Volumetric Oxygen Transfer Coefficient kLa Research Articles

Oxygen mass transfer and post-denitrification in a modified rotating drum biological contactor

A lab-scale rotating drum biological contactor (RDBC) was implemented and accessed concerning abiotic oxygen transfer rate and treatment capacity for domestic sewage. The dissolved oxygen (DO) transfer test showed that the rotational speed and the submergence level of drum greatly affected the DO in bulk liquid with the volumetric oxygen transfer coefficient KLa of 2.3-25.9 h−1, effectively achieving anaerobic/anoxic/aerobic conditions in the reactor. The post-denitrification process was proposed using the RDBC to degrade the synthetic nitrogen-containing sewage wastewater. When the hydraulic retention time was 16 h, the flow distribution ratio was 3:1 and the anoxic rotational speed was 6 rpm, the effluent NH4+-N and TN were 2.3 mg L−1 and 7.3 mg L−1 with the NH4+-N and TN removal efficiency of 94.1% and 80.9%, respectively. The Nitrosomonas europaea and Candidatus Kuenenia dominated in the anoxic carried-biofilm, indicating that the coexisting of aerobic and anaerobic ammonium oxidation bacteria enhanced the TN loss in the post-anoxic tank. This study provides an innovative non-aeration technology for biological wastewater treatment and has implications to simplify operation and save energy.

A dynamical model of an aeration plant for wastewater treatment using a phenomenological based semi-physical modeling methodology

Diffused aeration is a sensitive process for wastewater treatment. Because of the nonlinearity and complexity of aerator dynamics due to microorganism metabolism and oxygen transfer, reliable mathematical models are needed to perform control-oriented tasks. To this end, in this study we develop a phenomenological based semi-physical model (PBSM) to predict and describe the dynamic behavior of the oxygen transfer in a diffused aeration process by means of a formal modeling methodology. This model will then be validated by using data from an aeration pilot plant. In this paper, we also show a lack of agreement in the literature in terms of the different available ways to represent the volumetric oxygen transfer coefficient kLa. Reasonable agreement between the developed model and plant data is found by considering a phenomenological approach of the kLa instead of considering many of the available empirical correlations in the literature.

Modeling and simulation of xylitol production in bioreactor by Debaryomyces nepalensis NCYC 3413 using unstructured and artificial neural network models

This study examines the use of unstructured kinetic model and artificial neural networks as predictive tools for xylitol production by Debaryomyces nepalensis NCYC 3413 in bioreactor. An unstructured kinetic model was proposed in order to assess the influence of pH (4, 5 and 6), temperature (25°C, 30°C and 35°C) and volumetric oxygen transfer coefficient kLa (0.14h−1, 0.28h−1 and 0.56h−1) on growth and xylitol production. A feed-forward back-propagation artificial neural network (ANN) has been developed to investigate the effect of process condition on xylitol production. ANN configuration of 6-10-3 layers was selected and trained with 339 experimental data points from bioreactor studies. Results showed that simulation and prediction accuracy of ANN was apparently higher when compared to unstructured mechanistic model under varying operational conditions. ANN was found to be an efficient data-driven tool to predict the optimal harvest time in xylitol production.

MicrOLED-photobioreactor: Design and characterization of a milliliter-scale Flat-Panel-Airlift-photobioreactor with optical process monitoring

