Energy Considerations in Stock Preparation Refining Modified by Recycling

Transcription

1 Western Michigan University ScholarWorks at WMU Master's Theses Graduate College Energy Considerations in Stock Preparation Refining Modified by Recycling Rajendra D. Deshpande Western Michigan University Follow this and additional works at: Part of the Chemical Engineering Commons Recommended Citation Deshpande, Rajendra D., "Energy Considerations in Stock Preparation Refining Modified by Recycling" (1988). Master's Theses This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact

2 ENERGY CONSIDERATIONS IN STOCK PREPARATION REFINING MODIFIED BY RECYCLING by Rajendra D. Deshpande A thesis submitted to the Faculty of The Graduate College in partial fulfillment of the requirements for the Degree of Master of Science Department of Paper Science and Engineering Western Michigan University Kalamazoo, Michigan December 1988

3 ENERGY CONSIDERATIONS IN STOCK PREPARATION REFINING MODIFIED BY RECYCLING Rajendra D. Deshpande, M.S. Western Michigan University, 1988 Experiments were conducted to study the effect of refining on virgin fibers as well as on recycled fibers. Interaction effects of recycling and type of treatment of virgin fibers on characteristics of secondary fiber were studied. A significant reduction in drainage properties of the furnish for both hardwood and softwood pulps were observed. This reduction in properties was found proportional to the amount of refining. It is necessary to achieve required strength properties at higher freeness values, if fibers are to be recycled. This can be done by improvement in uniformity or homogeneity of refining and by gentler and less severe fiber treatment. This also results in higher power consumption. Existing refining theories were used to correlate strength properties with refining variables and thus to obtain optimum conditions for refining in terms of development of strength properties and power consumption.

4 ACKNOWLEDGEMENTS I wish to express sincere appreciation to Dr. Richard B. Valley, Dr. David K. Peterson and Dr. Ellsworth Shriver for their encouragement and suggestions. Without their valuable assistance, this study would not be completed. In addition, I would like to thank Mr. William Forester, Mr. Carl Shuster and Mr. Keith Manion for their useful help in conducting pilot plant experiments. Rajendra D. Deshpande. ii

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7 O rder N um ber E n erg y considerations in stock p rep a ra tio n refin in g m o dified by recycling Deshpande, Rajendra D., M.S. W estern M ichigan University, 1988 UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106

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9 TABLE OF CONTENTS ACKNOWLEDGEMENTS... ii LIST OF TABLES... v LIST OF FIGURES... vi CHAPTER I. INTRODUCTION... 1 II. REVIEW OF THE LITERATURE... 3 Theory of Refining... 3 Effect of Recycling on Sheet Properties... 6 Effect or Recycling on Fiber Characteristics... 8 Energy Considerations in Refining The Specific Edge Load Theory Severity and Number of Impacts Theory Change in Refining Response Due to Fiber Being Recycled III. PRESENTATION OF THE PROBLEM IV. EXPERIMENTAL APPROACH Introduction Description of the Experiments V. RESULTS AND DISCUSSION Effect of Energy per Impact on Handsheet Properties Effect of Recycling on Strength Properties of Handsheets... 49

10 Table of Contents- Continued CHAPTER Interaction Effect of Energy per Impact Level and Recycling Effect of Energy per Impact on Freeness Development of the Pulp Effect of Degree of Refining of Virgin Pulp on Recycled Fibers Optimization of Total Energy Input VI. CONCLUSIONS AND RECOMMENDATIONS LITERATURE CITED APPENDICES A. Experimental Data Pertaining to Fiber Treatment B. Effect of Recirculation on Development of Strength Properties C. Application of Pilot Plant Study to Full Scale Process BIBLIOGRAPHY iv

11 LIST OF TABLES 1. Maximum Tensile Data for Softwood Pulp Maximum Tensile Data for Hardwood Pulp Analysis of Variance for Tensile Data for Softwood Pulp Analysis of Variance for Tensile Data for Hardwood Pulp Effect of Refining of Pulp on Freeness of Subsequent Recycled Pulp Process Data for Softwood Virgin Pulp Analysis of Variance for Testing Significance of Regression Net Energy, Idling Energy and Number of Impacts Required for Corresponding Change in Tensile for Softwood Virgin Pulp Process Data for Hardwood Virgin Pulp Net Energy, Idling Energy and Number of Impacts Required for Corresponding Change in Tensile for Hardwood Virgin Pulp Process Data for Softwood Pulp(Recycled Once) Net Energy, Idling Energy and Number of Impacts Required for Corresponding Change in Tensile for Softwood Pulp(Recycled Once) Typical Optimum Energy Requirements in Stock Preparation Refining v

12 LIST OF FIGURES 1. A Simplified Structure of a Cellulose Fiber Fiber Swelling as a Result of Lamellae Separation Model of Ultrastructural Organization of a Microfibril Three Possible Means of Void Closure Schematic Description of Experimental Design Schematic Flow Diagram for Each Trial Schematic Flow Diagram for Each Run Plot of Tensile Versus C.S.F. for Softwood Pulp Plot of Density Versus C.S.F. for Softwood Pulp Plot of Burst Versus C.S.F. for Softwood Pulp Plot of Tear Versus C.S.F. for Softwood Pulp Plot of Scattering Coefficient Versus C.S.F. for Softwood Pulp Plot of Fiber Length Versus C.S.F. for Softwood Pulp Plot of Percentage of Short Fibers Versus C.S.F. for Softwood Pulp Plot of Tensile Versus C.S.F. for Hardwood Pulp Plot of Density Versus C.S.F. for Hardwood Pulp Plot of Burst Versus C.S.F. for Hardwood Pulp Plot of Tear Versus C.S.F. for Hardwood Pulp Plot of Scattering Coefficient Versus C.S.F. for Hardwood Pulp vi

