Drainage of high-consistency fiber-laden foams

Lightweight lignocellulosic fibrous mate- rials (LLFMs) offer a sustainable and biodegradable alternative in many applications. Enthusiastic interest in these materials has recently grown together with the newly risen interest in foam forming. Foam bubbles restrain fiber flocculation, and foam formed structures have high uniformity. Moreover, the bubbles support the fibrous structure during manufacturing enabling the formation of highly porous structures. Mechanical pressure cannot be applied in the manufacture of LLFMs as the materials would lose their porous structure. Water is therefore typically removed by a combination of drainage and thermal drying. Thermal drying of porous materials has been studied inten- sively. However, there are only a few studies on the drainage of fiber-laden foams. Thus, in this work, we conducted a systematic analysis of this topic. Our findings show that after drainage a stationary vertical moisture profile similar to that of pure foams is developed. Raising the initial fiber consistency was found to increase the final fiber consistency of the foam until the drainage ceased. Increasing mold height was found to increase the final consistency consider- ably. Without vacuum and heating, the shrinkage of samples during drainage was only slightly higher than the volume of the drained water. Drainage rate and final consistency increased clearly with increasing vacuum, but simultaneously sample shrinkage increased considerably. The best compromise was obtained with a vacuum of 0.5 kPa, which increased the final consistency by 60% without extra shrinkage. Using warm foam and heating the foam during drainage increased the final consistency considerably, but this also led to significant shrinkage of the sample

Dewatering and structural analysis of foam-formed, lightweight fibrous materials

This work studied the effect of the dewatering conditions on the behavior of fiber foams during dewatering and on the final structure of the formed, thick, lightweight lignocellulosic materials. The vacuum level, fiber type, consistency, and basis weight of the fiber foam were all varied. During dewatering, the time evolution of the thickness of the fiber foam in the mold was studied, and the dryness of the fiber foam immediately after dewatering was measured. The density and pore size profiles of the final dry materials was measured using X-ray microtomography (µCT)

Drainage of high-consistency fiber-laden foams

Lightweight lignocellulosic fibrous mate- rials (LLFMs) offer a sustainable and biodegradable alternative in many applications. Enthusiastic interest in these materials has recently grown together with the newly risen interest in foam forming. Foam bubbles restrain fiber flocculation, and foam formed structures have high uniformity. Moreover, the bubbles support the fibrous structure during manufacturing enabling the formation of highly porous structures. Mechanical pressure cannot be applied in the manufacture of LLFMs as the materials would lose their porous structure. Water is therefore typically removed by a combination of drainage and thermal drying. Thermal drying of porous materials has been studied inten- sively. However, there are only a few studies on the drainage of fiber-laden foams. Thus, in this work, we conducted a systematic analysis of this topic. Our findings show that after drainage a stationary vertical moisture profile similar to that of pure foams is developed. Raising the initial fiber consistency was found to increase the final fiber consistency of the foam until the drainage ceased. Increasing mold height was found to increase the final consistency consider- ably. Without vacuum and heating, the shrinkage of samples during drainage was only slightly higher than the volume of the drained water. Drainage rate and final consistency increased clearly with increasing vacuum, but simultaneously sample shrinkage increased considerably. The best compromise was obtained with a vacuum of 0.5 kPa, which increased the final consistency by 60% without extra shrinkage. Using warm foam and heating the foam during drainage increased the final consistency considerably, but this also led to significant shrinkage of the sample

Dewatering and structural analysis of foam-formed, lightweight fibrous materials

This work studied the effect of the dewatering conditions on the behavior of fiber foams during dewatering and on the final structure of the formed, thick, lightweight lignocellulosic materials. The vacuum level, fiber type, consistency, and basis weight of the fiber foam were all varied. During dewatering, the time evolution of the thickness of the fiber foam in the mold was studied, and the dryness of the fiber foam immediately after dewatering was measured. The density and pore size profiles of the final dry materials was measured using X-ray microtomography (µCT)

Pipe rheology of microfibrillated cellulose suspensions

The shear rheology of two mechanically manufactured microfibrillated cellulose (MFC) suspensions was studied in a consistency range of 0.2 - 2.0% with a pipe rheometer combined with ultrasound velocity profiling. The MFC suspensions behaved at all consistencies as shear thinning power law fluids. Despite their significantly different particle size, the viscous behavior of the suspensions was quantitatively similar. For both suspensions, the dependence of yield stress and the consistency index on consistency was a power law with an exponent of 2.4, similar to some pulp suspensions. The dependence of flow index on consistency was also a power law, with an exponent of -0.36. The slip flow was very strong for both MFCs and contributed up to 95% to the flow rate. When wall shear stress exceeded two times the yield stress, slip flow caused drag reduction with consistencies higher than 0.8%. When inspecting the slip velocities of both suspensions as a function of wall shear stress scaled with the yield stress, a good data collapse was obtained. The observed similarities in the shear rheology of both the MFC suspensions and the similar behavior of some pulp fiber suspensions suggests that the shear rheology of MFC suspensions might be more universal than has previously been realized

