Plug Flow Conditions Research Articles

Effect of liquid flow velocity on corrosion behavior of 20# steel at initial stage under gas-liquid two-phase plug flow condition

PurposeThe paper aims to study the effect of liquid flow velocity on corrosion behavior of 20# steel at initial stage under (CO2/aqueous solution) gas–liquid two-phase plug flow conditions.Design/methodology/approachWeight loss, scanning electron microscopy, energy-dispersive X-ray spectroscopy and XPS methods were used in this study.FindingsThe corrosion rate increased with the increasing liquid flow velocity at any different corrosion time. The corrosion rate decreased with the extension of corrosion time at the same liquid flow velocity. There was no continuous corrosion products film on the whole pipe wall at any different corrosion time. The macroscopic brown-yellow corrosion products on the pipe wall surface decreased with the increasing liquid flow velocity and the loose floccus corrosion products decreased gradually until these products were transformed into un-continuous needle-like dense products with the increasing liquid velocity. The main elements among the products film were Fe, C and O, and the main phases of products film on the pipe wall were Fe3C, FeCO3, FeOOH and Fe3O4. When the corrosion time was 1 h under different liquid–velocity condition, the thickness of local corrosion products film was from 3.5 to 3.8 µm.Originality/valueThe ion mass transfer model of corrosion process in pipe was put forward under gas–liquid two-phase plug flow condition. The total thickness of diffusion sublayer and turbulence sublayer decreased as well as the turbulence propagation coefficient increased with the increasing liquid velocity, which led to the increasing velocity of ion transfer during corrosion process. This was the fundamental reason for the increase of corrosion rate with the increasing liquid velocity.

Developing a novel biofiltration treatment system by coupling high-rate infiltration trench technology with a plug-flow porous-media bioreactor

The sequence of two infiltration steps combined with an intermediate aeration named ‘sequential managed aquifer recharge technology (SMART)’ proved to be a promising approach to replenish groundwater using treated wastewater effluents or impaired surface waters due to efficient inactivation of pathogens and improved removal of many trace organic chemicals. To minimize the physical footprint of such systems and overcome limitations through site-specific heterogeneity at conventional MAR sites, an engineered approach was taken to further advance the SMART concept. This study investigated the establishment of plug-flow conditions in a pilot scale subsurface bioreactor by providing highly controlled hydraulic conditions. Such a system, with a substantially reduced physical footprint in comparison to conventional MAR systems, could be applied independent of local hydrogeological conditions. The desired redox conditions in the bioreactor are achieved by in-situ oxygen delivery, to maintain the homogenous flow conditions and eliminate typical pumping costs. For the time being, this study investigated hydraulic conditions and the initial performance regarding the removal of chemical constituents during baseline operation of the SMARTplus bioreactor. The fit of the observed and simulated breakthrough curves from the pulse injection tracer test indicated successful establishment of plug-flow conditions throughout the bioreactor. The performance data obtained during baseline operation confirmed similar trace organic chemical biotransformation as previously observed in lab- and field-scale MAR systems during travel times of <13 h.

Characterization of a modular Temkin reactor with experiments and computational fluid dynamics simulations

The determination of reaction kinetics without the influence of transport limitations is challenging, especially under industrially relevant conditions, particle shapes, and sizes. The Temkin reactor concept, developed in the 1960s, allows to measure reaction kinetics with a small number of unmodified catalyst particles operating under assumed isothermal and plug-flow conditions. In this study, a recently developed modular Temkin reactor design is studied experimentally and with detailed CFD simulations in order to shed light on the residence time distribution without reaction. Cold fluid volumetric flow rates are varied in the range of 50–200 mlNmin-1 and two different particle shapes (spheres and rings) are tested. Pulse experiments and passive scalar computational fluid dynamics (CFD) simulations verify that this reactor design behaves like a cascade of stirred tank reactors with Bodenstein numbers of each chamber smaller than three. Mean age of air CFD simulations highlight potentials to re-design the entrance of the chamber in order to minimize stagnation zones.