13 List of Figures- continued 20. Plot of Fiber Length Versus C.S.F, for Hardwood Pulp Plot of Percentage of Short Fibers Versus C.S.F. for Hardwood Pulp Plot of Tensile, Density and Percentage of Short Fibers Versus Net Energy for Softwood(virgin) Pulp Plot of Tensile, Density and Percentage of Short Fibers Versus Net Energy for Softwood(recycled once) Pulp Plot of Tensile, Density and Percentage of Short Fibers Versus Net Energy for Softwood(recycled twice) Pulp Plot of Tensile, Density and Percentage of Short Fibers Versus Net Energy for Hardwood (virgin) Pulp Plot of Tensile, Density and Percentage of Short Fibers Versus Net Energy for Hardwood(recycled once) Pulp Plot of Tensile, Density and Percentage of Short Fibers Versus Net Energy for Hardwood(recycled twice) Pulp Plot of Development of C.S.F. Versus Net Energy Input for Softwood Pulp Plot of Development of C.S.F. Versus Net Energy Input Hardwood Pulp Effect of Recycling and Energy per Impact Level on C.S.F. Development Effect of Recycling and Energy per Impact Level on Drainage Time Contours of Constant Tensile for Softwood Virgin Pulp Contours of Constant Tensile for Hardwood Virgin Pulp Contours of Constant Tensile for Softwood Pulp(Recycled once) vii

14 List of Figures- continued 35. Plot of Tensile Versus Net Energy Input and Number of Impacts Plot of Residuals Versus Regressor Variable Plot of Fitted Values Versus Actual Values and Plot of Residuals Versus Fitted Values Plot of Change in Tensile Versus Net Energy Input and Number of Impacts for Softwood Pulp(Recycled Twice) Plot of Change in Tensile Versus Net Energy Input and Number of Impacts for Hardwood Pulp (Recycled once) Plot of Change in Tensile Versus Net Energy Input and Number of Impacts for Hardwood Pulp (Recycled twice) viii

15 CHAPTER I INTRODUCTION In many countries a high percentage of paper and board mills utilize waste paper as the source of fiber supply. Besides handling problems and product appearance, strength performance is not as good with recycled fiber. The negative effects of recycling on strength properties of paper can be minimized by proper fiber treatment in the stock preparation plant. Proper selection and application of refiners has been an ongoing task since the production demands outmoded Hollander beaters years ago and the industry went to continuous, high intensity units. However, there has been a general lack of attention to the energy consumption in these units mainly because there was more economic emphasis put on increasing productivity rather than on looking at energy consumption. Further, the refining response changes with type and quality of fiber. Quality of secondary fibers is most affected by contamination, action of drying and method of fiber treatment or.refining of fibers. This study is confined to the effect of refining. The ability of fibers to take up water is reduced due to recycling. Refining can normally be used to restore some of this lost ability. 1

16 A possible way to increase the potential of recycled fibers would be to treat the virgin stock so that the modifications due to recycling are minimized. The aim is to study how recycling modifies the refining response and by appreciating this, how to gain the full potential of recycled fiber source at optimum power consumption. This study will analyze the possibility of upgrading recycled fibers by refining.

17 CHAPTER II SUMMARY OF LITERATURE SEARCH Energy consumption is related to fiber characteristics but most of the literature cited treats energy consideration and fiber characteristics in two separate parts. The reason for this is that energy considerations are many and diversified due to the many and varied fiber furnishes. Operating economics leading to the best system can only be evaluated on a case-by-case study, one must look at the impact a change in the refiner area may make on the whole process. For convenience, this investigation has been treated in five parts. 1. Theory of refining. 2. Effect of recycling on sheet properties. 3. Effect of recycling on fiber characteristics. 4. Energy considerations in refining. 5. Change in refining response due to fiber being recycled. Theory of Refining Because of a fiber's hygroscopic nature, when cellulose fibers are immersed in water, the surface and porous section of the fiber will absorb water. Water enters into the fiber body via the cell wall pits and pores. Water causes internal swelling which is 3

18 resisted by the fiber structure. Mechanical treatment of fiber can not be accomplished without the presence of water. Without water the fiber rapidly disintegrates into useless debris. Water modifies the structural properties of fiber in such a way that mechanical treatment becomes less destructive and permits more selective disruption of the cell wall. LUMEN CXOSS SECTION Figure 1. A Simplified Structure of a Cellulose Fiber. The individual fibers are separated by primary laminae which contain about 70-90% of the total lignin. The fibers vary in length from 1.5 mm to 5.5 mm and the cross sectional diameter is about l/100th of their length. Inside the fiber there is an empty space called the lumen and the fiber wall is composed of several layers. The fiber has outer primary wall and inner secondary wall which is made up in three components. The inner components are rich in

19 cellulose and are considered to be the main body of fiber. In the refiner cellulose fibers are subjected to severe mechanical forces. The fibers are constantly being hit by crossing bar edges of the refiner plates. As a result, the bonds which hold the fiber structure together will fail and the fiber wall will be separated. As shown in the Figure 2., this will result in more swelling of the fibers and separation of the lamellae. m e r Figure 2. Fiber Swelling as a Result of Lamellae Separation (1_). Fibrils are raised on the fiber body, exposing new surfaces which then absorb more water and fibrillate further. Eventually

