Mass Transfer Studies in Stirred AirLift Reactor

In the present work, water and three phase compositions of Solka-Floc, a cellulose fiber for simulating the biomass in bacteria, yeast, and fungal fermentation were studied in a 1.4?m³ stirred airlift reactor. The fractional dispersed phase holdup and the overall volumetric mass transfer coefficients were measured. The dispersed phase riser gas holdup and overall volumetric mass transfer coefficients both increased with increasing riser superficial dispersed phase velocity (0.02–0.1?ms^?1) and agitator speed in the range of 0–5?rs^?1. An increase in the Solka-Floc concentration (1–3% w/v) was found to reduce ?_GR and K _L a _L . Empirical correlations have been developed for fractional dispersed phase gas holdup and overall volumetric mass transfer coefficients.     

D‐arabitol production from lactose by Kluyveromyces lactis at different aerobic conditions

BACKGROUND: The pentitol D‐arabitol has been produced from D‐glucose utilizing osmophilic yeast strains, however, there are remarkably few reports available on the production of D‐arabitol from lactose. Previous studies in the laboratory have shown that the osmophilic yeast Kluyveromyces lactis NBRC 1903 can convert lactose to extracellular D‐arabitol without extracellular accumulation of D‐glucose or D‐galactose. The present study was undertaken to determine the participation of aeration on the D‐arabitol synthesis in K. lactis NBRC 1903. RESULTS: The highest D‐arabitol concentration of 91.7 mmol L^?1 was achieved after 120 h cultivation in medium containing 555 mmol L^?1 of lactose with initial volumetric liquid‐phase mass transfer coefficient of oxygen (k_La)₀ of 85.5 h^?1. The fractional yield of D‐arabitol was affected by not only aeration but also growth phase. The highest fractional yield of D‐arabitol in terms of lactose consumption was 0.255 that was obtained at stationary phase with (k_La)₀ of 85.5 h^?1. CONCLUSION: It was found that oxygen supply is a key factor in the production of D‐arabitol. Patterns of metabolism were classified according to the level of oxygen supply and the growth phase. Copyright © 2010 Society of Chemical Industry     

Conversion efficiency of glucose/xylose mixtures for ethanol production using Saccharomyces cerevisiae ITV01 and Pichia stipitis NRRL Y‐7124

BACKGROUND: Efficient conversion of glucose/xylose mixtures from lignocellulose is necessary for commercially viable ethanol production. Oxygen and carbon sources are of paramount importance for ethanol yield. The aim of this work was to evaluate different glucose/xylose mixtures for ethanol production using S. cerevisiae ITV‐01 (wild type yeast) and P. stipitis NRRL Y‐7124 and the effect of supplying oxygen in separate and co‐culture processes. RESULTS: The complete conversion of a glucose/xylose mixture (75/30 g L^?1) was obtained using P. stipitis NRRL Y‐7124 under aerobic conditions (0.6 vvm), the highest yield production being Y_p/s = 0.46 g g^?1, volumetric ethanol productivity Q_pmax = 0.24 g L^?1 h^?1 and maximum ethanol concentration P_max = 34.5 g L^?1. In the co‐culture process and under aerobic conditions, incomplete conversion of glucose/xylose mixture was observed (20.4% residual xylose), with a maximum ethanol production of 30.3 g L^?1, ethanol yield of 0.4 g g^?1 and Q_pmax = 1.26 g L^?1 h^?1. CONCLUSIONS: The oxygen present in the glucose/xylose mixture promotes complete sugar consumption by P. stipitis NRRL Y‐7124 resulting in ethanol production. However, in co‐culture with S. cerevisiae ITV‐01 under aerobic conditions, incomplete fermentation occurs that could be caused by oxygen limitation and ethanol inhibition by P. stipitis NRRL Y‐7124; nevertheless the volumetric ethanol productivity increases fivefold compared with separate culture. Copyright © 2011 Society of Chemical Industry     

