Pulp Fibre Suspensions Research Articles

Influence of adsorbed and dissolved carboxymethyl cellulose on fibre suspension dispersing, dewaterability, and fines retention

The effect of adsorbed and soluble carboxymethyl cellulose (CMC) on dispersing, dewaterability, and fines retention of pulp fibre suspensions was investigated. CMC was added to a suspension in the presence of electrolytes, causing its adsorption to the fibre surfaces, or to a suspension without electrolytes, so that it stayed in the liquid phase. Both the CMC adsorbed on fibre surfaces and that in the liquid phase were able to disperse the fibre suspension due to the ability of CMC to reduce fibre-to-fibre friction in both phases. Adsorbed CMC promoted the formation of a water-rich microfibrillar gel on the fibre surfaces through the spreading out of microfibrils, leading to a decrease in friction at the fibre-fibre contact points and to the increased dispersion of fibres. CMC in the liquid phase of the suspension was in turn thought to prevent fibre-to-fibre contacts due to the large physical size of the CMC molecules. CMC in both phases had detrimental effects on dewatering of the pulp suspension, but adsorbed CMC caused more plugging of the filter cake, and this was attributed to its ability to disperse fibre fines, in particular. Thus, adsorbed CMC also reduced fines retention considerably more than did CMC in the liquid phase of a suspension.

Cavern formation in pulp suspensions using side-entering axial-flow impellers

Pulp fibre suspensions display non-Newtonian rheology, including a yield stress. In mixing operations, this creates regions of active motion around the impellers with the cavern size affecting the quality of mixing attained. Due to the opacity of the suspensions, two non-invasive techniques were evaluated for determining cavern dimensions: electrical resistance tomography (ERT) and ultrasonic Doppler velocimetry (UDV), with ERT chosen for most tests due to the speed of data acquisition. Cavern volume as a function of impeller speed is reported for a range of mixing conditions (hardwood and softwood pulp, suspension mass concentrations from one to five percent, two impeller offsets from the wall, and three suspension height-to-chest diameter ratios). A scaled version of a commercial axial flow impeller was used in a standard side-entering configuration. Measured cavern diameters were compared against model predictions available in the literature. The discrepancy between experimental data and model predictions were significant and were attributed to interaction between the developing cavern and the vessel walls. An alternative model was developed for predicting cavern volume taking this interaction into account.

Flow dynamics of pulp fiber suspensions

The transport between different equipment and unit operations plays an important role in pulp and paper mills because fiber suspensions differ from all other solid-liquid systems, due to the complex interactions between the different pulp and paper components. Poor understanding of the suspensions’ flow dynamics means the industrial equipment design is usually conservative and frequently oversized, thus contributing to excessive energy consumption in the plants. Our study aim was to obtain additional knowledge about the dynamic behavior of industrial pulp suspensions in order to evaluate the relative importance of the factors that influence pressure drop. To obtain mathematical correlations to quantify pressure drop for the transport of pulp suspensions, we studied four different industrial pulp suspensions (recycled pulp, eucalypt bleached kraft pulp, pine unbleached kraft pulp, and eucalypt [90%] + pine [10%] bleached kraft pulp) in a pilot rig specially designed and built for the effect.

Heat‐transfer fouling mitigation with model synthetic fibre suspensions

AbstractSuspensions of model polymer fibres at a fixed concentration and flow rate have been shown to mitigate heat‐transfer fouling. The results are in general agreement with previous studies using wood pulp fibre suspensions under the same conditions. Stiffer and shorter fibres were found to increase the initial induction period before the onset of fouling. Longer and more flexible fibres reduced the ultimate thickness of the fouling deposit as well as the ultimate asymptotic level of fouling resistance. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.

