carbohydrates % Glucose 1 % Xylose Control C/H C/H+P

88.6 85.6 85.6

% Arabinose |

11.1 14.1 14.2

Enzyme Applications in Conventional Kraft Pulping - American

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