Plug Flow Conditions Research Articles

A Note on Baffle Orientation in Long Ponds

This paper assesses the general idea of implementing transversal baffles to improve hydraulic efficiency of rectangular ponds used in water and wastewater treatment processes. The paper concentrates on long ponds with the length-to-width ratio of 4:1. Three different scenarios of plain (baffle-less) pond and ponds with four transversal and two longitudinal baffles were considered. A two dimensional Computational Fluid Dynamics (CFD) conservative tracer study was performed. Results showed that the longitudinal layout of the baffles yielded to a hydraulic regime closer to the plug-flow condition, while application of limited number of transverse baffles in long ponds showed an adverse effect on the overall hydraulic performance. Such baffles layout yielded to increased dispersion, as well as comparable dead zones in comparison with the plain pond. A three dimensional analysis was also conducted to evaluate the effect of volumetric dispersion, which yielded to the same results. Possible reasons for this phenomenon were discussed. Results suggest that application of limited number of transverse baffles in long ponds needs extra consideration.

Separating the initial growth rate from the rate of deactivation in the growth kinetics of multi-walled carbon nanotubes from ethene over a cobalt-based bulk catalyst in a fixed-bed reactor

The initial growth kinetics of multi-walled carbon nanotubes (CNTs) was investigated using a highly active Co-based mixed-oxide catalyst in a tubular fixed-bed reactor under plug-flow conditions with ethene as carbon source. The growth temperature and the ethene concentration were systematically varied in the range from 758 to 923K and from 5 to 45vol.%, respectively. The carbon mass accumulation was derived from the ethene conversion and analyzed by a kinetic model, from which the initial CNT growth rate and the mean lifetime of the active sites were derived permitting the prediction of the maximum theoretical CNT yield. With increasing growth temperatures up to 923K both the initial growth rate and the mean lifetime of active sites increased strongly with a significantly prolonged lifetime above 848K. The initial growth rate was slow at lower ethene concentrations, but the mean life time was very high. Increasing the ethene concentration up to 45vol.% led to a much higher initial growth rate, but shortened the mean lifetime strongly. Due to the fast deactivation at high ethene concentrations, the predicted maximum yield decreased considerably approaching the yield obtained after 5min of time on stream.

Biological processing in oscillatory baffled reactors: operation, advantages and potential.

The development of efficient and commercially viable bioprocesses is essential for reducing the need for fossil-derived products. Increasingly, pharmaceuticals, fuel, health products and precursor compounds for plastics are being synthesized using bioprocessing routes as opposed to more traditional chemical technologies. Production vessels or reactors are required for synthesis of crude product before downstream processing for extraction and purification. Reactors are operated either in discrete batches or, preferably, continuously in order to reduce waste, cost and energy. This review describes the oscillatory baffled reactor (OBR), which, generally, has a niche application in performing 'long' processes in plug flow conditions, and so should be suitable for various bioprocesses. We report findings to suggest that OBRs could increase reaction rates for specific bioprocesses owing to low shear, good global mixing and enhanced mass transfer compared with conventional reactors. By maintaining geometrical and dynamic conditions, the technology has been proved to be easily scaled up and operated continuously, allowing laboratory-scale results to be easily transferred to industrial-sized processes. This is the first comprehensive review of bioprocessing using OBRs. The barriers facing industrial adoption of the technology are discussed alongside some suggested strategies to overcome these barriers. OBR technology could prove to be a major aid in the development of commercially viable and sustainable bioprocesses, essential for moving towards a greener future.

Residence time study of a two-phase mixture in a horizontal cylindrical vessel

A cold model of a rotary holding furnace was studied using water and a kerosene-LIX® 973N organic mixture which are immiscible. The flow of the feed was found to behave similar to a gravity current where the feed preferentially moved along the liquid–liquid interface. Visual observation and residence time distribution obtained showed that the flow of the lighter feed mixture was similar to a laminar flow but with a preferential route along the wall with the outlet spout. In the commercial-scale rotary holding furnace, plug flow conditions are considered desirable while mixing or short-circuiting is considered undesirable. The flow in the cold model was fitted to a plug flow and three CSTRs all in series and a particular depth of the upper layer of organic was found where mixing with the bath fluid was a maximum. Air bubbling in the centre of the cold model showed that at low air flow rates, the air curtain acted to limit mixing but as the air flow rates increased, the increased circulation caused by the air flow increased mixing and negated the air curtain effect.

