Hydrodynamic characteristics and gas-liquid mass transfer in a biofilm airlift suspension reactor

The hydrodynamics and mass transfer, specifically the effects of gas velocity and the presence and type of solids on the gas hold-up and volumetric mass transfer coefficient, were studied on a lab-scale airlift reactor with internal draft tube. Basalt particles and biofilm-coated particles were used as solid phase. Three distinct flow regimes were observed with increasing gas flow rate. The influence of the solid phase on the hydrodynamics was a peculiar characteristic of the regimes. The volumetric mass transfer coefficient was found to decrease with increasing solid loading and particle size. This could be predominantly related to the influence that the solid has on gas hold-up. The ratio between gas hold-up and volumetric mass transfer coefficient was found to be independent of solid loading, size, or density, and it was proven that the presence of solids in airlift reactors lowers the number of gas bubbles without changing their size. To evaluate scale effects, experimental results were compared with theoretical and empirical models proposed for similar systems.     

Influence of sparger on energy dissipation, shear rate, and mass transfer to sea water in a concentric-tube airlift bioreactor

Data on volumetric mass-transfer coefficient, K_La_L, in a 12 × 10⁻³ m³ airlift bioreactor are reported. Measurements were made in sea water. The superficial gas velocity ranged up to 0.21 m/s. Four cylindrical spargers (60–1000 μm pore size) were tested. In bubbly flow, the sparger pore size strongly influenced the K_La_L; the highest K_La_L values were obtained with the smallest pore size. In contrast, in the transition and heterogeneous flow regimes, the pore size had little influence on K_La_L. The best correlation of the mass transfer data was obtained when both gas holdup and liquid superficial velocity were taken as independent variables. Shear rates were estimated in the different zones of the reactor. The highest values were found in the bottom zone of the reactor and in the gas-liquid separator. The penetration and isotropic turbulence models were used to develop a semi-theoretical equation relating the volumetric mass-transfer coefficient to shear rate; hence providing a better understanding of how the operational variables may be manipulated to attain a moderate shear rate and an appropriate level of mass transfer, two extremely important parameters for the growth of sensible microorganisms as those used in marine biotechnology.     

Hydrodynamics and mass transfer in a tubular airlift photobioreactor

In photobioreactors, which are usually operated under light limitation,sufficient dissolved inorganic carbon must be provided to avoid carbonlimitation. Efficient mass transfer of CO₂ into the culture mediumisdesirable since undissolved CO₂ is lost by outgassing. Mass transferof O₂ out of the system is also an important consideration, due tothe need to remove photosynthetically-derived O₂ before it reachesinhibitory concentrations. Hydrodynamics (mixing characteristics) are afunctionof reactor geometry and operating conditions (e.g. gas and liquid flow rates),and are a principal determinant of the light regime experienced by the culture.This in turn affects photosynthetic efficiency, productivity, and cellcomposition. This paper describes the mass transfer and hydrodynamics within anear-horizontal tubular photobioreactor. The volume, shape and velocity ofbubbles, gas hold-up, liquid velocity, slip velocity, axial dispersion,Reynoldsnumber, mixing time, and mass transfer coefficients were determined intapwater,seawater, and algal culture medium. Gas hold-up values resembled those ofvertical bubble columns, and the hydraulic regime could be characterized asplug-flow with medium dispersion. The maximum oxygen mass transfer coefficientis approximately 7 h^–1. A regime analysisindicated that there are mass transfer limitations in this type ofphotobioreactor. A methodology is described to determine the mass transfercoefficients for O₂ stripping and CO₂ dissolution whichwould be required to achieve a desired biomass productivity. This procedure canassist in determining design modifications to achieve the desired mass transfercoefficient.     

The influence of polymeric membrane gas spargers on hydrodynamics and mass transfer in bubble column bioreactors

Gas sparging performances of a flat sheet and tubular polymeric membranes were investigated in 3.1 m bubble column bioreactor operated in a semi batch mode. Air–water and air–CMC (Carboxymethyl cellulose) solutions of 0.5, 0.75 and 1.0 % w/w were used as interacting gas–liquid mediums. CMC solutions were employed in the study to simulate rheological properties of bioreactor broth. Gas holdup, bubble size distribution, interfacial area and gas–liquid mass transfer were studied in the homogeneous bubbly flow hydrodynamic regime with superficial gas velocity (U _G) range of 0.0004–0.0025 m/s. The study indicated that the tubular membrane sparger produced the highest gas holdup and densely populated fine bubbles with narrow size distribution. An increase in liquid viscosity promoted a shift in bubble size distribution to large stable bubbles and smaller specific interfacial area. The tubular membrane sparger achieved greater interfacial area and an enhanced overall mass transfer coefficient (K _La) by a factor of 1.2–1.9 compared to the flat sheet membrane.     

