Scale-up Methods for Fast Competitive Chemical Reactions in Pipeline Mixers

An experimental study investigated the scale-up methods for fast competitive/consecutive reactions in static mixers. Twelveelement Kenics helical element mixers of 1/8-, 1/4-, and 1/2-in.-diameters were tested using the fourth Bourne reaction system as a test reaction. The fourth Bourne reaction is a parallel reaction system where the acid-catalyzed hydrolysis of 2,2dimethoxypropane (DMP) is conducted in parallel with the neutralization of HCl with NaOH. With very rapid mixing, the HCl catalyst is neutralized by NaOH and minimal hydrolysis of DMP occurs. The main feed contained 200 mol/m3 of DMP and 210 mol/m3 of NaOH (5% stoichometric excess relative to HCl in the side stream). The side stream, which was introduced through two radial cylindrical feed ports at the midpoint of the third mixer element, contained 2000 mol/m3 of HCl. The two streams were combined in the mixer at a volumetric flow ratio of 10:1, main to side. By analysis of the reactor effluent for DMP hydrolysis products of acetone and methanol, the effectiveness of the mixer-reactor in promoting the acidbase reaction was quantified. The parameters used to evaluate the static mixer performance at different sizes are (1) the Reynolds number (experiment Re varied from 550 to 17 000), (2) the turbulent energy dissipation rate, and (3) the residence time in the mixer. At low flow rates and low energy dissipation, micromixing controlled and equal power dissipation was an acceptable scale-up criterion. At high flow rates and high energy dissipation, mesomixing controlled and equal residence time was the proper scale-up criterion. The recommended scale-up procedure is as follows: (1) Maintain geometrical similarity. (2) Maintain the following parameters constant: (A) ratio of the main to side feed rates, (B) ratio of the main to side feed velocities, (C) ratio of the reagent concentration in the main and side feeds, (D) number of mixing elements in the mixer, and (E) side feed orientation and location relative to the mixing elements. (3) Analyze the experimental data to determine the controlling mixing mechanism, either micromixing or mesomixing: (A) for micromixing control, scale-up using equal power dissipation; (B) for mesomixing control, scale-up using equal mixer residence time.