Exploration of the gasification of Spirulina algae in supercritical water

This study presents non-catalytic gasification of Spirulina algae in supercritical water using a plug flow reactor and a mechanism for feeding solid carbon streams into high pressure (>25MPa) environments. A 2III3-1 factorial experimental design explored the effect of concentration, temperature, and residence time on gasification reactions. A positive displacement pump fed algae slurries into the reactor at a temperature range of 550–600°C, and residence times between 4 and 9s. The results indicate that algae gasify efficiently in supercritical water, highlighting the potential for a high throughput process. Additional experiments determined Arrhenius parameters of Spirulina algae. This study also presents a model of the gasification reaction using the estimated activation energy (108kJ/mol) and other Arrhenius parameters at plug flow conditions. The maximum rate of gasification under the conditions studied of 53g/Ls is much higher than previously reported.

Computational Fluid Dynamics of Developing Avian Outflow Tract Heart Valves

Hemodynamic forces play an important role in sculpting the embryonic heart and its valves. Alteration of blood flow patterns through the hearts of embryonic animal models lead to malformations that resemble some clinical congenital heart defects, but the precise mechanisms are poorly understood. Quantitative understanding of the local fluid forces acting in the heart has been elusive because of the extremely small and rapidly changing anatomy. In this study, we combine multiple imaging modalities with computational simulation to rigorously quantify the hemodynamic environment within the developing outflow tract (OFT) and its eventual aortic and pulmonary valves. In vivo Doppler ultrasound generated velocity profiles were applied to Micro-Computed Tomography generated 3D OFT lumen geometries from Hamburger-Hamilton (HH) stage 16-30 chick embryos. Computational fluid dynamics simulation initial conditions were iterated until local flow profiles converged with in vivo Doppler flow measurements. Results suggested that flow in the early tubular OFT (HH16 and HH23) was best approximated by Poiseuille flow, while later embryonic OFT septation (HH27, HH30) was mimicked by plug flow conditions. Peak wall shear stress (WSS) values increased from 18.16 dynes/cm(2) at HH16 to 671.24 dynes/cm(2) at HH30. Spatiotemporally averaged WSS values also showed a monotonic increase from 3.03 dynes/cm(2) at HH16 to 136.50 dynes/cm(2) at HH30. Simulated velocity streamlines in the early heart suggest a lack of mixing, which differed from classical ink injections. Changes in local flow patterns preceded and correlated with key morphogenetic events such as OFT septation and valve formation. This novel method to quantify local dynamic hemodynamics parameters affords insight into sculpting role of blood flow in the embryonic heart and provides a quantitative baseline dataset for future research.

Analysis of Equilibrium-limited Dehydrogenation and Steam Reforming in Palladium Membrane Reactors

Palladium membrane reactors have the potential to achieve higher conversion as well as larger recovery of purified hydrogen in the steam reforming reaction, even at lower temperatures. However, increasing feed rate resulted in larger deviation from the ideal analytical model assuming plug flow and isothermal conditions. This study developed a computational fluid dynamics (CFD) model, which takes into account the concentration, temperature and velocity distributions due to mass, heat transfer and flow resistances in the membrane reactor. The CFD model clearly showed that large temperature and concentration distributions were formed in both the radial and axial directions, so reducing the reactor performance compared to the ideal reactor. The correctness of the model was verified for the dehydrogenation of cyclohexane in a single tube type as well as a multi-tube type of palladium membrane reactor. Furthermore, the multi-tube model was applicable for simulation of changes in the reactor design such as the membrane dimension. World-leading potentials of palladium membrane reactors in Japan are evaluated by introducing major projects.

