Dynamic measurement of the volumetric oxygen transfer coefficient in fermentation systems

A rapid and internally consistent technique has been developed to measure the volumetric oxygen transfer coefficient, k_La, in fermentation systems. The method consists of tracing the dissolved O₂ concentration of the fermentation broth during a short interruption of the aeration. The O₂ concentration trace thus obtained can be analyzed to determine the values of k_La. Additional experiments on prolonged O₂ starvation, carried out to find the limitation of the technique, suggest that O₂ uptake rate will vary if a prolonged (2–10 min.) O₂ starvation occurs.     

Dynamic measurement of the overall volumetric mass-transfer coefficient in air-sparged systems

The determination of the overall volumetric mass-transfer coefficient with the dynamic measurement technique involves modeling, parameter estimation, and experimental design. The combination and extension of previous efforts lead to some suggestions for improvements.     

Comparison of oxygen mass transfer coefficient in simple and extractive fermentation systems

Aeration and agitation are important variables to ensure effective oxygen transfer rate during aerobic bioprocesses; therefore, the knowledge of the volumetric mass transfer coefficient (k_La) is required. In view of selecting the optimum oxygen requirements for extractive fermentation in aqueous two-phase system (ATPS), the k_La values in a typical ATPS medium were compared in this work with those in distilled water and in a simple fermentation medium, in the absence of biomass. Aeration and agitation were selected as the independent variables using a 2² full factorial design. Both variables showed statistically significant effects on k_La, and the highest values of this parameter in both media for simple fermentation (241 s⁻¹) and extractive fermentation with ATPS (70.3 s⁻¹) were observed at the highest levels of aeration (5 vvm) and agitation (1200 rpm). The k_La values were then used to establish mathematical correlations of this response as a function of the process variables. The exponents of the power number (N³D²) and superficial gas velocity (V_s) determined in distilled water (α = 0.39 and β = 0.47, respectively) were in reasonable agreement with the ones reported in the literature for several aqueous systems and close to those determined for a simple fermentation medium (α = 0.38 and β = 0.41). On the other hand, as expected by the increased viscosity in the presence of polyethylene glycol, their values were remarkably higher in a typical medium for extractive fermentation (α = 0.50 and β = 1.0). A reasonable agreement was found between the experimental data of k_La for the three selected systems and the values predicted by the theoretical models, under a wide range of operational conditions.     

Comparison of oxygen mass transfer coefficient in simple and extractive fermentation systems

Aeration and agitation are important variables to ensure effective oxygen transfer rate during aerobic bioprocesses; therefore, the knowledge of the volumetric mass transfer coefficient (k_La) is required. In view of selecting the optimum oxygen requirements for extractive fermentation in aqueous two-phase system (ATPS), the k_La values in a typical ATPS medium were compared in this work with those in distilled water and in a simple fermentation medium, in the absence of biomass. Aeration and agitation were selected as the independent variables using a 2² full factorial design. Both variables showed statistically significant effects on k_La, and the highest values of this parameter in both media for simple fermentation (241 s⁻¹) and extractive fermentation with ATPS (70.3 s⁻¹) were observed at the highest levels of aeration (5 vvm) and agitation (1200 rpm). The k_La values were then used to establish mathematical correlations of this response as a function of the process variables. The exponents of the power number (N³D²) and superficial gas velocity (V_s) determined in distilled water (α = 0.39 and β = 0.47, respectively) were in reasonable agreement with the ones reported in the literature for several aqueous systems and close to those determined for a simple fermentation medium (α = 0.38 and β = 0.41). On the other hand, as expected by the increased viscosity in the presence of polyethylene glycol, their values were remarkably higher in a typical medium for extractive fermentation (α = 0.50 and β = 1.0). A reasonable agreement was found between the experimental data of k_La for the three selected systems and the values predicted by the theoretical models, under a wide range of operational conditions.     

