Microflotation performance for algal separation

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

Aerator design for microbubble generation

Fine bubbles are a key component in improving the performance of gas-liquid reactors, particularly in situations where reactions are mass transfer limited. Many aerator types exist for different reactor applications; however conventional aerators are mostly suited to coarse bubble generation. A new aerator suitable for microbubble generation by fluidic oscillation has been designed and tested with the view of getting a uniform bubble distribution across the aerator. Microbubbles generated from various membrane pore sizes and oscillation frequencies were characterized for this aerator to determine the optimum operating parameters. It was evident that the introduction of a flow distributor plate to the plenum chamber improved gas distribution from the inlet to the porous membrane leading to uniform bubble generation across the entire aerator The resultant average bubble size from this new design under oscillatory flow was found to be approximately 2-3 times the membrane pore size. This outcome has a great potential to promote the efficiency of multiphase reactors where mass transfer plays a key role

Evaporation dynamics of microbubbles

Until recently, generating clouds of microbubbles was a relatively expensive proposition, with the smallest bubbles requiring high energy density from either the saturation–nucleation mechanism or Venturi effect. Due to the expense of processing with microbubbles, exploration of the acceleration effects of microbubbles for physico-chemical processes are largely unstudied, particularly those that are combined effects. In this paper, the trade-off between heat transfer and evaporation on the microbubble interface are explored, largely by computational modelling but supported by some experimental evidence. The hypothesis is that both processes are inherently transient, but that during short residence times, vaporization is favoured, while at longer residence times, sensible heat transfer dominates and results in re-condensation of the initially vaporized liquid. The computational model address how thin a layer thickness will result in the maximum absolute vaporization, after which sensible heat transfer condenses the vapour as the bubble cools. This maximum vaporization layer thickness is estimated to be a few hundred microns, on the order of a few microbubble diameters at most. If the maximum vaporization estimate and the contact time necessary to achieve it are accurately estimated, these are engineering design features needed to design a vaporizing system to achieve maximum removal of vapour with minimum heat transfer. The modelling work presented here should be considered in light of the humidification experiments also conducted which showed the exit air at 100% saturation, but increasing gas temperature with decreasing layer height, and decreasing water temperature with decreasing layer height, all of which are consistent with the predictions of the computational model

Experimental investigation of helicity in turbulent swirling jet using dual-plane dye laser PIV technique

This paper reports a new method of generating two light sheets using a dye laser system and the use of this dual-plane dye laser system to analyse average helicity and energy dissipation in a turbulent swirling flow. The dual-plane PIV system that was used in this study consisted of three cameras and a single frequency Nd:YAG laser, which was used to generate two parallel light sheet planes with differing wavelengths(colour). The method of generating two different light sheet wavelengths using a single laser source is an innovative and new technique. Stereoscopic PIV measurements were obtained in one plane with the use of two CCD cameras, and standard PIV measurements were obtained in the other plane with the use of one CCD camera. The light scattered by the particles on two different light sheets were separated using appropriate optical filters. The measurements obtained were used to estimate the components of the velocity gradient tensor. The tensor components were then used to determine the average vorticity components and helicity quantities of the fluid that was investigated. To determine the average turbulent kinetic energy dissipation, the continuity equation was used to infer the out-of-plane gradient of the out-of-plane velocity. From the analysis of the results, it was found that regions with high helicity were correlated with regions of high turbulent kinetic energy dissipation. © 2008 Springer-Verlag

Design of an airlift loop bioreactor and pilot scales studies with fluidic oscillator induced microbubbles for growth of a microalgae Dunaliella salina

This study was conducted to test the feasibility of growing microalgae on steel plant exhaust gas, generated from the combustion of offgases from steel processing, which has a high CO content. Two field trials of batch algal biomass growth, mediated by microbubble transfer processes in an airlift loop bioreactor showed only steady growth of biomass with 100% survival rate. The gas analysis of CO uptake in the 2200L bioreactor showed a specific uptake rate of 0.1g/L/h, an average 14% of the CO available in the exhaust gas with a 23% composition of CO. This uptake led to a steady production of chlorophyll and total lipid constituency in the bioreactor, and an accelerating exponential growth rate of biomass, with a top doubling time of 1.8days. The gas analysis also showed anti-correlation of CO uptake and O production, which along with the apparent stripping of the O to the equilibrium level by the microbubbles, strongly suggests that the bioreactor is not mass transfer limited, nor O inhibited. Removing O inhibition results in high growth rates and high density of biomass. © 2011 Elsevier Ltd

Aerator design for microbubble generation

Fine bubbles are a key component in improving the performance of gas-liquid reactors, particularly in situations where reactions are mass transfer limited. Many aerator types exist for different reactor applications; however conventional aerators are mostly suited to coarse bubble generation. A new aerator suitable for microbubble generation by fluidic oscillation has been designed and tested with the view of getting a uniform bubble distribution across the aerator. Microbubbles generated from various membrane pore sizes and oscillation frequencies were characterized for this aerator to determine the optimum operating parameters. It was evident that the introduction of a flow distributor plate to the plenum chamber improved gas distribution from the inlet to the porous membrane leading to uniform bubble generation across the entire aerator The resultant average bubble size from this new design under oscillatory flow was found to be approximately 2-3 times the membrane pore size. This outcome has a great potential to promote the efficiency of multiphase reactors where mass transfer plays a key role

