Showing papers on "Mixing (process engineering) published in 2017"

Effective Thermo-Capillary Mixing in Droplet Microfluidics Integrated with a Microwave Heater

Abstract: In this study, we present a microwave-based microfluidic mixer that allows rapid mixing within individual droplets efficiently. The designed microwave mixer is a coplanar design with a small footprint, which is fabricated on a glass substrate and integrated with a microfluidic chip. The mixer works essentially as a resonator that accumulates an intensive electromagnetic field into a spiral capacitive gap (around 200 μm), which provides sufficient energy to heat-up droplets that pass through the capacitive gap. This microwave actuation induces nonuniform Marangoni stresses on the interface, which results in three-dimensional motion inside the droplet and thus fast mixing. In order to evaluate the performance of the microwave mixer, droplets with highly viscous fluid, 75% (w/w) glycerol solution, were generated, half of which were seeded with fluorescent dye for imaging purposes. The relative importance of different driving forces for mixing was evaluated qualitatively using magnitude analysis, and the effe...

Mixing and mass transfer in a pilot scale U-loop bioreactor

Abstract: A system capable of handling a large volumetric gas fraction while providing a high gas to liquid mass transfer is a necessity if the metanotrophic bacterium Methylococcus capsulatus is to be used in single cell protein (SCP) production. In this study, mixing time and mass transfer coefficients were determined in a 0.15 m3 forced flow U-loop fermenter of a novel construction. The effect on the impeller drawn power when a gas was introduced into the system was also studied. Mixing time decreased and mass transfer increased with increasing volumetric liquid flow rate and specific power input. This happened also for a large volume fraction of the gas, which was shown to have only minor effect on the power drawn from the pump impeller. Very large mass transfer coefficients, considerably higher than those obtainable in an STR and previous tubular loop reactors, could be achieved in the U-loop fermenter equipped with static mixers at modest volumetric liquid and gas flow rates. Biotechnol. Bioeng. 2017;114: 344-354. © 2016 Wiley Periodicals, Inc.

Biomass processing into ethanol: pretreatment, enzymatic hydrolysis, fermentation, rheology, and mixing

Abstract: Abstract Alternate energy resources need to be developed to amend for depleting fossil fuel reserves. Lignocellulosic biomass is a globally available renewable feedstock that contains a rich sugar platform that can be converted into bioethanol through appropriate processing. The key steps of the process, pretreatment, enzymatic hydrolysis, and fermentation, have undergone considerable amount of research and development over the past decades nearing the process to commercialization. In order for the commercialization to be successful, the process needs to be operated at high dry matter content of biomass, especially in the enzymatic hydrolysis stage that influences ethanol concentration in the final fermentation broth. Biomass becomes a thick paste with challenging rheology for mixing to be effective. As the biomass consistency increases, yield stress increases which limits efficiency of mixing with conventional stirred tanks. The purpose of this review is to provide features and perspectives on processing of biomass into ethanol. Emphasis is placed on rheology and mixing of biomass in the enzymatic hydrolysis step as one of the forefront issues in the field.

Mixing high-viscosity fluids via acoustically driven bubbles.

Abstract: We present an acoustofluidic micromixer which can perform rapid and homogeneous mixing of highly viscous fluids in the presence of an acoustic field In this device, two high-viscosity polyethylene glycol (PEG) solutions were co-injected into a three-inlet PDMS microchannel with the center inlet containing a constant stream of nitrogen flow which forms bubbles in the device When these bubbles were excited by an acoustic field generated via a piezoelectric transducer, the two solutions mixed homogenously due to the combination of acoustic streaming, droplet ejection, and bubble eruption effects The mixing efficiency of this acoustofluidic device was evaluated using PEG-700 solutions which are ~106 times more viscous than deionized (DI) water Our results indicate homogenous mixing of the PEG-700 solutions with a ~093 mixing index The acoustofluidic micromixer is compact, inexpensive, easy to operate, and has the capacity to mix highly viscous fluids within 50 milliseconds

Intensification of mixing of shear-thinning fluids possessing yield stress with the coaxial mixers composed of two different central impellers and an anchor