Viscous media in tower bioreactors: Hydrodynamic characteristics and mass transfer properties

Studies in tower reactors with viscous liquids on flow regime, effective shear rate, liquid mixing, gas holdup and gas/ liquid mass transfer (k _La) are reviewed. Additional new data are reported for solutions of glycerol, CMC, PAA, and xanthan in bubble columns with diameters of 0.06, 0.14 and 0.30 m diameter. The wide variation of the flow behaviour index (1 to 0.18) allows to evaluate the effective shear rate due to the gas flow. New dimensionless correlations are developed based on the own and literature data, applied to predict k _La in fermentation broths, and compared to other reactor types.List of Symbols a(a) m^–1 specific interfacial area referred to reactor (liquid) volume - Bo Bond number (g D _c ² _L/) - c _L(c _L ^* ) kmol m^–3 (equilibrium) liquid phase oxygen concentration - C coefficient characterising the velocity profile in liquid slugs - C _s m^–1 coefficient in Eq. (2) - d _B(d_vs) m bubble diameter (Sauter mean of d _B) - d ₀ m diameter of the openings in the gas distributor plate - D _c m column diameter - D _L m²s^–1 diffusivity - E _L(E_W) m² s^–1 dispersion coefficient (in water) - E ² square relative error - Fr Froude number (u _G/(g D_c)^0.5) - g m s^–2 gravity acceleration - Ga Gallilei number (g D _c ³ _L ² / _eff ² ) - h m height above the gas distributor the gas holdup is characteristic for - k Pasⁿ fluid consistency index (Eq. 1) - k _L m s^–1 liquid side mass transfer coefficient - k _La(k_La) s^–1 volumetric mass transfer coefficient referred to reactor (liquid) volume - L m dispersion height - n flow behaviour index (Eq. 1) - P W power input - Re liquid slug Reynolds number ( _L(u _G +u _L) D _c/_eff) - Sc Schmidt number ( _eff/( _L D _L )) - Sh Sherwood number (k _La D _c ² /D_L) - t s time - u _B(u_sw) m s^–1 bubble (swarm) rise velocity - u _G(u_L) m s^–1 superficial gas (liquid) velocity - V(V_L) m³ reactor (liquid) volume Greec Symbols W m^–2 K^–1 heat transfer coefficient - y(y _eff) s^–1 (effective) shear rate - _G relative gas holdup - s relaxation time of viscoelastic liquid - _L(_eff) Pa s (effective) liquid viscosity (Eq. 1) - _L kg m^–3 liquid density - N/m surface tension     

Characterisation of the gas-liquid mass transfer in shaking bioreactors

The maximum gas-liquid mass transfer capacity of 250ml shaking flasks on orbital shaking machines has been experimentally investigated using the sulphite oxidation method under variation of the shaking frequency, shaking diameter, filling volume and viscosity of the medium. The distribution of the liquid within the flask has been modelled by the intersection between the rotational hyperboloid of the liquid and the inner wall of the shaking flask. This model allows for the calculation of the specific exchange area (a), the mass transfer coefficient (k(L)) and the maximum oxygen transfer capacity (OTR(max)) for given operating conditions and requires no fitting parameters. The model agrees well with the experimental results. It was furthermore shown that the liquid film on the flask wall contributes significantly to the specific mass transfer area (a) and to the oxygen transfer rate (OTR).     

Comparison of stirred tank and airlift bioreactors in the production of polygalacturonases by Aspergillus oryzae

The production of endo and exo-polygalacturonase (PG) by Aspergillus oryzae IPT 301 was studied in a stirred tank bioreactor (STR) and an internal circulation airlift bioreactor. Using a factorial experimental design, a soluble culture medium was defined which allowed the production of exo- and endo-PG comparable to that obtained in a medium containing suspended wheat bran. The soluble medium was used in tests to compare the production of these enzymes in the STR and airlift bioreactor. In these tests, after 96 h, maximum enzymatic activity values achieved for exo- and endo-PG were 65.2 units (U) per mL and 91.3 U mL⁻¹, in the STR, with similar activity values of 60.6 U mL⁻¹ and 86.2 U mL⁻¹, respectively, being achieved in the airlift bioreactor. The airlift bioreactor also showed satisfactory results regarding the oxygen transfer rate in this process, indicating its potential to be used in an eventual larger scale production of exo- and endo-PG, with lower costs for both installation and operation.     