20 some of the fibrils will break off and form fines in the pulp. During this process of swelling and separation of fibrils the fiber becomes more flexible and is better able to withstand the stresses of refining. Refining is carried out using the principle of a rotor with bars operating in close proximity to a stationary plate also containing bars. While some degree of fiber treatment is obtained through the compressive forces on the flat bar surfaces, majority of fiber treatment takes place at the bar edges. the Three basic fiber changes experienced in refining are: 1. Cutting: rupturing the fiber in a plane perpendicular to the long axis. 2. Splitting: rupturing the fiber in a plane parallel to the long axis and 3. Brushing: internal crushing and flexing of the fiber. Fibrillation occurs as a result of the above fiber changes. Fibrillation in the presence of water increases flexibility and plasticity, so that when fiber surfaces are brought into close contact by pressure or surface tension, and subsequently dried, the fibers become attached at the points of contact, forming a coherent network having physical properties which can be varied according to requirements by variation in the method and intensity of refining. Effect of Recycling on Sheet Properties Some published information concerning changes in physical

21 properties of recycled fibers appear to conflict. McKee (2^) found substantial changes in physical strength properties after several recycles when beating to a constant freeness. He noted that sheet properties, which are a direct function of fiber-to-fiber bonding and fiber strength, decrease markedly with the number of times recycled. The loss in bonding potential had a greater effect than the loss in fiber strength. A study published recently by Guest and Westen (JL) shows similar results. They found that the strength loss of a sheet was due to a reduction in the number and strength of the inter-fiber bonds and it was the bonding strength and not the fiber strength that was lost. So, either the fiber surface is being modified or some other phenomena are occurring which affect the fiber bonding. On the other hand Wahren and Berg (3^), in comparing recycled sulphite and sulphate pulps at constant density, found that the elastic properties and tear remained relatively constant while breaking length decreased slightly. Bovin, Hartler and Teder( ) studied bleached and unbleached chemical pulps as well as mechanical pulps. Their results showed that all the properties of the chemical pulps as well as mechanical pulps were considerably affected by recycling. They also observed a large change in freeness if pulps where beaten again to the same breaking length after each cycle. Clidir and Howarth(.5) refined pulp to a constant high freeness on each recycle. They found that both tensile and tear declined with recycling, tear passing through a minimum. They attributed

22 the strength loss to a reduction in bonding, based on standard zero span tensile strength. Study of Cardwell and Alexander (js) reported conflicting results. They recycled pulp without significant decrease in strength properties such as burst, tear, zero span etc. provided that a functional sheet property, specifically tensile strength, rather than a drainage property, such as Canadian Standard Freeness, was selected to establish the appropriate amounts of beating for recycled pulps. They found that paper strength properties similar to those of the virgin pulp could be recovered after at least three recycles. In view of these findings and because mechanical attrition effects were small, there would be little difficulty in recovering adequate strength after several recycles. Thus, it is feasible to use clean high strength waste materials to replace significant quantities of virgin fiber without appreciable strength loss. A study by Koning and Godshall (2) determined the effect of repeated recycling of uncontaminated fibers on the compressive strength, scoring, and impact resistance of corrugated fiber board containers. In general, strength and performance were lowered when 100% recycled fiber were used. The greatest loss in strength occurred between the virgin material and the first recycle, rather than subsequent recycles. Effect of Recycling on Fiber Characteristics The swelling of fibers is more or less destroyed in the first

23 drying operation so that more mechanical action is necessary to swell the fiber in its second use. This yields an undesirable amount of fines which increase the drainage resistance of the suspensions. The present discussion, thus, deals with the changes in the cell wall that take place during the drying of virgin fibers, and the importance of these changes with regard to developing beating resistance in the fiber wall. The cell wall consists mainly of crystalline material characterized as microfibrils surrounded by matrix material. These microfibrils are oriented in certain directions and are laid down in several layers in the cell wall. The amorphous material in between is mainly lignin and hemicellulose.{j8) lot CUMENTAirr C E L L IA O M r e *. J o t " j jj 5.V.V- W C T O S E S ^ * & U C N I N ' ^ 1 Figure 3. Model of Oltrastructural Organization of a Microfibril. (a) cross section (b) model of a 120-A fibril in longitudinal section(8) There are essentially two ways in which the cell wall may react to mechanical treatment.(9) Either it may fragmentize into shorter

24 fibers or fibrils which are detached from the cell wall, whole cell wall may delaminate and develop yield cracks, or the whose dimensions may range from the molecular scale up to micro cracks separating the cell wall into layers. Opening up the cell wall of a once-dried fiber is a much more complex problem than it is for virgin fibers. A possible mechanism for void closure has been suggested by Guest and WestonU). There appears to be two stages to the closure of the voids during drying. The first type of void, representing about 50% of the fiber void volume, starts to close at about 50% moisture content (i.e., a solid: water ratio of 1: 1). the time that 30-40% moisture (approximately 2:1) is reached, By this first type of void is more or less closed. The second type starts to close in this 30-40% moisture region and continues closing up to complete dryness. Figure 4 shows the three types of void closure. 0p«n Void Tangential Radial Longitudinal Figure 4. Three Possible Means of Void Closure.(1)

