Todd Pugsley

Simulation and experimental validation of a freely bubbling bed of FCC catalyst

Abstract A validation and mesh refinement study has been performed for the simulation of a freely bubbling bed of fluid catalytic cracking (FCC) catalyst operating at superficial gas velocities in the range of 0.05 to 0.20 m/s, using the two-fluid computational fluid dynamics (CFD) code Multiphase Flow with Interphase eXchanges (MFIX). The simulation results have been compared to experimental data collected using an electrical capacitance tomography (ECT) imaging system as well as data from the literature. A comparison has been made based on the bed expansion, and bubble diameters and rise velocities. A novel method has been used to extract bubble properties from ECT data and simulation results. It was found that a modified gas–solids drag law, corresponding to an effective particle agglomerate diameter in the range of 135 to 170 μm for FCC particles of actual 75 μm mean diameter, was required to adequately predict the fluidization behavior observed experimentally. These findings support the argument that cohesive interparticle forces lead to agglomeration of FCC catalyst powder and significantly affect the fluidization quality. The model is the first presented in the open literature to successfully apply CFD to obtain realistic predictions of FCC bubbling bed behavior.

The influence of permittivity models on phantom images obtained from electrical capacitance tomography

Four different phantoms have been placed within the sensor field of an electrical capacitance tomography (ECT) system mounted on a bench-scale fluidized bed. The phantoms were created with fluid catalytic cracking catalyst particles having a packed-bed relative permittivity of 2.4. Linear back-projection (LBP) image reconstruction has been compared with iterative LBP for three permittivity models: parallel, series, and Maxwell model. The parallel permittivity model provides the best representation of the phantoms. Tubes of solids placed in an empty bed could only be reconstructed near the ECT wall, while voids created by immersion of an empty tube in a packed bed of solids could be imaged equally well at the centre or wall. After 500 iterations, the size of the phantom in the reconstructed image is in very good agreement with the actual size of the phantom.

EFFECT OF PARTICLE SIZE DISTRIBUTION ON LOCAL VOIDAGE IN A BENCH-SCALE CONICAL FLUIDIZED BED DRYER

ABSTRACT The effect of particle size distribution (PSD) on local voidage has been investigated in a conical fluidized bed containing dried placebo pharmaceutical granule. For each of the five PSDs examined, the static bed height was varied between 0.12 and 0.17 m and the superficial gas velocity was varied between 0.05 and 0.75 m/s. The local voidage was measured using a twin-plane electrical capacitance tomography (ECT) system. A wide PSD containing 12 wt% solids with a diameter of 2 mm or larger resulted in two different types of gas flow: an annular gas flow up to a gas velocity of 0.50 m/s and a centrally concentrated gas flow above 0.50 m/s. The mixtures containing less coarse material exhibited a centrally concentrated gas flow surrounded by a dense phase at the walls of the bed over the entire range of gas velocities and bed heights examined. Consideration of previous work by other researchers suggests that the behavior of the wide PSD mixture is due to segregation at the lower velocities. The local voidage was sensitive to small changes in static bed height. For the wide PSD mixture at a fixed gas velocity, the gas tended to spread more uniformly over the bed cross-section as static bed height increased. The opposite was true of the other mixtures, i.e., the gas flow became more centralized with increasing bed height.

The dynamic calibration of an electrical capacitance tomography sensor applied to the fluidized bed drying of pharmaceutical granule

Electrical capacitance tomographic data collected in a lab-scale fluidized bed used for the drying of pharmaceutical granule have been corrected for the influence of moisture on the permittivity of the drying material. The correction is based on a linear least-squares fit to measurements of capacitance in a packed bed of granule at various moisture contents. X-ray tomography has been used to independently verify this correction procedure. The influence of permittivity models and number of iterations used for the reconstruction of tomograms have also been examined. It has been determined that the Bottcher permittivity model performs best at bed moistures above approximately 5 wt% while the parallel model is superior at bed moisture below this value. The reconstruction technique based on iterative linear back-projection utilized for the reconstruction of ECT data required approximately 50 iterations to successfully reproduce the density behaviour seen in the x-ray tomographs. Instability in the reconstruction technique at higher numbers of iterations indicates that a linear least-squares fit does not completely capture the response of the sensor to moisture changes. For future applications, changes in bed voidage associated with the drying of pharmaceuticals must be addressed and included in this calibration procedure in order to implement this calibration technique throughout the drying process. Nevertheless, the viability of this technique for on-line calibration of an ECT sensor applied to the drying process has been demonstrated.

Monitoring the fluidized bed granulation process based onS-statistic analysis of a pressure time series

Pressure fluctuation measurements collected during the fluidized bed granulation of pharmaceutical granule have been analyzed using the attractor comparison technique denoted as theS-statistic. Divergence of the bed state from the reference during granulation is followed by a return to a condition statistically similar to the original state of the dry fluidized ingredients on drying. This suggests insensitivity of theS-statistic technique to the changes in particle size distribution occurring during the granulation process. Consequently, the monitoring of pressure fluctuations alone may provide an easily implemented technique for the tracking of granule moisture and process end-point determination.

