Design of a new FM01-LC reactor in parallel plate configuration using numerical simulation and experimental validation with residence time distribution (RTD)

The goal of this work was to develop new geometry design of inlet and outlet distributors of the FM01-LC in parallel plate configuration using Computational Fluid Dynamics (CFD). The new distributor geometry was experimentally evaluated with RTD experimental curves using the stimulus-response technique and approximated with axial dispersion model (ADM), plug dispersion exchange model (PDEM) and by solving the hydrodynamic (Reynolds average Navier–Stokes equation for low Reynolds number, RANS-LRN) and mass transport (convection–diffusion equation in transient and turbulent regimen) equations using computational fluid dynamics (F-tracer RTD method). Two sets of RTD experiments (common and new inlet and outlet distributors) in FM01-LC reactors with channel thickness of 0.011 m were carried out. The volumetric flows (Q) employed were from 0.5 to 3.5 L min⁻¹ (U₀ = 0.02-0.15 m s⁻¹). The new FM01-LC reactor had a more homogeneous velocity field in the entire reaction zone, as shown by axial dispersion values lower than those obtained with the common FM01-LC, at different Reynolds numbers. The RTD curves obtained with Comsol Multiphysics 4.3a are in agreement with RTD experimental curves, but deviations are observed at Reynolds numbers greater than “5991”.     

Liquid backmixing in oscillatory flow through a periodically constricted meso-tube

This paper deals with the measurement and modelling of axial liquid dispersion in a 4.5 mm internal diameter tube provided with smooth-periodic constrictions (meso-tube) in steady and oscillatory flow conditions. The residence time distribution (RTD) in the meso-tube was monitored for a range of fluid oscillation frequency (f) and amplitude (x₀) at laminar flow. The RTD response was modelled with three hydrodynamic models: (i) tanks-in-series, (ii) tanks-in-series with backflow and (iii) plug flow with axial dispersion. The steady flow through the meso-tube at flow rates up to 21.30 ml/min resulted in broad RTDs, mainly due to the parabolic velocity profile. The use of fluid oscillations allowed a fine-control of the axial liquid dispersion in the meso-tube due to generation of secondary flow in the regions between the constrictions. The axial dispersion coefficient D was reduced by up to 13-fold in comparison with the steady flow situation. Values of x₀ ≤ 1 mm and f = 10 Hz generally resulted in a maximum reduction in axial dispersion through, therefore maximum improvements in RTD. The tanks-in-series model was generally not capable of predicting RTDs in the meso-tube. The potential of this platform for the continuous, sustainable production of added-value products is herein demonstrated.     

RTD studies in trickle bed reactors packed with porous particles

The residence time distribution (RTD) for liquid phase in a trickle bed reactor (TBR) has been experimentally studied for air-water system. Experiments were performed in a 15.2 cm diameter column using commerical alumina extrudates with D/d_p ratio equal to 75 to eliminate the radial flow differences. The range of liquid and gas flow rates covered was 3.76 < Re_L < 9.3 and 0 < Re_G < 2.92. The axial dispersion model was used to compute axial dispersion coefficient. The effect of liquid and gas flow rates on total liquid holdup and axial dispersion was investigated. The total liquid holdup has been correlated to liquid and gas flow rates.     

Electrochemical incineration of indigo textile dye in filter-press-type FM01-LC electrochemical cell using BDD electrodes

This work shows results obtained in the incineration of 1 mM indigo textile dye (536 ppm COD) in 0.05 M NaCl aqueous media (which resembles a denim laundry industrial wastewater). Microelectrolysis and macroelectrolysis studies indicated that oxidation of indigo dye was carried out via hydroxyl radicals (OH) formed by water oxidation on the BDD surface, instead of active chlorine as usually occurs by using DSA. Electrolyses in a FM01-LC reactor was performed at Reynolds between 1600 < Re < 18300, and a fixed current density of 5.3 and 15 mA cm⁻². The experimental set-up achieved 100% efficiency in color removal, indigo mineralization and current efficiency. Estimated energy consumption, at Re = 12892 and J = 5.3 mA cm⁻², was 9 kWh m⁻³. Experimental data revealed that hydrodynamic conditions do not influence either the indigo degradation rate or the current efficiencies; therefore indigo degradation must involve a complex mechanism.     

