Net Flow Reynolds Number Research Articles

Optimization for Kinetic Model of Oxidative Desulfurization of Sour Naphtha Over a Natural Base Zeolite Catalyst in a Three Phase Oscillatory Baffled Reactor

The oxidative desulfurization (ODS) of Iraqi sour naphtha was studied in a novel design of a heterogeneous catalytic reactor. Molecular oxygen was used as an oxidizing agent and a clay-based zeolite was used as a catalyst. The present work of ODS was conducted in a three-phase oscillatory baffled reactor under different operating conditions; temperature =25, 35, 45, and 55°C, residence times (1 – 14 min), Reynold number of oscillation (175, 235, 315), and net flow Reynolds number (25, 50, 75). A zeolitic base ODS catalyst was prepared and applied as a heterogeneous catalyst in the OBR unit. The aim of this study is to obtain the ODS kinetic parameters and concentration profile of the sulfur compounds in the naphtha cut. The model developed was based on the properties of the feedstock, characteristics of the catalyst, and operation conditions inside the OBR according to the experimental observations. To obtain the kinetic model parameters mass transfer and flow conditions were applied in the model network. The equations were employed in gPROMS software to simulate the experimental results of sulfur concentration remaining after completion of the ODS process. The set of equations was successful in simulating the experimental results with a 5% absolute error. The predicted data were used for optimizing the kinetic parameters to get the ODS kinetic parameters based on minimizing sulfur concentration.

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.

Micromixing in oscillatory baffled flows

The mesoscale oscillatory baffled reactor (meso-OBR) is a novel screening platform that does not require significant optimisation of the operating conditions, making it ubiquitous. Although it is known that micromixing performance influences the observed reaction kinetics, the micromixing of meso-OBRs has not previously been characterised. Therefore, in this study we have measured the micromixing times in three meso-OBR configurations across a broad range of oscillatory (Reo = 50–1000) and net flow Reynolds numbers (Ren = 5–40) for the first time using the Villermaux-Dushman competing reaction scheme. Helical baffles exhibited the lowest and most consistent micromixing times, followed by the central axial baffle design and the integral baffle design. Using the micromixing times, design equations were developed that showed the ratio of the micromixing time to mean residence time was dependent only on the velocity ratio (ratio of oscillatory to net flow Reynolds numbers). In all baffle designs, micromixing times decreased with increasing velocity ratios (ψ > 25). Therefore, for the central baffle and integral baffle designs, the lowest micromixing times would not be accessible for flow chemistry applications, because the optimal velocity ratios for plug flow in these designs are only around 4–10. Whereas, the helical baffle can achieve plug flow across a much broader range of velocity ratios (up to ψ = 250), because of the additional swirling component to the flow, enabling the lowest micromixing times of the helical baffle to be exploitable for reaction engineering applications. The lowest micromixing time produced was 0.03 s, using the helical baffle with Ren = 10 and Reo = 750.

Oscillatory fluid motion unlocks plug flow operation in helical tube reactors at lower Reynolds numbers (Re ≤ 10)

Flow in helical tubes exhibits plug flow characteristics above a critical flow rate due to the formation of Dean vortices. In this study, we report the novel use of oscillatory flow inside coiled tubes to achieve high degrees of plug flow at lower flow rates. The plug flow enhancement appeared to ‘switch-on’ then ‘switch-off’ as the oscillation intensity was increased, corresponding to ‘oscillatory Dean numbers’ of Deo = 70–200, where secondary instabilities are expected. It is believed that oscillatory motion leads to the periodic formation and unravelling of Dean vortices. For a 5 mm tube diameter, the effects of radius of curvature (12.5–32.5 mm), tube pitch (7.5–12.5 mm), net flow rate (Ren = 10–50), oscillation frequency (2–8 Hz) and oscillation amplitude (1–8 mm) were studied. The prototype helical coil reactors were rapidly manufactured using 3D printing. The optimal conditions for plug flow corresponded to Deo/Ren = 2–8, at Strouhal numbers in the range St = 1–2. Plug flow was observed at net flow Reynolds numbers at least as low as Ren = 10, compared to Ren = 70–300 in the literature for coiled tubes subjected to steady flows.

