Mass transfer and mixing rates in fermentation vessels

The effects on mass-transfer and overall mixing rates of varying impeller geometry and operating speed have been studied for flat-bladed turbines in laboratory fermentors, in aerated aqueous solutions, and in unaerated and aerated suspensions (1.6% w/v) of paper pulp. In the absence of suspended solid, oxygen absorption rates could be correlated directly with power input. In the pulp suspension, oxygen absorption at a given power input was influenced by impeller geometry and operating speed. The data for the three-phase system can be correlated by a dimensionless equation relating oxygen-transfer rates and mixing times to the geometrical and operating parameters of the impellers.     

Mass transfer characteristics of a fermentation broth in a reactor: co-current downflow contacting reactor

Gas holdups, dispersion height, volumetric mass transfer coefficients and k_La of water and a yeast fermentation broth were studied in a co-current downflow contacting reactor. k_La and gas holdup increased with increasing superficial gas velocity and there was a parallelism between k_La and gas holdup. The values of k_La gas holdup, and dispersion height measured in the air/fermentation broth system were all lower than those in air/water system.     

Oxygen transfer in Streptomyces fermentation broths

The oxygen transfer coefficient has been investigated in S. noursci and S. lavendulae fermentation broths obtained from fermentors of different operating volumes (61., 30001., 20,0001.). Fermentors had K_La_s values ranging from 1.0 to 17.0 min^?1, calculated from sulphite oxidation rates. The dynamic measurement of the volumetric oxygen transfer coefficient. (K_La) has been performed in the different fermenting systems. As the fermentation progressed, especially in the first stages, K_La values have decreased in both fermentations and in each system of fermentors. In order to characterise the whole fermenting system an average K_La_s was calculated from the obtained K_La values. The average K_La grew with increasing K_La_s values and ranged from 0.03 to 3.72 min^?l. Some factors possibly having an influence on the, change of K_La have been studied. The oxygen transfer coefficients of the broths have been measured in falling films and ranged from 0.05 to 0.4 cm min^?1. The flow conditions have been characterized by Reynolds numbers of broths varying between 1.0 and 60.0. The average thickness of the falling films have been measured and plotted against Reynolds number. The Re⁺ which is the breaking-point of the plot increased as the fermentation proceeded. In the region of Re⁺ the values of the oxygen transfer coefficient increased rapidly. An approximate correlation could be established between the Re⁺ and the physical properties of fermentation broth.     

Heat transfer simulation in solid substrate fermentation

A mathematical model was developed and tested to simulate the generation and transfer of heat in solid substrate fermentation (SSF). The experimental studies were realized in a 1-L static bioreactor packed with cassava wet meal and inoculated with Aspergillus niger. A simplified pseudohomogeneous monodimensional dynamic model was used for the energy balance. Kinetic equations taking into account biomass formation (logistic), sugar consumption (with maintenance), and carbon dioxide formation were used. Model verification was achieved by comparison of calculated and experimental temperatures. Heat transfer was evaluated by the estimation of Biot and Peclet heat dimensionless numbers 5-10 and 2550-2750, respectively. It was shown that conduction through the fermentation fixed bed was the main heat transfer resistance. This model intends to reach a better understanding of transport phenomena in SSF, a fact which could be used to evaluate various alternatives for temperature control of SSF, i.e., changing air flow rates and increasing water content. Dimensionless numbers could be used as scale-up criteria of large fermentors, since in those ratios are described the operating conditions, geometry, and size of the bioreactor. It could lead to improved solid reactor systems. The model can be used as a basis for automatic control of SSF for the production of valuable metabolites in static fermentors.     

Mass transfer limitations in solid-state digestion

The biodegradation of waste placed in layers was studied in 500-ml lysimeters, without leachate recycle. All four became methanogenic within 100 days but the steady-state biogas output only reached 0.09 l kg^–1 d^–1, whereas an earlier study on a mixed feedstock had yielded 1.8 l kg^–1 d^–1. A novel physical model of solid-state digestion is deduced, implying a crucial role for mass transfer processes, with successful operation only within a limited range of diffusion rates.     

