Simulating air absorption in a hydraulic air compressor (HAC)

Abstract

A hydraulic air compressor (HAC) is a no moving parts compressor that uses hydropower to compress air that is inducted through a Venturi effect. The solubility of gases in water increases with pressure and provides a means for the dissolved gas to bypass the air-water separator and reduce the compressed air yield of a HAC. The model developed and fully described herein extends the approach of Chen and Rice (1983) to allow for a multiplicity of gas mixtures, predicts the compressed air yield reduction from gas solubility, and predicts the compressed air yield improvement by increasing the liquid temperature and the presence of pre-dissolved salts. The model simulated multiple scenarios that adjust the parameters that are known to affect air absorption in the downcomer and returns results consistent with the expected behaviour in each scenario. One of such scenarios was the simulation of the downcomer of the Ragged Chutes HAC installation. The results were compared with the single data point of oxygen concentration in compressed air delivered by a HAC and results from an equilibrium solubility model available in the literature. Under the same conditions, the model described herein predicts a concentration of oxygen of 0.181 mol/mol, a value that is 2.3% higher than the equilibrium condition. In order to match the results of the equilibrium model, the mass diffusivities of nitrogen, oxygen, argon and carbon dioxide needed to be artificially increased from their reported values in the literature by a factor of 2000. This suggests that the solubility kinetics are important to consider in the design of a HAC when predicting the compressed air yield but if the equilibrium assumption is adopted the estimate of compressed air yield will be conservative.