The Experts below are selected from a list of 65664 Experts worldwide ranked by ideXlab platform
S.s. Alves - One of the best experts on this subject based on the ideXlab platform.
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effect of bubble contamination on gas liquid mass transfer coefficient on co2 absorption in amine solutions
2008Co-Authors: Rocio Maceiras, S.s. Alves, Angeles M Cancela, Estrella AlvarezAbstract:Abstract An electrochemical method was used to follow CO 2 absorption both in water and in alkanolamine solutions in a bubble column (∼1 m tall). This method allows the determination of local mass transfer coefficients along the column. No special care was taken in avoiding trace contaminants. It was found that bubbles contaminate mostly at the gas distributor. Gas–liquid mass transfer coefficient decreases as bubbles rise along the column, taking values closer to those expected for clean bubbles with a mobile surface at the bottom of the column, and values closer to those expected for rigid bubbles, at the top of the column. If this is quantitatively interpreted within the framework of the stagnant Cap Model, it may be concluded that this decrease is mainly due to bubble shrinkage, which leads to a greater fraction of bubble area being covered by the stagnant Cap. Compared with this effect, the effect of further acquisition of contaminant molecules by the bubbles is negligible. The above conclusions can be drawn both for absorption with chemical reaction in amine solutions and for pure absorption of CO 2 in tap water, although in this case the shrinkage effect is less pronounced.
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Effect of bubble contamination on rise velocity and mass transfer
2005Co-Authors: S.s. Alves, Sandra Orvalho, Jorge M.t. VasconcelosAbstract:Abstract An apparatus where individual bubbles are kept stationary in a downward liquid flow was adapted to simultaneously (i) follow mass transfer to/from a single bubble as it inevitably gets contaminated; (ii) follow its shape; and (iii) periodically measure its terminal velocity. This apparatus allows bubbles to be monitored for much longer periods of time than does the monitoring of rising bubbles. Thus, the effect of trace contaminants on bubbles of low solubility gases, like air, may be studied. Experiments were done with air bubbles of 1–5 mm initial equivalent diameter in a water stream. The partial pressure of air in the liquid could be manipulated, allowing bubbles to be either dissolving or kept at an approximately constant diameter. Both drag coefficient and gas–liquid mass transfer results were interpreted in terms of bubble contamination kinetics using a simplified stagnant Cap Model. Drag coefficient was calculated from stagnant Cap size using an adaptation of Sadhal and Johnson's Model (J. Fluid Mech. 126 (1983) 237). Gas–liquid mass transfer Modelling assumed two mass transfer coefficients, one for the clean front of the bubble, the other for the stagnant Cap. Adjusted values of these coefficients are consistent with theoretical predictions from Higbie's and Frossling's equations, respectively.
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gas liquid mass transfer coefficient in stirred tanks interpreted through bubble contamination kinetics
2004Co-Authors: S.s. Alves, C I Maia, Jorge M.t. VasconcelosAbstract:Abstract Experimental data on the average mass transfer liquid film coefficient (kL) in an aerated stirred tank are presented. Liquid media used were tap water, electrolyte solutions and water with controlled addition of tensioactive material. Values of kL range from those expected for bubbles with a mobile surface to those expected for rigid bubbles. These data are quantitatively interpreted in terms of bubble contamination kinetics, using a stagnant Cap Model, according to which bubbles suddenly change from a mobile interface to a rigid condition when surface tension gradients, caused by surfactant accumulation, balance out shear stress.
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effect of contaminants on mass transfer coefficients in bubble column and airlift contactors
2003Co-Authors: Jorge M.t. Vasconcelos, S.s. Alves, Sandra Orvalho, J M L Rodrigues, Rui L Mendes, Alberto ReisAbstract:Abstract In this work, the effects of surface-active contaminants on mass transfer coefficients k L a and k L were studied in two different bubble contactors. The oxygen transfer coefficient, k L , was obtained from the volumetric oxygen transfer coefficient, k L a , since the specific interfacial area, a , could be determined from the fractional gas holdup, e , and the average bubble diameter, d 32 . Water at different heights and antifoam solutions of 0.5– 100 ppm were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and 0.85×10 −3 m 3 Capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k L a and k L values over a span of almost 400%. A theoretical interpretation is proposed based on Modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant Cap Model. Model results match experimental k L data within ±30%.
