Performance characteristics of yeast growth in a modified airlift fermenter

Two types of airlift fermenters, conventional (UT-ALF) and modified (CDT-ALF) were investigated to evaluate their performance with respect to baker's yeast growth. The riser tube of conventional external loop airlift fermenter is replaced by a converging-diverging tube, which is named as modified airlift fermenter having downcomer to riser cross-sectional area ratio A _d /A _r =1.8.The results were compared for the two types of airlift fermenter. A modified growth kinetics model for baker's yeast with oxygen as limiting substrate, has been proposed. The values of K _s and K _d of the growth model were determined from experimental data. The proposed model represented better for CDT-ALF system compared to UT-ALF. Compared to UT-ALF, CDT-ALF always showed higher cell mass concentration and low residual sugar concentration irrespective of the operating conditions. At optimum operating condition (initial glucose concentration 30 g/l, air flow rate 0.5 vvm and fermentation time 8 hrs.) 16.7% higher cell mass was observed in CDT-ALF compared to that in UT-ALF and yield (Y _x/s ) was found to be 0.51 which was theoretically very near to maximum achievable value.Symbols ALF Airlift fermenter - UT Uniform tube - CDT Converging-diverging tube - A _r Cross sectional area of riser - A _d Cross sectional area of downcomer - C _s Glucose cone, at any time, g/l - C _l Dissolved oxygen conc, at any time, g/l - _max Max. sp. growth rate, hr^–1 - Sp. growth rate, hr^–1 - X ₀ Initial cell mass cone. (dry wt.), g/l - X Cell mass conc. at any time t, g/l - C _s0 Initial glucose conc., g/l - C _s Glucose conc. at any time t, g/l - C _l Equilibrium conc. of oxygen, 0.0076 g/l - y _x/s Yield coefficient (dimensionless) - y _x/s gm cell mass produced/gm glucose consumed - Y _O2 gm cell produced/gm oxygen consumed - k _d maintenance coefficient, hr^–1 - K _L a volumetric mass transfer coefficient, hr^–1 - k _s saturation constant for the substrate, g/l - K _O2 saturation constant for the substrate of dissolved oxygen, g/l. This work was supported by a research grant from the Department of Biotechnology Govt. of India.     

Oxygen transfer measurements during yeast fermentations in a pilot scale airlift fermenter

Measurements of oxygen transfer were made during cultivation of the yeast Saccharomyces cerevisiae in a 90–250 litre working volume concentric tube airlift fermenter. Results demonstrated that the rate of oxygen transfer varies with position in the fermenter, being higher in the riser and top-section than in the downcomer and lowest near the base of the fermenter. The time for liquid circulation was generally smaller than the time constant for oxygen transfer (1/k_La) indicating that the rate of oxygen transfer was slow compared to the rate of liquid movement. Measured dissolved oxygen concentrations therefore did not represent the equilibrium arising from the balance between the rates of oxygen transfer and oxygen depletion. Hence measuredk _L a values were not representative of local oxygen transfer conditions but instead were indicators of the rate of mass transfer the liquid flow had encountered prior to reaching the point of measurement. Generally the individual rates of oxygen transfer in the vessel were found to increase with increasing vessel height.     

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.     

Effects of geometrical design on hydrodynamic and mass transfer characteristics of a rectangular-column airlift bioreactor

Gas holdup and gas–liquid mass transfer coefficients were measured in a 21-L rectangular-column airlift bioreactor with aspect ratio of 10 and working volumes ranging from 10 to 16 L. The effect of the bottom and top clearances was investigated using water and mineralized CMC solutions and covering a range of effective viscosity from 0.02 to 0.5 Pa s and surface tension from 0.065 to 0.085 N m⁻¹. The gas holdup and mass transfer results were successfully correlated using expressions derived via dimensional analysis. The separator gas holdup was found to be similar to the total gas holdup in the airlift bioreactor. The downcomer gas holdup (ɛ_d) increased two-fold when the bottom clearance (h_b) was increased from 0.014 to 0.094 m while the top clearance (h_t) had no effect. Increasing h_b decreased the mass transfer by 50% compared to 31% when the top clearance (h_t/D_hr) was increased. It was found that the gas–liquid separator diameter ratio (D_hs/D_hc) exerted the maximal influence of over 65% on mass transfer as compared to both clearances.     

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.     

Laboratory glass fermenter on the basis of the airlift principle

A 1.5-1 minifermenter consisting of RASOTHERM glass on the basis of the airlift principle was constructed and tested in the Institute of Biotechnology. The development of this fermenter bridged the gap between shake flasks and lab-scale fermenters and provided a possibility for the continuous cultivation of extremely halophilic bacteria. It was demonstrated that the fermenter can be used for cultivating bacteria that are sensitive to shear stress. It can also be recommended for purposes of cultivation of plant and animal cells because of this advantage and the possibility of this fermenter to be sterilized.     

