Oxygen transfer to mixed cultures in tower systems

A modified dynamic method is introduced to determine the oxygen transfer coefficient, K_L a, in aerobic fermentation systems which are not mechanically agitated. The dissolved oxygen concentration is measured continuously following a step down or a step up in aeration rate. The response curve is analyzed to obtain the value of K_La Experiments were carried out at several different air flow rates using mixed culture in concurrent tower fermentors with motionless mixers. The effect of sieve trays and Koch motionless mixers on oxygen transfer was investigated using a 3 in. diameter column. The values of K_L aobtained at the bottom of each column were found to be higher than those obtained at the top. Comparison of the results showed that the values ofK_L a were higher when the Koch mixers were used than when the sieve trays were employed. The oxygen uptake rate by the organisms rX, is also calculated by using the K_L a values obtained. They compare favorably withthe experimentally measured values.     

Characterization and improvement of oxygen transfer in pilot plant external air-lift bioreactor for mycelial biomass production

The oxygen transfer dynamics in a pilot plant external air-lift bioreactor (EALB) during the cultivation of mycelial biomass were characterized with respect to hydrodynamic parameters of gas holdup (), oxygen transfer coefficient (K_La) and superficial gas velocity (U _g), and dissolved oxygen (DO). An increased flow rate of air supply was required to meet the increased oxygen demand with mycelial biomass growth. Consequently, an increase in air flow rate led to an increase in , K_La and the DO level. The enhancement of oxygen transfer rate in the cultivated broth system, however, was limited with highly increased viscosity of the mycelial broth. An increase in air flow rate from 1.25 to 2.00 v/v/m resulted in a low increment of oxygen transfer. The newly designed pilot plant EALB with two air spargers significantly improved processing reliability, aeration rate and K_La. The pilot plant EALB process, operated under a top pressure from 0 to 1.0 bars, also demonstrated a significant improvement of oxygenation efficiency by more than 20% in DO and K_La. The performance of the two sparger EALB process under top pressure demonstrated an efficient and economical aerobic system with fast mycelial growth and high biomass productivity in mycelial biomass production and wastewater treatment.     

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

The time delay of oxygen probe response to the signal from a fermenter makes identification of the volumetric oxygen transfer coefficient k_La 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 k_La 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 k_La in loop fermenters.     

Investigation of the bacitracin biosynthesis in an airlift bioreactor

Results of pilot plant studies using an external-loop airlift bioreactor (170 l fermentation volume, liquid height-to-riser diameter: 27, loop-to-tower cross-section-area: 0.1225) have proven the relative merits of such a system in the bacitracin biosynthesis produced by the Bacillus licheniformis submerged aerobic cultivation. The results were compared to those obtained in a pilot-scale stirred-tank bioreactor with the same values of k_La. Excepting the aeration rate of 0.2 vvm, the fermentation process performed at 0.5 vvm and 1/0 vvm, respectively, unfolded similarly in the two fermentation devices with respect to the cell mass production, substrate utilization and bacitracin production during the fermentation process. In the riser section of the airlift bioreactor, the dissolved oxygen levels were higher, while in the downcomer section they were lower than those realized in the stirred tank bioreactor. Power requirements of the airlift fermenter were by 17–64% lower than those for a mechanically agitated system, depending on the aeration rates, which led to an important energy saving. Moreover, the lack of mechanical devices in the airlift system provides safety and a more gentle environment for the cultivation of microorganisms.     

The influence of geometry on hydrodynamic and mass transfer characteristics in an external airlift reactor for the cultivation of filamentous fungi

The influences of geometric configuration, mycelial broth rheology and superficial gas velocity (U_sg) were investigated with respect to the following hydrodynamic parameters: gas holdup (), oxygen transfer coefficient (K_La) and mixing time (t_m). Increases in U_sg and height of gas separator (H_t) caused an increase in and K_La, and a decrease in t_m. Consequently, a diameter ratio (D_d/D_r) of 0.71 and H_t 0.20 m were found to be the best geometry and operation parameters to achieve high aeration and mixing efficiency for the high viscous broth system in the cultivation of filamentous fungi. An external airlift reactor (EALR) was developed and designed for the cultivation of filamentous fungi. The EALR with two spargers excels in reliability and high aeration and mass transfer coefficiency, resulting in a fast mycelial growth and high biomass productivity in the cultivation of the fungus Rhizopus oryzae.     

