Turbulent Mixing Mechanism of Neutralization Reaction in a Semi-Batch Stirred Tank

This study investigated the turbulent mixing mechanism of liquid-phase neutralization reactions in a continuous-feed semi-batch stirred-tank reactor which affects the characteristics of the particulate products produced by the neutralization reaction. An experimental method for quantifying the overall neutralization reaction rate constant kaV was developed using pH visualization images, a pseudo-first-order reaction model, and the material balance equation for the base. The effects of impeller rotational speed N, feed flow rate Q and base inflow nozzle diameter din on the overall reaction rate constant were investigated. Relationships between kaV and Q, din, and N were derived based on the micro- (engulfment) and meso- (shedding) mixing models. Two types of neutralization experiments were conducted: (i) continuous liquid injection (jet mixing only), and (ii) continuous liquid injection and stirring (jet and stirring mixing). The results show that in the case of jet mixing only, kaV is determined by the turbulent dissipation rate ε. This indicates that the reaction was determined by the turbulent eddies elongating and entraining the surrounding unreacted fluid owing to viscous deformation (micro-mixing). For jet and stirring mixing with a nozzle set at the impeller tip, kaV is determined by micro- or meso-mixing induced by stirring depending on the strength of ε and turbulent energy k in the discharge flow from the impeller. With a nozzle set far from the impeller tip, kaV was affected by both jet and stirring mixing, depending on the injection and circulating flow in the reaction zone which varies with the nozzle position.