Modeling of antibiotics production in magneto three-phase airlift fermenter

A mathematical model is developed to describe the performance of a three-phase airlift reactor utilizing a transverse magnetic field. The model is based on the complete mixing model for the bulk of liquid phase and on the Michaelis-Menten kinetics. The model equations are solved by the explicit finite difference method from transient to steady state conditions. The results of the numerical simulation indicate that the magnetic field increases the degree of bioconversion. The mathematical model is experimentally verified in a three-phase airlift reactor with P. chrysogenum immobilized on magnetic beads. The experimental results are well described by the developed model when the reactor operates in the stabilized regime. At relatively high magnetic field intensities a certain discrepancy in the model solution was observed when the model over estimates the product concentration.     

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.     

Low-pressure airlift fermenter for single cell protein production: I. Design and oxygen transfer studies

The energy consumption of a fermenter constitutes a major part of the operating expense of a single cell protein process. A low-pressure airlift fermenter was designed to reduce this cost. In this new design, the fermenter broth is kept below 120 cm in depth, and air alone is employed to fulfill the need of supplying oxygen, and cooling and agitating the broth. The use of low-pressure air from air blowers instead of air compressors lowers the capital cost of air delivery and reduces the energy consumption in the fermenter section to below 1 kWh/kg protein, a saving of over 70% as compared to a conventional stirred tank fermenter. It also eliminates the investment of mechanical agitators, heat exchangers, and air compressors. Sulfite oxidation studies confirmed the design concepts.     

Hydrodynamics and mass transfer in an airlift loop fermentor

Summary The hydrodynamics and mass transfer behaviour of an airlift fermentor with an external loop (height 10m) has been investigated by measuring gas and liquid velocities, gas hold-up, liquid mixing and oxygen transfer coefficients. Liquid phase properties, i.e., ionic strength, viscosity and surface tension have been varied by altering the fermentation media. Results are compared with those from bubble column experiments performed in the same unit. It is shown, that more uniform two-phase flow in the airlift leads to advantages in scale-up and operation.Nomenclature a Specific interfacial area per volume of dispersion (m²/m³) - c Local concentration of tracer (kmol/m³) - c Concentration of tracer at infinite time (kmol/m³) - C_L Concentration of oxygen in the liquid bulk (kmol/m³) - C_L ^* Concentration of oxygen in the interface (kmol/m³) - D_ax Axial dispersion coefficient (cm²/s) - I Ionic strength (kmol/m³) - i Inhomogeneity [defined in Eq. (2)] - Rate of oxygen transfer (kmol/s) - t_c Circulation time (s) - t_M Mixing time (s) - V_R Volume of gas-liquid dispersion (m³) - V_SG Superficial gas velocity in up-flow column (m/s) Greek letter symbols _L Oxygen transfer coefficient (m/s) - Dynamic viscosity (m Pa s) - Surface tension (m N/m) Presented at the First European Congress on Biotechnology, Interlaken, September 25–29, 1978     

Growth and development of fern gametophytes in an airlift fermenter

Spores of the fernsPteridium aquilinum andAnemia phyllitidis were grown in an airlift fermenter and subsequent growth and development of gametophytes was monitored. Both species produced greater biomass than that generated in any other solid- or liquid-based culture system tested.Pteridium generated more tissue thanAnemia in every system. The morphology of airlift-grown gametophytes was similar to that of soil-grown plants; fewer gametophytes with perturbed development were observed in airlift cultures than in the other liquid-based systems. No attempt was made to optimise airlift conditions for the species and tissue employed, so it is concluded that airlift cultivation is a promising system for the bulk production of fern gametophytic tissue.     

Photoautotrophic growth of cell suspension cultures fromChenopodium rubrum in an airlift fermenter

Summary This communication describes the construction and operation of an airlift fermenter for the photoautotrophic growth of cell suspension cultures fromChenopodium rubrum. The basic batch culture unit provides a culture of 1.51 volume, sufficient to permit frequent aseptic sampling. It can be maintained at any desired temperature and aerated to different extents. Using an initial cell density of about 400,000 cells per ml suspension, the increase in cell number is 270% after a 14 days' growth period, although the stationary phase of growth is not yet reached. The transfer of photoautotrophic cell suspensions fromChenopodium rubrum from stationary growth into the large volume of fresh culture medium in the airlift fermenter results in an immediate protein formation, followed by an exponential phase of cell division, whereas rapid chlorophyll accumulation is delayed by 2 days.The growth capacities of photoautotrophic fermenter cultures including protein and chlorophyll formation as well asin vitro activities of the ribulosebisphosphate carboxylase and the phosphoenolpyruvate carboxylase are greatly lower as compared to photoautotrophic cells propagated in standard two-tier culture vessels using 30 ml culture medium. However the pattern of change in the activities of carboxylation enzymes is quite similar in both culture systems.Photoautotrophic cell suspensions fromChenopodium rubrum grown in an airlift fermenter assimilate about 90 mol CO₂/mg chlorophyll × hour. Dark CO₂ fixation is about 1.5% of the light values.Abbreviations PEF phosphoenolpyruvate - RuDP ribulosebisphosPhate - NS ground glass joints of standardized size made from Duran glass, Schott, Germany     

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.     

