Numerical multiphase modeling of CO₂ absorption and desorption in microalgal raceway ponds to improve their carbonation efficiency

The carbonation efficiency in raceway ponds was improved by modeling CO₂ desorption and absorption between the pond and the atmosphere. The Euler–Euler two-fluid method was used to model gas–liquid flow mixing with mass transfer in the raceway pond. The average gas hold-up, mass transfer coefficient, dissolved CO₂ concentration, CO₂ desorption rate to the atmosphere, and CO₂ absorption rate from the atmosphere were investigated using the effects of sump configuration, pond geometry, and gas–liquid hydrodynamic properties. The carbonation efficiency of the entire raceway pond was investigated by considering the effects of sump geometrical design, aspect ratio, water depth, paddle wheel rotational speed, gas bubble size, and gas mass flux. The CO₂ desorption and absorption rates were estimated using novel equations from the literature. Results showed that the CO₂ desorption rate was low in wide and shallow raceway ponds. The gas–liquid mass transfer increased in ponds with a low aspect ratio and small water depths. The high rotational speeds of the paddle wheel enhanced gas dissolution, and large amounts of CO₂ were desorbed to the atmosphere. Moreover, sump configuration as well as geometrical and gas–liquid hydrodynamic properties significantly affected the carbonation efficiency and algal productivity.