Chang Kyun Choi

Particle morphology of calcium carbonate precipitated by gas–liquid reaction in a Couette–Taylor reactor

Abstract The morphology and size of calcium carbonate particles produced by CO2–Ca(OH)2 reaction precipitation in a Couette–Taylor reactor were investigated experimentally. It was found that the change of particle morphology and size was caused dominantly by the excess species of reactants in the solution. As their measure, the relative enhancement factor Er involving the operation variables was introduced for the first time. The largest mean particle size and the most cube-like particle shape were observed at Er=1, implying the stoichiometric reaction condition. As the Er-value deviates from unity, particle morphology shifted to the spindle-like shape and mean particle size decreased with an increase in excess species concentration in the solution. The effect of excess species on particle size was predicted by the Bliznakov equation based on the monolayer adsorption. The correlation of particle size at various operating conditions represented the experimental data very well.

Synthesis of fluorine-containing graft copolymers of poly(perfluoroalkylethyl methacrylate)-g-poly(methyl methacrylate) by the macromonomer technique and emulsion copolymerization method

A new method of synthesizing graft copolymers having a poly(perfluoroalkylethyl methacrylate) (PFMA) backbone has been suggested and illustrated by preparing poly(perfluoroalkylethyl methacrylate)-g-poly(methyl methacrylate)s (PFMA-g-PMMA). These copolymers were prepared using the macromonomer technique and emulsion copolymerization method. PMMA macromonomers were synthesized by acylation of hydroxyl-group terminated PMMAs which were prepared by radical oligomerization of methyl methacrylate with a chain-transfer agent. The PMMA macromonomer was copolymerized with a comonomer of perfluoroalkylethyl methacrylate (FMA) by emulsion polymerization to obtain graft copolymers of PFMA-g-PMMA. Toluene was chosen as a suitable solvent to obtain stable emulsions containing PMMA macromonomer and FMA for copolymerization. Graft copolymers having various PMMA compositions and branch lengths could be prepared. It is confirmed that the desired graft copolymers were successfully prepared and that the method suggested for synthesizing PFMA-g-PMMAs is useful in the manufacture of various surface modification agents.

Effect of Taylor vortices on calcium carbonate crystallization by gas–liquid reaction

Abstract The effects of Taylor vortices on the mean size, size distribution, and morphology of calcium carbonate crystallized by the reaction of gaseous carbon dioxide and aqueous calcium hydroxide were experimentally investigated in a Couette–Taylor reactor. Via the absorption of CO 2 gas into pure water, the mass transfer coefficient of CO 2 at the gas–liquid interface was measured relative to the rotating speed of the inner cylinders in the Couette–Taylor reactor. Polymorphs of rhombohedral-, spindle-, and needle-shaped calcium carbonate crystals were observed with a change in the operating variables, including the reactant concentration and flow rate and the rotating speed of the inner cylinder in the Couette–Taylor reactor. Using a dimensionless parameter consisting of the operating variables, it was demonstrated that the morphological changes in the calcium carbonate crystals in the present reactor could be anticipated from the operating conditions. In the crystallization, the mean crystal size, predominantly controlled by crystal agglomeration, was most influenced by the fluid dynamic conditions related to the rotating speed of the inner cylinder and reactor geometry.

An analysis of the onset of compositional convection in a binary melt solidified from below

The onset of compositional convection during solidification of a two-component melt is analyzed by using the propagation theory we have developed. For the case of a plane interface the disturbance equations are produced under linear theory and these are transformed similarly by using the compositional boundary-layer thickness as the length scaling factor. The numerical results are compared favorably with existing experimental data.

The onset of mushy-layer-mode instabilities during solidification in ammonium chloride solution

Convective instabilities in the mushy layer during time-dependent solidification in aqueous ammonium chloride solution cooled from below are analyzed under linear stability theory by using propagation theory. The stability equations for liquid and mushy layers are transformed self-similarly by using the developing mushy-layer thickness as a length scaling factor. The critical conditions to mark convective motion in the mushy layer are obtained numerically when the Lewis number is zero. The present predictions compare reasonably well with existing experimental data.

Thermal instability in plane couette flow heated from below

The conditions marking the onset of natural convection due to buoyant forces are investigated in the thermal entrance region of horizontal plane Couette flow. The base temperature profile produced by pure forced convection is approximated as a fourth order polynomial using the integral method. With this approximate base-temperature profile stability criteria are obtained by means of the local stability analysis, its modification, and the natural-amplification analysis. The last one takes into consideration the axial amplification rate of disturbances at the onset of thermal instability. This new concept is tested here for the first time. The consideration of axial amplification of disturbances makes the system more stable. It is shown that the results of the natural-amplification analysis agree well with the existing experimental data.