Model development,parameter identification,and simulation of SCP production processes in air lift tower reactors with external loop—II: Extended culture modelling and quasi steady state parameter identification

Yeast was cultivated in extended culture in a bench-scale 275 cm high air lift tower reactor 15 cm dia. with an external loop. Longitudinal dissolved oxygen concentration profiles, substrate and cell mass concentrations in the medium, O₂ and CO₂ concentrations in the gas phase, as well as gas flow rates and liquid recirculation rates were measured. A distributed parameter model was used to describe the cultivation process variation along the column, cell mass, substrate and oxygen balances in the medium, O₂ and CO₂ balances in the gas phase, variation of the volumetric mass transfer coefficient along the column due to bubble coalescence, as well as double substrate Monod kinetics. Based on simulation runs it was assumed that under non limited and oxygen transfer limited growth conditions, the cell mass and substrate concentrations are uniform in the reactor. The simulation was carried out by a hybrid computer. The unknown model parameters (volumetric mass transfer coefficient at the gas entrance, k_La^E, and coalescence factor K_ST) and two kinetic parameter R_Omax and K_O were identified by means of experimental results with quasi steady state simulation methods.     

Removal of Cu2+ from Water Using Liquid-Liquid Microchannel Extraction

The very good extraction selectivity of Cu²⁺ from water was demonstrated with a new microchannel equipment, by employing di-(2-ethylhexyl)phosphoric acid (D2EHPA) as an extractant and kerosene as a solvent. The effects of different experimental parameters on the extraction efficiency E, the volumetric mass transfer coefficient K_La, and the entrainment were experimentally investigated. The results showed that the extraction efficiency increased with increasing temperature, extractant concentration, phase ratio (organic/aqueous), and pH. The total flow rate, phase ratio, and pH were found to have a great effect on the mass transfer, whereas the temperature and the extractant concentration showed little effect.     

Nonsteady-state simulation of extended cultures in tower loop reactors

Hansenula polymorpha was cultivated in extended culture in an air lift tower loop reactor with the substrates enthanol and glucose, respectively. The spacial and time variations of process variables were simulated by a hybrid computer by means of a distributed parameter (dispersion) model, taking into account the spacial variations of pressure, volumetric mass transfer coefficient, dissolved oxygen concentration, oxygen gas phase mole fraction and gas velocity. The volumetric mass transfer coefficient at the gas entrance, k_La^E, its variation near the gas distributor, described by a coalescence factor, K_ST, and the oxygen saturation constant, K_O, were identified by quasi-steady-state methods.The kinetic parameters, maximum specific growth rate, μ_max, and oxygen yield coeflicient, Y_X/O, were identified during nonstationary simulation. The distributed parameter cultivation processes were simulated in a z (dimensionless longitudinal distance)—t (cultivation time)- The agreement between calculated and measured process variables is excellent.     

Mass Transfer Characteristics in a Rotor‐Stator Reactor

Mass transfer characteristics in a rotor‐stator reactor in terms of the overall volumetric mass‐transfer coefficient (K_xa) using the N₂‐H₂O‐O₂ system were investigated. The effects of various operating parameters including rotation speed, liquid volumetric flow rate, and gas volumetric flow rate on K_xa were systematically examined, and a gas‐liquid mass transfer model was established to predict K_xa. Results reveal that K_xa increased with higher rotation speed, liquid volumetric flow rate, and gas volumetric flow rate. The results also confirm that the predicted values of K_xa were in agreement with the experimental values with deviation within 15 %. The results contribute to a better understanding of mass transfer characteristics in rotor‐stator reactors.     

