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The migration of non-Brownian particles inside a droplet subjected to shear flow is investigated. The viscosity ratio, particle concentration, particle to droplet size ratio and capillary number are systematically varied. It was observed that particles migrate in the vorticity direction either towards or away from the equatorial plane. The main factor determining the migration direction is the viscosity ratio. The particle concentration and applied capillary number have no influence on the equilibrium distribution and only affect the time necessary to reach this distribution. In order to identify the flow profile, both velocity and shear rate magnitudes to which the particles inside the droplet are subjected, are numerically simulated using the Volume-of-Fluid method in a particle-free droplet/matrix system. The numerical predictions of the flow are used to evaluate the presence of possible driving mechanisms for cross-streamline particle motion, such as particle inertia and shear rate gradients. Depending on the conditions, the simulations reveal significant differences in the streamline profiles and shear rate gradients within the droplet. The observed steady state particle distributions can partially be explained by shear-induced migration towards zones of low shear rates and/or low streamline curvature. However, in complex flows, such as inside a deformed droplet, different migration mechanisms can occur simultaneously. Hence, confinement inside the droplet and potential secondary flows may also play a role.

Photochemical reactors with conventional homogeneous illumination suffer from a light efficiency problem, which is inherent to their design: Dark zones arise near the reagent‐rich inlet whereas the reagent depleted outlet is over‐illuminated. Any attempt to mitigate dark zones at the inlet will only increase photon losses further downstream. This study reports the principles and model equations for co‐ and counter‐current illumination in photochemical reactors, along with an optimization study to determine the most efficient and productive operating point. This work proves that the use of co‐ and counter‐current illuminated reactors increases the energy efficiency while easing scalability by implementing larger path lengths, without altering the reactor's geometry. We report a simple model to determine the conversion obtained by such novel illumination techniques and compare it to the current state‐of‐the‐art. Two non‐dimensional groups where derived that describe all possible reactor configurations, these are the initial absorbance (A) and the quantum photon balance (ρϕ). Variation of both parameters leads for non‐competitive photochemical reactions to an optimal point for the current state‐of‐the‐art as well as the novel co‐axial illumination. Ultimately, we recommend the use of an initial absorbance value (AA) of at least 1, and a photon quantum balance (ρϕ) equal to 1 to introduce sufficient light and enable near complete absorption of light. ispartof: Journal of Advanced Manufacturing and Processing status: Published online

Weakly-supported electrolyses are frequently encountered in organic electrosynthesis in small scale flow reactors. Sometimes “self-supported” electrolyses are reported where no supporting electrolyte is deliberately added at all. To interpret the steady-state and transient behavior of these electrochemical micro- and millireactors, an orthogonal collocation method based on Chebyshev polynomials was developed for calculating the concentration and current distributions in parallel plate and annular geometries. This method takes into account diffusion, migration, and convection in unidirectional flow for an arbitrary number of dilute ionic species. In particular, the method is flexible with regard to the electrode and homogenous reactions so that intricate reaction schemes are conveniently simulated. As a case study, the effect of the supporting electrolyte concentration and the stability of the generated reactive intermediate ions on a generic paired electrosynthesis is investigated. The numerically calculated concentration profiles and current responses are then compared with asymptotic approximations. This reveals that fundamentally different operating regimes exist for a self-supported paired electrolysis compared to more conventional electrolyses where an excess of supporting electrolyte is added. The Matlab code and a detailed user guide is freely available via the Supplementary Information. ispartof: Electrochimica Acta vol:292 pages:914-934 status: published

Translucent structured reactors have proven to be an effective design to scale up microreactors. By generating surface area, this flexible reactor design allows to increase the catalyst loading without increasing the catalyst layer thickness, which is beneficial in tackling diffusion limitations in single-channel reactors. However, adding more depth to such a structure by replicating the channels increases the number of scattering boundaries which leads to energy losses. As a result, there is a design problem which seeks to define the optimal catalyst layer thickness and optimal number of repeating boundaries on a light path. Most of the models are numerically solved and very specific to the reactor type being modeled. In this work, a catalyst layer mass balance model is used to construct a model of a translucent structured reactor which takes into account internal mass transfer effects and which can be used to design an optimal structure. The model is simplified to obtain a graphical tool and an analytical model which is validated to estimate the overall reactor kinetics as a function of dimensionless groups. For a conventional range of parameters, the optimal catalyst layer thickness and optimal number of structural layers was equal to 2 µm and 4, respectively. The presented tools in this work are a step forward in the fabrication of design methods for photocatalytic reactor structures. This way, the designer can easily estimate the design outcome without any complex calculations.

We present a novel technique to quantify interfacial mass transfer in gas–liquid flows in capillaries. A model was developed which allowed the relationship between absorption fluxes and bubble size changes to be established. Depending on the observed absorption rate, two limiting and simplifying cases for low and high absorption rates were suggested. For both cases, the flux was viewed as an input function which optimally tracked the observed bubble velocity, bubble volume, and unit cell volume change. For the case of low absorption rates, the mass transfer coefficient can be assumed to be constant and the model was fitted to experimental data by adjusting the value of this mass transfer coefficient. The bubble velocities were extracted from flow images using the cross-correlation between measured signal intensities for nearby pixels. Bubble and unit cell volumes were not determined by individual tracking, but a probability density function was estimated for each location in the capillary which contains the bubble and unit cell sizes of all bubbles passed. This allows the model to be fitted to an ensemble of bubbles making the procedure more robust. The developed model was validated for CO2 absorption in a buffer solution, and then applied to CO2 absorption using the ionic liquid 1-ethyl-3-methylimidazolium acetate ([Emim][Ac]), which exhibits high absorption rates and less known thermophysical properties. The developed technique is able to quantify interfacial fluxes for a wide range of gas and liquid flow rates.

Coupling photochemistry with flow microreactors enables novel synthesis strategies with higher efficiencies compared to batch systems. Improving the reproducibility and understanding of the photochemical reaction mechanisms requires quantitative tools such as chemical actinometry. However, the choice of actinometric systems which can be applied in microreactors is limited, due to their short optical pathlength in combination with a large received photon flux. Furthermore, actinometers for the characterization of reactions driven by visible light between 500 and 600 nm (e.g. photosensitized oxidations) are largely missing. In this paper, we propose a new visible-light actinometer which can be applied in flow microreactors between 480 and 620 nm. This actinometric system is based on the photoisomerization reaction of a diarylethene derivative from its closed to the open form. The experimental protocol for actinometric measurements is facile and characterized by excellent reproducibility and we also present an analytical estimation to calculate the photon flux. Furthermore, we propose an experimental methodology to determine the average pathlength in microreactors using actinometric measurements. In the context of a growing research interest on using flow microreactors for photochemical reactions, the proposed visible-light actinometer facilitates the determination of the received photon flux and average pathlength in confined geometries. ispartof: SCIENTIFIC REPORTS vol:8 issue:1 ispartof: location:England status: published