Peter Walzel

Spray Drying of Mannitol as a Drug Carrier—The Impact of Process Parameters on Product Properties

Powders intended for the use in dry powder inhalers have to fulfill specific product properties, which must be closely controlled in order to ensure reproducible and efficient dosing. Spray drying is an ideal technique for the preparation of such powders for several reasons. The aim of this work was to investigate the influence of spray-drying process parameters on relevant product properties, namely, surface topography, size, breaking strength, and polymorphism of mannitol carrier particles intended for the use in dry powder inhalers. In order to address this question, a full-factorial design with four factors at two levels was used. The four factors were feed concentration (10 and 20% [w/w]), gas heater temperature (170 and 190°C), feed rate (10 and 20 L/h), and atomizer rotation speed (6,300 and 8,100 rpm). The liquid spray was carefully analyzed to better understand the dependence of the particle size of the final product on the former droplet size. High gas heater temperatures and low feed rates, corresponding to high outlet temperatures of the dryer (96–98°C), led to smoother particles with surfaces consisting of smaller crystals compared to those achieved at low outlet temperatures (74–75°C), due to lower gas heater temperatures and higher feed rates. A high solution concentration of the feed also resulted in the formation of comparably rougher surfaces than a low feed concentration. Spray-dried particles showed a volume-weighted mean particle size of 71.4–90.0 µm and narrow particle size distributions. The mean particle size was influenced by the atomizer rotation speed and feed concentration. Higher rotation speeds and lower feed concentrations resulted in smaller particles. Breaking strength of the dried particles was significantly influenced by gas heater temperature and feed rate. High gas heater temperatures increased the breaking strength, whereas high feed rates decreased it. No influence of the process parameters on the polymorphism was observed. All products were crystalline, consisting of at least 96.9% of mannitol crystal modification I.

An Experimental and Numerical Study of Transversal Dispersion of Granular Material on a Vibrating Conveyor

The mixing of thin granular layers transported on the surface of an oscillating trough is experimentally and numerically examined. The particle dispersion was experimentally quantified by an image processing system recording the growth of the mixing layer thickness of two differently colored but otherwise identical sand particle streams along the longitudinal position within the transporting channel. Granular flow and dispersion on the vibrating conveyor were studied numerically based on a three-dimensional discrete element code. Both experiments and simulations were used to derive quantities characterizing the transversal dispersion. The mixing was found to be directly proportional to the vertical acceleration of the conveyor and inversely proportional to the mass flow of the transported material. Keeping the above-mentioned parameters constant, the dispersion increases with increasing mean particle diameter. When performing the experiments with materials of different mean particle diameters and tuning the mass flow to achieve the same level of dimensionless bed height, the magnitude of the dispersion coefficient remains constant, as was also confirmed by the numerical simulation.

Impact of excipients on coating efficiency in dry powder coating.

Dry powder coating is a technique to coat substrates without the use of organic solvent or water. The polymer powder is directly applied to the cores to be coated. Liquid additives are often used to lower the glass transition temperature of the polymer and to enhance the adhesion of the powder to the cores. This leads to an increase in coating efficiency of the process. The impact of various liquid additives and their properties like spreading behavior, viscosity and plasticizing activity were investigated with respect to their influence on the coating efficiency of the process. Ethylcellulose and hydroxypropyl methylcellulose acetate succinate were used as coating polymers. Spreading behavior of the liquid additive on the polymer was the most influencing parameter and could be successfully predicted with contact angle measurements on polymer films. Calculations of works of adhesion and spreading coefficients also revealed to be promising predictive techniques for choosing suitable additives to improve process efficiency. Isopropyl myristate showed the best spreading behavior resulting in the highest coating efficiency. Based on these results, a formulation for ethylcellulose containing isopropyl myristate was developed and film formation was examined using dissolution testing and imaging techniques to evaluate the optimum curing conditions.

Adaptation of a new pneumatic nebulizer for sample introduction in ICP spectrometry

A new pneumatic nebulizer, the pneumatic extension nozzle (PEN), originally developed in technical laboratories, mainly for the production of rock and glass wool, is applied for sample introduction in ICP-OES. The droplet formation process under various operating conditions and geometries was investigated using a transparent enlarged model of the PEN. For the application in ICP spectrometry the geometry of the PEN was optimized and miniaturized with the aid of similarity theory. In comparison with a standard concentric nebulizer the miniaturized PEN generates an aerosol with a twice higher mass fraction of droplets with diameters D < 10 μm. An empirical model for the prediction of the mean droplet diameter is presented, using the nozzle diameter as a linear size scale. This model enables a better fit to experimental data compared with existing models. Applying the miniaturised PEN to a simultaneous ICP-OES instrument lowers the detection limits up to a factor of 3.5 depending on the element. The new pneumatic extension nozzle can be easily adapted to existing ICP-OES instruments.

