The mass transfer from a stagnant CO2 gas pocket to the flowing water in a horizontal pipe was investigated experimentally, considering the application of pH adjustment by injecting gaseous CO2 for raw water in water treatment industries. In the experiments, the variation of the CO2 gas pocket volume and the corresponding pH values of the pipe flow with time under different conditions were recorded. The mass transfer coefficient of the CO2 gas pocket in the pipe flow was then calculated. The results showed that the injection of gaseous CO2 into the pipe flow could effectively adjust the pH. The volume of the CO2 gas pocket decreased exponentially. Different from most studies on bubble mass transfer, it was found that the ambient CO2 concentration could not be neglected in this study due to the large volume of the gas pocket and the restricted space in the pipe. The mass transfer coefficient increased with the increasing ratio of the CO2 injection rate to the water flow rate and exhibited a sharp reduction as the volume of the CO2 gas pocket decreased by about 80%. The outcomes of this paper can contribute to a better understanding of gas bubble mass transfer in pipe flows.

  • The mass transfer of a large CO2 gas pocket in the pipe flow was investigated, which has been hardly reported before.

  • The pH change can be divided into three regimes, and the mass transfer coefficient of the CO2 gas pocket exhibited two stages of values, which is new knowledge.

  • The CO2 concentration in the pipe flow is important to mass transfer due to the large volume of the gas pocket and the restricted space in the pipe.

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