Determination of volumetric oxygen transfer coefficient by off-gas analysis

In various aerobic bioreactors including activated sludge aeration tanks, the volumetric mass transfer coefficient K_La is frequently used as an estimate of the rate of oxygen dissolution into the liquid phase. The K_La measurement in such bioreactors is widely applied with the aid of sodium sulfite (Na₂SO₃) as an oxygen-consuming substance used to maintain low dissolved oxygen concentration. In the present study, the effect of the addition of Na₂SO₃ on K_La, determined by an off-gas analysis, was investigated specifically from the viewpoint of variations in the size of air bubbles and the enhancement factor associated with the change in sulfite concentration. Experiments were conducted in a draft-tube bubble column, using a zirconia electrode oxygen analyzer for measurement of the O₂ mole fraction in the exhaust gas and a dual electrical resistivity probe for measurement of the bubble size. It was found that the increase in the specific gas-liquid interfacial area, resulting from bubble size reduction effected by Na₂SO₃ functioning as an electrolyte, is more pronounced than the enhancement of the absorption rate through the interface. The upper limit of Na₂SO₃ concentration for sustaining physical absorption, in the absence of any catalyst, ranges from 30 to 70 mol/m³, while that for preventing the average bubble size from decreasing is about 15 mol/m³. Furthermore, to secure a reliable K_La measurement, the K_La value should not exceed 50 h⁻¹ for the liquid depth of 3 m even when the limiting conditions are not exceeded. The off-gas analysis proposed in this study for K_La determination is expected to be extremely useful provided that the above conditions are fulfilled, since it only requires moderate addition of the sulfite as the oxygen-consuming substance and will not interrupt the reactor operation as long as oxygen uptake occurs in the system.     

The volumetric oxygen mass transfer coefficient in antibiotic biosynthesis liquids

This paper approaches the problem of oxygen mass transfer. This transfer is in antibiotic biosynthesis liquids produced by microorganisms belonging to the actinomycete and fungi classes, which exhibit a shear thinning non-Newtonian rheological behaviour. The volumetric oxygen mass transfer coefficients in these liquids (k_L a_b) change during biosynthesis processes. The change is mainly due to rheological parameter modifications, such as increasing the consistency index (K) and decreasing the flow behaviour index (n). The values of k_L a_b were 3.0–6.5 times lower than those recorded in water, and their decreasing depended on the k_L a values obtained without biological liquid and on the nature of fermentation broths, as well. Starting from experimental data, two correlations were established between k_L a_b and P/V,υ_SG and P/V,υ_SG, N, respectively. These correlations contain a dimensionless factor (η_am/η_g), which takes into account the rheological properties of the liquid phase and offers the possibility for a fast and sufficiently accurate estimation of k_L a_b. The empirical correlations developed in the paper correspond reasonably well with the relatively wide variety of experimental data, as in the model proposed by PEREZ and SANDALL , and allow for the comparison of the fermentation batches of the same or different microorganisms; also, they may be applied to the workings of design, scale-up, control and monitoring of bioreactors.     

Dynamic measurement of the volumetric mass transfer coefficient in agitated vessels: Effect of the start-up period on the response of an oxygen electrode

The response of a polarographic oxygen electrode to a step change and to an exponential change in bulk oxygen concentration was studied theoretically and experimentally for the case where there is a significant liquid film resistance at the outerside of the membrane-covered electrode. The probe response has been described considering the start-up period of the concentration changes (the period of time that will elapse before the new concentration level is established and/or before the volumetric mass transfer coefficient k_La regains its steady-state value after the gas supply is opened to the fermentor). A linear change of the pertinent characteristics is assumed during this start-up period. It is shown that a substantial error could be introduced by neglecting the start-up period for cases frequently occurring in practice. In addition, the dependences of the probe response on the direct contact of bubbles with an electrode and on the fluid flow field around it were discussed.     

Determination of power consumption and volumetric oxygen transfer coefficient in bioreactors

A torque meter has been developed for determining the power consumption in a bench stirred tank. The device has been bonded in the stirrer shaft inside a commercial bench fermentor, in order to avoid frictional losses in the mechanical seal. Power consumption measurements in ungassed and gassed systems were obtained at different agitation and aeration conditions, for Newtonian and non-Newtonian fluids. Also, a "simple modified sulfite method" for volumetric oxygen transfer coefficient (k_La) determination was developed and the experimental data were correlated with the gassed power (P_g) by using well-known correlations presented in the literature.     