25 The virgin fines obtained by beating are highly swollen and they loose about half of their swelling ability in the drying process. The fines obtained in beating consist to a large extent of fibrils which are strongly coupled to each other by the bonding mechanism that takes place during drying. Thus when once more liberated they do not have the same ability to re-swell as fines from virgin fiber and they contribute more to the specific surface area of the suspension than to the swelling potential. In the sense they start to behave more like fillers with less effect on the strength and more on the drainage properties.(9) This study conflicts with the study done by Haws and Joshi.(10) Hawes and Joshi (1 ) found that recycled pulp fines, both primary and secondary, are almost as effective as the corresponding virgin kraft fines in improving handsheet strength properties. Fines from recycled pulp, provided it does not contain excessive amount of de-bonders like ink, stickies, and fillers, could contribute significantly in increasing the strength properties of the paper and board. Energy Considerations in Refining It has generally been accepted that refining is inefficient and that a considerable amount of energy is dissipated in water surrounding the fibers and does not contribute to useful work(ll). There have been many discussions of the efficiency in the refining process in recent years.

26 Representative of early thoughts on the subject is the review and estimate by Van den Akker (JL2), which leads to efficiency figures of 0.1 to 1%. Van den Akker's estimate was based on the work required to separate individual elementary fibrils from microfibrils. Nissan (JL1) has examined the refining process in terms of fracture mechanics, where the strain energy per unit volume is the drive which creates the voids, fibrillation, and other fractures in fiber during their passage through a representative refining system. More recently, the work published by Atalla and Wahren(1J) set an upper limit to the amount of energy that can be consumed in the fibrillation of cellulose in cell walls. Instead of considering the strain energy driving the creation of voids and cracks, they concentrated on the energy absorbed in the generation of new surfaces. They estimated that the efficiency of refining process may be as high as 25%, which is considerably above previously published estimates. Development of directly applicable theories to describe the process of refining has lead to a more complete understanding of what happens during this process as well as a direct means of optimization of stock preparation in terms of fiber properties and energy consumed. Fundamental to all these theories is the realization that the primary parameter in refining is the cutting edge length of any unit. Brecht, Athanassoulas, and Suewert(^4) proved that the

27 leading edge length was the dominant factor in refining as opposed to the bar width. The mathematical description of this process have taken three forms: 1. The specific edge load theory: Brecht, Athanassoulas and Suewert (14_) (15J, Brecht and Suewert (16) (17) 2. The concept of severity and frequency of impacts: Lewis and Danforth(]J[), Danforth and Miadota(1JU, Danforth and Koon(20), Danforth (2_1) (22_) 3. A combination of first two theories; Leider and Rihs(23), Leider(24) The Specific Edge Load Theory The fundamental parameter in this theory is the cutting edge length, that is the total intersecting length of rotor bars per unit time. Dividing the net power applied by cutting edge length defines the intensity of the refining action. Dividing the power applied (net or gross) by throughput gives the energy input of refining. Cutting Edge Length = n, nr L S) (2.1) Intensity = Specific Edge Load «* Net power Cutting Edge Length * t - P o n f n r L Cl...(2.2)

28 14 Energy input = Power ( net or gross ) Throughput Where, P t P c = Gross power, kw = Idling power or no load power, kw n r, n. = Number of rotor and stator bars which actually cross L = Length of each rotor and stator crossing, m 2 = Rotational speed, s'1 The specific edge load theory has following major advantages: 1. No empirical constants are required 2. The equations used are simple The major limitation of this theory is, that it neglects the effect of stock characteristics such as consistency and flow rate. Severity and Number of Impact Theories Fiber refining can be quantitatively characterized by two factors, relative severity of impacts and relative number of impacts received per fiber, which can be shown to be functions of certain refiner design and operating parameters.(18)-(22) Specifically the relations as presented by Lewis and Danforth(18^) and Danforth and Miadota(19) are: ( HP. - HPB ) At S = K 2 (2.3) D (RPM) L t L. C

29 15 and, IN = L L (RPM) c Kj (2.4) X R Where, S N HP a HP (HP.-HPJ K Relative severity of impacts Relative number of impacts Total horsepower applied No load horsepower Net horsepower Total area of refining zone Total length of rotor edges L. D RPM Total length of stator edges Effective diameter Rotor RPM Stock consistency Average bar contact length Throughput rate KjjKj = Appropriate constant K x and K 2 are constants specific to individual systems. The need to derive them accurately has restricted the utility of these equations. Leider and Rihs(23) and Leider(2 ) has further developed these equations to replace the constants with measurable terms. The basic equation is as follows :

30 4 16 E N (2.5) Note that the units for the right hand terms in equation (2.5) are: Horsepower x day Horsepower x day impacts fiber ton impact fiber ton E _ E N 1/M The average number of impacts experienced by the fiber is passing through refiner is given by : N = n r n, fl x p f... (2.6) Where, nr = Number of bars on the rotor n, = Number of bars on the stator T = Residence time of the fiber in the refiner p = Probability that a fiber will be in a position to be impacted With the further work equation (2.6) becomes: It3 D a 4 _ W a _ 3. L. Df. Lf (2.7) plate system fiber dimensions variables dimensions

31 Where, Dm = mean diameter of refining zone = ( D x + D 2 ) / 2 Wm = ( bar width + groove width ) / 2 L fl q D f = groove length = number of revolution per unit time = volumetric flow rate = Average fiber diameter L t = Average fiber length This theory, though it is mathematically more complex, has the potential of describing the process of refining completely by the use of true fundamental refining variables (i.e., number of impacts and energy per impact). The above theory represents only a first order approximation to the complex process of disk refining. There are limitations of this theory as well as specific edge load theory. The theory does not take in to account the following factors: 1. The influence of flow rate and consistency at constant throughput. 2. The influence of fiber size distribution in the initial pulp and the influence of its change as a result of refining. 3. Distribution of the number of impacts and energy per impact. 4. Type of impacts. 5. Other secondary effects such as temperature, pressure, ph etc.

