1989 Volume 1
A concept for the fibre flocculation process in turbulent flow is presented including both rupture and aggregation of fibre flocs. Based on results from the literature, a mechanism is proposed for the breaking-up and the building-up of flocs in interaction with turbulent eddies of different length scales in the turbulence spectrum. The mechanism has been formalized into a two-way hierachical concept with analogy to the decay of a scalar fluctuation.
Deformation and break-up of flocs by turbulent eddies with a length scale similar to the floc scale dominates the rupture process. Eddies of a smaller scale agitate the floc causing weakening and activation of the network. Floc aggregation happens due to floc collisions when the flocs are transported by turbulent eddies of a larger scale. The smaller scales activate the floc surface, and increase the probability of forming a floc. Erosion and deposition of single fibres takes place in the turbulent fine structure.
This concept finds support when tested and compared with the results in the literature. Experiments have been conducted in a vertical pipe flow with a varying flow rate, fibre length and consistency, taking measurements at various positions downstream of an orifice . Results were obtained using high speed movie film, image analysis of still photos and measurements of flocculation and turbulence spectra using a laser . Increasing the turbulent kinetic energy or decreasing the macro scale gives lower floc intensity at large length scales .
During web formation, the difference between the speed of the pulp suspension and that of the wire gives rise to a smear force that orients fibres preferentially along the machine direction . This has been known for a long time. It is less clear, however, haw the distribution of fibre orientations is fly related to the conditions of web formation and to the properties of papermaking fibres. We have studied this question using simple theoretical models and experimental data inc1-tiding irk-We analysis results.
We find that fibre-to-fibre interactions determinehow much individual fibres are rotated by the oriented shear field. In this way the fibre orientation distribution obtained for a given speed cliff an the pulp properties. On the other hand, the turbulence of the pulp flow seems to be the mechanism through. which the conditions of web formation affect fibre orientation. In particular, we suggest that the on set of hear-induced turbulence in the suspension flow determine the maximum fibre orientation anisotropy that can be achieved.
The model calculations show that the distribution of fibre orientations cannot be represented in terns of a simple functional form with only one adjustable parameter. The shape of the distribution depends an the fibre p and especially the wet fibre flexibility. In paper the orientation distribution of curly fibres is quite different from that of straight fibres. On the other hand, no diff in the distribution is observed if long and short fibre are examined separately.
Cambridgepp 121-152Effects of Pulp Drying and Reﬁning on Softwood Fibre Wall Structural OrganisationsAbstractPDF
Structural organisations of a range of softwood kraft and other pulps are described in terms of their response to pulp drying and refining. Fibre widths, thicknesses, wall thicknesses, and wall areas, as well as cross-sectional shapes can be very different depending on whether pulps are dried and/or refined. Fibres can also respond to refining in different ways depending on whether the fibres have thin or thick walls.
Actual cross-sectional wall areas (including void space associated with delaminations) of undried kraft fibres normally remain unchanged with refining whereas those of dried and rewetted fibres increase. For refined fibres, delaminated walls are envisaged as consisting of several concentrically oriented lamellae aggregates or coarse lamellae (containingwall substance plus water) interdispersed with void space filled predominantly with water. Thus, effects of pulp refining could be to make the structural organisation of the coarse lamellae but not the walls of undried kraft fibres more dense, and the walls of dried and rewetted fibres less dense. Furthermore, the thicknesses of uncollapsed, undried fibres normally decrease with pulp refining while those of collapsed, dried and rewetted fibres increase. Finally, cross-sectional shapes and dimensions of dried and rewetted, thick- and thin-walled fibres are modified in different ways and at different rates.
Wall organisations of kraft, soda-AQ and neutral sulphite-AQ (NSAQ) fibres are very different. Kraft fibre diameters (widths and thicknesses), wall thicknesses, wall areas and hemicellulose contents are substantially lower than those of NSAQ and soda-AQ fibres when compared at either the same lignin content or pulp yield. Effects of refining are to cause kraft fibre diameters to be decreased further, and walls to delaminate and expand into fibre lumens. The unique properties of kraft fibres are explained by a tightening and contraction of the fibrillar and lamella wall organisations.
Observed changes in fibre dimensions and behaviours are discussed in terms of fibre wall structural organisations, compositions and behaviours .
The article reviews the state of knowledge of the action and effect of beating on chemical pulps. The view is put forward that the problem is so complex that researchers have been tempted to oversimplify it, but some of these simplifications now may represent a barrier to progress. New approaches incorporating newly developed research techniques are proposed for future research efforts.
