Gas holdup, power consumption, and oxygen absorption coefficient in a stirred-tank fermentor under foam control

For a laboratory stirred-tank fermentor (STF) with foaming system of 0.5M sulfite solution containing an anionic soft detergent, the performing of a foam-breaking apparatus with a rotating disk (FARD)fitted to the STF was evaluated. The gas holdup in a mechanical foam-control system (MFS), i.e., the stirred-tank fermentor with the rotating disk foambreaker, was confirmed to be larger than that in a nonfoaming system (NS), i.e., the STF with an antifoam agent added. The agitation power in the mechanical foam control system was found to be smaller compared with the agitation power in the nonfoaming system, due to the increased gas holdup. Comparison of the oxygen absorption coefficient between the mechanical foam control system and the nonfoaming system in terms of the specific power input also demonstrated the superiority of the mechanical foam control system, not only in oxygen transfer performance but also in power input economy.     

Relation between critical disk speed for foam-breaking of rotating-disk mechanical foam-breakers and foaming characteristics of solutions

For tower fermentors, i.e., bubble columns (BCs), treating various foaming liquids, the critical disk rotational speed N_c required for foam-breaking of rotating-disk mechanical foam-breakers (MFRDs) fitted to BCs was studied in relation to the foaming characteristics of the liquids. An empirical equation was presented for predicting N_c in foam-breaking regions. With this equation, it was found that prediction of N_c of MFRDs fitted to BCs treating various liquids was possible regardless of the type of foaming liquid or its concentration.     

Influence of operating variables on liquid circulation in a 10.5-m(3) jet loop bioreactor

A 10.5-m(3) concentric tube jet loop reactor was used to study the influence of the working liquid volume, mean superficial air velocity, operating pressure, downcomer aeration, liquid jet velocity, and two ratios of draft tube/reactor diameter (D(t)/D) on liquid circulation time (T(c)). The experiments were carried out in a water-air system with the use of the acid pulse method. Results showed that circulation time was independent of the working liquid volume over a certain minimum liquid level, whereas downcomer aeration and D(t)/D ratio appeared as amenable parameters to achieve a high degree of control over liquid circulation and mixing efficiency, and to optimize the overall reactor performance. Increasing the operating pressure caused a reduction of the liquid circulation rate. However, ionger residence times of the air bubbles and the higher mass transfer driving force that result at higher pressures improve oxygen utilization. The relationship between T(c) and air load was independent of the operating pressure, provided the correlation is given as a function of the mean superficial air velocity. Neither liquid circulation nor gas holdup were significantly affected by liquid jet velocity. (c) 1995 John Wiley & Sons, Inc.     

A method for the control of the dissolved oxygen tension in microbial cultures

A method for the control of dissolved oxygen tension in growing microbial cultures is described. The apparatus consists of a motor-driven air sparge pipe which may be lowered or raised to give a variable point of entry of the air stream into the culture liquid and hence a variable gas dispersion and gas–liquid contact time. Control of the sparge pipe position is by means of a feedback control loop consisting of a dissolved oxygen probe, an on/off controller, and a reversing electric motor which drives the sparge pipe. The difficulty presented by the relatively slow response of the oxygen probe has been overcome by incorporating an adjustable rate of control action.     

Improvement of foam breaking and oxygen-transfer performance in a stirred-tank fermenter

This study examined a stirred-tank fermenter (STF) containing low-viscosity foaming liquids with an agitation impeller and foam-breaking impeller mounted on the same shaft. Results showed that the performance of the foam-breaking impeller can be improved by changing a conventional six-blade turbine impeller into a rod impeller as the agitation impeller. The volumetric oxygen-transfer coefficient, k _L a, in the mechanical foam-control method (MFM) using a six-blade vaned disk as the foam-breaking impeller in the STF with the rod impeller was approximately five times greater than that of the chemical foam-control method (CFM) adding an anti-foaming agent in the STF with the six-blade turbine impeller. Application of the present method to the cultivation of Saccharomyces cerevisiae K-7 demonstrated that the cultivation time up to the maximum cell concentration was remarkably shorter than that achieved using a conventional CFM.     