Dual-plane PIV investigation of acoustically excited jets in a swirl nozzle

© Author(s) 2016.A novel dual-plane dye laser particle image velocimetry (PIV) technique used to analyze helicity and energy dissipation in an unexcited turbulent swirling jet of pressurized cold air has established that regions within the flow field of the jet exhibiting high helicity are correlated regions of high turbulent kinetic energy dissipation. This PIV configuration provides estimates of all components of the velocity gradient tensor, facilitating calculation of the helicity from the vorticity components. Application of this novel dual-plane PIV technique is extended in this study to investigate helical structures in a turbulent swirling jet where the underlying shear flow is subjected to external acoustic sinusoidal forcing in a plane perpendicular to the central axis of the jet. It was found that acoustic excitation had a significant effect on the mean velocity profile parallel to the direction of the jet. The horizontal forcing resulted in the generation of vorticity that was skewed with a pitch that favored a distribution of angles around 90° with respect to the velocity vector. The distribution of the time-averaged helicity angle indicated organized helical activity, but such activity is not dominated by large-scale coherent structures of maximal helicity

CO mass transfer induced through an airlift loop by a microbubble cloud generated by fluidic oscillation

Carbon dioxide is one of the most common gases produced from biological processes. Removal of carbon dioxide from these processes can influence the direction of biological reactions as well as the pH of the medium, which affects bacterial metabolism. Kinetics of carbon dioxide transfer mechanisms are investigated by sparging with conventional fine bubbles and microbubbles. The estimate of the concentrations of CO (aq), H CO , HCO3 , and CO from pH measurement in an airlift loop sparged mixer is derived. The canonical estimate of overall mass transfer coefficient of CO has been estimated as 0.092 min for a microbubble size of 550 m compared with 0.0712 min for a fine bubble (mean bubble size of 1.3 mm) sparging. It is observed that the efficiency of CO removal has increased up to 29% by microbubble sparging compared with fine bubble sparging. Laminar bubbly flow modeling of the airlift loop configuration correctly predicts the trend of the change in overall mass transfer in both gas stripping with nitrogen and gas scrubbing for CO exchange, while demonstrating the expected separated flow structure. The models indicate that the macroscale flow structure is transient and pseudoperiodic. This latter feature should be tested by flow visualization, as preferential frequencies in the flow can be exploited for enhanced mixing. © 2011 American Chemical Society

Experimental investigation of helicity in turbulent swirling jet using dual-plane dye laser PIV technique

This paper reports a new method of generating two light sheets using a dye laser system and the use of this dual-plane dye laser system to analyse average helicity and energy dissipation in a turbulent swirling flow. The dual-plane PIV system that was used in this study consisted of three cameras and a single frequency Nd:YAG laser, which was used to generate two parallel light sheet planes with differing wavelengths(colour). The method of generating two different light sheet wavelengths using a single laser source is an innovative and new technique. Stereoscopic PIV measurements were obtained in one plane with the use of two CCD cameras, and standard PIV measurements were obtained in the other plane with the use of one CCD camera. The light scattered by the particles on two different light sheets were separated using appropriate optical filters. The measurements obtained were used to estimate the components of the velocity gradient tensor. The tensor components were then used to determine the average vorticity components and helicity quantities of the fluid that was investigated. To determine the average turbulent kinetic energy dissipation, the continuity equation was used to infer the out-of-plane gradient of the out-of-plane velocity. From the analysis of the results, it was found that regions with high helicity were correlated with regions of high turbulent kinetic energy dissipation. © 2008 Springer-Verlag

Influence of Surface Wettability on Microbubble Formation

The production and utilization of microbubbles are rapidly becoming of major importance in a number of global applications, from biofuel production to medical imaging contrast agents. Many aspects of bubble formation have been studied, with diffuser characteristics (such as pore size, pore orientation) and gas flow rate all being shown to influence the bubble formation process. However, very little attention has been paid to the influence of surface wettability of the diffuser and the detailed role it plays at the triple interface of gas–liquid–diffuser. Here, we investigate how the wettability of the diffuser surface impacts upon the dynamics of the bubble formation process and examine the effect both at the orifice and upon the bubble cloud produced as a result of the engineered wetting variations. Experimental data shown here indicate the presence of a switching point at a contact angle of θ = 90°, where bubble size vastly changes. When a surface exhibits a contact angle below 90°, bubbles emitted from it are considerably smaller than those emitted from a surface with an angle in excess of 90°. This effect is observable over flow rates ranging from 2.5 to 60 mL min^–1 from a single pore, an array of controlled pores, and the industrially relevant and commercially available sintered metals and sintered ceramic diffusers. It is also observed for both thiol and silane modified surfaces, encompassing a range of contact angles from 10° to 110°. In addition, the importance of the diffuser plate’s surface topography is discussed, with elevated roughness acting to reduce the effect of surface chemistry in some instances