The heat transfer characteristics of a mesoscale continuous oscillatory flow crystalliser with smooth periodic constrictions

The heat transfer performance of a 5 mm internal diameter (I.D.) mesoscale continuous oscillatory flow crystalliser with smooth periodic constrictions (herein called SPC meso-tube) is herein reported for the first time for both steady flow and unsteady oscillatory flow conditions. Experimental values of the tube-side Nusselt number, Nut, accompanied by an estimability analysis, emphasized the key role played by smooth constrictions and bulk flow velocity in controlling tube-side heat transfer, while revealing a weaker influence of oscillatory flow on heat transfer enhancement in the SPC meso-tube. Although the presence of smooth constrictions provided an increased surface area to volume (SAV) ratio, and recirculation zones which promoted heat transfer rates, a maximum 1.7-fold heat transfer augmentation was obtained when fluid oscillations were combined with smooth constrictions. The behaviour of the SPC meso-tube was such that increasing the net flow Reynolds number, Ren, from 11 up to 54 with the combination of smooth constrictions and oscillatory flow resulted in the attainment of higher rates of heat transfer up to a maximum Nut of 3.09. The Strouhal number, St, was also found to have a more significant effect on the heat transfer performance than oscillatory frequency, f. An empirical correlation was for the first time developed to describe the heat transfer characteristics of the SPC meso-tube, and predict Nut based on experimental data for the range of net flow and oscillatory flow conditions investigated. A parameter estimability approach was also implemented to enhance the prediction capability of the correlation. The approach was based on a sequential orthogonalisation, thanks to which the most influential factors affecting the tube-side heat transfer were identified given the available experimental data. Overall, the results accentuate the efficient heat transfer capabilities of the SPC meso-tube in the laminar flow regime, and its suitability for performing cooling crystallisations where tight temperature control of supersaturation is essential.

Effect of oscillation amplitude on the residence time distribution for the mesoscale oscillatory baffled reactor

<p>A recent development in oscillatory baffled reactor technology is down-scaling the reactor, so that it can be used for the applications such as small-scale continuous production of bioethanol. A mesoscale oscillatory baffled reactor (MOBR) with central baffle system was developed and fabricated at mesoscales (typically 5 mm diameter). This present work aims to analyse the mixing conditions inside the MOBR by evaluating the residence time distribution (RTD) against the dynamic parameters of net flow Reynolds number (<em>Re</em><em><sub>n</sub></em>) at 4.2, 8.4 and 12.6 corresponding to flow rates of 1.0, 2.0 and 3.0 ml/min respectively, oscillatory Reynolds number (<em>Re</em><em><sub>o</sub></em>) between 62 to 622, and Strouhal number (<em>Str</em>) between 0.1 to 1.59. The effect of oscillation frequency and amplitude on RTD performance were studied at frequency, amplitude, and velocity ratio ranging from 4 to 8 Hz, 1 to 4 mm and 1 to 118, respectively. Effect of oscillation frequency has resulted in the variance of the RTD increased as the oscillation frequency increased from 5 Hz to 8 Hz and peak at 6 Hz of 0.264. A further increase in the frequency above 5 Hz caused the RTD to slightly broaden and positively skewed. At frequency of 5 Hz, the RTD profiles were close to Gaussian form for all tested amplitude values from 1 mm to 4 mm. At low amplitudes, i.e. xo = 1 mm, the variance exhibited its minimum around 0.842 at <em>Re</em><em><sub>o</sub></em><em> </em>=156. An increase in <em>Re</em><em><sub>o</sub></em><em> </em>above 300 resulted in increased in the variance rapidly to 1.28, and later eliminated the plug flow behaviour and the reactor behaved similar to a single continuous stirred tank reactor.</p><p>Chemical Engineering Research Bulletin 19(2017) 111-117</p>

Scale‐Up of Oscillatory Helical Baffled Reactors Based on Residence Time Distribution

AbstractFor the first time, the scale‐up of oscillatory helical baffled reactors (OHBR) was studied. Residence time distributions were used to assess the scale‐up over a wide range of net flow Reynolds numbers and oscillation conditions. A simple scale‐up correlation based on oscillation and net flow conditions was established to predict the plug flow behavior at various reactor scales. It is suggested that operating conditions can be easily extrapolated from the laboratory scale to the pilot scale, and even to industrial scales, as the mixing behavior is similar.

