Optimizing dissolved air flotation design system

Abstract

Dissolved Air (Pressure) Flotation-DAF, is a well-established separation process that employs micro-bubbles as a carrier phase. This work shows results concerning bubble generation at low working pressures in modified DAF-units to improve the collection of fragile coagula by bubbles. DAF of Fe (OH)3 (as model) was studied as a function of saturation pressure in the absence and presence of surfactants in the saturator. DAF was possible at 2 atm by lowering the air/water surface tension. This fact, which leads to substantial energy savings, was explained in terms of decreasing the "minimum" energy required for bubble nucleation and cavity in the nozzle. More, bubbles-fragile coagula attachment was improved by dividing the recycling water into two: 1) the inclined inlet to the cell (traditional) and 2) inside the separation tank through a water flow inlet situated below the floating bed using a "mushroom" type diffuser. Because of the reduction observed in the degree of turbulence in the conventional collection zone, DAF performance improved yielding high precipitate recoveries

Microbubble Generation with the Aid of a Centrifugal Pump

“… Intermediate values for the independent variables led to maximum separation efficiency (oil‐in‐water); this indicates the correct ranges for these variables and working conditions. The maximum value of the obtained separation efficiency (98 %) corresponds to oil recovery levels with the aid of destabilizing substances such as surfactants or biosurfactants 11, 18. Minimum values led to minimum separation efficiency; this was likely to be due to the increase in microbubble size and low quality of air for microbubble formation. Maximum pressure with an intermediate air and water flow rate ratio for microbubble production led to a low degree of efficiency; this was likely to be due to the reduction in water flow as a result of the small opening of the pressure control valve. Such as that described by Féris et al 11, a maximum air and water flow rate ratio for microbubble production with intermediate pressure led to considerable, but not maximum, efficiency; this was likely to be due to the excessive quantity of microbubble formation components for adequate opening of the pressure control valve. …”

“…The maximum value of the obtained separation efficiency (98 %) corresponds to oil recovery levels with the aid of destabilizing substances such as surfactants or biosurfactants 11, 18.…”

“…Smaller or larger values for the ratios between the air and water flow rates did not produce microbubbles and subjected the system to intense instabilities. The literature provides average values for the pressures recommended for the production of microbubble in DAF systems 11.…”

“…The microbubbles attach to the oil droplets, forming a larger, lighter structure that rises to the surface of the flotation chamber. The efficiency of floc formation is directly proportional to the intensity of the microbubble flow 11. Herein, the microbubble production efficiency and water‐oil separation efficiency were correlated through visual observations and quantitative analysis of the microbubble production conditions.…”

“…Despite the effectiveness of DAF in the water‐oil separation process, the production of microbubbles is costly 10 because the use of compressors and pressurized tanks or the use of multiphase pumps for the production of saturated effluence involves a considerable expenditure of energy 11. Thus, more compact, economical equipment is needed.…”