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Esters are among the highest volume of industrial organic compounds produced. They are frequently employed in various domestic and industrial processes. Fischer esterification regarded as the most common and widely practiced process of ester synthesis, faces serious limitations of low conversion and high reaction time attributed largely to establishment of equilibrium. Ester hydrolysis, reverse reaction to esterification, starts by supply of a byproduct- water. Several approaches have been developed to avoid equilibrium establishment and to improve overall conversion and rate of reaction, a significant difference exists between the current industrial practices and optimum esterification process/conditions. In the current review, a critical analysis of esterification techniques is conducted. Catalytic, non-catalytic thermal esterification, enzymatic esterification, along with factors affecting their productivity are discussed in detail. The current barriers, future challenges and potential of the esterification technologies are analyzed. Based on the comprehensive-data analysis, a novel technology-based solution is proposed.

Waste resources are an attractive option for economical the production of biodiesel; however, oil derived from waste resource contains free fatty acids (FFA). The concentration of FFAs must be reduced to below 1 wt.% before it can be converted to biodiesel using transesterification. FFAs are converted to fatty acid methyl esters (FAMEs) using acid catalysis, which is the rate-limiting reaction (~4000 times slower than transesterification), with a low conversion as well, in the over biodiesel production process. The study is focused on synthesizing and using a bifunctional catalyst (7% Sr/ZrO2) to carry out esterification and transesterification simultaneously to convert waste cooking oil (WCO) into biodiesel using microbubble-mediated mass transfer technology. The results reveal that a higher conversion of 85% is achieved in 20 min using 7% Sr/ZrO2 for biodiesel production. A comprehensive kinetic model is developed for the conversion of WCO in the presence of a 7% Sr/ZrO2 catalyst. The model indicates that the current reaction is pseudo-first-order, controlled by the vapor–liquid interface, which also indicates the complex role of microbubble interfaces due to the presence of the bifunctional catalyst. The catalyst could be recycled seven times, indicating its high stability during biodiesel production. The heterogeneous bifunctional catalyst is integrated with microbubble-mediated mass transfer technology for the first time. The results are unprecedented; furthermore, this study might be the first to use microbubble interfaces to “host” bifunctional metallic catalysts. The resulting one-step process of esterification and transesterification makes the process less energy-intensive and more cost-efficient, while also reducing process complexity.

This work explores the following applications of graph theory to plasma chemical reaction engineering: assembly of a weighted directional graph with the key addition of reaction nodes, from a published set of reaction data for air; graph visualisation for probing the reaction network for potentially useful or problematic reaction pathways; running Dijkstra’s algorithm between all species nodes; further analysis of the graph for useful engineering information such as which conditions, reactions, or species could be enhanced or supressed to favour particular outcomes, e.g. targeted chemical formation. The use of reaction-nodes combined with derived parameters allowed large amounts of key information regarding the plasma chemical reaction network to be assessed simultaneously using a leading open source graph visualisation software (Gephi). A connectivity matrix of Dijkstra’s algorithm between each two species gave a measure of the relative potential of species to be created and destroyed under specific conditions. Further investigation into using the graph for key reaction engineering information led to the development of a graph analysis algorithm to quantify demand for conditions for targeted chemical formation: Optimal Condition Approaching via Reaction-In-Network Analysis (OCARINA). Predictions given by running OCARINA display significant similarities to a well-known electric field strength regime for optimal ozone production in air. Time dependent 0D simulations also showed preferential formation for O· and O3using the respective conditions generated by the algorithm. These applications of graph theory to plasma chemical reaction engineering show potential in identifying promising simulations and experiments to devote resources.

Product inhibition and the cost of downstream separations are two main barriers in using lignocellulosic biomass for bioethanol production. If bioethanol can be continuously removed from fermentation broth without affecting the fermentation process, significant gains can be achieved with bioethanol yields and process efficiency. Hot microbubble clouds generated by energy efficient means have been used to remove ethanol from dilute ethanol–water mixtures (∼4% [v/v]) maintained at 60 °C, and the effect of key operating parameters on the stripping rate has been studied. Numerical simulations of a hot microbubble rising in a dilute ethanol–water mixture were also performed to understand the instantaneous concentrations within the gas phase. Increasing the inlet gas temperature from 90 to 150 °C and decreasing the liquid height in the unit from 50 to 5 mm both increased the ethanol stripping rate. However, the benefit of increasing the gas temperature for maximum ethanol removal depended on the liquid height in the unit. Under all operating conditions, ethanol concentration was reduced below ∼2% [v/v] within ∼25 min of operation, demonstrating the potential of hot microbubble stripping for product removal from lignocellulosic fermenters. Implemented effectively in a fermenter, this technology could intensify the bioethanol production process and improve process economics.

