Electrostatic charge generation in gas-solid fluidized beds is a significant industrial problem. Associated problems include particle agglomeration and particle wall fouling. In the polymerization industry this may result in "sheets" of fused polymer, due to exothermic reaction causing the melting of the polymer, which can fall off and block the distributor plate disrupting fluidizing gas flow. Additionally, blockage of the catalyst feed or the polymer removal system can take place or the product can become non-uniform. All of these problems require shut-down of the reactor which results in lost production time. While this phenomena has been identified for many years, the mechanisms involved are not well understood, especially wall fouling and the distribution of charge within the bed. Isolation of individual parameters such as hydrodynamics, operating conditions, and material involved is necessary to evaluate how each parameter impacts charge generation during fluidization. In this thesis, the fluidization system consisted of a stainless steel column, two online Faraday cups, and a retractable distributor plate. This system allowed for the simultaneous measurement of charge within different regions of the bed: the entrained fine particles, the particles adhered to the column wall, and the bulk of the bed. Additionally, mass and particle size distributions were measured and images of the layer of particles adhered to the column wall were taken for comparison. This allowed for a charge distribution comparison and evaluation of wall fouling. Three different parameters were investigated: duration of fluidization, column wall material, and relative humidity of fluidizing gas. Fluidization time was studied for 15, 30, 60, 120, 180, and 360 min; relative humidity was investigated for 0%, 20%, 40%, 60%, and 80% relative humidity. Both fluidization time and relative humidity were evaluated at four different fluidization gas velocities, two each in the bubbling and slugging flow regimes. Column wall material was evaluated for a stainless steel and carbon steel column at two gas velocities, one each in the bubbling and slugging flow regimes. Fluidization time was found to influence wall fouling in the bubbling flow regime as the particle layer continued to build as fluidization progressed. In the slugging flow regime, the particle layer developed within 15 minutes of the onset of fluidization. The bubbling flow regime was shown to have a greater capacity for charge generation than the slugging flow regime. This was due to the vigorous mixing in the bubbling flow regime resulting in more particle-particle interactions. Column wall material was shown to influence wall fouling in the slugging flow regime due to the differences in surface roughness of the columns. This was due to the particle-wall contacts resulting in frictional charging which is the predominant charging mechanism in this flow regime. Charge was also impacted in the bubbling flow regime in those particles that were adhered to the column wall. Relative humidity was found to influence wall fouling at the lowest gas velocity tested. However, variations in generation of charge occurred at all fluidization gas velocities tested; the charge-to-mass ratios for the particles adhered to the column wall in the slugging flow regime decreased with high relative humidities. This was due to either the formation of a water film layer on the column wall or instantaneous surface water films on the particles throughout fluidization.

This thesis is an experimental study of the fluidization of binary mixture in particulate flows. A fluidized bed with two distributors was built with water being used as carrying fluid. Three types of solid particles of nylon, glass and aluminum of the same size and different densities are used in the experiments. The wall effect on a single particle fluidization, the fluidization of binary mixture of large density difference (nylon and aluminum of density ratio of 0.42), and the fluidization of binary mixture of close density (glass and aluminum with density ratio of 0.91) were investigated. Also, the effect of distributors on mono-disperse and bi-disperse particle fluidization was investigated. Results show that the presence of narrow walls reduces the minimum fluidization velocity for a single particle by as much as nearly 40%. Also, in the case of binary mixture of close density particles, uniform mixing was easily achieved and no segregation was observed, but in the case of large density difference particles, there exists significant segregation and separation. At high velocity, the uniform distributor behaves like a transport bed. To achieve a full bed in the single jet, it requires 1.5 times velocity of the uniform distributor. This behavior determines their application in the industries.

