Carbon dioxide degassing in fresh and saline water. II: Degassing performance of an air-lift

A study was undertaken to measure the efficiency with which carbon dioxide was stripped from freshwater (0‰) and saline water (35‰ NaCl) passing through an air-lift at 15 °C. The air-lift was constructed of 50 mm (OD) PVC pipe submerged 95 cm in a tank, had an adjustable air injection rate, and could be adjusted to three lifting heights: 11, 16 and 25 cm.

The gas to liquid ratio (G:L) was high (1.9–2.0) at low water discharge rates (Qw) and represented the initial input energy required to raise the water up the vertical riser section to the discharge pipe. The air-lift increased in pumping efficiency rapidly thereafter, to a G:L minima of 0.3–0.6 at 60–70 L min−1.

After this point the G:L ratio increased with Qw, representing decreasing air-lift pumping efficiency. The CO2 concentration of the influent and effluent water was measured using submersible infrared CO2 probes over a range of influent CO2 concentrations. The CO2 mass transfer coefficient [(kLa)20] ranged from 0.025 to 0.468.

Increasing lift height increased mass transfer, which was attributed to both the increased G:L ratio and the contact time inside the air-lift. The relative effect of lift height and pumping rate on mass transfer was such that a 5 cm increase in lift height was approximately equal to a G:L increase of 0.5.

The CO2 stripping efficiency was effectively the same between salinities, and the influent CO2 concentration only had a modest effect on CO2 stripping efficiency. At an influent concentration of 40 mg L−1 the CO2 stripping efficiency was 1–3% higher than at an influent of 10 mg L−1. The relatively minor effects of salinity and influent CO2 concentration on stripping efficiency contrasted with a companion study investigating the stripping efficiency of a cascade column.

The difference was attributed to the low-to-moderate mass transfer efficiencies of the air-lift. A general equation was derived for the airlift that allows one to calculate the mass transfer coefficient for a given lift height, Qw, or G:L ratio. The mass transfer coefficient can then be used to calculate the CO2 stripping efficiency for any water type (i.e. temperature, alkalinity, salinity and influent CO2 concentration).