Increase Of kLa Research Articles

Effect of ultrasonic waveforms on gas–liquid mass transfer in microreactors

AbstractThis study aims to investigate the effect of ultrasonic waveforms on the gas–liquid mass transfer process. For a given load power (P), continuous rectangular wave yielded stronger bubble oscillation and higher mass transfer coefficient (kLa) than continuous triangular and sinusoidal wave. For pulsed ultrasound, the kLa decreased monotonically with decreasing duty ratio (D), resulting in weak enhancement at low D (≤33%). For a given average load power (PA), concentrating the P for a shorter period resulted in a higher kLa due to stronger cavitation behavior. For a given PA and D, decreasing the pulse period (T) led to an increase in kLa, which reached a constant high level when the T fell below a critical value. By optimizing the D and T, a kLa equivalent to 92% of that under continuous ultrasound was obtained under pulsed ultrasound at a D of 67%, saving 33% in power consumption.

Laboratory scale cultivation of Salinispora tropica in shake flasks and mechanically stirred bioreactors.

Marine actinomycetes from the genus Salinispora have an unexploited biotechnological potential. To accurately estimate their application potential however, data on their cultivation, including biomass growth kinetics, are needed but only incomplete information is currently available. This work provides some insight into the effect of temperature, salinity, nitrogen source, glucose concentration and oxygen supply on growth rate, biomass productivity and yield of Salinispora tropica CBN-440T. The experiments were carried out in unbaffled shake flasks and agitated laboratory-scale bioreactors. The results show that the optimum growth temperature lies within the range 28-30 °C, salinity is close to sea water and the initial glucose concentration is around 10 g/L. Among tested nitrogen sources, yeast extract and soy peptone proved to be the most suitable. The change from unbaffled to baffled flasks increased the volumetric oxygen transfer coefficient (kLa) as did the use of agitated bioreactors. The highest specific growth rate (0.0986 h-1) and biomass productivity (1.11 g/L/day) were obtained at kLa = 28.3 h-1. A further increase in kLa was achieved by increasing stirrer speed, but this led to a deterioration in kinetic parameters. Improvement of S. tropica biomass growth kinetics of was achieved mainly by identifying the most suitable nitrogen sources and optimizing kLa in baffled flasks and agitated bioreactors.

N2O generation via nitritation at different volumetric oxygen transfer levels in partial nitritation-anammox process

Volumetric oxygen transfer coefficient (KLa) is an important parameter affecting oxygen transfer and nitrogen conversion during biological wastewater management. However, how the generation of nitrous oxide (N2O) responds to the variation of KLa levels is unclear. In this study, N2O generation via the main pathway, which was confirmed to be nitritation, at different KLa levels was investigated in a partial nitritation-anammox (PN-A) reactor run as a sequencing batch reactor. The results showed that N2O generation in the PN-A process, which was described using the maximum N2O generation rate (MGR), the duration of N2O generation (DG), and the N2O generation factor, was affected by KLa levels and the limiting factors of the nitritation process, i.e., oxygen transfer and nitritation activity. DG always decreased with the increase of KLa, indicating that the process of N2O generation lasted for a shorter time at higher KLa. With the increase of KLa, MGRs remarkably increased when oxygen transfer was the limiting factor, while slightly reduced when nitritation activity became the limiting factor. As a result, with the increase of KLa levels, the N2O generation factor remarkably increased at lower KLa levels when oxygen transfer was the limiting factor, but significantly reduced at higher KLa levels when nitritation activity became the limiting factor. These results revealed how N2O generation via nitritation responded to different KLa levels and the limiting factors of the nitritation process. Based on these results, practical suggestions to mitigate the N2O generation in PN-A process operated in a batch mode were recommended.

