Superficial Gas Flow Rates Research Articles

Hydrodynamic study of the operating window of a stator-rotor vortex chamber reactor

The gas-solid vortex reactor is promising for process intensification while the huge gas consumption and relatively low solids loading can be a hurdle for some industrial applications. Therefore, an improved design, the stator-rotor vortex chamber (STARVOC) has been proposed. To construct the operating window, five materials with sizes of 300–1000 μm and densities of 700–2330 kg/m3 are tested at varying rotational speeds from 300 to 600 RPM and superficial gas flow rates from 0 to 2.3 m/s. A quantitative assessment of the bed's axial uniformity is conducted and a minimum rotational speed is determined. A semi-empirical correlation is developed for the terminal velocity of particles, enabling obtaining maximum solids loading and identifying maximum rotational speed. It is found that the operating window can be divided into six zones and the appropriate zone for uniform fluidization is identified. This framework provides operational guidelines for STARVOC implementation in drying and reactive applications.

Filamentous organisms degrade oxygen transfer efficiency by increasing mixed liquor apparent viscosity: Mechanistic understanding and experimental verification

Recent findings have demonstrated that activated sludge morphology significantly impacts oxygen transfer efficiency (OTE) in the activated sludge process. In this study, we developed a mechanistic understanding of this impact. Mixed liquor samples collected from a domestic wastewater treatment plant (WWTP) were blended with a bulking activated sludge from a bench scale reactor (BSR) cultured on synthetic wastewater to manipulate various morphological parameters such as the settled sludge volume (SV), the sludge volume index (SVI), and the specific filament length (SFL). The filaments that were present in the blended sludges consisted largely of Type 0041 and Type 021N, which are commonly found in WWTPs that treat domestic wastewater. Variations in sludge morphology, as quantified by settled sludge volume after 30 min (SV30), SVI, and SFL, systematically affected the mixed liquor apparent viscosity (μapp), which consequently impacted OTE. An increase in the SFL from 9.61 × 106 μm g−1 to 6.88 × 107 μm g−1 resulted in a 41.4% increase in apparent viscosity and a 24.6% decrease in volumetric mass transfer coefficient (KLa). A new parameter, named the ultimate settleability (SVULT), was developed by curve fitting the SV versus time data and found to relate with μapp through an expanded form of the Einstein Equation for the viscosity. Therefore, SVULT is a corollary for the particle volume fraction that incorporates effects of both the sludge morphology and mass concentration on μapp. Theoretical derivation revealed that an increase in SVULT resulted in an increase in μapp, which reduced oxygen transfer by increasing the air bubble size and reducing refreshment of the liquid at the gas-liquid interface. The KLa was found to be inversely proportional to μapp0.75 through fitting the experimental data with the theoretical model. Using a variance-based global sensitivity analysis, three operating parameters that have the most impact on oxygen transfer were identified: the power input per unit volume, the superficial gas flowrate, and the μapp.

Multiphysics simulation of algal growth in an airlift photobioreactor: Effects of fluid mixing and shear stress

A multiphysics model has been developed to predict the effects of fluid mixing and shear stress on microalgal growth in an airlift photobioreactor. The model integrates multiphase flow dynamics, radiation transport, shear stress, and algal growth kinetics using an Eulerian approach. The model is first validated by comparing its predictions with experimental data, and then the radiation transport and algal growth kinetics submodels are added to predict biomass accumulation under different flow conditions. The simulations correctly predict biomass growth curves for a wide range of superficial gas flow rates and demonstrate that biomass productivity increases with increased gas flow rate due to better light delivery to microorganisms. However, at the higher gas flow rates considered, shear stress on microorganisms inhibits biomass growth. Lastly, it is shown that the Eulerian approach used here provides a less cumbersome computational approach and provides better predictions than the circulation time and Lagrangian approaches.

