Membrane sparger in bubble column,airlift, and combined membrane–ring sparger bioreactors

The bubble column and the two internal loop airlift reactors (riser/downcomer area ratios of 0.11 and 0.58) characterized in this study were equipped with a rubber membrane sparger, which produced small bubbles, giving high mass transfer coefficients. The low mixing intensity in the bubble column was increased by an order of magnitude in the airlift reactors. We designed a novel aeration and mixing system by adding a ring sparger to the membrane sparger in the bubble column and maintained the advantages of both airlift configuration (good mixing properties) and bubble column configuration (efficient aeration, without any internal constructions). The combined membrane–ring sparger system has unique features with respect to the efficiency of utilization of substrate gasses and energy. Model experiments showed that the small bubbles from the membrane sparger do not coalesce with the large bubbles from the ring sparger. If different gases were added through the two spargers it was possible to transfer a hazardous or expensive gas quantitatively to the liquid through the membrane sparger (dual sparging mode). In the combined membrane–ring sparger system the energy input for mixing and mass transfer is divided. Therefore, the energy consumption can be minimized if the flow distribution of air through the membrane and ring sparger is controlled by the oxygen demand and the inhomogeneity of the culture, respectively (split sparging mode). The dual sparging mode was used for mass production of the alga Rhodomonas sp. as the first step in aquatic food chains. Avoiding mechanical parts removes an important risk of malfunction, and a continuous culture could be maintained for more than 8 months. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 64: 452–458, 1999.     

Scaling oxygen mass transfer in agitated fermentors

There are many scaling formulas that predict the oxygen mass transfer coefficient as k(L).a = constant.(Hp/V)(alpha)Vs(beta) Exponents alpha and beta frequently are scale dependent themselves. A general formula has been derived from the work of Calderbank,(1) Miller,(2) and Tilton,(3) resulting in k(L).a = C(1) phi + C(2) log (Pm/V) phi where phi equals the gas-holdup fraction and Pm/V equals the effective mechanical power input per unit of volume. This formula is consistent with the formula of Westerterp(4) modified by Miller.(2) Gas holdup can be predicted in several ways. Gas-sparged isothermal expansion power input, used for predicting phi, demonstrates that scaling can be done by using either superficial air velocity or volume per volume per minute for aeration.The importance of mixing in replenishing oxygen at the boundary layers of microorganisms will be assessed and compared with the k(L).a as the oxygen transfer ratelimiting step.     

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.     

Process-engineering characterization of small-scale bubble columns for microbial process development

Volumetric oxygen transfer rates and power inputs were estimated by a model of the formation of primary gas bubbles at the static sparger (sinter plate) of small-scale bubble columns and a common mass-transfer correlation for bubbles rising in a non-coalescent Newtonian electrolyte solution of low viscosity. Estimations were used to assess the dimensioning and possibilities of small-scale bubble column application with an height/diameter ratio of about 1. Estimations of volumetric oxygen transfer rates (<0.16 s^-1) and power inputs (<100 W m^-3) with a mean pore diameter of the static sparger of 13 µm were confirmed as function of the superficial air velocity (<0.6 cm s^-1) by measurements using an Escherichia coli fermentation medium. Small-scale bubble columns are thus to be classified between shaking flasks and stirred-tank reactors with respect to the oxygen transfer rate, but the maximum volumetric power input is more than one magnitude below the power input in shaking flasks, which is of the same order of magnitude as in stirred-tank reactors. A small-scale bubble columns system was developed for microbial process development, which is characterized by handling in analogy to shaking flasks, high oxygen transfer rates and simultaneous operation of up to 16 small-scale reactors with individual gas supply in an incubation chamber.     