Gas–liquid interfacial area and its influence on oxygen transfer coefficients in a simulated hydrocarbon bioprocess in a bubble column reactor

AbstractBACKGROUNDThe overall volumetric oxygen transfer coefficient (KLa) is a critical parameter in evaluating the oxygen transfer in any aerobic bioprocess, e.g. a hydrocarbon‐based bioprocess. However, KLa comprises the oxygen transfer coefficient (KL) and the gas–liquid interfacial area, which are commonly affected by operating conditions of a bioprocess system. Therefore, this study has experimentally measured KLa and interfacial area in a model hydrocarbon‐based bioprocess under a range of operating conditions such as alkane (n‐C14–20) concentration (HC), solids (cornflour) loading (SL) and superficial gas (air) velocity (UG) in a bubble column reactor (BCR).RESULTSIt was found that the value of KLa increased with increasing UG in the BCR. The most significant impact on KLa was obtained through an increase in UG, where an increase of UG from 1 to 3 cm s−1 resulted in a 133% increase in KLa. Furthermore, a relationship between KLa and SL was identified and showed an optimal level at SL of 3 g L−1, indicating increased dispersion viscosity in the system as SL increased up to a certain point. However, HC showed an insignificant change in both the interfacial area and KLa with the range considered (2.5 to 20% v/v) of HC.CONCLUSIONSThis study expands on previous work by including an inert solid (cornflour) phase, which ultimately furthers our understanding of the effect of the solids type and size, and operational conditions (HC, SL and UG) on the behaviour of overall oxygen transfer in a bubble column hydrocarbon bioprocess.

TiO2 nanofluid for oxygen mass transfer intensification in pulsed plate column

Oxygen mass transfer coefficient (kLa) is a key parameter in biochemical, photochemical, or other reactors. Pulsed plate column (PPC) has a potential application as a gas-liquid contactor. In the present work, TiO2 nanofluid mediated oxygen mass transfer intensification in PPC is studied. The pulsing velocity (A × f) and nanoparticle loading in volume percent (Φ) have an interacting effect on kLa. The effect of Φ on kLa, showed a dual regime of an increase in kLa with an increase in Φ up to a critical loading (Φ CL ) and decrease in kLa on further increase in Φ. The effect of (A × f) on kLa showed three regimes. Φ CL & kLa were found to decrease as the nanoparticle's size increased. Dimensionless correlations based on multiple regression analysis and ANN models were developed to predict the kLa values in PPC with TiO2 nanofluids.

Power consumption and gas–liquid mass transfer in a hot-sparged three-phase stirred reactor

The volumetric mass transfer coefficient (kLa) is one of the most important parameters for design and scale-up of gas–liquid–solid three-phase mechanically stirred tank reactors. In this work, the effects of agitation speed, superficial gas velocity, temperature, and volumetric solid concentration (CV) on the power consumption and kLa were discussed and quantitatively analyzed. In an air-water-glass beads system, the gassed power number (NPG) and kLa increase remarkably with the temperature. At ambient temperature, kLa decreases significantly with increasing CV with a power law exponent of 3.10, but this decreasing effect becomes weaker at higher temperatures. In this hot-sparged three-phase system, the addition of solids slows down the increasing tendency of NPG with the temperature, but enhances the increasing tendency of kLa. The increase in temperature weakens the decreasing effect of CV on kLa. Compared with the effect of temperature on NPG and kLa, the effect of CV is nearly negligible. As a practical guide to industrial applications, it is more effective to enhance the gas–liquid mass transfer by increasing the temperature or power consumption than by changing CV or increasing superficial gas velocity. The empirical correlations of power consumption and kLa obtained in this work can provide helpful guidance for the industrial design and operation of hot-sparged three-phase stirred tank reactors.

Intensification of liquid-liquid extraction in a tubular sono-extractor using 1.7 MHz ultrasound and SiO2 nanoparticles

The purpose of the present study is to investigate the effect of high-frequency ultrasound propagation in a sono-extractor to augment liquid-liquid mass transfer in the presence of SiO2 nanoparticles. For this purpose, four 1.7 MHz ultrasound transducers are placed on the body of the sono-extractor and the effect of transducer locations on the mass transfer characteristics are evaluated. Using ultrasound and nanoparticles showed a higher effect on the increase in the mass transfer characteristics compared with only using ultrasound or nanoparticles. Activation of two bottom transducers is more efficient from viewpoint of lower energy consumption. The effect of increasing the power of ultrasound (P) is investigated and measurable increase in KLa and E are obtained. The effect of various weight fraction of nanoparticles (ϕ = 0.1–2% wt.) on KLa and E is evaluated and ϕ = 2% wt showed higher values. Maximum values of KLa (2.81 × 10−3 1/s) and E (0.943) are achieved for activation of two bottom ultrasound transducers with P = 9.5 W and ϕ = 2% wt.

