Residence time distribution of dispersed liquid and solid phase in a continuous oscillatory flow baffled crystallizer

The use of a continuously operated oscillatory flow baffled crystallizer (COBC) has been promoted as a promising alternative crystallizer design for continuous crystallization because of the claim, based on dispersion of liquid, that plug flow can be achieved. Plug flow can lead to uniformity in product quality, if good control over nucleation and the growth of crystals is also achieved. In this study a residence time distribution (RTD) analysis was made for both homogeneous (methylene blue-water) and heterogeneous tracer system (melamine-water). In literature it is proposed, on the basis of homogeneous tracer experiments only, that the velocity ratio Ψ (the ratio between the oscillatory velocity and the superficial velocity of the imposed flow) is sufficient to identify optimal operating conditions for plug flow in COBC. Multiple combinations of amplitude and frequency result in the desired Ψ value. Our results show that operating at high amplitudes increases dispersion, reducing the plug flow like mixing. Thus Ψ alone is not sufficient for optimizing the mixing. Our study for the first time compares dispersion of homogenous and heterogeneous tracer in the commercially available DN15 system, addressing the knowledge gap in handling solids in COBC. Comparable responses were obtained with both the tracer systems for changes in the oscillatory flow variables. Operation at relative low amplitudes was optimal to obtain plug flow like behavior, even with 10% (w/w) slurry with no problems of the particles settling. The optimal operating condition for minimal dispersion was clearly different for the homogenous and the heterogeneous system.