Electrostatic charge generation in gas-solid fluidized beds is a significant industrial problem. Associated problems include particle agglomeration and particle wall fouling. In the polymerization industry this may result in "sheets" of fused polymer, due to exothermic reaction causing the melting of the polymer, which can fall off and block the distributor plate disrupting fluidizing gas flow. Additionally, blockage of the catalyst feed or the polymer removal system can take place or the product can become non-uniform. All of these problems require shut-down of the reactor which results in lost production time. While this phenomena has been identified for many years, the mechanisms involved are not well understood, especially wall fouling and the distribution of charge within the bed. Isolation of individual parameters such as hydrodynamics, operating conditions, and material involved is necessary to evaluate how each parameter impacts charge generation during fluidization. In this thesis, the fluidization system consisted of a stainless steel column, two online Faraday cups, and a retractable distributor plate. This system allowed for the simultaneous measurement of charge within different regions of the bed: the entrained fine particles, the particles adhered to the column wall, and the bulk of the bed. Additionally, mass and particle size distributions were measured and images of the layer of particles adhered to the column wall were taken for comparison. This allowed for a charge distribution comparison and evaluation of wall fouling. Three different parameters were investigated: duration of fluidization, column wall material, and relative humidity of fluidizing gas. Fluidization time was studied for 15, 30, 60, 120, 180, and 360 min; relative humidity was investigated for 0%, 20%, 40%, 60%, and 80% relative humidity. Both fluidization time and relative humidity were evaluated at four different fluidization gas velocities, two each in the bubbling and slugging flow regimes. Column wall material was evaluated for a stainless steel and carbon steel column at two gas velocities, one each in the bubbling and slugging flow regimes. Fluidization time was found to influence wall fouling in the bubbling flow regime as the particle layer continued to build as fluidization progressed. In the slugging flow regime, the particle layer developed within 15 minutes of the onset of fluidization. The bubbling flow regime was shown to have a greater capacity for charge generation than the slugging flow regime. This was due to the vigorous mixing in the bubbling flow regime resulting in more particle-particle interactions. Column wall material was shown to influence wall fouling in the slugging flow regime due to the differences in surface roughness of the columns. This was due to the particle-wall contacts resulting in frictional charging which is the predominant charging mechanism in this flow regime. Charge was also impacted in the bubbling flow regime in those particles that were adhered to the column wall. Relative humidity was found to influence wall fouling at the lowest gas velocity tested. However, variations in generation of charge occurred at all fluidization gas velocities tested; the charge-to-mass ratios for the particles adhered to the column wall in the slugging flow regime decreased with high relative humidities. This was due to either the formation of a water film layer on the column wall or instantaneous surface water films on the particles throughout fluidization.

Call number: LD2668 .T4 1979 G36 / Master of Science

Call number: LD2668 .T4 1986 C44 / Master of Science / Chemical Engineering

This study focuses on the effect that particle properties (size, density and shape) have on fluidized bed hydrodynamics and entrainment rate. The experimental work was carried out using two Plexiglas columns having internal diameters of 0.05 m and 0.14 m respectively and a total height of 6 m from the gas distributor. The particle density was in the range 1300 to 4600 kg/m3, the Sauter mean diameter was in the range 23 to 60 um and the fines content (% < 22 um) was in the range 1 to 29 %. Particle shapes, which ranged from angular to spherical, were characterized by image analysis of SEM photographs. Air was used, as the fluidizing gas and the superficial velocity was kept constant at 0.38 m/s in the 0.05 m column. In the 0.14 m column it was varied in the range 0 to 0.8 m/s. The dense-phase voidage, bubble fraction and entrainment rate of the powders were measured at ambient conditions. In general it was found that the bubble fraction and entrainment rate increased with an increase in the superficial gas velocity. The dense phase voidage was found to increase with an increase in the fines content of the powder and it was only a weak function of the superficial gas velocity. Most importantly, it was found that angular-shaped particles had a higher dense phase voidage, a lower bubble fraction and a lower entrainment rate when compared to spherical-shaped particles having similar particle density and size. Possible reasons for the lower entrainment rate for the angular-shaped particles are given. The measured dense phase voidage, bubble fraction and entrainment flux was compared with predictions from published correlations and it was found that none of the correlations provided a good fit to the data obtained in this work and that different correlations predicted widely different entrainment rates for the same system. It is therefore recommended that literature correlations should be used with caution in the absence of experimental data. Empirical correlations for the dense phase voidage and bubble fraction are developed. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2007.