32 Change in Refining Response due to Fiber being Recycled When pulp is refined, the fibers are subjected to mechanical stresses. According to Guest and WestonU), the ability of the cell wall to withstand these stresses depends on the existence of viscoelastic (amorphous) regions to absorb the energy. The drying process causes the sealing of the pores. It is also likely to increase the crystallinity of the cell wall. Fibers in which paracrystalline regions dominate will not be able to withstand the mechanical stresses as well and will tend to favor fines production if treated in the same way as its amorphous counterpart. This would explain the difference in response of recycled fibers to refining. Several questions still remain to be answered with regard to the future usage of recycled fibers. A first is whether we can produce virgin fibers that are more suitable for secondary use. Another is concerned with the treatment of secondary fibers in the paper mill in order to improve their strength performance. A third is whether we can design equipment for, and recommend strategies of beating which to some extent counteract the development of drainage resistance in these fibers.

33 CHAPTER III PRESENTATION OF THE PROBLEM The refining process is one in which a considerable amount of energy is transferred from the refining shaft to the pulp. Thus the energy transfer mechanism is closely related to the fiber treatment. It is possible to simplify the study of the results of refining as related to fibers. The number of impacts experienced by a fiber, N, and the energy per impact imparted to a fiber, E, are the only two independent stock refining variables after the choice of refining unit and tackle are made.(14_)- (24J Danforth(21) has further mentioned that use of refiners offers an excellent way of optimizing specific qualities of secondary fiber stock. In order to get the best from the recycled fiber sources, they must be treated appropriately. variations in recycled fibers. Different virgin stocks will produce The way in which they are treated should reflect this. Changes in the cell wall structure demand the use of different treatment from virgin stock.(1)-(13) Most of the literature tells about what happens to fibers when paper is reused a number of times. For example recycled fibers do not develop burst or tensile strength as high as virgin fibers. There is little literature to explain what causes these property losses. None of the literature reviewed discusses a method or 19

34 strategy of fiber treatment to restore these properties. The objectives of this study were to answer the following questions: 1. Is there a significant effect of method of refining or fiber treatment on quality of recycled fibers? 2. Should we treat virgin fibers differently if we have to recycle the paper? 3. Should the refining of virgin fibers differ from the refining of recycled fibers in order to get improved strength properties? 4. Is it possible to develop some strategies for refining in order to get required strength properties at optimum power consumption?

35 CHAPTER IV EXPERIMENTAL APPROACH Introduction Most of the research done in the recycling area has been carried out in the laboratory. In laboratory experiments although it is easy to measure and control fiber characteristics, quantitative measurement of energy transfer is difficult. Further it is difficult to correlate energy consumption data obtained from laboratory scale experiments to that of actual energy consumption in paper mill. In such cases pilot plant experiments give practical results which are more directly applicable to mill conditions. Pilot plant facilities at Western Michigan University, Kalamazoo are ideal for this project. The Double Disk refiner in the pilot plant can simulate actual mill operating conditions. This refiner has been hooked to the computer for the measurement of stock flow and energy consumption. It is easy to make paper on pilot plant paper machine, where again we can maintain actual mill operating conditions and re-slush paper for further steps of the experiments. Description of Experiments Characteristics of virgin fibers such as development of slowness and strength properties at different refining conditions 21

36 were determined in order to provide a reference for the refining responses of secondary fibers. The main approach of the experimental design was to vary the amount of treatment and severity of treatment for a given fiber (virgin or recycled) in order to determine development of pulp properties (tensile or slowness for example) and corresponding energy input to the refiner. For this study all the system variables such as stock flow, stock consistency, refiner plate design, refiner rpm, etc. were kept constant. Thus most of the limitations(discussed on pp. 14 and 17) of the Specific Edge Load Theory or Severity and Frequency of Impact Theory were eliminated and the results obtained such as development of pulp properties and energy input were functions of fiber characteristics only. It is evident from equation (2.5) that there are basically two ways to increase net energy input (En t) or the amount of refining. Either the number of impacts or the energy per impact can be increased. In mill applications the number of impacts is usually fixed so that the energy per impact increases as the net refining energy is increased. On the other hand in the Valley Beater, energy per impact is fairly constant and the amount of refining is increased by increasing the number of impacts. There is no reason to expect that the functional relationship between two pulp properties (tensile and slowness for example) would be the same for these two types of refining. Two types of chemical pulps were used for this study: 1. Bleached softwood(kraft) pulp consisting of

37 a. 60% Black Spruce b. 40% Jack Pine 2. Bleached hardwood(kraft) pulp consisting of a. 75% Aspen and Poplar b. 25% Spruce and Jack Pine For each type of furnish, experiments were conducted for 1. Virgin fibers. 2. Fibers recycled once. 3. Fibers recycled twice. The schematic description of the experimental design is shown in Figure 5. In this study the refining was done at fixed energy per impact levels and for a given trial, EE.t (Net energy input) was increased by increasing N (Number of impacts). Samples were collected at six different levels of E. or N. The schematic n t diagram for each trial is shown in the Figure 6(pp. 26). As shown in the Figure 6, pulp was recycled in the stock chest. Stock samples were collected at 0, 5, 10, 15, 20, and 25 minute time intervals from the chest. After 25 minutes of refining, paper was made on paper machine and was used for subsequent recycling experiments. The energy per impact level was varied between the trials. Three levels corresponding to low, medium and high values of energy were used (26 kw, 36 kw and 49 kw respectively). The schematic flow diagram for each run is shown in Figure 7. As shown in the Figure 7, 525 lb. of O.D. pulp was split into three batches. (One batch corresponded to one trial and each run included three trials. Approximately 20 % of the fibers were lost for the

