While the experiments are thus conceptually straightforward, this is not always the case with respect to the interpretation and extraction of the true mass accommodation coefficient because of the simultaneous occurrence of all of the processes depicted in Fig. 5.12. The approach to extracting a from the measurements of the net gas uptake was treated above in Section E.l.
For gas-liquid combinations with relatively small uptake coefficients 10~4-10~7), longer interaction times between the gas and liquid are needed than can be obtained with the falling-droplet apparatus. These are provided in a bubble apparatus, a typical example of which is shown in Fig. 5.24. The gas of interest as a mixture with an inert carrier gas is introduced as a stream of bubbles into the liquid of interest. The interaction time is varied by moving the gas injector relative to the surface. The composition of the gas exiting the top of the liquid is measured as a function of the interaction time (typically 0.1-1 s), e.g., by mass spectrometry. The interaction time is limited by the depth in the liquid at which the bubbles are injected and their buoyancy. Longer interaction times and better control over them have been achieved using a modified apparatus in which the bubbles are generated and transported horizontally (Swartz et al., 1997).
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