Design of a Bench-Scale Tubular Reactor Similar to Plug Flow Reactor for Gas-Phase Kinetic Data Generation-Illustration with the Pyrolysis of Octanoic Acid

The material described in this article deals with waste conversion into energy vectors by pyrolysis, steam cracking, or oxidation of liquid biomass, carried out at small to medium scale. The design of a bench-scale experimental setup devoted to gas phase kinetic data generation in a tubular reactor under laminar regime close to plug flow is detailed based on a very simple approach. Validation of the designed bench-scale setup was successfully carried out within the context of octanoic acid pyrolysis by generating kinetic data with satisfactory measurement repeatability and material balances. The key to this positive result is that axial dispersion coefficient is much smaller in gas-phase than in liquid-phase, thus allowing the designed small sized tubular reactor to be close to the plug flow reactor. Such a feature of the axial dispersion coefficient is not well known by the wider public. Besides, octanoic acid was selected as surrogate for carboxylic acids because of their key role in various industrial applications (combustion of ethyl biodiesel; production of biofuel and biosourced chemicals).

The Hydrodynamics and Mixing Performance in a Moving Baffle Oscillatory Baffled Reactor through Computational Fluid Dynamics (CFD)

Oscillatory baffled reactors (OBRs) have attracted much attention from researchers and industries alike due to their proven advantages in mixing, scale-up, and cost-effectiveness over conventional stirred tank reactors (STRs). This study quantitatively investigated how different mixing indices describe the mixing performance of a moving baffle OBR using computational fluid dynamics (CFD). In addition, the hydrodynamic behavior of the reactor was studied, considering parameters such as the Q-criterion, shear strain rate, and velocity vector. A modification of the Q-criterion showed advantages over the original Q-criterion in determination of the vortices’ locations. The dynamic mesh tool was utilized to simulate the moving baffles through ANSYS/Fluent. The mixing indices studied were the velocity ratio, turbulent length scale, turbulent time scale, mixing time, and axial dispersion coefficient. We found that the oscillation amplitude had the most significant impact on these indices. In contrast, the oscillatory Reynolds number did not necessarily describe the mixing intensity of a system. Of the tested indices, the axial dispersion coefficient showed advantages over the other indices for quantifying the mixing performance of a moving baffle OBR.

Hydrodynamics of pulsed sieve-plate extraction columns

This paper presents a review of some hydrodynamic parameters in pulsed sieve-plate extraction columns. The hydrodynamic parameters in liquid-liquid systems in these columns were analyzed regarding the effects of operating and geometrical parameters. The values of Sauter mean drop diameter were function of the existing work flow regimes in the column device defined as mixer-settler, emulsion and dispersion regimes. It was concluded that the dispersed-phase holdup was a function of the mean drop diameter and dispersed-phase superficial velocity. An increase in the dispersed-phase holdup induced an increase in the interface area in the liquid-liquid system. Knowledge of the value of the dispersed-phase holdup can be used for calculation of the volumetric mass transfer coefficient, one of the important factor in the design of the column extractor. It was concluded that the increase in the dispersed-phase superficial velocity is causing a decrease in axial dispersion. On the other hand, an increase in the continuous-phase superficial velocity is causing the increase in the axial dispersion coefficient. Some of the empirical equations proposed in literature for calculations of the hydrodynamic parameters were presented. These correlations, derived for determination of the hydrodynamic parameters in pulsed sieve-plate extraction columns, can be used for the design of these liquid-liquid extraction columns.

Hydrodynamics in spray and packed liquid-liquid extraction columns: A review

This work provides a review of hydrodynamic characteristics such as the slip velocity, the dispersed-phase holdup, mean drop size, and axial dispersion of non-mechanically agitated liquid-liquid (L-L) extractors, with special reference to spray and packed bed columns. The complexity and importance of hydrodynamic behavior in designing and scaling up L-L extractors was a driving force to analyze, compare and discuss some important experimental findings available in the literature. The effects of phase velocities and the dispersed-phase holdup on the slip velocity, the mean drop size and the axial dispersion coefficient were studied and presented. Empirical correlations for slip velocity, the Sauter mean drop diameter and the axial dispersion coefficient, which were taken from the literature, were commented in terms of their applicability.

New regimes of dispersion in microfluidics as mediated by travelling temperature waves

We unveil new regimes of dispersion in miniaturized fluidic devices, by considering fluid flow triggered by a travelling temperature wave. When a temperature wave travels along a channel wall, it alters the density and viscosity of the adjacent fluid periodically. Successive expansion–contraction of the fluid volume through a spatio-temporally evolving viscosity field generates a net fluidic current. Based on the temporal evolution of the axial dispersion coefficient, new regimes of dispersion—such as a short-time ‘oscillating regime’ and a large-time ‘stable regime’—have been identified, which are absent in traditionally addressed flows through miniaturized fluidic devices. Our analysis reveals that the oscillation of axial dispersion persists until the variance of species concentration becomes equal to half of the square of the wavelength of the thermal wave. The time period of oscillation in the dispersion coefficient turns out to be a unique function of the thermal wavelength and net flow velocity induced by thermoviscous pumping. The results of this study are likely to contribute towards the improvement of microscale systems that are subjected to periodic temperature variations, including microreactors and DNA amplification devices.

Axial dispersion and mixing of coolant gas within a separate-effect prismatic modular reactor

Multiphase Reactors Engineering and Applications Laboratory performed gas phase dispersion experiments in a separate-effect cold-flow experimental setup for coolant flow within heated channels of the prismatic modular reactor under accident scenario using gaseous tracer technique. The separate-effect experimental setup was designed on light of local velocity measurements obtained by using hot wire anemometry. The measurements consist of pulse-response of gas tracer that is flowing through the mimicked riser channel using air as a carrier. The dispersion of the gas phase within the separate-effect riser channel was described using one-dimensional axial dispersion model. The axial dispersion coefficient and Peclet number of the coolant gas phase and their residence time distribution within were measured. Effect of heating intensities in terms of heat fluxes on the coolant gas dispersion along riser channels were mimicked in the current study by a certain range of volumetric air flow rate ranging from 0.0015 to 0.0034 m3/s which corresponding to heating intensity range from 200 to 1400 W/m2. Results confirm a reduction in the response curve spreads is achieved by increasing the volumetric air velocity (representing heating intensity). Also, the results reveal a reduction in values of axial dispersion coefficient with increasing the air volumetric flow rate.

Hydrodynamics of a pilot plant spray extraction column

The hydrodynamic characteristics of the liquid-liquid system of toluene-water in a pilot plant spray extraction column were experimentally determined. The experimental data for hydrodynamic characteristics such as the dispersed phase holdup, mean droplet size, and the axial dispersion coefficient were obtained. The dispersed phase superficial velocity had a great influence on toluene holdup. At the same time, a strong effect of the continuous phase superficial velocity on the dispersed phase holdup was evident. The dispersed phase holdup had a tendency to increase when the ratio of the dispersed phase superficial velocity and characteristic velocity increased. The Sauter mean droplet diameter decreased with increasing dispersed phase superficial velocity when the continuous phase superficial velocity remained constant. In contrast, it was not affected by the changes in the continuous phase superficial velocity while the dispersed phase superficial velocity remained constant. It was concluded that the Peclet number increased as a result of an increase of the Reynolds number.