James N. Michaels

Use of X-ray tomography to study the porosity and morphology of granules

Abstract X-ray computed tomography (XRCT) is a technique that uses X-ray images to reconstruct the internal microstructure of objects. Known as a CAT scan in medicine, it has found wide application for whole-body and partial-body imaging of hard tissues (e.g., bone). A modern tabletop XRCT system with a resolution of about 4 μm was used to characterize some pharmaceutical granules. Total porosity, pore size distribution, and geometric structure of pores in granules produced using different conditions and materials were studied. The results were compared to data obtained from mercury porosimetry. It was found that while XRCT is less precise in the determination of total porosity in comparison to mercury porosimetry, it provides detailed morphological information such as pore shape, spatial distribution, and connectivity. The method is nondestructive and accurate down to the resolution of the instrument. Tomographic images show that the pore network of individual granules comprises relatively large cavities connected by narrow pore necks. The major structural difference between granules produced at different conditions of compaction and shear is a reduction in the pore neck diameter; the cavity size is relatively insensitive to these conditions. Comparison of pore size distributions determined from tomographic images and mercury porosimetry indicates that mercury intrusion measures the pore neck size distribution, while tomography measures the true size distribution of pores ca. 4 μm or larger (the instrument resolution).

Liquid distribution in wet granulation: dimensionless spray flux

This study investigates binder distribution in wet granulation and focuses on the nucleation zone, which is the area where the liquid binder and powder surface come into contact and form the initial nuclei. An equipment independent parameter, dimensionless spray flux Psi (a), is defined to characterise the most important process parameters in the nucleation process: solution flowrate, powder flux, and binder drop size. Ex-granulator experiments are used to study the relationship between dimensionless spray flux, process variables and the coverage of binder fluid on the powder surface. Lactose monohydrate powder on a variable speed riffler passed under a flat spray once only. Water and 7% HPC solution at two spray pressures were used as binders. Experiments with red dye and image analysis demonstrate that changes in dimensionless spray flux correlate with a measurable difference in powder surface coverage. Nucleation experiments show that spray flux controls the size and shape of the nuclei size distribution. At low Psi (a), the system operates in the drop controlled regime, where one drop forms one nucleus and the nuclei size distribution is narrow. At higher Psi (a), the powder surface cakes creating a broader size distribution. For controlled nucleation with the narrowest possible size distribution, it is recommended that the dimensionless spray flux be less than 0.1 to be in the drop-controlled regime

Scale-up of mixer granulators for effective liquid distribution

There is considerable anecdotal evidence from industry that poor wetting and liquid distribution can lead to broad granule size distributions in mixer granulators. Current scale-up scenarios lead to poor liquid distribution and a wider product size distribution. There are two issues to consider when scaling up: the size and nature of the spray zone and the powder flow patterns as a function of granulator scale. Short, nucleation-only experiments in a 25L PMA Fielder mixer using lactose powder with water and HPC solutions demonstrated the existence of different nucleation regimes depending on the spray flux Ψa—from drop-controlled nucleation to caking. In the drop-controlled regime at low Ψa values, each drop forms a single nucleus and the nuclei distribution is controlled by the spray droplet size distribution. As Ψa increases, the distribution broadens rapidly as the droplets overlap and coalesce in the spray zone. The results are in excellent agreement with previous experiments and confirm that for drop-controlled nucleation, Ψa should be less than 0.1. Granulator flow studies showed that there are two powder flow regimes—bumping and roping. The powder flow goes through a transition from bumping to roping as impeller speed is increased. The roping regime gives good bed turn over and stable flow patterns. This regime is recommended for good liquid distribution and nucleation. Powder surface velocities as a function of impeller speed were measured using high-speed video equipment and MetaMorph image analysis software. Powder surface velocities were 0.2 to 1 ms−1—an order of magnitude lower than the impeller tip speed. Assuming geometrically similar granulators, impeller speed should be set to maintain constant Froude number during scale-up rather than constant tip speed to ensure operation in the roping regime.

Kinetics, limit cycles, and mechanism of the ethylene oxidation on platinum

The oxidation of ethylene on polycrystalline Pt films was studied in a CSTR at atmospheric pressure and temperatures between 200 and 400 °C. The new technique of solid electrolyte potentiometry (SEP) was used to monitor the activity of oxygen on the metal catalyst. To this end the platinum film catalyst also served as one of the electrodes of a solid-electrolyte oxygen concentration cell and the open-circuit emf of the cell was monitored during reaction. It was found that the steady-state surface oxygen activity a0 satisfies the equation a0 = KsPO2PET, where Ks depends on temperature only. The reaction rate is first order in ethylene and adsorbed oxygen. Over a certain range of temperature and gas-phase composition both the surface oxygen activity and the reaction rate exhibit self-sustained oscillations. Limit cycles appear only over a well-defined range of surface oxygen activity a0 values. The oscillations can be explained in terms of the stability of a surface platinum oxide. The reaction mechanism is discussed in light of these observations.

