Oscillatory Baffled Reactors Research Articles

Green energy using oscillatory baffled reactors: advances in biodiesel production from high free fatty acid karanja oil

Abstract The global demand for sustainable and ecologically friendly energy sources has spurred biodiesel research. Civilization, urbanization, and population growth have drastically boosted transportation energy consumption. Energy demand is causing fossil fuel crises. Replace petroleum diesel to solve energy and environmental problems. Biofuels have very vast scope in the area of energy sector. Karanja oil, derived from the seeds of the Karanja tree (Pongamia pinnata), is a promising biodiesel feedstock due to its high Free Fatty Acid content. This study uses Oscillatory Baffled Reactor (OBR) to produce biodiesel from Karanja oil. The feedstock is high-FFA Karanja oil trans-esterified in two steps. In OBR, reaction time is only 15 min, whereas in conventional reactor it takes 1–2 h to produce biodiesel. Optimizing biodiesel synthesis involved varying H2SO4 concentrations from 1 vol% to 5 vol% and 0.3 to1.0 wt% of NaOH concentration for first and second step respectively. Oil-to-alcohol molar ratios 1:6, 1:9 and 1:12. Optimization occurs at 1:12 oil-to-alcohol ratio, 1 vol% H2SO4 catalyst, and 0.6 wt% NaOH catalyst. Studies have also examined how catalyst concentration and oil: alcohol molar ratio affect biodiesel conversion and physical qualities such kinetic viscosity, specific gravity, and acid value. This study shows that OBR can increase biodiesel production from high-FFA Karanja oil, which further opens up opportunities for additional study and improvement.

Modeling of suspension polymerizations in continuous oscillatory baffled reactors (COBR) - Part I: Vinyl acetate polymerization

In this work, a phenomenological model to represent suspension polymerizations in a continuous oscillatory baffled reactor is presented for the first time. The reactor is described as a series of tanks of equal volume connected. Additionally, a backflow stream is also included to simulate a backmixing mechanism. The proposed model considers mass, energy, statistical moments balances and regulatory control loops. Furthermore, the heat transfer coefficient is modeled as a function of the oscillatory conditions and the properties of the suspension. The model predictions are compared with available experimental data and good agreement is observed. The effects of key model parameters on model responses are also investigated. It is shown that initiator type and injection points must be carefully planned to avoid excessive temperature gradients along the reactor length. Given the thorough description of kinetic and heat transfer phenomena, the model can be used for reactor design and process scale up.

Non-dimensional analysis of experimental pressure drop and energy dissipation measurements in Oscillatory Baffled Reactors

An experimental study is performed to characterize the pressure drop and the power consumption in Oscillatory Baffled Reactors, using dimensionless numbers: the oscillatory Fanning friction factor (fosc) and the Power number (Po), respectively. Two baffle geometries (one-orifice and three orifices) are tested, for different fluids and oscillating amplitudes. The range of oscillatory Reynolds numbers (Reosc) tested is 10–1000. Data reduction based on the statistical fitting and the FFT of the pressure drop and velocity signals is introduced to assess the maximum pressure drop and the phase lag between both signals. The new set of experimental data proves the limitations of the conventional models available in the open literature. fosc and Po provide consistent dimensionless results for the different working fluids tested and their trends are clearly related to different flow behaviours: laminar or chaotic flow. Correlations for fosc and Po as a function of Reosc and dimensionless amplitude are obtained.

Continuous process for the epoxidation of terpenes using mesoscale oscillatory baffled reactors

