Robert Pfeffer - Academia.edu (original) (raw)

Papers by Robert Pfeffer

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2005

Droplet formation, droplet interaction and coagulation together with droplet mass transfer are ma... more Droplet formation, droplet interaction and coagulation together with droplet mass transfer are major sub-processes in the developing technology of nanoparticle production by means of solute nucleation inside the emulsion droplet. The solvent (ethanol) droplets containing the solute form during the solvent jet dispergation in the pressurized flow of solvent CO 2 . In the formed two phases flow of solvent-antisolvent emulsion, the solvent diffuses from droplets into antisolvent, while antisolvent dissolves inside solvent droplets. The solvent replacement by the antisolvent causes droplet supersaturation by solute when it occurs near the critical point of solvent-antisolvent emulsion (∼80 bar and 31 • C) and the intradroplet nucleation of solute. To provide the same droplet lifetime and the uniform droplet supersaturation, the hydrodynamic relaxation time for droplets and for two phases flow have to be shorter than their relaxation time of the mass transfer. Above a critical volume fraction of solvent, droplets dissolve partially. Afterwards, i.e. downstream, antisolvent is saturated with solvent, i.e. phase equilibrium establishes within two-phase flow with uniform solute supersaturation inside droplets. Under these conditions, an additional mechanism of the supersaturation is identified, which is droplet specific (the supersaturation caused by increasing solute concentration), and is favorable for small particle production. As long as droplets move along the last quasi-equilibrium section of uniform two-phase flow with length L, the nucleation and a solute precipitation proceed within droplets, i.e. the dimension of formed particles is controlled by droplet residence time τ res which is proportional to L and inversely proportional to stream velocity. The maximal τ res and the precipitation time which affect nanoparticle dimension is restricted by the rate of turbulent droplet coagulation.

Advances in Colloid and Interface Science, 2007

The use of a supercritical Solvent (S)-Antisolvent (AS) process (SAS) for fine particle productio... more The use of a supercritical Solvent (S)-Antisolvent (AS) process (SAS) for fine particle production is finding widespread industrial applications. The perfection of this technology requires insight into many basic laws of interface and colloid science. In SAS the solute is dissolved in an organic solvent and the solution is sprayed into a near critical AS stream. SAS is a complex process involving the interaction of jet hydrodynamics, droplet formation, mass transfer, phase equilibrium, intra-droplet nucleation, and microcrystal growth. A complete description would have to take into account all of these processes; however, such a model is not currently available. In the two-phase flow of an S/AS emulsion, S diffuses from droplets into AS, while AS dissolves inside the S droplets. S replacement by AS (Supercritical CO 2 ) causes solute supersaturation in the droplets. When it occurs near the critical point of the S/AS emulsion (80 bar, 32°C), intra-droplet nucleation and precipitation of the solute occurs. The possibility of solute particle production and the particle size is controlled by the droplet size and by the interrelationship between three time scales. These are the droplet mass transfer time τ N , the nucleation time τ N , i.e., the time necessary for one particle nucleus to form in one droplet, and the droplet residence in the supersaturated stream τ res .

Journal of Fluid Mechanics, Nov 30, 1988

... 8xV(rj3~-~,, 2.19b) Using (2.4), the Cartesian components of the force and torque exerted by ... more ... 8xV(rj3~-~,, 2.19b) Using (2.4), the Cartesian components of the force and torque exerted by the fluid on each particle is given by 4 = -4x[Ejll i+F,lllj+Ejol k], (2.20 a) ... The balance between bouyancy and Stokes drag gives -4x[Ej11 i+Ejllj+EjOl k] = -QEu~( 3 Ps, - P) gk, (2.21a) ...

We have previously reported that the fluidization of nanoparticle agglomerates can be enhanced by... more We have previously reported that the fluidization of nanoparticle agglomerates can be enhanced by the addition of external force fields such as vibration, acoustic waves, centrifugal force, and magnetic particles. The criteria usually used to evaluate the enhancement in fluidization quality are the fluidized bed expansion, pressure drop, and visual appearance of the fluidized bed to determine the presence of bubbles, large heavy agglomerates and/or channeling and spouting.