Mass transfer and liquid mixing in an airlift reactor with a net draft tube

Mass transfer and liquid mixing in an airlift reactor with a net draft tube were experimentally investigated. Four different column diameters were considered. The mass transfer was measured using the volumetric gas-liquid mass transfer coefficient which was determined by the dynamic method. The mass transfer coefficients in the airlift reactors with different column diameters were not always higher than those in the bubble columns. The liquid mixing was measured using mixing time which was determined by a pulse technique. Under the same superficial gas velocity, the mixing times of the airlift reactors with a net draft tube were always less than those of the bubble columns.List of Symbols C mol·dm^–3 bulk concentration of dissolved oxygen - C ₀ mol·dm^–3 initial concentration of dissolved oxygen - C _e mol·dm^–3 saturated concentration of dissolved oxygen - ¯C dimensionless dissolved oxygen concentration - D _c cm diameter of column - D _N cm diameter of the nozzle hole - D _T cm diameter of the net draft tube - H _L cm static liquid height - H _T cm height of the net draft tube - k _L a hr^–1 volumetric mass transfer coefficient - L _T cm length of the net draft tube - t _M sec mixing time of the liquid phase - t ₀ sec mixing time of the liquid phase in a bubble column - V _L dm³ volume of the liquid phase - U _g cm/s superficial air velocity     

Bubble characteristics and mass transfer in an airlift reactor with multiple net draft tubes

A pilot scale airlift reactor with multiple net draft tubes was developed. The reactor, 29?cm in diameter and 300?cm height, had four modules of double net draft tubes. Bubble size, bubble number, gas holdup, and volumetric mass transfer coefficient were measured under different superficial air velocities. The air velocity had little effect on bubble size but had significant influence on bubble number. A bubble column was also investigated for comparison. The airlift reactor had a higher gas holdup and volumetric mass transfer coefficient than those in the bubble column. The draft tubes in the airlift reactor substantially improved the reactor performance.     

Liquid-phase dispersion in an airlift reactor with a net draft tube

Liquid-phase dispersion in an airlift reactor with a net draft tube was considered. Four net tubes with different ratios of draft tube to reactor diameters and superficial air velocities ranged from zero to 6.05 cm/s were investigated. The sparger was a porous plate. The parameter of the dispersion effect, axial dispersion coefficient, was characterized by measuring the residence time distribution in the liquid phase with single-pulse tracer input. The values of the dispersion coefficient of the proposed airlift reactor were much higher than those of the bubble column under the same operating conditions.     

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.     

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.     

High oxygen transfer rate in a new aeration system using hollow fiber membrane

A new type of bubble aeration column called a hollow fiber membrane (HFM) aeration column was proposed, which was featured in the use of hollow fiber membranes and gave a high bubble density in the column. The value of k(L)a was increased by modifying the membrane surface for making the pore size smaller. The Sauter mean diameter of bubbles (D(vs)) was 2.0 +/- 0.1 mm in the range of the superficial gas velocity from 0.02 m s(-1) to 0.065 m s(-1), while that obtained for the bubbles near the membrane was 811 mum at the superficial gas velocity of 4.0 x 10(-4) m s(-1). The difference was ascribed to the effect of coalescence of bubbles. The value of K(L)a increased in proportion to the superficial gas velocity up to 0.02 m s(-1), and was almost constant above 0.03 m s(-1). The maximum value of k(L)a, 2.5 s(-1), was higher than those of the other aeration columns reported previously. The pneumatic power consumption per unit liquid volume (P(v)) for obtaining the same k(L)a was the smallest in the HFM aeration columns. P(v), for obtaining the same interfacial area of bubbles per liquid volume, was also lower than those for other types of aeration columns. It was suggested from the measurement of bubble diameter that the larger interfacial area generated in the HFM aeration column ascribes to the larger gas holdup than the smaller D(vs). (c) 1992 John Wiley & Sons, Inc.     

Hydrodynamic and mass transfer characteristics of three-phase gaslift bioreactor systems.

This review focuses on the hydrodynamic and mass transfer characteristics of various three-phase, gaslift fluidized bioreactors. The factors affecting the mixing and volumetric mass transfer coefficient (k(L)a), such as liquid properties, solid particle properties, liquid circulation velocity, superficial gas velocity, bioreactor geometry, are reviewed and discussed. Measurement methods, modeling and empirical correlations are reviewed and compared. To the authors' knowledge, there is no 'generalized' correlation to calculate the volumetric mass transfer coefficient, instead, only 'type-specific' correlations are available in the literature. This is due to the difficulty in modeling the gaslift bioreactor, caused by the variation in geometry, fluid dynamics, and phase interactions. The most important design parameters reported in the literature are: gas hold-up, liquid circulation velocity, 'true' superficial gas velocity, mixing, shear rate, aeration rate and volumetric mass transfer coefficient, k(L)a.     