With 140MT of ammonia produced per annum, over 2% of the world's total energy output is used for ammonia production. Consequently, the remediation and recovery problem is daunting. Ammonia is a pernicious pollutant with an extremely high affinity for water. Less than 2mgL−1 is harmful to flora and fauna but most waste streams contain between 500 mgL−1-5000mgL−1. Physico-chemical processes recover ammonia but are not highly effective in terms of efficiencies for such dilute solutions. Hot microbubble stripping is investigated here. It involves their injection into thin liquid layers for selective vaporization with limited sensible heating of the substrate liquid phase. The method results in several phenomena not previously observed in literature: (i) 100% separation efficiency in 30 minutes of processing time as opposed to 95% efficiency in 30 hours for the benchmark of industrial stripping; (ii) stripping of ammonia at a pH less than 9, i.e. without addition of alkaline pH modifiers and with air as a carrier gas; (iii) using cold microbubbles to strip ammonia is effective, just slower. The process is enhanced due to high mass transfer coefficients: 1000-3000 times faster than conventional stripping. This effect is explained by a new non-equilibrium chemical thermodynamics evaporation – condensation model based on Langmuir's vapour pressure prediction from the kinetic theory of gases that explains the first order kinetics, its magnitude, and the very strong temperature dependence.

Depleting fossil fuel resources and related environmental concerns have focused the world’s attention on finding sustainable and renewable energy resources such as biofuels. Biodiesel, produced from vegetable oil, animal fats, waste cooking oil, and greases, shows a promising alternative for petroleum-diesel. The conventional biodiesel production methods are time consuming and energy intensive due to mass transfer limitations. This limitation can be reduced by carrying out reaction at a bubble interface which supplies a “local” excess of alcohol to push equilibrium in the forward direction while simultaneously increasing the rate of reaction. During this process, methanol vapors, fed through a sintered borosilicate diffuser, react with the free fatty acids (oleic acid in this case) and catalyst. Owing to the lower flow rates of methanol and pore size of the diffuser (15–40 μm) used, the bubbles produced are significantly smaller 1 mm and lies in the range of 200–600 μm. Kinetic study shows that reaction follows first order kinetics w.r.t. oleic acid. The esterification reaction through bubble reactor achieved 97% conversion of oleic acid in 30 min as compared with 80% in 312 min using conventional methods.

This work focuses on the development of a bespoke rig which allows for the simultaneous use of non-thermal plasmas (NTPs), oscillating gas flow via a Desai-Zimmerman Fluidic Oscillator (DZFO) and a Solid Oxide Cell (SOC) to create a highly efficient energy conversion device to facilitate the reduction of CO2 to CO. Both fluidic oscillation and NTPs have the potential to reduce resistances in a SOC, key to furthering their commercialisation. The potential role of NTPs in improving the kinetics and efficiency of reactions relevant to CO2 reduction, such as the dissociation of CO2, is presented. Performance improvements using a rapidly oscillating gas flow, provided by the DZFO, to minimise concentration polarisation resistance by disrupting boundary layer formation and increasing overall efficiency are also discussed. The intersection of these technologies provide a path for a paradigm shift in the ability to convert waste CO2 into high value feedstock using renewable energy.

Esterification, a reaction extensively used in chemical processing, is limited by the establishment of kinetic equilibrium, i.e. marginally exer/endergonic. The reaction is generally slow with low yield making downstream separation cost intensive. A new heterogeneous contacting method for the synthesis of ethyl acetate through fine bubbles tests the hypothesis that reactive distillation can “pull” the reaction nearer to completion, reducing the downstream separation requirements. It achieves a high yield of ethyl acetate, 79.9% in 35 min, as compared with 64% conversion in 350 min using a conventional method. The kinetics of esterification reaction under bubbly flow conditions are studied – entirely different from the conventional bulk model. The alcohol is fed as vapor within the bubbles which means alcohol is always in deficit, providing an opportunity to convert an equilibrium limited reaction to nearly irreversible one. As the bubbles flow upwards, the reaction proceeds at the “skin” of the bubble. If the esterification reaction occurs at or near the microbubble interface, ethanol is in large excess in the bubble phase, as acetic acid is well below its boiling point in the liquid phase. By Le Chatelier's principle, the local excess ethanol will push the equilibrium towards completion. Similarly, removal of water and ethyl acetate via the “dry” bubble pulls the equilibrium towards completion.

In this study, the transient numerical simulation of the flow in a fluidic oscillator has been performed. The proposed device includes several geometrical modifications of a previously patented apparatus intended for the synthesis of ozone-rich bubbles in an oxygen plasma. Prior to the experimental construction of the proposed fluidic oscillator, the present work performs a numerical study of the internal flow in the proposed design, aimed to determine its feasibility. The unsteady simulations are based on the unsteady Reynolds averaged Navier–Stokes equations coupled to the transition Shear Stress Transport (transition SST) turbulence model due to the low Reynolds numbers considered (3500 and 5000 based on flow bulk velocity). The behavior of the complex fluid flow inside the device, where four main vertical structures develop and interact, along one cycle is described in detail including the turbulent kinetic energy and intermittency in the analysis. Moreover, the effect of increasing the Reynolds number on the pressure oscillation frequency and amplitude is analyzed. In particular, the frequency is increased around a 38% and the amplitude a 100% when switching from a Reynolds number of 3500–5000. The numerical results obtained are encouraging, and the evaluated fluidic oscillator design will be fabricated and analyzed in an upcoming experimental study.