Comparison of the hydrodynamics and mass transfer characteristics in internal-loop airlift bioreactors utilizing either a novel membrane-tube sparger or perforated plate sparger

Two gas spargers, a novel membrane-tube sparger and a perforated plate sparger, were compared in terms of hydrodynamics and mass transfer (or oxygen transfer) performance in an internal-loop airlift bioreactor. The overall gas holdup ε _T, downcomer liquid velocity V _d, and volumetric mass transfer coefficient K _L a were examined depending on superficial gas velocity U _G increased in Newtonian and non-Newtonian fluids for the both spargers. Compared with the perforated plate sparger, the bioreactor with the membrane-tube sparger increased the values of ε _T by 4.9–48.8 % in air–water system when the U _G was from 0.004 to 0.04 m/s, and by 65.1–512.6 % in air–CMC solution system. The V _d value for the membrane-tube sparger was improved by 40.0–86.3 %. The value of K _L a was increased by 52.8–84.4 % in air–water system, and by 63.3–836.3 % in air–CMC solution system. Empirical correlations of ε _T, V _d, and K _L a were proposed, and well corresponding with the experimental data with the deviation of 10 %.     

Studies on transfer processes in mixing vessels: effect of particles on gas-liquid mass transfer using modified Rushton turbine agitators

The influence of carbon dioxide concentration in liquid medium on elemental sulphur oxidation by Thiobacillus thiooxidans bacteria presented in this paper can be divided into 3 differing relationships. First relationship shows increase of sulphur biooxidation rate with increase of carbon dioxide concentration in liquid medium. Second one shows decrease of S⁰ oxidation rate with increase of CO₂ concentration in nutrient and in the third relationship there is no influence of carbon dioxide concentration on sulphur oxidation by Thiobacillus thiooxidans bacteria. The influence of carbon dioxide concentration in liquid nutrient on alive bacteria concentration in liquid medium is similar to those described above.     

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.     

Mixing time and oxygen transfer characteristics of double draft tube airlift fermentor

Despite the increasing importance of airlift fermentors, very little published information is available on how the geometric configurations of the draft tubes and the air-sparging system affect the mixing and oxygen transfer characteristics of the fermentor. A 14-L air-lift fermentor was designed and build with a fixed liquid height to diameter ratio of 1.5 utilizing four equally spaced air jets at the bottom. Two jet orifice sizes were used, 1.27 and 3.81 mm i.d., and for each jet size the following four geometric configurations were used: Single inner concentric draft tube, single outer concentric draft tube, two concentric draft tubes, and no draft tubes where the fermentor was operated as a shallow bubble column. It was found that the presence of draft tubes stabilized liquid circulation patterns and gave systemically higher mixing times than those obtained in the absence of draft tubes. In addition, the double draft tube geometry resulted in higher mixing times than the single draft tubes. For the power unit volume range 20 to about 250 W/m³ the larger 3.81-mm orifices gave systemically higher k_L a values than the smaller 1.27-mm i.d. orifices. At 200 W/m³ the use of a single outer draft tube with the 3.81-mm orifices resulted in 94% increase in k_L a values over that obtained with no draft tubes. However, the effect of draft tube geometry on k_L a values when the 1.27-mm orifices were used was not significant. The air bubble formation characteristics at the jet orifices were found to be different, which reflected the differences observed in mass transfer and mixing characteristics. The power economy for oxygen transfer was found to be depend strongly on the orifice size and less on the geometric configuration of draft tubes.     

Oxygen mass transfer characteristics in a membrane-aerated biofilm reactor

Immobilization of pollutant-degrading microorganisms on oxygen-permeable membranes provides a novel method of increasing the oxidation capacity of wastewater treatment bioreactors. Oxygen mass transfer characteristics during continuous-flow steady-state experiments were investigated for biofilms supported on tubular silicone membranes. An analysis of oxygen mass transport and reaction using an established mathematical model for dual-substrate limitation supported the experimental results reported. In thick biofilms, an active layer of biomass where both carbon substrate and oxygen are available was found to exist. The location of this active layer varies depending on the ratio of the carbon substrate loading rate to the intramembrane oxygen pressure. The thickness of a carbon-substrate-starved layer was found to greatly influence the mass transport of oxygen into the active biomass layer, which was located close to, but not in contact with, the biofilm-liquid interface. The experimental results demonstrated that oxygen uptake rates as high as 20 g m-2 d-1 bar-1 can be achieved, and the model predicts that, for an optimized biofilm thickness, oxygen uptake rates of more than 30 g m-2 d-1 bar-1 should be possible. This would allow membrane-aerated biofilm reactors to operate with much greater thicknesses of active biomass than can conventional biofilm reactors as well as offering the further advantage of close to 100% oxygen conversion efficiencies for the treatment of high-strength wastewaters. In the case of dual- substrate-limited biofilms, the potential to increase the oxygen flux does not necessarily increase the substrate (acetate) removal rate.