Effect of surface contaminants on oxygen transfer in bubble column reactors

Gas hold-up (ɛ_g), sauter mean bubble diameter (d₃₂) and oxygen transfer coefficient (k_La) were evaluated at four different alkane concentrations (0.05, 0.1, 0.3 and 0.5 vol.%) in water over the range of superficial gas velocity (u_g) of (1.18–23.52) × 10⁻³ m/s at 25 °C in a laboratory-scale bubble column bioreactor. Immiscible hydrocarbons (n-decane, n-tridecane and n-hexadecane) were utilized in the experiments as impurity. A type of anionic surfactant was also employed in order to investigate the effect of addition of surfactant to organic-aqueous systems on sauter mean bubble diameter, gas hold-up and oxygen transfer coefficient. Influence of addition of alkanes on oxygen transfer coefficient and gas hold-up, was shown to be dependent on the superficial gas velocity. At superficial gas velocity below 0.5 × 10⁻³ m/s, addition of alkane in air–water medium has low influence on oxygen transfer coefficient and also gas hold-up, whereas; at higher gas velocities slight addition of alkane increases oxygen transfer coefficient and also gas hold-up. Increase in concentration of alkane resulted in increase in oxygen transfer coefficient and gas hold-up and roughly decrease in sauter mean bubble diameter, which was attributed to an increase in the coalescence-inhibiting tendency in the presence of surface contaminant molecules. Bubbles tend to become smaller with decreasing surface tension of hydrocarbon, thus, oxygen transfer coefficient increases due to increasing of specific gas–liquid interfacial area (a). Empirical correlations were proposed for evaluating gas hold-up as a function of sauter mean bubble diameter, superficial gas velocity and interfacial surface tension as well as evaluating Sherwood number as a function of Schmidt, Reynolds and Bond numbers.     

Enhancing gas-liquid mass transfer rates in non-newtonian fermentations by confining mycelial growth to microbeads in a bubble column

The performance of a penicillin fermentation was assessed in a laboratory-scale bubble column fermentor, with mycelial growth confined to the pore matrix of celite beads. Final cell densities of 29 g/L and penicillin titres of 5.5 g/L were obtained in the confined cell cultures. In comparison, cultures of free mycelial cells grown in the absence of beads experienced dissolved oxygen limitations in the bubble column, giving only 17 g/L final cell concentrations with equally low penicillin titres of 2 g/L. The better performance of the confined cell cultures was attributed to enhanced gas liquid mass transfer rates, with mass transfer coefficients (k(L)a) two to three times higher than those determined in the free cell cultures. Furthermore, the confined cell cultures showed more efficient utilization of power input for mass transfer, providing up to 50% reduction in energy requirements for aeration.     

Promotion of oxygen transfer in three-phase fluidized-bed bioreactors by floating bubble breakers

The objective of the present study was to investigate a method to enhance the volumetric rate of oxygen transfer in three-phase fluidized-bed bioreactors. The rates of oxygen transfer from air bubbles to viscous liquid media were promoted by floating bubble breakers in three-phase fluidized beds operated in the bubble coalescing regime. The liquid-phase volumetric oxygen transfer coefficient has been recovered by fitting the axial dispersion model to the resultant data, and its dependence on the experimental variables, such as the gas and liquid flow rates, particle size, concentration of bubble breakers, and liquid viscosity, has been examined. The results indicate that the liquid-phase volumetric oxygen transfer coefficient can be enhanced up to 20-25%. The coefficient exhibits a maximum with respect to the volume ratio of the floating bubble breakers to the fluidized solid particles; it increases with increases in the gas and liquid flow rates and size of fluidized particles, while it decreases with an increase in the liquid viscosity. An expression has been developed to correlate the liquid-phase volumetric oxygen transfer coefficient with the experimental variables.     