Pipe rheology of microfibrillated cellulose suspensions

The shear rheology of two mechanically manufactured microfibrillated cellulose (MFC) suspensions was studied in a consistency range of 0.2 - 2.0% with a pipe rheometer combined with ultrasound velocity profiling. The MFC suspensions behaved at all consistencies as shear thinning power law fluids. Despite their significantly different particle size, the viscous behavior of the suspensions was quantitatively similar. For both suspensions, the dependence of yield stress and the consistency index on consistency was a power law with an exponent of 2.4, similar to some pulp suspensions. The dependence of flow index on consistency was also a power law, with an exponent of -0.36. The slip flow was very strong for both MFCs and contributed up to 95% to the flow rate. When wall shear stress exceeded two times the yield stress, slip flow caused drag reduction with consistencies higher than 0.8%. When inspecting the slip velocities of both suspensions as a function of wall shear stress scaled with the yield stress, a good data collapse was obtained. The observed similarities in the shear rheology of both the MFC suspensions and the similar behavior of some pulp fiber suspensions suggests that the shear rheology of MFC suspensions might be more universal than has previously been realized

Pipe rheology of wet aqueous application foams

Chemical Engineering Science https://doi.org/10.1016/j.ces.2023.119282 Foam application of chemicals to the wet web is currently being developed for the paper and board industry. An important part of this work is to understand the rheology of the used application foams. Polyvinyl alcohol (PVOH) is widely used as a strength additive in paper and board, and it was the main surfactant in this study. The PVOH foam density varied between 100 kg/m3 and 300 kg/m3 and the dosage of PVOH varied between 0.5% to 6%. The foam viscosity and slip flow were determined with a pipe rheometer using three pipe diameters. The slip velocity was quantified by recording the foam motion in the vicinity of the wall of an acrylic pipe with a high-speed video camera. A measurement setup was also built for measuring the slip flow indirectly in opaque pipes. General formulas for the foam viscosity and slip flow, based on several physical quantities describing both the foam and the base liquid, were obtained using dimensional analysis. Specifically, dimensionless shear stress and dimensionless wall shear stress were found to be proportional to certain powers of the capillary number and slip capillary number, respectively. The contribution of the slip flow to the total flow rate was significant, especially with lower flow rates when most of the volumetric flow was due to the slip. In the literature, many papers have suggested that there is no slip flow in steel pipes. Our results suggest that this is due to the high pipe roughness used in those works. In our measurements, the slip velocity of a smooth-walled steel pipe was equal to the slip in an acrylic pipe. The obtained viscosity and slip models form a solid basis for developing and running various industrial processes including foam application processes. For new foam recipes, quite a small number of rheological measurements are needed to determine the model parameters

Pipe rheology of wet aqueous application foams

Chemical Engineering Science https://doi.org/10.1016/j.ces.2023.119282 Foam application of chemicals to the wet web is currently being developed for the paper and board industry. An important part of this work is to understand the rheology of the used application foams. Polyvinyl alcohol (PVOH) is widely used as a strength additive in paper and board, and it was the main surfactant in this study. The PVOH foam density varied between 100 kg/m3 and 300 kg/m3 and the dosage of PVOH varied between 0.5% to 6%. The foam viscosity and slip flow were determined with a pipe rheometer using three pipe diameters. The slip velocity was quantified by recording the foam motion in the vicinity of the wall of an acrylic pipe with a high-speed video camera. A measurement setup was also built for measuring the slip flow indirectly in opaque pipes. General formulas for the foam viscosity and slip flow, based on several physical quantities describing both the foam and the base liquid, were obtained using dimensional analysis. Specifically, dimensionless shear stress and dimensionless wall shear stress were found to be proportional to certain powers of the capillary number and slip capillary number, respectively. The contribution of the slip flow to the total flow rate was significant, especially with lower flow rates when most of the volumetric flow was due to the slip. In the literature, many papers have suggested that there is no slip flow in steel pipes. Our results suggest that this is due to the high pipe roughness used in those works. In our measurements, the slip velocity of a smooth-walled steel pipe was equal to the slip in an acrylic pipe. The obtained viscosity and slip models form a solid basis for developing and running various industrial processes including foam application processes. For new foam recipes, quite a small number of rheological measurements are needed to determine the model parameters