In this overview, forming is defined’more generally to include all processes from thick stock dilution with recirculated white water (mix production), to the final dewatering of the wet web on the wire.
Grammage variations in the finished paper are generated mainly during the forming process. These variations can be expressed as mass formation in the small scale range and MD-, CD- and residual variance for large scale variations.
Mass formation can be evaluated off-line using beta radiography, combined with micro densitometry or image analysis . A new technique involving direct re cording of electron beam transmission is under development, with promises of faster processing, perhaps even on-line, and higher geometrical resolution. Characterization techniques based on the co-occurence matrix, especially suitable in image analysis, can be a useful complement to the traditional power spectra.
It has recently been conclusively demonstrated that in flowing fibre suspensions, flocs are kept together by the bending forces of the fibres involved. To study the dynamic behaviour of flowing fibre suspensions, modern video techniques and image analysis are being applied.
The build-up of the fibre mat is considered to be a filtration process under normal conditions. This process posesses an inherent “self healing” effect on local grammage variations . The mass formation of a laboratory sheet is for this reason superior to that of a random sheet. The higher mix consistencies used for machine made sheets cause floc generation, and usually results in a worse large scale mass formation than that of the random sheet.
When evaluating the mechanical and optical characteristics of a machine made paper sample, its properties relative to those of a laboratory sheet formed from the same furnish may be expressed as the Forming Efficiency.
To improve grammage uniformity, the mix should be fed to the headbox directly after the dilution of thick stock with white water. No processes like screening or cleaning from which uncontrolled reject flows are drawn from the measured fibre flow should be allowed . Further, the material content in the recirculated white water should be controlled to a constant level if the composition of the paper produced is to be held constant.
In todays headboxes, the tapered manifold is the dominating method of distributing the mix flow into a tube bundle across the entire wire width. The distributing tubes can exhaust either into a stilling chamber, or directly into the outlet nozzle. In the latter case, the tube outlet area must be maximized to avoid excessive wake effects. Further, the nozzle contraction ratio must be large enough to reduce the degree of relative turbulence to an acceptable level.
There are two basic headbox designs for stratified forming . In one of them, thin, pointed vanes separate the different furnishes. In the other, thicker separation walls generate “air wedges”, which may separate the furnishes all the way to the initial dewatering point . In the first case, layer mixing can begin be-fore dewatering; in the latter case, four new interfaces between air and mix are created, and all are potential sources for disturbance generation.
Mathematical methods are now being applied to the calculation of water flow patterns in headboxes, which may eventually lead to designs with improved flow stability.
High consistency headboxes have been developed to form paper according to an extrusion process. To obtain acceptable mass formation, various channel shapes, causing mix deflocculation, are used.
Because of the complex interactions between dewatering forces, water flow, material movement and resulting web structure and elasticity, no models have so far been developed for the prediction of filtration dewatering rates. The Kozeny-Carman equation, describing flow through porous beds, is a too simplified model to be of any great value in this situation, and therefore empirical equations are applied instead.
The development of forming wires have lead to multi-layer designs where the topography of the paper and the wear sides can be optimized simultaneously.
For Fourdrinier dewatering, several new dewatering elements have been introduced, allowing a better control of the activity in the mix on the wire, and thus also of the mass formation.
Fourdinier dewatering can be especially sensitive to pressure pulses from the hydraulic headboxes since amplification due to standing wave generation on the wire can create considerable MD grammage variations.
Local slice lip adjustments, especially on hydraulic headboxes which normally have a low conver-gence nozzle, can cause considerable cross flows on the Fourdrinier wire, and this will have a large effect on the grammage profile. Further, local changes in fibre alignment will be generated, a problem which has not yet been given due consideration.
In twin wire forming, the dewatering pressure is generated by wire tension according to either of two basic principles : roll dewatering with constant; or blade dewatering with pulsating dewatering pressure. A combination of these two principles has resulted in the best mass formation and retention. Recently a new method has been demonstrated, in which the dewatering pressure is not generated by wire tension but instead can be controlled to the desired pressure event along the forming zone.
A rapid method to measure the distribution of mass density of paper and paper board was developed. Paper is irradiated with electron beams with energies from 50 to 200 keV and the attenuation of the electron beams by the paper is detected by a photographic film and a real-time TV monitor system. The image of the attenuation of the electron beams (electrograph) is converted into the distribution of mass density through a basis weight calibration. By this method, the basis weight distribution of paper and paper board from 0 to 400 g/m2 was obtained in less than 1 second with both detectors. It was found that the electrograph gives very similar information as beta-radiograph. Experiments to optimize the imaging of the distribution of mass density of paper by electron beams are described along with the relevant physics.
