Chapter 9 - Modeling of particle formation in supercritical fluids (SCF)

Chapter 8 - State of the Art Modeling of Particle Formation in Supercritical Fluids

The effect of initial drop size on particle size in the supercritical antisolvent precipitation (SAS) technique

Experimental studies of particle formation from solution droplets were conducted using a newly developed monodisperse spray drying process. Solutes beclomethasone dipropionate and caffeine were dissolved in ethanol, pressurized hydrofluoroalkane propellant 134a, and mixtures thereof. Solutions were atomized into monodisperse microdroplets using a custom droplet generator installed in a laboratory scale spray dryer, enabling drying and collection of the resulting monodisperse microparticles. The effects of droplet diameter, solution concentration, solvent composition, and drying rate on the physical properties of the dried particles were evaluated. Particle morphology and size were assessed using ultramicroscopy and image analysis of micrographs. Extent of crystallinity and polymorphism were investigated using Raman spectroscopy. The drying temperature was found to have a large effect on the morphology of amorphous beclomethasone dipropionate particles. Particles dried near room temperature were spheroidal to ellipsoidal with prevalent surface concavities and evidence of shell buckling; increasing the drying temperature for fixed droplet size and composition resulted in a transition to more spherical, smooth-surfaced particle morphologies. Crystalline caffeine microparticles were made up of assemblies of multiple crystallites. The measured length and breadth of these crystallites was found to be correlated with the time available for crystal nucleation and growth as calculated using a particle formation model. The results highlight the abilities and limitations of currently available particle formation models in elucidating the relationships between the size, composition, and evaporation rate of drying solution droplets and the physical properties of the resulting particles. The work demonstrates the suitability of monodisperse spray drying as an experimental technique for investigating the fundamentals of particle formation from solution droplets.© 2018 American Association for Aerosol Research

Simple model of particle formation by homogeneous and heterogeneous nucleation

Are classical nucleation theory and the 1950 LaMer model of particle formation supported for a wide range of particle formations, or do competing models in the form of chemical reaction mechanisms have better experimental support? Read on to find out.

In this work, a model for particle formation and growth is used to simulate the aluminum particle synthesis by an evaporation-condensation process. The effect of commonly employed process parameters (vaporization temperature, cooling rate, system pressure) is investigated. The model to be solved is the general dynamic equation (GDE) that accounts for particle nucleation, condensation and coagulation at non-isothermal conditions. The GDE was solved using the nodal method. The methodology approximates the particle size distribution to a few nodes by introducing size-splitting operators. The simulation results show that particle formation and growth take place in a short temperature range. The coagulation increases the particle size while maintaining the number concentration. On the other hand, the surface condensation tends to shift the particles size distribution towards a larger size. It was shown that production of nanoparticles with a more uniform size distribution and smaller particles are favored using low pressure and low vaporization temperature. The particle size distribution is nearly influenced by the cooling rate in the range study here.

This article reports on detailed models of soot particle formation in combustion. First, we present the polycyclic aromatic hydrocarbon-primary particle (PAH-PP) model where soot particles are described by primary particles which are made up of PAHs. The model describes the formation, growth and oxidation of soot in laminar premixed ethylene flames. The connectivity between primary particles is stored to calculate the rounding of soot particles due to surface growth and condensation. We then show that a model intermolecular potential based on the simple Lennard-Jones potential supports the physical binding of PAHs as a viable mechanism for soot formation. We subsequently present the kinetic Monte Carlo-aromatic site (KMC-ARS) model which describes the structure and growth of planar PAHs. The PAH processes are represented as jump processes, and the energetics and kinetics were determined by quantum chemistry calculations. Lastly, we use molecular dynamics with a new potential specifically developed for PAH interaction and the combined PAH-PP/KMC-ARS model to show that pyrene dimerisation is unlikely to be the critical soot formation step at flame temperatures of about 1500–2000 K.