Hydrodynamic Study of Bubbles in a Bubble Column Reactor Part II – Numerical Study

The present work deals with the use of CFD analysis and the validation of the experimental work carried out on the artificial splitting of an air bubble in a bubble column reactor. In Part I of this work, artificial splitting of bubble in a bubble column rector is experimentally studied by using a high speed camera. Image processing technique was used to identify bubble size and bubble velocity. In present work CFD simulations are carried out using ANSYS FLUENT software using Volume of Fluids (VOF) method. VOF is based on a surface tracking technique applied to a fixed Eulerian space. The phase fraction in physical quantities that can be used to distinguish the distribution of gas hold up in a bubble Column reactor. The numerical study of splitting of bubble into two bubbles of nearly equal size is considered. In the bubble column reactor, the liquid phase is stationary and gas flow rate in it is varied. The superficial gas flow rates are 10 lph, 15 lph, 20 lph and 25 lph. The characteristics of bubble after splitting which include its shape, size and velocity for various gas flow rates mentioned above are studied numerically and are compared with experimental results. These hydrodynamic characteristics play a pivotal role in the reactions occurring between the liquid and gas phases in the bubble column reactor.

10.2478/s11814-009-0242-3

Anaerobic and aerobic biodegradation were integrated in an internal airlift loop reactor (IALR) by adding porous microbial carriers. In this bioreactor, aerobic activated sludge was suspended in the liquid bulk, while the anaerobic microbes were attached within the core of carriers. The integrated IALR was applied to the treatment of synthetic phenol wastewater. After 50 days’ acclimation according to co-substance strategy, the influent COD decreased from 3,700 mg/L to 400 mg/L (phenol removal rate was over 99%) with the residence time of 24 h. High performance could be achieved under the operation condition of superficial gas flow rate higher than 0.07 cm/s, temperature beyond 15°C and the microbial carrier volume fraction larger than 5%. Integration of anaerobic/aerobic biodegradation in IALR enhanced the synergetic effects between aerobic and anaerobic degradation; therefore, it has great potential in the treatment of phenol wastewater and other wastewater containing hard biodegradable organics.

Integrated anaerobic/aerobic biodegradation in an internal airlift loop reactor for phenol wastewater treatment

Anaerobic and aerobic biodegradation were integrated in an internal airlift loop reactor (IALR) by adding porous microbial carriers. In this bioreactor, aerobic activated sludge was suspended in the liquid bulk, while the anaerobic microbes were attached within the core of carriers. The integrated IALR was applied to the treatment of synthetic phenol wastewater. After 50 days’ acclimation according to co-substance strategy, the influent COD decreased from 3,700 mg/L to 400 mg/L (phenol removal rate was over 99%) with the residence time of 24 h. High performance could be achieved under the operation condition of superficial gas flow rate higher than 0.07 cm/s, temperature beyond 15°C and the microbial carrier volume fraction larger than 5%. Integration of anaerobic/aerobic biodegradation in IALR enhanced the synergetic effects between aerobic and anaerobic degradation; therefore, it has great potential in the treatment of phenol wastewater and other wastewater containing hard biodegradable organics.

Effect of Temperature and Suspended Solids on Local Void Fraction in a Sparged Reactor with a Multi-Impeller Agitator

The void fraction distribution in the vertical direction in a tall, sparged, gas–liquid reactor of 0.48 m diameter is measured at various temperatures with and without glass beads in the suspension. The agitator comprises two up-pumping wide-blade hydrofoils positioned above a hollow-blade dispersing turbine. Operating temperatures range from 24 to 81°C with solid concentrations of up to 21 vol%. Detailed comparisons of the void distributions at 24 and 81°C in the presence of 9 and 15 vol% solids are presented. All the experiments reported here are performed at a constant agitator speed of N = 6 s–1.Temperature affects the overall and integrated local void distributions in a similar manner. At all temperatures, the local gas fraction is the maximum in the region just above the levels of the top and bottom impellers. At fixed agitator speeds and superficial gas flow rates the overall and local gas holdup values are always less at high temperature in the absence of solids. At ambient temperature, the local gas fraction is reduced at high solid concentrations. At 81°C, on the other hand, the effect of solid concentration on the void fraction is almost nil although the local extremes in the void distribution are somewhat high. The measured local void fractions show a less uniform gas distribution in the presence of solids and at high temperatures.The results of this study help improve our understanding of the differences between two- and three-phase reactors operating under cold- and hot-sparged conditions.