The role of interphase nitrogen transport in the dynamic measurement of the overall volumetric mass transfer cefficient in air-sparged systems

It has been shown both theoretically and experimentally that interphase nitrogen transport may have a significant influence on the rate of interphase oxygen transport, and thereby also on the value of the volumetric mass transfer coefficient of oxygen, k_la, determined in mechanically agitated bubble fermentors using the variants of dynamic method presented in the literature. The experiments were carried out in 1M KCI solution at five stirrer frequencies and two gas inlet levels. The gas interchanges were performed either without interrupting the aeration and agitation of the charge (A) or with the aeration and agitation of the charge turned on at the same time (B). The applied variants of the interchange were N₂→ O₂→, O₂→ N₂, N₂→ air, air→ N₂, O→ O₂, and O→ air. In the two last variants the oxygen dissolved in the charge was removed by reacting with sulfite ions. The k_la values calculated by allowing for the nitrogen transport for procedure A were approximately equal to the values obtained by disregarding the nitrogen transport, whereas those for procedure B were higher (up to 40%), than the values obtained disregarding the nitrogen transport.     

Production of bacterial cellulose by Acetobacter xylinumwith an air-lift reactor

Bacterial cellulose was produced by Acetobacter xylinum subsp. surcrofermentans BPR2001 in a 50 liter air-lift reactor using fructose as the main carbon source. When air was supplied, the production of the cellulose was only 2.3 g/l in 80 h but when O -fortified air was supplied, the cellulose concentration increased to 5.63 g/l in 28 h and the productivity of the cellulose in an air-lift reactor with O -fortified air supply was comparable to that in a mechanically agitated jar fermenter.     

Vertical thin-layer photoreactor for controlled cultivation of cyanobacteria

Nostoc sp. was cultivated in an air-lift reactor with continuous recirculation of the head gas phase that aerated and agitated the cyanobacterial suspension at regulated flow rates. The supply of inorganic carbon for growth was coupled with pH control, in the range of 7.7 to 8.1, by intermittent sparging of CO₂-head gas mixtures. The formation of irregular bubbles with swirling motion at the photostage of the reactor promoted efficient CO₂ transference in dense populations of Nostoc sp. (1.1 g/l) when bubbling at flow rates of 10 l/min. Biomass productivity was almost six-fold higher in the photoreactor (16.4 mg/l.h) than in a conventional system (2.8 mg/l.h). The exponential growth phase of cultures in the photoreactor amounted to 60% of the total growth period.The authors are with the Laboratorio de Alimentos, Area Microbiologia, Facultad de Quimica Bioquimica y Farmacia, Universidad Nacional de San Luis, Chacabuco y Pedernera, 5700 San Luis, Argentina     

Oxygen-transfer rates and efficiencies in one- and two-stage airlift towers

The primary consideration in fermentor design is the supply of oxygen to the growing microorganisms. The oxygen-transfer characteristics of a two-stage splitcylinder airlift tower were compared to those of a similar single-stage airlift tower of equal liquid volume using a sodium sulfite–air system. At superficial gas velocities, from 720 to 1200 cm/min, no difference in K_La was apparent. The K_La was significantly larger in the two-stage tower for a gas velocity between 1200 and 2728 cm/min. At 2728 cm/min a K_La of 25.2 min^?1 was achieved in the two-stage system, and at 2262 cm/min the two-stage tower had a 54% larger K_La than the single stage. A comparison of dispersion-volume based K_La showed a 27% larger value at a gas velocity of 2262 cm/min. The performance ratios for the two-stage tower were larger than those for the single-stage tower at oxygen-transfer rates greater than 180 mmol/liter hr. A comparison of the data with literature values is presented.     

Different bioreactor configurations for the decolourisation of the azo dye reactive black 5 by Geotrichum sp. CCMI 1019

Decolourisation of the azo dye Reactive Black 5 by Geotrichum sp. CCMI 1019 was studied using stirred tank reactors (STR) and two types of bubble columns (porous plate (PP) bubble column and aeration tube (AT) bubble column). For the bubble columns, the k_La increased with the gas fractional hold-up (ε_G) and the aeration rate. A linear relationship between ε_G and superficial gas velocity was obtained for all reactors. At same aeration rates, the PP bubble columns showed higher k_La and hold-up values than the AT bubble column. In the STRs, large and dense aggregates were formed which adhered to surfaces whereas bubble columns gave smaller and less compact pellets.

Manganese peroxidase and laccase were detected in the extracellular media in all reactors. However, laccase was only detected after the onset of decolourisation, suggesting that additional enzymes may be involved. Mn peroxidase activity was detected (about 46 U/ml) in both the STRs and AT bubble columns but higher values (110 U/ml) were obtained with the PP bubble columns.

Out of the three reactor systems studied, the AT bubble columns gave the most favourable results for Reactive Black 5 decolourisation. Rapid and complete colour removal was obtained throughout the visible spectrum. Bubble columns are simple in design as well as operation and may be useful for the bioremediation of textile wastewater.     