Highlights • Continuous removal of ethanol from dilute mixtures using hot-microbubbles. • Ethanol concentration maintained below the inhibition threshold for organism TM242. • An ethanol productivity up to 14.9 g L−1h−1 was supported by the stripping unit., Product inhibition is a barrier to many fermentation processes, including bioethanol production, and is responsible for dilute product streams which are energy intensive to purify. The main purpose of this study was to investigate whether hot microbubble stripping could be used to remove ethanol continuously from dilute ethanol–water mixtures expected in a bioreactor and maintain ethanol concentrations below the inhibitory levels for the thermophile Parageobacillus thermoglucosidasius (TM242), that can utilize a range of sugars derived from lignocellulosic biomass. A custom-made microbubble stripping unit that produces clouds of hot microbubbles (~120 °C) by fluidic oscillation was used to remove ethanol from ~2% (v/v) ethanol–water mixtures maintained at 60 °C. Ethanol was continuously added to the unit to simulate microbial metabolism. The initial liquid height and the ethanol addition rate were varied from 10 to 50 mm and 2.1–21.2 g h−1 respectively. In all the experiments, ethanol concentration was maintained well below the inhibition threshold of the target organism (~2% [v/v]). This microbubble stripping unit has the potential to operate in conjunction with a 0.5–1.0 L fermenter to allow an ethanol productivity of 14.9–7.8 g L−1h−1 continuously.

A new theory for additional heat transfer convected by a dispersed phase of microbubbles was posited recently. An additional convection term in the heat transport equation reflects the latent heat of vapor of the liquid carried by the microbubbles from hot zones that vaporize more liquid to cold zones where condensation releases the latent heat. This theory was shown to be consistent with analysis of observations of freezing times measured by in the original Mpemba effect study, by inferring heat transfer coefficients fitted by Newton’s law of cooling. In this paper, the scaling analysis, leading to the proposition that the additional heat flux is proportional to the phase fraction of microbubbles, is tested by steady state solutions of the canonical hot wall / cold wall buoyant convection problem. For phase fractions 0.02 and 0.1, the maximum ratio of additional Nusselt number emergent is five, occurring in the microfluidic regime. Increasing the characteristic length of the domain maintains the monotonicity of the increase in additional Nusselt number ratio over the case of no microbubbles present. The additional heat transfer due to the microbubble dispersion, ranging from 5-50%, is found to be nearly proportional to the microbubble phase fraction for the range of 0.02 to 0.2. However, larger characteristic lengths introduce insufficient heat flux from the hot wall to maintain a “driven cavity” flow structure, so that the steady state structure that emerges is a stable stratification with thin boundary layers near the hot and cold walls, with weak shear flow convection. The stable stratification resultant at higher characteristic lengths suppresses the additional heat flux due to microbubble mediation, but only moderately deviating from proportionality.

Background Industrial biotechnology will play an increasing role in creating a more sustainable global economy. For conventional aerobic bioprocesses supplying O2 can account for 15% of total production costs. Microbubbles (MBs) are micron-sized bubbles that are widely used in industry and medical imaging. Using a fluidic oscillator to generate energy-efficient MBs has the potential to decrease the costs associated with aeration. However, little is understood about the effect of MBs on microbial physiology. To address this gap, a laboratory-scale MB-based Saccharomyces cerevisiae Ethanol Red propagation–fermentation bioethanol process was developed and analysed. Results Aeration with MBs increased O2 transfer to the propagation cultures. Titres and yields of bioethanol in subsequent anaerobic fermentations were comparable for MB-propagated and conventional, regular bubble (RB)-propagated yeast. However, transcript profiling showed significant changes in gene expression in the MB-propagated yeast compared to those propagated using RB. These changes included up-regulation of genes required for ergosterol biosynthesis. Ergosterol contributes to ethanol tolerance, and so the performance of MB-propagated yeast in fed-batch fermentations sparged with 1% O2 as either RBs or MBs were tested. The MB-sparged yeast retained higher levels of ergosteryl esters during the fermentation phase, but this did not result in enhanced viability or ethanol production compared to ungassed or RB-sparged fermentations. Conclusions The performance of yeast propagated using energy-efficient MB technology in bioethanol fermentations is comparable to that of those propagated conventionally. This should underpin the future development of MB-based commercial yeast propagation.

Microbubbles have been involved in industrial processing since the 1970s with the introduction of dissolved air flotation into common practice. The turn of the century saw microbubbles become regularly used in medical imaging. But in bioprocessing, only this decade has seen rapid advances in R&D, with some bioprocesses, particularly in wastewater treatment, adopted at full industrial scale, and others at pilot scale, such as anaerobic digestion and fermentation, which is full industrial scale for many biomanufacturing and pharmaceutical processes. This article reviews the methods of microbubble generation only briefly, as it turns out only one generation method, fluidic oscillation through microporous diffusers, has the requisite features for introduction into full scale fermentation processes. Subsequently, six fundamental physicochemical hydrodynamics mechanisms that have been exploited by microbubble innovations in bioprocessing are presented and analyzed, particularly for the roles they play in bioprocessing applications. Some examples are drawn with applications to microalgal and yeast processing, as well as usage in wastewater treatment processes. Because the smallest microbubbles can increase rates in some of these six fundamental processes by several orders of magnitude over conventional processing methods, with the optimal contacting patterns, the promise for wider exploitation in bioprocessing is substantial.