Simulation of Cold Flow FCC Stripper Hydrodynamics at Small Scale Using Computational Fluid Dynamics

The refining industry trend toward increased throughput of heavier feeds, accompanied by increased catalyst circulation rates in the fluid catalytic cracking (FCC) unit, has created flow problems in the FCC stripper. This has reduced stripper efficiency, leading to loss of valuable product to the regenerator. This adversely affects the FCC unit heat balance in some cases, requiring the catalyst circulation rate to be reduced, thereby limiting the flow of feed to the unit. Research on FCC strippers published in the open literature is scarce. The purpose of the present study is to model the hydrodynamics of a cold flow, laboratory scale FCC stripper using computational fluid dynamics (CFD). Simulations were performed using the two-fluid CFD code MFIX (www.mfix.org) for operating conditions of gas superficial velocities between 0.1 and 0.33 m/s and solid circulation fluxes ranging between 28 and 90 kg/m2s. A 2-D cold-flow annular FCC stripper was simulated since this study is a first step in modeling stripper hydrodynamics. The simulations were in good qualitative and quantitative agreement with the limited cold-flow stripper hydrodynamic data published in the open literature. The model was also shown to predict the onset of flooding in the baffled stripper, and the addition of downcomers to the baffles widened the stable operating envelope.

Effect of conditions and protectants on the survival of Penicillium bilaiae during storage

Abstract Penicillium bilaiae is a fungal microorganism used to promote soil-bound phosphorous uptake in several crop species such as wheat, canola, and pulse crops. In this study, an air-dried granular starch-based form of the inoculant was studied for improved longevity. Experiments were performed to determine optimal water activity, packaging atmosphere, and the effects of several protective chemicals. Viable spore fractions were measured at 1, 3, and 6 months. Longevity of the microorganism was found to suffer at intermediate values of water activity and with the addition of glycerol. However, skim milk powder, sucrose, MSG, and proprietary PBX3000 were found to improve survival, with a maximum survival fraction of 15% at 6 months using the best formulation. Cell concentration was not found to influence shelf-life. High viability loss during storage in a nitrogen environment resulted in less than 1% survival.

Experimental determination of viability loss of Penicillium bilaiae conidia during convective air-drying

A study was conducted on the drying of Penicillium bilaiae, a fungal micro-organism used to promote soil-bound phosphorous uptake in several crop species, such as wheat, canola and pulse crops. A wet pellet formed from a mixture of the inoculant and a starch-based carrier was air-dried to the appropriate water activity to extend the shelf-life of the viable fungal conidia. Convective air-drying was examined as a low-energy alternative to the more expensive freeze-drying technology that is currently in use. Experiments were conducted to measure the loss of conidia viability during drying in a fixed-bed, thin-layer convective dryer. The dryer air inlet temperature and relative humidity were controlled in experiments to determine the effect of thermal and dessicative stresses on conidial viability. The measured survivor fraction was determined to be dependent on solids temperature, moisture content and drying rate. Thermal stresses became significant for process temperatures above 30°C, while the survivor fraction fell sharply below a dry basis moisture ratio of 30%. Slower drying kinetics associated with high inlet air relative humidity were found to significantly improve the recovery of viable conidia. By minimising environmental stresses, survivor fractions of up to 75% could be achieved, but this result fell dramatically with the introduction of more severe conditions. A general linear statistical model is used to quantify experimental error and the significance level of each factor.

Direct Comparison of Fluidized and Packed Bed Bioreactors for Bioremediation of an Air Pollutant

A gas-solid fluidized bed bioreactor has been successfully used for the bioremediation of ethanol (a model volatile organic compound, VOC) contaminated air. A key objective of this fluidized bioreactor study was to compare the performance of fluid bed operation to packed bed operation. A fluid bed system increased homogeneity and improved upon operating problems such as plugging and channelling normally associated with packed bed bioreactors. The bioreactor bed was comprised of a mixture of moist sawdust particles and glass spheres. Depending on the superficial velocity of the waste gas stream, the bioreactor could be operated in either packed or fluidized mode. During fluid bed operation, the sawdust and glass sphere mixture was maintained in a bubbling/slugging regime. As expected, fluid bed operation demonstrated significantly higher mass transfer rates but the maximum elimination capacity was 75 g m-3sawdust h-1 as compared to 225 g m-3sawdust h-1 for packed bed operation. In packed bed mode, higher ethanol concentrations were used in order to have comparable ethanol loadings and this may have contributed to faster growth rates and thus faster bioremediation rates.

Distributor Induced Hydrodynamics in a Conical Fluidized Bed Dryer

The hydrodynamics induced by perforated, punched, and mesh (Dutch weave) distributor plates have been studied using dry placebo pharmaceutical granule in a conical fluidized bed dryer at inlet superficial gas velocities of 0.5 to 3.5 m/s. For superficial gas velocities up to 2.0 m/s, the punched plate design leads to improved hydrodynamics based on reduced bubble frequencies and limited segregation. Beyond 2.0 m/s, the influence of gas velocity supersedes that of distributor design, as coalescence dominates the hydrodynamic behavior resulting in low-frequency, high-intensity spectral density distributions for all distributor designs.