Monodisperse Primary Particles Research Articles

Influence of polydispersity and breakage on stochastic simulations of spray fluidized bed agglomeration

Monte Carlo (MC) model is a stochastic and discrete method to enhance the understating of spray fluidized bed (SFB) agglomeration in the field of food, chemical and pharmaceutical industry. Previous studies focused on agglomerates composed of monodisperse primary particles. Size dispersity of primary particles is, though, an important aspect to fully assess the morphological features of an agglomerate. In the current work, a polydisperse tunable aggregation model is therefore developed. The radius of gyration, porosity, surface area and agglomeration rate increase with primary particle dispersity. Furthermore, the breakage of already formed agglomerates is introduced in the MC model. The influence of breakage decreases the overall growth rate and the arithmetic mean diameter of the particle size distribution by around 11%. The overall growth rate and the diameter of agglomerates with a similar number of primary particles increase with decreasing temperature of fluidization gas and increasing mass fraction of binder.

Self-organization of colloidal particles during drying of a droplet: Modeling and experimental study

Formation of structurized micro/nanoparticle aggregates in spray drying process is analyzed theoretically and experimentally. Colloids of mono- and bimodal particle size distribution are used as the precursors to demonstrate different patterns of particle self-organization inside the drying droplet. In case of monodisperse primary particles their self-organization in the final aggregate results in either a hollow or a full (packed) spherical structure. For primary particles with bimodal size distribution, either the layered structure of aggregates is formed (with smaller particles forming outer layer and the bigger particles captured inside) or the ordering of bigger particles on the aggregate surface is observed, depending on process parameters. Numerical investigations allow to predict and explain the conditions at which self-assembling of particles within powder aggregates takes place.

Coagulation of Agglomerates Consisting of Polydisperse Primary Particles.

The ballistic agglomeration of polydisperse particles is investigated by an event-driven (ED) method and compared to the coagulation of spherical particles and agglomerates consisting of monodisperse primary particles (PPs). It is shown for the first time to our knowledge that increasing the width or polydispersity of the PP size distribution initially accelerates the coagulation rate of their agglomerates but delays the attainment of their asymptotic fractal-like structure and self-preserving size distribution (SPSD) without altering them, provided that sufficiently large numbers of PPs are employed. For example, the standard asymptotic mass fractal dimension, Df, of 1.91 is attained when clusters are formed containing, on average, about 15 monodisperse PPs, consistent with fractal theory and the literature. In contrast, when polydisperse PPs with a geometric standard deviation of 3 are employed, about 500 PPs are needed to attain that Df. Even though the same asymptotic Df and mass-mobility exponent, Dfm, are attained regardless of PP polydispersity, the asymptotic prefactors or lacunarities of Df and Dfm increase with PP polydispersity. For monodisperse PPs, the average agglomerate radius of gyration, rg, becomes larger than the mobility radius, rm, when agglomerates consist of more than 15 PPs. Increasing PP polydispersity increases that number of PPs similarly to the above for the attainment of the asymptotic Df or Dfm. The agglomeration kinetics are quantified by the overall collision frequency function. When the SPSD is attained, the collision frequency is independent of PP polydispersity. Accounting for the SPSD polydispersity in the overall agglomerate collision frequency is in good agreement with that frequency from detailed ED simulations once the SPSD is reached. Most importantly, the coagulation of agglomerates is described well by a monodisperse model for agglomerate and PP sizes, whereas the detailed agglomerate size distribution can be obtained by scaling the average agglomerate size to the SPSD.

The effect of primary particle polydispersity on the morphology and mobility diameter of the fractal agglomerates in different flow regimes

Properties of colloidal and aerosol agglomerates depend on their morphology. Accurate estimation of the mobility-equivalent diameter dm in different flow regimes is essential in many industrial processes and measurements. Previous work on the hydrodynamic properties of clusters focussed on agglomerates composed of monodisperse primary particles. However aggregates formed in real processes, e.g. soot particles, are usually formed from polydisperse monomers. Using numerically-generated agglomerates it is shown here that the radius of gyration, surface area, and mass of the agglomerates increase with primary particle polydispersity (given constant geometric mean primary particle size dpg). Here, dm is taken as the projected area-equivalent diameter for the free molecular regime; Stokesian Dynamics is used to compute dm in the continuum flow regime. For fixed number of primaries and dpg, dm increases with polydispersity in both free molecular and continuum regimes (>20% for large particles at high polydispersity). Considering an aerosol population with polydisperse primary particles, this increase is found to depend on whether the variations in primary particle size occur within aggregates or between aggregates; this can be important in the interpretation of measurements. Finally, mobility diameters are correlated with total number, median diameter and its geometric standard deviation of the primary particles.

