Mixtures of xanthan and guar gum in aqueous solution were studied in two flow situations: simple shear and porous media. In addition, solids transport in vertical annular flow of sand suspensions was ...explored. The zero shear rate viscosity of the solutions displayed a pronounced synergy: the viscosity of the mixture is higher than that of the polymer solutions in a wide range of relative concentrations of the two polymers, in agreement with previous literature. However, at relatively high shear rates, the viscosity approaches the value of the more viscous xanthan gum solutions at mass fractions of xanthan gum between 0.1 and 0.15, and the degree of synergy substantially decreases. Stress relaxation experiments in simple shear indicate that the polymer mixtures exhibit a well-defined yield stress after relaxation that is absent in solutions of pure polymers. In porous media flow experiments, a synergistic behavior mimicking the shear flow results was obtained for the polymer mixtures at low shear rates. However, at a critical shear rate, the apparent viscosity in porous media flows exceeds the shear viscosity due to the elongational nature of flow in the pores. The solids transport capacity in annular flows is well-represented by trends in shear viscosity and stress relaxation behavior. However, the lack of viscosity synergy at high shear rates limits the applicability of the mixtures as a way to improve solids suspension capacity in annular flows.
In this work we present an experimental study of non-Newtonian solid–liquid suspension flow in the annular space of a concentric-pipe system that simulates the geometry of wells during drilling for ...petroleum exploration and production, excluding effects of drill rotation. The liquids used were solutions of biodegradable guar gum at various concentrations and the solid phase was siliceous sand. We studied how the mean annular velocity, polymer concentration and degree of polymer cross-linking affect axial solids distribution. The results show that the solids suspension capacity of the solutions is enhanced by the presence of polymer: the addition of guar gum decreases the minimum annular velocity needed to achieve a homogeneous axial solids distribution in the annular region. However, the amount of solids that is carried by the solutions is not always increased in the presence of polymer. The chemical cross-linking of guar gum with borax promotes a substantial increase of transported solids in the annular space. Overall, the results presented show that relatively high viscosities, high polymer relaxation times under shear flow and non-zero residual shear stresses are desired rheological characteristics for a drilling fluid to obtain a uniform solids distribution in the annulus, as well as high solids carrying capacity.
In this work we perform an experimental study of the hydrodynamics of semibatch slurry bubble columns in which the liquid phase is a solution of a high molecular weight hydrolyzed polyacrylamide. ...Experiments were conducted in bubble columns with two different diameters (10 and 29 cm.) In gas-liquid operation, the gas holdup decreases as the polymer concentration increases due to the formation of large stable bubbles. In the presence of high solids concentration, this trend is reversed in the larger column due to an increase in the number of small bubbles and an appreciable increase in the residence times of the large bubbles brought about by an increase in slurry circulation. The distribution of solid phase becomes more uniform as the polymer concentration increases. This trend is quantified by means of the application of a modified form of the sedimentation-dispersion model. The macromolecular conformation of the polymer in solution is altered by adding NaCl, whose positive ions screen negative charges in the polymer molecule producing a contraction of the coil. At the concentration levels used in this work, the addition of salt decreases the ability of the polymer to produce a more uniform solids distribution.
This work presents experimental data on gas holdup in slurry bubble columns with a foaming liquid. The effects of solids concentration, solid particle size, superficial phase velocities and column ...dimensions on the gas holdup are analyzed. At low superficial gas velocities (less than 4cm/s), for which the liquid does not foam, the presence of solids with small particle size does not affect the gas holdup whereas solids with large particle size induce foam formation and thus their presence increases the gas holdup. In the foaming regime, an increase of solids concentration decreases the gas holdup. The operating mode has a strong effect on the gas holdup: the semi-batch operating mode (stagnant liquid-solid suspension) increases the ability of the liquid to foam with respect to the continuous mode. Regarding the effect of column dimensions, the results presented show that the height of the bubble column does not affect at an appreciable extent the gas holdup in the range 6 < LID < 12. At high gas velocities (greater than 6 cm/s) the gas holdups obtained in a 30 cm-internal diameter column are the same as those measured in a 10 cm-internal diameter column.