Cambridgepp 195–218Scaling, Percolation and Network Theories: New Insights into Papermaking?AbstractPDF
Recent advances in the theory of condensed matter physics have furnished us with powerful theoretical methods for understanding the structural and dynamical properties of in homogeneous materials, such as the fibre network which constitutes paper. We discuss the concepts of universality and scaling, on which all the new theoretical arguments are based. In understanding the properties of forming and of paper, the percolation transition, i.e. the unique state where an infinitely connected network of fibres is formed, is of particular interest. The physical aspects and the power of the percolation theory are discussed in the presentation. Applications of these new concepts to paper have so far been few.
We show how the percolation theory yields practical qualitative results explaining the relationship between the consistency of the suspension in the headbox and the formation. The complicated structure of the turbulence on the wire and on the jet, which manifests itself in the residual variations of basis weight, is discussed using some scaling ideas related to the universality of the nonlinear dynamical systems. The effect of formation on the mechanical properties, especially ultimate strength, can be viewed theoretically with the aid of scaling, percolation and network theories. The reinforcing effect of chemical fibres in newsprint can be judged by a rather simple scaling argument,which,when developed further, gives insight into the nonlinear relationship between strength and the amount of chemical fibres in newsprint.
Scaling and percolation are qualitative methods. When studied and applied properly, these concepts help us get a picture of the in homogeneous materials and understand the basic principles behind their properties. It is then easier to decide on the right quantities to measure when quantitative information is needed in papermaking.
Cambridgepp 219–249Applications of Flow Visualization Technique in Wet End ChemistryAbstractPDF
A microcomputer–aided flow visualization system consisting of Strobovision analyzer(SVA) and image processing apparatus was applied to the analysis of the effect of a variety of parameters in wet end chemistry on the flocculation behavior of model stock systems. These are (a)single polymer addition systems and dual component additive systems,(b)low and high filler content suspensions,and (c)colloidal force and hydrodynamic shear force.
Analysis by digital geometry was carried out to represent features of projected images of flocs and fiber network supporting flocs. The flocculation states were represented numerically. It was attempted to test visually and numerically a flocculation hypothesis in a dual component additive system.
Within the scope of the present investigation,there is some limitations in that this system can be applied only for dilute suspensions with concentrations far lower than those of conventional paper stock in a headbox.
Cambridgepp 67-86The Effect of Reﬁning on Wet Fiber Flexibility and its Relationship to Sheet PropertiesAbstractPDF
The wet fiber flexibilities of several softwood and hardwood species were measured by Steadman’s method (5). Softwoods showed a broader range of wet fiber flexibilities than hardwoods. Refining* decreased the spread as measured by the IQR and increased the median values. The wet fiber flexibility and its distribution was sensitive to the type of refiner, beating load and refiner consistency. Refining at high intensity by increasing the refiner load in a Valley Beater, resulted in the production of pulps with inferior strength properties. When the pulps were at constant WFF, those prepared under low intensity exhibited superior tensile strength. The main effect on the fibers beaten at high consistency was to sharply reduce their fiber length. Changing loads in the PFI mill had less of an impact on the WFF and paper properties . Refining with the low load resulted in maximizing fiber flexibility. However, the tensile strength was inferior. The tensile-density relationship did not change as a function of refining load in the PFI mill, indicating that the quality of refining did not change. The WRV was useful in understanding the relationship between fiber and sheet properties.
Cambridgepp 87-104Response of Pulp Fibres to Mechanical Treatment during MC FluidizationAbstractPDF
Medium consistency (MC) unit operations are becoming increasingly widespread in pulping and bleaching processes. Many MC devices, notably MC pumps, mixers andscreens,relyon creation of a fluid-like state in the pulp suspension. This fluidization requires the dissipation of considerable energy and exposes the pulp fibres to mechanical treatment. The treatment alters pulp physical properties.
The changes in properties of a never-dried semi-bleached softwood kraft pulp with mechanical treatment were determined in a batch operated concentric-cylinder device. A fluid-like state was created in medium and low consistency pulp suspensions with treatment energy varying up to 10 MJ/kg pulp. The treatment energies encompassed the range used in mill and batch laboratory operations and the extensive exposure given during batch MC fluidization studies. MC treatment was found to beat, curl and microcompress the pulp fibres. The implications of MC treatment in mill devices are discussed.