Oxygen Mass Transfer Coefficient kLa Research Articles

Influence of Interfacial Force Models and Population Balance Models on the kLa Value in Stirred Bioreactors

Optimal oxygen supply is vitally important for the cultivation of aerobically growing cells, as it has a direct influence on cell growth and product formation. A process engineering parameter directly related to oxygen supply is the volumetric oxygen mass transfer coefficient kLa. It is the influences on kLa and computing time of different interfacial force and population balance models in stirred bioreactors that have been evaluated in this study. For this investigation, the OpenFOAM 7 open-source toolbox was utilized. Firstly, the Euler–Euler model with a constant bubble diameter was applied to a 2L scale bioreactor to statistically examine the influence of different interfacial models on the kLa value. It was shown that the kL model and the constant bubble diameter have the greatest influence on the calculated kLa value. To eliminate the problem of a constant bubble diameter and to take effects such as bubble breakup and coalescence into account, the Euler–Euler model was coupled with population balance models (PBM). For this purpose, four coalescence and five bubble breakup models were examined. Ultimately, it was established that, for all of the models tested, coupling computational fluid dynamics (CFD) with PBM resulted in better agreement with the experimental data than using the Euler–Euler model. However, it should be noted that the higher accuracy of the PBM coupled models requires twice the computation time.

Microbubbles intensification and mechanism of wet air oxidation process of MDEA-containing wastewater

ABSTRACT In order to the intensification of gas–liquid mass transfer of MDEA-containing wastewater during wet air oxidation (WAO) process, the microbubbles and millimetre bubbles were applied by fine-pore sparger (5 and 20–30 μm) and single pore sparger (6.35 mm), respectively. Effect of the superficial gas velocity on the average microbubble size, gas holdup and oxygen mass transfer coefficient (KLa) of MDEA-containing wastewater at the ambient conditions was studied. The results showed that the microbubbles (less than 1 mm) were beneficial to enhance mass transfer process and had a higher dissolved oxygen concentration during WAO process of MDEA-containing wastewater owing to higher gas holdup and larger oxygen mass transfer coefficient. The COD removal ratio was 66% at low superficial gas velocity (ug = 0.3 cm/s) in WAO process by microbubbles, while it achieved at high superficial gas velocity (ug = 3.0 cm/s) by millimetre bubbles. The critical oxygen mass transfer coefficient KLa was 0.183 min−1 of MDEA-containing wastewater by 20–30 and 5 μm fine pore sparger, which was 2∼5 times more than that of single pore sparger (<0.1 min−1). The microbubbles could improve dissolved oxygen concentration and enhance the formation of hydroxyl radical at short time with atmospheric pressure. During the WAO process, the MDEA would be converted into intermediates including formic acid, acetic acid, ammonium, nitrite and nitrate. The WAO process with microbubbles could significantly improve the gas–liquid mass transfer performance at low superficial gas velocity and greatly reduce air consumption for MDEA-containing wastewater.

Do wastewater pollutants impact oxygen transfer in aerated horizontal flow wetlands?

Aerated treatment wetlands are an increasingly recognized nature-based technology for wastewater treatment that relies heavily on mechanical aeration. Although aeration-mediated oxygen transfer into the wastewater can be impeded by wastewater pollutants, little is known about the link between the volumetric oxygen mass transfer coefficient kLa and the organic carbon concentration of the wastewater in aerated wetlands. In this study, oxygen transfer experiments were carried out in a lab-scale gravel column using clean water and wastewater from a pilot-scale horizontal flow (HF) aerated wetland treating domestic sewage. The α-factor, which describes the ratio of the volumetric oxygen mass transfer coefficient kLa in wastewater to clean water, was reduced by increasing soluble chemical oxygen demand (CODs). The derived regression equation α=1.066-1.372E-3mgCODsL-1 was incorporated into a numerical process model to simulate the impact of the reduced oxygen transfer on a hypothetical HF aerated wetland. The simulations revealed that α and treatment efficacy for nitrogen were substantially reduced by CODs at low aeration (kLa of 1 h−1) and high influent wastewater strength (CODs of 300 mg L−1). At the same kLa and influent CODs concentration, longitudinal gradients of α and concentrations for dissolved oxygen (DO), NH4-N and NOx-N in the simulated wetland were shifted up to 21% of wetland length downstream. These effects decreased with increasing kLa and were found to be negligible at kLa > 3 h−1, which corresponds to an air flow rate of approximately 400 L m−2 h−1. Following this, higher organic carbon concentrations can reduce oxygen transfer in HF aerated wetland systems, thus resulting in decreased treatment efficacy.