Modeling Ozone Mass Transfer in Reclaimed Wastewater

Ozone mass transfer in reclaimed water was evaluated at pilot scale to determine mass-transfer characteristics and reaction kinetics and to assess the use of oxygen as a surrogate to measure this process. Tests were conducted in a 40-L/min pilot plant over a 3-year period. Nonsteady-state mass-transfer analyses for both oxygen and ozone were performed for superficial gas flow rates ranging from 0.13m/min to 0.40m/min. The psi factor, which is the ratio of volumetric mass-transfer coefficients of ozone to oxygen, was determined. The decrease in oxygen transfer rate caused by contaminants in reclaimed water was only 10 to 15% compared to tap water. A simple mathematical model was developed to describe transfer rate and steady state ozone concentration. Ozone decay was modeled accurately as a pseudo first-order reaction between ozone and ozone-demanding materials.

Phenol and 4-chlorophenol biodegradation by yeast Candida tropicalis in a fluidized bed reactor

The objective of this work was to evaluate (i) the hydrodynamics and oxygen transfer characteristics of a fluidized bed reactor (FBR) and (ii) their performance in the continuous biodegradation of a phenol (Phe) and 4-chlorophenol (4-CP)-containing influent when the FBR was loaded with Candida tropicalis yeast immobilized onto granular activated carbon (GAC) particles. The first part was carried out in terms of the behavior of mixing time ( t m95), bubble diameter ( d B), gas hold-up ( ɛ G), gas–liquid interfacial area ( a′) and oxygen transfer coefficient ( k L a) in biphasic (gas–liquid) and triphasic (gas–liquid–carbon) systems, both batch-wise operated in pseudohomogeneous regime at superficial gas flow rates U G below 1.72 cm s −1. Mixing time was determined using a tracer technique, ɛ G was measured by the volume expansion method; ( d B) was determined by the procedure reported by Poulsen and Iversen and k L a was determined by a transient gassing-in technique. It was found that hydrodynamic variables were severely influenced by GAC particles. The d B values were smaller in triphasic than in biphasic system, with the consequent increase in the ɛ G, a′ and k L a values. The removal efficiencies of both Phe and 4-CP in the bioreactor challenged at increasing volumetric loading rate of pollutants were determined in the second part of the study. Phe and 4-CP was determined by HPLC. Immobilized biomass on GAC was estimated from their total nitrogen content. Regarding continuous biodegradation experiments, the FBR was capable of efficiently removing Phe at volumetric loading rates as high as 60 mg Phe L −1 h −1 when it was fed as the sole carbon source. Beyond this point, removal efficiencies drastically decreased. When operated with a feed consisting of a mixture of Phe and 4-CP, the FBR was able to remove more than 98% of 4-CP in the range of volumetric loading rates of 4.1 mg 4-CP L −1 h −1 and 55 mg Phe L −1 h −1 with no apparent deterioration of bioreactor performance.

Neutralization of static surface charges by an ac ionizer in a nitrogen and dry air environment

To elucidate the dynamics of static charge elimination, we measured charge decay and residual potential (balance or offset voltage) in gases with various ion mobilities. It was observed that surface charge decay, especially for positive charges, occurs much faster in nitrogen than in air. The residual potential on the probe is negative in pure nitrogen and increases toward positive values with the injection of small quantities of air in front of the ionizer. The fluctuations in the residual potential are generally less than 3 V peak-to-peak. For ionizer operations in nitrogen environments, the charge decay rate increases with superficial gas-flow rate. The results are consistent with a theory of the interaction between bipolar ions and a charged object.