Optimizing the primary particle size distributions of pressurized metered dose inhalers by using inkjet spray drying for targeting desired regions of the lungs

Conventional suspension pressurized metered dose inhalers (pMDIs) suffer not only from delivering small amounts of a drug to the lungs, but also the inhaled dose scatters all over the lung regions. This results in much less of the desired dose being delivered to regions of the lungs. This study aimed to improve the aerosol performance of suspension pMDIs by producing primary particles with narrow size distributions. Inkjet spray drying was used to produce respirable particles of salbutamol sulfate. The Next Generation Impactor (NGI) was used to determine the aerosol particle size distribution and fine particle fraction (FPF). Furthermore, oropharyngeal models were used with the NGI to compare the aerosol performances of a pMDI with monodisperse primary particles and a conventional pMDI. Monodisperse primary particles in pMDIs showed significantly narrower aerosol particle size distributions than pMDIs containing polydisperse primary particles. Monodisperse pMDIs showed aerosol deposition on a single stage of the NGI as high as 41.75 ± 5.76%, while this was 29.37 ± 6.79% for a polydisperse pMDI. Narrow size distribution was crucial to achieve a high FPF (49.31 ± 8.16%) for primary particles greater than 2 µm. Only small polydisperse primary particles with sizes such as 0.65 ± 0.28 µm achieved a high FPF with (68.94 ± 6.22%) or without (53.95 ± 4.59%) a spacer. Oropharyngeal models also indicated a narrower aerosol particle size distribution for a pMDI containing monodisperse primary particles compared to a conventional pMDI. It is concluded that, pMDIs formulated with monodisperse primary particles show higher FPFs that may target desired regions of the lungs more effectively than polydisperse pMDIs.

The Structure of Agglomerates Consisting of Polydisperse Particles

Agglomeration is encountered in many natural or industrial processes, like growth of aerosol particles in the atmosphere and during material synthesis or even flocculation of suspensions, granulation, crystallization and with colloidal particle processing. These particles collide by different mechanisms and stick together forming irregular or fractal-like agglomerates. Typically, the structure of these agglomerates is characterized with the fractal dimension, Df , and pre-exponential factor, kn , of simulated agglomerates of monodisperse primary particles (PP) for ballistic or diffusion-limited particle-cluster and cluster-cluster collision mechanisms. Here, the effect of PP polydispersity on Df and kn is investigated with agglomerates consisting of 16 - 1024 PP with closely controlled size distribution (geometric standard deviation, σ g = 1-3). These simulations are in excellent agreement with the classic structure (Df and kn ) of agglomerates consisting of monodisperse PPs made by four different collision mechanisms as well as with agglomerates of bi-, tri-disperse and normally distributed PPs. Broadening the PP size distribution of agglomerates decreases monotonically their Df and for sufficiently broad PP distributions (σ g > 2.5) the Df reaches about 1.5 and kn about 1 regardless of collision mechanism. Furthermore with increasing PP polydispersity, the corresponding projected area exponent, Dα , and pre-exponential factor, ka , decrease monotonically from their standard values for agglomerates with monodisperse PPs. So Df as well as Dα and ka can be an indication for PP polydispersity in mass-mobility and light scattering measurements, if the dominant agglomeration mechanism is known, like diffusion-limited and/or ballistic cluster-cluster coagulation in aerosols.