Concentration effects of main components of synthetic culture media on oxygen transfer in bubble column bioreactors

The effects of the concentration of three kinds of constituents of aqueous culture media (mixed electrolytes, glucose, sucrose, and commercial antifoam) on oxygen transfer were studied in a bubble column bioreactor. The volumetric mass transfer coefficient (kLa) and the specific interfacial area (a) were measured at five superficial gas velocities under the homogeneous bubbling regime. The mixed electrolytic salts (0 ≤ ionic strength ≤ 0.4126 M) and, to a lesser extent, the glucose (0 ≤ c ≤ 0.222 M) caused the increase of kLa up to a maximum value, followed by a either a slight decrease, or no significant change. The higher values of kLa corresponded to critical concentrations, indicating a maximum inhibition of bubble coalescence. The kLa was governed by changes in interfacial area up to the critical concentrations, where the dominant contribution shifted to kL; at the studied concentrations, the sucrose had no significant effect on kLa. Finally, low concentrations of commercial antifoam caused an overall decrement in kLa due entirely to the hindering effects of antifoams on kL.

Dynamics and mass transfer characteristics of CO2 absorption into MEA/[Bmim][BF4] aqueous solutions in a microchannel

Abstract The evolution of the size and velocity of CO2 bubble absorbed into MEA/[Bmim][BF4] aqueous solutions was in-line investigated by a high-speed camera. The coupling effect between the variation of volume and the velocity of bubbles was highlighted. According to the evolution of average mass transfer coefficient, the mass transfer performance in the whole flow process was studied. The results indicated that both the length and velocity of bubble decreased gradually in the flow process due to mass transfer. A linear correlation between the relative velocity (the ratio of average bubble velocity to two-phase superficial velocity) and the relative lost length (the ratio of the lost length to the initial length) was proposed. The increase in overall liquid-phase volumetric mass transfer coefficient (kLa) at higher gas flow rate could be ascribed to the shorter residence time and larger specific surface area. While the increase of kLa at higher liquid flow rate and MEA or [Bmim][BF4] concentration was resulted from intensified mass transfer. An empirical correlation of overall liquid-phase volumetric mass transfer coefficient was proposed taking the gas-phase Reynolds number, liquid-phase Reynolds number and Damkohler number into account.

Enhancing recovery of ammonia from rare earth wastewater by air stripping combination of microwave heating and high gravity technology

Currently, a larger amount of NH3-N wastewater is generated in the rare earth manufacture process. Air stripping is one of the effective technologies for ammonia removal, but low separation efficiency and huge stripping tower rendering the process techno-economically inefficient. Hence, microwave heating and high gravity separation are employed to intensify air stripping process. Microwave dielectric characteristics for this solution indicate strong absorption, which can reduce the heating time rapidly. The effect of main operating parameters such as the rotational speed (ω), liquid flow rate (QL), gas flow rate (Qg), and stripping temperature (T)) on the volumetric liquid mass transfer coefficient (KLa) and stripping efficiency (η) are assessed. Results show that KLa increase significantly with increasing gas flow rate, followed by rotating speed and liquid flow rate. However, the η decrease with increase in QL due to reduction in the liquid hydraulic retention time. Under the optimal conditions, KLa and η could reach 0.0061 s−1 and 99.3%, respectively. Dimensionless empirical correlations are developed relating process parameters with the KLa and η, which shows a good agreement of the model equation with the experimental data. In addition, high gravity air stripping and conventional technologies are compared.

