No abstract

Microbubbles increase the mixing efficiency in airlift bioreactors. Dispersal of gas phase throughout the ALR occurs with decreasing the bubble size. Phase slip velocity decreases with smaller bubble size as gas rise rate decreases. a b s t r a c tAirlift bioreactors can provide an attractive alternative to stirred tanks, particularly for bioprocesses with gaseous reactants or products. Frequently, however, they are susceptible to being limited by gas-liquid mass transfer and by poor mixing of the liquid phase, particularly when they are operating at high cell densities. In this work we use CFD modelling to show that microbubbles generated by fluidic oscillation can provide an effective, low energy means of achieving high interfacial area for mass transfer and improved liquid circulation for mixing.The results show that when the diameter of the microbubbles exceeded 200 mm, the "downcomer" region, which is equivalent to about 60% of overall volume of the reactor, is free from gas bubbles. The results also demonstrate that the use of microbubbles not only increases surface area to volume ratio, but also increases mixing efficiency through increasing the liquid velocity circulation around the draft tube. In addition, the depth of downward penetration of the microbubbles into the downcomer increases with decreasing bubbles size due to a greater downward drag force compared to the buoyancy force. The simulated results indicate that the volume of dead zone increases as the height of diffuser location is increased. We therefore hypothesise that poor gas bubble distribution due to the improper location of the diffuser may have a markedly deleterious effect on the performance of the bioreactor used in this work.

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 CO2(aq), H2CO3, HCO3 –, and CO3 2– from pH measurement in an airlift loop sparged mixer is derived. The canonical estimate of overall mass transfer coefficient of CO2 has been estimated as 0.092 min –1 for a microbubble size of 550 m compared with 0.0712 min –1 for a fine bubble (mean bubble size of 1.3 mm) sparging. It is observed that the efficiency of CO2 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 CO2 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.

In this study, the effect of microfluidic microbubbles on overall gas-liquid mass transfer (CO 2 dissolution and O 2 removal) was investigated under five different flow rates. The effect of different liquid substrate on CO 2 mass transfer properties was also tested. The results showed that the K L a can be enhanced by either increasing the dosing flowrate or reducing the bubble size; however, increasing the flow rate to achieve a higher K L a would ultimately lower the CO 2 capture efficiency. In order to achieve both higher CO 2 mass transfer rate and capture efficiency, reducing bubble size (e.g. using microbubbles) has been proved more promising than increasing flow rate. Microbubble dosing with 5% CO 2 gas showed improved K L a by 30% -100% across different flow rates, compared to fine-bubble dosing. In the real algal culture medium, there appears to be two distinct stages in terms of K L a, divided by the pH of 8.4.

No abstract

BACKGROUNDInactivation processes can be classified into non‐thermal inactivation methods such as ethylene oxide and γ‐radiation, and thermal methods such as autoclaving. The ability of carbon dioxide enriched microbubbles to inactivate Pseudomonas putida suspended in physiological saline, as a non‐thermal sterilisation method, was investigated in this study with many operational advantages over both traditional thermal and non‐thermal sterilisation methods.RESULTSIntroducing carbon dioxide enriched microbubbles can achieve ∼2‐Log reduction in the bacterial population after 90 min of treatment, addition of ethanol to the inactivation solution further enhanced the inactivation process to achieve 3, 2.5 and 3.5‐Log reduction for 2%, 5% and 10 %( v/v) ethanol, respectively. A range of morphological changes was observed on Pseudomonas cells after each treatment, and these changes extended from changing cell shape from rod shape to coccus shape to severe lesions and cell death. Pseudomonas putida KT 2440 was used as a model of gram‐negative bacteria.CONCLUSIONUsing CO2 enriched microbubbles technology has many advantages such as efficient energy consumption (no heat source), avoidance of toxic and corrosive reagents, and in situ treatment. In addition, many findings from this study could apply to other gram‐negative bacteria. © 2017 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The cost of microalgae harvesting constitutes a heavy burden on the commercialization of biofuel production. The present study addressed this problem through economic and parametric comparison of electrochemical harvesting using a sacrificial electrode (aluminum) and nonsacrificial electrode (graphite). The harvesting efficiency, power consumption, and operation cost were collected as objective variables as a function of applied current and initial pH of the solution. The results indicated that high harvesting efficiency obtained by using aluminum anode is achieved in short electrolysis time. That harvesting efficiency can be enhanced by increasing the applied current or the electrolysis time for both electrodes materials, where 98% of harvesting efficiency can be obtained. The results also demonstrated that the power consumption with graphite anode is higher than that of aluminum. However, at 0.2 A the local cost of operation with graphite (0.036US$/m3), which is distinctly lower than that of aluminum (0.08US$/m3). Furthermore, the harvesting efficiency reached its higher value at short electrolysis time at an initial pH of 6 for aluminum, and at an initial pH of 4 for graphite. Consequently, the power consumption of the harvesting process could be reduced at acid- nature conditions to around 0.46KWh/Kg for aluminum and 1.12KWh/Kg for graphite.

Article Highlights• Highest adsorption efficiencies were obtained at low concentrations and low weights • Increasing the adsorption time can possibly have a negative effect on efficiency • Adsorption time parameter can be a positive factor at high concentrations and biomass Abstract Albizia lebbeck biomass was used as an adsorbent material in the present study to remove methyl red dye from an aqueous solution. A central composite rotatable design model was used to predict the dye removal efficiency. The optimization was accomplished under a temperature and mixing control system (37 °C) with different particle size of 300 and 600 μm. Highest adsorption efficiencies were obtained at lower dye concentrations and lower weight of adsorbent. The adsorption time, more than 48 h, was found to have a negative effect on the removal efficiency due to secondary metabolites compounds. However, the adsorption time was found to have a positive effect at high dye concentrations and high adsorbent weight. The colour removal efficiency was found to increase with the increase in methyl red concentration and decrease the adsorption time at constant adsorbent weight.