Macro-kinetic investigation on phenol uptake from air by biofiltration: Influence of superficial gas flow rate and inlet pollutant concentration

The macro-kinetic behavior of phenol removal from a synthetic exhaust gas was investigated theoretically as well as experimentally by means of two identical continuously operating laboratory-scale biological filter bed columns. A mixture of peat and glass beads was used as filter material. After sterilization it was inoculated with a pure strain of Pseudomonas putida, as employed in previous experimental studies. To determine the influence of the superficial gas flow rate on biofilter performance and to evaluate the phenol concentration profiles along the column, two series of continuous tests were carried out varying either the inlet phenol concentration, up to 1650 mg . m(-3), or the superficial gas flow rate, from 30 to 460 m(3) . m(-2) . h(-1). The elimination capacity of the biofilter is proved by a maximum volumetric phenol removal rate of 0.73 kg . m(-3) . h(-1). The experimental results are consistent with a biofilm model incorporating first-order substrate elimination kinetics. The model may be considered a useful tool in scaling-up a biofiltration system. Furthermore, the deodorization capacity of the biofilter was investigated, at inlet phenol concentrations up to 280 mg . m(-3) and superficial gas flow rates ranging from 30 to 92 m(3) . m(-2) . h(-1). The deodorization of the gas was achieved at a maximum inlet phenol concentration of about 255 mg . m(-3), operating at a superficial gas flow rate of 30 m(3) . m(-2) . h(-1). (c) 1996 John Wiley & Sons, Inc.

Local measurement of interfacial area, interfacial velocity and liquid turbulence in two-phase flow

Double sensor probe and hotfilm anemometry methods were developed for measuring local flow characteristics in bubbly flow. The formulation for the interfacial area concentration measurement was obtained by improving the formulation derived by Kataoka and Ishii. The assumptions used in the derivation of the equation were verified experimentally. The interfacial area concentration measured by the double sensor probe agreed well with one by the photographic method. The filter to validate the hotfilm anemometry for measuring the liquid velocity and turbulent intensity in bubbly flow was developed based on removing the signal due to the passing bubbles. The local void fraction, interfacial area concentration, interfacial velocity, Sauter mean diameter, liquid velocity, and turbulent intensity of vertical upward air–water flow in a round tube with an inner diameter of 50.8 mm were measured by using these methods. A total of 54 data sets were acquired consisting of three superficial gas flow rates, 0.015–0.076 m s −1, and three superficial liquid flow rates, 0.600, 1.00, and 1.30 m s −1. The measurements were performed at the three locations: L/ D=2, 32, and 62. This data is expected to be used for the development of reliable constitutive relations which reflect the true transfer mechanisms in two-phase flow.

Evaluation of phenol diffusivity through Pseudomonas putida biofilms: application to the study of mass velocity distribution in a biofilter

Phenol diffusivity through Pseudomonas putida biofilms with thickness ranging from about 9AE 10~6 to 90AE 10~6 m has been evaluated in order to carry out a kinetic study on phenol aerobic degradation in a biofilter. An average effective diffusivity of 8.92.10~12 m2 s~1 has been calculated at 20 iC, with no appreciable dependence on biofilm thickness. This value, that is only 0.6 % of that calculated in water at the same temperature, has been used to carry out a comparison between diffusion, convection and bioreaction mass velocities along the biofilter fed with air streams contaminated with different levels of phenol. Although diffusion through the biofilm is the limiting step at local level, biomass grows so abundantly within the support pores at high residence time that the most superficial active layers of biofilm are enough to transform nearly completely the substrate fed. At low residence time, on the contrary, the system is not able to face an evident situation of substrate overloading. Deodorization tests have also been carried out varying the support porosity, the superficial gas flow rate, and the starting phenol concentration in the polluted gaseous stream. This study could provide a general tool to model fixed-bed columns. List of symbols A m2

Macro‐kinetic investigation on phenol uptake from air by biofiltration: Influence of superficial gas flow rate and inlet pollutant concentration

Macro‐kinetic investigation on phenol uptake from air by biofiltration: Influence of superficial gas flow rate and inlet pollutant concentration