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

Abstract

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 ( Q w) 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 Q w, representing decreasing air-lift pumping efficiency. The CO 2 concentration of the influent and effluent water was measured using submersible infrared CO 2 probes over a range of influent CO 2 concentrations. The CO 2 mass transfer coefficient [( k L a) 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 CO 2 stripping efficiency was effectively the same between salinities, and the influent CO 2 concentration only had a modest effect on CO 2 stripping efficiency. At an influent concentration of 40 mg L −1 the CO 2 stripping efficiency was 1–3% higher than at an influent of 10 mg L −1. The relatively minor effects of salinity and influent CO 2 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, Q w, or G:L ratio. The mass transfer coefficient can then be used to calculate the CO 2 stripping efficiency for any water type (i.e. temperature, alkalinity, salinity and influent CO 2 concentration).