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.     

Hydrodynamics, mass transfer and rheological studies of gibberellic acid production in an airlift bioreactor

Although a lot of research has been done into modelling microbial processes, the applicability of these concepts to problems specific for bioreactor design and optimization of process conditions is limited. This is partly due to the tendency to separate the two essential factors of bioreactor modelling, i.e. physical transport processes and microbial kinetics. The deficiencies of these models become especially evident in industrial production processes where O₂ supply is likely to become the limiting factor, e.g. production of gibberellic acid and other organic acids. Hydrodynamics, mass transfer and rheology of gibberellic acid production by Gibberella fujikuroi in an airlift bioreactor is presented in this work. Important hydrodynamic parameters such as gas holdup, liquid velocity in the riser and in the downcomer, and mixing time were determined and correlated with superficial gas velocity in the riser. Mass transfer was studied evaluating the volumetric mass transfer coefficient, which was determined as a function of superficial gas velocity in the riser and as a function of fermentation time. Culture medium rheology was studied through fermentation time and allowed to explain the volumetric mass transfer coefficient behaviour. Rheological behaviour was explained in terms of changes in the morphology of the fungus. Finally, rheological studies let us obtain correlations for gas holdup and volumetric mass transfer coefficient estimation using the superficial gas velocity in the riser and the culture medium apparent viscosity.     

Hydrodynamics and mass transfer in aqueous two-phase protein extraction using a continuous perforated rotating disc contactor

A continuous perforated rotating disc contactor was used to extract bovine serum albumin (BSA) with aqueous two-phase systems based on polyethylene glycol (PEG) and phosphate salts. The dispersed phase holdup and mass transfer coefficient were determined. It was found that the dispersed phase holdup increased with increasing PEG phase velocity. The overall mass transfer coefficient for BSA was independent of the PEG phase velocity.     

Generation of small bubbles and small bubble-liquid mass transfer in airlift reactors containing highly viscous liquids

The time-dependent gas hold-up is investigated during the aeration of the Saccharomyces cerevisiae suspension, the aqueous saccharose solutions and the glycerol solutions in the external loop airlift reactor. Due to the time-dependent bubble size distribution the fraction of the small bubble hold-up in the total gas hold-up decreases with an increase of the gas flow rate and with a decrease of the viscosity. The course of the accumulation process of the small bubbles is described by the first-order kinetic equation. The small bubble accumulation rate is investigated in the airlift reactor and the bubble column. It is showed that the small bubbles form and disappear exclusively in the riser of the airlift reactor. It is found that the small bubble-liquid mass transfer coefficient is several times larger than the overall oxygen transfer coefficient.     

Scale-up impacts on mass transfer and bioremediation of suspended naphthalene particles in bead mill bioreactors

Scale-up effects on mass transfer and bioremediation of suspended naphthalene particles have been studied in 20 and 58L bead mill bioreactors and compared to data generated earlier with a laboratory scaled bioreactor. The bead mill bioreactor performance with respect to naphthalene mass transfer rate was dependent on the size and loading of the inert particles, as well as the rotational speed of the roller apparatus. The optimum operating conditions were found to be 15mm glass beads at a loading of 50% (total volume of particles/working volume of bioreactor: v/v%) and a bioreactor rotational speed of 50rpm. The highest naphthalene mass transfer coefficients obtained in the large scale system under these optimum conditions (19.6 and 22.4h(-1) for 20 and 58L vessels, respectively) were higher than those determined previously in a 2.5L bead mill bioreactor (0.7h(-1)). The acute toxicity tests indicated that the bioreactor effluent was less toxic than the untreated naphthalene suspension. Biodegradation rates obtained in these large scale bead mill bioreactors under optimum conditions (36-37.4mgL(-1)h(-1)) were higher than those achieved in the control bioreactors of similar sizes (11.4 and 11.6mgL(-1)h(-1)) but were slower than those previously determined in a 2.5L bead mill bioreactor (59-61.5mgL(-1)h(-1)). The limitation of oxygen in the large scale systems and damage of the bacterial cells due to the crushing effects of the large beads are likely contributing factors in the lower observed biodegradation rates. The optimum conditions with respect to naphthalene mass transfer might not necessarily translate to optimum performance with regard to bioremediation.     

