Jonathan Seville

Positron emission particle tracking studies of spherical particle motion in rotating drums

Positron emission particle tracking has been used to track the motion of a single radioactively labelled tracer particle within a bed of similar particles in a partially filled horizontal rotating drum. Runs were performed using 1.5 mm glass spheres in a 136 mm diameter drum and using 3 mm glass spheres in 100 and 144 mm diameter drums, at drum rotation speeds from 10 to 65 rpm. An active surface layer approximately two-thirds as thick as the underlying bed layer was apparent in all cases. Considerable slip of the bed at the walls was observed in most runs, which is attributed to a rolling motion of the outermost layer of spheres. The axial dispersion coefficient was determined for each run and was found to be proportional to the frequency at which particles circulate around the bed, and to be strongly dependent on particle size but independent of drum diameter.

Fluid trajectories in a stirred vessel of non-newtonian liquid using positron emission particle tracking

Abstract The technique of positron emission particle tracking (PEPT) enables the position of a small radioactive particle tracer to be detected many times per second and its trajectory followed. The technique was employed to determine the flow field of viscous non-Newtonian CMC solutions inside a 290 mm diameter vessel using a 600 μm almost neutrally buoyant tracer. Two axial flow impellers, Lightnin A410 and A320, were used. Their mixing effectiveness was characterised qualitatively by a study of the flow patterns generated within the vessel, and quantitatively by a statistical analysis of the trajectories of the Lagrangian tracer tracked over a test period of about an hour. Zones of effective fluid agitation as well as zones of stagnation and poor mixing were clearly identified. Analysis of particle trajectory-length distributions showed that an increase in agitation speed gave rise to a wider trajectory distribution in the tank. Both agitators behaved predominantly as radial flow impellers as the fluid viscosity was increased. This feature seems to be specific to highly viscous fluids. The ratio of the mean fluid-circulation velocity and the impeller tip velocity is proposed as an index of agitation.

Kinetics of HCl reactions with calcium and sodium sorbents for IGCC fuel gas cleaning

Abstract Experiments for HCl sorption by calcium and sodium sorbents have been carried out with simulated gases of compositions typical for “air-blown” and “oxygen-blown” IGCC fuel gases. The concentration of CO2 affects the behaviour of HCl sorption by Ca-sorbents but does not influence the performance of Na-sorbents in the temperature range 300–600°C. The sorbent particle size has a slight effect on HCl sorption by Na2CO3 in the range 90–250 μm. The optimum temperature for the reaction was found to be between 400 and 500°C. This has been attributed to a combination of reaction kinetics and sintering. A mathematical model has been developed to simulate gas solid reactions in fixed beds. The coefficient of gas diffusion through the product layer (NaCl) was obtained at 300–600°C.

Scaling relationships for rotating drums

Rotating drums are extensively used in the chemical and process industries as kilns, mixers, dryers and reactors. Despite challenges from the development of newer and more specialised technologies such as fluidised beds, rotating drums continue to find applications. This is mainly due to their ability to handle varying feed stocks, in particular granular materials with broad particle size distribution and significant difference in physical properties. However, the scale-up methodology for such devices is still largely empirical and no general and systematic method has been established. This work is therefore devoted to the development of a set of scaling relationships which can be used to properly design rotating drums. The scaling relationships are obtained by non-dimensionalising the differential equations governing the behaviour of solids motion in rotating drums. The derived dimensionless groups include Froude number, pseudo-Reynolds number, pseudo-Euler number, drum geometric ratios, drum inclination, drum fill percentage, size distribution of particles, and physical properties of the particles and drum wall such as restitution coefficient and elasticity modulus. At relatively low pseudo-Reynolds numbers, the effect of velocity fluctuation and hence the granular temperature can be neglected, the granular flows are thus in the quasi-static regime. At relatively low pseudo-Euler numbers, the effect of the frictional contribution to the total stress tensor is negligible and the flow is in the rapid granular flow regime. The upper limit of the quasi-static regime is evaluated and the adequacy of the application of granular flow kinetic theory to rotating drums is discussed. It is shown that granular flows in large drums operated at low to medium rotational speeds are often in the quasi-static regime, whereas those in small drums operated at medium rotational speeds may be in the transition flow regime. Preliminary experiments have been carried out with the aim of reducing the total number of controlling dimensionless groups. The dimensionless groups investigated include the drum length-to-diameter ratio, rotational Froude number, fill percentage, particle-to-drum diameter ratio, particle restitution coefficient, and the relative drum wall roughness. It is shown that the Froude number, particle-to-drum diameter ratio, drum fill percentage, and particle restitution coefficient can be combined to give a single dimensionless parameter if drums are operated in a rolling mode and the Froude number is greater than 0.003.

The effect of lubrication on density distributions of roller compacted ribbons

Roller compaction is a continuous dry granulation process for producing free flowing granules in order to increase the bulk density and uniformity of pharmaceutical formulations. It is a complicated process due to the diversity of powder blends and processing parameters involved. The properties of the produced ribbon are dominated by a number of factors, such as the powder properties, friction, roll speed, roll gap, feeding mechanisms and feeding speed, which consequently determine the properties of the granules (size distribution, density and flow behaviour). It is hence important to understand the influence of these factors on the ribbon properties. In this study, an instrumented roller press developed at the University of Birmingham is used to investigate the effect of lubrication on the density distribution of the ribbons. Three different cases are considered: (1) no lubrication, (2) lubricated press, in which the side cheek plates of the roller press are lubricated, and (3) lubricated powder, for which a lubricant is mixed into the powder. In addition, how the powders are fed into the entry region of the roller press and its influence on ribbon properties are also investigated. It is found that the method of feeding the powder into the roller press plays a crucial role in determining the homogeneity of the ribbon density. For the roller press used in this study, a drag angle (i.e., the angle formed when the powder is dragged into the roller press) is introduced to characterise the powder flow pattern in the feeding hopper. It is shown that a sharper drag angle results in a more heterogeneous ribbon. In addition, the average ribbon density depends upon the peak pressure and nip angle. The higher the peak pressure and nip angle are, the higher the average ribbon density is. Furthermore, the densification behaviour of the powder during roller compaction is compared to that during die compaction. It has been shown that the densification behaviour during these two processes is similar if the ribbons and the tablets have the same thickness.