A Composite Formation Route to Well‐Crystalline Manganese Oxide Nanocrystals: High Catalytic Activity of Manganate–Alumina Nanocomposites

AbstractManganese oxide nanocrystals are combined with aluminum oxide nanocrystals to improve their crystallinity via calcination without a significant increase of crystal size. A nanocomposite, consisting of two metal oxides, can be synthesized by the reaction between permanganate anions and aluminum oxyhydroxide keggin cations. The as‐prepared manganese oxide–aluminum oxide nanocomposite is X‐ray amorphous whereas heat‐treatment gives rise to the crystallization of an α‐MnO2 phase at 600 °C and Mn3O4/Mn2O3 and γ‐Al2O3 phases at 800 °C. Electron microscopy and N2 adsorption‐desorption‐isotherm analysis clearly demonstrate that the as‐prepared nanocomposite is composed of a porous assembly of monodisperse primary particles with a size of ∼20 nm and a surface area of >410 m2 g−1. Of particular interest is that the small particle size of the as‐prepared nanocomposite is well‐maintained up to 600 °C, a result of the prevention of the growth of manganate grains through nanoscale mixing with alumina grains. The calcined nanocomposite shows very‐high catalytic activity for the oxidation of cyclohexene with an extremely high conversion efficiency of >95% within 15 min. The present results show that the improvement of the crystallinity without significant crystal growth is very crucial for optimizing the catalytic activity of manganese oxide nanocrystals.

Generation and Geometrical Analysis of Dense Clusters with Variable Fractal Dimension

The generation and geometrical analysis of clusters composed of rigid monodisperse primary particles with variable fractal dimension, df, in the range from 2.2 to 3 are presented. For all generated aggregate populations, it was found that the dimensionless aggregate mass, i, and the aggregate size, characterized by the radius of gyration, Rg, normalized by the primary particle radius, Rp, follow a fractal scaling, i = kf(Rg/Rp)df. Furthermore, the obtained prefactor of the fractal scaling, kf, is related to df according to kf = 4.46df-2.08, which is in agreement with literature data. For cases when df cannot be directly determined from light scattering or confocal laser scanning microscopy, it can be estimated from its relation with a perimeter fractal dimension, dpf, or a chord fractal dimension, dcf, both obtained from 2D projection of aggregates. A relation between df and dpf of the form df = or-1.5dpf + 4.4 was developed by fitting data obtained in this work for 2.2 < df < 3 together with data of Lee and Kramer [Adv. Colloid Interface Sci. 2004, 112(1-3), 49-57] for 1.8 < df < 2.4. It was found that the method of determining df via dpf is very robust with respect to an artificially introduced blur. In contrary, a relation between df and dcf could only be established for the case of ideal optical analysis, while the introduction of blur results in a significant effect on the chord length distribution (and its moments), up to the point of impeding the evaluation of dcf. Hence, for compact aggregates, it is recommended to determine df from dpf by applying the proposed relation, which is valid in a broad range of df relevant for industrial praxis, with little effect of blur on it. Apart from scaling relations with respect to aggregate mass and size, it was found that the 3D quantities, i and Rg, can be directly related to the area squared over perimeter, A2/P, and the 2D radius of gyration, Rg,2D, respectively, which are obtained from 2D projections. In particular, the following two relations are provided: i = 4.5(A2/P)0.9 and Rg/Rp = 1.47 (Rg,2D/Rp)0.99.

Analysis of light scattering from soot using optical cross sections for aggregates

Soot formed in flames usually consists of aggregates (clusters or agglomerates) of a variable number of nearly spherical, monodisperse primary particles (monomers or spherules). In this work, the optical properties of polydisperse aggregates are used to analyze light scattering data from a coannular ethene diffusion flame. In previously reported studies, data have been obtained on the local extinction and volumetric scattering cross sections from laser scattering experiments, on the flame velocity field from laser velocimetry, and on the primary particle sizes determined by electron microscopy. The present analysis yields the average number of primary particles per aggregate, the mean-square radius of gyration, the soot volume fraction and the aggregation rate. It is found that sustained collisional growth of the aggregates occurs while their primary particles growth through heterogeneous reactions low in the flame, and contract through surface oxidation in the upper half of the flame. A recent value of the refractive index gives internally consistent moment ratios of the aggregate size distribution function. This method of analysis provides a more detailed and complete description of the formation, growth and oxidation of soot aggregates in a diffusion flame.