A three-phase Robinson-Mahoney reactor as a tool for intrinsic kinetic measurements: Determination of gas-liquid hold up and volumetric mass transfer coefficient

The liquid hold-up, εL, as well as the volumetric gas-liquid mass transfer coefficient, kLa, have been measured in a bench-scale, gradientless, three-phase Robinson-Mahoney reactor at high temperature and pressure conditions using realistic feeds to verify its suitability for the acquisition of intrinsic chemical kinetics. kLa has been determined via the so called ‘dynamic gas absorption technique’. Even without stirring, the reactor internals cause already interaction between the gas and the liquid, resulting in a mass transfer coefficient amounting to 1.17×10−2s−1. Initially, a small increase in the agitator speed results in a moderate increase of kLa. However, once an agitator speed of 10rps is reached, kLa was found to increase more rapidly. This observation is captured in the following correlation of the volumetric gas-liquid mass transfer with the power input per volume: kLa=1.06×10−2(P/V)1.13+1.17×10−2. The trend as well as the order of magnitude of our results are similar to those reported by Pitault et al. at ambient conditions, albeit that the kLa in the absence of stirring is significantly higher and the variation with the agitator speed is less pronounced in our case. The liquid hold-up was investigated at ambient conditions using a H2-water mixture as well as at high temperature and pressure (HTP) conditions using a H2-Halpasol™ mixture by injecting an impulse of tracer or by imposing a step change in the liquid phase fed to the reactor, respectively. At HTP conditions, the temperature and total pressure, which were varying between 523–583K and 2.5–5.5MPa respectively, were found to only exert a minimal impact on the liquid hold-up, i.e., the liquid hold-up remained relatively constant at about 70%. A decrease to a liquid hold-up of about 50% was observed by changing the volumetric inlet gas-liquid ratio from 5 to 250m3NTPm−3 at these high temperature and pressure conditions. At ambient conditions, a change in the volumetric inlet gas-liquid ratio from 0 to 580m3NTPm−3 was necessary to achieve a similar reduction in liquid hold-up. This difference was attributed to differences in surface tension of the fluids used rather than in operating conditions such as temperature and total pressure. The experimentally observed liquid hold-up was higher compared to that obtained from vapour-liquid equilibrium calculations at similar volumetric inlet gas liquid ratio. The evolution of the liquid hold-up with increasing volumetric inlet gas-liquid ratio suggests the phase within the reactor changes from completely filled liquid with gas bubbles dispersed at low inlet gas-liquid ratio and potentially to continuous gas phase with entrained liquid droplets at very high inlet gas-liquid ratio.

Liquid–liquid mass transfer property of two inline high shear mixers

The liquid–liquid mass transfer properties of two inline high shear mixers (HSMs) were investigated by the liquid–liquid extraction (LLE) method using the water-benzoic acid-kerosene system. The characteristic parameters of the extraction efficiency (E) and volumetric mass transfer coefficient (KLa) were measured under different conditions involving the rotor speed, two liquid phase flow rates, and surfactant concentration. The results indicate that both E and KLa increase with the rotor speed and aqueous-phase flow rate. The value of E decreases sharply but KLa increases slightly with the organic-phase flow rate. Both E and KLa increase first and then decrease with the surfactant concentrations. Dimensionless correlations for the Sherwood number were obtained to provide guidance on the optimization and scale-up design of these kinds of inline HSMs.

Investigation of extraction fraction in confined impinging jet reactors for tri-butyl-phosphate extracting butyric acid process

The extraction fraction E and overall volumetric mass transfer coefficient kLa of TBP extracting butyric acid process in confined impinging jet reactors (CIJR) with two jets were investigated. The main variables tested were the concentration of tri-butyl-phosphate (TBP) and butyric acid, the impinging velocity V, the impinging velocity ratio of two phases Vorg/Vaq, the nozzle inner diameter di and the distance L between the jet axes and the top wall of the impinging chamber. The results showed that E and kLa increase with an increase of the impinging velocity V, the concentration of TBP Corg, and the impinging velocity ratio Vorg/Vaq. However, E and kLa decrease with an increase of the inner diameter di from 1 to 2mm, the concentration of butyric acid Caq from 0.5% (v/v) to 2% (v/v). The factor L ranging from 3 to 11mm has a negligible effect on E and kLa. A correlation on these variables and kLa was proposed based on the experimental data. These results indicated good mass transfer performance of CIJR in the extraction operation.