Continuous epoxidation of terpenes with H2O2 was carried out using mesoscale oscillatory baffled reactors (meso-OBRs) in a solvent-free environment. The performance of the new 3D-printed single, tri- and multi-orifice baffles was compared with helical and integral baffles. The performance investigated includes mixing intensity, induction period, steady-state attainment, and heat removal capability of the meso-OBR. Moreover, passive isothermalisation was also investigated using meso-OBR in a heat pipe assembly. The tri- and multi-orifice baffles were able to overcome mixing limitations and achieved a comparable rate of reaction to batch at mixing conditions of Reo > 850 and Reo > 500, respectively. Both baffles exhibited rapid steady-state attainment, shorter induction period (t = 1.5τ) and better reproducibility with product variation of ∼1.3%. The meso-OBR designs demonstrated effective heat transfer capability, allowing the reaction to being operated isothermally with ±1 °C temperature variation in solvent-free conditions. This removes the need for a solvent, thus reducing reaction volume by a 5-fold. The timescale for the reaction was reduced from ∼8 h in a conventional process to 30 min in the multi-orifice meso-OBR, a 16-fold reduction. A better process has been developed for a continuous epoxidation of terpenes with H2O2 using multi-orifice meso-OBRs, with a potential intensification factor of ∼80.

Electrochemical Oscillatory Baffled Reactors Fabricated with Additive Manufacturing for Efficient Continuous-Flow Oxidations.

Electrochemical continuous-flow reactors offer a great opportunity for enhanced and sustainable chemical syntheses. Here, we present a novel application of electrochemical continuous-flow oscillatory baffled reactors (ECOBRs) that combines advanced mixing features with electrochemical transformations to enable efficient electrochemical oxidations under continuous flow at a millimeter distance between electrodes. Different additive manufacturing techniques have been employed to rapidly fabricate reactors. The electrochemical oxidation of NADH, a very sensitive substrate key for the regeneration of enzymes in biocatalytic transformations, has been employed as a benchmark reaction. The oscillatory conditions improved bulk mixing, facilitating the contact of reagents to electrodes. Under oscillatory conditions, the ECOBR demonstrated improved performance in the electrochemical oxidation of NADH, which is attributed to improved mass transfer associated with the oscillatory regime.

Design, performance characterization and applications of continuous oscillatory baffled reactors

The continuous oscillatory baffled reactor (OBR) is a particular type of tubular reactor, which has drawn increasing attention over the past few decades due to the benefits it provides in terms of intensification of heat and mass transfer, as well as equipment compactness compared with stirred tank reactors. This process enhancement is principally due to the interaction of the oscillatory flow with internal baffles and the consequent generation of transverse flows and eddies. Continuous OBRs are already applied in several industrial sectors, however these reactors present certain limitations in terms of operating conditions and the range of possible applications. This review presents and discusses the current knowledge on continuous OBR design guidelines, performance characterization and applications. It aims to guide the selection of the most appropriate continuous OBR design, as well as the characterization criteria, according to the type of application and final process objective.

The effect of particle size on flow in a continuous oscillatory baffled reactor using CFD

AbstractExperimental and numerical characterizations of flow in continuous oscillatory baffled reactors (COBR) have mainly been based on single phase in past decades. By coupling a primary Eulerian liquid phase with a secondary discrete Lagrangian phase consisting of solid particles of given density and size, this work investigates the effects of particle size on axial dispersion, evaluates residence times and velocities of particles, and quantifies the oscillation dampening caused by the presence of solid particles in a COBR.

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.

Analytical calculation of the flow superposition effect on the power consumption in oscillatory baffled reactors

The aim of this communication is to clarify some aspects related to the power consumption in continuous Oscillatory Baffled Reactors (OBRs). The first aspect studied is the effect of the flow superimposition on the power consumptions associated to the net flow and the oscillatory flow. The quasi-steady model is used to obtain an expression for the oscillatory flow effect on the net flow power consumption, and vice versa. The expression obtained for the oscillatory flow effect is in good agreement with the one available in the literature, whose deduction is still unknown. The second aspect is the effect of the energy recovery in the power consumption. An analytical expression, function of the pressure drop-velocity phase lag, is derived, showing that the power consumption difference between a system with and without energy recovery can be significant.