Mass Transfer Reaction Kinetics of β‐Isophorone Oxidation by Air in an Agitator Bubbling Reactor

The mass transfer reaction kinetics of β‐isophorone (β‐IP) oxidation reaction by air was investigated in a lab‐scale agitator bubbling reactor. This reaction can be regarded as instantaneous and there exists a critical concentration of β‐IP. When the catalyst concentration is kept unchanged, the reaction rate is only controlled by the gas film and the reaction kinetics is of zero order with respect to β‐IP when the β‐IP concentration lies over the critical concentration. The reaction rate is controlled by the dual film and the kinetics is of first order with respect to β‐IP when its concentration is below the critical concentration. Under the gas film‐controlling condition, the effect of temperature, agitator speed, and aeration on the reaction rate is evaluated. A correlation equation of gas phase volumetric mass transfer coefficients combining superficial gas velocity and agitator speed is defined.     

Process kinetics of UASB reactors treating non‐inhibitory substrate

The degradation of a non‐inhibitory substrate (sucrose) in upflow anaerobic sludge bed (UASB) reactors with different superficial flow velocites (u_s) was performed to generate experimental data. Additionally, a kinetic model accounting for the mass fraction of methanogens (f) and granule size distribution in UASB reactors is also proposed. At the volumetric loadings of 2.65–21.16 g COD dm^?3 day^?1, both the COD removal efficiency and granule size of the UASB reactors increase with increasing u_s. The f values determined experimentally increase from 0.13–0.24 to 0.27–0.43 if the volumetric loading is increased from 2.65 to 5.29 g COD dm^?3 day^?1. With a further increase in volumetric loading, the f values decline because of the accumulation of volatile fatty acids (VFAs). The predicted residual concentrations of VFAs and COD are in fairly good agreement with the experimental data. From the calculated effectiveness‐factor values, the influence of mass transfer resistance of the substrate sucrose on the overall substrate removal rate should not be neglected. From parametric sensitivity analyses together with the simulated concentration profiles, methanogenesis is the rate‐limiting step. Copyright © 2003 Society of Chemical Industry     

Improved mass transfer and biodegradation rates of naphthalene particles using a novel bead mill bioreactor

One of the main challenges in the treatment of polycyclic aromatic hydrocarbons (PAHs) in controlled bioreactors is the hydrophobicity and low solubility of these compounds in the aqueous phase, resulting in appreciable mass transfer limitations within the bioreactor. To address this challenge, we have developed a modified roller bioreactor (Bead Mill Bioreactor) in which inert particles are used to improve mass transfer from the solid phase to the aqueous phase. Experimental results with naphthalene as a model PAH and Pseudomonas putida as a candidate bacterium indicate that both the mass transfer rate from the solid phase to liquid phase and the biodegradation rate in the Bead Mill Bioreactor (BMB) were significantly higher than those in a conventional roller bioreactor (20‐fold and 5.5‐fold, respectively). The enhancement of mass transfer was dependent on the type, size and volumetric loading of the inert particles, as well as concentration of particulate naphthalene. The highest mass transfer coefficient (k_La = 2.1 h^?1) was achieved with 3 mm glass beads at a volumetric loading of 50% (particle volume/working volume) with 10 000 mg dm^?3 particulate naphthalene. The maximum biodegradation rate of naphthalene attained in the bead mill bioreactor (59.2 mg dm^?3 h^?1 based on the working volume and 118.4 mg dm^?3 h^?1 based on the liquid volume) surpasses most other rates published in the literature and is equivalent to values reported for more complex bioreaction systems. The bead mill bioreactor developed in the present work not only enjoys a simple design but shows excellent performance for treatment of PAHs suspended in an aqueous phase. This fundamental information will be of significant value for future studies involving soil‐bound PAHs. Copyright © 2005 Society of Chemical Industry     

Kinetic study on ethanol production using Saccharomyces cerevisiae ITV‐01 yeast isolated from sugar cane molasses