Impact of suspended solids on the activated sludge non-newtonian behaviour and on oxygen transfer in a bubble column

This paper presents the experimental study and analysis performed in order to better understand the link between activated sludge properties, rheological behaviour and oxygen transfer. The experimental set-up consists of a bubble column of 0.3m3 continuously fed with activated sludge and a capillary rheometer, installed in two different wastewater treatment plants: a conventional activated sludge plant and a membrane bioreactor. In the bubble column, equipped with a fine bubble diffuser, the overall gas hold-up (ϵG) and volumetric oxygen transfer coefficients (kLa) were measured. A fraction of the column outflow was sent to the capillary rheometer where the activated sludge rheological behaviour was investigated. Several properties of the studied activated sludge were characterised (MLSS, MLVSS, soluble COD, surfactants, surface tension, soluble cations) and their impact on rheology and oxygen transfer was examined.The experimental data showed that the parameters K and n, from the Ostwald-de Waele rheological model, were strongly related to the suspended solids concentration (in terms of MLSS or MVLSS). An increase in kLa was observed as the superficial gas velocity (UG) was increased. Compared to clean water, the kLa coefficient was lower in activated sludge and still reduced with an increase of the MLSS concentration. This reduction could be partially attributed to a lower gas holdup (εG) associated with an increase in the sludge apparent viscosity (μapp) leading to a reduction of the specific interfacial area (a). Subsequently, an estimation of the mean shear rate exerted by the bubble swarm in the column allowed to evaluate the sludge apparent viscosity (μapp) of the mixed liquor at a given superficial gas velocity and MLSS concentration. Finally an empirical correlation linking kLa to the superficial gas velocity (UG) and the sludge apparent viscosity was obtained for both types of sludge. The good agreement between the experimental and the fitted data suggests that kLa can be estimated from the superficial gas velocity and the rheological behaviour.

Kinetic evidence for the effect of salts on the oxygen solubility using laboratory prototype aeration system

This study is dedicated to an accurate evaluation of thermodynamic and kinetic aspects in the oxygenation process of water/salt solutions. Oxygenation has been analyzed by means of kinetic study of oxygen dissolution from the oxygen mass transfer coefficient (KLa) and oxygen transfer rate (SOTR). A stirred, submerged aerated 4-liter system has been designed and the operational conditions have been optimized by studying the influence of hydraulic head, air flow and salinity of water using an optical oxygen sensor. Concerning the thermodynamic phase equilibria, experimental and modeling results are obtained from different binary systems (water/air) and ternary systems (water/air/salts). This information is necessary to predict the composition of the gas phase during the process and is also important for an implementation in a process simulation. The oxygen mass transfer coefficients were firstly measured, by monitoring in the time the oxygen concentration in various synthetic liquid phases containing different salts (NaCl, KCl, LiCl). When compared to clean water, noticeable increase of KLa was observed; the variation of KLa and SOTR with the solution salinity was modeled and found dependent on the nature of cation in the salt added. For all cases, an increase of KLa with increasing salinity was observed but, when the results are correlated with salt molar concentration, very important relation attributed to different salts has been obtained. The present study clearly confirmed the importance to define the experimental conditions before to describe and to model appropriately the gas–liquid mass transfer phenomena.

MASS TRANSFER STUDIES IN AN AGITATED VESSEL WITH RADIAL-AXIAL IMPELLER COMBINATION

The effect of radial-axial impeller combination in dual configuration was tested for gas liquid mass transfer coefficient (KLa) and compared with that of dual axial impeller (30o Pitched blade) combination. The trials were conducted at gas rates of 2litre/min to 10litre/min and agitation rates of 180rpm to 360rpm.Good mass transfer coefficient was obtained by replacing the lower axial impeller with a radial impeller. Rushton Turbine and Curved blade (half pipe) impeller were used in replacing the lower axial impeller. Amongst the two radial impellers, curved blade resulted in higher KLa than Rushton Turbine at highest gassing rate tested. About 15-35% and 20-48% increase in KLa was observed by replacing lower pitched blade impeller with Rushton and Curved blade impeller each. The results from the present study shows the capability of replacing lower axial impeller with Radial impeller and retrofitting existing lower Rushton Turbine with Curved blade impeller.