In this work we perform an experimental study of the spatial distribution of phases in slurry bubble columns with conical distributors that have a volume comparable to that of the cylindrical ...section. Three different distributors were used whose apex angles were 13°, 22° and 34°. In gas-liquid operation, the gas holdups are axially uniform in the cylindrical section and decrease towards the wall, whereas in the conical section they increase towards the inlet. These trends are observed in the three cones for all the operating conditions explored. The solids distributions in the conical sections are qualitatively different depending on whether the operation is semibatch or continuous with respect to the flow of solid-liquid suspension: in semibatch operation, the concentration monotonically increases towards the bottom of the cone and exhibits a slight increase as the wall is approached; in continuous operation, an absolute maximum in solids concentration is obtained at a point located on the wall of the cone and intermediate height. The location of this maximum moves upwards as the total solids content in the column increases and as the apex angle decreases. The maximum in solids concentration signals the most probable site for the onset of solids sedimentation and the presence of low mixing levels and reduced mass transfer rates in a slurry reactor. In the range of conditions explored in the present work, the lowest apex angle (13°) yields a more uniform solids distribution throughout the system
This work presents an experimental study of solids suspension in upward annular flow using non-Newtonian biopolymer solutions as liquid phases. The biopolymers employed were xanthan and guar gums, ...and the solid phase consisted of silica sand with a mean particle size of 250 um. The experimental equipment is a vertical concentric pipe system, with 3 m of height and a hydraulic diameter of 0.15 m, simulating the drilling process at room temperature without the drill rotation effects. The annular velocity, concentration of load and polymer were the most important variables studied. It was found that the transport ratio allows a simple quantification of the effects of the polymer as a viscosifying additive in drilling muds, independently of the solids loading. xanthan gum provides a better suspension capacity than Guar gum_at similar concentrations, especially at low flow velocities, while at high velocities the solids transport is similar for both polymeric solutions, due to their pseudoplasticity.
In Alzheimer's disease (AD), the distribution of the amyloid precursor protein (APP) and its fragments other than amyloid beta, has not been fully characterized. Here, we investigate the distribution ...of APP and its fragments in human AD brain samples and in mouse models of AD in reference to its proteases, synaptic proteins, and histopathological features characteristic of the AD brain, by combining an extensive set of histological and analytical tools. We report that the prominent somatic distribution of APP observed in control patients remarkably vanishes in human AD patients to the benefit of dense accumulations of extra‐somatic APP, which surround dense‐core amyloid plaques enriched in APP‐Nter. These features are accentuated in patients with familial forms of the disease. Importantly, APP accumulations are enriched in phosphorylated tau and presynaptic proteins whereas they are depleted of post‐synaptic proteins suggesting that the extra‐somatic accumulations of APP are of presynaptic origin. Ultrastructural analyses unveil that APP concentrates in autophagosomes and in multivesicular bodies together with presynaptic vesicle proteins. Altogether, alteration of APP distribution and its accumulation together with presynaptic proteins around dense‐core amyloid plaques is a key histopathological feature in AD, lending support to the notion that presynaptic failure is a strong physiopathological component of AD.
Objective
Memory impairment is a hallmark cognitive deficit in Parkinson’s disease (PD). However, it remains unclear which processes underlie this deficit in PD. Also, little is known on these ...patients’ subjective experiences of memory difficulties and their relationship with objective measures. We aim to portray memory deficits in PD by combining objective and subjective memory measures.
Methods
Fifteen PD patients and 15 controls were assessed with an extended version of the Face-Name Associative Memory Exam (FNAME) and the Memory Failures of Everyday Questionnaire (MFE-28). We also explored the relationship among clinical and cognitive variables.
Results
Participants with PD presented with more memory complaints. On the FNAME, these patients exhibited lower performance in free recall, as well as in name recognition and matching. Importantly, when controlling for initial learning, group effects disappeared, except for matching. Associative memory therefore was significantly compromised in PD and correlated with subjective memory complaints (SMC).
Conclusion
Our findings suggest that associative memory may constitute a sensitive measure to detect subtle memory deficits in PD. Moreover, the current study further clarifies the source of memory impairment in PD. Thus, our study highlights the clinical value of including associative memory tests such as the FNAME in PD neuropsychological assessment.
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