Mass transfer enhancement in moving biofilm structures

Biofilms are layers of microbial cells growing on an interface and they can form highly complex structures adapted to a wide variety of environmental conditions. Biofilm streamers have a small immobile base attached to the support and a flexible tail elongated in the flow direction, which can vibrate in fast flows. Herein we report numerical results for the role of the periodical movement of biofilm streamers on the nutrient uptake and in general on the solute mass transfer enhancement due to flow-induced oscillations. We developed what to our knowledge is a novel two-dimensional fluid-structure interaction model coupled to unsteady solute mass transport and solved the model using the finite element method with a moving mesh. Results demonstrate that the oscillatory movement of the biofilm tail significantly increases the substrate uptake. The mass transfer coefficient is the highest in regions close to the streamer tip. The reason for substrate transfer enhancement is the increase in speed of tip movement relative to the surrounding liquid, thereby reducing the thickness of the mass transfer boundary layer. In addition, we show that the relative mass transfer enhancement in unsteady conditions compared with the rigid static structure is larger at higher flow velocities, and this relative increase favors a more flexible structure.     

Amino acid fermentation and hydrogen transfer in mixed cultures

Abstract The degradation of the following amino acids was investigated in mixed cultures obtained from a waste water purification plant: aspartate, glutamate, serine, alanine, valine and leucine. Inhibition of sulfate-reducing bacteria in these mixed cultures by molybdate was found to inhibit amino acid degradation. The degradation of serine, alanine, valine and leucine was accelerated considerably by active sulfate reduction. The fermentation of aspartate and glutamate was not stimulated by the presence of sulfate-reducing bacteria. The existence of species which are able to ferment valine and leucine by coupling their oxidation to the reduction of exogenous acetate to butyrate was demonstrated.     

Enhancement of oxygen transfer rates in fermentation using oxygen-vectors

Oxygen transfer is one of the bottlenecks in conventional fermentation technology and it has so far been almost totally overlooked with regards to high cell densities and immobilized cells. This review presents some new concepts to improve oxygen supply in aerobic fermentations, especially the use of oxygen-vectors. The oxygen-vectors generally used are liquids which are insoluble in the fermentation media. Their utilization in an emulsified form can significantly increase the oxygen transfer coefficient between gas and aqueous phases. It seems that the vector acts as an active intermediate in the oxygen transport from gas bubbles to aqueous phase, but the mechanisms involved in this unconventional technique of aeration are not yet known.     

Mass transfer limitation of sulfate in methanogenic aggregates

The role of mass transfer limitation of sulfate as a factor governing the competition between sulfate reducing and methane producing bacteria in methanogenic aggregates was theoretically evaluated by the calculation of steady-state sulfate microprofiles using a reference set of parameters obtained from the literature. The shooting method was used as a numerical technique for solving the mathematical model. The effect of the parameters on mass transport limitation was tested by varying each reference value of the parameters with a factor of 3. Sulfate limitation within granules prevailed at moderate (0.1 kg m(-3)) and low sulfate concentrations in the bulk liquid, at high maximum sulfate utilization rates (3.73 x 10(-5) kg SO(4) (2-) kg(-1) VSS S(-1) or biomass concentrations (40 KG VSS m(-3)), and in large aggregates (radius of 7.5 10(-4) m). The effective diffusion coefficient of sulfate and the affinity constant were less determinative for the penetration depth of sulfate within a granule. (c) 1994 John Wiley & Sons, Inc.     

Mass transfer in aqueous two-phases system packed column

SCH 201781 is a direct thrombin inhibitor recently under study in clinical trials to determine its safety and efficacy for the treatment of venous and arterial thrombosis. In aqueous solution, SCH 201781 exists as three forms, a ring-opened hydrated form and two ring-closed diastereomers. An automated solid-phase extraction LC-MS/MS method that chromatographically separates and measures each form was developed and validated from 1 to 1000 ng/mL in human plasma. For calibration curve standards, within- and between-run precision (%CV) ranged from 0.6 to 13.7%, while accuracy (%bias) ranged from -4.8 to 13.1%. For quality control samples, within- and between-run %CV ranged from 1.5 to 9.9% while %bias ranged from -9.1 to 4.9%. The method requires a sample volume of 0.8 ml and utilizes 2H6-labeled SCH 201781 as the internal standard. For sample processing, an Isolute C-8 96-well solid phase extraction plate and a Tomtec Quadra 96 sample processor is employed. Separation of the three forms of SCH 201781 is achieved using a 5 microm, 2 mm x 100 mm Asahipak C8 HPLC column and gradient elution. A Sciex API 365 equipped with a turbo ionspray source is used in the selected reaction monitoring mode for detection. The validated method was used to support clinical studies.     