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 number of turbine impellers on surface aeration in laboratory fermentor

The effect of one and two four-flat-bladed turbine impellers on the surface aeration intensity in a laboratory tormentor was investigated at different agitation speeds and sparge rates. The surface aeration was found to be generally more intensive with two impellers for the same other operating conditions because of higher power consumption and the position of the upper impeller closer to the free liquid surface. The surface aeration intensity was successfully correlated in terms of power consumption and sparge rate. The aeration number alone cannot be used for general predicting surface aeration intensities.     

Surface aeration in a small,agitated, and sparged vessel

In a recent publication, a technique was outlined for measuring surface aeration rates in an agitated vessels while sparging, and it was shown that surface aeration rates fall rapidly with increasing sparge rates. That work was conducted in a 0.61 m diam vessels. The work reported here was done in a small vessel (0.22 m diam) where surface aeration has been reported to be of particular significance. In general, the results obtained in the small vessel confirmed those in the large one and in addition were generally in good agreement with those recently published elsewhere for an almost identical geometry. For typical practical power inputs and sparge rates, the rate of surface aeration was never more than 20% of the sparge rate and generally less than 5%. These results indicate that surface aeration is of considerably less importance than has generally been believed following the findings of workers who estimated its effect by comparing KL a values under unsparged conditions with those when sparging.     

Flow pattern visualization in a mimic anaerobic digester using CFD

Three-dimensional steady-state computational fluid dynamics (CFD) simulations were performed in mimic anaerobic digesters to visualize their flow pattern and obtain hydrodynamic parameters. The mixing in the digester was provided by sparging gas at three different flow rates. The gas phase was simulated with air and the liquid phase with water. The CFD results were first evaluated using experimental data obtained by computer automated radioactive particle tracking (CARPT). The simulation results in terms of overall flow pattern, location of circulation cells and stagnant regions, trends of liquid velocity profiles, and volume of dead zones agree reasonably well with the experimental data. CFD simulations were also performed on different digester configurations. The effects of changing draft tube size, clearance, and shape of the tank bottoms were calculated to evaluate the effect of digester design on its flow pattern. Changing the draft tube clearance and height had no influence on the flow pattern or dead regions volume. However, increasing the draft tube diameter or incorporating a conical bottom design helped in reducing the volume of the dead zones as compared to a flat-bottom digester. The simulations showed that the gas flow rate sparged by a single point (0.5 cm diameter) sparger does not have an appreciable effect on the flow pattern of the digesters at the range of gas flow rates used.     

Production of extracellular protease from crude substrates with dregs in an external-loop airlift bioreactor with lower ratio of height to diameter

Bacillus subtilis AS1.398 was cultivated in a 11.5-L total volume external-loop airlift bioreactor with a low height-to-diameter ratio of 2.9 and a riser-to-downcomer diameter ratio of 6.6 for the production of protease from crude substrates with dregs. The influence of aeration rate, liquid volume, and sparger hole diameter on protease production was investigated. An average of 8197 u/mL protease activity was obtained after a total fermentation time of 32 h in the external-loop airlift bioreactor with a liquid volume of 8.5 L, air flow rate of 1.5 vvm, and sparger hole diameter of 1.5 mm. The addition of one stainless steel sieve plate in the riser of the airlift bioreactor increased productivity of protease. After 32 h of fermentation, an average of 8718 u/mL protease activity was achieved in the external-loop airlift bioreactor with one sieve plate and an air flow rate of 1.2 vvm, liquid volume of 8.5 L, and gas sparger hole diameter of 1.5 mm. This was 9.0% higher than the typical averages of about 8000 u/mL protease activity in the mechanically stirred tank bioreactors of the enzyme factory using the same microorganism. It is possible to make a scale-up of the external-loop airlift bioreactor and feasible to operate it for production of protease from crude substrate with dregs.     