Enhancement of inline mixing with lateral synthetic jet pairs at low Reynolds numbers: The effect of fluid viscosity

In this paper, mixing between two liquid streams of the same flow rate in a planar mixing channel enhanced by means of three lateral synthetic jet pairs is examined using PLIF and PIV at net flow Reynolds numbers of Ren=2, 10 and 83. The changes in the flow Reynolds numbers are produced with the use of fluids with different dynamic viscosities. The synthetic jet pairs are operated 180° out-of-phase and at a range of actuation frequencies (characterized by the dimensionless Strouhal number Str) and displacements (characterized by the dimensionless stroke length L). It is found that at a sufficiently high frequency or dimensionless stroke length, a homogenous mixing can be achieved. Our experimental evidence shows that the synthetic jet pairs enhance mixing via two key mechanisms, i.e. vortex interaction and entrainment; tearing and stretching of liquid interface. A functional relationship among Ren, Str and L to ensure a nearly homogenous mixing is also obtained by best fitting the experimental data. It can be used for selecting the synthetic jet operating conditions to ensure a good mixing for a scaled version of this fluid mixer. This correlation indicates that the effectiveness of mixing has a weak dependence on Ren, implying that the fluid mixers of such a design can be effective over a wide range of net flow Reynolds numbers and for fluids of different viscosities.

Enhancement of laminar flow mixing using a pair of staggered lateral synthetic jets

In the work reported in this paper, laminar flow mixing between two parallel water streams, which are injected into a rectangular channel at a net flow Reynolds number of 83, is enhanced using a pair of staggered lateral synthetic jets (LSJ) located on the opposite walls of the mixing channel. The synthetic jet pair is operated 180° out-of-phase at a range of actuation frequencies, f, and dimensionless stroke length, L. Our results show that an excellent mixing is obtained when a relatively high value of f or L is used. The dominant mixing enhancement mechanisms are identified, which are found to change as the LSJ actuation parameters vary. Quantitative comparison with the opposing LSJ pair configuration reveals that the staggered configuration slightly outperforms the opposing configuration at moderate levels of f or L. As f or L increase further, however, a similar mixing performance is obtained from both configurations.

Liquids mixing enhanced by multiple synthetic jet pairs at low Reynolds numbers

In this paper, the effect of three synthetic jet pairs on the mixing between two fluid streams in a planar mixing channel is examined at a net flow Reynolds number of 2. They are mounted transversely to the incoming flow and operated 180° out-of-phase. The PLIF technique is used to provide a visual impression of the effect of synthetic jets on mixing as their operating condition varies, whereas PIV is used to provide the complementary information about the flow structures produced by the synthetic jets and their role in promoting mixing. Our experimental results demonstrate that this inline fluid mixer with a simple design produces an excellent mixing at low Reynolds numbers. It is found that such a good mixing is the result of a significantly increased interfacial area created by these synthetic jet pairs. Based on the degree of mixing deduced from the PLIF data at some distances downstream of the synthetic jets, a functional relationship between the synthetic jet actuation frequency and amplitude is also obtained. This functional relationship can be used for selecting the synthetic jet operating conditions to ensure a good mixing for a scaled version of this fluid mixer.

A PLIF and PIV study of liquid mixing enhanced by a lateral synthetic jet pair

In this paper, enhancement of mixing between two water streams of the same flow rate in a planar channel by means of a lateral synthetic jet pair is studied at a net flow Reynolds number of 83 using PLIF and PIV. The synthetic jet pair is operated 180° out-of-phase at a range of actuation frequencies and displacements, with the latter being characterized by the dimensionless stroke length. The extent of mixing is evaluated using PLIF data at a location further downstream in the mixing channel. It is found that at a fixed actuation frequency a higher dimensionless stroke length produces a better mixing, and as the actuation frequency increases a lower dimensionless stroke length is required to achieve a given mixing degree. At a sufficiently high frequency or dimensionless stroke length, a nearly homogenous mixing with a mixing degree greater than 0.9 can be obtained. A functional relationship between actuation frequency and dimensionless stroke length is also obtained by best fitting the experimental data, which can be used for selecting the synthetic jet operating conditions to ensure a good mixing. Furthermore, both PLIF and PIV results show that each synthetic jet actuation cycle produce two opposing vortex pairs, which play an important role in prompting mixing between the two fluid streams. The excellent mixing obtained at a high frequency or a high dimensionless stroke length is found to be largely caused by a strong interaction between these opposing vortex pairs.