Evaluation of Frother Types for Improved Flotation Recovery and Selectivity

A laboratory study was conducted to evaluate and compare the effectiveness of nine different frother types when used in a three-phase, continuously operating froth flotation system. The frothers included several that are commonly used in the industry (e.g., MIBC, 2EH, and F-1) as well as unique frother types (e.g., F-3). The tests were conducted in a 5-cm diameter laboratory flotation column that provided near plug-flow mixing conditions due to a length-to-diameter ratio of around 50:1. Test results indicate that F-1, MIBC, and MPC (in order of decreasing effectiveness) provided the weakest performance in terms of combustible recovery while F-2, MAC, and 2EH were the top three generating the highest separation efficiencies. When processing ultrafine coal, the ash content of the flotation concentrate ranged from 10% to 15% while recovering over 80% of the combustible material. F-3, F-4, and DIBC provided over 80% recovery of combustibles at the expense in the amount of hydraulic entrainment. The flotation performances were also closely examined in accordance with the fundamental properties of the nine tested frothers, and their correlations were addressed in detail.

Assessment of volumetric scale-up law for processing of a sustained release formulation on co-rotating hot-melt extruders.

In this research, the volumetric scale-up law was assessed for its applicability to scale-up from a laboratory-scale extruder (11 mm diameter) to a pilot-scale extruder (16 mm diameter) with geometric similarity using low feed rates (0.1-0.26 kg/h at lab-scale). A sustained release formulation was extruded on both scales using scaled feed rates according to the volumetric scale-up law. The specific mechanical energies, drug solid-state, drug dissolution and the residence time distribution responses (i.e. axial mixing degree, mean residence time, width of distribution) were compared between both scales. The results showed that the difference in mean residence time between both scale extruders reduced with higher throughput and thus fill level. Overall, the specific mechanical energies (SME) were comparable between scales when using the volumetric scale-up law (i.e. applying scaling factor q = 3) and were exactly matching with a scaling factor of q = 2.6. Furthermore, plug flow conditions at lab-scale should be avoided before scaling up to obtain similar SMEs. The same degree of axial mixing (represented by the Peclet number) was demonstrated at a scaling factor of q = 2. If drug solid-state is a critical quality attribute (CQA), focus should be on the screw speed and cooling capacity of the larger scale extruder. The drug dissolution showed similarity between scales and was independent of drug solid-state for this formulation, indicating that successful scale-up was possible.

Hydrodynamic, Mass Transfer and RTD Studies of Fluid Flow in a Spiral Microreactor

In this article, the spiral microreactor made of copper tube with U-junction, mixing region, was used to study the reactor performance on sodium hydroxide-n-butyl acetate system. The internal diameter of microreactor is 1.5 mm and length is 2 m. The performance of spiral microreactor was studied considering hydrodynamic flow, mass transfer characteristics, RTD and computational analysis of fluid flow using n-butyl acetate and sodium hydroxide system. Pressure drop was estimated in spiral geometry using dean number and compared with simulation results for laminar flow. The volumetric mass transfer coefficient obtained was in the range of 0.02–0.37 s−1. The dispersion number calculated from the RTD study was equal to 0.0526 indicating the plug flow conditions.

Effect of flow behavior in extra-column volumes on the retention pattern of proteins in a small column

Experimental and theoretical analysis of deformation of band profiles in extra-column volumes (ECV) was performed, and its influence on the retention pattern of proteins in a small chromatographic column was quantified. Several macromolecule and small-molecule compounds, and their mixtures were eluted from a chromatographic system in the absence and presence of the column. The peak deformation in ECV was attributed to non-uniform velocity distribution in the radial direction in connecting capillaries. The phenomenon enhanced with increasing molecular weight of the model compound, when radial diffusion dominated the mechanism of band spreading. The band shape was also affected by the geometry of the injection system used, i.e., an injection loop capillary or a superloop. The phenomenon vanished for a small molecule compound, for which plug flow conditions could be established. The difference in flow behaviour of the macromolecule and small-molecule compounds caused them to migrate with different velocities in ECV, which resulted in partial separation of their bands. The ECV effect influenced the retention behaviour of macromolecules in a small column; it caused tailing of peaks and asymmetry of breakthrough curves. To describe the elution profiles in ECV and in the column, a mathematical model was used which accounted for non-ideality of the flow pattern. The model reproduced accurately band profiles of macromolecules within a range of relatively low velocities, typical however for protein chromatography.