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gas liquid mass transfer to single bubbles effect of surface contamination
2002Co-Authors: Jorge M.t. Vasconcelos, Sandra Orvalho, S.s. AlvesAbstract:The dissolution of single bubbles of gases of low solubility kept stationary in a downward stream of water was studied. In “clean” water, two regimes are identified. Initially, the process is fast, consistent with the theory for circulating bubbles. Then, the mass-transfer rate falls sharply to that predicted for solid spheres. Transition times and transition diameters vary widely with experimental conditions. In untreated water, only the second regime is found. Results are explained in terms of the kinetics of trace surfactant accumulation at the interface. An adaptation of the stagnant-Cap Model is proposed, with surface immobilization expressed in terms of interface dynamics. The Model yields good prediction of the transition point for a very large set of conditions, including different gases at various concentrations in the liquid stream and a wide range of initial bubble diameters.
Jorge M.t. Vasconcelos - One of the best experts on this subject based on the ideXlab platform.
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Effect of bubble contamination on rise velocity and mass transfer
2005Co-Authors: S.s. Alves, Sandra Orvalho, Jorge M.t. VasconcelosAbstract:Abstract An apparatus where individual bubbles are kept stationary in a downward liquid flow was adapted to simultaneously (i) follow mass transfer to/from a single bubble as it inevitably gets contaminated; (ii) follow its shape; and (iii) periodically measure its terminal velocity. This apparatus allows bubbles to be monitored for much longer periods of time than does the monitoring of rising bubbles. Thus, the effect of trace contaminants on bubbles of low solubility gases, like air, may be studied. Experiments were done with air bubbles of 1–5 mm initial equivalent diameter in a water stream. The partial pressure of air in the liquid could be manipulated, allowing bubbles to be either dissolving or kept at an approximately constant diameter. Both drag coefficient and gas–liquid mass transfer results were interpreted in terms of bubble contamination kinetics using a simplified stagnant Cap Model. Drag coefficient was calculated from stagnant Cap size using an adaptation of Sadhal and Johnson's Model (J. Fluid Mech. 126 (1983) 237). Gas–liquid mass transfer Modelling assumed two mass transfer coefficients, one for the clean front of the bubble, the other for the stagnant Cap. Adjusted values of these coefficients are consistent with theoretical predictions from Higbie's and Frossling's equations, respectively.
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gas liquid mass transfer coefficient in stirred tanks interpreted through bubble contamination kinetics
2004Co-Authors: S.s. Alves, C I Maia, Jorge M.t. VasconcelosAbstract:Abstract Experimental data on the average mass transfer liquid film coefficient (kL) in an aerated stirred tank are presented. Liquid media used were tap water, electrolyte solutions and water with controlled addition of tensioactive material. Values of kL range from those expected for bubbles with a mobile surface to those expected for rigid bubbles. These data are quantitatively interpreted in terms of bubble contamination kinetics, using a stagnant Cap Model, according to which bubbles suddenly change from a mobile interface to a rigid condition when surface tension gradients, caused by surfactant accumulation, balance out shear stress.
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effect of contaminants on mass transfer coefficients in bubble column and airlift contactors
2003Co-Authors: Jorge M.t. Vasconcelos, S.s. Alves, Sandra Orvalho, J M L Rodrigues, Rui L Mendes, Alberto ReisAbstract:Abstract In this work, the effects of surface-active contaminants on mass transfer coefficients k L a and k L were studied in two different bubble contactors. The oxygen transfer coefficient, k L , was obtained from the volumetric oxygen transfer coefficient, k L a , since the specific interfacial area, a , could be determined from the fractional gas holdup, e , and the average bubble diameter, d 32 . Water at different heights and antifoam solutions of 0.5– 100 ppm were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and 0.85×10 −3 m 3 Capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k L a and k L values over a span of almost 400%. A theoretical interpretation is proposed based on Modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant Cap Model. Model results match experimental k L data within ±30%.