Zimmerman [Chem.Eng.Sci. 238: 116618, 2021] developed a theory for additional convection of latent heat by microbubbles from regions of hot water to regions of cold water, a modified heat transport equation that incorporates this convective effect, and a scaling analysis that gives a rough prediction for how this additional heat transfer is proportional to the phase fraction of microbubbles. This theory is supported by arguing that Mpemba and Osborne (1969) had such microbubbles dispersed, resulting in a strong correlation between the dissolved gas solubility at saturation, dependent on the sole controlled variable – the initial temperature of the freezing experiment – and the inferred heat transfer coefficient. The argument was made that the source of the microbubbles was exogeneous due to vigorous boiling. In this paper, a range of systematic errors and uncertainties concerning the Mpemba effect is explored. The two largest uncontrolled variables are the level of vigorous boiling and the level of dissolved gases in tap water. Using the same approach for estimation of the heat transfer coefficients, the experiments of Bregovic (2012) which had no boiling are analyzed. The resulting heat transfer coefficients are shown to pass the accepted Mpemba line criteria for the occurrence of a Mpemba effect. Employing the scaling analysis for additional heat transfer shows reasonable agreement with the hypothesis that source of microbubbles in Bregovic’s experiments is endogeneous: the saturation-nucleation mechanism for spontaneous growth of microbubbles in supersaturated water. The parallel analysis applied to the experiments of Mpemba and Osborne show consistency with predictions of endogenous microbubble generation, too.

The initial aim of the paper is to dramatically improve the pretreatment stage of biodiesel production, which converts problematic free fatty acids to fatty acid methyl esters, by introduction of a microbubble mediated reactive distillation stage instead of acid pretreatment. This will shift the conventional esterification process towards completion with a yield higher than 80%, even without high excess methanol. Application of ozone microbubbles has the advantage over acid gas catalysis in that it gives higher conversion and leaves no catalyst residue and requires no further catalyst recovery separation steps (a “phantom” catalyst). Unreacted ozone breaks down into oxygen, so the off-gases are just a humid air stream that can be vented. Importantly, ozonolysis breaks carbon–carbon double bonds into aldehydes and carboxylic acids. Many ester species were found after contacting the feedstock with ozone-rich microbubbles, depending on the molecular structure of the alcohols for the ozonolysis of oleic acid with alcohols, i.e., methanol, ethanol, n-propanol, iso-propanol, and n-butanol. In the case of ozonolysis of used cooking oil mixed with methanol, the results from the GC–MS show that all saturated free fatty acids (including palmitic acid, stearic acid, and myristic acid) are converted to methyl esters within 20 h of 60 °C ozonolysis, whereas trace amounts of these chemicals remain at lower temperatures. The results also show that the conversion of oleic acid to form oleic acid methyl ester is 91.16% after 32 h of ozonolysis at 60 °C. Therefore, the free fatty acid content in used cooking oil is less than 1.33%, which makes it suitable as a reactant for biodiesel production via transesterification. However, this result is different from the result provided by ASTM D974 in that the acid numbers decrease dramatically by 25% at the beginning of ozonolysis followed by a plateau. Moreover, if the fluidic oscillator is used to generate bubbles in ozonolysis of oleic acid mixed with methanol, the results show that the yields of ozonolysis product 1-nonanal increase by 30%. This observation means that ozonolysis of oleic acid is relative to the specific interfacial area, and favoured at low liquid temperatures.

Impurities and colloidal substances are two of many fouling conditions that reduce the membrane filtration performance used in wastewater treatment. This study investigates the potential of fluidic-oscillation-generated microbubbles (MBs) to defoul the filtration membrane. Cartridge filters for microfiltration (MF) of 1 μm pore size were fouled using surface seawater collected from the Hull coastal area. The seawater was circulated at 5.8 L/min to actuate colloidal substance deposition on the membrane surface. The recorded feed channel pressure drop (ΔP) across the membrane filters showed rapid fouling occurred in the first 8 hrs of the circulation. Fluctuations of ΔP during the next 8 hrs were observed showing the colloids filling the pores of the membrane, and remaining steady for 2 hrs showing the membrane was completely fouled. The filtration membrane was cleaned and defouled using fluidic-oscillator-generated MBs. The fouled membranes were sparged with 1 L/min of air scouring for ∼1 to ∼2 hrs to remove the deposited colloids and impurities on the surface of the membrane. The membrane, analysed by Scanning Electron Microscopy (SEM), UV254 and Electrical Conductivity (EC) meter, showed the extent of MBs-mediated removal of the deposited colloidal particle from the membrane surfaces. This study found that the highest defouling rate occurs with MBs generated by fluidic oscillator (closed vent), followed by MBs generated by fluidic oscillator (opened vent) and MBs generated without fluidic oscillator at 9.53, 6.22, and 3.41 mbar/min, respectively.