Mixing performance in continuous oscillatory baffled reactors

In the current literature, there is limited information on the influence of operating parameters on spatial mixing quality and how a secondary feed should be introduced into continuous oscillatory baffled reactors (COBR) to achieve good mixing quality. This work explores for the first time the impact of the position of a secondary feed (passive non-reactive tracer) on spatial mixing performance in a COBR using transient laminar CFD simulations. Three theoretical feed positions are studied covering a range of net flow and oscillatory Reynolds numbers (Renet=6-27/Reo=24-96), the range being chosen to ensure that the flow field remains two-dimensional in all cases. Macromixing is evaluated by analysing the spatial uniformity of the tracer with the areal distribution method developed by Alberini et al. (2014a). Introduction of the secondary stream at the reactor wall or upstream of the edge of the first baffle greatly improves mixing quality due to the recirculation eddies, which assist radial mixing. However, introduction of the secondary feed at the centreline results in high axial dispersion with limited radial mixing. With an adequate introduction position, mixing quality typically increases with an increment in the velocity ratio. Nevertheless, if the net flow is too low, mixing performance decreases because the secondary stream is pushed upstream of the baffles, where it does not benefit from flow recirculation.

Predicting power consumption in continuous oscillatory baffled reactors

Continuous oscillatory baffled reactors (COBRs) have been proven to intensify processes, use less energy and produce fewer wastes compared with stirred tanks. Prediction of power consumption in these devices has been based on simplistic models developed for pulsed columns with single orifice baffles several decades ago and are limited to certain flow conditions. This work explores the validity of existing models to estimate power consumption in a COBR using CFD simulation to analyse power density as a function of operating conditions (covering a range of net flow and oscillatory Reynolds numbers: Renet=6-27/Reo=24-96) in a COBR with a single orifice baffle geometry. Comparison of computed power dissipation with that predicted by the empirical quasi-steady flow models shows that this model is not able to predict correctly the values when the flow is not fully turbulent, which is common when operating COBRs. It has been demonstrated that dimensionless power density is inversely proportional to the total flow Reynolds number in laminar flow and constant in turbulent flow, as is the case for flow in pipes and stirred tanks. For the geometry studied here P/V∗=330/ReT in laminar flow and P/V∗=1.92 in turbulent flow.

Computational fluid dynamic enabled design optimisation of miniaturised continuous oscillatory baffled reactors in chemical processing

The first CFD-enabled multi-objective design optimisation methodology for continuous oscillatory baffled reactors (COBRs), used for flow chemistry-based process development, is described, where performance is quantified in terms of two metrics: a mixing efficiency index and the variance of the residence time distribution. The effect of cross-validation approaches on the surrogate modelling of these performance metrics is examined in detail and the resultant surrogate models used to demonstrate the influence of key design variables. Pareto fronts of non-dominated solutions are presented to illustrate the available design compromises for COBR performance and it is shown that these can give a narrow Residence Time Distribution and good mixing within the final design. The novel feature of offset baffles within a channel, explored here for the first time, is identified as a key parameter in improving the performance of COBRs.

Smoothed particle hydrodynamics – A new approach for modeling flow in oscillatory baffled reactors

Existing numerical models of mixing processes in oscillatory baffled reactors (OBR) are mainly Eulerian-based. An alternative Lagrangian based methodology, Smoothed-Particle Hydrodynamics (SPH), for predicting flow patterns and assessing mixing performance is presented in this paper. A bespoke SPH solver is developed for single phase modeling, as it is, for the first time, applied to OBR, and the results are compared with those from Eulerian modeling, i.e. Finite Volume (FV) method. SPH has successfully captured the expected flow characteristics in OBR as clearly and equally as its Eulerian counterpart, validating the SPH method. Since SPH provides historical information of individually tracked fluid packets/particles in the domain of interest, it allows for readily quantitative assessments of mixing without additional models. Two new indexes to assess mixing and plug flow efficiency have been proposed by making full use of SPH's capabilities.