Residence Time Distribution Study of a Living/Controlled Radical Miniemulsion Polymerization System in a Continuous Tubular Reactor

AbstractResidence time distribution (RTD) studies were done to determine the flow characteristics in a continuous tubular reactor. Pulse tracer experiments were done at different flow rates and temperatures, and a comparison was made between a homogeneous aqueous salt mixture versus a heterogeneous miniemulsion mixture. The heterogeneous system was studied under two different conditions, one with a monomer‐in‐water droplet dispersion and one with fully formed polymer particles dispersed in water. There were differences observed between all of the systems tested and none of them matched an ideal plug flow condition. The reactor contains stagnant zones of varying volume and tracer spreading was observed in all cases. The dispersion model was found to model the system quite well in most cases.magnified image

Depositional processes, bedform development and hybrid bed formation in rapidly decelerated cohesive (mud–sand) sediment flows

AbstractFlows with high suspended sediment concentrations are common in many sedimentary environments, and their flow properties may show a transitional behaviour between fully turbulent and quasi‐laminar plug flows. The characteristics of these transitional flows are known to be a function of both clay concentration and type, as well as the applied fluid stress, but so far the interaction of these transitional flows with a loose sediment bed has received little attention. Information on this type of interaction is essential for the recognition and prediction of sedimentary structures formed by cohesive transitional flows in, for example, fluvial, estuarine and deep‐marine deposits. This paper investigates the behaviour of rapidly decelerated to steady flows that contain a mixture of sand, silt and clay, and explores the effect of different clay (kaolin) concentrations on the dynamics of flow over a mobile bed, and the bedforms and stratification produced. Experiments were conducted in a recirculating slurry flume capable of transporting high clay concentrations. Ultrasonic Doppler velocity profiling was used to measure the flow velocity within these concentrated suspension flows. The development of current ripples under decelerated flows of differing kaolin concentration was documented and evolution of their height, wavelength and migration rate quantified. This work confirms past work over smooth, fixed beds which showed that, as clay concentration rises, a distinct sequence of flow types is generated: turbulent flow, turbulence‐enhanced transitional flow, lower transitional plug flow, upper transitional plug flow and a quasi‐laminar plug flow. Each of these flow types produces an initial flat bed upon rapid flow deceleration, followed by reworking of these deposits through the development of current ripples during the subsequent steady flow in turbulent flow, turbulence‐enhanced transitional flow and lower transitional plug flow. The initial flat beds are structureless, but have diagnostic textural properties, caused by differential settling of sand, silt and cohesive mud, which forms characteristic bipartite beds that initially consist of sand overlain by silt or clay. As clay concentration in the formative flow increases, ripples first increase in mean height and wavelength under turbulence‐enhanced transitional flow and lower transitional plug‐flow regimes, which is attributed to the additional turbulence generated under these flows that subsequently causes greater lee side erosion. As clay concentration increases further from a lower transitional plug flow, ripples cease to exist under the upper transitional plug flow and quasi‐laminar plug flow conditions investigated herein. This disappearance of ripples appears due to both turbulence suppression at higher clay concentrations, as well as the increasing shear strength of the bed sediment that becomes more difficult to erode as clay concentration increases. The stratification within the ripples formed after rapid deceleration of the transitional flows reflects the availability of sediment from the bipartite bed. The exact nature of the ripple cross‐stratification in these flows is a direct function of the duration of the formative flow and the texture of the initial flat bed, and ripples do not form in cohesive flows with a Reynolds number smaller than ca 12 000. Examples are given of how the unique properties of the current ripples and plane beds, developing below decelerated transitional flows, could aid in the interpretation of depositional processes in modern and ancient sediments. This interpretation includes a new model for hybrid beds that explains their formation in terms of a combination of vertical grain‐size segregation and longitudinal flow transformation.

