Microbubble Dispersion Research Articles

Immersion tomographic study of the motion of bubbles in a flooded granular bed

Immersion tomography is used to study the rise of single bubbles and bubble ensembles in a liquid-saturated granular bed. The bed is a monodisperse packing of glass beads 3 and 6 mm in diameter. The behavior of rising bubbles differs significantly depending on the ratio of the bubble size to the size of packing elements. Filming and video recording of the bubble rise process are performed. Numerous consecutive frames of rising gas cavities are shown and commented on. A model is proposed for the rise velocity and transverse dispersion of microbubbles.

Microbubble fermentation of recombinant Pichia pastoris for human serum albumin production

The high cell density fermentation of recombinant Pichia pastoris for human serum albumin (HSA) production is a high oxygen demand process. The oxygen demand is usually met by increased agitation rate and use of oxygen-enriched air. However, fermentation using a microbubble dispersion (MBD) can supply adequate oxygen to the microorganisms at relatively low agitation rates because of improved oxygen mass-transfer of the microbubbles used for the sparging. Conventional air-sparged fermentations were conducted for the production of HSA using P. pastoris at agitation rates of 350, 500, 750 rpm. These experiments were compared to MBD-sparged fermentation at 350 and 500 rpm agitation rates. After 97 h of air-sparged fermentation, cell masses were 27.9, 48.9, and 138.2 g/l at agitation rates of 350, 500, and 750 rpm, respectively. For MBD-sparged fermentation, cell mass concentrations after 97 h were 129.9 and 137.7 g/l at 350 and 500 rpm, respectively. Protein concentrations were 36.3 and 67.8 mg/l for air-sparged at 350 and 500 rpm, while 267.3 and 301.2 mg/l for MBD-sparged experiments at 350 and 500 rpm, respectively. The volumetric oxygen mass-transfer coefficient, k L a for the MBD system at 500 rpm was 268.6 h −1 compared to 76.1 h −1 for the conventional sparger at 500 rpm. There was significant improvement in oxygen transfer to the microorganism relative to the conventional sparger.

Microbubble fermentation of Trichoderma reesei for cellulase production

A laboratory-scale microbubble dispersion (MBD) generator was shown to improve oxygen transfer to aerobic microorganisms when connected to a conventional air sparger. However, this process setup had not been tested with fungi, where the morphology of microorganisms causes a very viscous broth rheology that impedes the oxygen mass transfer. Trichoderma reesei RUT C30 was chosen as a model fungus as it shows both the challenging rheology and a cellulase enzyme complex product that is shear-sensitive. Conventional and MBD sparged fermentations where carried out to compare the effect of aeration methods on oxygen transfer and product formation. MBD sparged fermentations showed fivefold higher k L a values and nearly twofold higher cell mass productivities. The cellulase enzyme activities were similar for both the MBD and the conventional sparged fermenters.

Detection of individual microbubbles of ultrasound contrast agents: imaging of free-floating and targeted bubbles.

During echo examinations with microbubble contrast, individual "dots" of ultrasound reflection can be visualized. To address the question whether these signals represent individual microbubbles, very dilute suspensions of ultrasound contrast agents or individual microbubbles attached to Petri dishes were prepared and studied by ultrasound imaging. Microbubble suspensions were diluted in saline and evaluated by a clinical ultrasound imaging system. Microbubble concentration was verified by Coulter counter. Single microbubble preparation on a Petri dish was established by streptavidin-biotin interaction under microscopy control and subjected to ultrasound imaging. Ultrasound of dilute microbubble dispersions demonstrated distinct white foci; concentration of these sites was consistent with signals from individual microbubbles as determined by Coulter. Individual microbubbles immobilized on polystyrene were also visualized by ultrasound. Ultrasound medical systems can resolve backscatter signals from individual microbubbles of ultrasound contrast, both in solution and in the targeted immobilized state, implying picogram sensitivity.