Existence of a scaling relation in continuous cultures of Scheffersomyces stipitis: the steady states are completely determined by the ratio of carbon and oxygen uptake rates

BackgroundRecently, we showed that steady-state continuous cultures of S. stipitis follow the principles of growth on mixture of two complementary substrates. More precisely, when such cultures are fed with progressively higher concentrations of glucose sf at fixed dilution rate D = 0.1 h−1, oxygen mass-transfer coefficient kla = 50 h−1, and oxygen solubility c_{text{o}}^{*}, they transition from glucose- to oxygen-limited growth through an intermediate dual-limited regime in which both glucose and oxygen are limiting, and ethanol is produced without loss of glucose. It is, therefore, of considerable interest to characterize the dual-limited regime. We found that the dual-limited regime occurs precisely when the operating parameters D, sf, kla, and c_{text{o}}^{*} satisfy the relation Y_{text{os}} < Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right) < Y_{text{os}}^{prime }, where Yos and Y_{text{os}}^{prime } denote g of glucose consumed per g of oxygen consumed in the carbon- and oxygen-limited regimes. In this work, our goal was to determine if the above characterization of the dual-limited regime holds over a wider range of D, kla, and to understand why the dual-limited regime is determined by the dimensionless ratio Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right).ResultsTo this end, we performed the foregoing experiments at three additional dilution rates (D = 0.07, 0.15, and 0.20 h−1) and one additional mass-transfer coefficient (kla = 100 h−1). We find that the above characterization of the dual-limited regime is valid for these conditions as well. Furthermore, the boundaries of the dual-limited regime are determined by the dimensionless ratio Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right), because the steady-state concentrations are completely determined by this ratio. More precisely, if the steady-state concentrations of biomass, glucose, oxygen, and ethanol are suitably scaled, they collapse into a single curve with Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right) as the independent variable.ConclusionThe dual-limited regime is characterized by the relation Y_{text{os}} < Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right) < Y_{text{os}}^{{prime }} over the entire range of operating condition 0.07 h−1 ≤ D ≤ 0.20 h−1 and 50 ;{text{h}}^{ - 1} le k_{text{l}} a le 100;{text{h}}^{ - 1}. Since the effect of all operating parameters is embedded in the single parameter Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right), the dimensionless plot provides a powerful tool to compare, with only a handful of data, various ethanol-producing strains over a wide range of operating conditions.

Impact of nozzle operation on mass transfer in jet aerated loop reactors. Characterization and comparison to an aerated stirred tank reactor.

The impact of mass transfer on productivity can become a crucial aspect in the fermentative production of bulk chemicals. For highly aerobic bioprocesses the oxygen transfer rate (OTR) and productivity are coupled. The achievable space time yields can often be correlated to the mass transfer performance of the respective bioreactor. The oxygen mass transfer capability of a jet aerated loop reactor is discussed in terms of the volumetric oxygen mass transfer coefficient kLa [h-1] and the energetic oxygen transfer efficiency E [kgO2kW-1h-1]. The jet aerated loop reactor (JLR) is compared to the frequently deployed aerated stirred tank reactor. In jet aerated reactors high local power densities in the mixing zone allow higher mass transfer rates, compared to aerated stirred tank reactors. When both reactors are operated at identical volumetric power input and aeration rates, local kLa values up to 1.5 times higher are possible with the JLR. High dispersion efficiencies in the JLR can be maintained even if the nozzle is supplied with pressurized gas. For increased oxygen demands (above 120 mmolL-1h-1) improved energetic oxygen transfer efficiencies of up to 100 % were found for a JLR compared to an aerated stirred tank reactor operating with Rushton turbines.

The Study of Micro-Pressure Inner-Loop Bioreactor Oxygen Mass Transfer Characteristics

The oxygen mass transfer characteristics in a Micro-Pressure Inner-Loop bioreactor (MPR) were studied by clean water oxygenation experiment, the results show that when the aeration adopt by 0.1, 0.2, 0.4, 0.6 m3·h-1, respectively, the oxygen mass transfer coefficient KLa(20) in the reactor increases with the increase of the aeration. KLa(20) shows a good linear correlation with the aeration. The rate is 0.2128 h·m-3·min-1 and the correlation coefficient R=0.993. However, the trend of EO2 increases first and then decreases with the increase of aeration. When the aeration increased to 0.4 m3·h-1, the EO2 reaches the maximum. If aeration increases constantly, EO2 begin to decrease excessive aeration may lead to an increase in energy waste during reactor operation.

