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Channeling of gas can reduce mass transfer performance in multiphase micropacked-bed reactors. Viscous and capillary forces cause this undesired and often unpredictable phenomenon in systems with catalyst particle sizes of hundreds of micrometers. In this work, we acoustically modify flow in a micropacked-bed reactor to reduce gas channeling by applying high-power sonication at low ultrasonic frequencies (~40 kHz). Experimental residence time distributions reveal two orders of magnitude reduction in dispersion with ultrasound, allowing for nearly plug-flow behavior at high flow rates in the bed. Sonication appears to partially fluidize the packed-bed under pressurized cocurrent two-phase flow, effectively improving dispersion characteristics. [ABSTRACT FROM AUTHOR]

Handling hazardous multiphase reactionsin flow brings not onlysafety advantages but also significantly improved performance, dueto better mass transfer characteristics. In this paper, we presenta continuous microreactor setup, capable of performing olefin autoxidationswith O2, under solvent-free and catalyst-free conditions.Owing to the transparent reactor design, consumption of O2can be visually followed and exhaustion of the gas bubbles marksa clear end point along the channel length coordinate. Tracking theposition of this end point enables measuring effective rate constants.The developed system was calibrated using the well-studied β-pinenesubstrate, and was subsequently applied to the synthetically interestingtransformation of (+)-valencene to (+)-nootkatone. For the latter,a space-time yield was obtained that is at least 3 orders of magnitudelarger than that realized with established biotechnology approaches. [ABSTRACT FROM AUTHOR]

Theidea of applying the gas–liquid–solid (GLS) three-phasecirculating fluidized bed to CO2capture by aqueous ammoniais proposed in this work. A three-phase circulating fluidized bedtest rig was built, and a series of experiments were carried out tostudy the multiphase flow and gas–liquid mass transfer behavior.Compared with the conventional reactors, the solids in the three-phasecirculating fluidized bed can break up bubbles into much smaller onesand distribute them more uniformly. The experimental results showa three-phase circulating fluidized bed can provide larger interfacearea, longer residence time, and higher mass transfer efficiency.Based on these results, the process for CO2capture byaqueous ammonia using a three-phase circulating fluidized bed wasdesigned conceptually and its feasibility was studied preliminarilyby Apsen Plusa simulation software. When the mole fractionof aqueous ammonia is about 5.16% in the three-phase circulating fluidizedbed, more than 90% of the CO2can be absorbed and the ammoniaescaping ratio is very small. The three-phase circulating fluidizedbed presents great potential in CO2separation by aqueousammonia. [ABSTRACT FROM AUTHOR]

Thedownflow slurry bubble column reactor is gaining importanceas a simple and inexpensive means of achieving efficient multiphasecontact and mass-transfer operations. Knowledge about the effectsof solids on gas–liquid–solid systems is minimal inthe literature. In the present work, the effect of the presence ofsolids on the mass-transfer characteristics for gas–liquid–solidsystem is experimentally investigated in a downflow slurry bubblecolumn with an internal diameter of 0.05 m. The mass-transfer coefficientwas evaluated using the electroreduction of ferricyanide ions as themodel reaction. The effects of superficial slurry velocity, gas velocity,slurry viscosity, and gas holdup on the mass-transfer coefficientwere analyzed. The experimental results showed that the mass-transfercoefficient in the column is influenced more by the superficial slurryvelocity than the superficial gas velocity. In this work, the mass-transferefficiency in the gas–liquid–solid downflow bubble columnreactor was analyzed for different concentrations of the slurry (0.1–1.0wt %) and different sizes of the particles (2.21–96.0 μm).Empirical correlations were developed for the downflow slurry systemwith the different variables to interpret the mass-transfer phenomena.The developed correlation for mass-transfer efficiency as a functionof quality of mixedness was also interpreted by the information entropytheory. The present analysis on mass-transfer characteristics andmodels for predicting the mass-transfer coefficient can provide insightinto a further understanding of multiphase reactors in industrialapplications. [ABSTRACT FROM AUTHOR]

We present a pH-sensitive laser-induced fluorescence(LIF) techniqueto investigate mass transfer in reactive flows. As a fluorescent dye,we used 5-(and-6)-carboxy SNARF-1, which, when excited with a pulsedNd:YAG laser at 532 nm, provides good sensitivity in the range 4 ≤pH ≤ 12. For validation, we first applied the dye to single-phasereactive flows by investigating the neutralization of sodium hydroxidewith hydrochloric acid. Comparison to the classical passive mixingcase showed that this dye was able to capture the reaction progressand to quantify the mass transport. Next, we investigated the absorptionof CO2in an alkaline solution using gas–liquidflow and found that the LIF technique is able to quantify the localmass-transfer rate in microfluidic systems. Results for differentmicrochannel geometries highlight the strong connection between localmass transfer and secondary flow structures in gas–liquid Taylorflow. [ABSTRACT FROM AUTHOR]