Scale-Up of Microbubble Dispersion Generator for Aerobic Fermentation

A laboratory-scale microbubble dispersion (MBD) generator was shown to improve oxygen transfer to aerobic microorganisms when coupled to the conventional air-sparger. However, the process was not demonstrated on a large scale to prove its practical application. We investigated the scale-up of a spinning-disk MBD generator for the aerobic fermentation of Saccharomyces cerevisiae (baker's yeast). A 1-L spinning-disk MBD generator was used to supply air for 1- and 50-L working volume fermentation of baker's yeast. For the two levels investigated, the MBD generator maintained an adequate supply of surfactant-stabilized air microbubbles to the microorganisms at a relatively low agitation rate (150 rpm). There was a significant improvement in oxygen transfer to the microorganism relative to the conventional sparger. The volumetric mass transfer coefficient, kLa, for the MBD system at 150 rpm was 765 h(-1) compared to 937 h(-1) for the conventional sparger at 500 rpm. It is plausible to surmise that fermentation using larger working volumes may further improve the kLa values and the dissolved oxygen (DO) levels because of longer hold-up times and, consequently, improve cell growth. There was no statistically significant difference between the cell mass yield on substrate (0.43 g/g) under the MBD regime at an agitation rate of 150 rpm and that achieved for the conventional air-sparged system (0.53 g/g) at an agitation rate of 500 rpm. The total power consumption per unit volume of broth in the 50-L conventional air-sparged system was threefold that for the MBD unit for a similar product yield. Practical application of the MBD technology can be expected to reduce power consumption and therefore operating costs for aerobic fermentation.

Behaviour of dispersed micro-bubbles in a rotating pipe section

Behaviour of dispersed micro-bubbles in a rotating pipe section

Behaviour of dispersed micro-bubbles in a rotating pipe section

A study is carried out for swirling flow of a fluid containing dispersed micro-bubbles (nominal size 80 μm in diameter). From experimental results the location of the stagnation point is found to be a strong function of the swirl ratio, but independent of the aspect ratio of the rotating pipe section when the actual position is normalized to the length of the section. It is also found from experimental results, with an aid of a numerical simulation, that a parabolic surface is formed by some larger bubbles which are caught in orbits of the rotating flow field. It is further verified that smaller-sized bubbles pass along limited stream lines.

Attenuation of ultrasound in porous media with dispersed microbubbles

The dispersion relation for a granular bed with a small amount of fine bubbles is formulated and analyzed. It is assumed that the grain size is much larger than the bubble's radius and that their volume concentration is small. The study is motivated by the problem of acoustic diagnostics of fixed bed chemical reactors operating in multiphase flow regime.

Reactor Design Issues for Synthesis-Gas Fermentations.

Synthesis gas is readily obtained by gasifying coal, oil, biomass, or waste organics and represents an abundant, potentially inexpensive, feedstock for bioprocessing. The primary components of synthesis gas, carbon monoxide and hydrogen, can be converted into methane, organic acids, and alcohols via anaerobic fermentations. Bioconversion of synthesis gas is an attractive alternative to catalytic processing because the biological catalysts are highly specific and often more tolerant of sulfur contaminants than inorganic catalysts. However, because the aqueous solubilities of carbon monoxide and hydrogen are low, synthesis-gas fermentations are typically limited by the rate of gas-to-liquid mass transfer. Consequently, a major engineering challenge in commercial development of synthesis-gas fermentations is to provide sufficient gas mass transfer in an energy-efficient manner. This paper reviews recent progress in the development of synthesis-gas fermentations, with emphasis on efforts to increase the efficiency of gas mass transfer. Metabolic properties of several microbes able to ferment synthesis gas are described. Results of synthesis-gas fermentations conducted in various bioreactor configurations are summarized. Recent results showing enhancement of synthesis-gas fermentations using microbubble dispersions are presented, and studies of the mass-transfer and coalescence properties of microbubbles are described.

Protein recovery using gas–liquid dispersions

Two separation techniques, foam separation and colloidal gas aphrons (CGAs), both of which are based on gas–liquid dispersions, are compared as potential applications for protein recovery in downstream processing. The potential advantages of each method are described and the concentration and selectivity achieved with each method, for a range of proteins is discussed. The physical basis of foam separation is the preferential adsorption of surface active species at a gas–liquid interface, with surface inactive species remaining in bulk solution. When a solution containing surface active species is sparged with gas, a foam is produced at the surface: this foam can be collected, and upon collapse contains surface active species in a concentrated form. CGAs are microbubble dispersions (bubble diameters 10–100 μm) with high gas hold ups (>50%) and relatively high stability, which are formed by stirring a surfactant solution at speeds above a critical value (typically around 5000 rpm). It is expected that when proteins are brought into contact with aphrons, protein adsorbs to the surfactant through electrostatic and/or hydrophobic forces. The aphron phase can be separated easily from the bulk solution due to its buoyancy, thus allowing separation of protein in a concentrated form.