One of the greatest challenges faced by humanity is the changing global environmental trends. Evidence points out to the increased greenhouse gas emission by humans as the main cause. One of the possible solutions to reduce CO 2 emissions is to produce value added products or the precursors which can be used as the feedstock for the former. In the current study CO 2 is converted to CO by dielectric barrier discharge (DBD)-corona hybrid plasma treatment while maintaining low power consumption. The kinetics of CO formation by CO 2 plasmolysis is studied and analysed in detail. A well mixed system kinetics model has been proposed to estimate the reaction time for the experimental studies. Optical emission spectroscopy has been used to study CO 2 plasma and mixtures of CO 2 /N 2 , CO 2 /water vapour and CO 2 /Ar plasma. A parametric study varying length of electrode, voltage, frequency, CO 2 flow rate and pressure in plasma chamber is conducted. CO formation was found to be favoured at 600 mL/min flow rate, 1 atm pressure, 4.8 kV voltage at 40 kHz of frequency with 9 cm electrode.

Sustainability and the price of feedstock are the key parameters governing the economics of biodiesel production. Higher price of biodiesel is the major obstacle in its commercialization. Chicken fats, otherwise a waste and environmental nuisance, are an attractive feedstock for cleaner biodiesel production because of low cost and worldwide availability. Moreover, Establishment of thermodynamic equilibrium in esterification reaction limits the overall conversion and reduces the rate of reaction. A new vapor (methanol bubble)-liquid contacting method has been proposed to analyze the hypothesis that bubble reactor can “push” the biodiesel production near completion. Simultaneous removal of water from the system would “pull” the equilibrium in the forward direction. Kinetic investigation shows that the reaction occurs, indeed, at the vapor-liquid interface and consequently yielding a higher conversion of 89.90% in 30 min -- significantly higher than any previously reported studies for chicken fat based biodiesel. Furthermore, the use of, an economical and already used at commercial grade catalyst, p-Toluene sulfonic acid eliminates the need to produce a catalyst at commercial scale before it could be used in an industrial process. These factors make microbubble mediated biodiesel production from chicken fat oil using p-Toluene sulfonic acid a lucrative option.

Viscoelastic fluids can be difficult to model due to the wide range of different physical behaviors that polymer melts can exhibit. One such feature is the viscous elastic boundary layer. We address the particular problem of a viscoelastic shear-dependent fluid flowing past a corner and investigate how the properties of the boundary layer change for a White-Metzner fluid. The boundary layer equations are derived and the upstream layer is matched with the far-field flow. It was found that if the fluid is sufficiently shear thinning then the viscoelastic boundary layer formulation fails due to the inertial forces becoming dominant. The depth of the boundary layer is controlled by the shear-thinning parameters. These effects are not a feature of other shear-thinning models, such as the Phan-Thien-Tanner model. This study provides insight in the different effects of some commonly used viscoelastic models in corner flows in the upstream boundary layer, the downstream boundary layer is not addressed.

Microbubbles dispersions in aqueous solutions can be long lived. For instance, 20micron size microbubbles take on the order of a day to rise one meter. Consequently, any currents in a reasonably sized vessel would be expected to entrain such a microbubble dispersion as the buoyant force is exceeded by the inertial force of liquid currents. This paper argues for the advantages of a microbubble dispersion mediated condenser with two benefits. The obvious advantage over fine bubble direct contact heating or cooling is that the microbubble phase, which can be engineered with a throughput of approximately a hectare per second of interfacial area flux per cubic meter of solution volume, should not be limited by heat transfer to and from the liquid and microbubble phase. Rather the limitation will be on the wetted area for heat transfer of the vessel to its heat exchange configuration. The second potential advantage follows from the theory proposed in this paper. Arranging the condenser in the microbubble mediated airlift configuration will introduce additional heat transfer from microbubbles vaporizing hotter water near the central plume and convecting that additional latent heat to the cold wall, which condenses the water vapor and releases the latent heat. This additional convection of latent heat is proposed as an additional source term for heat transport equation, and the magnitude of the effect is shown to be proportional to the phase fraction of microbubbles. This theory is shown to be consistent with analysis of observations of freezing times measured by Mpemba and Osborne [Phys. Educ. 4:172-5, 1969], that infer heat transfer coefficients from fitting Newton’s law of cooling. The inferred heat transfer coefficient ratio from the presumed highest microbubble phase fraction to the lowest is ~7.4:1. Whether or not that enhancement level persists to a microbubble condenser in an airlift vessel, the promise of additional heat transfer should be explored.