Response of <Emphasis Type="Italic">Tisochrysis lutea</Emphasis> [Prymnesiophycidae] to aeration conditions in a bench-scale photobioreactor

The effects of superficial gas velocity (U_gr), gas entrance velocity (ν), and bubble size on the growth of Tisochrysis lutea was investigated in 600-mL photobioreactors operated with airlift pumps. Superficial gas velocities, calculated from measured air flow rates, ranging from 7 to 93 mm s^?1 were created using a 1.6-mm diameter syringe. We tested the effects of sparger velocity over a range of 2.48 to 73.4 m s^?1 and the effects of bubble size by using two styles of air stones and an open glass pipette, which created a bubble sizes in the range of 0.5 to 5 mm. We calculated oxygen mass transfer coefficient, k_La, values for all experimental conditions. Cell growth increased linearly with increased superficial gas velocity and decreased with increased sparger velocity. Results indicated that smaller bubble size leads to some initial cell damage, but after time, the increased gas transfer as reflected by the k_La value produced higher growth than larger bubbles. Two mechanisms were observed to correlate with cell damage in T. lutea: increasing velocity at the sparger tip and bubble bursting at the surface. These results demonstrate a method to test sensitivity of T. lutea to aeration, which is important for the design of airlift systems.     

Hydrodynamics in bubble column bioreactors with fermentation broths having a yield stress

Summary The hydrodynamics in a bubble column bioreactor with fermentation broths having a yield stress are studied. Specifically, the liquid velocity at the reactor axis, the axial dispersion coefficient, and the gas hold-up are examined. The liquid velocity at the reactor axis and the gas hold-up are measured in a 40-1 bench-scale bubble column fermentor using carboxypolymethylene (Carbopol) aqueous solutions as simulated broths. Theoretical correlations for the liquid velocity at the reactor axis, the axial dispersion coefficient, and the gas hold-up are derived on the basis of an energy balance and the mixing length theory. The correlations are compared with the present data and a reasonable agreement is found. The theoretical predictions are also in satisfactory agreement with the re-examined data for actual fermentation broths which are Chaetomium cellulolyticum and Neurospora sitophila cultured in a 1000-1 pilot-plant scale airlift fermentor.     

Hydrodynamic and Mass Transfer Characteristics of Three-Phase Gaslift Bioreactor Systems

ABSTRACT:?

This review focuses on the hydrodynamic and mass transfer characteristics of various three-phase, gaslift fluidized bioreactors. The factors affecting the mixing and volumetric mass transfer coefficient (k_La), such as liquid properties, solid particle properties, liquid circulation velocity, superficial gas velocity, bioreactor geometry, are reviewed and discussed. Measurement methods, modeling and empirical correlations are reviewed and compared. To the authors' knowledge, there is no 'generalized' correlation to calculate the volumetric mass transfer coefficient, instead, only 'type-specific' correlations are available in the literature. This is due to the difficulty in modeling the gaslift bioreactor, caused by the variation in geometry, fluid dynamics, and phase interactions. The most important design parameters reported in the literature are: gas hold-up, liquid circulation velocity, 'true' superficial gas velocity, mixing, shear rate, aeration rate and volumetric mass transfer coefficient, k_La.     

Oxygen transfer kinetics of riboflavin fermentation by Ashbya gossypii in agitated fermentors

Effects of agitation and aeration rates on volumetric oxygen transfer coefficient and oxygen uptake rate of a riboflavin broth containing Ashbya gossypii were investigated in three batch, sparged, and agitated fermentors having the working volumes of 0.42, 0.85, and 2.5 l. The change of oxygen uptake rate with time at 250 rev min⁻¹ stirring and vvm aeration rates was shown. The volumetric oxygen transfer coefficients and maximum oxygen uptake rates obtained have been correlated to mechanical power inputs per unit volume of the fermentation broth and the superficial air velocities.     

Gas holdup and overall volumetric oxygen transfer coefficient in airlift contactors

The two major types of airlift contactors, concentric-tube and external-loop, were investigated for their gas holdup (riser and downcomer) and overall mass transfer characteristics. Results obtained in batch charges of tap water and 0.15 kmol/m(3) NaCl solution are reported for external-loop airlift contactors having downcomer-to-riser cross-sectional area ratios, A(d)/A(r), ranging from 0.11 相似文献   

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.