Characterization of the two-phase systems in airlift tower-loop bioreactors during the cultivation of E. coli

During the cultivation of E. coli in an airlift tower-loop bioreactor, the following properties were measured: transverse profiles of Sauter bubble diameter, d(S); local relative gas holdup, E(G); bubble rise velocity, u(BS); local mean velocity, ū turbulence intensity, u'; macrotime scale, T(EL); dissipation time scale, tau(E); power spectrum, E(n); and energy dissipation spectrum D(n) at different distances from the aerator. The influence, distance from the aerator, absence and/or presnece of cells, and batch and/or continuous-culture operation on the behavior of the two-phase system are discussed on the basis of these properties.     

Gas hold-up in external-loop airlift bioreactors

A new model of gas hold-up is proposed for external-loop airlift bioreactors. It is based on the similarity between the liquid circulation due to the local variation of gas hold-up in airlift bioreactors and the natural convection due to temperature difference. The model is developed to include the case of non-Newtonian fermentation media which are involved in many industrially bioprocesses. The capability of the model is examined using a wide range of experimental results including the present data. Reasonable agreement is obtained between the proposed model and the experimental data both for Newtonian and non-Newtonian media.     

Liquid circulation in external-loop airlift bioreactors

A simple model for prediction of liquid velocity in external-loop airlift bioreactors has been developed. Theoretical correlations for friction factor of gas-non-Newtonian two-phase flows and for liquid velocity in the riser were derived using the concept of an eddy diffusivity. The predictions of the proposed model were compared with the available experimental data for the friction factor and the liquid velocity in the riser of external-loop airlift contactors. Satisfactory agreement was obtained.     

Enclosed bioreactors for the mass cultivation of photosynthetic microorganisms: the future trend

Through the years, large numbers of high value chemicals and potential pharmaceutical agents have been identified in photosynthetic cells and cultures. In order that algal biotechnology can be upgraded to the current level of bacterial and fungal biotechnologies, the adoption of fully controllable, enclosed fermentation systems is a prerequisite.     

Modelling mixing parameters in concentric-tube airlift bioreactors

The mixing behaviour of the liquid phase in concentric-tube airlift bioreactors of different scale (RIMP: V_L=0.070 m³; RIS-1: V_L=2.50 m³; RIS-2: V_L=5.20 m³) in terms of mixing time was investigated. This mixing parameter was determined from the output curves to an initial Dirac pulse, using the classical tracer response technique, and analyzed in relation to process and geometrical parameters, such as: gas superficial velocity, x_SGR; top clearance, h_S; bottom clearance, h_B, and ratio of the resistances at downcomer entrance, A_d/A_R. A correlation between the mixing time and the specified operating and geometrical parameters was developed, which was particularized for two flow regimes: bubbly and transition (x_SGRА.08 m/s) and churn turbulent flow (x_SGR> 0.08 m/s) respectively. The correlation was applied in bioreactors of different scale with a maximum error of ᆲ%.     

Modelling mixing parameters in concentric-tube airlift bioreactors

Axial dispersion of the liquid phase was investigated in a concentric-tube airlift bioreactor (RIMP: V _L=0.70?m³) as a whole and in the separate zones (riser, downcomer, gas-separator) using the axial dispersion model. The axial dispersion number Bo and the axial dispersion coefficient, D _ax were determined from the output curves to an initial Dirac pulse, using the tracer response technique. They were analyzed in relation to process and geometrical parameters, such as: gas superficial velocity, ν_SGR; top clearance, h _S; bottom clearance, h _B, and resistances at downcomer entrance expressed as A _d/A _R ratio. Correlations between Bodenstein numbers in the overall bioreactor and riser and downcomer sections (Bo_T,Bo_R,Bo_D) and the geometrical and process parameters were developed, which can allow to assess the complex influence of these parameters on liquid axial dispersion.     