Comparative investigation of fine bubble and macrobubble aeration on gas utility and biotransformation productivity

The sufficient provision of oxygen is mandatory for enzymatic oxidations in aqueous solution, however, in process optimization this still is a bottleneck that cannot be overcome with the established methods of macrobubble aeration. Providing higher mass transfer performance through microbubble aerators, inefficient aeration can be overcome or improved. Investigating the mass transport performance in a model protein solution, the microbubble aeration results in higher k_La values related to the applied airstream in comparison with macrobubble aeration. Comparing the aerators at identical k_La of 160 and 60 1/h, the microbubble aeration is resulting in 25 and 44 times enhanced gas utility compared with aeration with macrobubbles. To prove the feasibility of microbubbles in biocatalysis, the productivity of a glucose oxidase catalyzed biotransformation is compared with macrobubble aeration as well as the gas‐saving potential. In contrast to the expectation that the same productivities are achieved at identically applied k_La, microbubble aeration increased the gluconic acid productivity by 32% and resulted in 41.6 times higher oxygen utilization. The observed advantages of microbubble aeration are based on the large volume‐specific interfacial area combined with a prolonged residence time, which results in a high mass transfer performance, less enzyme deactivation by foam formation, and reduced gas consumption. This makes microbubble aerators favorable for application in biocatalysis.     

Venturi aeration and thermophilic aerobic sewage sludge digestion in small-scale reactors

Summary The aeration performance of two venturi aeration reactors (operating volumes 381 and 251) was studied for an air-water system. It was found that the mass transfer coefficient (k _la) could be described in terms of the superficial gas velocity (V _s) alone by the simple expressionk _La=aV _infb ^supS with constantsa=0.313,b=0.579 for the 38-1 reactor anda=0.214,b=0.534 for the 25-1 reactor. A similar relationship was obeyed when the 38-1 reactor was aerated with a diffuser tile (a=17.0,b=1.52). A linear relationship betweenk _La and gas hold-up was observed for the 38-1 reactor with both venturi and diffuser aeration. The 25-1 reactor was used successfully for the thermophilic aerobic digestion of sewage sludge. A mean sludge temperature rise of 30°C was observed. Chemical oxygen demand, pH, and total solids content of the digested sludge differed significantly from the feed sludge and were similar to values obtained for full-scale thermophilic aerobic digestion. No significant differences between inorganic solids content, dissolved oxygen concentration, or redox potential were observed between feed and digested sludge.     

The application of a new type of sintered glass carriers for the cultivation of anchorage-dependent mammalian cells

Cylinder-shaped sintered glass carriers were successfully used for immobilizing and cultivating cells of murine line L₉₂₉. The process has been performed in a fixed-bed bioreactor (working volume of 150 ml) with perfusion and aeration of the medium (total volume of 3.5 l). The maximum cell density of 1.23 × 10⁷cells/ml was achieved when the basal medium (MEM + 10% bovine serum) was enriched with glucose (up to 2 g/l) and L-glutamine (up to 4 mM). The carriers have proved to be nontoxic, mechanically stable and reusable.     

Influence of agitation and aeration on growth and antibiotic production by <Emphasis Type="Italic">Xenorhabdus nematophila</Emphasis>

The effect of agitation and aeration on the growth and antibiotic production by Xenorhabdus nematophila YL001 grown in batch cultures were investigated. Efficiency of aeration and agitation was evaluated through the oxygen mass transfer coefficient (K _L a). With increase in K _L a, the biomass and antibiotic activity increased. Activity units of antibiotic and dry cell weight were increased to 232 U ml⁻¹ and 19.58 g l⁻¹, respectively, productivity in cell and antibiotic was up more than 30% when K _L a increased from 115.9 h⁻¹ to 185.7 h⁻¹. During the exponential growth phase, DO concentration was zero, the oxygen supply was not sufficient. So, based on process analysis, a three-stage oxygen supply control strategy was used to improved the DO concentration above 30% by controlling the agitation speed and aeration rate. The dry cell weight and activity units of antibiotic were further increased to 24.22 g l⁻¹ and 249 U ml⁻¹, and were improved by 24.0% and 7.0%, compared with fermentation at a constant agitation speed and a constant aeration rate (300 rev min⁻¹, 2.5 l min⁻¹).     