Comparison of a production process in a membrane-aerated stirred tank and up to 1000-L airlift bioreactors using BHK-21 cells and chemically defined protein-free medium

The applicability of a protein-free medium for the production of recombinant human interleukin-2 with baby hamster kidney cells in airlift bioreactors was investigated. For this purpose, a BHK-21 cell line, adapted to grow and produce in protein-free SMIF7 medium without forming spheroids in membrane-aerated bubble-free bioreactors, was used as the producer cell line. First, cultivation of the cells was established at a 20-L scale using an internal loop airlift bioreactor system. During the culturing process the medium formulation was optimized according to the specific requirements associated with cultivation of mammalian cells under protein-free conditions in a bubble-aerated system. The effects of the addition of an antifoam agent on growth, viability, productivity, metabolic rates, and release of lactate dehydrogenase were investigated. Although it was possible to establish cultivation and production at a 20-L scale without the use of antifoaming substances, the addition of 0.002% silicon-oil-based antifoaming reagent improved the cultivation system by completely preventing foam formation. This reduced the release of lactate dehydrogenase activity to the level found in bubble-free aerated stirred tank membrane bioreactors and led to a reduction in generation doubling times by about 5 h (17%). Using the optimized medium formulation, cells were cultivated at a 1000-L scale, resulting in a culture performance comparable to the 20-L airlift bioreactor. For comparison, cultivations with protein-containing SMIF7 medium were carried out at 20- and 1000-L scales. The application of protein supplements did not lead to a significant improvement in the cultivation conditions. The results were also compared with experiments performed in a bubble-free aerated stirred tank membrane bioreactor to evaluate the influence of bubbles on the investigated culture parameters. The data implied a higher metabolic activity of the cells in airlift bioreactors with a 150% higher glucose consumption rate. The results of this study clearly demonstrate the applicability of a protein-free chemically defined medium for the production of recombinant proteins with BHK cells in airlift bioreactors.     

Polysaccharide concentration and molecular weight effects on the oxygen mass transfer in a reciprocating plate bioreactor

In most polysaccharide fermentations, the nature of the fermentation broth changes drastically with time and, as a result, the overall oxygen mass transfer coefficient (K(L)a) can vary by orders of magnitude. To obtain a better understanding of this phenomenon, an experimental program was devised to study the respective influence of molecular weight and concentration of dextran solutions on K(L)a. Experiments were conducted in a reciprocating plate bioreactor. This bioreactor uses a stack of perforated plates that is reciprocated axially in the column and it is therefore well suited for mixing viscous liquid broths and providing uniform overall mass transfer coefficients. The variation of K(L)a with the power input per unit volume and the superficial gas velocity were obtained for three ranges of molecular weights and five concentrations of dextran. In every medium, two regimes of operation were observed as a function of the power input per unit volume: a first regime, at low power inputs per unit volume where K(L)a remains constant until a threshold of power input is attained; and a second regime, which is characterized by a steep increase of K(L)a as a function of the power input per unit volume. The presence of dissolved biological macromolecules, not only because of their effect on the rheology of the medium but also because their effect on the gas-liquid interface, has a significant impact on K(L)a. It was found that, generally, small concentrations of polysaccharide favor oxygen mass transfer despite the increase in medium viscosity. However, the respective influence of polysaccharide concentration and molecular weight was different for the two regimes of operation. (c) 1996 John Wiley & Sons, Inc.     

Hydrodynamic characteristics and microalgae cultivation in a novel flat‐plate photobioreactor

Flat‐plate photobioreactors (FPPBRs) are widely reported for cultivation of microalgae. In this work, a novel FPPBR mounted with inclined baffles was developed, which can make the fluid produce a “spirality” flow. The flow field and cell trajectory in the photobioreactor were investigated by using computational fluid dynamics. In addition, the cell trajectory was analyzed using a Fast Fourier transformation. The influence of height of the baffles, the angle α between the inclined baffle and fluid inlet flow direction (z), and the fluid inlet velocity on the frequency of flashing light effect and pressure drop were examined to optimize the structure parameters of the inclined baffles and operating conditions of the photobioreactor. The results showed that with inclined baffles built‐in, significant swirl flow could be generated in the FPPBR. In this way, the flashing light effect for microalgal cell could also be achieved and the photosynthesis efficiency of microalgae could be promoted. In outdoor cultivation of freshwater Chlorella sp., the maximum biomass productivity of Chlorella sp. cultivated in the photobioreactor with inclined baffles was 29.94% higher than that of the photobioreactor without inclined baffles. © 2012 American Institute of Chemical Engineers Biotechnol. Prog., 2013     

