Peter Harriott

Solid-liquid mass transfer in turbulent pipe flow

Mass transfer rates were measured for smooth pipe sections of benzoic acid dissolving in glycerine-water solutions. For Reynolds numbers of 10,000 to 100,000 and Schmidt numbers of 430 to 100,000, the data were correlated with an average deviation of 5·4 per cent by the equation NSh = 0·0096 NRe0.913NSc0·346 The data fall 5 to 10 per cent below the correlation of Friend and Metzner [2] and 20 to 100 per cent above other published correlations (1, 3, 4). The 0·346 exponent for the Schmidt number appears significantly different from the value of 13 predicted by simple theories. Mass transfer coefficients were also measured for Methocel solutions of 1 to 9 cP viscosity but almost the same diffusivity. The coefficients were only 0 to 6 per cent greater than those given by the above equation, giving added support to the significance of the Schmidt number. Stoffubergangskoeffizienten wurden auch in Methocel-Losungen der Viskositat 1 bis 9 cP, jedoch bei gleichen Diffusionskoeffizienten, gemessen. Die Werte liegen hochsten 6% uber denjenigen, die sich aus obiger Gleichung ergeben; damit scheint die Gultigkeit des Sc-Exponenten erwiesen.

The kinetics of methanation on nickel catalysts

The kinetics of methanation of carbon monoxide were studied with 2% Ni/SiO2 and 10% Ni/SiO2 catalysts in a differential reactor. The role of carbon as an intermediate was explored by making transient tests of carbon deposition, carbon gasification, and methane formation. If carbon is an intermediate, neither the normal dissociation of CO nor reactions of H with C seem to be controlling. The limiting step may be the surface reaction between adsorbed carbon monoxide and hydrogen atoms to form carbon and water. The data suggest surface heterogeneity with over a 10 fold range in activity for adsorbed CO.

Critical entry pressure for liquids in hydrophobic membranes

Abstract Porous membranes made from a hydrophobic polymer have a minimum entry pressure for water and for some liquids with a contact angle less than 90°. A model was developed to account for this behavior by treating the membrane as a square array of intersecting cylindrical fibers. Experimental data for air-liquid systems in polytetrafluoroethylene membranes showed good agreement with the model, but some liquid-liquid systems gave anomolously low entry pressures.

The effect of crystallite size on the activity and selectivity of silver catalysts

Abstract The effect of crystallite size on the activity and selectivity of silver catalysts for the oxidation of ethylene was studied in a differential reactor at 220 °C, P O 2 = 0.264 atm, P E = 0.264 atm. The activities per unit area for ethylene oxide formation and for carbon dioxide formation decreased with increasing crystallite size, but not at the same rate. The selectivity was nearly zero for 20 A particles and gradually increased to 60% for 500 A particles. Two types of active sites are believed to be formed by adsorption of oxygen at steps and faces of the silver crystals, and reaction occurs between ethylene and atomic or molecular oxygen competitively adsorbed on these sites.

The oxidation of ethylene using silver on different supports

Abstract The partial oxidation of ethylene to ethylene oxide was studied using silver deposited on aluminas or silicas with moderate to high surface area. The selectivity was very low for silver on activated alumina or dessicant-grade silica gels, but selectivities of 30–60% were obtained for silver on nonporous silicas, heat treated silica gel, and heat treated aluminas. Very high activity and good selectivity were obtained with a catalyst having silver particles 30–70 A in diameter. Even smaller particles are believed responsible for the very low selectivities observed with some catalysts.

Kinetics of oxidation and chemisorption of oxygen for porous carbons with high surface area

Oxidation and chemisorption rates were measured for several powdered porous carbons with surface areas of 500–2400 m2/g. A thermal balance was used. Activation energies of adsorption and intrinsic adsorption rate constants were obtained from initial adsorption rate data at 300–400°C. The data fitted a second-order Langmuir adsorption isotherm, suggesting that oxygen molecules dissociated on the surface. The data for the oxidation runs indicated a clear transition from a high activation energy regime (at low to moderate temperature) to an adsorption control regime. The activation energy and oxidation rate at high temperature (above 625°C) agreed with an extrapolation of the adsorption data. The reactivity of the carbons correlated with the active surface area from the chemisorption data but not with the total surface area measured by low temperature adsorption of nitrogen.

Process dynamics of starch-based microcellular foams produced by supercritical fluid extrusion. I: model development

Abstract Supercritical fluid extrusion (SCFX) has been used successfully to produce biopolymeric foams with bubble size in the range of 50–200 microns, and bubble density to the order of 106 per cm3. Final bubble size and expansion ratio of extrudates depend on process and material parameters like CO2 injection rate, nozzle temperature, oven temperature, melt viscosity, melt yield stress, etc. The objective of this study was to describe SCFX process dynamics and post-extrusion drying mechanism by using a mathematical model for bubble growth at the microscopic level in conjunction with a macroscopic model for flow of starch melt through the extruder nozzle, bulk diffusion of CO2 and water to the atmosphere and heat transfer in the extrudate. The model was written in Visual Basic. The model provided the basis for a good understanding of the mechanisms of bubble growth and collapse, post-extrusion drying and open cell formation during SCFX processing, and is the only such one developed so far for extrusion puffed products. Simulation results and comparison with experimental data are presented in Part II of this paper.

Diffusion effects in the preparation of impregnated catalysts

Abstract The distribution of metal within silver-alumina catalysts was studied using autoradiography. When the solute in the impregnating solution was strongly adsorbed on a high-surface-area support, the distributions agreed with predictions from the transient diffusion equations. For low-surface-area supports, mass transfer during drying gave irregular distributions. Theory and results are also given for distribution of metal resulting from fast reaction of a dissolved salt and a reducing agent within the support particle.

Mass transfer resistances in the palladium-catalyzed hydrogenation of methyl linoleate

Abstract The hydrogenation of methyl linoleate to methyl oleate and methyl stearate was studied in a 6-inch stirred batch reactor at 121°C and 45 psia. Several particle size fractions of a 1 per cent Pd-carbon catalyst were used to help separate the gas absorption resistance, the external mass transfer resistance, and the internal resistance to pore diffusion and reaction. Corrected to the same concentration of hydrogen, the initial rate of reaction for linoleate was 30 times that for oleate. This difference in reactivity and stronger adsorption of linoleate account for the high selectivity of 50–80.

Viscosities of fatty acids and methylated fatty acids saturated with supercritical carbon dioxide

The viscosities of several types of lipids saturated with supercritical carbon dioxide (SC-CO2) were measured with a high-pressure capillary viscometer. Oleic acid and linoleic acid were evaluated from 85 to 350 bar at 40 and 60°C. The more SC-CO2-soluble methylated derivatives of these fatty acids were evaluated from 90 to 170 bar at 40 and 60°C. The complex mixture of anhydrous milk fat (AMF) was evaluated from 100–310 bar at 40°C. The viscosities of the methylated fatty acids saturated with SC-CO2 decreased between 5 and 10 times when the pressure increased from 1 to 80 bar, followed by a further decrease by a factor of 2 to 3 when the pressure was increased from 80 to 180 bar. The viscosities of the fatty acids and AMF saturated with SC-CO2 had viscosity reduction similar to the methylated fatty acids between 1 and 80 bar, but they decreased much less between 80 and 350 bar. At constant pressure, the viscosity of the fatty acids and AMF decreased with increasing temperature, whereas the viscosity of the methylated fatty acids increased with increasing temperature. The lipid/SC-CO2 mixtures were Newtonian, and their viscosities were best interpreted by using the mass concentration of dissolved SC-CO2 in the lipids and the pure component viscosities.