Dusk cake detachment from gas filters

Abstract Most forms of barrier filter for particulate removal from gases are cleaned periodically by administering a short pulse of pressurized gas to the downstream side. This is intended to remove the dust cake which deposits during filtration. The mechanisms of dust cake detachment from both rigid and flexible filter media are considered, with particular reference to the effect of the ‘cake loading’, the cake mass per unit area of filtration medium. Small-scale experimental methods for the quantitative determination of the conditions necessary for cake detachment are reviewed, and experimental data on cake detachment at ambient temperature are summarized. At low cake loadings (particularly below 300 g/m 2 ) experiments carried out under conditions of reverse gas flow and celeration show different trends: in general, the ‘cake detachment stress’ measured in an acceleration test increases with increase in cake loading while the opposite is true for cake detachment by reverse flow. At higher cake loadings (about 1000 g/m 2 ) the results of the two methods converge. Cakes are removed much more easily from rigid media than from flexible ones under corresponding conditions. Experiments on rigid media show that if the cake detachment stress under reverse flow conditions is taken at the first point of significant cake removal, (the ‘burst pressure’), the resulting values are in good agreement with median detachment stresses obtained by acceleration. It is likely that cake removal by reverse flow is influenced by ‘hinging’ of cake patches, which remain loosely attached to the surface after their apparent detachment stress has been overcome.

Identification of pre-existing cracks on soil fracture surfaces using dye

Abstract A significant strength-controlling property of aggregated soils is structural porosity such as cracks. A technique is presented to identify structural porosity on fracture surfaces using methylene blue dye. Soil aggregates were immersed in the stain, returned to their original water content, and equilibrated. They were then fractured under a dynamic load using an impact rig. The fragments were collected and sieved to selected size ranges. The proportion of the fragment surface that was stained from each size range was measured using a colour image analyser adapted for this experiment. The first set of samples examined was formed by compacting aggregate beds to selected macroporosities to obtain samples with a statistically homogeneous macrostructure. Natural soil aggregates, collected from zero and conventional traffic treatments, were also tested. Results obtained illustrate that compaction reduces both structural porosity and fragmentation. An increase in the impact energy resulted in a decrease in the proportion of the fracture surface that was stained, showing that more ‘new’ surface area was being produced.

Modelling of sintering of iron particles in high-temperature gas fluidisation

A recently published theory of de-fluidisation of gas fluidised beds has been extended to the case of sintering produced by surface and volume diffusion. The theory was tested against a recently published investigation of the sintering and de-fluidisation behaviour of iron particles at temperatures above 700 K. The theory successfully predicted the dependence on temperature of the gas velocity required to prevent de-fluidisation as a function of the activation energy for surface diffusion. The assumptions made in the theory and the effects of scale are discussed.

EXPERIMENTAL VERIFICATION OF THE SCALING RELATIONSHIPS FOR BUBBLING GAS FLUIDISED BEDS USING THE PEPT TECHNIQUE

Abstract An experimental verification of the scaling relationships for bubbling gas-fluidised beds has been carried out by measuring particle cycle frequency. The Positron Emission Particle Tracking technique was used, which enabled non-intrusive measurements of solids motion in three dimensions with a spatial resolution of ∼2 mm under the conditions of this work. Three cylindrical beds (70, 141 and 240 mm ID) equipped with multiple orifice-type distributors were tested. It was shown that for geometrically similar beds, the pair of Froude numbers based on the minimum fluidisation and excess gas velocities was sufficient for similarity at the viscous limit. The experimental results showed little influence of the gas-to-particle density ratio, but a small effect of particle-to-bed diameter ratio when the change in the ratio was significant. The experimental results also indicated a possible relaxation of the viscous limit up to a particle Reynolds number of about 100. More experimental studies are needed to confirm the validity of such a relaxation.

Some aspects of heat transfer in rolling mode rotating drums operated at low to medium temperatures

Abstract Rotating drums in industries are often operated at elevated temperatures. Heat transfer in such devices therefore plays a prime role. However, the controlling steps in heat transfer have not been fully clarified though a considerable amount of work has been carried out. In this paper, heat transfer in rotating drums operated in a rolling mode is assessed. The focus is on direct heating operations at low to medium temperatures, where thermal radiation is unimportant. It is shown that heat transfer from the covered wall to the particle bed is the dominant mechanism in supplying heat to the bed. Heat transfer between the freeboard gas and the exposed surface of the free cascading layer accounts for only a small portion, in contradiction to our previously held opinion. The heat transfer rate between the freeboard gas and the uncovered drum wall may be comparable to that between the covered drum wall and the particle bed indicating that both steps could be controlling. If heat is supplied to the boundaries of the particle bed rapidly, and macroscopic particle mixing is rapid, then heat transfer within the bed may be controlled by either or both of gas–solid heat transfer and heat conduction within individual particles depending upon the physical properties of both interstitial gas and particles. Although theoretical analyses are principally used in this work, comparison with experiments is also made where appropriate.