Model-based design of carousel ion-exchange processes
Abstract
A detailed rate model and simulation package were developed for equilibrium and nonequilibrium mass action ion exchange, the periodic movement of feed and product ports in the multicolumn carousel process, and extracolumn dispersion effects. The model was specifically applied to the development of carousel processes for selective removal of $\rm\sp{137}Cs\sp+$ from alkaline nuclear waste solutions (supernates). Equilibrium data for supernates and a resorcinol-formaldehyde (R-F) cation-exchange resin can be correlated well by an equation of the Freundlich-Langmuir type when Na$\sp+$ is assumed primarily responsible for modulation of Cs$\sp+$ selectivity. The data can not be accurately described by ideal mass action. For supernates with high levels of Na$\sp+$ and K$\sp+,$ a multicomponent Langmuir correlation is used and shows the selectivity order to be $\rm{Cs\sp+}\gg{K\sp+} > Na\sp+.$ Comparison of Cs$\sp+$ breakthrough data, obtained for supernate loading to R-F resin columns, and rate model simulations show that high downflow linear velocities are needed to produce well-packed columns. Under well-packed conditions, intraparticle diffusion is a controlling mass-transfer resistance, and constant pattern breakthrough curves develop that are sharp and symmetric. Development is quicker and breakthrough curves are sharper for lower flow rates, smaller particle sizes, and higher Cs$\sp+$ feed concentrations. At low downflow velocities, intracolumn dispersion effects (i.e., poorly packed columns) are significant resulting in early, asymmetric breakthrough curves with slow approach to equilibrium and slow constant pattern development. Extracolumn dispersion effects are ruled out because they are insignificant in comparison to intracolumn dispersion unless unrealistically large extracolumn volumes are used that produce delayed, asymmetric breakthrough curves. Apparently, loading at high downflow velocities helps minimize buoyancy effects caused by high supernate densities. This is important since throughput and column utilization in the carousel process can be significantly reduced when intracolumn dispersion is controlling. Under well-packed conditions, 100% utilization of Cs$\sp+$ capacity in the lead column and maximum throughput can be achieved while containing the mass-transfer zone in the downstream columns of the carousel process. Higher throughputs can be achieved for smaller particle sizes.
Degree
Ph.D.
Advisors
Wang, Purdue University.
Subject Area
Chemical engineering
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