Abstract: The performance of different configurations of coaxial mixers composed of a wall scraping anchor impeller in combination with two different or identical central high speed impellers in the agitation of the shear-thinning fluids with yield stress (e.g. xanthan gum solution) was investigated. These multi-impeller mixers are more compact for the larger scale of mixing operations. The coaxial mixers employed in this study were the Scaba-Scaba-anchor, Scaba-Rushton-anchor, Rushton-Scaba- anchor, Scaba-pitched blade-anchor, and pitched blade-Scaba-anchor. The quality of mixing achieved by these five coaxial mixers was assessed for the Reynolds numbers in the range of the laminar to the transitional regime in the co-rotating mode through computational fluid dynamic (CFD) and electrical resistance tomography (ERT) techniques. A new correlation was introduced for these complex configurations of the coaxial mixers by incorporating the Metzner-Otto constants ( K s ) of the different types of the central impellers into the Reynolds number. The experimental and CFD data were employed to evaluate the mixing intensification attained by these five coaxial mixers in regard to the power consumption, mixing time, velocity profiles, shear strain rate profiles, flow number, power number, and pumping effectiveness. The analysis of the collected data indicated that the intensification of mixing of the highly viscous non-Newtonian fluids achieved by the Scaba-pitched blade-anchor coaxial mixer was the highest among the mixing systems explored in this study.

Microreactor technology for biodiesel production: a review

Abstract: Due to a rise in global demand for energy and an increase in greenhouse gases, biodiesel has been accepted as an alternative fuel because of its biodegradability, low environment detrimental effect, better quality of exhaust gas emission, and renewability. Biodiesel is a mixture of monoalkyl esters of long-chain fatty acids also known as fatty acid methyl esters. Transesterification is the most commonly adopted technique for the production process. But the conventional biodiesel technology has its own disadvantages. Process intensification technologies can overcome these drawbacks. Some novel reactors such as microchannel reactor, static mixers, oscillatory flow reactors, and spinning tube reactors have been developed so as to improve mass transfer and mixing. These technologies can achieve rapid and high reaction rates due to the high surface area/volume ratio and short diffusion distance thus intensifying the transesterification process. Various factors such as alcohol to triglyceride molar ratio, microchannel size, residence time, reaction temperature, mixing mechanism, and catalyst affect the production process. Although biodiesel production has been commercialized in several countries, it still requires a clean, effective, and environment-friendly technology to make it cost-effective and increase its competency against conventional fossil fuels. Microreactor technology, however, has proved to be a benchmark to serve this purpose. The current review paper provides an overview about different types of microreactors used in biodiesel production and the parameters affecting biodiesel production in microreactors. The microreactor technology discussed in this paper aims to improve the production process by decreasing the reaction time from hours to minutes.

Thermal characterization of a series of lignin-based polypropylene blends

Abstract: Polypropylene (PP), due to its chemical stability, is considered one of the main responsible of the increasing amount of plastic wastes on earth. To overcome this problem and to reduce the dependence of oil feedstocks, the use of lignocellulosics as fillers or reinforcements in thermoplastic materials has been increasing enormously in the last decades. In the present work, Liquid Wood (a mixture of cellulose, hemp, fax and lignin) was used to prepare, by mechanical mixing followed by thermal extrusion, blends of various PP/Liquid Wood ratios. Differential scanning calorimetry and thermogravimetric analysis experiments were performed in order to verify whether and how much the composition of the blends affects the thermal properties of the obtained compounds. Both calorimetric and thermogravimetric results indicate that the application of PP as a matrix does not limit the processing temperature of Liquid Wood, which may lead to a perfect marketable composite from these components. The addition of Liquid Wood also resulted in enhanced mechanical properties for the PP/Liquid Wood blends.

An easily fabricated three-dimensional threaded lemniscate-shaped micromixer for a wide range of flow rates.

Abstract: Mixing fluid samples or reactants is a paramount function in the fields of micro total analysis system (μTAS) and microchemical processing. However, rapid and efficient fluid mixing is difficult to achieve inside microchannels because of the difficulty of diffusive mass transfer in the laminar regime of the typical microfluidic flows. It has been well recorded that the mixing efficiency can be boosted by migrating from two-dimensional (2D) to three-dimensional (3D) geometries. Although several 3D chaotic mixers have been designed, most of them offer a high mixing efficiency only in a very limited range of Reynolds numbers (Re). In this work, we developed a 3D fine-threaded lemniscate-shaped micromixer whose maximum numerical and empirical efficiency is around 97% and 93%, respectively, and maintains its high performance (i.e., >90%) over a wide range of 1 < Re < 1000 which meets the requirements of both the μTAS and microchemical process applications. The 3D micromixer was designed based on two distinct mixing strategies, namely, the inducing of chaotic advection by the presence of Dean flow and diffusive mixing through thread-like grooves around the curved body of the mixers. First, a set of numerical simulations was performed to study the physics of the flow and to determine the essential geometrical parameters of the mixers. Second, a simple and cost-effective method was exploited to fabricate the convoluted structure of the micromixers through the removal of a 3D-printed wax structure from a block of cured polydimethylsiloxane. Finally, the fabricated mixers with different threads were tested using a fluorescent microscope demonstrating a good agreement with the results of the numerical simulation. We envisage that the strategy used in this work would expand the scope of the micromixer technology by broadening the range of efficient working flow rate and providing an easy way to the fabrication of 3D convoluted microstructures.