Gas holdup and overall volumetric oxygen transfer coefficient in airlift contactors

The two major types of airlift contactors, concentric-tube and external-loop, were investigated for their gas holdup (riser and downcomer) and overall mass transfer characteristics. Results obtained in batch charges of tap water and 0.15 kmol/m(3) NaCl solution are reported for external-loop airlift contactors having downcomer-to-riser cross-sectional area ratios, A(d)/A(r), ranging from 0.11 相似文献   

Hydrodynamic characteristics and overall volumetric oxygen transfer coefficient of a new multi-environment bioreactor

The hydrodynamic characteristics and the overall volumetric oxygen transfer coefficient of a new multi-environment bioreactor which is an integrated part of a wastewater treatment system, called BioCAST, were studied. This bioreactor contains several zones with different environmental conditions including aerobic, microaerophilic and anoxic, designed to increase the contaminant removal capacity of the treatment system. The multi-environment bioreactor is designed based on the concept of airlift reactors where liquid is circulated through the zones with different environmental conditions. The presence of openings between the aerobic zone and the adjacent oxygen-depleted microaerophilic zone changes the hydrodynamic properties of this bioreactor compared to the conventional airlift designs. The impact of operating and process parameters, notably the hydraulic retention time (HRT) and superficial gas velocity (U _G), on the hydrodynamics and mass transfer characteristics of the system was examined. The results showed that liquid circulation velocity (V _L), gas holdup (ε) and overall volumetric oxygen transfer coefficient ( $ k_{\text{L}} a_{\text{L}} $ ) increase with the increase of superficial gas velocity (U _G), while the mean circulation time (t _c) decreases with the increase of superficial gas velocity. The mean circulation time between the aerobic zone (riser) and microaerophilic zone (downcomer) is a stronger function of the superficial gas velocity for the smaller openings (1/2 in.) between the two zones, while for the larger opening (1 in.) the mean circulation time is almost independent of U _G for U _G ≥ 0.023 m/s. The smaller openings between the two zones provide higher mass transfer coefficient and better zone generation which will contribute to improved performance of the system during treatment operations.     

Effects of oxygen volumetric mass transfer coefficient on transglutaminase production by Bacillus circulans BL32

The effects of oxygenation in cultures of Bacillus circulans BL32 on transglutaminase (TGase) production and cell sporulation were studied by varying the agitation speed and the volume of aeration. Kinetics of cultivations has been studied in batch systems using a 2 L bioreactor, and the efficiency of agitation and aeration was evaluated through the oxygen volumetric mass transfer coefficient (k_La). It was adopted a two-stage aeration rate control strategy: first stage to induce biomass formation, followed by a second stage, in which cell sporulation was stimulated. A correlation of TGase production, spores formation, and oxygen concentration was established. Under the best conditions (500 rpm; 2 vvm air flow, followed by no air supply during stationary phase; k_La of 33.7 h⁻¹), TGase production reached a volumetric production of 589 U/L after 50 h of cultivation and the enzyme yield was 906 U/g cells. These values are 61% higher than that obtained in shaker cultures and TGase productivity increased 82%, when k_La varied from 4.4 to 33.7 h⁻¹. The maximal cell concentration increased four times in relation to shaker cultures and the cultivation time for the highest TGase activity was reduced from 192 h to just 50 h. These results show the importance of bioprocess design for the production of microbial TGase, especially concerning the oxygen supply of cultures and the induction of cell sporulation.     

Liquid phase volumetric mass transfer coefficient in dairy effluent stream

The effluent from a dairy cold storage plant was studied for the levels of dissolved oxygen saturation. The effluent to the aerobic digester mainly contains lactose, milk fat, protein and lactic acid. The study was conducted at five different temperatures in a two litre laboratory fermentor. The volumetric liquid phase mass transfer coefficient was correlated to temperature with an average absolute deviation of 6.2%.     

Effects of oxygen volumetric mass transfer coefficient and pH on lipase production by Staphylococcus warneri EX17

The principal objectives of this study were to evaluate the kinetics of lipase production by Staphylococcus warneri EX17 under different oxygen volumetric mass transfer coefficients (k_La) and pH conditions in submerged bioreactors, using glycerol (a biodiesel by-product) as a carbon source. Cultivations were conducted at different k_La (26, 38, 50, and 83 h⁻¹) and pH values (6.0, 7.0, and 8.0). The optimal k_La and pH were 38 h⁻¹ and 7.0, respectively. Under these conditions, the maximal cell production obtained was 8.0 g/L, and the volumetric and specific lipase production reached high levels of activity, approximately 800 U/L and 150 U/g cell, respectively, after 12 h of cultivation. This result was approximately five times higher than that obtained in the shake flask cultures. The relationship between cell growth and lipase production was found to be associated with growth by the Luedeking-Piret model.     

