D. Mark Martinez

Linear stability of a Berman flow in a channel partially filled with a porous medium

The temporal stability of similarity solutions for an incompressible fluid moving in a channel partially filled with a porous medium is analyzed. A constant wall suction acting on the bottom surface of the porous medium drives the fluid; the upper wall of the channel is impermeable. This work extends the work of King and Cox [“Asymptotic analysis of the steady-state and time-dependent Berman problem,” J. Eng. Math. 39, 87 (2001)] to a wider class of similarity solutions where coupled flow, both above and through a porous medium, is considered. In this work, a similarity transform is proposed which satisfies both the Navier–Stokes equation in the clear fluid portion of the domain and the Brinkman extended Darcy law relationship in the porous medium. The boundary conditions between the clear fluid and porous regions are those outlined by Ochoa-Tapia and Whitaker [“Momentum transfer at the boundary between a porous medium and a homogeneous fluid I: theoretical development,” Int. J. Heat Mass Transfer 38, 263...

Increasing efficiency of enzymatic hemicellulose removal from bamboo for production of high-grade dissolving pulp

To improve the efficiency of enzymatic hemicellulose removal from bamboo pre-hydrolysis kraft pulp, mechanical refining was conducted prior to enzyme treatment. Refining significantly improved the subsequent hemicellulose removal efficiency by xylanase treatment. Results showed that when PFI refining was followed by 3h xylanase treatment, the xylan content of the bamboo pre-hydrolysis kraft pulp (after first stage oxygen delignification) could be decreased to 2.72% (w/w). After bleaching of enzyme treated pulp, the alpha-cellulose content was 93.4% (w/w) while the xylan content was only 2.38%. The effect of refining on fibre properties was investigated in terms of freeness, water retention value, fibre length and fibrillation characteristics. The brightness, reactivity and viscosity were also determined to characterize the quality of final pulp. Results demonstrated the feasibility of combining refining and xylanase treatment to produce high quality bamboo dissolving pulp.

Insight into the evolution of the proton concentration during autohydrolysis and dilute-acid hydrolysis of hemicellulose

BackgroundDuring pretreatment, hemicellulose is removed from biomass via proton-catalyzed hydrolysis to produce soluble poly- and mono-saccharides. Many kinetic models have been proposed but the dependence of rate on proton concentration is not well-defined; autohydrolysis and dilute-acid hydrolysis models apply very different treatments despite having similar chemistries. In this work, evolution of proton concentration is examined during both autohydrolysis and dilute-acid hydrolysis of hemicellulose from green bamboo. An approximate mathematical model, or “toy model”, to describe proton concentration based upon conservation of mass and charge during deacetylation and ash neutralization coupled with a number of competing equilibria, was derived. The model was qualitatively compared to experiments where pH was measured as a function of time, temperature, and initial acid level. Proton evolution was also examined at room temperature to decouple the effect of ash neutralization from deacetylation.ResultsThe toy model predicts the existence of a steady-state proton concentration dictated by equilibrium constants, initial acetyl groups, and initial added acid. At room temperature, it was found that pH remains essentially constant both at low initial pH and autohydrolysis conditions. Acid is likely in excess of the neutralization potential of the ash, in the former case, and the kinetics of neutralization become exceedingly small in the latter case due to the low proton concentration. Finally, when the hydrolysis reaction proceeded at elevated temperatures, one case of non-monotonic behavior in which the pH initially increased, and then decreased at longer times, was found. This is likely due to the difference in rates between neutralization and deacetylation.ConclusionsThe model and experimental work demonstrate that the evolution of proton concentration during hydrolysis follows complex behavior that depends upon the acetyl group and ash content of biomass, initial acid levels and temperature. In the limit of excess added acid, pH varies very weakly with time. Below this limit, complex schemes are found primarily related to the selectivity of deacetylation in comparison to neutralization. These findings indicate that a more rigorous approach to models of hemicellulose hydrolysis is needed. Improved models will lead to more efficient acid utilization and facilitate process scale-up.

A novel low inertia shear flow instability triggered by a chemical reaction

We present an experimental investigation of a novel low Reynolds number shear flow instability triggered by a chemical reaction. An acid-base reaction taking place at the interface between a Newtonian fluid and carbopol-940 solution leads to a strong viscosity stratification, which locally destabilizes the flow. Our experimental observations are made in the context of a miscible displacement flow, for which the flow instability promotes local mixing and subsequently improves the displacement efficiency. The experimental study is complemented by a simplified normal mode analysis to shed light on the origin of the instability.

