Effects of temperature on the flow and tensile strengths of powders

Measurements have been made of the flow rates and tensile strengths of a variety of powders — magnesia, lactose, fatty acids, organic drugs — over a range of temperatures from ?20° to 200 °C. The results have been explained in terms of the effects of temperature on the hardness and elasticities of the materials concerned, and the distances between the particles.Under compression, the asperities on their surfaces deform plastically and may melt, if the temperature is raised above about 0.9 of the conventional melting point in K, to form welded bonds.The activation energy of bonding for the different materials is between about 8 and 11 kJ mole^?1.     

The effect of large particles on the flow properties of powders

This paper is presented to fill a gap in the knowledge of the effect of larger particles on the flow behaviour of finer powders. Very little has appeared in the literature on the matter and the study is justified in that it is common practice to remove coarse particles before testing on a shear tester. The assumption is made in removing larger particles that they do not affect behaviour and that it is the fines which cause the binding and hence the flowability problems.Four different dry fine powders, a fine white sand, an electrostatic precipitator dust, a mixture of zircon and pyrophyllite (ZAP75) and a local steaming coal were investigated. No generalisation can be made about the addition of various volumetric percentages of spheres of varying sizes or of irregular particles. Thus, with fine sand, balls had little effect on the shear strength whereas irregular coal particles produced marked effect increasing consistently with the relative amount added. On the other hand, with the ZAP75, an increase in strength at low normal loads was exhibited but a decrease (over strength of powder alone) was found at higher normal loads.Experiments were conducted on a whole coal with varying limits of upper size (and also with varying limits of lower size) and the shear strength was found to be strongly dependent on those size limits. It is concluded that whereas much more experimental work is necessary before generalisation can be made of the effect of the presence (or absence, depending on the viewpoint) of large particles on the flowability of a powder, such an effect cannot be ignored.     

Hold-up of fine particles in the packed dense bed of the multisolid pneumatic transport bed

The hold-up of fine particles in the packed bed of dense particles in the multisolid pneumatic transport bed was experimentally examined. In the experiments, three types of fine particles were used including FCC particles (65 μm), glass beads (105 μm) and fine sand (155 μm), while three types of dense particles were used including aluminum particles (5.50 mm and 6.96 mm) and iron oxide particles (11.08 mm). The hold-up of fine particles in the packed bed of dense particles was found to be higher by a factor as large as 6 compared with that observed in systems without dense particles. An interaction coefficient between fine particles and dense particles was defined based on the momentum balance equation of fine particles in the packed bed. This interaction coefficient, which is empirically correlated, was utilized to account for the hold-up of fine particles in the packed bed.     

Effects of combined humidity and temperature on the friction of non-compacted iron,copper and hydroscopic powders

The effects of humidity and temperature on the interparticle friction of iron, copper, Portland cement and plaster of Paris powders have been investigated using a torsional apparatus. It is found that at ambient temperature (22 ± 2 °C), the interparticle friction of iron and copper powders, of a given particle shape, size distribution and size, first decreases then increases and again decreases with increasing humidity. At a constant relative humidity of ～30%, the friction of iron and copper powders is relatively insensitive to temperatures up to ～20 °C and then increases slowly but approximately linearly with increasing temperature. At 22 ± 2 °C, the interparticle friction of Portland cement and plaster of Paris powders increases gradually and then decreases with increasing humidity.     

Pressure fluctuations and choking criterion for vertical pneumatic conveying of fine particles

Pressure fluctuations and bed voidages are obtained in the dilute phase flow and dense phase flow regimes of vertical pneumatic conveying systems of fine particles. Statistical parameters of the pressure fluctuations are utilized to study the transition from the dilute phase flow to the dense phase flow. Out of the four types of fine particle used in the experiments, three types of fine particle exhibit a choking transition, which is characterized by a sharp change in the bed voidage. At the choking transition, sharp changes in the power spectral density function and standard deviation of the pressure fluctuations are also observed. The fourth type of fine particle does not exhibit any sharp changes in bed voidage, power spectral density function, or standard deviation of the pressure fluctuations over a wide range of air velocities used in the experiments. It is shown that the power spectral density function and the standard deviation of pressure fluctuations can be effectively used to accurately determine the choking transition. The experimentally determined choking velocities and bed voidages in the slugging dense phase flow are compared with available correlations.     

The measurement of emulsion phase voidage in gas fluidized beds of fine powders

The results of two different experimental methods, X-ray absorption and bed collapse, for the determination of the emulsion-phase voidage of gas fluidized beds are compared. It is shown that good agreement exists between them over the range of gas velocities studied, but that possible differences in voidage in an axial direction are only revealed by the X-ray technique. The bed collapse method applied to a Geldart Group A powder confirms that there is little difference between the surface settling rates of a bubbling bed and of a uniformly expanded bed of the same voidage.     

On the correlation of minimum porosity with particle size distribution

It has long been recognised that the major factors which influence the density of cohesionless material are the type of distribution of particle size, and particle shape. However, the difficulty of measuring particle shape and of expressing a particle size distribution comprehensively has proved a stumbling block to the determination of a successful correlation with porosity. None of the correlations so far has yielded an empirical relationship which can be employed with confidence to a wide range of types of particle size distribution.This paper examines some of the difficulties which are encountered with such correlations when attempting to describe the particle size distribution by Gaussian measures, and develops a new method of expression, by a single ‘offset‘ value, of a particle size distribution. A good empirical relationship is found between minimum porosity and this value, having taken due account of particle shape. It is suggested that the method will give engineers greater appreciation of the grading curve of cohesionless materials.     