Abstract. The formation and growth of atmospheric aerosol particles is considered using an exact discrete method with molecular resolution in size space. The method is immune to numerical diffusion problems that are a nuisance for typical simulation methods using a sectional representation for the particle size distribution. For condensational growth, a slight modification is proposed for the Fuchs-Sutugin expression, which improves the prediction of the growth rate of nano-sized particles by as much as a factor of two. The presented method is applied to particle formation in a Finnish Boreal forest and is shown to capture the essential features of the dynamics quite nicely. Furthermore, it is shown that the growth of the particles is roughly linear, which means that the amount of condensable vapour is constant (of the order 1013 1/m3).

When dissolved in N,N-dimethylformamide and then dialyzed against phosphate-buffered saline, A-B-A block copolymers composed of poly [N5-(2-hydroxyethyl)-L-glutamine]-block-poly(gamma-benzyl-L-glutamate)- block-poly [N5-(2-hydroxyethyl)-L-glutamine] form particles. The particles are cage-like structures with average diameters of 300 nm (average polydispersity, 0.3-0.5). They are stable in aqueous solution at 4 degrees C for up to 3 weeks, at which time flocculation becomes apparent. Negative staining and freeze-fracture electron microscopy suggest that cage-like particles are formed by selective association of segregated micelle populations. A model of particle formation is presented in which B blocks form micelles in dimethylformamide. On dialysis against an aqueous solution, the extended A blocks then associate intermolecularly to form rod-shaped micelles, which connect the B block micelles. The result is a meshed cage-like particle. The implications of these observations on the aggregation behavior of polymeric surfactants in dilute solution are discussed.

A model has been developed to predict the formation and growth of metallic particles in a flame incinerator system. Flow fields and temperature profiles in a cylindrical laminar jet flame have been used to determine the position and physical conditions of the species along the flame. The size distribution of the particles formed was approximated by a unimodal lognormal function to describe the aerosol behavior in the flame. The effects of inlet metallic vapor feed concentrations, initial seed sizes and seed concentrations on the resultant particle size distribution are presented. The model has potential for further development to be used as a predictive technique for applications in design and operation of incinerator systems.

Modeling of particle formation in pan granulators with sieve-mill recycle

Modeling of particle formation by spray pyrolysis using droplet internal circulation

ABSTRACTA simulation method is presented which encompasses all relevant mechanisms, which are necessary for the description of the early stages of particle formation in arc discharges. Next to discrete coagulation and nucleation events, a continuous surface growth process is included into the simulation, making thus the description of the evaporation of thermodynamic unstable particles possible. The driving force for the nucleation and growth/evaporation is coupled to the monomer concentration in the gaseous phase and thus subject to change in the further course of the simulation. It is shown, that the simulation results gained by the incorporation of all three of these processes cannot be reproduced, if one of those processes is not simulated.

Modeling of particle formation by spray drying process

Porous coordination polymers, in particular flexible porous coordination polymer networks that change their network structure on guest adsorption, have enormous potential in applications involving selective storage, separation and sensing. Despite the expected significant differences in their adsorption properties, porous coordination polymer nanocrystals remain largely unexplored, and there have been no reports about studies on flexible porous coordination polymer nanocrystals, mainly due to a lack of preparation methods. Here, we present a new technique for the rapid preparation of porous coordination polymer nanocrystals that combines non-aqueous inverse microemulsion with ultrasonication. Uniform nanocrystals of {[Zn(ip)(bpy)]}(n) (ip = isophthalate, bpy = 4,4'-bipyridyl; CID-1), a flexible porous coordination polymer, have been prepared by this method and analysed using field-emission scanning electron microscopy, energy-dispersive X-ray analysis, infrared spectroscopy, Raman spectroscopy and X-ray powder diffraction. A model for particle formation and growth is presented and discussed. Adsorption experiments with methanol show that the overall adsorption capacities of nanoparticles and bulk are almost identical, but the shapes of the sorption isotherms differ significantly and the adsorption kinetics increase dramatically.