Oxygen transfer to mycelial fermentation broths in an airlift fermentor

Oxygen transfer rates and gas holdups were measured in mycelial fermentation broths of Chaetomium cellulolyticum and Neurospora sitophila, each cultured in a 1300-L pilot-plant-scale airlift fermentor. These cultures exhibited highly non-Newtonian flow behavior coupled with a substantial decrease in oxygen transfer rates. The volumetric mass transfer coefficients in these cultures were found to be 65-70% lower than those in water. The data were compared with the available correlations obtained for simulated fermentation broths. In general, the data for C. cellulolyticum are in satisfactory agreement with the correlations for the model media but the data for N. sitophila are higher than that predicted by the correlations. Model media based correlations are found to be applicable to the fermentation processes if the culture medium does not possess a high yield stress.     

Enhanced oxygen transfer rates in fermentation using soybean oil-in-water dispersions

Summary The effect of soybean oil on the volumetric oxygen transfer coefficient during the cultivation ofAerobacter aerogenes cells is presented. For our aeration-agitation conditions (0.278 vvm and 500 rpm), it has been demonstrated that the use 19% (v/v) of soybean oil enabled a 1.85-fold increase of thek _l a coefficient (calculated on a per liter aqueous phase basis). For smaller volumetric oil fractions,k _L a increased linearly with the oil loading. Because of the oxygen-vector properties of soybean oil, this oil is able to significantly increase thek _L a of a bioreactor.Nomenclature C^*, C saturation and actual dissolved oxygen concentrations respectively (g/m³) - K_La volumetric oxygen transfer coefficient (h^–1) - K_La_initial k _La measured before the oil addition (h^–1) - M_O2 molar mass of oxygen (dalton) - N oxygen transfer rate (g/m³. h) - P_O2. P_N2 partial pressures ofO ₂ andN ₂ in the gas (atm) - P_H2OT partial pressure of water in air at the temperatureT (atm) - P_T total pressure (atm) - Q₀ volumetric flow rate of outlet air before seeding (m³/h) - Sp spreading coefficient (dynes/cm) - T absolute temperature of outlet gas (K) - V_i volume of the liquidi in the fermentor (m³) - V_M molar volume at 273 K and 1 atm (m³/mole) - _ij interfacial tension betweeni andj componants (dynes/cm) - _v volumetric fraction of the oil (v/v) - G gas - O oil - W water - i inlet - o outlet     

Enhancement of oxygen transfer in highly viscous non-newtonian fermentation broths

The influence of short draft tubes covered by perforated plates on gas-liquid mass transfer was examined in external-loop airlift bioreactors. The volumetric mass transfer coefficients in a model external-loop airlift bioreactor were measured with water and non-Newtonian media. It was found that introduction of draft tubes covered with perforated plates in the riser significantly improved the mass transfer rate, particularly in higher viscous non-Newtonian fermentation media. The enhancement of mass transfer rate might be due mainly to an increase in bubble coalescence and redispersion. (c) 1994 John Wiley & Sons, Inc.     

Mechanism of enhanced oxygen transfer in fermentation using emulsified oxygen-vectors

Limitations of oxygen transfer in fermentation can be solved using auxiliary liquids immiscible in the aqueous phase. The liquids (called oxygen-vectors) used in this study were hydrocarbon (n-dodecane) and perfluorocarbon (forane F66E) in which oxygen is highly soluble (54.9 mg/L in n-dodecane and 118 mg/L in forane F66E at 35 degrees C in contact with air at atmospheric pressure). It has been demonstrated that the use of n-dodecane emulsion in a culture of Aerobacter aerogenes enabled a 3. 5-fold increase of the volumetric oxygen transfer coefficient(k(L)a) calculated on a per-liter aqueous phase basis. The droplet size of the vector played a crucial role in the phenomena. When a static contact between gas bubble and vector droplet was established in water, the vector covered the bubble, in agreement with positive values of the spreading coefficient for these fluids. The determination of the oxygen transfer coefficients (k(L)) in a reactor with a definite interfacial area enabled the main resistance to be located in the boundary layer of the waterside either for a gas-water or a vector-water interface. Because oxygen consumption by weakly hydrophobic cells can only occur in the aqueous phase, the oxygen transfer is achieved according to the following pathway: gas-vector-water-cell. Finally, a mechanism for oxygen transfer within this four-phased system is proposed.