Study on Contrast Effect of Microbubbles as Ultrasound Contrast Agents

In this study, the contrast effect from different sized microbubbles is investigated both theoretically and experimentally. Theoretically, the reference backscattered power is defined to evaluate the contrast effect of different sized microbubbles dispersed in the heart. In this study, the calculation results of two kinds of microbubbles of sizes 35 µm and 2.2 µm are discussed. From the calculation results obtained, it is found that the contrast effect from microbubbles of size 35 µm is better than that from microbubbles of size 2.2 µm. The reason being that as the size of microbubbles decreases, a large portion of ultrasonic energy is lost due to absorption. Based on the same reason, microbubbles less than 10 µm will cause an acoustic shadowing effect at a higher concentration of microbubbles, according to the calculation results. Experimentally, microbubbles of an average size of 35 µm are produced, and by filtration, microbubbles of an average size of 2.1 µm are collected. Then, the contrast effect produced from microbubbles before and after filtration are compared. It is observed that for both kinds of microbubbles, a good contrast effect is observed at a relatively low concentration of microbubble dispersion. However, distortion of images occurs at high concentrations. The acoustic shadowing effect is observed for microbubbles of an average size of 2.1 µm which can also be expected from the calculation results. On the other hand, the multiple-scattering effect which results in blurred images, occurs for microbubbles of an average size of 35 µm.

Mass-transfer properties of microbubbles. 1. Experimental studies.

Synthesis-gas fermentations have typically been gas-to-liquid mass-transfer-limited due to low solubilities of the gaseous substrates. A potential method to enhance mass-transfer rates is to sparge with microbubble dispersions. Mass-transfer coefficients for microbubble dispersions were measured in a bubble column. Oxygen microbubbles were formed in a dilute Tween 20 solution using a spinning disk apparatus. Axial dispersion coefficients measured for the bubble column ranged from 1.5 to 7.2 cm2/s and were essentially independent of flow rate. A laser-diffraction technique was used to determine the interfacial area per unit gas volume, a. The mass-transfer coefficient, KL, was determined by fitting a plug-flow model to the experimental, steady-state, liquid-phase oxygen-concentration profile. The KL values ranged from 2.9 x 10(-5) to 2.2 x 10(-4) m/s. Volumetric mass-transfer coefficients, KLa, for microbubbles with an average initial diameter of 60 microns ranged from 200 to 1800 h-1. Enhancement of mass transfer using microbubbles was demonstrated for a synthesis-gas fermentation. Butyribacterium methylotrophicum was grown in a continuous, stirred-tank reactor using a tangential filter for total cell recycle. The fermentation KLa values were 14 h-1 for conventional gas sparging through a stainless steel frit and 91 h-1 for microbubble sparging. The Power number of the microbubble generator was determined to be 0.036. Using this value, an incremental power-to-volume ratio to produce microbubbles for a B. methylotrophicum fermentation was estimated to be 0.01 kW/m3 of fermentation capacity.

Simulations of Contrast Effects from Free Microbubbles in Relation to Their Size, Concentration and Acoustic Properties

To gain a deeper insight into the diagnostic images produced by microbubbles as ultrasound contrast agents, the basic acoustical properties such as the scattering cross section, extinction cross section and the expected backscattered power from different sized microbubbles are calculated. The results proved that the ratio of scattering to extinction cross section, size and concentration of microbubbles are the factors determining the maximum backscattered power received from microbubble dispersions. When microbubbles decrease in size, the energy loss by microbubbles upon ultrasound exposure gradually changes from loss by scattering to loss by absorption. As a result, the concentration of microbubbles required to produce a B mode image of the comparable brightness is increased. However, an increased concentration of microbubbles does not necessarily produce a clearer image. In this paper, it is further established that an optimum concentration exists at which the microbubbles give maximum backscattered power. The optimum microbubble concentration and the maximum backscattered power also depend on the depth of the microbubbles in the microbubble dispersion. In addition, the concentration of microbubbles necessary to produce the maximum backscattered power during resonance is less than that at other frequencies.

Effect of surfactants on carbon monoxide fermentations byButyribacterium methylotrophicum

Butyribacterium methylotrophicum has been grown on carbon monoxide as its carbon and energy source in the presence of various surfactants that are capable of forming microbubble dispersions for the screening of surfactants for use in microbubble-sparged synthesis gas fermentations. In the range of 0-3 times the critical micelle concentration, the presence of Tween surfactants was not significantly inhibitory to growth, final cell density, and fermentation stoichiometry, although some of the Brij surfactants caused significant inhibition. As the batch fermentations entered the stationary phase both the pH and the ratio of acetate to butyrate decreased.