Corrosion behavior of 20# steel at initial stage under the (CO2/aqueous solution) gas-liquid two-phase plug flow condition

The corrosion behavior of 20# steel at initial stage under CO2/H2O gas-liquid two-phase plug flow conditions was studied through weight loss, SEM, EDS, XPS methods. The corrosion rate results indicated that there was an obvious deceleration transition at the early stage of corrosion process. The difference of corrosion rate under different CO2 pressures decreased with the extension of corrosion time. The precipitation of corrosion products started from the defects area of the pipe wall surface or the crystal interface, and the corrosion products extended to contact each other with the extending time. The corrosion products grew into the flocculated corrosion products on the surface of acicular corrosion products layer deposited preliminary with the extending corrosion time. The corrosion products on the bottom pipe wall grew faster than that on the top pipe wall. The main phases among the corrosion products layer were Fe3C, FeCO3, FeOOH and Fe3O4, and there was no polygonal FeCO3 appeared.

Ultrafiltration Modeling Allowing for Axial Dispersion in a Solution under Purification

Formulas for ultrafiltration operating and geometric parameters that take into account the longitudinal diffusion of solute molecules in a solution under treatment have been derived based on differential equations of material balance. The effect of longitudinal diffusion on the concentration of solute molecules in the permeate and the retentate in comparison with plug flow and perfect mixing conditions has been evaluated.

Domestic wastewater treatment by modified bio-rack wetland system

The conventional Bio-Rack Wetland System (CBRWS) is a comparatively a novel technique for treating wastewater. It has potential to overcome clogging problem in constructed wetlands. Modifications are possible to enhance treatment potential of CBRWS with relatively lower cost. In the present study, modifications were implemented by planting multispecies (Canna indica and Typha angustata) vegetation in bio-racks made up of discarded non-perforated plastic water bottles in which plug-flow was induced through baffling. The performance of CBRWS and Modified BRWS (MBRWS) was evaluated for the removal of dissolved organics (Carbon and Nitrogen) from domestic wastewater. CBRWS and MBRWS were operated in sequential continuous [1.5 h, 4–24 h (in multiples of 4) HRT] and MBRWS in continuous (4, 12 and 24 h HRT) mode. The systems were operated at Organic Loading Rate (OLR) of 0.25–3 kg COD/m3 d. The removal efficiency of MBRWS was more than CBRWS by “4–9%, 4–13% and 3–16%” for “COD, BOD3 and TKN”, respectively. Dissolved oxygen level was also increased by 0.6–1 mg/L in MBRWS at all HRTs. The contribution of baffling used to induce plugflow conditions in MBRWS was found to be more significant than the use of multispecies vegetation for the removal of dissolved organics. The study demonstrated potential of MBRWS to maintain better aerobic conditions and treat wastewater at lower HRTs.

CFD based compartment-model for a multiphase loop-reactor

In multiphase devices, fluid dynamics have a high impact on concentration profiles and mass transfer between the phases and therefore influence efficiency. Standard models often assume ideally mixed conditions or plug flow. The application of such models for multiphase devices with complex flow patterns causes inaccuracies, if the flow deviates from ideally mixed or plug flow conditions. Therefore, for a precise model based design and operation parameter determination of devices with complex flow patterns, the local fluid dynamics should be considered. CFD simulations for multiphase systems including mass transfer, population balance equations for coalescence and breakage as well as reactions are still time consuming. Thus, we developed a compartment-model based on prior calculated CFD flow-data. In the CFD simulations, the time consuming population balance equations for coalescence and breakage, mass transfer and reactions are neglected. These phenomena are considered in the compartment-model. Thereby we reduce the overall computing time.This paper presents the CFD based compartment-model applied on a loop-reactor. First, a three-phase CFD model of the developed multiphase loop-reactor is introduced. Following, the paper presents the compartment-model and the application of a time-driven constant-number Monte-Carlo approach to solve population balances. Finally, the compartment-model is applied to the liquid-liquid extraction part of the loop-reactor calculating the drop size distribution and mass transfer based on previously calculated CFD data.