Froth flotation is the most widely used method for beneficiation of fine coals. In flotation, the size of air bubbles is an important factor affecting the separation efficiency. The size of air bubbles generated by passing air through a bubble diffuser/sparger can be reduced by changing the air flow pattern from steady to oscillatory. In the present work, coal flotation with oscillatory air supply generated by a fluidic oscillator was investigated and the results were compared with those obtained with steady air flow. The flotation tests were carried out at semi-continuous mode using a laboratory-scale flotation column. The results suggest that use of the fluidic oscillator can lead to substantial reagent cost savings and help improve coal flotation efficiency. Further experiments were carried out to fundamentally understand the effect of different air flow patterns on the flotation performance. Compared to steady air flow, oscillatory air flow results in better gas dispersion, higher gas holdup, higher foam stability, and less axial mixing of fluid inside the flotation column.

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.

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As an environmental friendly cleantech, micro/nano bubbles (MNBs) have been increasingly used for pollution control in cleaner production and water renewability improvement, and many works used optical density (OD) as proxy for the concentration of chemical/biological substances in liquid samples. This study reported that MNBs induced considerable light scattering and impacted the measured OD of liquid samples. The experimental results showed that bubble aeration increased the OD at wavelengths of 300–600 nm by 0.14–0.50 in commonly used bio-cultural medium and deionized water. The scattered light that fell outside the effective forward range increased the OD, and this increase was dependent on the bubble size and light wavelength. The Mie calculations showed that the scattering of smaller bubble is stronger and results in more OD increase. By defining the OD deviation 263 μm in water. The finite-difference time-domain simulation illustrated the highly nonuniform electric field near the bubble, and verified the higher scattering efficiencies at shorter wavelength. De-bubbling samples from MNB systems via short-term sonication eliminated light scattering. Findings from this work delineated the optical properties of MNBs, highlighted resulting impact on optical measurement and proposed a de-bubbling method in water treatment.

Yeast requires and consumes a high amount of oxygen rapidly during growth. Maintaining yeast cultures under sufficient aeration, however, is a significant challenge in yeast propagation. Due to their high surface area, microbubbles are more efficient in mass transfer than coarse bubbles. The performance of an airlift loop bioreactor equipped with a fluidic oscillator generated microbubbles in yeast propagation is presented here. The approach is compared with a conventional bubble generation method that produces coarse bubbles. Dosing with microbubbles transferred more oxygen to the cultures, achieving non-zero dissolved O2 levels and consequently, eliminating the starvation state of yeast in contrast to coarse bubble sparging. The average cell growth yield obtained under microbubble sparging reached 0.31 mg/h (±0.02) while 0.22 mg/h (±0.01) was recorded for cells grown with coarse bubbles during the log phase. The percent difference in average growth yield after 6 hours was 18%. Additionally, the use of microbubbles in yeast harvest from growth medium proved effective, yielding >99% cell recovery. The result of this study is crucial for the biofuel industry but also, the food, nutraceutical and pharmaceutical industry for which end product purity is premium.

The wider scale use of aerated water for irrigation has been limited by dis-uniformity of aeration in the field, limited longevity of oxygen in irrigation water and lack of knowledge of what oxygen concentration brings optimal growth. Two options are presented for increasing dissolved oxygen (DO) in irrigation water: Venturi (VT) and fluidic oscillator (FO) aeration systems. Sweet corn was grown in pots to evaluate the effect of aerated water. Compared to the control treatment (CK), VT and FO showed an increase in irrigated water DO in the irrigation tank and feeding pots after aeration. Incorporation of surfactant in the irrigation water significantly improved DO level for the VT and FO treatment. Typically the DO levels reached a peak when the pump is turned off and then decline to a minimum after 24 h. The VT and FO systems had a higher magnitude and duration of the DO level in the water in all treatments compared to the CK system. The FO aeration maintained the longest duration of elevated DO in the water by a factor of two compared to the VT aeration. Compared to the CK, the best result on longevity and DO concentration was achieved using 4 ppm of a non-ionic surfactant. The corn biomass was significantly greater for the VT compared to both the FO and CK treatments.

The scaling up and downstream processing costs of biofuels from microalgae are major concerns. This study focuses on reducing the cost by using energy efficient methods in the production of microalgae biomass and the downstream processes (biomass harvesting and lipid extraction). Ozonation of Dunaliella salina (green alga) and Halomonas (Gram-negative bacterium) mixed cultures for 10 min at 8 mg/L resulted in a reduction in the bacterial contaminant without harming the microalga. Harvesting of D. salina cells through microflotation resulted in a 93.4% recovery efficiency. Ozonation of the harvested microalgal cells for 60 min produced three main saturated hydrocarbon compounds (2-pentadecanone, 6, 10, 14-trimethyl; hexadecanoic acid; octadecanoic acid) consisting of 16 to 18 carbons. By systematically switching the carrier gas from CO2 to O3, the microbubble-driven airlift loop bioreactor (ALB) delivers nutrient to the culture and in-situ disinfection respectively. Further, modulating the bubble size to match particle size ensures recovery of the cells after culture. All three key operations (disinfection, harvesting and lipid extraction) are assembled in a scalable, relatively energy efficient process.