Modelling of the process yields of a whey fermentation

Summary The biomass yields (y) and COD reduction efficiencies (η) of a whey fermentation by Kluyveromyces fragilis were studied in a 100-1 fermenter at various stirrer speeds and lactose concentrations, and compared to those obtained in 10-1 and 15-1 fermenters at constant values of the oxygen transfer coefficient (k_La) and air velocity. The empirical models previously constructed by using the 15-1 fermenter data could be used to predict the yields on the other scales by calculating for each run the 15-1 fermenter which would provide the same oxygen transfer coefficient measured by the sulphite method on each fermenter under study. To make this model independent of stirrer speeds used in each generic fermenter, the effect of aeration and mixing was incorporated into an overall parameter (k_La) and the values of y and η were correlated only with temperature, lactose level and k_L a, since these variables were approximately orthogonal. The validity of this model was finally checked against the yields reported by Wasserman et al. (1961) in a 6-m³ fermenter, thus confirming the capability of the model to provide a reliable basis for further scale-up on the production scale.     

Flow characteristics of a pilot-scale airlift fermentor

The effects of aeration on the flow characteristics of water in a glass pilot-scale airlift fermentor have been examined. The 55-L capacity fermentor consisted of a 15.2-cm-i.d. riser column with a 5.1-cm-i.d. downcomer tube. It was found that the average bubble size diminished with increased aeration. Typically, average bubble sizes ranged from 4.32 mm at a superficial gas velocity of 0.64 cm/s to 1.92 mm at 10.3 cm/s. A gas holdup of 0.19 was attained with superficial gas velocities (v_s) on the order of 10 cm/s, indicating the highly gassed nature of the fluid in the riser section of the fermentor. Circulation velocities of markers placed in the fermentor decreased with increasing aeration rates due to increased turbulence and axial liquid back mixing within the riser section. Actual volumetric liquid circulation rates remained relatively constant (0.36–0.49 L/s) for values of (v_s) up to 10 cm/s. Based on theoretical calculations, the ascending velocity of bubbles in a swarm reached 54 cm/s in the range of (v_s) values studied.     

Effects of pluronic F-68 on oxygen transport in an agitated,sparged bioreactor

The gassing out method was used to study the effect of Pluronic F-68 on oxygen transport in an agitated, sparged bioreactor. At a low air sparge rate (0.07 vvm), the addition of Pluronic F-68 had a minimal effect on K_La in both water and medium at low agitation rates. As the agitation rate was increased at this low sparge rate the relative inhibitory effect of Pluronic F-68 on K_La increased with increasing agitation rate. At an air sparge rate of 0.80 vvm, the addition of Pluronic F-68 resulted in a significant reduction in K_La at all agitation rates studied.     

Effect of aeration and mixing in various types of cultivation devices on the cell cycle of budding yeasts

When investigating the effect of aeration capacityK _L a of a cultivation device on the cell cycle of daughter cells ofCandida utilis it was found that the length of a phase (S + G₂) of the cell cycle is influenced by the rate of oxygen transfer. An increase ofK _L a, of a cultivation device achieved by increasing the specific output of mechanical energy for air dispersion and mixing may lead to cell damage and to changes in the cell cycle. The effect of high intensity of aeration and mixing is thus invalidated.     

Bubble coalescence behaviour in biological media

Summary A twin bubble column was used to measure the k_La values for oxygen in model and cultivation media using the steady state method described previously (Adler et al. 1980). Desmophen and soy oil were used as antifoam agents together with model and/or cultivation media for Chaetomium cellulotyticum, Trichoderma reesei, Hansenula polymorpha, Saccharomyces cerevisiae and Escherichia coli. The bubble coalescence behavior is mainly influenced by antifoam agents and somewhat by protein and alcohol additives. In the range investigated (0.01 to 0.1%.), the k_La values are not influenced by the Desmophen concentration and only slighthly by the soy oil concentration (0.5 to 1.5%.). The coalescence behaviour was characterized by the ratio m_corr=(k_La)_corr/(k_La)_ref. A nutrient salt solution with Desmophen was used as a reference. The k_La measured in the investigated media were corrected by considering the differences in k_La's in the investigated and reference media. These m_corr values can directly be used for bubble columns close to the optimum aeration rate.Symbols a Specific gas/liquid interfacial area - c Concentration - k_L Mass transfer coefficient - k_La Volumetric mass transfer coefficient - W_SG Superficial gas velocity - E_G Relative gas hold-up     

The optimization of L-lysine fermentation and a mass transfer model

The efficiency of L-lysine biosynthesis is essentially determined by the power input and aeration ratio in the stirred fermenter. A mass transfer model was developed by means of the results of lysine fermentations in four geometrically similar fermenters with working volumes of 10 1, 50 1, 100 1 and 2500 1 which allows the optimization of lysine fermentation from the energetical point of view. The usefullness of this k_La-model is demonstrated with an example where the power input for an unknown fermenter is calculated.