Effect of Porous Baffles on the Energy Performance of Contact Tanks in Water Treatment

Three-dimensional numerical simulations are performed to evaluate the effect of porous baffles on the efficiency of water treatment contact tanks. A second-order accurate numerical model is employed for the solutions of unsteady flow and tracer transport through the porous baffles. The flow through the porous medium is characterized while using the Darcy-Forchheimer relationship. Large Eddy Simulation (LES) model is used to simulate the instantaneous mixing of the tracer in the chambers of the contact tank. Three different porosities are considered to evaluate the effect of porosity on the hydraulic and mixing efficiencies of the contact tank. Simulated time-averaged flow field shows that porous baffles that are placed at the entrance of each chamber could successfully mitigate short-circuiting and yield plug-flow conditions through the system for low porosities. Flow in the contact tank becomes laminar as the flow velocities decrease due to viscous effects and inertial resistance in the porous zone. For this case, the tracer is transported with bulk flow through the system and leaves the contact tank with a high peak seen in the Residence Time Distribution (RTD) plot. Porous layer increases the hydraulic efficiency of the conventional design from “poor” to “good” according to the baffling factor and increases the overall efficiency from “compromising” to “good” according to the AD index. Comparison of the performance of the porous layer with the previously developed slot-baffle design shows that the slot-baffle design increases the efficiency of the tank with increasing dispersion effects, whereas the porous design increases hydraulic efficiency and reduces the dispersion effects. While the porous design reduces energy efficiency by 33% due to a drastic increase in drag in the flow through porous zone, the slot-baffle design increases the energy efficiency of the conventional design by 67%.

Optical determination of oxygen mass transfer in a helically-coiled pipe compared to a straight horizontal tube

The gas-liquid mass transfer of oxygen has been examined experimentally in a helically-coiled pipe using an optical colorimetric method. Two tracer redox-reactions have been used for this purpose: (i) methylene blue and (ii) resazurin. The gas hold-up has been varied from 0.1 to 0.6, and liquid flow rate from 0.37 to 0.68 l/min, both leading to plug-flow conditions in the helix for all parameter combinations. A progress variable has been defined to measure the advancement of the reaction and thus, oxygen transfer to the liquid. The resazurin reaction turned out to be more appropriate for mass transfer studies, due to the bigger time constant difference of oxidation and reduction. Therefore, it can be adapted more easily to the specific conditions of the set-up. It also possesses more intense colours and higher fluorescence intensity, which might be useful for other mass transfer studies. As expected, the oxygen mass transfer increases with gas hold up, but liquid flow rate has little influence. A comparison with a horizontal tube of the same diameter shows the drastically increased mass transfer in the helix, due to better radial mixing and different flow pattern. Depending on the experimental conditions, oxygen concentration in the helix was up to twice that of the horizontal tube. This can be explained by the much higher liquid side mass transfer velocity, which is determined in this study from the experimental kla-values and specific bubble surface per turn of the helix. For this purpose, bubble dimensions and gas-liquid cell volumes have been determined from the experimental images. The paper thus presents a locally resolved characterization of gas-liquid mass transfer in a helically coiled reactor. The high mass transfer especially during the first turns of the helix shows its advantages compared to a straight tube.

Experimental Study on Pressure Drop and Flow Dispersion in Packed Bed of Natural Zeolite

The use of conventional correlation for pressure drop and dispersion coefficient calculation may result in inaccurate values for zeolite packed bed as the correlations are generally developed for regularly shaped and uniformly sized particles. To support the research on the application of modified natural zeolite as tar cracking catalyst, the research on the hydrodynamic behaviour of zeolite packed bed has been conducted. Experiments were carried out using a glass column with diameter of 37.8 mm. Natural zeolite with particle size of about 2.91 to 6.4 mm was applied as packing material in the column, and the bed height was varied at 9, 19 and 29 cm. Air was used as the fluid that flows through the bed and nitrogen was used as a tracer for residence time distribution determination. Air flow rates were in the range of 20 to 100 mL/s which correspond to the laminar-transitional flow regime. The pressure drops through the bed were in the range of 1.7 to 95.6 Pa, depending on the air flow rate and bed height. From these values, the parameters in the Ergun equation were estimated, taking into account the contribution by wall effect when the ratio of column to particle diameter is low. The viscous and inertial term constants in the Ergun equation calculated ranges from 179 to 199 and 1.41 to 1.47 respectively while the particle sphericity ranges from 0.56 to 0.59. The reactor Peclet number were determined to range from 5.2 to 5.5, which indicated significant deviation from a plug flow condition.