Turbulently flowing liquid–liquid dispersions. Part I: Drop breakage and coalescence

A successful attempt to simulate turbulently flowing liquid–liquid dispersions was undertaken in this work where the turbulent dispersion/coalescence of drops was accurately predicted over a wide range of operating conditions using the model developed by Coulaloglou and Tavlarides (1977) [36]. Experimental data obtained from an intensified liquid–liquid reactor/contactor in which screen-type static mixers were used to superimpose an adjustable uniformly distributed turbulence field on the nearly plug flow conditions encountered in high velocity pipe flows were used to validate the model predictions.Drop size distribution and the Sauter mean diameter (when quasi-steady state conditions were assumed to be reached) were compared with the experimental results measured by photographic techniques and good agreement was obtained at different flow velocities and diverse screen geometries.The use of mutli-stage screen-type static mixers where alternating breakage-dominated and coalescence dominated regions exist allowed the development of accurate model parameters that may be used for simulating other more complex liquid–liquid contacting conditions such as those encountered in mechanically agitated tanks.

CFD Analysis of a Single Palladium Membrane Tube Reactor for the Dehydrogenation of Cyclohexane as a Chemical Hydrogen Carrier

A palladium membrane reactor has been applied as a promising chemical hydrogen carrier to recover the hydrogen from cyclohexane. However, it is found that increasing feed rate resulted in a larger deviation from the ideal analytical model assuming plug flow and isothermal conditions. This study, therefore, presents a CFD (computational fluid dynamics) model development, which takes into account the mass and heat transfer, and its validation for the dehydrogenation of cyclohexane in a shell-and-tube type of palladium membrane reactor. The CFD model clearly shows that the temperature and concentration distributions were formed both in the radial and axial directions. Simulation results with cyclohexane dehydrogenation were in good agreement with the experimental data. It is expected that the model developed in this study will be applicable to more complicated reactor design and analysis.

Tuning the cycle length in the NO x storage-reduction process and its contribution to the real-flow scenario

The influence of reductant supply timing on the performance of a Ba/Pt–Al 2O 3 model storage-reduction catalyst was analysed. This study was developed in terms of NO x and CO conversion, CO availability for NO x ad-species reduction and temperature profile analysis. A relevant CO–O 2 mixing effect was noticed during lean-to-rich and rich-to-lean transitions. On the basis of constant overall amount of reductant, the intensity of this effect was dependent on the number of transitions per time unit. Then, an excessive cycle length reduction led to operation states with CO defect, that subsequently controlled NO x conversion and temperature profiles over the catalyst. This mixing effect was related to the deviation from ideal plug flow condition. In this real-flow context, and due to the inherently transient NSR operation, the need for a quantitative description of those departures from idealised flow was evidenced. A first approach, just for the case of a discrete lean-to-rich transition, was presented in this work.

Size Selective Synthesis of Superparamagnetic Nanoparticles in Thin Fluids under Continuous Flow Conditions

AbstractContinuous flow spinning disc processing (SDP), which has extremely rapid mixing under plug flow conditions, effective heat and mass transfer, allowing high throughput with low wastage solvent efficiency, is effective in gaining access to superparamagnetic Fe3O4 nanoparticles at room temperature. These are formed by passing ammonia gas over a thin aqueous film of Fe2+/3+ which is introduced through a jet feed close to the centre of a rapidly rotating disc (500 to 2500 rpm), the particle size being controlled with a narrow size distribution over the range 5 nm to 10 nm, and the material having very high saturation magnetizations, in the range 68–78 emu g−1.