This Article presents the derivation and validation of a rigorous model for the prediction of multicomponent Maxwell-Stefan (MS) diffusion coefficients. The MS theory provides a sound framework for modeling mass transport in gases and liquids. Unfortunately, MS diffusivities are concentration dependent, and this needs to be taken into account in practical applications. There is therefore a considerable interest in models describing the concentration dependence of MS diffusivities. While current practice employs empirical models for this purpose, recent work on molecular simulations favor the physically based Darken equation. The Darken equation, however, is limited to binary mixtures and is not predictive. In this study, a multicomponent Darken model for MS diffusivities is derived from linear response theory and the Onsager relations. In addition, a predictive model for the required self-diffusivities in the mixture is proposed, leading to the predictive Darken-LBV model. We compare our novel model to the existing generalized Vignes equation and the generalized Darken equation using molecular dynamics (MD) simulation. Two systems are considered: (1) ternary and quaternary systems in which particles are interacting using the Weeks-Chandler-Andersen (WCA) potential; (2) the ternary system n-hexane-cyclohexane-toluene. Our results show that, in all studied systems, the novel predictive Darken-LBV equation describes the concentration dependence better than the existing models. The physically based Darken-LBV model provides a sound and robust framework for prediction of MS diffusion coefficients in multicomponent mixtures. [ABSTRACT FROM AUTHOR]

The hydrodynamic and drying mechanisms of a dilute-phase, gas−solid flow have been predicted in a laboratory and a large-scale pneumatic dryer. A computational fluid dynamics (CFD) method based on the two-fluid multiphase flow model has been adopted. A control-volume based technique implemented in the FLUENT CFD package was applied along with the kinetic theory of granular flow (KTGF) to simulate the flow pattern and heat and mass transfer processes for wet PVC and sand particles by hot air. The mass-weighted averages of theoretical predictions for temperature and moisture content agree well with the experimental data for similar models. User-defined subroutines were added to extend FLUENT capability to account for mixture properties and to simulate the drying rate for surface moisture evaporation and moisture diffusion inside the particulate phase. The present model can be extended to simulate the drying of various temperature sensitive materials. [ABSTRACT FROM AUTHOR]

Multiphase flow microreactors benefit from rapid mixing and high mass transfer rates, yet their application in enzymatic catalysis is limited due to the fast inactivation of enzymes used as biocatalysts. Enzyme inactivation during segment flow is due to the large interfacial area between aqueous and organic phases. The Peclet number of the system points to strong convective forces within the segments, and this results in rapid deactivation of the enzyme depending on segment length and flow rate. Addition of surfactant to the aqueous phase or enzyme immobilization prevents the biocatalyst from direct contact with the interface and thus stabilizes the enzyme activity. Almost 100% enzyme activity can be recovered compared to 45% without any enzyme or medium modification. Drop tensiometry measurements point to a mixed enzyme−surfactant interfacial adsorption, and above a certain concentration, the surfactant forms a protective layer between the interface and the biocatalyst in the aqueous compartments. Theoretical models were used to compare adsorption kinetics of the protein to the interface in the segment flow microreactor and in the drop tensiometry measurements. This study is the basis for the development of segment flow microreactors as a tool to perform productive enzymatic catalysis. [ABSTRACT FROM AUTHOR]

The molecular diffusion of an oil−gas system is a key factor to determine the mass transfer rate. The actual mass transfer between oil and gas is an unsteady-state process. In this paper, the diffusion coefficients of the N2-rich gas−crude oil, CO2-rich gas−crude oil, and CH4-rich gas−crude oil systems were determined at 60 °C and 20 MPa by the mutual diffusion model developed. Results of the coefficient measurements show obvious transit characteristics of the gas−oil diffusion processes. Both the required equilibrium time and the diffusion rate depend on not only the diffusion gas but also the diffusion system. The diffusion gas concentration affects the diffusion coefficients slightly. [ABSTRACT FROM AUTHOR]

The manufacture of chemicals in microstructured reactors (MSR) has become recently a new branch of chemical reaction engineering focusing on process intensification and safety. MSR have an equivalent hydraulic diameter up to a few hundreds of micrometers and, therefore, provide high mass- and heat-transfer efficiency increasing the reactor performance drastically, compared to the conventional one. This article provides a comprehensive overview of the state of the art of the MSR applied for multiphase reactions. The reactions are classified based on the number of phases involved: fluid−fluid, fluid−solid, and three phase reactions. In the first part of the review, limitations of conventional reactors are discussed in brief. Furthermore, different types of MSR and their advantages with respect to their conventional counterparts are described. Particular attention is given to the identification of the parameters that control the flow pattern formed in microcapillaries regarding the mass-transfer efficiency. Case studies of various multiphase reactions carried out in MSR are discussed in detail. [ABSTRACT FROM AUTHOR]