Small-scale cultivation systems with real-time-monitoring of suspension parameters are important for high-throughput bioprocess development. This manuscript describes the design and characterization of a new photobioreactor (PBR) approach using 3D-printing and organic light emitting diodes (OLEDs) in the design step. The structurally complex miniaturized PBR periphery was manufactured from polyamide using the selective laser sintering technology. The MicrOLED-PBR - the first Flat-Panel-Airlift photobioreactor (FPA-PBR) with a working volume below 20mL - was equipped and validated with non-invasive optical sensors for cell- (microalgal dry weight concentration and chlorophyll fluorescence) and suspension parameters (pH, dO2 and dCO2) allowing multiparametric high-resolution physiological studies of microalgae growth at low photon flux densities. The OLED modules used in the MicrOLED-PBR were characterized with respect to their spectral photosynthetically active radiation efficiency (35.31%), maximum photon flux density (83μmolm−2s−1) and resulting photon flux density profiles across the layer thickness of the FPA-cultivation chamber (10mm) according to Lambert-Beers law (150μmolm−2s−1 for dual-plane external illumination). The hydrodynamic properties of the FPA-cultivation chamber, i.e. its volumetric oxygen transfer coefficient kLa (1.5–57h−1), superficial gas velocity (0.8–42mh−1), mixing time (1.5–34.5s) and gas hold-up (0.016–0.2) were comparable to those for lab- and production-scale FPA-PBRs at volumetric aeration rates of 0.5–5.0Lh−1. The application of the MicrOLED-PBR was demonstrated for optimizing the CO2 conditions during batch-mode growth of Chlamydomonas reinhardtii 11-32b. By analyzing the suspension dynamics in real-time limitations of dissolved carbon dioxide were identified at a CO2 amount of 0.1vol% whereas 2.0vol% CO2 was identified as optimum conditions for growing C. reinhardtii 11-32b in the MicrOLED-PBR.

Micro-bubble aeration in turbulent stirred bioreactors: Coalescence behavior in Pluronic F68 containing cell culture media

Micro-bubble aeration in stirred bioreactors represents a suitable method that allows sufficient oxygen transfer without the necessity of intense stirring. However, understanding of micro-bubble behavior in real culture media remains a crucial factor in the interpretation of possible cell/bubble interactions and the scale up of bioprocesses. The present study focuses the impact of various stirrer configurations, sparging rates, as well as 2 different bioreactor scales on micro-bubbles in SF-900 II insect culture medium, which represents a suitable example for Pluronic F68 supplemented culture media commonly applied in cell culture processes. Obtained results for the first time demonstrate a coalescence determined increase in bubble Sauter diameter d32 from 187 to 211µm with increasing stirring intensities. The development of an empirical model reveals d32∝V0.4(P/V)0.4, were P represents the stirrer induced power input and V the medium volume. The results were interpreted a consequence of micro-bubble entrapment in turbulent flow structures. Additional studies on the volumetric oxygen transfer coefficient kLa demonstrate a strong impact of axial flow impellers and indicate that increases in kLa can be attributed to an improvement of the gas–liquid oxygen transfer rather than an increased specific gas surface area due to bubble breakup.

Impact of energy supply and oxygen transfer on selective lipopeptide production by Bacillus subtilis BBG21

The influence of power dissipation and volumetric oxygen transfer coefficient kLa on Bacillus subtilis productivity of lipopeptides surfactin and fengycin was studied in shake flasks in view of scaling-up of this fermentation process. The experiments performed with different flask sizes, relative filling volumes, and shaking frequencies confirmed clearly that lipopeptide production changed in function of power dissipation, via interfacial gas–liquid contact surface and oxygen supply. It was demonstrated that kLa is the key parameter controlling the productivity and the selectivity of the bioreaction. Varying the oxygen transfer conditions, the synthesis could be oriented to mixed production or to surfactin mono-production. The fraction of surfactin towards total lipopeptides produced and the maximal surfactin production both increased with kLa increase (surfactin concentration about 2gL−1 at kLa=0.04–0.08s−1), while the maximal fengycin production (fengycin concentration about 0.3gL−1) was obtained at moderate oxygen supply (kLa=0.01s−1).

Effect of silicone oil fraction and stirring rate on methane degradation in a stirred tank reactor

AbstractBACKGROUND: Methane is an important greenhouse gas and its biological oxidation constitutes a cost‐effective alternative for low concentration emissions. Nevertheless, due to its low aqueous solubility (similar to oxygen), gas mass transfer is often the limiting step. Two‐phase partition bioreactors (TPPBs), have been proposed to increase the biodegradation of poorly soluble compounds. In this work, the effect of stirring rate and silicone oil fraction on abiotic volumetric oxygen transfer coefficient kLa and methane elimination capacity (EC) by a methanotrophic consortium in a stirred tank was studied.RESULTS: In abiotic conditions, the kLa increased when increasing both factors. Nevertheless, a negative effect on kLa was observed with silicone oil at the highest stirring rate (800 rpm). During methane biodegradation experiments, EC also improved when increasing both factors, but contrary to abiotic kLa, it was not affected at the highest stirring rate. While stirring rate was the dominant variable determining methane biodegradation, the effect of oil fraction was only significant above 200 rpm. A maximum EC improvement of 700% was observed when increasing stirring rate from 200 to 800 rpm with 10% of silicone oil.CONCLUSIONS: Increasing both stirring rate and oil fraction enhanced EC. The results were different from those observed with kLa suggesting that kLa is not sufficient to describe properly the mass transfer in TPPB because other factors (interfacial contact between the phases and physicochemical properties of organic phase) should be considered. Copyright © 2010 Society of Chemical Industry