Characterisation of mesoscale oscillatory helical baffled reactor—Experimental approach

A novel mesoscale helical baffled design of oscillatory baffled reactor (OBR) has been constructed and characterised in the net flow laminar regime net flow Reynolds numbers (Re n) (net flow Reynolds numbers Re n ≤ 10). A high degree of plug flow can be achieved in this design of OBR. In conventional OBR designs, plug flow is generated by the formation of well-mixed volumes in series due to toroidal vortex formation generated by the interaction of the baffle geometry with an oscillatory motion superimposed upon the net flow. The helical baffled design represents a significant development in oscillatory baffled reactors, as this system can provide plug flow behaviour over a wider range of oscillatory Reynolds number (Re o), (50–800). This is due to the combined effect of the swirling flow and vortex production. The oscillation amplitude was demonstrated to have a strong effect on the flow behaviour. The results revealed that the helical baffled design exhibited plug flow at high oscillation amplitudes ( x o ≥ 2 mm or Strouhal number (Str), Str ≤ 0.2).The highest number of equivalent tanks-in-series was obtained at Re o = 400–700.

Effect of geometrical parameters on fluid mixing in novel mesoscale oscillatory helical baffled designs

The effect of geometrical parameters, including helical pitch and wire diameter, on the fluid mixing inside a novel mesoscale helical baffled design of an oscillatory baffled reactor (OBR) was for the first time experimentally investigated at a net flow rate of 1.72 ml/min (net flow Reynolds numbers Re n of 7.2). The results show that the influence of helical wire diameter on the fluid mixing was negligible at Strouhal numbers above 0.2. However, the degree of plug flow increased 2-fold at oscillatory Reynolds numbers beyond 300 and Strouhal number below 0.2 when the wire diameter increased. The fluid mixing was found to be a function of the ratio of oscillation amplitude to helical pitch. The results showed that plug flow behaviour can be achieved when the ratio of oscillation amplitude to helical pitch was in a range of 0.2–0.6.

Development and evaluation of novel designs of continuous mesoscale oscillatory baffled reactors

Oscillatory baffled reactors (OBRs) are a form of plug flow reactor, ideal for performing long reactions in continuous mode, as the mixing is independent of net flow rate leading to more compact and practical designs. Mesoscale OBRs are currently being developed for laboratory-scale processes. The systems are designed to scale-up to industrial scale directly, or to be used as small-scale production platforms in their own right. Three different meso-reactor baffle designs (integral baffles, helical baffles and axial circular baffles (or “central”) baffles) were developed. These designs were chosen, as they are easily fabricated at “mesoscales” (here typically ∼5 mm diameter) and can be operated at low flow rates (μl/min to ml/min), whereas conventional designs of OBRs cannot. It was found that an increase in the net flow Reynolds number increased the optimum range of oscillatory Reynolds numbers over which plug flow can be achieved. The oscillation conditions had little effect on the residence time distribution behaviour at net flow Reynolds numbers above 25. These designs are effective and robust in scaled-down production of high added value products and decreasing the reagents required.

Mass transfer characteristics in an axisymmetric wavy-walled tube for pulsatile flow with backward flow

Under the pulsatile flow with backward flow (PFBF) conditions, flow mixing and mass transfer characteristics are experimentally investigated in an axisymmetric wavy-walled tube at a net flow Reynolds number from 50 to 1,000. An electrochemical technique is employed to measure the mass transfer rate. An optimal Strouhal number corresponding to the peak value of the mass transfer enhancement factor is observed, which is independent of the oscillatory fraction of the flow rate, but decreases with the increasing net flow Reynolds number. It was found that the mass transfer enhancement under PFBF has the similar characters of resonant enhancement in two-dimensional (2-D) channels, but there also exists an essential difference since no self-sustained oscillation occurs in the wavy-walled tube.