Mechanical properties of agglomerates

Abstract The mechanical properties of dry and wet agglomerates are reviewed in the context of continuum solid and fluid mechanics and fracture mechanics. The focus is on practical measurements of tensile strength, yield strength, hardness and fracture toughness, and how they define the attrition behavior of agglomerates. Well-established mechanical testing methods can be applied to agglomerates, but certain limitations apply due to the nature of agglomerates being inherently non-equilibrium (glassy), anisotropic, and compressible. The mechanical response of agglomerates may vary from brittle, elastic-plastic (for most dry agglomerates) to elastoviscoplastic and fully plastic (for wet agglomerates) depending on preparation method, environment, structure and loading conditions. This transition from solid to liquid-like behavior can be followed by applying solid/fracture mechanics and rheology-based testing, respectively. It is clear that most available practical measures of agglomerate mechanical behavior are not intrinsic, i.e. independent of test specimen geometry and the manner in which stress is applied. Therefore, selection and execution of measurements must be guided by loading conditions and agglomerate size and structure from the process of concern. Micromechanical modeling addresses some of the dependence of mechanical properties on the structure of agglomerates [e.g., porosity] and the properties of their primary constituents, but it cannot describe quantitatively bulk deformation and fracture of agglomerates. For this reason, agglomerate formulations are still tailored to achieve desired performance by empirical correlation of primary particle and agglomerate structure to mechanical properties.

Effect of primary particle size on granule growth and endpoint determination in high-shear wet granulation

The effect of primary particle size on granule growth and endpoint determination during high-shear wet granulation was investigated. Three different grades of lactose monohydrate, having different volume mean particle sizes (39, 84, and 127 μm), were granulated with water in a 25-l high-shear mixer. Increasing primary particle size results in larger, less porous wet granules. This is consistent with the expectation that both the capillary and viscous interparticle forces decrease with increasing primary particle size, and the resulting granules become more deformable. Increasing the volume of granulating liquid reduces the porosity, but has only a minor influence on wet granule size. In contrast, the apparent dry granule size increases markedly with increasing granulating liquid. Changes in the impeller torque correlated reasonably well with changes in the wet granule size distribution, although torque is not a state function of wet granule size. It is also influenced by primary particle size and the chaotic nature of wall build-up and collapse. Impeller torque correlated poorly with apparent dry granule size. This is because of the changing nature of interparticle forces upon drying. Thus, understanding the relationship between impeller torque and dry granule size requires understanding both wet and dry granule interparticle forces and how they are influenced by pore saturation and primary particle size. One needs to be keenly aware of these limitations if using impeller torque to determine granulation endpoint.

Carbon and oxygen atom mobility during activation of Mo2C catalysts

Abstract Temperature-programmed desorption (TPD) and reduction (TPR) were used to study the activation of Mo 2 C for ethylene hydrogenation at 298 K. Mo 2 C can incorporate large amounts of oxygen, and since oxygen is a poison for the reaction, it must be removed from the surface to activate the catalyst. As oxygen is removed from the surface by evacuation or reduction, it is replenished by diffusion of oxygen from the bulk. Therefore, the equivalent of several monolayers must be removed to obtain an oxygen-free surface, although it is not necessary to remove all the bulk oxygen. Furthermore, the oxygen cannot be removed by evacuation or reduction without also removing a significant amount of bulk carbon. Although the initial activity of the catalyst for ethylene hydrogenation is independent of the amount of carbon removed, the rate of deactivation of the Mo 2 C strongly depends on the extent of carbon removal. As more carbon is removed, the rate of deactivation increases and the shape of the activity-time curve (deactivation curve) changes from an exponential decay to a sigmoidal shape. TPR spectra of oxygen chemisorbed on the surface after two different activation procedures showed that the surface of the catalyst depends on the extent of carbon removal. As more carbon is removed from the Mo 2 C, the oxygen is bound more tightly to the surface. This suggests that ethylene also might be bound more tightly on the carbon-deficient catalyst, which is consistent with the higher rate of deactivation.

Growth rates and mechanism of electrochemical vapor deposited yttria-stabilized zirconia films

Abstract Electrochemical vapor deposition (EVD) is a leading technology to deposit thin gas tight films of yttria-stabilized zirconia (YSZ) over porous substrates for solid oxide electrolyte fuel cell or steam electrolyzer applications. EVD occrus via a tarnishing-type process where oxygen is supplied to the growing film face in the form of anions permeating the film while the zirconium and yttrium are supplied as gaseous metal chlorides. Films of YSZ were deposited onto porous substrates; film growth followed a parabolic rate law, indicating a growth mechanism which is limited by charge transport though the film. The activation energy of film growth, 3.9 eV, indicates film growth is limited by electron transport. A model for film growth is presented which is consistent with these observations.

The role of PtOx in the isothermal rate oscillations of ethylene oxidation on platinum

Abstract A kinetic model has been developed to explain the oscillatory phenomena observed during the oxidation of ethylene on polycrystalline Pt films in a CSTR. Direct measurement of the oxygen activity on the Pt catalyst indicates that rate and oxygen activity oscillations are caused by the periodic formation and decomposition of surface platinum oxide. The model explains semiquantitatively all the experimental observations, i.e., that (1) oscillations occur on the fuel rich side only; (2) increasing rates correspond to decreasing surface oxygen activity; (3) there exist an upper and a lower temperature limit for oscillations; (4) the frequency of oscillations is a linear function of both the ethylene/O2 ratio and the residence time in the CSTR. The thermodynamic properties of PtOx are estimated from the oxygen activity measurements. The model may be applicable to other Ptcatalyzed oscillatory reactions as well.

Double Layer Capacitance of Porous Platinum Electrodes in Zirconia Electrochemical Cells

This paper reports on the capacitance of the double layer at the interface between porous platinum electrodes and yttria-stabilized zirconia measured by potential step chronoampermetry. The capacitance is independent of oxygen partial pressure and electrode potential and increases from 0.2 {mu}F/cm{sup 2} at 555{degrees}C to 1.3 {mu}F/cm{sup 2} at 695{degrees}C. These value are at least an order of magnitude smaller than capacitances extracted from the low-frequency portion of ac impedance spectra. This indicates that the capacitive behavior of platinum electrodes in zirconia cells is dominated by time-dependent faradaic processes.