Influence of fluid-mechanical parameters on volumetric mass transfer coefficient in a spout–fluid bed with a draft tube

In order to apply a spout–fluid bed reactor with a draft tube for the nitrification process of wastewater treatment, the influence of fluid-mechanical parameters on volumetric mass transfer coefficient was analyzed. Experiments were carried out in a 1000mm high 2D semi-column with a draft tube. The draft tube had a cross section of 50×50mm2 and the annular region of the reactor had a cross section of 50×140mm2. The work described in this paper reports the influence of several fluid-mechanical parameters including liquid and gas flow rates, particle diameters and particle circulation on volumetric mass transfer coefficient, kLa. The liquid flow ranged from 3.5–4.5m3/h and gas flow ranged from 300 to 800L/h. The particles used were glass spheres with diameters of 3, 4, 5 and 6mm. Tap water and air were used as the liquid and gas phase, respectively. The experimental results have shown that particle circulation through the draft tube contributes to kLa increase as well as the increase in the particle diameter, the gas and liquid flow rates.

DISTRIBUTION OF OXYGEN TRANSFER RATES IN STIRRED BIOREACTOR FOR DIFFERENT FERMENTATION BROTHS - OXYGEN-VECTOR DISPERSIONS

The oxygen transfer into the fermentation broths could be improved in presence of oxygen-vectors, without intensification of mixing or aeration. The experimental results for simulated, P. shermanii and S. cerevisiae broths indicated the significant increase of kLa, by adding n-dodecane, but the magnitude of this effect depends especially on the cells affinity for hydrocarbon droplets. Therefore, due to the higher affinity of yeasts cells for hydrocarbon droplets during their entire growth cycle, the increase of oxygen mass transfer rate was lower and the influence of specific power input was different than those recorded for simulated or bacterial broths. By means of the experimental data, mathematical correlations describing the influences of the main parameters on kLa have been proposed for each studied fermentation systems at different positions on the broths height, with an average deviations varying between ±6.72% and ±6.93%.

Relationship between acceleration of hydroxyl radical initiation and increase of multiple-ultrasonic field amount in the process of ultrasound catalytic ozonation for degradation of nitrobenzene in aqueous solution

The synergetic effect between ozone and ultrasound can enhance the degradation of nitrobenzene and removal efficiency of TOC in aqueous solution, and the degradation of nitrobenzene follows the mechanism of hydroxyl radical (OH) oxidation. Under the same total ultrasonic power input condition, the degradation rate of nitrobenzene (kNB), the volumetric mass transfer coefficient of ozone (kLa), and the initiation rate of OH (kOH) increases with introduction of additional ultrasonic field (1–4) in the process of ozone/ultrasound. The increasing amount of ultrasonic fields accelerates the decomposition of ozone, leading to the rapid appearance of the maximum equilibrium value and the decrease in the accumulation concentration of ozone in aqueous solution with the increasing reaction time. The increase in mass transfer of gaseous ozone dissolved into aqueous solution and the acceleration in the decomposition of ozone in aqueous solution synchronously contribute to the increase of kLa. The investigation of mechanism confirms that the increasing amount of ultrasonic fields yields the increase in cavitation activity that improves the mass transfer and decomposition of ozone, resulting in acceleration of OH initiation, which determines the degradation of nitrobenzene in aqueous solution.

Gas–Liquid Mass Transfer Characteristics in Two Inline High Shear Mixers

The mass transfer characteristics of a gas–liquid continuous flow system accompanied by reaction in two commercial inline high shear mixers (HSMs) were evaluated by the sulphite oxidation method. The gas–liquid specific interfacial area, a, and the volumetric mass transfer coefficient, kLa, were measured under different operating conditions involving the rotor speed, the liquid flow rate, the gas flow rate and the surface tension. The results indicate that a and kLa increase with the rotor speed and liquid flow rate due to an increase of turbulence intensity. Both parameters increase slightly first but then decrease with the gas flow rate. For either the dual rows ultrafine-toothed or the single-row blade-screen configuration of HSMs, a increases while kLa decreases with the surfactant concentration, and both of them tend to be changeless at the surfactant concentration higher than 30 mg/L. Correlations for the specific interfacial area and the Sherwood number are obtained to guide the process design and ...