The relationship between contrasting ages of groundwater and streamflow

AbstractTracer data demonstrate that waters in aquifers are often much older than the stream waters that drain them. This contrast in water ages has lacked a general quantitative explanation. Here we show that under stationary conditions, the age distribution of water stored in a catchment can be directly estimated from the age distribution of its outflows, and vice versa. This in turn implies that the storage selection function, expressing the catchment's preference for the release or retention of waters of different ages, can be estimated directly from the age distribution of outflow under stationary conditions. Using gamma distributions of streamflow ages, we show that the mean age of stored water can range from half as old as the mean age of streamflow (for plug flow conditions) to almost infinitely older (for strongly preferential flow). Many streamflow age distributions have long upper tails, consistent with preferential flow and implying that storage ages are substantially older than streamflow ages. Mean streamflow ages reported in the literature imply that most streamflow originates from a thin veneer of total groundwater storage. This preferential release of young streamflow implies that most groundwater is exchanged only slowly with the surface and consequently is relatively old.

Methane steam reforming at low steam to carbon ratios over alumina and yttria-stabilized-zirconia supported nickel-spinel catalyst: Experimental study and optimization of microkinetic model

An experimental work to acquire reaction kinetic data of methane steam reforming (MSR) to produce syngas was carried out over alumina and yttria-stabilized-zirconia (YSZ) supported nickel-spinel catalyst (Ni-spinel). With the aim of making MSR more energy efficient, the catalyst was tested at 1.25 and 1.50 steam to carbon (SC) ratios, slightly higher than the stoichiometric ratio of 1.0 but far less than normally considered 3.0. The experiments were conducted at isothermal plug-flow conditions and at near atmospheric pressure in a laboratory scale quartz reactor for five different space times (STP) between 55ms and 277ms at three different temperatures of 973K, 1073K, and 1123K. Reaction kinetic data of MSR over Ni-spinel catalyst are presented in terms of methane conversion and products yield or selectivity. To simulate measured kinetic data in a 1D plug-flow reactor model, a surface microkinetic model of MSR over nickel based catalyst was adopted from the literature and validated by optimizing the kinetic parameters of the most influential elementary reaction steps. A stepwise approach based on a partial equilibrium analysis and a local sensitivity analysis was implemented to scrutinize the most influential elementary reaction steps. With the adjustment of only 12 parameters out of 78, the optimized microkinetic model predicted the exit flow rates of chemical species accurately for the entire operating space. Adsorption of H2O, desorption of H2(s), and reaction pairs of surface dehydrogenation of CH4(s) and CH2(s) or formation of CH(s) were found to be the most influential reaction steps for the studied operating conditions. Surface coverages of H(s), CO(s) and H2O(s) were increased in the case of optimized microkinetic model of Ni-spinel catalyst compared to the original microkinetic model. Increased surface coverage of H2O(s) in the case of optimized microkinetic model supports the experimental finding in the literature that the introduction of yttria to zirconia increases the activity of the oxygen pumping component.

A high-pressure plug flow reactor for combustion chemistry investigations

A plug flow reactor (PFR) is built for investigating the oxidation chemistry of fuels at up to 50 bar and 1000 K. These conditions include those corresponding to the low temperature combustion (i.e. the autoignition) that commonly occurs in internal combustion engines. Turbulent flow that approximates ideal, plug flow conditions is established in a quartz tube reactor. The reacting mixture is highly diluted by excess air to reduce the reaction rates for kinetic investigations. A novel mixer design is used to achieve fast mixing of the preheated air and fuel vapour at the reactor entrance, reducing the issue of reaction initialization in kinetic modelling. A water-cooled probe moves along the reactor extracting gases for further analysis. Measurement of the sampled gas temperature uses an extended form of a three-thermocouple method that corrects for radiative heat losses from the thermocouples to the enclosed PFR environment.Investigation of the PFR’s operation is first conducted using non-reacting flows, and then with isooctane oxidation at 900 K and 10 bar. Mixing of the non-reacting temperature and species fields is shown to be rapid. The measured fuel consumption and CO formation are then closely reproduced by kinetic modelling using an extensively validated iso-octane mechanism from the literature and the corrected gas temperature. Together, these results demonstrate the PFR’s utility for chemical kinetic investigations.