3D‐Printed Electrodes with Improved Mass Transport Properties

Abstract: Today's electrochemical reactor design is a less developed discipline as compared to electrocatalytic synthesis. Although catalysts show increasing conversion rates, they are often operated without measures for the reduction of concentration polarization effects. As a result, a stagnant boundary layer forms at the electrode-electrolyte interface. This stagnant boundary layer presents an additional voltage drop and reduces the energy efficiency. It is generally accepted that this phenomenon is caused by a combination of fast electrode reactions and slow diffusion of the reacting species. Our earlier work demonstrated the potential of non-conducting static mixers to reduce concentration polarization effects. However, there are few studies on conductive static mixers applied as electrodes. In this study, we present a new concept of additive manufactured flow through electrode mixers. Our electrode geometry combines a high surface area with mixing properties, diminishing concentration polarization effects of transport-limited reactions. Mass transport properties of these conductive static mixers are evaluated in an additive manufactured electrochemical reactor under controlled conditions by applying the limiting-current method.

Organic Condensation and Particle Growth to CCN Sizes in the Summertime Marine Arctic Is Driven by Materials More Semivolatile Than at Continental Sites

Abstract: Ship-based aerosol measurements in the summertime Arctic indicate elevated concentrations of ultrafine particles with occasional growth to CCN sizes. Focusing on one episode with two continuously growing modes, growth occurs faster for a large, pre-existing mode (dp ≈ 90 nm) than for a smaller nucleation mode (dp ≈ 20 nm). We use microphysical modeling to show that growth is largely via organic condensation. Unlike results for mid-latitude forested regions, most of these condensing species behave as semi-volatile organics, as lower-volatility organics would lead to faster growth of the smaller mode. The magnitude of the CCN hygroscopicity parameter for the growing particles, ~0.1, is also consistent with organic species constituting a large fraction of the particle composition. Mixing ratios of common aerosol growth precursors, such as isoprene and sulfur dioxide, are not elevated during the episode, indicating that an unidentified aerosol-growth precursor is present in this high-latitude marine environment.

Experimental and CFD studies of power consumption in the agitation of highly viscous shear thinning fluids

Abstract: Mixing of non-Newtonian fluids is widely encountered in the process industries. In this research, we obtained a constitutive expression that relates the viscosity of a mixture of glycerol and a gel formed of polyethylene glycol and Carbomer to the shear rate, temperature and mass fraction of one of the two components. We found that the mixtures of these two fluids were well characterized by a non-Newtonian power law model. We then used a number of homogeneous mixtures of the two fluids at different temperatures and mass fractions in a simple stirred tank agitated mechanically by a Rushton turbine to derive experimental power curves, which we then derived numerically in a CFD model by replicating the experimental conditions. We used a combination of an air bearing and a load cell to precisely measure the power required by the impeller to agitate the non-Newtonian mixtures. The computational and experimental results are in good agreement, indicating that the rheological data and the CFD model are accurate.

Magnetic fluidized bed for solid phase extraction in microfluidic systems.

Abstract: Fluidization, a process in which a granular solid phase behaves like a fluid under the influence of an imposed upward fluid flow, is routinely used in many chemical and biological engineering applications. It brings, to applications involving fluid-solid exchanges, advantages such as high surface to volume ratio, constant mixing, low flow resistance, continuous operation and high heat transfer. We present here the physics of a new miniaturized, microfluidic fluidized bed, in which gravity is replaced by a magnetic field created by an external permanent magnet, and the solid phase is composed of magnetic microbeads with diameters ranging from 1 to 5 μm. These beads can be functionalized with different ligands, catalysts or enzymes, in order to use the fluidized bed as a continuous purification column or bioreactor. It allows flow-through operations at flow rates ranging from 100 nL min-1 up to 5 μL min-1 at low driving pressures (<100 mbar) with intimate liquid/solid contact and a continuous recirculation of beads for enhanced target capture efficiencies. The physics of the system presents significant differences as compared to conventional fluidized beds, which are studied here. The effects of magnetic field profile, flow chamber shape and magnetic bead dipolar interactions on flow regimes are investigated, and the different regimes of operation are described. Qualitative rules to obtain optimal operation are deduced. Finally, an exemplary use as a platform for immunocapture is provided, presenting a limit of detection of 0.2 ng mL-1 for 200 μL volume samples.