Abstract not available

Graduation date: 1998

A gas-fluidized bed bioreactor was developed in this research as a new method for treating polluted air. The fluidization characteristics of selected packing materials were investigated. Then, bioremediation was tested using two types of packing in a fluidized bioreactor, as well as in a comparable packed bed. Microorganisms on the particles biodegrade contaminants in the polluted air, which flows up through the bed. At high flowrates, the polluted air fluidizes the particles, while at low velocities the operation is in packed bed mode.<p>Initially, sawdust was selected for use as a packing material. Due to the poor fluidization properties of sawdust, glass spheres were added. A mixture of sawdust and glass spheres remained well mixed during fluidization. In the mixture, interparticle forces increased with increasing moisture in the sawdust, eventually causing defluidization of the bed. In the absence of bioremediation, mass transfer was studied between ethanol-contaminated air and sawdust/glass sphere packing, and found to be higher in the fluidized versus packed mode. In bioremediation experiments, ethanol removal efficiencies were as high as 95% in both operating modes. The maximum elimination capacities (EC) of ethanol were 75 and 225 g m^-3 sawdust h^-1 in the fluidized and packed beds respectively.<p>The packing of the fluidized bed bioreactor was optimized in order to boost bioremediation rates. Experiments showed that peat granules fluidized well in a bubbling regime, likely due to their relatively high density and sphericity. In peat bioremediation trials, the fluidized mode outperformed the packed bed; the maximum ECs were 1520 and 530 g m^-3 peat h^-1, respectively. Removal efficiency in the fluidized mode decreased with velocity, because the size and amount of large bubbles increased.<p>A steady-state model of the fluidized bioreactor was developed. By taking account of bubble properties during fluidization, the model helps to explain how bubble size, microbial properties and bioreactor residence time affect removal efficiency and elimination capacity of the bioreactor.<p>A peat gas-fluidized bioreactor shows promise as an efficient, low-cost technology for air treatment. Particle mixing in the fluidized bed may prevent operating problems associated with the packed bed bioreactor. Fluidized bioreactors are ideal for the treatment of high volume, low concentration air emissions.

Fluidized bed dryers (FBDs) are used in the pharmaceutical industry to remove excess moisture from granule prior to tablet formation. As granule moisture content is reduced from its initial to final state, the velocity required to fully fluidize the granule decreases and the bed voidage decreases. The change in these fluidization properties are attributed to the decrease in the interparticle force load created by a reduction in liquid bridging as moisture is removed. During constant velocity drying, these fluidization properties result in a bubbling fluidization state, which evolves into a bubble coalescing regime as drying proceeds. This behaviour was identifiable using pressure fluctuation time-series analysis techniques.<p> Distributor design studies using dry and wet granule in a conical fluidized bed suggest that the punched plate design limits bubble coalescence when compared to the perforated plate and Dutch weave mesh designs. Furthermore, the Dutch weave results in extensive segregation, which is undesirable from a fluidization perspective. Local drying hydrodynamic measurements using x-ray densitometry found that the punched and perforated plates generate a centralized bubbling core region during drying with a defluidized bed periphery. This fluidized core region grows as drying proceeds until the defluidized region disappears. Under the same operating conditions, a porous plate distributor creates extensive channelling and defluidization across the entire bed cross-section during the constant rate period of drying. These poor fluidization characteristics are a result of the porous plate introducing the gas into the bed as a fine dispersion.<p> Lastly, the hydrodynamics associated with the conical vessel geometry improves the circulation and mixing patterns in fluidized bed dryers. This is especially the case in the entry region of the conical bed where the high inlet gas velocity prevents defluidization around the periphery of the bed. The straight walled geometry of the cylindrical bed resulted in defluidization in this area. As a result, the hydrodynamics associated with bubbling differ significantly between the geometries over the course of drying.