Influence of temperature on the reactivity of limestone particles with sulfur dioxide

A modified version of the grain theory was used to analyse the effect of temperature on the sulfation reaction. The theory combines the influence of temperature on the grain size, its effect on the rate of transport of SO₂ and the chemical reaction. An efficient numerical procedure is presented for solving the model equations which take into account the radial gradient of the effective diffusivity in a particle.     

Effect of the solvent properties on the polarographic behaviour of the Zn2+ and Cd2+ complexes with glycine,N-acetyl and N-Benzoyl glycine

The polarographic behaviour of Zn²⁺ and CD²⁺ and their complexes with glycine, N-acetyl and N-benzoylglycine has been studied in DMSO, CH₃CN and DMF; the results have been compared with those previously obtained in H₂O and C₂H₅OH. The effect of the solvent properties on the reduction of those systems has been explained in terms of the electron pair donor (EPD) and of electron pair acceptor (EPA) strength of the solvent itself. The higher the donor number (DN), the more negative the E_{$\text{1}\text{2}$} value both of the free metal ions and the complexes. Moreover, the stability constants of the complexes, polarographically determined, seem to be related to the acceptor number (AN) of the solvent.     

Longitudinal dispersion of solid particles in fluid beds with horizontal baffles

The axial pressure profiles, allowable gas velocities and temperature distributions are measured for the fluidization of air—FCC cracking catalyst systems in 12- and 19-cm-diam. eight-stage fluid beds equipped with seven horizontal baffles. From these measurements, gas bubble holdup, apparent longitudinal dispersion and intermixing velocity of solid particles through the baffles are studied as functions of baffle design. It is shown that the gas bubble holdup increases, the operational range of gas flow decreases and the flow pattern of solid particles approaches plug flow with decreasing free area of baffles.     

Initial flux decline of the cellulose acetate butylate membranes with time under RO performance

Initial flux decline, i.e. compaction, has been investigated for the cellulose acetate butylate membrane prepared by varying casting condition. The compaction behavior of membranes prepared in a short evaporation period was different from that prepared in a long evaporation period. Compression tests showed that the membrane thinned by 30 ～ 40% instantaneously when pressurized; afterwards it thinned gradually. This corresponded to the behavior of decline. It appears that a change in decline occurs reflecting membrane structure depending on casting condition. The results were discussed for a pore model based on Kozeny-Carman equation and on the viscoelastic deformation model.     

Isothermal diffusion—reaction in a slab catalyst with bimolecular Langmuir-Hinshelwood kinetics: Connections with negative first-order kinetics

The problem of isothermal diffusion-reaction in a slab catalyst with bimolecular Langmuir—Hinshelwood kinetics is studied. Analytic solutions for the concentration profile and the catalytic effectiveness factor are reported for the full range of Thiele modulus values, in the limit of large adsorption inhibition constant. In this limit, connections with negative first-order kinetics are shown, and rather good explicit estimates of the region of multiple steady states are provided. Comparisons with numerical calculations are also made.     

Electrochemistry of ferrocene in anionic,cationic and nonionic micellar solutions. Effet of the micelle solubilization of the half-wave potentials

The redox properties of the ferrocene/ferricinium Fc/Fc⁺ couple were studied in aqueous 0.1 M NaCl solution of sodium dodecylsulphate (SDS), N-cetyltrimethylammonium bromide (CTAB) and polyoxyethylene-23 lauryl ether (Brij 35). Solubilities of ferrocene were measured by spectrophotometry and by limiting current measurements in direct current voltammetry. The reversible half-wave potentials were determined by cyclic and differential pulse voltammetry. The results are discussed on the basis of the micelle solubilization equilibrium of both the reductant Fc and the oxidant Fc⁺. Indeed, diffusion coefficient ratios of the oxidant to the reductant, measured by coulometry, show that the micelle solubilization of the electrogenerated ferricinium cation Fc⁺ must also be considered in the nonionic and the anionic micellar solutions. Thus in every solution studied, a standard potential of Fc/Fc⁺ in water may be obtained from the experimental values of the half-wave potentials, the partition coefficients of Fc and Fc⁺ and the ratio of their diffusion coefficients. The behaviour of ferrocene in the SDS solutions and the effect produced by the addition of pentanol seem to confirm that SDS forms micelles with a less ordered structure than the other surfactants studied. The invariance of the half-wave potentials of ferrocene in pure SDS solutions whatever the surfactant and ferrocene concentrations suggests the use of this solute-solvent couple as a reference potential system.     

Substrate-inhibited enzyme reaction in a tubular reactor with axial dispersion

For a fixed set of physico-chemical parameters, an isothermal substrate-inhibited enzyme reaction in a tubular reactor with axial dispersion can give rise to multiple steady states. Criteria previously developed for non-isothermal reactions are applied to this case to develop analytic conditions which ensure uniqueness of steady states. The criteria compare well with values obtained from exact numerical calculations.     

On the modelling of continuous emulsion polymerization using a population balance with instantaneous radical termination

An analytical solution of a population balance is used to mathematically describe continuous emulsion polymerization. Steady state performance is examined. The assumption of instantaneous free radical termination within particles is made. The desorption mechanism is included. Particle size distribution information is obtained, and the effect of the desorption mechanism is noted. A desorption rate constant is calculated when the model is fit to data found in the literature.