Large-scale cultivation ofSolanum chrysotrichum cells: Production of the antifungal saponin SC-1 in 10-I airlift bioreactors

Cells of two different cell lines:ccvx (cotyledon derived) andccvz (hypocotyl derived) ofSolanum chrysotrichum were cultivated in 10-1 airlift bioreactors for the production of the human antimycotic compound SC-1. When using 3 g l^-1 dry weight inoculum in a batch culture, higher levels of biomass were achieved with theccvx cell line (14.6 g l^-1) than withccvz (7.7 g l^-1), resulting in 23 and 12 mg g^-1 of SC-1 after 17 days in culture forccvx andccvz, respectively. The maximum productivity of SC-1 in bioreactors was 0.025 g l^-1 day^-1 after 9 days in culture. When using a draw-fill mode, the productivity increased by 60% to a value of 0.041 g l^-1 day, 4 days after 50% of the cell suspension was removed and replaced with fresh medium. This latter bioreactor system is a feasible alternative for the production of the antimycotic metabolite ofS. chrysotrichum on a large scale.Abbreviations DW Cell dry weight - FW Fresh weight - MS Murashige and Skoog (1962) medium - T _d Doubling time     

Hydrodynamics of non-Newtonian liquids in external-loop airlift bioreactors

In order to obtain further information on the behaviour and optimal design of external-circulation-loop airlift bioreactors, the liquid circulating velocity was studied using highly viscous pseudoplastic solutions of starch and antibiotic biosynthesis liquids of Penicillium chrysogenum, Streptomyces griseus, Streptomyces erythreus, Bacillus licheniformis and Cephalosporium acremonium. Measurements of liquid circulation velocity were made in laboratory and pilot plant external-loop airlift bioreactors, under various conditions concerning gas flow rate, riser liquid height at constant downcomer height, A _D /A _R ratio, using the impulse-response technique. It has been found that these parameters had a significant effect on liquid circulation velocity together with the apparent viscosity and dry weight of the solid phase in the biosynthesis liquids. For the tested liquids, the superficial liquid velocity in the riser section of an external-loop airlift bioreactor may be described by the following equation: where the exponents and the constant c take different values depending on the liquid phase properties and flow regime.     

Hydrodynamics of non-Newtonian liquids in external-loop airlift bioreactors

Gas holdup investigation was performed in two external-loop airlift bioreactors of laboratory (V _L =1.189·10^?3? 1.880·10^?3 m³; H _R =1.16 ? 1.56 m; H _D = 1.10 m; A _D /A _R = 0.111 ? 1.000) and pilot scale (V _L =0.157?0.170 m³; H _R =4.3?4.7 m; H _D =4.0?4.4 m;A _D /A _R =0.04?0.1225), respectively, using as liquid phase non-Newtonian starch solutions of different concentration with K=0.061?3.518 Pa sⁿ and n=0.86?0.39 and fermentation broths of P. chrysogenum, S. griseus, S. erythreus, B. licheniformis and C. acremonium at different hours since inoculation and from different batches. The influence of bioreactor geometry, liquid properties and the amount of introduced compressed air was investigated. The effect of sparger design on gas holdup was found to be negligible. It was found that gas holdup depends on the flow media index, ?_GR decreasing with the increase of liquid pseudoplasticity, A _D /A _R ratio and H _R /H _D ratio. The experimental data are in agreement with those presented in literature by Popovic and Robinson, which take into account liquid properties, geometric parameters and gas superficial velocity, with a maximum error of ±30%. It was obtained a correlation for gas holdup estimation taking into account the non-Newtonian behaviour of the fermentation broths and the dry weight of the solid phase, as well. The concordance between the experimental data and those calculated with the proposed correlation was good, with a maximum error of ±17%. Also, a dimensionless correlation for gas holdup involving superficial velocities of gas and liquid, cross sectional areas ratio, dispersion height to riser diameter ratio, as well as Froude and Morton numbers, was obtained.     