Drop breakup in liquid-liquid dispersions at an orifice plate observed in a large-scale model

Fine emulsions, i.e., a dispersion of fine liquid droplets in another liquid, can be obtained by forced flow of a coarse predispersion through small orifices. Direct observations of droplets at the original scale is nearly impossible. A promising solution is a large scale-up, considering the most important scale factors. The experiments presented in this paper were conducted on a model 250 times the original size, while maintaining strong hydromechanical similarity. The breakup of single oil droplets in water was documented with a highspeed camera.

Reversing granular flow on a vibratory conveyor

Experimental results are presented for the transport properties of granular materials on a vibratory conveyor. For circular oscillations of the shaking trough nonmonotonous dependence of the transport velocity on the normalized acceleration Γ is observed. Two maxima are separated by a regime where the granular flow is much slower and, in a certain driving range, even reverses its direction. Similar behavior is found for a single solid body with a low coefficient of restitution, whereas an individual glass bead of 1 mm diameter is propagated in the same direction for all accelerations.

Influence of pore structure and impregnation-drying conditions on the solid distribution in porous support materials

ABSTRACT Deposition of solids within porous materials from a drying solution is an important phenomenon in numerous natural and industrial processes. A profound knowledge about influences of different parameters on the solid distribution in the material is required for an effective targeted impregnation process. Experimental investigations and simulations are used to study the influence of pore structure, drying conditions, and solute concentration on the solid distribution in porous support materials after impregnation and drying. It is found that low drying rates lead to strong solid accumulation at the material surface, whereas high drying rates reduce the solute transport to the surface and result in more uniform solid distributions. A small pore diameter and distribution width reduce solute migration during drying and lead to uniform solid distributions without being influenced by the drying conditions. A higher initial concentration of the impregnation solution causes pronounced surface accumulation, while low initial solute concentrations result in more uniform distributions. Fundamental effects during drying are captured in an existing pore network model by adaption of experimental pore structures and impregnation–drying conditions, resulting in a good general agreement of experiments with simulations.

Aerodynamically assisted jetting and threading for processing concentrated suspensions containing advanced structural, functional and biological materials

In recent years material sciences have been interpreted right across the physical and the life sciences. Essentially this discipline broadly addresses the materials, processing, and/or fabrication right up to the structure. The materials and structures areas can range from the micro‐ to the nanometre scale and, in a materials sense, span from the structural, functional to the most complex, namely biological (living cells). It is generally recognised that the processing or fabrication is fundamental in bridging the materials with their structures. In a global perspective, processing has not only contributed to the materials sciences but its very nature has bridged the physical with the life sciences. In this review we discuss one such swiftly emerging fabrication approach having a plethora of applications spanning the physical and life sciences.

Experimental Investigation of the Effects of Fluid Properties and Geometry on Forced Convection in Finned Ducts With Flow Pulsation

An experimental study was conducted on the effects of flow pulsation on the convective heat transfer coefficients in a flat channel with series of regular spaced fins. Glycerol-water mixtures with dynamic viscosities in the range of 0.001-0.01 kg/ms were used as working fluids. The device contains fins fixed to the insulated wall opposite to the flat and smooth heat transfer surface to avoid any heat transfer enhancement by conduction of the fins. Pulsation amplitude x o = 0.37 mm and pulsation frequencies f in the range of 10 Hz<f<47 Hz were applied, and a steady-flow Reynolds number in the laminar range of 10 <Re <1100 was studied. The heat transfer coefficient was found to increase with increasing Prandtl number Pr at a constant oscillation Reynolds number Re osc . The effect of the d h /L ratio was found to be insignificant for the system with series of fins and flow pulsation due to proper fluid mixing in contrast to a steady finned flow. A decrease in heat transfer intensification was obtained at very low and high flow rates. The heat transfer was concluded to be dynamically controlled by the oscillation.

Residence Time Distribution of Segregating Sand Particles in a Rotary Drum

The residence time distribution of single quartz sand fractions in a continuously operated rotary drum (D = 0.32 m, L = 2.6 m) was measured with the pulse method within the cascading regime at rotation speeds n ≤ 10 min−1. The particle size distribution (PSD) was composed of single fractions within the particle size range of 0.6 to 1.5 mm to obtain an RRSB-PSD (Rosin, Rammler, Sperling, and Bennet particle size distribution, see, e.g., Stieß, 2009) with a size parameter of x′ = 1.0 mm and a scatter parameter of nRRSB = 3.5. The particles were dosed in a mixed state at flow rates , collected at the drum outlet in samples, and evaluated by a color identification method. From the response curves, the mean residence time of single fractions could be obtained as well as the axial dispersion coefficient. The mean residence time was found to increase for particles with smaller diameters, as these particles are forced inward into the particle layer due to segregation with lower axial transportation displacements per revolution. Typically, the largest fraction was found to move about 1.4 times faster through the drum than the smallest fraction. Reducing the feed flow rate as well as the rotation speed of the drum, the overall residence time increases. Fractions at the border of the PSD exhibit much smaller scatter of residence time and show smaller dispersion coefficients than particles in the middle of the PSD, preferably moving at median trajectories within the layer. The dispersion coefficients were found to be within the range of 1.0 × 10−6 m2/s < Dax < 2.5 × 10−5 m2/s.