Population balance simulation of gas–liquid contacting

A successful attempt to simulate turbulently flowing gas–liquid dispersions was undertaken in this work where the turbulent dispersion/coalescence of bubbles was accurately predicted over a wide range of operating conditions by incorporating the effect of virtual mass into the phenomenological model developed by Coulaloglou and Tavlarides [1977. Description of interaction processes in agitated liquid–liquid dispersions. Chemical Engineering Science, 32, 1289–1297] for liquid–liquid dispersions. The population balance equation was numerically solved and the results obtained were compared with the experimental data obtained from an intensified gas–liquid reactor/contactor in which screen-type static mixers were used to superimpose an adjustable uniformly distributed turbulence field on the nearly plug flow conditions encountered in high velocity pipe flows. The model was also found to be capable of predicting gas–liquid contacting for the case of industrial streams where the presence of amphiphilic constituents was found to retard coalescence and result in average interfacial areas as high as 2100 being achieved. The fact that the model was capable to match experimental results obtained under very demanding/extreme conditions (where the flowing dispersion is successively exposed to breakage-dominated and coalescence dominated regions with local energy dissipation ratios as high as 400) suggests that it may be used for simulating other more complex gas–liquid contacting conditions such as those encountered in MAT.

Diffusion broadening of DMA transfer functions. Numerical validation of Stolzenburg model

This study investigates the validity of the assumptions inherent in the semianalytical expression derived by Stolzenburg [(1988). An ultrafine aerosol size distribution measuring system. Ph.D. thesis, Department of Mechanical Engineering, University of Minnesota, Minnesota] for the differential mobility analyzer (DMA) transfer function. For this purpose, a fourth-order finite-difference numerical method is employed to calculate the diffusion-broadened transfer function of two widely used DMA units: the “long” DMA (TSI Model 3081) and the “nano”-DMA (TSI Model 3085). Stolzenburg's results were in good agreement with the results of this study. Deviations were only observed near the boundaries of the transmitted mobility window. Under fully developed laminar flow, the area calculated with both methods agreed within 0.3%. Under plug-flow conditions, the same area was calculated 6.3% less than Stolzenburg's estimate for particles at the low detection limit of both DMAs. This corresponds to particle losses within the DMA classification region. The results confirm that Stolzenburg transfer functions can be used with good confidence in typical DMA applications and that two commercial DMA systems exhibit limited particle losses inside their classification region.

Structural and dynamical properties for confined polymers undergoing planar Poiseuille flow

The authors present the results from nonequilibrium molecular dynamics simulations for the structural and dynamical properties of highly confined linear polymer fluids undergoing planar Poiseuille flow. They study systems confined within pores of several atomic diameters in width and investigate the dependence of the density profiles, the mean squared radius of gyration, the mean squared end-to-end distance, streaming velocity, strain rate, shear stress, and streaming angular velocity as functions of average fluid density and chain length. Their simulation results show that, sufficiently far from the walls, the radius of gyration for molecules under shear in the middle of the pore follows the power law Rg=ANbnu, where Nb is the number of bonds and the exponent has a value of 0.5 which resembles the value for a homogeneous equilibrium fluid. Under the conditions simulated, the authors find the onset of flat velocity profiles but with very little wall slippage. These flat profiles are most likely due to the restricted layering of the fluid into just one or two molecular layers for narrow pore widths compared to chain length, rather than typical plug-flow conditions. The angular velocity is shown to be proportional to half the strain rate in the pore interior when the chain length is sufficiently small compared to the pore width, consistent with the behavior for homogeneous fluids in the linear regime.

Influence of Reactant Mixing in a Laminar Flow Reactor: The Case of Gas Reburning. 1. Experimental Study

An experimental work of the reburn process in a laminar flow reactor with two coaxial feeding streams has been performed, with the objective of analyzing the influence of reactant mixing in this reactive system. The ways of reaching different mixing conditions have been diverse: by means of changing the velocity ratio among the two streams through which reactants are fed into the reactor, and by introducing the reactants into the reactor through different conducts (fuel introduced through the inner stream and oxidant introduced through the outer stream and vice versa). Moreover, the influence of the main variables of the reburn processaverage residence time, temperature, and stoichiometryhas been studied under laminar flow conditions. In addition, the results achieved in the present work have been compared with data from the literature obtained from experiments conducted under ideal plug-flow reactor conditions. The results show an important influence of the reactant injection mode, which affects the rea...

