Comparison of ultrasonic mixing with mechanical mixing in a continuous flow system
Mixing effects produced by ultrasonics in a flow process have been compared with the mixing obtained by a propeller stirrer at different speeds. These studies were carried out in an ultrasonic tank coupled with an ultrasonic generator. The generator produced sound waves at 90 kc./sec. at an average output of 80 watts. Stimulus response technique was used to measure the residence time distributions. Water was allowed to flow through the tank at flow rates of 56, 125, 188, 250 and 320 ml./min. Residence time distributions were measured for each flow rate at the following five conditions: 1) zero r.p.m. 2) ultrasonic mixing 3) with stirrer at 200 r.p.m. 4) stirrer at 265 r.p.m. 5) stirrer at 1800 r.p.m. A slug of sodium hydroxide solution was used as a tracer and the tracer output was measured by a pH meter. A comparison of the residence times thus obtained gave a relative idea of the degrees of mixing obtained for various experiments. For a better understanding of the mixing taking place in the vessel under different experimental conditions, a model was proposed for the various exit age distribution curves obtained experimentally. The new model was compared with the models proposed by Levenspiel, and Bischoff and McCracken. It was found to predict better the curves for stirrer mixing. However, the curves obtained at zero r.p.m. and with ultrasonic mixing were predicted slightly better by Bischoff and McCrackens' model. All these models had dead space and bypass flow as common parameters. The amounts of dead space and bypass flow at the flow rate 250 ml./min. were as follows: 1) zero r.p.m. dead space 20 per cent; bypass flow 20 per cent 2) ultrasonic mixing dead space 4.6 per cent; Bypass flow 5 per cent 3) propeller at 200 r.p.m. Dead space 5 per cent; Bypass flow 5 per cent 4) propeller at 265 r.p.m. Dead space 3 per cent; Bypass flow 3 per cent 5) propeller at 1800 r.p.m. Dead space 1 per cent; Bypass flow 1 per cent Thus, as expected, the mixing obtained with ultrasonics was better than that at zero r.p.m. It is comparable with the mixing obtained at 200 r.p.m. Also, the amounts of dead silence and bypass flow decreased as the intensity of mixing increased. From the mean residence times obtained with ultrasonic mixing, an index has been developed to estimate the degree of mixing produced by ultrasonics in equivalent r.p.m. for a given flow. This index has been named "Sonomixing number". The results show that sonomixing number is significantly affected by the flow rate. As the flow rate decreases, the sonomixing number increases. It is postulated that the sonomixing number may be related to the ultrasonic intensity. Finally a correlation is developed for the mean residence time in terms of the sonomixing number. It should be noted that the results reported in this thesis are applicable under the limitations of the present experiemental setup, namely, the mixing vessel geometry, the flow inlet and outlet positions, the stirrer shape and size, and the present ultrasonic intensity and frequency.