Light Transfer And Gas-liquid Mass Transfer In Microalgae Cultivation Systems

Microalgae are considered as one of the most potential alternatives to fossil fuels. To date, large scale cultivation of microalgae has become a main bottlenecks of restricting the industrialization of microalgal biotechnology. To our knowledge, previous studies on microalgae cultivation system has been conducted from the cultivation conditions of biology, such as lighting, temperature, concentration of inorganic carbon and oxygen, and so on. Many kinds of open and closed cultivation systems have been developed for microalgae cultivation. However, the research on the effects of transport phenomena, such as light transfer, light/dark cycle, gas-liquid mass transfer on the microalgae cultivation is few. In this dissertation, based on the structure of microalgae cultivation system, the light transfer in open cultivation system and gas-liquid mass transfer in closed cultivation system were studied by theoretical analysis, experiments and numerical simulations.Firstly, based on the radiative transfer theory, a 1-dimensional mathematical model of light distributions in microalgae cultivation systems was developed via obtaining the light scattering characteristics of microalgae and micro-bubble by Mie theory. The accuracy and feasibility of mathematical model was verified with the light distribution experimental measurements. The results showed that the method in this paper is suitable to describe and predict the light distributions in microalgae cultivation system with a wide range of concentrations of microalgae cells.Secondly, the effects of light intensity, light wavelength and intermittent light of medium frequency ?light/dark cycle and the ratio of light-to-dark? on the cultivation of Chlorella pyrenoidosa were experimentally measured. The results showed that the light intensity plays a dominated role in cultivation of Chlorella pyrenoidosa. The optimal light intensity for cultivation of Chlorella pyrenoidosa ranged from 50 ?E·m-2·s-1 to 220 ?m·m-2·s-1. At a higher light intensity, the red light could enhance the growth rate of Chlorella pyrenoidosa. While, at a lower light intensity, the blue light should be more suitable for cultivation of Chlorella pyrenoidosa. Moreover, the results indicated that the light/dark cycle had little impacts on the growth of Chlorella pyrenoidosa in medium frequency of intermittent light. The increase in the ratio of light-to-dark could improve the growth rate of Chlorella pyrenoidosa effectively.Thirdly, the light/dark characteristics in open cultivation systems ?raceway pond? were investigated by numerical simulations. Based on the light distributions in microalgae cultivation system, the light/dark characteristics of microalgae cells was obtained. Then, the effects of structures of raceway pond and operation conditions on light/dark characteristics in raceway pond were discussed. The results indicated that the decrease of the rotational speed of paddlewheel, installation of flow-deflector baffles and V-shape raceway pond could improve the light cycle time and the ratio of light-to-dark in raceway pond, and then enhance the productivity of microalgae.Finally, for closed cultivation systems, the CO₂ gas-liquid mass transfer in flat-plate airlift photobioreactor was investigated. The gas holdup distribution in 3-dimensional flat-plate airlift photobioreactor were simulated by CFD method, and the overall volumetric mass transfer coefficient of CO₂ were calculated and validated by experimental measurements. Then, the effects of the number of flat and waved baffle, and the position of baffle installation on the gas-liquid mass transfer coefficient in flat-plate airlift photobioreactor were investigated by numerical simulations. The results showed that with the increase of Ad/Ar the gas-liquid mass transfer coefficient increased firstly and then decreased for single baffle. While for double baffles, it decreased with the increase of Ad/Ar. In addition, the waved baffles could have higher gas-liquid mass transfer coefficient only at lower Ad/Ar than flat baffle for the baffle numbers investigated. The effects of wave numbers and wave amplitudes of waved baffle on the gas-liquid mass transfer coefficient are related to Ad/Ar.The research fruits of this studies not only for enhancing the productivity of microalgae but also has important implications for designing, optimization and scale-up of cultivation systems.