Heat transfer, pressure drop, and flow pattern recognition during condensation inside smooth, helical micro-fin, and herringbone tubes

This paper presents a study of flow regimes, pressure drops, and heat transfer coefficients during refrigerant condensation inside a smooth, an 18° helical micro-fin, and a herringbone tubes. Experimental work was conducted for condensing refrigerants R-22, R-407C, and R-134a at an average saturation temperature of 40 °C with mass fluxes ranging from 400 to 800 kg m −2 s −1, and with vapour qualities ranging from 0.85 to 0.95 at condenser inlet and from 0.05 to 0.15 at condenser outlet. These test conditions represent annular and intermittent (slug and plug) flow conditions. Results showed that transition from annular flow to intermittent flow, on average for the three refrigerants, occurred at a vapour quality of 0.49 for the smooth tube, 0.29 for the helical micro-fin tube, and 0.26 for the herringbone tube. These transition vapour qualities were also reflected in the pressure gradients, with the herringbone tube having the highest pressure gradient. The pressure gradients encountered in the herringbone tube were about 79% higher than that of the smooth tube and about 27% higher than that of the helical micro-fin tube. A widely used pressure drop correlation for condensation in helical micro-fin tubes was modified for the case of the herringbone tube. The modified correlation predicted the data within a 1% error with an absolute deviation of 7%. Heat transfer enhancement factors for the herringbone tube against the smooth tube were on average 70% higher while against the helical micro-fin tube it was 40% higher. A correlation for predicting heat transfer coefficients inside a helical micro-fin tube was modified for the herringbone tube. On average the correlation predicted the data to within 4% with an average standard deviation of 8%.

Bioreactor Configurations for Ex‐Situ Treatment of Perchlorate: A Review

The perchlorate anion has been detected in the drinking water of millions of people living in the United States. At perchlorate levels equal to or greater than 1 mg/L and where the water is not immediately used for household purposes, ex-situ biotreatment has been widely applied. The principal objective of this paper was to compare the technical and economic advantages and disadvantages of various bioreactor configurations in the treatment of low- and medium-strength perchlorate-contaminated aqueous streams. The ideal bioreactor configuration for this application should be able to operate efficiently while achieving a long solids retention time, be designed to promote physical-chemical adsorption in addition to biodegradation, and operate under plug-flow hydraulic conditions. To date, the granular activated carbon (GAC) or sand-media-based fluidized bed reactors (FBRs) and GAC, sand-, or plastic-media-based packed bed reactors (PBRs) have been the reactor configurations most widely applied for perchlorate treatment. Only the FBR configuration has been applied commercially. Commercial-scale cost information presented implies no economic advantage for the PBR relative to the FBR configuration. Full-scale application information provides evidence that the FBR is a good choice for treating perchlorate-contaminated aqueous streams.

Inactivation of Escherichia coli by ozone under bench-scale plug flow and full-scale hydraulic conditions

To determine the disinfection efficacy of ozonation, water companies can apply several disinfection calculation methods. The goal of this study was to evaluate the use of the T10 and continuous stirred tank reactor (CSTR) method to extrapolate inactivation rates of ozone sensitive microorganisms observed in laboratory tests to full-scale ozonation in drinking water treatment. The inactivation efficacy of the ozonation at the Amsterdam water treatment works was assessed by determining Escherichia coli concentrations in large volume samples before and after ozonation over a period of 1 year. The inactivation of dosed E. coli WR1 was tested in a bench-scale dissolved ozone plug flow reactor (DOPFR) on the same feed water as the full-scale ozonation in which a concentrated ozone solution in Milli-Q ® water was dosed. Applying the T10 method on the inactivation rates observed in the DOPFR strongly overestimated the inactivation capacity of the full-scale ozonation. The expected inactivation based on the CSTR method (LT2ESWTR) approached the observed inactivation at full-scale. Therefore, the CSTR method should be preferred to calculate inactivation of ozone sensitive organisms such as E. coli, viruses, Giardia and Campylobacter by full-scale ozonation.