In the current study, kinetics of hydrogen formation from water vapour plasmolysis and ozone formation from air plasma are analysed. The hypothesis of this paper is that operating a plasma microreactor with residence time in the reactor approximately that of the timescale predicted by chemical kinetics simulation for the formation of hydrogen leads to a low power consumption mode of operation. However, this objective is difficult to assess with hydrogen formation from plasmolysis, as hydrogen concentration is difficult to monitor continuously as discussed in previous work and summarised here in section one as well. Ozone, however, is shown to have similar chemical kinetics and is readily analysed continuously, so is used as a proxy for the hydrogen hypothesis. Ozone was generated from air plasma using a Dielectric barrier discharge (DBD)-Corona hybrid discharge reactor. It was experimentally verified that time scale to achieve highest concentration of ozone is of the order of 10 −2 s at 0.8 L min −1 , 4.8 kV, 37 kHz, 3 cm electrode length and atmospheric pressure, in agreement with Lozano-Parada and Zimmerman [1] . This is the time scale to produce 99% of ozone. In previous work [2] , using kinetic modelling it was proposed a similar time scale to produce 99% hydrogen from water vapour plasmolysis under well mixed, batch reactors and hence supporting the claim made by Zimmerman and Lozano Parada [3] . Pressure and flow rate of the air, electrode material, length of external electrode and its configuration, applied power and frequency have been varied in this study, the results of which confirm the low power consumption mode is achieved by matching the residence time to the kinetics timescale.

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.

Microbubbles are bubbles below 1 mm in size and have been extensively deployed in industrial settings to improve gaseous exchange between gas and liquid phases. The high surface to volume ratio offered by microbubbles enables them to enhance transport phenomena and therefore can be used to reduce energy demands in many applications including, waste water aeration, froth flotation, oil emulsion separations and evaporation dynamics. Microbubbles can be produced by passing a gas stream through a micro-porous diffuser placed at the gas–liquid interface. Previous work has shown that oscillating this gas steam can reduce the bubble size and therefore increase energy savings. In this work we show that it is possible to further reduce microbubble size (and consequently maximise the number of bubbles) by varying the frequency of the oscillating gas supply. Three different microbubble generation systems have been investigated; an acoustic oscillation system and a mesh membrane, a fluidic oscillator coupled to a single orifice membrane and a fluidic oscillator coupled to a commercially available ceramic diffuser. In all three bubble generation methods there is an optimum oscillation frequency at which the bubble size is minimised and the number of microbubbles maximised. In some cases a reduction in bubble size of up to 73% was achieved compared with non-optimal operating frequencies. The frequency at which this optimum occurs is dependent on the bubble generation system; more specifically the geometry of the system, the type micro-porous diffuser and the gas flow rate. This work proves that by tuning industrial microbubble generators to their optimal oscillation frequency will result in a reduction of microbubble size and increase their number density. This will further improve gaseous exchange rates and therefore improve the efficiency of the industrial processes where they are being employed to produce bubbles, leading to a reduction in associated energy costs and an increase in the overall economic and energetic feasibility of these processes.

Airlift 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 µm, 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.

Over reliance on fossil fuels to meet the ever increasing energy demands has adversely affected the economy and environment of Pakistan. The transportation sector is the major consumer of these imported petroleum products. Among these petroleum products, high speed diesel and gasoline are the most consumed. Bio-fuels, produced from indigenous biomass, can surmount the twofold problem of deteriorating environment and sustainable supply of energy resource. As an agricultural country, Pakistan prodigiously produces biomass (e.g. animal dung, molasses and bagasse) which can replace natural gas, gasoline and electricity conventionally produced from fossil fuels. However, an alternative to high speed diesel has not been identified yet. This review paper is focused on developing a strategy for identifying potential biodiesel feed-stocks suitable for Pakistan and their contribution towards “the solution” of sustainable energy and environment. The selection parameters for feedstock, land, irrigation, oil yield, composition and maturation duration are discussed in detail. Based on agro-climatic conditions of the Pakistan, castor and jatropha cultivated on water scarce lands are lucrative feed stocks for bio-diesel. Moreover, by-products of both these feed stocks like seed cake and glycerine are explored for production of gaseous fuel such as bio-gas or hydrogen, electricity generation and animal meal production which can render biodiesel economically viable. An integrated approach for biodiesel production associated with by-products utilization has been proposed.