Study of the liquid circulation velocity in external-loop airlift bioreactors

Liquid circulation velocity was studied in externalloop air-lift bioreactors of laboratory and pilot scale, respectively for different gas input rates, downcomer-to-riser cross-sectional area ratio, A _D/A_R and liquid phase apparent viscosities.It was found that, up to a gas superficial velocity in the riser v _SGR 0.04 m/s the dependency of v _SLR on v _SGR is in the following form: v _SLR = a v _SGR ^b , with the exponent b being 0.40. Over this value of v _SGR, only a small increase in liquid superficial velocity, v _SLR is produced by an increase in v _SGR. A _D/A_R ratio affects the liquid superficial velocity due to the resistance in flow and overall friction.For non-Newtonian viscous liquids, the circulation liquid velocity in the riser section of the pilot external-loop airlift bioreactor is shown to be dependent mainly on the downcomer-to-riser cross-sectional area ratio, A _D/A_R, the effective (apparent) liquid viscosity, _eff and the superficial gas velocity, v _SGR.The equation proposed by Popovic and Robinson [11] was fitted well, with an error of ± 20%.List of Symbols A _D m² downcomer cross-sectional area - A _Rm² riser cross-sectional area - a = coefficient in Eq. (7) - b = exponent in Eq. (7) - c _s m^–1 Coefficient in Eq. (3) - D _D m downcomer diameter - D _R m riser diameter - g m²/s gravitational acceleration - H _D m dispersion height - H _L m ungassed liquid height - K Pa s ⁿ consistency index - K _B = friction factor at the bioreactor bottom - K _F = friction factor - K _T = friction factor at the bioreactor top - V _L m³ liquid volume in the bioreactor - V _D m³ liquid volume in downcomer - V _R m³ liquid volume in riser - v _LDm/s downcomer linear liquid velocity - v _LR m/s riser linear liquid velocity - v _SGR m/s riser superficial liquid velocity - v _SLR m/s riser superficial liquid velocity - s^–1 shear rate - _GD = downcomer gas holdup - _GR = riser gas holdup - _eff Pa s effective (apparent) viscosity - Pa shear stress The authors wish to thank Mrs. Rodica Roman for the help in experimental data collection and to Dr. Stefanluca for the financial support.     

The specific interfacial area in external-loop airlift bioreactors

The specific interfacial areas in two external-loop airlift bioreactors of laboratory and pilot scale were determined, mainly by the chemical reaction method (sulphite oxidation). The parameter studied in water/salt and starch/salt solutions was greately affected by gas superficial velocity, A _D /A _R ratio, by H _R ?H _D /H _D ratio and η _ap , respectively. Correlations for the specific interfacial area in the two systems, considering the effects of the above-mentioned parameters, were proposed.     

Hydrodynamics in external-loop airlift bioreactors with static mixers

Liquid circulation superficial velocity and gas holdup behaviours were investigated in an external-loop airlift bioreactor of 0.170?m³ liquid volume in gas-induced and forced-circulation-loop operation modes, in the presence of static mixers made of corrugated stainless steel pieces, resulting in packets with the height-to-diameter ratio equal to unity and using non-Newtonian starch solutions as liquid phase. The static mixers were disposed in the riser in three blocks, each with three mixing packets, successively turned 90° to the adjacent mixing element. It was found that in the presence of static mixers and forced-loop operation mode, liquid circulation superficial velocity in the riser section was significantly diminished, while gas holdup increased in a great measure. It was considered that static mixers split the fluid into individual streams and break up the bubbles, resulting in small bubble sizes with a relative homogeneous bubble distribution over riser cross section. They act as supplementary resistances in liquid flow, reducing riser cross sectional area, equivalent with A _D /A _R area ratio diminishing.     

Two-stage cultivation of Digitalis lanata cells: semicontinuous production of deacetyllanatoside C in 20-litre airlift bioreactors.

A two-stage cultivation method was employed to develop a semicontinuous biotransformation process for the production of deacetyllanatoside C, a cardenolide of the important digoxin series. Digitoxin was used as the substrate for biotransformation. The process was optimized in 1-l shake flasks and then established on the 20-l scale using two airlift bioreactors, one for cell growth (working volume 12 litres) and another for deacetyllanatoside C production (working volume 18 litres). Growth and production phases were synchronized and the process finally ran semicontinuously in 7-d cycles. Six consecutive production runs were performed yielding a total of 43.8 g deacetyllanatoside C.     

