Determination of the mechanisms causing and limiting separations by column crystallization

Abstract

The process of column crystallization at total reflux and steady state was analyzed mathematically and experimentally in order to determine the mechanisms by which separations are achieved and limited. The results of the study indicate that separation is produced by the formation of crystals in the freezing section and by interphase mass-transfer in the adjacent purification section. The separation is mainly limited by axial eddy diffusion in the liquid phase. Data from three binary chemical systems were used in the study. The first system, m -chloronitrobenzene- m -bromonitrobenzene, forms a solid solution with a small phase separation (less than 6 wt. %) and was the primary subject of this study. The second system, azobenzenestilbene, forms a solid solution with a phase separation which exceeds 20 wt. %. The third system, benzene-cyclohexane, has essentially no solid solubility in the range of compositions for which data on separation have been reported (½ to 3 wt. % cyclohexane). Four mathematical models of column crystallization were developed and compared with experimental data. These models were based on different sets of assumptions as to the mechanisms which control the overall separation. The model which is consistent with experimental data was used to evaluate mass transfer coefficients and effective liquid-phase diffusivities. Diffusivities varied between 1.3 and 4.6 sq. cm./sec., and mass transfer coefficients varied between 0.07 and 0.64 cm./sec. in reasonable agreement with results obtained in studies of liquid-liquid extraction in pulsed columns.