38 subsequent run). For each pulp sample, Canadian Standard Freeness, net refining energy, and handsheet properties were obtained and can be found tabulated in Appendix A. Refiner used for this study was manufactured by Beloit Jones Corporation, Dalton, Massachusetts, having following dimensions: Refiner type: 12" Double Disk Model: DD 3000 Plates: Diameter cm Angle Bar width cm Groove width cm Average bar length cm Number of bars on rotor Number of bars on stator Rotor RPM revolutions/min

39 25 Bleached Kraft Pulp (Softvood/Hardwood) 525 lt>. O.D. Trial It. 0.1' Pulp Refining Lev Energy/Impact ' Sample Collection Refining Medium Energy/Impact Sample.Collection Trial It. 0D Pulp Refining High Energy/Impact ' Sample Collection Refining Lov Energy/Impact ' Sample ' Collection, Plefining Medium Energy/Impact Paper Malting KSample- ^ Collection,) Refining High Energy/Impact Refining Lov Energy/Impact f Paper ^ { Sample ^ I Malang ) I Collection.) Refining Medium Energy/Impact f Paper ^ / Sample 'l I Malting ) ^Collection; Refining High Energy/Impact Sample Collection, Figure 5. Schematic Description of Experimental Design.

40 26 A A Agitator B- Back /Storage chest V' Stock valve P- Stock pump R - Refiner r Elov m eter: - Sample collection points Figure 6. Schematic Flow Diagram for Each Trial.

41 27 T1 SI _ it S2 T2 SI _ a & S2 7Ts /T\ /Is T3 r 4 SI n T S 2 H fr-ar- TVF VF/RF : Virgin fiber or recycled fiber H :Beater [ for re-slushing] S1.S2 : Storage chests. II. 12. T3 : Trial 1.Trial 2 and Trial?. M :Paper machine. R :Refiner Figure 7. Schematic Flow Diagram for Each Run.

42 CHAPTER V RESULTS AND DISCUSSION In the refining process, a certain amount of energy is transferred to the pulp, resulting in a change in physical properties. The effectiveness of the refining depends on how much energy we apply and also on how it is applied. The following analysis is designed to provide a means of evaluating the effect of refining and recycling in terms of energy input, stock development and physical properties of handsheets. Data obtained from run (1) were not used for analysis because the computer was not connected for recording the data. Data obtained from run (2) were used for analysis since the computer was connected for recording energy consumption accurately. This was also useful for reducing variations in raw material and operating conditions. Experimental data for this analysis are shown in Appendix A. Results obtained from the experiments are summarized in the plots. Effect of energy per impact level and recycling on development of softwood and hardwood pulp is shown in the Figures 8 to 14 and Figures 15 to 21 respectively. Figures 22 to 27 show the effect of energy per impact level on net energy input versus development of tensile index, density and percentage of short fibers for each type of pulp. 28

43 Virgin C.S.F. 700 Recycled once > 80 - Z X 0 1 u c I Low Energy/Impact Medium Energy/Impact n High Energy/Impact C.S.F. 90 Recycled twice t 2 x 1 ««ce K C.S.F. 300 Figure 8. Plot of Tensile versus C.S.F. for Softwood Pulp.

44 Virgin? I 5k 'in 0.5- o Low Energy/Impact Medium Energy/Impact High Energy/Impact C.S.F. 600 Recycled once 700? 0.7- O) >. 'in C Q Medium Energy/Impact High Energy/Impact C.S.F Recycled twice n< 5k W c < C.S.F Figure 9. Plot of Density versus C.S.F. for Softwood Pulp.

45 31 Low Energy/Impact Medium Energy/Impact High Enargy/lmpact C.S.F. Recycled once a Low Enargy/lmpact a Madium Enargy/lmpact B High Enargy/lmpact C.S.F. Recycled twice : 6 3 CD y a 1 a a a L ^ M \ o a B Low Energy/Impact Madium Energy/impact High Enargy/lmpact B C.S.F. Figure 10. Plot of Burst versus C.S.F. for Softwood Pulp.

46 32 Virgin Low Energy/Impact Medium Energy/Impact High Enargy/lmpact C.S.F. x" «g Recycled once E x 0.7- (9 g (D i B a B Low Enargy/lmpact Madium Enargy/lmpact High Enargy/lmpact C.S.F. Recycled twice 500 c? < E k. (8 Madium Energy/Impact High Energy/Impact C.S.F Figure 11. Plot of Tear versus C.S.F. for Softwood Pulp.

47 33 Virgin ' "3 o O 28- n * «- 0 Low Enargy/lmpact Madium Enargy/lmpact B High Enargy/lmpact C.S.F. Recycled once Q <8 Low Energy /Impact Madium Enargy/lmpact High Enargy/lmpact Recycled twice C.S.F. s High Enargy/lmpact C.S.F Figure 12. Plot of Scattering coefficient versus C.S.F. for Softwood Pulp.