CO2 in large-scale and high-density CHO cell perfusion culture

Productivity in a CHO perfusion culture reactor was maximized when pCO₂ was maintained in the range of 30–76 mm Hg. Higher levels of pCO₂ (> 150 mm Hg) resulted in CHO cell growth inhibition and dramatic reduction in productivity. We measured the oxygen utilization and CO₂ production rates for CHO cells in perfusion culture at 5.55×10^-17 mol cell^-1 sec^-1 and 5.36×10^-17 mol cell^-1 sec^-1 respectively. A simple method to directly measure the mass transfer coefficients for oxygen and carbon dioxide was also developed. For a 500 L bioreactor using pure oxygen sparge at 0.002 VVM from a microporous frit sparger, the overall apparent transfer rates (k_La+k_AA) for oxygen and carbon dioxide were 0.07264 min^-1 and 0.002962 min^-1 respectively. Thus, while a very low flow rate of pure oxygen microbubbles would be adequate to meet oxygen supply requirements for up to 2.1×10⁷ cells/mL, the low CO₂ removal efficiency would limit culture density to only 2.4×10⁶ cells/mL. An additional model was developed to predict the effect of bubble size on oxygen and CO₂ transfer rates. If pure oxygen is used in both the headspace and sparge, then the sparging rate can be minimized by the use of bubbles in the size range of 2–3 mm. For bubbles in this size range, the ratio of oxygen supply to carbon dioxide removal rates is matched to the ratio of metabolic oxygen utilization and carbon dioxide generation rates. Using this strategy in the 500 L reactor, we predict that dissolved oxygen and CO₂ levels can be maintained in the range to support maximum productivity (40% DO, 76 mm Hg pCO₂) for a culture at 10⁷ cells/mL, and with a minimum sparge rate of 0.006 vessel volumes per minute.A = volumetric agitated gas-liquid interfacial area at the top of the liquid, 1/mB = cell broth bleeding rate from the vessel, L/minCER = carbon dioxide evolution rate in the bioreactor, mol/min[CO₂] = dissolved CO₂ concentration in liquid, M[CO₂]^* = CO₂ concentration in equilibrium with sparger gas, M[CO₂]^** = CO₂ concentration in equilibrium with headspace gas, MCO₂(1) = dissolved carbon dioxide molecule in water[C_T] = total carbonic species concentration in bioreactor medium, M[C_T]_F = total carbonic species concentration in feed medium, MD = bioreactor diameter, mD_I = impeller diameter, mD_b = the initial delivered bubble diameter, mF = fresh medium feeding rate, L/minH_L = liquid height in the vessel, mk_A = carbon dioxide transfer coefficient at liquid surface, m/mink _infA ^supO = oxygen transfer coefficient at liquid surface, m/minNomenclature     

Performance of a continuous foam separation column as a function of process variables

Enrichment and recovery of bovine serum albumin has been examined in a continuous foam separation column. The effects of the operating factors, superficial air velocity, feed flow rate, feed concentration and pH on the above characteristics was investigated. The protein enrichment decreased with the increase in the value of each of these parameters. Protein recovery increased with increasing air velocity, decreased with increasing feed flow rate and did not change very much with increasing feed concentration. Maximum protein recovery was obtained at the isoelectric point (pH 4.8) of the protein. Maximum protein recovery was found to be a strong function of the air velocity in the range 0.05-0.15 cm/s. Further increase in air velocity did not have much effect on recovery because of very large bubbles formed as a result of coalescence. Bubble size was determined as a function of the above factors in the liquid and foam sections of the column. It was found to be dependent on protein concentration, feed flow rate and solution pH. The effect was more significant in the foam section of the column. The bubbles in the foam section were significantly larger (about 3-10 times) than those in the liquid, with a sharp change at the foam-liquid interface. The bubble size measurements were used to calculate the interfacial area and it was shown that the rate of protein removal increases with increasing interfacial area.     