Computational Fluid Dynamics–Discrete Element Method Investigation of Pressure Signals and Solid Back-Mixing in a Full-Loop Circulating Fluidized Bed

Abstract: Computational fluid dynamics combined with discrete element method is employed to investigate the pressure signals and solid back-mixing behavior in a three-dimensional full-loop circulating fluidized bed operating in fast fluidization (FF) and dilute phase transport (DPT) regimes. The minimum fluidization velocity is successfully predicted after model validation. The gas–solid full-loop hydrodynamics is accurately captured. Pressure signals under different fluidization regimes shed light on the flow dynamics. The wider solid residence time distribution (RTD) curve with a longer tail in the FF regime indicates that solid flow closes to perfect mixing flow, and more severe solid back-mixing is due to solid internal circulation existing in the riser. The smaller solid RTD curve with a short tail in the DPT regime suggests that the solid flow deviates a little from plug flow, and a small-scale solid back-mixing is due to geometry restraint and recirculating gas–solid flow occurring in the lower and upper reg...

LES of n -Dodecane Spray Combustion Using a Multiple Representative Interactive Flamelets Model

Abstract: A single-hole n -dodecane spray flame is studied in a Large-Eddy Simulation (LES) framework under Diesel-relevant conditions using a Multiple Representative Interactive Flamelets (MRIF) combustion model. Diesel spray combustion is strongly affected by the mixture formation process, which is dominated by several physical processes such as the flow within the injector, break-up of the liquid fuel jet, evaporation and turbulent mixing with the surrounding gas. While the effects of nozzle-internal flow and primary breakup are captured within tuned model parameters in traditional Lagrangian spray models, an alternative approach is applied in this study, where the initial droplet conditions and primary fuel jet breakup are modeled based on results from highly resolved multiphase simulations with resolved interface. A highly reduced chemical mechanism consisting of 57 species and 217 reactions has been developed for n -dodecane achiving a good computational performance at solving the chemical reactions. The MRIF model, which has demonstrated its capability of capturing combustion and pollutant formation under typical Diesel conditions in Reynolds-Averaged Navier-Stokes (RANS) simulations is extended for the application in LES. In the standard RIF combustion model, representative chemistry conditioned on mixture fraction is solved interactively with the flow. Subfilter-scale mixing is modeled by the scalar dissipation rate. While the standard RIF model only includes temporal changes of the scalar dissipation rate, the spatial distribution can be accounted for by extending the model to multiple flamelets, which also enables the possibility of capturing different fuel residence times. Overall, the model shows good agreement with experimental data regarding both, low and high temperature combustion characteristics. It is shown that the ignition process and pollutant formation are affected by turbulent mixing. First, a cool flame is initiated at approximately stoichiometric mixture and propagates towards the rich side. Hence, heat and radicals are transported away from the most reactive mixture and thus the ignition is delayed. At the same time, the transported heat and radicals increase the reactivity of rich mixtures, which strongly affects the CO formation. NO was found to increase compared to the no transport case due to enhanced mixing, which is related to a broader high-temperature zone and the additional transport of oxygen from lean into high-temperature regions.

Multiphase injector modelling for automotive SCR systems

Abstract: Advances in emission control technologies have seen the introduction of Selective Catalyst Reduction (SCR) systems as a method for NOx decontamination in light and heavy duty vehicles. SCR systems make use of a ureawater solution (UWS) injected directly into the exhaust gas stream for the reduction of NOx contaminants to Nitrogen (N2) over a monolith catalyst [2]. The effectiveness of an SCR system depends on many factors including the type of catalysts, the injection and mixing pattern of the UWS, temperature and more [1].Spray analysis involves multi-phase flow phenomena and requires the numerical solution of the conservation and transport equations for the gas and the liquid phase simultaneously. Spray/wall interaction mechanisms such as droplet splash, spread, rebound or stick are complex to model and directly affected by the injection parameters [4]. The accurate modelling of the UWS injector can help in the prediction of phenomena such as wall film formation, droplet evaporation and urea crystallization [7].This study presents a series of multi-phase numerical analyses, computed with the commercial software AVL Fire 2014 v, to measure the impact of injection velocity, spray angle and droplet size, in the overall performance of an SCR system. The analysis consisted of a completely mixed turbulent flow, solved using a two equations turbulence model (kzetaf). The interaction of the injected particles was solved with an Euler/Lagrange approach, the liquid phase calculation was based on the statistical Discrete Droplet Method interacting with the numerical solution of the conservation equations of the flow pattern.It was found that the injection parameters had an impact on the final results of the ammonia uniformity index (NH3UI) and wall film formation on the SCR system. The method applied in this work successfully predicted the performance of an SCR system. Moreover, a series of response surfaces were created based on a linear regression model which allowed for further design optimization outside of the initial experimental space.