Mass‐transfer enhancement in a wavy‐walled tube by imposed fluid oscillation

AbstractThe present experimental study deals with mass‐transfer enhancement and fluid dynamic behavior in a wavy‐walled tube for pulsatile flow. Three flow parameters are considered here: the net flow Reynolds number, the oscillatory fraction of the flow rate, and the Strouhal number. Among them, we especially focused on the effect of Strouhal number on mass‐transfer enhancement under the condition of no reverse flow. Time‐averaged transport enhancement by means of imposed fluid oscillation is found to vary with the net flow Reynolds number and with the oscillatory fraction of the flow rate. The most effective mass‐transfer enhancement is registered when the net flow falls on just before the transitional flow regime for steady flow. Furthermore, there is an optimum Strouhal number, corresponding to the maximum transport enhancement, that depends on the oscillatory fraction of the flow rate. The results of time variation for flow behavior show that stable and unstable flow states exist during one oscillation cycle. The duration of the unstable flow state, corresponding to the optimum Strouhal number, is found to be the longest, and it leads to a remarkable fluid exchange between the mainstream and the recirculation zone, thus contributing to the mass transfer enhancement. These results indicate that the transport enhancement mechanism for the wavy‐walled tube is quite different from the resonant transport enhancement observed in the two‐dimensional (2‐D) wavy‐walled channel. © 2004 American Institute of Chemical Engineers AIChE J, 50: 762‐770, 2004

Mass Transfer Enhancement in a Sinusoidal Wavy-Walled Tube for Pulsatile Flow.

This study describes experimentally mass transfer enhancement in a wavy-walled tube for pulsatile flow. Three flow parameters are considered, i. e. net flow Reynolds number, oscillatory fraction and oscillatory frequency. The attention is paid especially to the effect of oscillatory frequency on mass transfer enhancement under conditions of no reverse flow. We show that the mass transfer enhancement factor varies with the net flow Reynolds number and oscillatory fraction. When the net flow is located just before the transitional flow regime for steady flow, the effective mass transfer enhancement is available. Furthermore we found that the resonant transport enhancement observed in a grooved channel, does not exist in the wavy-walled tube. A peak transport enhancement takes place in an optimum Strouhal number, which depends on the oscillatory fraction. The pulsatile flow state tends to be stable in the acceleration phase and unstable in the deceleration phase. The unstable flow state is found to lead to a strong fluid exchange between the mainstream and the recirculation vortex, and thus contributes to the mass transfer enhancement.

A Mixing-Based Design Methodology for Continuous Oscillatory Flow Reactors

Oscillatory flow reactors (OFR) have been actively developed over the last fifteen years at a number of UK universities, including the department of Chemical Engineering, University of Cambridge. OFRs exploit the uniform and efficient vortex mixing that can be achieved when an oscillatory fluid motion interacts with orifice plate baffles in a tube. One particularly advantageous application area that has been investigated is for performing ‘long’ (usually over 10 minutes) reactions in configurations which are substantially more compact than batch reactors, and which have substantially smaller length to diameter ratios than conventional tubular reactors. The development of OFRs is at a stage where laboratory and pilot scale feasibility is proven, but where no design methodology exists to implement the OFR on a production scale. In this paper, one such methodology is presented, based on research done over the last four years to develop specific applications for OFRs. The design method is based on a shell-and-tube configuration, whereby a series of standard tube diameters are chosen, and the tube-side steady flow (throughput) and oscillatory mixing conditions are chosen to ensure optimal mixing conditions. The scale-up principle is one of maintaining the same geometric ratios as laboratory scale devices, and ensuring similar values for the oscillatory and net flow Reynolds numbers. Following this protocol should ensure an optimal design, with a near plug flow RTD, efficient heat and mass transfer, effective dispersion (in multi-phase systems) and minimization of power dissipation. Once these criteria have been applied, the reactor is sized on the basis of required residence time, which then fixes the maximum throughput and length for each tube diameter. A choice is then made for the final configuration, based on the throughput requirement, the criteria mentioned above and practical considerations. This method is applied to two industrial case studies, and details are given of feasible designs for full-scale reactors using information from pilot studies already performed.

Droplet size distribution in a continuous oscillatory baffled reactor

We report our experimental investigation of droplet size distribution (DSD) in a continuous oscillatory baffled reactor (COBR). The droplet experiments were carried out over a range of oscillation amplitudes and frequencies and for three different net flow Reynolds numbers in the absence of surfactants and coalescence inhibitors. Our experimental results show that the oscillation amplitude and frequency play a more dominant role in controlling the mean droplet size and size distribution than the net flow. A correlation linking the mean droplet size, d 32, with energy dissipation due to fluid oscillation, ε, and energy dissipation due to net flow, ε n , has been established.