On the evaluation of power density models for oscillatory baffled reactors using CFD

While continuous oscillatory baffled reactors (COBR) have been proven a viable alternative to traditional batch reactors for organic synthesis and crystallization, research into the estimation of power density for this type of device has largely been stagnated for the past 25 years. This work reports, for the first time, detailed analysis and examination of the applicability, capability and deficiencies of two existing models using CFD methodology. The “quasi-steady” model (QSM) over-estimates power dissipation rates due to the inaccurate formulation of two of its geometric parameters for modern COBRs. By using a revised power law dependency on the number-of-baffles term (nx) and an appropriate orifice discharge coefficient (CD), we demonstrate that the updated QSM can not only be used for a much wider application range than previously outlined, but also for both batch and continuous operations. The “eddy enhancement” model (EEM) generally provides better predictions of power density for the conditions tested; however, its accuracy can substantially be enhanced by applying the aforementioned power law dependency on n and an empirical correlation proposed in this work to estimate EEM’s “mixing length”. After full validation, both models give very similar power density estimations and can be used interchangeably with high confidence.

Combining continuous flow oscillatory baffled reactors and microwave heating: Process intensification and accelerated synthesis of metal-organic frameworks

We have constructed a continuous flow oscillatory baffled reactor (CF-OBR) equipped with a homogeneous and controllable microwave applicator in an entirely novel design. This affords a new route to chemical production incorporating many of the principles of process intensification and allows, for the first time, investigation of the synergistic benefits of microwave heating and CF-OBRs such as; faster and continuous processing; improved product properties and purity; improved control over the processing parameters; and reduced energy consumption. The process is demonstrated by the production of a metal-organic framework (MOF), HKUST-1, a highly porous crystalline material with potential applications in gas storage and separation, catalysis, and sensing. Our reactor enabled the production of HKUST-1 at the 97.42 g/h scale, with a space time yield (STY) of 6.32 × 105 kg/m3/day and surface area production rate (SAPR) of 1.12 × 1012 m2/m3/day. This represents the highest reported STY and fastest reported synthesis (2.2 s) for any MOF produced via any method to-date and is an improvement on the current SAPR for HKUST-1 by two orders of magnitude owing to the superior porosity exhibited by HKUST-1 produced using our rig (Langmuir surface area of 1772 compared to 600 m2/g).

Thermal performance of meso-scale oscillatory baffled reactors

This study reports the evaluation of heat transfer characteristics and pressure drops in three designs of meso-scale oscillatory baffled reactors over the net flow range Ren = 60–2500 and the oscillatory flow range Reo = 0-1600. The three designs were the oscillatory central baffled reactor (OCBR), the oscillatory helical baffled reactor (OHBR), and the oscillatory orifice baffled reactor (OOBR). The designs were evaluated in terms of “thermal performance” to take the heat transfer and pressure drop into account.Interestingly, in all designs the pressure drop was observed to decrease with increasing oscillation, due to some degree of energy recovery. In all cases, oscillating the flow significantly enhanced the Nusselt number by upto a maximum of around 10-fold. The meso-OBRs were shown to achieve higher thermal performances than steady flows in smooth tubes in all cases. Up to 480%, 460%, and 300% improvements in thermal performance was achieved for the OHBR, OCBR, and OOBR, respectively. The OHBR exhibited the lowest pressure drop, resulting in the highest thermal performance of the designs investigated here. Hence, on the basis of this study, the OHBR design is recommended. New empirical correlations for predicting pressure drop and Nusselt number were established for each design, with data mostly lying in the range ±30% of the correlations.

Continuous reactive coupling of glycerol and acetone – A strategy for triglyceride transesterification and in-situ valorisation of glycerol by-product