48 Virgin 2.4- o> c IS I* a.o 2.3- LL 0 Low Energy/Impact Madium Energy/Impact a High Enargy/lmpact E 2.2- E ? 300 Recycled once C.S.F Recycled twice C.S.F JD ^ ? a w a LL a B Low Energy/Impact Medium Energy/Impact High Enargy/lmpact C.S.F Figure 13. Plot of Fiber Length versus C.S.F. for Softwood Pulp.

49 35 Virgin <n o r c CO B Low Energy/Impact Medium Energy/Impact High Energy/Impact C.S.F w "3 High Energy/Impact C.S.F. Recyded twice </> -O VS r 40-2 CO B Low Energy/Impact Medium Energy/Impact a High Energy/Impact C.S.F Figure 14. Plot of % of Short Fibers versus C.S.F. for Softwood Pulp.

50 36 90 Virgin f Z x c Low Energy/Impact Medium Energy/Impact B High Energy/Impact C.S.F. f z c e 70 B Low Enargy/lmpact Madium Enargy/lmpact High Enargy/lmpact C.S.F. Racyclad twice I z x 3 c <n c B B Low Energy/impact Medium Energy/Impact High Energy/Impact C.S.F Figure 15. Plot of Tensile versus C.S.F. for Hardwood Pulp.

51 0.9 Virgin 37 0 Low E nergy/lmpact Medium E nergy/lmpact o High E nergy/lmpact C.S.F. < E u O) w u> si V) J Low Energy/Impact Medium Energy/Impact B High Energy/Impact C.S.F Recycled twice O Low Energy/Impact Medium Energy/Impact B High Energy/Impact m 0.84 Io - H S ' S C.S.F Figure 16. Plot of Density versus C.S.F. for Hardwood Pulp.

52 38 s Virgin n 0. JC x ' g c 4 3 Medium Energy/Impact C.S.F. < E Cl j c a o c a Low Energy/Impact Medium Energy/Impact High Energy/Impact C.S.F < Era Q. JC 4.0» S 3.8 (A <a Low Energy/Impact Medium Energy/Impact a High Energy/Impact C.S.F Figure 17. Plot of Burst versus C.S.F. for Hardwood Pulp.

53 Virgin & o-h < E Z E X* <D 0.5- k. (0 fl I C.S.F S5 c\j % 0.4- Z EX* a a ~ a Low Energy/Impact Medium Energy/Impact O High Energy/Impact Recycled twice C.S.F o> E & B a Low Energy/Impact Medium Energy/Impact High Energy/Impact C.S.F Figure 18. Plot of Tear versus C.S.F. for Hardwood Pulp.

54 Virgin O 1 24: 22 - Q Low Energy/Impact Medium Energy/Impact a High Energy/Impact C.S.F = (3 o to Low Energy/Impact Medium Energy/Impact High Energy/Impact C.S.F Recycled twice 22 - S 20- O " <55. d Low Energy/Impact Medium Energy/impact a High Energy/Impact C.S.F Figure 19. Plot of Scattering coefficient versus C.S.F. for Hardwood Pulp.

55 41 Virgin CD Low Energy/Impact Medium Energy/Impact High Energy/Impact U C.S.F. Recycled once E E c 0.9- <B o B Low Energy/Impact Medium Energy/Impact High Energy/Impact C.S.F. 0.9 E E U) c fi 0.7- B Low Energy/Impact Medium Energy/Impact o High Energy/Impact C.S.F. Figure 20. Plot of Fiber Length versus C.S.F. for Hardwood Pulp.

56 42 Virgin 18 - (A 9 J3 C w o a Low Energy/Impact Medium Energy/Impact High Energy/Imp act C.S.F Recycled once 24 - E W B a Low Energy/Impact Medium Energy/Impact High Energy/Impact C.S.F Recycled twice r o C V) o 3* a e B Low Energy/Impact Medium Energy/Impact High Energy/Impact Figure C.S.F. Plot of % of Short Fibers versus C.S.F. for Hardwood Pulp.

57 80 o Low Energy/Impact Medium Energy/Impact B High Energy/impact Net Energy, HPD/T 0.7? >s C a Q 0.5 Low Energy/Impact Medium Energy/Impact b High Energy/Impact Net Energy, HPD/T o Low Energy/Impact Medium Energy/impact b High Energy/Impact 24 -O Net Energy, HPD/T Figure 22. Plot of Tensile, Density and Percentage of Short Fibers versus Net Energy for Softwood(virgin) Pulp.

58 O) 80 - '» -TA _S T3 70 " Q Low Energy/Impact Medium Energy/Impact a High Energy/Impact H 60-* Net Energy, HPD/T 20 3K </> e CD a a Low Energy/Impact Medium Energy/Impact B High Energy/Impact Net Energy, HPD/T 20 in «34- «1 32- co a Low Energy/Impact Medium Energy/Impact B High Energy/Impact Net Energy, HPD/T Figure 23. Plot of Tensile, Density and Percentage of Short Fibers versus Net Energy for Softwood(recycled once) Pulp.

59 90 45 t Z x" «X I c c o Low Energy/Impact Medium Energy/Impact a High Energy/Impact Net Energy, HPD/T? 0.8- E I) I -7 i n Low Energy/Impact Medium Energy/Impact High Energy/Impact Net Energy, HPD/T 20 I i l o 4 0 -! ' 'o 3? o Low Energy/Impact Medium Energy/Impact High Energy/Impact Net Energy, HPD/T Figure 24. Plot of Tensile, Density and Percentage of Short Fibers versus Net Energy for Softwood(recycled twice) Pulp.