Prediction of mass transport profiles in a laboratory filter-press electrolyser by computational fluid dynamics modelling

A commercial computational fluid dynamics code (Fluent) has been used to analyze the performance of a unit cell laboratory; the filter-press reactor (FM01-LC) operating with characteristic linear flow velocities between 0.024 m s⁻¹ and 0.110 m s⁻¹. The electrolyte flow through the reactor channel was numerically simulated using a finite volume approach to the solution of the Navier-Stokes equations. The flow patterns in the reactor were obtained and the mean linear electrolyte velocity was evaluated and substituted into a general mass transport correlation to calculate the mass transport coefficients. In the region of 150 < Re < 550, mass transport coefficients were obtained with a relative error between 5% and 29% respect to the experimental k_m values. The differences between theoretical and experimental values are discussed.     

CFD evaluation of internal manifold effects on mass transport distribution in a laboratory filter-press flow cell

The internal manifold geometry strongly influences the flow distribution inside an electrochemical reactor. The mass transport coefficient is a function of the flow pattern and is a key parameter in successful electrochemical reactor design and scale-up. In this work, a commercial computational flow dynamics (CFD) package was used to describe the flow pattern in the FM01-LC reactor at controlled volumetric flow rates (corresponding to mean linear flow velocities past the electrode surface between 0.024 and 0.11 m s^?1). Numerical Re numbers were obtained for each local flow velocity at different positions in the reactor channel. From a known mass transport correlation (based on dimensionless groups, i.e. Sh, Re, Sc), numerical k _m values were obtained (in the range 200 < Re < 1,000) at different positions in the reactor channel. Computed k _m numbers are compared against experimental values. This computational approach could be useful in reactor design or selection since it facilitates a fast, preliminary reactor flow and mass transport characterisation without experimental electrochemical measurements.     

Axial dispersion in laminar flow in helical coils

The results of an experimental study of dispersion of solute in helical coils under laminar flow conditions are reported. The ranges of variables covered are 10< λ < 280, 10 <N_Re < 1600 and 1·5 × 10³ <N_Sc < 8·7 × 10³. Under the conditions where the dispersion model holds the data are correlated with a standard deviation of 19·6 per cent. The conditions under which the dispersion model holds for helical coils are found to be less severe than those for straight tubes. It is shown that coiling can reduce axial dispersion under laminar flow conditions by a factor of upto 500.     

A comparison of the electrochemical recovery of palladium using a parallel flat plate flow-by reactor and a rotating cylinder electrode reactor

The production of catalytic converters generates large amounts of waste water containing Pd²⁺, Rh³⁺ and Nd³⁺ ions. The electrochemical treatment of these solutions offers an economic and effective alternative to recover the precious metals in comparison with other traditional metal recovery technologies. The separation of palladium from this mixture of metal ions by catalytic deposition was carried out using a rotating cylinder electrode reactor (RCER) and a parallel plate reactor (FM01-LC) with the same cathode area (64 cm²) and electrolyte volume (300 cm³). The study was carried out at mean linear flow velocities of 1.27 < ν < 11.36 cm s⁻¹ (120 < Re = υd_e/v < 1080) for the FM01-LC reactor and 20 < ν < 140 cm s⁻¹ (7390 < Re = υd/v < 51,700) for the RCER. The morphology of the palladium deposits at the entrance and at the exit of the electrolyte compartment of the FM01-LC reactor showed the effect of the manifold distributors during the electrolysis; the manifolds generate micro turbulences, increasing the mass transport coefficient in these areas and favouring rapid recovery of palladium ions. More uniform high purity palladium deposits were obtained on the surface of the RCER. The cumulative current efficiency to recover 99% of Pd²⁺ ions in the parallel plate electrode reactor was 35% while the recovery of 97% of Pd²⁺ in the RCER was 62%. The volumetric energy consumption during the electrolysis was 0.56 kW h m⁻³ and 2.1 kW h m⁻³ for the RCER and the FM01-LC reactors, respectively. Using a three-dimensional stainless steel electrode in the FM01-LC laboratory reactor, 99% of palladium ions were recovered after 30 min of electrolysis while in the RCER, 120 min were necessary.     

Solids mixing in a down-flow circulating fluidized bed of 0.418-m in diameter

The axial and lateral solids mixing in a down-flow circulating fluidized bed of 0.418-m diameter was investigated by a pneumatic injection phosphor tracer technique (PIPTT). The axial and lateral solids dispersion were determined by measuring the solids RTD at same axial but different lateral positions using point sources for tracer injection. A two-dimensional dispersion model described the measured RTD curves satisfactorily. The results were compared to those obtained in the small scale downers and the scale-up effect was investigated. The axial solids Peclet number Pe_a is around 110 and invariable with changing U_g, G_s and ?_s, while the lateral solids Peclet number Pe_r is linearly increasing with ?_s. And Pe_r is found to decrease with the square root of inner diameter (ID) in comparison with the results obtained in small ID downers. Correlation of Pe_r, Pe_r = (15 + 70.7 ?_s)D^− 0.5, is proposed.     