Pulp consistency determined by a combination of optical and acoustical measurement techniques

In this study, methods based on ultrasonic attenuation and optical time-of-flight measurements are used simultaneously in determining both the fibres and fines mass fractions, respectively, of a cellulose pulp fibre suspension. The optical measurements are done by a laser radar and the acoustical measurements are based on ultrasonic attenuation measurements in a pulse-echo set-up. Two kinds of long-fibre fractions are studied, thermo-mechanical pulp and chemical softwood pulp. Fibre and fines mass fraction ranges are 0.25–1.0% and 0–0.75%, respectively. The results show that the fibres are the predominant source for absorption and scattering of ultrasonic waves and are thus mainly contributing to the attenuation of ultrasound in the pulp. It is also found that the fines are the predominant source for optical scattering and fines are thus mainly contributing to the propagation delay of the light pulse in the laser radar set-up. By combining the ultrasonic attenuation and the optical time-of-flight measurements, it is shown that the mass fraction of fines and the mass fraction of fibres in a pulp sample could be determined, respectively.

Numerical simulation of flow of pulp fiber suspensions in stock pump with semi-open impeller

The flow of pulp fiber suspensions with a consistency below 5% through a pump falls into the turbulent region. The 2-phase 3D turbulent flow of the mixture is simulated in the hydraulic components composed by suction chamber, front board, semi-impeller and volute casing for the first time.The pseudo-fluid model and the k-e turbulent model are modified. Phase coupled SIMPLE algorithm is applied to make pressure-velocity correction equation. Based on the results of the numerical simulation, the distribution of velocity, pressure and density of liquid and particle phase are compared and analyzed. The flow analysis in the tip between front board and impeller is emphasized, which reflects the advantage of semi-open impeller when it transports.2-phase medium. This calculation result will be helpful for the design of pump impeller and the establishment of design method of centrifugal stock pump.

Velocity measurements in a cylindrical hydrocyclone operated with an opaque fiber suspension

Two novel techniques to measure velocity in opaque pulp fiber suspensions have been evaluated in a hydrocyclone. A pitometer equipped with a medical micropressure transducer was used to record the tangential velocity. The pressure opening of the pitometer was kept clean with a small purge flow. The small size of the probe minimized its influence on the flow field. The pitometer was able to measure the tangential velocity in the fiber suspension at different levels in the hydrocyclone. Additionally, a non-intrusive ultrasonic velocity profiler (UVP) was used to measure the radial particle velocity. The UVP proved to be capable of measuring radial particle velocity profiles.

Velocity measurements in a cylindrical hydrocyclone operated with an opaque fiber suspension

Two novel techniques to measure velocity in opaque pulp fiber suspensions have been evaluated in a hydrocyclone. A pitometer equipped with a medical micropressure transducer was used to record the tangential velocity. The pressure opening of the pitometer was kept clean with a small purge flow. The small size of the probe minimized its influence on the flow field. The pitometer was able to measure the tangential velocity in the fiber suspension at different levels in the hydrocyclone. Additionally, a non-intrusive ultrasonic velocity profiler (UVP) was used to measure the radial particle velocity. The UVP proved to be capable of measuring radial particle velocity profiles.

MISAPPLICATION OF THE METZNER-REED NON-NEWTONIAN CORRELATION METHOD

The Metzner-Reed friction factor-generalized Reynolds number method was developed originally for flow conditions that are truly laminar and where suspensions are homogeneous. The method has been misapplied when correlating rheological data for inhomogeneous, nonlaminar flow wood pulp fiber suspensions and other systems. A closer examination has subsequently shown that a linear correlation on logarithmic coordinates will always be obtained for any set of actual rheological data or fictitious data using this procedure, irrespective of the consideration of Reynolds number. It is shown that a linear correlation can still be found at Re > 5 × 105. The method is still being misapplied and results are being reported in the literature and textbooks without the restriction of the laminar flow/homogeneous conditions.

Fiber Orientation and Flow Properties of Pulp Fiber Suspensions under Shear Flow Conditions

The rheological properties of mixed pulp fiber suspensions were measured with a parallel-plates type rheometer. The effects of shear rate and mixture ratios of softwood and hardwood kraft pulp fibers on flocculation of the fibers were evaluated using a high speed CCD camera. The flow curves of the suspensions showed individual three zones derived from different flocculation conditions of the systems. The effect of the long fibers on the flocculation is more remarkable than the short ones. The two-dimensional fast Fourier transform technique was used to analyze the fiber orientation in polar coordinate. The fiber orientation in flow direction is rather irregular, even under high shear rate conditions. The present method is useful for evaluating the flow behavior of the fiber suspensions and the fiber orientation.