The effects of the oxygen transfer coefficient and substrate concentration on the xylose fermentation by Debaryomyces hansenii

Relevant production of xylitol by Debaryomyces hansenii requires semiaerobic conditions since in aerobic conditions the accumulated reduced adenine dinucleotide coenzyme is fully reoxidized leading to the conversion of xylitol into xylulose. For oxygen transfer coefficient values from 0.24 to 1.88 min^-1, in shake flasks experiments, biomass formation increased proportionally to the aeration rate as shown in the oxygen transfer coefficient and xylose concentration isoresponse contours. The metabolic products under study, xylitol and ethanol were mainly growth associated. However, for oxygen transfer coefficient above 0.5 min^-1 higher initial xylose concentration stimulated the rate of production of xylitol. This fact was less evident for ethanol production. The direct relationship between increased biomass and products formation rate, indicated that the experimental domain in respect to the aeration rate was below the threshold level before the decreasing in metabolic production rates reported in literature for xylose-fermenting yeasts. The fact that ethanol was produced, albeit in low levels, throughout the experimental design indicated that the semiaerobic conditions were always attained. Debaryomyces hansenii showed to be an important xylitol producer exhibiting a xylitol/ethanol ratio above four and a carbon conversion of 54% for xylitol.Abbreviations K_La oxygen transfer coefficient - DO(T) dissolved oxygen (tension) - OUR oxygen uptake rate - NAD(H) oxidised (reduced) nicotinamide adenine dinucleotide - NADP(H) oxidised (reduced) nicotinamide adenine dinucleotide phosphate - CRC catabolic reduction charge - C oxygen concentration in the culture medium - C^* oxygen concentration at saturation conditions - Y_i response from experiment i - parameters of the polynomial model - x experimental factor level (coded units) - R² coefficient of multiple determination - t time     

Mathematical model based estimation of volumetric oxygen transfer coefficient(s) in the production of proteolytic enzymes in Bacillus amyloliquefaciens

Alkaline protease is a class of important hydrolytic enzymes having wide applications in bioprocess industries. Their optimum pH in the alkaline range and stability at higher temperatures make them ideal in detergent and leather processing industries. These enzymes have excellent depilating capacity. The present study aims at process optimization for the production of alkaline protease from Bacillus amyloliquefaciens ATCC 23844. Information on the optimal operating temperature and pH were elicited from specific growth rates and alkaline protease yields. It was also observed that besides pH and temperature, the oxygen transfer rate is another important limiting variable for the production of protease. Volumetric oxygen transfer coefficient (k _L a) was estimated at various impeller speeds and aeration rates. The optimal impeller speed and aeration rates were determined from k _L a and the relative protease yield data. It was understood that the oxygen transfer rate is one of the crucial parameters for the production of proteolytic enzymes by B. amyloliquefaciens.     

On the methodology of oxygen transfer coefficient measurments

A method for the determination of the k_La in microbial cultures is described. It is based on the relation that expresses the k_La in function of the oxygen transfer rate and the oxygen driving force. Its application is demonstrated by showing the influence of the stirring geometry and intensity as well as the aeration rate and the salts concentration on the k_La of a bioreactor.     

Dynamic measurement of oxidase activity based on oxygen consumption in open systems

Oxidases catalyze the oxidation of a variety of substrates with the concomitant reduction of molecular oxygen as a final electron acceptor. UV‐visible spectrophotometry is a simple and high‐throughput method commonly used to measure oxidase activities. However, drawbacks such as light scattering exist especially concerning the activity assessment of enzymes immobilized on supports. Monitoring of the universal cosubstrate O₂ circumvents these drawbacks. This study aimed at developing a methodology that allows activity measurement of many types of oxidases based on O₂ consumption applicable to various open systems. Dissolved oxygen in the reaction medium was monitored by an O₂ sensor and the reaction rate was deduced from the O₂ mass balance equation correcting for atmospheric diffusion. Common activity units (μM_product min^?1 or U/L) could be subsequently derived using calibration curves. The sensitivity of the method toward temperature, atmospheric pressure, and ionic strength variations was evaluated, and made it possible to define operating windows for the simplification of the proposed methodology.