Estimation of diffusion and solubility coefficients for water and CO2 in reaction injection molded parts

AbstractThe evolution of gases from Reaction Injection Molded (RIM) parts during painting causes “pinhole” surface defects, which result in scrap. The transport of gases plays a major role in this outgassing. The diffusion rates of CO2 and water through reaction injection molded parts is measured in this work. Despite the presence of glassy hard domains and dispersed microbubbles of size <25 μm (nucleated parts), the diffusion showed Fickian characteristics, and diffusion coefficients were independent of the concentration of water and CO2 in nucleated and non‐nucleated RIM parts. The presence of microbubbles enhances diffusion, which could be predicted from the diffusion rate through the non‐nucleated material by using a simplistic one‐dimensional model. The diffusion coefficients of water and CO2 follow an Arrhenius relationship. The solubility coefficients follow a van't Hoff relationship over a wide range of 20 to 150°C. Apart from predicting diffusion and solubility coefficients, for various RIM materials and processing conditions, the estimated parameters will be used to interpret the outgassing phenomenon in a subsequent paper.

Effect of Calcium Chloride on Sodium Alginate Microbubbles as Ultrasound Contrast Agent

In order to stabilize microbubbles without coating layers to be used as ultrasound contrast agent, the viscosity of the liquid for storage has to be increased. Sodium alginate is a material which forms viscous liquid when its powder is dissolved in water. The viscosity may vary from 1 cP to about 100 cP when the amount of material used is increased from 0.1% to 1%. However, the viscosity is still not enough to make stable microbubbles which can be stored for over several weeks. In this paper, by adding calcium chloride to the sodium alginate solution, a gel-type solution is produced which has a high viscosity capable of holding microbubbles for a long time. By measuring the viscosity of the sodium alginate solution and the solution with calcium chloride solution added, it is known that the solution is changed from a Newtonian liquid to a non-Newtonian liquid when calcium chloride solution is added. Since the attenuation coefficient of microbubble dispersion is proportional to the number of microbubbles, the disappearance of microbubbles lowers the attenuation coefficient. Therefore, in this study, the attenuation coefficient is used as an estimating parameter for the lifetime of microbubbles.

Formation and coalescence properties of microbubbles

Fermentations that involve a gaseous substrate are inherently rate-limited by mass transfer from the gaseous to the liquid phase. Aerobic and synthesis-gas fermentations are primary examples of such mass-transfer-limited fermentations. The traditional approach to enhance gas-to-liquid mass transfer is to increase the impeller rate, thereby increasing the interfacial area available for mass transfer. However, this approach results in a dramatic increase in power consumption, especially for large-scale systems, because power consumption is proportional to the impeller rate to the third power and the impeller diameter to the fifth power. Microbubble dispersions offer an alternative, providing high interfacial area with the potential for low power input. In aerobic fermentations with yeast, the transport of oxygen from a gas to a liquid, reported in terms of k{sub 1}a, was found to be greater by an order of magnitude using microbubble sparging oven conventional air sparging. Microbubbles are surfactant-stabilized, spherical bubbles with diameters on the order of 100 {mu}m. By comparison, gas bubbles found in conventional sparging units range from 3-5 mm. The surfactant layer surrounding the microbubble generates a diffuse electric double layer that acts to repel other bubbles and prevent coalescence. A microbubble dispersion exhibits colloidal properties and is stable enoughmore » to be pumped. This article examines the effects of surfactant type and concentration on microbubble formation, drainage, and stability. Effects of salts on microbubbles dispersions are examined as well. This information is needed to evaluate the utility of microbubble dispersions for biotechnological applications.« less

Increased oxygen transfer in a yeast fermentation using a microbubble dispersion

A microbubble dispersion (MBD) was used to supply oxygen for aerobic fermentations in a standard 2 L stirred tank fermenter. The microbubble dispersion was formed using only surfactants produced naturally. Growth rates ofSaccharomyces cerevisiae cultures were found to be equal or greater with MBD sparging than with gas sparging. The oxygen transfer coefficent with MBD sparging was found to be 190/h and independent of impeller speed from 100–580 rpm. The oxygen transfer coefficient with air sparging rose from 55 to 132/h over the same range of impeller speeds. Power requirements for the fermenter systems were estimated.