Methyl esters of fatty acids are widely used as biodiesel, a sustainable replacement for petro-diesel. The conventional biodiesel process produces crude glycerol, which constitutes about 10 wt% of the total products. This has led to a surplus of crude glycerol due to global increase in biodiesel use, necessitating increased research into sustainable processes that could convert the crude glycerol into higher value-added products. This study investigates biodiesel processes for continuous transesterification of triglycerides to methyl esters, coupled to conversion of the glycerol by-product into solketal, a value-added product, via reaction with acetone in situ. The study was carried out using one-stage and two-stage catalytic transesterification of triacetin and methanol in mesoscale oscillatory baffled reactors (meso-OBRs). The two-stage process involved two meso-OBRs in series packed with AmberlystTM resin catalysts: a basic AmberlystTM A26-OH in the first stage to catalyse transesterfication of triacetin with methanol, and an acidic AmberlystTM 70-SO3H in the second stage to catalyse the coupling of glycerol and acetone to form solketal. One-stage triacetin transesterification and glycerol coupling with acetone was carried out in a meso-OBR packed with the acidic AmberlystTM 70-SO3H resin. In the two-stage process, the triacetin was converted to 99.1 ± 2.0% methyl acetate and 98.0 ± 1.3% glycerol after 25 min residence time in the first reactor and the glycerol was reacted with acetone in the second reactor to achieve 76.5 ± 2.8% solketal conversions after 35 min. The single-stage process achieved 48.5 ± 2.7% solketal conversion after 30 min. The meso-OBR was operated continuously to achieve high quality steady states and consistent triacetin conversions. The triglyceride transesterification with reactive coupling of glycerol with acetone produces less crude glycerol by-product. This process strategy could be optimised for future biodiesel production.

Development of a more robust correlation for predicting heat transfer performance in oscillatory baffled reactors

The oscillatory baffled reactor has been well-characterised in most areas of flow reactor performance (mixing, mass transfer, multi-phase operation etc.), with the exception of heat transfer, where comparatively few data exist in the literature. Here, a robust investigation of heat transfer in the “standard”, 26 mm diameter, oscillatory baffled reactor is presented which goes beyond the parametric limitations of previous studies.5-fold Nusselt number increases over steady unbaffled flows are shown to be achievable. The maximum enhancement attributed to the oscillatory flow component alone compared to the steady-flow, baffled case is shown to be 1.7-fold. The degree of heat transfer enhancement is shown to plateau when the oscillatory flow Reynolds number exceeds 1300, indicating that a radial mixing limit has been reached.A new correlation for predicting heat transfer coefficients in oscillatory baffled reactors has been developed. Based on the data generated here, it is accurate to ±30% across the experimental range of the study. The correlation has been further validated using literature data, and shown to be the most robust correlation to date for predicting heat transfer performance in oscillatory baffled reactors.

Advanced control of a continuous oscillatory flow crystalliser

This paper presents the application and challenges of achieving Model Predictive Control (MPC) on two continuous oscillatory baffled crystallisation reactors, delivering precise product quality control in the face of raw material fluctuations. A key advantage of MPC is that it effectively deals with multivariable interactions and constraints that appear within the continuous crystallisation process. Using a flexible real-time software package, a control scheme is proposed that incorporates three MPC blocks for controlling reactor cooling profile, API (Active Pharmaceutical Ingredient) concentration and crystal size distribution, respectively. Furthermore, the solution is customisable and transferable to different crystallisation reactors as well as various APIs.

A study of the flow structures generated by oscillating flows in a helical baffled tube

Oscillatory baffled reactors (OBRs) are able to generate plug flow at laminar net flow conditions, providing appropriate oscillation conditions are selected. Mesoscale OBRs containing helical baffles exhibit wider “operating windows” (i.e. a broader oscillation intensity range) for plug flow than other baffle designs. It has been hypothesised that additional swirling in the flow provides another mechanism to limit axial dispersion. These swirling flows have previously been qualitatively identified, but in this study these flows were investigated both numerically and experimentally using CFD and PIV for the first time. The flow structures obtained via simulation (laminar solver) were visualised using isosurfaces of the Q-criterion and 3D streamlines. The characteristic feature of the flow is a helically shaped vortex that forms behind the baffle. Streamlines move both radially (wrapping around the vortex structure) and tangentially. Using the swirl number and analogous ‘radial’ number, a transition between vortex-dominated and swirl-dominated mixing was observed providing evidence that the hypothesis is valid. It was found that when the oscillation intensity is increased, the tangential motion of the flow increases faster than the increase in radial flow because the vortex sizes are bounded by the column diameter.