New milliliter‐scale stirred tank bioreactors for the cultivation of mycelium forming microorganisms

A novel milliliter‐scale stirred tank bioreactor was developed for the cultivation of mycelium forming microorganisms on a 10 milliliter‐scale. A newly designed one‐sided paddle impeller is driven magnetically and rotates freely on an axis in an unbaffled reaction vessel made of polystyrene. A rotating lamella is formed which spreads out along the reactor wall. Thus an enhanced surface‐to‐volume ratio of the liquid phase is generated where oxygen is introduced via surface aeration. Volumetric oxygen transfer coefficients (k_La) > 0.15 s^?1 were measured. The fast moving liquid lamella efficiently prevents wall growth and foaming. Mean power consumption and maximum local energy dissipation were measured as function of operating conditions in the milliliter‐scale stirred tank bioreactor (V = 10 mL) and compared to a standard laboratory‐scale stirred tank bioreactor with six‐bladed Rushton turbines (V = 2,000 mL). Mean power consumption increases with increasing impeller speed and shows the same characteristics and values on both scales. The maximum local energy dissipation of the milliliter‐scale stirred tank bioreactor was reduced compared to the laboratory‐scale at the same mean volumetric power input. Hence the milliliter impeller distributes power more uniformly in the reaction medium. Based on these data a reliable and robust scale‐up of fermentation processes is possible. This was demonstrated with the cultivation of the actinomycete Streptomyces tendae on both scales. It was shown that the process performances were equivalent with regard to biomass concentration, mannitol consumption and production of the pharmaceutical relevant fungicide nikkomycin Z up to a process time of 120 h. A high parallel reproducibility was observed on the milliliter‐scale (standard deviation < 8%) with up to 48 stirred tank bioreactors operated in a magnetic inductive drive. Rheological behavior of the culture broth was measured and showed a highly viscous shear‐thinning non‐Newtonian behavior. The newly developed one‐sided paddle impellers operated in unbaffled reactors on a 10 milliliter‐scale with a magnetic inductive drive for up to 48 parallel bioreactors allows for the first time the parallel bioprocess development with mycelium forming microorganisms. This is especially important since these kinds of cultivations normally exhibit process times of 100 h and more. Thus the operation of parallel stirred tank reactors will have the potential to reduce process development times drastically. Biotechnol. Bioeng. 2010; 106: 443–451. © 2010 Wiley Periodicals, Inc.     

Stochastic modelling of axial dispersion in external-loop airlift bioreactors

The paper presents a model of the motion of a particle subjected to several transport processes in connection with mixing in two phase flow. A residence time distribution technique coupled with a one-dimensional dispersion model was used to obtain the axial dispersion coefficient in the liquid phase, D_ax. The proposed model of D_ax for an external-loop airlift bioreactor is based on the stochastic analysis of the two-phase flow in a cocurrent bubble column and modified for the specific flow in the airlift reactor. The model takes into account the riser gas superficial velocity, the riser liquid superficial velocity, the Sauter bubble diameter, the riser gas hold-up, the downcomer-to-riser cross sectional area ratio. The proposed model can be applied with an average error of ᆨ.     

Dynamic modeling and simulation of continuous airlift bioreactors

For dynamic behaviors of continuous airlift bioreactors, a mathematical model based on a tanks-in-series model with backflow has been developed. The equations describing the dynamics of airlift bioreactors are material balances for micro-organism, substrate, dissolved oxygen and oxygen in gas-phase and heat balances. Non-ideal mixing of liquid and gas phases is taken into account using a tanks-in-series model with backflow. The batch operation, startup operation and the consequence of plant failure were simulated and the effects of design and operating parameters for an airlift bioreactor on its dynamic behaviors were discussed. The concentration profiles of micro-organism, substrate, dissolved oxygen and oxygen in gas-phase and the temperature profile in an airlift bioreactors and their dynamics were obtained. The computational results indicate that the transients of a chemostat in the case of bubble column bioreactor are slower compared with those in the case of airlift bioreactor. The proposed simulator is more precise as compared with models published previously in the literature and therefore provides more reliable and rational examination of continuous airlift bioreactor performance.     