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gas liquid mass transfer to single bubbles effect of surface contamination
2002Co-Authors: Jorge M.t. Vasconcelos, Sandra Orvalho, S.s. AlvesAbstract:The dissolution of single bubbles of gases of low solubility kept stationary in a downward stream of water was studied. In “clean” water, two regimes are identified. Initially, the process is fast, consistent with the theory for circulating bubbles. Then, the mass-transfer rate falls sharply to that predicted for solid spheres. Transition times and transition diameters vary widely with experimental conditions. In untreated water, only the second regime is found. Results are explained in terms of the kinetics of trace surfactant accumulation at the interface. An adaptation of the stagnant-Cap Model is proposed, with surface immobilization expressed in terms of interface dynamics. The Model yields good prediction of the transition point for a very large set of conditions, including different gases at various concentrations in the liquid stream and a wide range of initial bubble diameters.
Bruno C Hancock - One of the best experts on this subject based on the ideXlab platform.
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a study on the sensitivity of drucker prager Cap Model parameters during the decompression phase of powder compaction simulations
2010Co-Authors: Tuhin Sinha, Jennifer S Curtis, Bruno C Hancock, Carl WassgrenAbstract:Abstract The compaction of pharmaceutical powders can be simulated using phenomenological elasto-plastic continuum Models adopted from soil mechanics. These Models are typically implemented in finite element codes and have been used recently to investigate the macroscopic property distributions in powders during compaction. The present study demonstrates the importance of obtaining accurate yield surface parameters for use in such Models. A commercial finite element code implementing the Drucker–Prager Cap (DPC) Model was used to Model the compression and decompression stages of powder compaction in a tabletting operation. The parameters used in the DPC Model were obtained from the literature. Although the compression stage of the process gave expected behavior, the decompression response was unrealistic for at least one set of published data. Small values for the friction and cohesion parameters resulted in a significant elastic recovery during decompression. This study demonstrates the need to obtain accurate parameter data in order to Model the decompression stage of powder compaction.
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a modified drucker prager Cap Model for die compaction simulation of pharmaceutical powders
2008Co-Authors: James A Elliott, A C Bentham, A Mills, Gregory E Amidon, Bruno C HancockAbstract:Abstract In this paper, we present a modified density-dependent Drucker-Prager Cap (DPC) Model to simulate the compaction behaviour of pharmaceutical powders. In particular, a nonlinear elasticity law is proposed to describe the observed nonlinear unloading behaviour following compaction. To extract the material parameters for the modified DPC Model, a novel experimental calibration procedure is used, based on uniaxial single-ended compaction tests using an instrumented cylindrical die. The Model is implemented in ABAQUS by writing a user subroutine, and a calibration process on microcrystalline cellulose (MCC) Avicel PH101 powders is detailed. The calibrated parameters are used for the manufacturing process simulation of two kinds of typical pharmaceutical tablets: the flat-face tablet and the concave tablet with single or double radius curvatures. The Model developed can describe not only the compression and decompression phases, but also the ejection phase. The Model is validated by comparing finite element simulations with experimental loading–unloading curves during the manufacture of 8 and 11 mm round tablets with flat-face (FF), single radius concave (SRC) and double radius concave (DRC) profiles. Moreover, the density and stress distributions during tabletting are used to analyse and explain the failure mechanism of tablets. The results show that the proposed Model can quantitatively reproduce the compaction behaviour of pharmaceutical powders and can be used to obtain the stress and density distributions during compression, decompression and ejection.
Roger Lawcock - One of the best experts on this subject based on the ideXlab platform.
