Hydrodynamics and mass transfers in bubbly two-phase flow systems with applications to efficient gas compression in hydraulic gas compressors

Abstract

A steady one-dimensional integral drift-flux formulation for co-current two-phase (water-air) bubbly flow tubes of devices known as hydraulic air compressors (HAC) is presented. The model simultaneously evaluates hydrodynamics with interphase mass transfers, including psychrometric aspects. The model is verified with reported experimental observations of CO2 absorbed in bubble columns as well as from a 5-metre-high pilot HAC fabricated to support the research. The model shows good agreement with the observations of gas volume fraction and gas-phase concentration of CO2 along the axis of the bubble column. The model is extended to deal with transient cases to predict liquid-phase and gas-phase concentrations of gas species. Allowing for the transients to steady state, the bubbly flow model can predict the operating conditions of the pilot HAC (within 10 % of measured) for varying inlet gas-phase concentrations of CO2 from 1 % to 10 % by mole in the gas phase and 20 ppm to 80 ppm by mass in the liquid phase. Some of the work considered a ∼30 m demonstrator scale HAC, constructed to support the re- search. A wire-to-air audit of the previously published performance led an upward revision of the peak efficiency of the installation from 72.9 % to 74.4 % applying when an in-line air-water mixing head was deployed. An updated performance map of the HAC compression efficiency of the demonstrator is presented. Application of the verified model to simulation of the downcomer flow tube in the various HAC’s of varying scales and ratings considered indicate that the psychrometric effects provide an upper limit of 38 ◦C on process temperature due to losses arising as compressed water vapour condenses. Model and observations confirm that the compression process is isothermal to within ∼10 mK. Performance of a novel air compressor featuring a Venturi injector is presented, designed based on understandings gained from the HAC investigations. The pilot unit was able to produce compressed air at a variable and controllable pressure up to 690 kPa (g) (100 psi (g)) while the pump(s) faced a near constant head as the system pressure increased. Electrical power required was estimated through modification of the pump performance curve due to the observed pump wear. The Venturi injector system compressed was found to compress air with an efficiency of 6.9 % when compressing air to the same pressure as the HAC Demonstrator (approximately 30 psi (g)) and 3.2 % when compressing to 620 kPa (g) (90 psi (g)). Optimization of the injector geometry to maximize air flow and compression efficiency is necessary to realize commercially viable designs.