Mathematical modeling of non-ideal mixing continuous flow reactors for anaerobic digestion of cattle manure

Most conventional digesters used for animal wastewater treatment include continuously stirred-tank reactors. While imperfect mixing patterns are more common than ideal ones in real reactors, anaerobic digestion models often assume complete mixing conditions. Therefore, their applicability appears to be limited. In this study, a mathematical model for anaerobic digestion of cattle manure was developed to describe the dynamic behavior of non-ideal mixing continuous flow reactors. The microbial kinetic model includes an enzymatic hydrolysis step and four microbial growth steps, together with the effects of substrate inhibition, pH and thermodynamic considerations. The biokinetic expressions were linked to a simple two-region liquid mixing model, which considered the reactor volume in two separate sections, the flow-through and the retention regions. Deviations from an ideal completely mixed regime were represented by changing the relative volume of the flow-through region (a) and the ratio of the internal exchange flow rate to the feed flow rate (b). The effects of the hydraulic retention time, the composition of feed, the initial conditions of the reactor and the degree of mixing on process performance can be evaluated by the dynamic model. The simulation results under different conditions showed that deviations from the ideal mixing regime decreased the methane yield and resulted in a reduced performance of the anaerobic reactors. The evaluation of the impact of the characteristic mixing parameters (a) and (b) on the anaerobic digestion of cattle manure showed that both liquid mixing parameters had significant effects on reactor performance.     

Determination of kinetic and retention properties of cartridge and disk devices for solid-phase extraction

The kinetic properties of cartridge and disk solid-phase extraction devices are determined by forced-flow liquid chromatography. Typical cartridges provide about 5–15 theoretical plates per cm of bed height and particle-loaded membranes provide about 4–9 theoretical plates for a 0.5-mm-thick membrane. It is shown that cartridge devices fail to provide their maximum trapping performance because of inadequate packing density and that the required packing density could be easily achieved in practice with particles of a standard size. The retention properties of common sorbents for extraction from water and air are characterized with the solvation parameter model. For predominantly aqueous solutions a favorable cavity term results in increased retention while polar interactions tend to reduce retention. Retention on porous polymer sorbents is more complicated because of their capacity to absorp significant amounts of the sample processing solvent resulting in solvent-dependent changes in retention properties. For trapping organic volatiles from air cavity formation and dispersion interactions are important, and in the case of Tenax its capacity for induction interactions is also significant.     

Design and performance of a trickling air biofilter for chlorobenzene and o-dichlorobenzene vapors.

From contaminated industrial sludge, two stable multistrain microbial enrichments (consortia) that were capable of rapidly utilizing chlorobenzene and o-dichlorobenzene, respectively, were obtained. These consortia were characterized as to their species composition, tolerance range, and activity maxima in order to establish and maintain the required operational parameters during their use in biofilters for the removal of chlorobenzene contaminants from air. The consortia were immobilized on a porous perlite support packed into filter columns. Metered airstreams containing the contaminant vapors were partially humidified and passed through these columns. The vapor concentrations prior to and after biofiltration were measured by gas chromatography. Liquid was circulated concurrently with the air, and the device was operated in the trickling air biofilter mode. The experimental arrangement allowed the independent variation of liquid flow, airflow, and solvent vapor concentrations. Bench-scale trickling air biofilters removed monochlorobenzene, o-dichlorobenzene, and their mixtures at rates of up to 300 g of solvent vapor h(-1) m(-3) filter volume. High liquid recirculation rates and automated pH control were critical for stable filtration performance. When the accumulating NaCl was periodically diluted, the trickling air biofilters continued to remove chlorobenzenes for several months with no loss of activity. The demonstrated high performance and stability of the described trickling air biofilters favor their use in industrial-scale air pollution control.     

