The performance of microflotation, dispersed air flotation with microbubble clouds with bubble size about 50 µm, for algae separation using fluidic oscillation for microbubble generation is investigated. This fluidic oscillator converts continuous air supply into oscillatory flow with a regular frequency to generate bubbles of the scale of the exit pore. Bubble characterization results showed that average bubble size generated under oscillatory air flow state was 86 µm, approximately twice the size of the diffuser pore size of 38 µm. In contrast, continuous air flow at the same rate through the same diffusers yielded an average bubble size of 1,059 µm, 28 times larger than the pore size. Following microbubble generation, the separation of algal cells under fluidic oscillator generated microbubbles was investigated by varying metallic coagulant types, concentration and pH. Best performances were recorded at the highest coagulant dose (150 mg/L) applied under acidic conditions (pH 5). Amongst the three metallic coagulants studied, ferric chloride yielded the overall best result of 99.2% under the optimum conditions followed closely by ferric sulfate (98.1%) and aluminum sulfate with 95.2%. This compares well with conventional dissolved air flotation (DAF) benchmarks, but has a highly turbulent flow, whereas microflotation is laminar with several orders of magnitude lower energy density. Biotechnol. Bioeng. 2012; 109:1663–1673. © 2012 Wiley Periodicals, Inc.

No abstract

In general, there are three ways of generating microbubbles. The most common class uses compression of the air stream to dissolve air into liquid, which is subsequently released through a specially designed nozzle system, to nucleate small bubbles as potentially nanobubbles, based on the cavitation principle. These bubbles subsequently grow into much larger bubbles through the rapid dissolution of the supersaturated liquid. The second class uses power ultrasound to induce cavitation locally at points of extreme rarefaction in the standing ultrasonic waves. The third class uses an air stream delivered under low offset pressure, and airs to break off the bubbles due to an additional feature, whether it be mechanical vibration, or flow focussing, or fluidic oscillation. Conventional air diffusers rely on the structure of porous material for the nozzles to generate small bubbles, but fluidic oscillation in general promises to break off the forming bubble while it is still a hemispherical cap-the smallest shape for which bubble formation from a pore is likely to occur given the strong adverse affect of surface tension at higher curvatures. The first two classes of microbubble generation are usually associated with high power densities and power consumption by either the compression or ultrasonic treatment. The third class should have the lowest power consumption, provided it achieves the application targets of bubble size distribution, air phase holdup, and bubble dispersion. In this paper, recent patents in microbubble generation are categorized into the first and the third classes above. The subject area is reviewed for its importance in several fields of application, particularly generalized flotation processes and bioreactor treatments.

Microbubble generation by a novel fluidic oscillator driven approach is analyzed, with a view to identifying the key design elements and their differences from standard approaches to airlift loop bioreactor design. The microbubble generation mechanism has been shown to achieve high mass transfer rates by the decrease of the bubble diameter, by hydrodynamic stabilization that avoids coalescence increasing the bubble diameter, and by longer residence times offsetting slower convection. The fluidic oscillator approach also decreases the friction losses in pipe networks and in nozzles / diffusers due to boundary layer disruption, so there is actually an energetic consumption savings in using this approach over steady flow. These dual advantages make the microbubble generation approach a promising component of a novel airlift loop bioreactor whose design is presented here. The equipment, control system for flow and temperature, and the optimization of the nozzle bank for the gas distribution system are presented. §1 Introduction Airlift reactors are perceived to have performance advantages over bubble columns and stirred tank bioreactors for many applications, biorenewables production in particular.Where the product is a commodity biochemical or biofuel, energy efficiency is the primary concern. There are multiple objectives for the optimization of energy efficiency, however. The hydrodynamics of stirring is an important consideration, as are the phase transfer of nutrient influx and the efflux of inhibitor products and byproducts. Finally, the metabolism of cells or microbes engaged in the biochemical production are a major constraining factormass transfer from the bulk liquid to the bioculture must be maintained. There are two important reasons to use airlift loop bioreactors (ALB) that arise from the airlift effects: flotation and flocculation. Small bubbles attached to particles or droplets significantly lower the density of the aggregrate. Grammatika and Zimmerman (2001) describe these generalized flotation effects. Such aggregates are susceptible to floc formation. Typically, microbes or cells that sediment out of the suspension accumulate in stagnation zones at the bottom expire.Given the importance of energy usage in the operation of ALBs, it is surprising that the sparging system, which is the central power consumption feature of the ALB, has not received more attention. Jones (2007) gives a good review of the major features of ALB, including the conventional types of sparger design. Chisti and Moo-Young (1987) classify the spargers used in the ALB as dynamic and static. Dynamic spargers use injection through nozzles to disperse the gas introduced. Static spargers are typically less reliant on the momentum of the jet, and the gas is introduced typically through a perforated plate (see Deshpande and Zimmerman, 2005a,b) or less commonly, through a porous baffle (Heijnen and Van't Riet, 1984). This study was motivated by the development of a novel microbubble generation technique based on fluidic oscil...