Powder Technology, 2001

. A model is developed for estimating the coating time in a magnetically assisted impaction coati... more . A model is developed for estimating the coating time in a magnetically assisted impaction coating MAIC device. The mixture of the host, guest and magnetic particles is assumed to be in a fluidized state where the distribution of velocities is a Maxwell-Boltzman type. It is assumed that the collisions among the particles are important for impinging the guest particles onto the surface of host particles, and thus forming a semi-permanent coating on the surface of host particles. The coating time is shown to depend on several parameters, including the number density of host particles, the diameter ratio of the host and guest particles, the height of the fluidized particle bed and the material properties of the host and guest particles. Our model shows that there is an optimal value of the bed height for which the coating time is a minimum. The coating time increases sharply when the bed height is smaller or larger than the optimal value, and also when the diameter of host particles is increased. A comparison of the model results with experimental data shows surprisingly good agreement for the coating time as a function of host particle surface covered considering the many assumptions that were made in deriving the model. q

Chem Eng Commun, 1996

... I - exp (8 + 2)/lQ Authors Robinson (1967 [8]) Deutsch-Anderson (1919,1921 in [I]) Matts-Ohnf... more ... I - exp (8 + 2)/lQ Authors Robinson (1967 [8]) Deutsch-Anderson (1919,1921 in [I]) Matts-Ohnfeldt (1963 in [7]) Cooperman Cooperman (1971 [9]) Kunz-Hanna (1972 [10]) Cooperman-Cooperman (1982 [11]) (1984 [6]) Petersen (1990 [12]) Zhao-Pfeffer (Present model) 2 No. ...

Coating or encapsulation of nanoparticles is a major challenge due to the extremely small size, h... more Coating or encapsulation of nanoparticles is a major challenge due to the extremely small size, high surface energy, and high surface area of the nanoparticles. In this paper we describe a new method using supercritical CO 2 as an anti-solvent (SAS) for nanoparticle coating/encapsulation. A model system, using silica nanoparticles as host particles and Eudragit polymer as the coating material, was chosen for this purpose. The SAS process causes a heterogeneous polymer nucleation with the nanoparticles acting as nuclei and a subsequent growth of polymer on the surface of the nanoparticles induced by mass transfer and phase transition. A polymer matrix structure of encapsulated nanoparticles is formed by agglomeration of the coated nanoparticles. Field emission scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy and Fourier transform infrared spectroscopy were used to characterize the coated/encapsulated silica nanoparticles.

Powder Technology, Apr 1, 2008

An experimental study on filtration of submicron solid and liquid aerosol particles by using a fi... more An experimental study on filtration of submicron solid and liquid aerosol particles by using a filter media composed of agglomerates or granules of nanoparticles is described. Fumed silica nanoagglomerates, carbon black granules, silica shells, activated carbon granules, glass beads and nanoporous hydrophobic aerogel were among the granular filter media tested and compared to a commercially available HEPA fiber-based filter. Other than the glass beads which were used for comparison purposes, the primary particle size of the agglomerates/granules is of nanometer scale, but they agglomerate to form porous structures of about several hundreds of microns which were customized as packed (deep bed) or fluidized bed filters and challenged against submicron solid and liquid aerosols. For packed bed filters, the size of the granules has been optimized to a range of 150-500 µm with a filter thickness of about 1-3 in. and superficial gas velocities of less than 4 cm/s. Fluidized beds required granules smaller than 150 µm and the height of the bed was in the range of 15-40 cm.

Industrial & Engineering Chemistry Fundamentals, 1970

The fluidization of nanoparticle agglomerates can be largely improved by using downward pointing ... more The fluidization of nanoparticle agglomerates can be largely improved by using downward pointing micronozzles, creating a high-velocity jet, as experimentally shown. By discrete particle simulations -treating the agglomerates as single particles -we show that the main reason is probably the reduction of the agglomerate size by agglomerate-agglomerate collisions.