An increased focus on renewable technologies is necessary to meet the level of carbon emission reduction required to achieve the 2°C scenario (2DS). Carbon containing materials are at the root of most industrial value chains and currently 90% of organic chemicals are made from fossil fuels and 5-10% of global crude oil demand is for the manufacture of chemicals. By providing a process to convert waste CO2 into high value CO we can take steps towards the goal of reducing emissions and provide an important link between various sectors to turn waste materials into new products, improving resource efficiencies, and reducing negative environmental factors. This presents a significant opportunity to leverage CO2 as a feedstock while supporting global goals in green energy, manufacturing, etc. Solid oxide cells (SOCs) provide an effective mode of simultaneously reducing CO2 and separating O2, however, over 80% of the cost to co-electrolyse CO2 and H2O comes from electrical input necessary to drive a SOEC. While changes in cell geometry and composition have been used to improve performance, energy inputs need to be reduced to increase overall device efficiency for widespread commercialisation to be realised. In this work we focus on the use of oscillating the gas flow into an SOC to create a highly efficient energy conversion device to facilitate the reduction of CO2 to CO. Fluidic oscillation is known to cause disruptive flow which promotes turbulence in small flow fields, thereby increasing the overall efficiency – key to furthering their commercialisation. Fluidic oscillators are one of a variety of fluidic switching devices developed in the field of microfluidics. A single gas supply inlet guides an incoming gas flow into a destabilising microfluidic feature, generating the necessary disruption that results in jet diversion between two identical channels. This switching results in a pulsed flow in each of the outlet channels, alternating between full forward flow, and very slight backflow. The flow in the forward regime bears similarity to laminar flow, whereas the slight backflow regime more closely resembles turbulence. This effect can be observed even in extremely small channel diameters and causes a disruptive effect which interrupts the formation of a boundary layer of stagnant gas flow along flow field chamber walls. While conventional oscillators require high flow rates on the order of 50-80 L/min, for this particular work we are utilising a novel fluidic oscillator capable of producing an oscillatory flow at low onset flow rates of as low as 50 ml/min, which is necessary for operation with lab scale SOCs. Experimental results show improved performance with the addition of oscillating gas flow and no other changes to cell geometry, materials composition, or gas composition. The magnitude of improvement in polarization resistance was dependent on operating conditions, the reasons for which will be discussed in the paper.

Harvesting and extracting lipids from the microalgal biomass are the most expensive processes in biodiesel production. This study focuses on reducing the lipid extraction cost using ozone-rich microbubbles technique. The lipid extraction of Dunaliella salina slurry with methanol (1:2 v/v) was performed in a 0.2 L bioreactor at room temperature with direct ozonation (8 mg L−1). When the temperature was increased (60 C) and smaller bubbles were introduced during extraction, the concentration of products increased significantly to around 156%, 88.9% and 150% for 6,10,14-trimethylpentadecan-2-one, palmitic acid and stearic acid, respectively. The energy usage for extracting D. salina lipid with ozone has been estimated to be around 2.16 MJ kg−1 dry algae (36% energy) which is a small fraction of the energy that is used in the production of biodiesel, unlike centrifugation and solvent extraction methods, which consume more than 90% of the energy.

Bubble measurement has been widely discussed in the literature and comparison studies have been widely performed to validate the results obtained for various forms of bubble size inferences. This paper explores three methods used to obtain a bubble size distribution&mdash, optical detection, laser diffraction and acoustic inferences&mdash, for a bubble cloud. Each of these methods has advantages and disadvantages due to their intrinsic inference methodology or design flaws due to lack of specificity in measurement. It is clearly demonstrated that seeing bubbles and hearing them are substantially and quantitatively different. The main hypothesis being tested is that for a bubble cloud, acoustic methods are able to detect smaller bubbles compared to the other techniques, as acoustic measurements depend on an intrinsic bubble property, whereas photonics and optical methods are unable to &ldquo, see&rdquo, a smaller bubble that is behind a larger bubble. Acoustic methods provide a real-time size distribution for a bubble cloud, whereas for other techniques, appropriate adjustments or compromises must be made in order to arrive at robust data. Acoustic bubble spectrometry consistently records smaller bubbles that were not detected by the other techniques. The difference is largest for acoustic methods and optical methods, with size differences ranging from 5&ndash, 79% in average bubble size. Differences in size between laser diffraction and optical methods ranged from 5&ndash, 68%. The differences between laser diffraction and acoustic methods are less, and range between 0% (i.e., in agreement) up to 49%. There is a wider difference observed between the optical method, laser diffraction and acoustic methods whilst good agreement between laser diffraction and acoustic methods. The significant disagreement between laser diffraction and acoustic method (35% and 49%) demonstrates the hypothesis, as there is a higher proportion of smaller bubbles in these measurements (i.e., the smaller bubbles &lsquo, hide&rsquo, during measurement via laser diffraction). This study, which shows that acoustic bubble spectrometry is able to detect smaller bubbles than laser diffraction and optical techniques. This is supported by heat and mass transfer studies that show enhanced performance due to increased interfacial area of microbubbles, compared to fine bubbles.

A cost effective, fast, and accurate technique was needed to measure the vapor composition of a binary system (ethanol-water) and also that of a liquid composition in a ternary system (acetic acid-acetol&ndash, water) in a microbubble distillation unit. Cheap TGS-series gas sensors were used for this purpose with both calibrations and measurements carried out in a specially designed chamber. A single parameter polynomial regression was fitted to the binary system, and a two parameter polynomial with an interaction term was fitted to the ternary system. The correlation coefficient, R-squared, was found to be greater than 0.99 for both systems, thus validating the implementation of this novel sensor.