Floc Structure and Flow Properties of Pulp Fiber Suspensions.

The rheological properties of pulp and PET fiber suspensions were measured with a parallel-plates type rheometer. A high speed CCD camera was used to observe the changes in the floc structures that were produced during the rheological measurement. The flow curves of the pulp fiber suspensions showed Newtonian flow in the low shear rate range. With increasing shear rate, the shear stress increased and then became unstable, namely, jumps in the flow curves were observed, which are due to the formation of fiber flocs. At a higher shear rate, the flocs disappeared and the systems showed Newtonian flow again. The flow curves of the polyethylene terephthalate (PET) fiber suspensions showed non-Newtonian flow even in the low shear rate range. We found that the uniform distribution of fibers became uneven and then the flocs began to form at the critical shear rate; and over the critical shear rate, the fibers began to uniformly disperse again, i.e., the flocs disappeared. The two-dimensional fast Fourier transform (FFT) technique and Guinier approximation were used to obtain the radius of gyration, Rg, of the flocs. It is considered that the Rg of the flocs is useful to characterize the mechanism of fiber flocculation.

A thermodynamics-based model and data for Ca, Mg, and Na ion partitioning in kraft pulp fibre suspensions

Article A thermodynamics-based model and data for Ca, Mg, and Na ion partitioning in kraft pulp fibre suspensions was published on May 1, 2000 in the journal Nordic Pulp & Paper Research Journal (volume 15, issue 2).

Heat transfer in the drag reducing regime of wood pulp fibre suspensions

Heat transfer to suspensions of different grades of wood pulp fibres at different concentrations were investigated over a range of flow rates. It was observed that a small amount of fibre in suspension causes drag reduction and reduces the heat transfer coefficient below that for water. Both effects are a function of concentration, flow rate and fibre type. The more flexible fibres produce a greater level of drag reduction and a greater lowering of heat transfer coefficient. The measurement of heat transfer coefficient provides a method of comparing fibres of different flexibility for papermaking.

The use of a mixing-sensitive chemical reaction for the study of pulp fibre suspension mixing

The competitive, consecutive chemical reactions between 1-naphthol and diazotized sulfanilic acid were utilized to study the mixing of a pulp fibre suspension in a 22 L stirred tank reactor. Mixing quality was determined from the distribution between the mono and bis substituted reaction products once a correlation was made for the adsorption of the product dyes onto the suspended fibres. The technique was found to be adequate for assessing micromixing and turbulence intensity within a fibre suspension provided the measured product distribution, Xs, was between 0.4 and 0.01. Thus the mixing conditions that could be assessed depended on both the energy dissipation within the mixer and the amount of the fibre present. For the experimental conditions chosen for this study energy dissipation rates would typically have to be less than 80 W/kg and the suspension mass concentration less than 2.5%. When compared with water, a reduction in turbulence levels at both the impeller zone and a remote zone in the stirred vessel was observed for fibre mass concentrations as low as 0.5%. The turbulence decreased as the suspension mass concentration was increased. This decrease is attributed to energy dissipation by friction at fibre-fibre contact points as the fibres move relative to one another in the flow. This sink removes energy from the turbulence cascade which never shows up as small-scale fluid deformations leading to better mixing. On a utilise les reactions chimiques consecutives concurrentes entre le 1-naphtol et I'acide sulfanilique diazotise dans le but d'etudier le melange d'une suspension de fibres de pulpe dans un reacteur agite de 22 L. La qualite du melange a ete determinee a partir de la distribution des produits de reaction mono et bi-substitues apres qu'une correlation a ete etablie pour I'adsorption des matieres colorantes des produits dans les fibres suspendues. La technique s'est averee adequate pour l'estimation du micromelange et de I'intensite de la turbulence dans une suspension de fibres lorsque la distribution de produit mesuree, Xs, est comprise entre 0,4 et 0,0l. Ainsi, les conditions de melange qui peuvent etre estimees dependent la afois de la dissipation de I'energie dans le melangeur et de la quantite de fibre. Pour les conditions expeimentales choisies dans cette etude, les taux de dissipation de I'energie devraient etre typiquement inferieurs a 80 W/kg et la concentration des matieres de suspension inferieure a 2.5%. Si l'on fait des comparaisons avec I'eau, une eduction des niveaux de turbulence dans la zone de la turbine et loin de la turbine a ete observee pour des concentrations en fibres aussi faibles que 0,5%. La turbulence diminue lorsque la concentration de matieres en suspension est augmentee. Cette baisse est attribuable a la dissipation de l'energie par la friction aux points de contact fibres-fibres lorsque que les fibres se deplacent l'une par rapport a l'autre dans I'ecoulement. Ces interactions puisent de Ietnergie de la cascade de turbulence qui ne peut donc pas &re utilisee pour les deformations de fluides a petite echelle assocites a un meilleur melange.