Electrochemical incineration of p-cresol and o-cresol in the filter-press-type FM01-LC electrochemical cell using BDD electrodes in sulfate media at pH 0

This work shows a comparative study of the incineration of 2-mM p-cresol and o-cresol in 1 M-H₂SO₄ in aqueous media. Microelectrolysis studies indicated that both the p-cresol and o-cresol oxidation were carried out via hydroxyl radicals (OH) formed by water oxidation in the boron-doped diamonds (BDD)-H₂O-H₂SO₄-p-cresol and o-cresol interface. In both cases, the potential and current density ranges, where great amounts of OH are formed, were between 2.3 V ≤ E ≤ 2.75 V versus SHE and J = 10 mA cm⁻². Electrolyses in an undivided FM01-LC reactor were performed at different Reynolds values 27,129 ≤ Re ≤ 42,631, and at J = 10 mA cm⁻². For p-cresol and o-cresol, the rate of degradation was slow, however it increases slightly as a function of the Re, indicating that the oxidation involves a complex pathway; current efficiency also rises as a function of the Re. For p-cresol, the mineralization at Re = 42,631 reached 90%, with 71% current efficiency and an energy consumption of 7.84 kWh m⁻³; whereas o-cresol was mineralized to 84%, with 67% current efficiency and an energy consumption of 6.56 kWh m⁻³. The results obtained in this work demonstrated that o-cresol is more recalcitrant than p-cresol.     

Axial dispersion in gases flowing through small bore helical column

The results of an experimental study on the axial dispersion of gases flowing in helical columns under laminar flow conditions are reported. The ranges of variables covered are 26.6 < λ < 98; 10 < N_Re < 100; 0.176 < N_Sc < 1.359. The measured dispersion coefficients are correlated with Reynolds, Schmidt and Dean groups. A single dimensionless parameter, N_Dc (N_Se^0,5, was found to correlate the data well. Up to N_De (N_Sc)^0,5 =10, dispersions in straight and coiled tubes exhibit very similar axial dispersion behavior.     

Characterisation of fluid mixing in novel designs of mesoscale oscillatory baffled reactors operating at low flow rates (0.3-0.6 ml/min)

For the first time two mesoscale oscillatory baffled designs (central and integral baffles with their volumes of 5.2 ml and 4.4 ml, respectively) were experimentally characterised at net flow rates as low as 0.3 ml/min (Re_n ∼ 1.25), giving a residence time of around 15-17 min over a wide range of oscillation conditions. The purpose was to identify the lower limits of operability, thereby determining the maximum residence time per unit reactor volume for these mesoscale units. The characteristics of fluid flow were found to be strongly affected by Strouhal number at these low net flows. For the integral baffles, the oscillation conditions exhibited little influence on the fluid mixing. For the central baffles, there were three distinct flow regimes, depending on Strouhal numbers which affect the fluid characteristics differently. At two regimes of Sts, St ≥ 0.8 and 0.13 ≤ St ≤ 0.2, an increase in frequency did not alter the axial dispersion. At St ≥ 0.8, the fluid experienced less uniform mixing, representing by right-skewed residence time distribution (RTD) curves. At 0.20 ≤ St ≤ 0.13, the fluid mixing was significantly improved, indicated by narrow and symmetrical RTD curves with Re_o up to 700. At 0.4 ≤ St ≤ 0.27 and St ≤ 0.1, the degree of plug flow was a function of Re_o. The maximum number of tanks achieved at these low flow rates was in the range 30-35, occurring at a velocity ratio (Re_o/Re_n) of 39-40.     

Liquid phase axial mixing in bubble columns operated at high pressures

Liquid phase axial mixing was measured in a 100 mm i.d. bubble column operated in the pressure range of 0.1-0.5 MPa. Water, ethanol and 1-butanol were used as the liquid phase and nitrogen as the gas phase. The temperature and superficial gas velocity were varied in the range of 298-323 K and 0.01-0.21 m/s, respectively. The axial dispersion coefficient increased with an increase in the gas density due to pressure. The temperature had surprisingly a small effect. A CFD model was developed for the prediction of flow pattern in terms of mean velocity and eddy diffusivity profiles. The model was further extended for the prediction of residence time distribution and hence the axial dispersion coefficient (D_L). The predictions of axial dispersion coefficient agree favorably with all the experimental data collected in this work as well as published in the literature. The model was extended for different gas-liquid systems. The predicted values of axial dispersion coefficient were found to agree very well with all the experimental data.     