Oxygen Mass Transfer in Bubble Column Bioreactor

In the present study volumetric oxygen mass transfer coefficient kLa has been determined for biodegradation of phenol in a bubble column bioreactor. Experimental studies have been carried out at different i) feed concentrations of phenol, ii) air flow rates and iii) feed flow rates. Dynamic method has been used to determine the oxygen mass transfer coefficient. The mass transfer coefficient for oxygen obtained is in the range of 0.00513 – 0.01793 s-1. kLa was found to increase with increase in air flow rates and decrease with increase in feed concentration of phenol. The values obtained in this work are compared with the values available in literature. A mathematical correlation is derived for kLa in terms of dimensionless numbers.

Aerobic biodegradation of a mixture of sulfonated azo dyes by a bacterial consortium immobilized in a two‐stage sparged packed‐bed biofilm reactor

AbstractThe biodegradation of the sulfonated azo dyes, Acid Orange 7 (AO7) and Acid Red 88 (AR88), by a bacterial consortium isolated from water and soil samples obtained from sites receiving discharges from textile industries, was evaluated. For a better removal of azo dyes and their biodegradation byproducts, an aerobically operated two‐stage rectangular packed‐bed biofilm reactor (2S‐RPBR) was constructed. Because the consortium's metabolic activity is affected by oxygen, the effect of the interstitial air flow rate QGI on 2S‐RPBR's zonal values of the oxygen mass transfer coefficient kLa was estimated. In the operational conditions probed in the bioreactor, the kLa values varied from 3 to 60 h−1, which roughly correspond to volumetric oxygen transfer rates, dcL/dt, ranging from 20 to 375 mg O2 L−1h−1. Complete biodegradation of azo dyes was attained at loading rates BV,AZ up to 40 mg L−1d−1. At higher BV,AZ values (80 mg L−1 d−1), dye decolorization and biodegradation of the intermediaries 4‐amino‐naphthalenesulphonic acid (4‐ANS) and 1‐amino‐2‐naphthol (1‐A2N) was almost complete. However, a diminution in COD and TOC removal efficiencies was observed in correspondence to the 4‐aminobenzenesulfonic acid (4‐ABS) accumulation in the bioreactor. Although the oxygen transport rate improved the azo dye mineralization, the results suggest that the removal efficiency of azo dyes was affected by biofilm detachment at relatively high QGI and BV,AZ values. After 225 days of continuous operation of the 2S‐RFBR, eight bacterial strains were isolated from the biofilm attached to the porous support. The identified genera were: Arthrobacter, Variovorax, Agrococcus, Sphingomonas, Sphingopyxis, Methylobacterium, Mesorhizobium, and Microbacterium.

Carrier effects on oxygen mass transfer behavior in a moving‐bed biofilm reactor

AbstractThis study investigates the carrier effects on the oxygen mass transfer behavior of a gas–liquid biofilm surface, and aims to provide evidence for parameter optimization in the practical operation of a moving‐bed biofilm reactor (MBBR) during the coking‐plant wastewater process. By using the dynamic oxygen dissolution method, the volumetric oxygen mass transfer coefficient KLa was measured by varying the suspended carrier stuffing rate and the intensity of aeration. Within the range of fluidizable flow rate, the efficiency of oxygen mass transfer increased with suspended carrier stuffing rate, and KLa reached its peak value when the stuffing rate was 40%. KLa has an increasing trend with an increase of the aeration intensity, but high aeration intensity was not favorable for reactor operation. Better oxygen mass transfer effect and higher oxygen transfer efficiency could be achieved when the aeration intensity was 0.3 m3 h−1 and the suspended carrier stuffing rate was 30–50%. The possible mechanisms that can account for carrier effects on oxygen mass transfer are the changes in the gas–liquid interfacial area. The ammonia nitrogen removal performance of the coking‐plant wastewater in MBBR was satisfied by using the above‐suggested conditions. Copyright © 2009 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Scale-up and application of a cyclone reactor for fermentation processes

A cyclone reactor for microbial fermentation processes was developed with high oxygen transfer capabilities. Three geometrically similar cyclone reactors with 0.5 l, 2.5 l and 15 l liquid volume, respectively, were characterized with respect to oxygen mass transfer, mixing time and residence time distribution. Semi-empirically correlations for prediction of oxygen mass transfer and mixing times were identified for scale-up of cyclone reactors. A volumetric oxygen mass transfer coefficient k L a of 1.0 s−1 (available oxygen transfer rate with air: 29 kg m−3 h−1) was achieved with the cyclone reactor at a volumetric power input of 40 kW m−3 and an aeration gas flow rate of 0.2 s−1. Continuous methanol controlled production of formate dehydrogenase (FDH) with Candida boidinii in a 15 l cyclone reactor resulted in more than 100% improvement in dry cell mass concentration (64.5 g l−1) and in about 100% improvement in FDH space-time yield (300 U l−1 h−1) compared to steady state results of a continuous stirred tank reactor.