An analytical approach of thermodynamic behavior in a gas target system on a medical cyclotron

An analytical model has been developed to study the thermo-mechanical behavior of gas targets used to produce medical isotopes, assuming that the system reaches steady-state. It is based on an integral analysis of the mass and energy balance of the gas-target system, the ideal gas law, and the deformation of the foil. The heat transfer coefficients for different target bodies and gases have been calculated. Excellent agreement is observed between experiments performed at TRIUMFs 13 MeV cyclotron and the model.

Dewatering of fibre suspensions by pressure filtration

A theoretical and experimental study of dewatering of fibre suspensions by uniaxial compression is presented. Solutions of a one-dimensional model are discussed and asymptotic limits of fast and slow compression are explored. Particular focus is given to relatively rapid compression and to the corresponding development of spatial variations in the solidity and velocity profiles of the suspension. The results of complementary laboratory experiments are presented for nylon or cellulose fibres suspended in viscous fluid. The constitutive relationships for each suspension were measured independently. Measurements of the load for different fixed compression speeds, together with some direct measurements of the velocity profiles using particle tracking velocimetry, are compared with model predictions. The comparison is reasonable for nylon, but poor for cellulose fibres. An extension to the model, which allows for a strain-rate-dependent component in the network stress, is proposed, and is found to give a drama...

Modeling the pressure rise of a liquid target on a medical cyclotron: Steady-state analysis

The steady-state behaviour of a liquid target used to produce medical isotopes by low-energy cyclotrons is studied. A model based on the conservation of mass and energy is proposed to describe the pressure rise of the target assuming equilibrium between liquid and vapour phases during irradiation. The effects of water radiolysis are taken into account. Excellent agreement is achieved between the model, and both constant-temperature bath tests and experiments conducted on a 13MeV cyclotron at TRIUMF.

Pumping performance increase through the addition of turbulent drag-reducing polymers to pulp fibre suspensions

The addition of a small amount of long chain polymers to a turbulent fluid is known to reduce the wall shear stress and drag. Similarly, the addition of pulp fibres to a turbulent suspension is also turbulent-drag reducing despite pulp fibres having a length scale that is 1000 times larger than polymer molecules. The mechanism of drag reduction and its impact on centrifugal pump performance is poorly understood, especially when there is a combination of polymer and fibres in suspension. Centrifugal (slurry) pump performance was measured as a function of pulp fibre and PAM polymer concentration. Both the pump best efficiency and maximum head rise were greater when pumping modest concentrations of polymer solutions and low consistency pulp fibre than pure water. We measured an efficiency increase of 22 percent and a maximum head increase of 4.3 percent with the addition of 150 ppm PAM polymer relative to pure water. We measured an increase of 8 percent and 2.3 percent in pump efficiency and maximum head coefficient, respectively, with 2 percent pulp fibres over that of water alone. With both 1 percent consistency pulp fibres and 100 ppm of PAM polymers, we measured a 12 percent increase in efficiency over that of pulp suspension alone. With both 2 percent consistency pulp fibres and 100 ppm of PAM polymers, we measured an 8 percent increase in efficiency over that of pulp fibre suspension alone. The reasons for the increased pump efficiency with addition of additives is not known but are thought to be due to the turbulent-drag-reducing properties associated with flow of these suspensions.Copyright

Investigation of low consistency reject refining of mechanical pulp for energy savings

Abstract In this study, the effects of low consistency refining (LCR) energy and intensity on mechanical pulp properties have been studied for three different types of reject pulps (softwood TMP, softwood CTMP and hardwood CTMP), which were refined at varying intensity. Resulting pulp properties have been compared with high consistency refining (HCR) of the same reject pulps. For all furnish types, it was shown that LCR can develop pulp properties matching those developed through HCR with significantly less energy. The resulting pulp properties were found to be affected not only by refining intensity and energy, but also by initial fibre morphology. Pilot LCR trials demonstrated that high freeness reject pulp is initially insensitive to refining intensity as specific energy is applied. This enables the first stage of LCR to be carried out at a higher specific energy and intensity, which can reduce the number of stages of LCR required to reach a target quality. This work shows that low intensity LCR is capable of achieving the same tensile index as HCR pulp at a target freeness of 200 ml CSF.

Pressure rise in medical cyclotron liquid targets: Transient analysis

Transient behavior of proton-beam bombarded liquid-targets are studied at various initial conditions at the TR13 cyclotron at TRIUMF. Depending on the initial condition, experiments show a range of different responses from steady-state to self-sustained oscillations. To address this, a system of equations based on the conservation of mass and energy is proposed. Coupling between the beam and fluid-density and chemical reactions driven by the beam (radiolysis) are identified as the main reasons to describe this behavior. Excellent qualitative agreements are achieved.