Liquid re-circulation in turbulent vertical pipe flow behind a cylindrical bluff body and a ventilated cavity attached to a sparger

The turbulent flow field (Re=60024) in the wake of a cylindrical bluff body in a 0.105 m internal diameter pipe with an area blockage ratio of 82% in turbulent single-phase flow was studied using laser Doppler velocimetry (LDV). The results for the time-averaged velocity showed a toroidal vortex below the bluff body. The axial location below the bluff body where both the time-averaged radial and axial velocity components were zero (eye of the vortex) was found at approximately 0.72D. The end of the re-circulation region as defined by a stagnation point on the centreline of the pipe was found at an axial location below the bluff body of approximately 1.3D. These two locations did not change when altering the liquid superficial velocity confirming that the geometry (i.e., size) of the toroidal vortex is not dependent on the superficial liquid velocity or the speed of the vortex.Similar measurements using LDV were taken in the wake of a ventilated cavity in a vertical 0.105 m internal diameter pipe, with an area blockage ratio of 80%. The flow beneath the cavity was turbulent two-phase bubbly flow and the liquid-only flow ahead of the cavity was turbulent (Re=45618). The cavity was attached to a (central) sparger, which is a scale-up of the design used by Bacon (1995). The average gas void fraction in the wake of the cavity was 7%. The results for the time-averaged velocity confirmed the formation of a toroidal vortex remarkably similar to the vortex formed below the bluff body. The eye of the vortex and the end of the re-circulation region were found at an axial location below the ventilated cavity of 0.78 and 1.35D, respectively, i.e., almost identical to the results for the bluff body.The LDV results of the cylindrical bluff body and the ventilated cavity were compared with the fully predictive model of the velocity distribution in the vortex proposed by Thorpe et al. (2001) and good agreement was found in both cases. The model also agreed well with the data of van Hout et al. (2002) for a Taylor bubble rising in stagnant liquid in a 0.025 m internal diameter pipe. The CFX simulations of Thorpe et al. (2001) carried out for a 0.050 m internal diameter pipe, agreed well with the experimental data of the cylindrical bluff body, the ventilated cavity and the data obtained by van Hout et al. (2002) when correlating the results in the appropriate dimensionless form. Our analysis showed that the maximum axial re-circulation velocity in the centre of the vortex ring was directly proportional to the mean velocity in the annulus at the base of the cylindrical bluff body, the ventilated cavity or the Taylor bubble. The proportionality constant for all cases was found to be approximately 0.38 confirming the value proposed by Thorpe et al. (2001).     

Overall map and correlation of dispersion data for flow through granular packed beds

Measured values of the coefficients of transverse and longitudinal dispersion (D_T and D_L, respectively) are reported for liquid flow through granular packed beds. Measurements of D_T were made for 50<Sc<1950 and 300<Pe_m<10⁵, working with water at temperatures between 278 and 373 K, and values of D_L were measured for Sc≅750 and 1<Pe_m<45, working with water at 293 K; nearly two hundred new data points are reported.The data obtained, together with data from other sources, both for gas and liquid flow, are reported in plots of Pe_T vs Pe_m and Pe_L vs. Pe_m, in order to stress the influence of Sc on dispersion and elucidate the difference between liquid and gas behaviour.Empirical correlations are presented for the prediction of the dispersion coefficients (D_T and D_L) over the entire range of practical values of Sc and Pe_m, and they are shown to give the dispersion coefficients with very good accuracy.     

Residence time distribution in a packed bed bioreactor containing porous glass particles: Influence of the presence of immobilized cells

An experimental investigation of the liquid phase residence time distribution (RTD) in a packed bed bioreactor containing porous glass particles is presented. For Re < 1, intraparticle forced convection is negligible and only diffusion, characterized by an effective diffusion coefficient, must be considered to describe the mass transfer process between the extraparticle and the intraparticle fluid phase. For Re > 1, the mass transfer rate becomes dependent on the liquid flow rate, indicating the existence of intraparticle convection. A model including axially dispersed flow for the external fluid phase and an ‘apparent’ effective diffusivity that combines diffusion and convection, predicts experimental RTD data satisfactorily. Yeast cells immobilized inside the porous glass beads did not affect the mass transfer rate at low biomass loading. At high biomass loading (0·02 g yeast cells g^?1 carrier), the mass transfer rate between the extraparticle and intraparticle fluid phase was significantly decreased. Comparison of the RTD data from experimets performed in the presence and absence of cells in the external fluid phase revealed that the mass transfer rate is influenced by the cells immobilized inside the porous particles and not by the cells present in the external fluid phase.     