Oxygen transfer and mixing in mechanically agitated airlift bioreactors

Gas holdup, mixing, liquid circulation and gas–liquid oxygen transfer were characterized in a large (∼1.5 m³) draft-tube airlift bioreactor agitated with Prochem^® hydrofoil impellers placed in the draft-tube. Measurements were made in water and in cellulose fiber slurries that resembled broths of mycelial microfungi. Use of mechanical agitation generally enhanced mixing performance and the oxygen transfer capability relative to when mechanical agitation was not used; however, the oxygen transfer efficiency was reduced by mechanical agitation. The overall volumetric gas–liquid mass transfer coefficient declined with the increasing concentration of the cellulose fiber solids; however, the mixing time in these strongly shear thinning slurries was independent of the solids contents (0–4% w/v). Surface aeration never contributed more than 12% to the total mass transfer in air–water.     

Continuous biotransformation of glycopeptide antibiotic A40926 in a cascade of three airlift bioreactors using immobilized Actinoplanes teichomyceticus cells

Immobilized cells of Actinoplanes teichomyceticus ATCC 31121 were used to selectively cleave the acyl group of A40926 yielding the deacylated form of the molecule. The feasibility of this particular biotransformation in a series of three perfectly mixed airlift bioreactors with immobilized cells was examined. A continuously operated airlift cascade was designed using a model for a series of reactors with immobilized biocatalyst beads obeying Michaelis–Menten kinetics. In independent experimental runs the cascade bioreactor system was operated continuously for 56 days with an overall conversion of 99%. Model estimates for reactor volumes and relative conversions were found to be in a good agreement with the experimental results.     

Stem cell cultivation in bioreactors

Cell-based therapies have generated great interest in the scientific and medical communities, and stem cells in particular are very appealing for regenerative medicine, drug screening and other biomedical applications. These unspecialized cells have unlimited self-renewal capacity and the remarkable ability to produce mature cells with specialized functions, such as blood cells, nerve cells or cardiac muscle. However, the actual number of cells that can be obtained from available donors is very low. One possible solution for the generation of relevant numbers of cells for several applications is to scale-up the culture of these cells in vitro. This review describes recent developments in the cultivation of stem cells in bioreactors, particularly considerations regarding critical culture parameters, possible bioreactor configurations, and integration of novel technologies in the bioprocess development stage. We expect that this review will provide updated and detailed information focusing on the systematic production of stem cell products in compliance with regulatory guidelines, while using robust and cost-effective approaches.     

Microbial biomass production from rice straw hydrolysate in airlift bioreactors

Rice straw is a by-product of rice production, and a great bioresource as raw biomass material for manufacturing value-adding protein for animal feedstock, which has been paid more and more attention. In the present work, utilizing rice straw hydrolysate as a substrate for microbial biomass production in 11.5L external-loop airlift bioreactors was investigated. Rice straw hydrolysate obtained through acid-hydrolyzing rice straw was used for the culture of yeast Candida arborea AS1.257. The influences of gas flow rate, initial liquid volume, hole diameter of gas sparger and numbers of sieve plates on microbial biomass production were examined. The best results in the external-loop airlift bioreactor were obtained under 9.0 L initial liquid volume, 1.1 (v/v)/min gas flow rate during culture time of 0-24 h and 1.4 (v/v)/min gas flow rate of 24-48 h at 29+/-1 degrees C. The addition of the sieve plates in the riser of the external-loop airlift bioreactor increased productivity. After 48 h, under optimized operation conditions, crude protein productivity with one sieve and two sieves were 13.6 mg/mL and 13.7 mg/mL, respectively, comparing 12.7 mg/mL without sieves in the airlift bioreactor and 11.7 mg/mL in the in the 10-L mechanically stirred tank bioreactor. It is feasible to operate the external-loop airlift bioreactors and possible to reduce the production cost for microbial biomass production from the rice straw hydrolysate.