Linear and nonlinear dynamic viscoelasticity of pulp fiber suspensions

Linear and nonlinear dynamic viscoelasticity of pulp fiber suspensions

Water-cellulose interaction in wood pulp fiber suspensions studied by oxygen-17 and deuterium NMR relaxation. The effect of beating

Water-cellulose interaction in wood pulp fiber suspensions studied by oxygen-17 and deuterium NMR relaxation. The effect of beating

Motion of pulp fibre suspensions in rotary devices

AbstractSuspensions of semi‐bleached kraft pulp at low and medium mass concentrations were sheared in a rotary shear tester at angular velocities up to 524 rad/s (5000 rpm). The use of baffled housings, with rotors having lugs, imposed shear within the body of the suspensions and prevented slip at the vessel walls. The torque was measured as a function of angular velocity, and high speed photographs were taken through a transparent plate to aid in interpretation of the findings. The torque vs. angular velocity curves and flow phenomena were found to depend on the mass concentration of the suspension, the gap width between the rotor and housing, and the air content of the suspension. With little gas present, the flow initially involved tangential flow in an active cavity whose radial extent increased with increasing angular velocity. When the flow reached the outer baffles, a marked flow transition occurred, probably explaining what has been called “fluidization” by Gullichsen and Harkonen (1981). As the gas content of the suspension increased, the gas congregated around the rotor, impeding the transfer of momentum to the suspension. The apparent viscosity of a 10% by mass suspension was of the order of 16 Pa · s under the test conditions.

EFFECT OF SUSPENDED FIBRES ON MACRO-MIXING AND MICRO-MIXING IN A STIRRED TANK REACTOR

Abstract The effects of suspended fibre on macroscale and microscale mixing in a small stirred tank reactor were studied under batch conditions using the competitive, consecutive azo coupling between 1-naphthol and diazotized sulfanilic acid. The mixing quality was determined from the distribution between the mono and bis substituted reaction products. Nylon (2 and 3 millimeters in length) and fully bleached softwood kraft pulp (FBK) fibre suspensions were examined at volumetric concentrations up to the limit that complete suspension motion could be maintained in the vessel at impellet rotational speeds of 7 and 10s−1. The adsorption of the product dyes on the fibre was found to be proportional or very nearly proportional to their concentrations in the aqueous phase and did not interfere with the assessment of mixing in the suspension. Suspended fibres were found to slightly increase the ‘critical feed time’ of the diazotized sulfanilic acid, corresponding to an increase in the macromixing or bulk blend lime of the vessel. Continued increase in the fibre concentration led to the formation of a well mixed cavern centered on the impeller and the creation of stagnant regions adjacent to the vessel walls. The departure from the Newtonian fibre-free case is due to changes in the flow through and distribution of the energy dissipative regions in the vessel. The most effective location for chemical addition to ensure good microscale mixing remains in the impeller vicinity.