Effect of Operating Parameters on the Spray Drying of Tomato Paste

The production of tomato powder from tomato paste using the spray drying technique has been investigated in this work. The influence of a number of process variables, namely, feed total solids, feed flow rate, feed temperature, air temperature, air flow rate, and starch addition on the physical properties of spray‐dried tomato powder was investigated. The product properties studied were total solids, average particle diameter, bulk density, and solubility. The increase in the feed total solids increased tomato powder total solids, particle size and bulk density and decreased its solubility, while the increase in the feed flow rate decreased tomato powder total solids and solubility, and increased the average particle size and bulk density.     

Oil drop size and gas hold-up in an oil–water airlift system with motionless mixers

The degree of emulsification, measured as surface area of oil generated, was studied. The effect of interfacial tension, volume fraction of oil, and power per unit volume on the Sauter mean diameter of the oil drops was determined in an airlift system with motionless mixers. A mathematical expression to predict the Sauter mean diameter was developed using regression techniques. From this equation another equation, which will predict the surface area of oil in terms of the same variables, was derived. The effects of water air surface tension and power per unit volume on the gas hold-up were obtained using similar techniques. The results show that the interfacial tension and the surface tension are important variables when hydrocarbon fermentations are carried out in airlift systems.     

A fluid dynamic study of the retrofitting of large agitated bioreactors: Turbulent flow

Studies were conducted in three 19-m(3) fermentors (14 m(3) working volume, aspect ratio = 3:1), one fitted with four Rushton turbines (D/T = 0.35), one with three Lightnin' A315 hydrofoil impellers (D/T = 0.46). The power drawn under the same aerated conditions relative to the unaerated ones was always greater with the hydrofoils, which gives them the potential for enhanced mass transfer rates under practical operating conditions. However, the power draw was also sensitive to the magnitude of the unaerated power. Indeed, at low unaerated specific power ( approximately 0.6 W-kg) and high air flow rates ( approximately 1vvm), the relative power draw with the hydrofoils could be even greater than 1. The hold-up with each of the impellers was broadly similar at the same aeration rate and power input, though the later had a much smaller impact in these large vessels than has been reported in the literature based on smaller scale work. As usual, repressed coalescence caused increased hold-up, and, with the hydrofoils, this increase was associated with a lower power draw. Because of the greater mechanical vibration of the reactors with the hydrofoils, vibration characteristics of the vessels were measured and they were very similar. The results showed that provided care is taken in the mechanical design of the system, such impellers can operate reliably in large-scale fermentations with the potential for enhanced biological performance. (c) 1994 John Wiley & Sons, Inc.     

Effective bioreactor pH control using only sparging gases

pH control is critical in bioreactor operations, typically realized through a two-sided control loop, where CO₂ sparging and base addition are used in bicarbonate-buffered media. Though a common approach, base addition could compromise culture performance due to the potential impact from pH excursions and osmolality increase in large-scale bioreactors. In this study, the feasibility of utilizing control of sparge gas composition as part of the pH control loop was assessed in Chinese hamster ovary (CHO) fed-batch cultures. Fine pH control was evaluated in multiple processes at different setpoints in small-scale ambr®250 bioreactors. Desired culture pH setpoints were successfully maintained via air sparge feedback control. As part of the pH control loop, air sparging was increased to improve CO₂ removal automatically, hence increase culture pH, and vice versa. The effectiveness of this pH control strategy was seamlessly transferred from ambr®250 to 200 L scale, demonstrating scalability of the proposed methodology. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2743, 2019     

Equipment for hydrocarbon fermentations

Although air-lift fermentors have been employed industrially and in the laboratory, little information has been published on the effects of design on performance. With respect to both liquid circulation and mass transfer, not only the actual rates but the efficiency in relation to power consumption is strongly influenced by lift height and diameter, submergence ratio, and air-flow rate. Relatively wide tubes, operating at high submergence ratios and rather low air-flow rates favor high efficiency. Since these conditions lead to rather low absolute values of circulation and mass-transfer rates, the microbial population which can be supported will also be rather small. Mass transfer can be increased by the insertion of an orifice some distance; above the air-inlet point and by suitable arrangement of the discharge from the air lift into the headspace of the fermentor.