Comparison of the cellular internalization of antibodies used either as immunotoxins or in ADEPT

The internalization into tumor cells of two antibodies (C242 and 454A12), which make potent immunotoxins when linked to ricin A-chain, and an antibody (A5B7), which does not make a potent immunotoxin but has proven useful in ADEPT, was evaluated. The 454A12 antibody was rapidly taken into the cells, 50% of the antibody being internalized after 2 h. The C242 antibody was internalized more slowly, approx 50% being taken up by the cells in 24 h. With A5B7, less than 10% of the antibody was internalized after 24 h. Internalization of the C242 antibody was accompanied by the appearance of antibody degradation products in the cell medium after 2 h, and this degradation could be inhibited by addition of a metabolic inhibitor that prevented cell internalization. In contrast, minimal degradation of the A5B7 antibody could be detected up to 24 h after binding to the cells. In conclusion, both 454A12 and C242 antibodies, which make potent immunotoxins, were internalized into tumor cels. The A5B7 antibody, which does not make a potent immunotoxin, was not internalized, and this property may be one reason why A5B7 has proved useful for delivery of enzymes in ADEPT.     

A comparison of the results following oromandibular reconstruction using a radial forearm flap with either radial bone or a reconstruction plate

Fifteen cases of oromandibular reconstruction using a radial osteocutaneous flap were compared with 16 in which the mandible was replaced with a reconstruction plate and a forearm flap was used for intraoral lining. All cases involved oral cancer; most had been irradiated. Nine survived in each group. Complications included one infected nonunion in addition to two bone exposures in the bone group, compared with three cases of plate exposure and two bone exposures in the plate group. Functional results were similar in both, but osteointegrated implants were possible only in the patients receiving bone. Cosmesis seemed somewhat better in the plate group. Donor-site problems were common but minor, and long-term forearm function was slightly reduced in both groups. Although the sample sizes were small, the reconstruction plate together with a radial forearm flap appeared to provide effective reconstruction following composite resection. However, we would not recommend this for the younger patient or in benign disease.     

Comments on the communication: on the calculation of shear rate and apparent viscosity in airlift and bubble column bioreactors

The determination of the shear rate in bubble column and airlift bioreactors is an important question from both the perspective of cell damage and the correlation of hydrodynamic parameters in non-Newtonian fluids in these contractors. In the context of correlating hydrodynamic parameters in non-Newtonian fluids, a common approach involves assuming that there exists an average shear rate in the column that is proportional to the superficial gas velocity. This average shear rate is then used to evaluate an effective viscosity of the non-Newtonian fluid that is subsequently used to quantify the fluid's rheological behavior in correlation. Contrary to a recent communication, this report illustrates that this approach, which has mainly been applied to bubble columns, can also be applied to external loop airlift contractors, replacing the superficial gas velocity by the superficial gas velocity by the superficial gas velocity supplied to the riser of the contractor. This extension is based upon consideration of the relevant characteristic velocity in the active zone (i.e., the riser section) of the reactor.     

Assessment of mass transfer and mixing in rigid lab‐scale disposable bioreactors at low power input levels

Mass transfer, mixing times and power consumption were measured in rigid disposable stirred tank bioreactors and compared to those of a traditional glass bioreactor. The volumetric mass transfer coefficient and mixing times are usually determined at high agitation speeds in combination with sparged aeration as used for single cell suspension and most bacterial cultures. In contrast, here low agitation speeds combined with headspace aeration were applied. These settings are generally used for cultivation of mammalian cells growing adherent to microcarriers. The rigid disposable vessels showed similar engineering characteristics compared to a traditional glass bioreactor. On the basis of the presented results appropriate settings for adherent cell culture, normally operated at a maximum power input level of 5 W m^?3, can be selected. Depending on the disposable bioreactor used, a stirrer speed ranging from 38 to 147 rpm will result in such a power input of 5 W m^?3. This power input will mix the fluid to a degree of 95% in 22 ± 1 s and produce a volumetric mass transfer coefficient of 0.46 ± 0.07 h^?1. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 30:1269–1276, 2014