A Gain-Scheduling Approach for Control of Dissolved Oxygen in Stirred Bioreactors

This paper discusses control of dissolved oxygen in a bioreactor when the stirrer speed is used as control signal. During batch and fed-batch cultivations the operating conditions change significantly which causes tuning problems, especially when performance requirements are high. A linearized process model reveals that the variations in the oxygen dynamics are mainly due to changes in the volumetric oxygen transfer coefficient KLa. To account for the process variations, a control strategy based on PID control and gain scheduling from the stirrer speed is suggested. Controller parameters in different operating regions are obtained using auto-tuning experiments. The approach is easy to implement and does not require any exhaust-gas analysis. Experimental results from a laboratory reactor are presented.

A kLa identification technique for a dynamic model of the coupled system: Air‐lift fermenter — oxygen probes

AbstractThe time delay of oxygen probe response to the signal from a fermenter makes identification of the volumetric oxygen transfer coefficient kLa by the dynamic method more complicated. A coupled model involving the transient‐state oxygen balance of the fermenter together with the dynamic model of the oxygen probe must be then formulated, solved and identified.In this paper two simple models of air‐lift loop fermenters have been proposed and a coupled mathematical model of the fermenter – oxygen probe system has been developed. The identification procedure was used to estimate kLa values in the fermenter with internal circulation flow on the basis of experimental measurements. A comparison of evaluated and experimental indications of the probes placed at various heights of the column proves that the model presented gives a possibility of the first‐step approximation of kLa in loop fermenters.

Oxygen transfer in broths of plant cells at high density

The rheological properties of the culture broths of some plant cells (Cudrania tricuspidata, Vinca rosea, and Agrostemma githago) at high density (10-18 g dry wt/L) were measured, and oxygen transfer in the broths in various bioreactors was investigated. The rheological properties of the broths were dependent on the size, specific gravity, and concentration of the cell aggregates contained in the broths. The broths were non-Newtonian and pseudoplastic fluids. The flow behavior index n was fairly constant (0.53) and the consistency index K varied in proportion to the sixth-to-seventh power of the cell mass concentration M. The apparent viscosity mu(a) of the broths was in proportion to the 6.5th power of M. The oxygen transfer in the broths was discussed on the basis of the results obtained for suspensions of granulated agars (agar concentration, 5.8%) in water, which were similar to the broths in rheological properties. The volumetric oxygen transfer coefficient k(L)a in the broths was dependent on mu(a)(k(L)a proportional, variant mu(a) (-m)) and decreased greatly at a certain apparent viscosity, mu(ac). The values of m and mu(ac) were closely related to the aeration-agitation mechanisms of the bioreactors. The values of mu(ac) in aeration-agitation type bioreactors was larger than that in aeration-type bioreactors, whereas for m, the reverse was true.

Method for the determination of oxygen transfer coefficients (KLa) with the correction for the actual cultivation conditions

A method for determination of the volumetric oxygen transfer coefficient KLa from dynamic measurement data under conditions of both excess oxygen and oxygen-limited growth of microorganisms is described. The KLa determination follows from the integrated form of the oxygen balance equation and involves a correction for the actual cultivation conditions during measurement.

Method for the determination of oxygen transfer coefficients (KLa) with the correction for the actual cultivation conditions

AbstractA method for determination of the volumetric oxygen transfer coefficient KLa from dynamic measurement data under conditions of both excess oxygen and oxygen‐limited growth of microorganisms is described. The KLa determination follows from the integrated form of the oxygen balance equation and involves a correction for the actual cultivation conditions during measurement.