Influence of successive and independent arrangement of Kenics mixer units on light/dark cycle and energy consumption in a tubular microalgae photobioreactor

The design of mixers in tubular photobioreactors is a key part to reduce the cost of microalgal culture. There have been several investigations of the static Kenics mixer in tubular photobioreactors, but how to use it efficiently has been rarely reported. In this work, the efficiency of light/dark cycle enhancement, defined as the ratio of the dimensionless increment of light/dark cycle frequency to the dimensionless increment of energy consumption, is introduced to evaluate economic aspects of adding a mixer. Then, the 4-unit Kenics mixer consisting of four successive left-right-twisting units and 1-unit Kenics mixer consisting of only one unit are compared in terms of the efficiency using computational fluid dynamics, which shows that an increase of the unit number (from 1 to 4) does not bring a considerable improvement ( < 7.5%) in the L/D cycle frequency, while leads to a much higher pressure drop ( > 36%), and thus a much lower efficiency. Finally, two approaches to increasing the efficiency are developed. One is to adjust the boundary of light and dark zones in the tube to approximate the high vorticity region, and the other is to adopt more effective types of arrangement of units (e.g., the interval arrangement, instead of the already-existing successive arrangement).