The continuous mixing of particulate solids

A method has been developed by which the process engineer will be able to design continuous mixing systems and to explore mixing strategies, thus enabling him to take advantage of the benefits of the continuous mixing of particulate solids. The method is based on the prediction of the degree of smoothing out of periodic fluctuations in composition from a knowledge of the residence time distribution of the mixer and the nature of the fluctuations. Experimental verification of the method has been carried out in a ribbon blade continuous mixer using both free flowing sand and cohesive chalk. The residence time distributions were measured by adding an impulse of coloured tracer and measuring the outgoing composition, continuously, by a reflectivity device. Models involving a proportion of piston flow and of simple (or perfect) mixers in series were found to fit the experimental distributions. Periodic fluctuations were approximated by a Fourier Series and the amplitude reduction of rectangular periodic impulses were computed and compared with the measured amplitude reduction. With few exceptions the agreement was satisfactory for engineering purposes. It is claimed that the method can be used for designing continuous mixing systems and it is shown how a minor ingredient can be added in periodic impulses rather than a continuous stream to produce a satisfactory mixture.