Liquid‐phase axial dispersion of turbulent gas–liquid co‐current flow through screen‐type static mixers

This article discusses the characteristics of turbulent gas–liquid flow through tubular reactors/contactors equipped with screen‐type static mixers from a macromixing perspective. The effect of changing the reactor configuration, and the operating conditions, were investigated by using four different screen geometries of varying mesh numbers. Residence time distribution experiments were conducted in the turbulent regime (4500 < Re < 29,000). Using a deconvolution technique, the RTD function was extracted to quantify the axial/longitudinal liquid‐phase dispersion coefficient. The findings highlight that axial dispersion increases with an increasing flow rate and/or gas‐phase volume fraction. However, regardless of the number and geometry of the mixing elements, reactor configuration, and/or operating conditions, the recorded liquid‐phase axial dispersion coefficients in the presence of screens was lower than that for an empty pipe. Furthermore, the geometry of the screen was found to directly affect the axial dispersion coefficient in the reactor. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1390–1403, 2017     

Numerical study of particle dispersion in the wake of gas-particle flows past a circular cylinder using discrete vortex method

A numerical investigation on the particle dispersion in the wake of particle-laden gas flows past a circular cylinder at Reynolds number of 10⁵ is presented. In the numerical method, the Discrete Vortex Method with the diffusion velocity model is employed to calculate the unsteady gas flow fields and a Lagrangian approach is applied to track individual particles. A dispersion function is defined to represent the dispersion scale of the particle. The distributions of gas velocities and vortex blobs, the trajectories and dispersion functions as well as distributions for particles with various Stokes numbers ranging from 0.01 to 1000 are obtained. The numerical results show that: (1) very small sized particles with St = 0.01 can distribute both in the vortex core and around the vortex periphery, whereas intermediate sized particles with St = 1.0, 10 are distributed around the vortex periphery, and very large sized particles with St = 1000 do not feel the gas flow; (2) only at small Stokes number (St = 0.01, 0.1) the particles do not impact with the cylinder; (3) the particle's dispersion intensity decreases precipitously as St is increased from 0.01 to 10.     

Mixing in bubble columns: a new approach for characterizing dispersion coefficients

Use of bubble columns as photobioreactors requires a quantitative knowledge of radial mixing in these columns. A complete model of liquid-phase dispersion was used to simultaneously characterize axial and radial mixing in a relatively large (0.06 m³, 2.3 m tall, 0.193 m in diameter) bubble column photobioreactor. Axial and radial dispersion coefficients and mixing times were determined in tap water and sea water for superficial aeration velocities of up to . The measured axial dispersion coefficients (D_z) were generally consistent with the predictions of the well established correlations, thus validating the complete dispersion model used in the analysis. The D_z values ranged from ∼150 to and were highly reproducible. There was evidence that the existing literature data on D_z in bubble columns are slightly underestimated, as consistent underestimation was found to be a characteristic of the widely used dispersion model that disregards radial dispersion. The value of the radial dispersion coefficient was typically about 1% of the D_z value under any given condition. Except at incipient aeration, the radial dispersion coefficient was not as sensitive to the magnitude of the aeration rate as was the axial dispersion coefficient. The mixing time data were generally consistent with the existing correlations.     

Green process technology for peroxycarboxylic acids: Estimation of kinetic and dispersion parameters aided by RTD measurements: Green synthesis of peroxycarboxylic acids

Synthesis of peroxypropionic and peroxyacetic acids from hydrogen peroxide and their carboxylic acids was carried out in a continuous fixed-bed reactor, demonstrating the concept of green process technology: biodegradable chemicals are made with the aid of heterogeneous catalysts. This reactor was constructed with several sampling locations along its length, allowing a better kinetic investigation. A profound study of the mean residence time distribution (RTD) was performed to analyze the behavior of the flow pattern at different liquid flow rates and catalyst loadings by using step and impulse methods with different tracer molecules. It was found that impulse experiments with HCl as a tracer is the most reliable method to study the RTD, because of this tracer does not interact with the catalyst. Based on the RTD study, it was demonstrated that the temperature (in the range 30–60 °C) and the catalyst distribution inside the reactor do not affect the flow pattern, but the ratio of the catalyst mass-to-liquid ratio affects the mean residence time and the liquid dispersion. Furthermore, it was shown that the presence of an axial dispersion is negligible when the average particle diameter is <1.38 mm, e.g., Péclet number exceeds 100. The kinetic study was carried out at different mean residence times (from 10 to 55 min), at different wet catalyst loadings (from 25 to 49 g) and reactor temperatures (from 30 to 60 °C). Mathematical models comprising kinetic and dispersion parameters were developed to compare the estimated parameters obtained from the continuous fixed-bed reactor with the ones obtained from a batch reactor.