Concentration Impedance in Testing of Solid Oxide Cells Revisited

Concentration impedance in the context of solid oxide cells (SOC) has been discussed in the literature [1-6]. We divide the concentration impedance into diffusion impedance and conversion impedance. Diffusion impedance is defined as impedance due to a reactant concentration gradient caused by a slow (relative to the electrode reaction rate) diffusion through a porous boy of a stagnant gas layer. Conversion impedance is defined as impedance caused by a decrease in an electrode reactant concentration due the conversion of the reactant by electrode reaction. Mainly the diffusion impedance has been treated in spite of the fact that the conversion impedance often is of greater importance. Actually not all workers within the SOC field recognize the concept of conversion impedance. Other workers claim that the two kinds of concentration impedance cannot be distinguished. It is correct that the two phenomena will interact, and thus often it may be difficult to separate them, but we have tools that make it possible to distinguish between the two types of impedance. The diffusion impedance due to diffusion through a porous body, e.g. an electrode support layer, will not be sensitive to reactant gas flow rate, whereas the conversion impedance will be very sensitive to the reactant flow rate. Diffusion impedance results in the simple, clean case in a Warburg element, i.e. a 45 ° slope followed by a relative sharp decrease by decreasing low frequencies down to the x-axis in a Nyquist plot. In contrast to this, clean conversion impedance can always be modeled by a resistor in parallel with an ideal capacitor, which most often has a rather huge value in the order of 0.1 – 1 F cm-2. The paper and presentation will explain how the two types of concentration impedance may be extracted from impedance spectra in a manner that they are separated. Both simple examples and more difficult cases with very high fuel utilization at high current density, where the interaction between diffusion and conversion impedance is in particular strong, will be given. Special emphasis will be put on the case of plug flow condition, which is the relevant case for practical cells and stacks.

Feed-forward-based software sensor for outlet turbidity of coagulation process considering plug flow condition

Physical online sensors are widely used in wastewater treatment plants. The high costs of acquisition and maintenance as well as the delayed response due to long hydraulic retention times, applications of some physical online sensors are limited. Consequently, studies on developing software sensors have been drawn much attentions these years. Aiming to predict treatment results after water is coagulated, and this paper focuses on developing a software sensor of outlet turbidity for coagulation process. Solution to address the potential non-plug flow conditions in sedimentation tanks is discussed. Model validation results show that the proposed software sensor has 86% of accuracy within the main working range. This paper expresses a novel concept that what the proposed software sensor predicted has been not formed in the coagulation process. Since the software sensor enables to know the treatment results without waiting for long hydraulic retention time of sedimentation tank, it not only shortens the response time of manual dosing control but also serves as a feedback parameter to define optimal dosage for coagulant dosing control systems.

Mathematical Modeling of Electrolyte Flow in a Segment of Flow Channel over Porous Electrode Layered System in Vanadium Flow Battery with Flow Field Design

In this work, a two-dimensional mathematical model is developed to study the flow patterns and volumetric flow penetrations in a segment of flow channel over porous electrode layered system in vanadium flow battery with flow field design. The flow distributions at the interface between the flow channel and porous electrode are examined. It is found that the non-linear pressure distributions can distinguish the interface flow distributions under the ideal plug flow and ideal parabolic flow inlet boundary conditions. Nevertheless, the volumetric flow penetration within the porous electrode beneath the flow channel through the integration of interface flow velocity reveals that this value is identical under both ideal plug flow and ideal parabolic flow inlet boundary conditions. The volumetric flow penetrations under the advection effects by both flow channel and landing/rib are estimated. The maximum current density achieved in a flow battery with a flow field architecture can be predicted based on the 100% amount of electrolyte flow reactant consumption through the porous electrode beneath flow channel and landing/rib. The corresponding theoretical maximum current densities achieved in vanadium flow battery with one and three layers of SGL 10AA carbon paper electrode have reasonable agreement with experimental results under a proper permeability.