60 46 80 * fi 70- s' 60- '1 50- (2 a Low Energy/Impact Medium Energy/Impact High Energy/Impact 0 10 Net Energy, HPD/T ? 0.8- E 5* W 0.7- n b Low Energy/Impact Medium Energy/Impact High Energy/Impact Net Energy, HPD/T 18 - (0 J3 a U. a Low Energy/Impact Medium Energy/Impact B High Energy/Impact Net Energy, HPD/T Figure 25. Plot of Tensile, Density and Percentage of Short Fibers versus Net Energy for Hardwood(virgin) Pulp.

61 47 80 t Low Energy/Impact Medium Energy/Impact High Energy/Impact Net Energy, HPD/T o o> Vi o Low Energy/Impact Medium Energy/Impact n High Energy/Impact Net Energy, HPD/T 26 - u. 22 «20 a Low Energy/Impact Medium Energy/Impact B High Energy/Impact Net Energy, HPD/T Figure 26. Plot of Tensile, Density and Percentage of Short Fibers versus Net Energy for Hardwood(recycled once) Pulp.

62 70 64 <1 a Low Energy/Impact Medium Energy/Impact High Energy/Impact 0.88 Net Energy, HPD/T ? E & ^ -82i *35 i B Low Energy/Impact Medium Energy/Impact High Energy/Impact Net Energy, HPD/T cn 26- si u. Low Energy/Impact Medium Energy/Impact B High Energy/Impact 0 10 Net Energy, HPD/T Figure 27. Plot of Tensile, Density and Percentage of Short Fibers versus Net Energy for Hardwood(recycled twice) Pulp. 20

63 The least square procedure was used for curve fitting. Curves with maximum value of coefficient of determination(r2) and minimum value of standard error of prediction were selected. Effect of Energy per Impact on Handsheet Properties A significant dependence of strength development on the energy per impact was observed for all pulps. Refiner trials at low values of energy per impact level produced pulps with much higher handsheets strength properties at a given freeness than trials at a high energy per impact level. The degree of response varied with specific physical properties. Density and tear index did not appear to be greatly influenced by the energy per impact level except for twice recycled pulp as can be seen in the Figure 9, 11, 16 and 18. Fiber length and percentage of short fibers (less than 20% of the average fiber length) were strongly affected by energy per impact level. As expected, high energy levels resulted in greater reduction of average fiber length. The effect was more significant for virgin pulp (both softwood and hardwood), than for recycled pulp (Figures 13, 14, 20 and 21) Effect of Recycling on Strength Properties of Handsheets It is more difficult to compare the strength properties of handsheets from re-pulped fibers than that of virgin fibers because of different degree of freeness. Still, Figures 8, 10, 15 and 18 show that maximum obtainable strength index for recycled paper is

64 less. A greater significant difference was observed for hardwood pulp and for the pulp recycled at high energy per impact level. This may also be due to greater degree of refinement at high energy per impact level. Interaction Effect of Energy per Impact Level and Recycling Figures 8, 10, 11, 15, 17 and 18 show that strength properties of pulp are affected by: 1. Method of refining (energy per impact level) 2. Number of recycles 3. Degree of refining A method of analysis of variance was used to find the significance of energy per impact level, effect of recycling and their interaction effect on strength properties of handsheets. Development of tensile strength, for example, was studied for different energy per impact levels (e.g., low, medium and high) and for different quality of pulps (e.g., virgin, recycled once and recycled twice). As shown in the Figures 8 and 15, polynomial model of degree two was fitted to describe functional relationship between development of tensile strength and freeness. For example, the fitted regression model obtained for virgin softwood pulp refined at high energy impact level was: y = (x) (x2) (5.1) Where; y = Tensile, N.m/g x C.S.F. The Coefficient of Determination(R2) value for this model is

65 one. That is, 100% of the variation in the response data was explained by the model. Taking derivatives on both sides of equation (5.1) with respect to x, we get: dy/dx = (x) (5.2) For maximum value of tensile index dy/dx = 0 (5.3) i.e. From equation (5.2) 0 = (x) or, x = (5.4) By substituting this value of x in equation (5.1), we get: or, y = y = (76.49) (76.49) 2 (5.5) Thus the maximum obtainable tensile strength for softwood pulp refined at high energy per impact was Similarly the maximum obtainable tensile strength was calculated for each type (softwood and hardwood) and quality (virgin and recycled) of pulp, refined at various energy levels. The data obtained are shown in Tables Table 1 Maximum Tensile Data for Softwood Pulp Quality of pulp Energy level per impact Low Medium High Virgin Recycled once Recycled twice

66 52 Table 2 Maximum Tensile Data for Hardwood Pulp Quality of pulp Energy level per impact Low Medium High Virgin Recycled once Recycled twice As shown in the Tables 1 and 2, this is a two factor (energy level and quality of pulp) factorial design with only a single replicate; that is, one observation per cell. The linear statistical model for this is: Vij = ** + *4j +...(5.6) i = 1, 2, 3. and, j = 1, 2, 3. where {TB)43 is an interaction term. A test developed by Tukey (1949) ( 5.) was helpful in determining the effect of interactions between energy level and quality of pulp. The complete analysis of variance is summarized in tables 3 and 4. Table 3 shows that for softwood pulp only the effect of energy per impact level was significant on tensile index. Surprisingly, the effect of recycling on softwood pulp properties was not significant at the 5% a level. Effect of interaction is also not significant at the 5% a level. In other words, softwood pulp