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experimental calibration of density dependent modified drucker prager Cap Model using an instrumented cubic die for powder compact
2010Co-Authors: Baosheng Zhang, Mukesh Jain, Chenghao Zhao, Michael Bruhis, Roger LawcockAbstract:Theory and experimental calibration of density dependent modified Drucker-Prager/Cap (DPC) Model are presented by using a novel instrumented cubic die in powder compaction tests. The cubic die is designed for directly determining the loading and unloading forces and displacements of powder compact inside the die in compaction and transverse directions without any additional calibration. The Cap surface parameters and elastic properties are characterized by fitting stress and strain curves recorded during loading and unloading at different green density values and the plastic material parameters for failure surface are obtained by additional radial and axial compressive tests. The experimental data is subsequently used in the simulation of cubic die compaction to verify the results from the density dependent modified DPC Model.
Sandra Orvalho - One of the best experts on this subject based on the ideXlab platform.
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Effect of bubble contamination on rise velocity and mass transfer
2005Co-Authors: S.s. Alves, Sandra Orvalho, Jorge M.t. VasconcelosAbstract:Abstract An apparatus where individual bubbles are kept stationary in a downward liquid flow was adapted to simultaneously (i) follow mass transfer to/from a single bubble as it inevitably gets contaminated; (ii) follow its shape; and (iii) periodically measure its terminal velocity. This apparatus allows bubbles to be monitored for much longer periods of time than does the monitoring of rising bubbles. Thus, the effect of trace contaminants on bubbles of low solubility gases, like air, may be studied. Experiments were done with air bubbles of 1–5 mm initial equivalent diameter in a water stream. The partial pressure of air in the liquid could be manipulated, allowing bubbles to be either dissolving or kept at an approximately constant diameter. Both drag coefficient and gas–liquid mass transfer results were interpreted in terms of bubble contamination kinetics using a simplified stagnant Cap Model. Drag coefficient was calculated from stagnant Cap size using an adaptation of Sadhal and Johnson's Model (J. Fluid Mech. 126 (1983) 237). Gas–liquid mass transfer Modelling assumed two mass transfer coefficients, one for the clean front of the bubble, the other for the stagnant Cap. Adjusted values of these coefficients are consistent with theoretical predictions from Higbie's and Frossling's equations, respectively.
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effect of contaminants on mass transfer coefficients in bubble column and airlift contactors
2003Co-Authors: Jorge M.t. Vasconcelos, S.s. Alves, Sandra Orvalho, J M L Rodrigues, Rui L Mendes, Alberto ReisAbstract:Abstract In this work, the effects of surface-active contaminants on mass transfer coefficients k L a and k L were studied in two different bubble contactors. The oxygen transfer coefficient, k L , was obtained from the volumetric oxygen transfer coefficient, k L a , since the specific interfacial area, a , could be determined from the fractional gas holdup, e , and the average bubble diameter, d 32 . Water at different heights and antifoam solutions of 0.5– 100 ppm were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and 0.85×10 −3 m 3 Capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k L a and k L values over a span of almost 400%. A theoretical interpretation is proposed based on Modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant Cap Model. Model results match experimental k L data within ±30%.
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gas liquid mass transfer to single bubbles effect of surface contamination
2002Co-Authors: Jorge M.t. Vasconcelos, Sandra Orvalho, S.s. AlvesAbstract:The dissolution of single bubbles of gases of low solubility kept stationary in a downward stream of water was studied. In “clean” water, two regimes are identified. Initially, the process is fast, consistent with the theory for circulating bubbles. Then, the mass-transfer rate falls sharply to that predicted for solid spheres. Transition times and transition diameters vary widely with experimental conditions. In untreated water, only the second regime is found. Results are explained in terms of the kinetics of trace surfactant accumulation at the interface. An adaptation of the stagnant-Cap Model is proposed, with surface immobilization expressed in terms of interface dynamics. The Model yields good prediction of the transition point for a very large set of conditions, including different gases at various concentrations in the liquid stream and a wide range of initial bubble diameters.