BACKGROUND: Bio‐ethanol production from renewable sources, such as sugar cane, makes it a biofuel that is both renewable and environmentally friendly. One of the strategies to reduce production costs and to make ethanol fuel economically competitive with fossil fuels could be the use of wild yeast with osmotolerance, ethanol resistance and low nutritional requirements. The aim of this work was to investigate the kinetics of ethanol fermentation using Saccharomyces cerevisiae ITV‐01 yeast strain in a batch system at different glucose and ethanol concentrations, pH values and temperature in order to determine the optimum fermentation conditions. RESULTS: This strain showed osmotolerance (its specific growth rate (µ_max) remained unchanged at glucose concentrations between 100 and 200 g L^?1) as well as ethanol resistance (it was able to grow at 10% v/v ethanol). Activation energy (Ea) and Q₁₀ values calculated at temperatures between 27 and 39 °C, pH 3.5, was 15.6 kcal mol^?1 (with a pre‐exponential factor of 3.8 × 10¹² h^?1 (R² = 0.94)) and 3.93 respectively, indicating that this system is biologically limited. CONCLUSIONS: The optimal conditions for ethanol production were pH 3.5, 30 °C and initial glucose concentration 150 g L^?1. In this case, a maximum ethanol concentration of 58.4 g L^?1, ethanol productivity of 1.8 g L^?1 h^?1 and ethanol yield of 0.41 g g^?1 were obtained. Copyright © 2010 Society of Chemical Industry     

Aeration and mixing in vortex fermenters

The overall apparent volumetric gas—liquid mass transfer coefficient (k₉₅a) and the mixing time (t₉₅) were determined in a 240 dm³ vortex aerated fermenter over stirrer speed and air flow ranges of 300–800 rpm and 10–45 normal dm³ min^?1, respectively. The mass transfer data obtained in an aqueous salt solution (2.5 kg m^?3 NaCl in water) compared well with the measurements in a fermentation medium used in culture of certain microaerophilic bacteria. Over the ranges examined, the gas-liquid mass transfer coefficient depended only on air flow rate; the dependence was linear with flow. Mixing time declined with increasing agitation according to a power-law relationship. The mixing and mass transfer characteristics of the vortex aerated system were compared with that of a ‘standard’ stirred tank fermenter (27 dm³). The mixing time variations with respect to agitation rate were remarkably similar for the two types of fermenters examined.     

Gas Holdup and Volumetric Mass Transfer Coefficient in a Slurry Bubble Column

The gas holdup, ?, and volumetric mass transfer coefficient, k_La, were measured in a 0.051 m diameter glass column with ethanol as the liquid phase and cobalt catalyst as the solid phase in concentrations of 1.0 and 3.8 vol.‐%. The superficial gas velocity U was varied in the range from 0 to 0.11 m/s, spanning both the homogeneous and heterogeneous flow regimes. Experimental results show that increasing catalyst concentration decreases the gas holdup to a significant extent. The volumetric mass transfer coefficient, k_La, closely follows the trend in gas holdup. Above a superficial gas velocity of 0.04 m/s the value of k_La/? was found to be practically independent of slurry concentration and the gas velocity U; the value of this parameter is found to be about 0.45 s^–1. Our studies provide a simple method for the estimation of k_La in industrial‐size bubble column slurry reactors.     

Influence of oxygen and medium conditions on the performance of a tower reactor with an external loop

Hansenula polymorpha was cultivated using ethanol and/or glucose as substrate in a bubble column, air lift, loop reactor. The optimum aeration rate was determined for extended culture operation. Considerable influence of the ethanol concentration on the volumetric mass transfer coefficient (k_La), and the specific gas-liquid inter-facial area (a) was found. Glucose concentration had no comparable influence on k_La and a. The influence of lack of substrate and/or dissolved oxygen on the culture was investigated. For 30 min the absence of substrate did not influence μm, whereas a lack of dissolved oxygen for 2.5 min reduced μm to one half of its original value.     

Discrete system identification and self-tuning control of dissolved oxygen concentration in a stirred reactor

This work presents the applications of discrete-time system identification and generalized minimum variance (GMV) control of dissolved oxygen (DO) level in a batch bioreactor in which Saccharomyces cerevisiae is produced at aerobic condition. Air flow rate and mixing rate were varied to determine the maximum local liquid phase volumetric mass transfer coefficient (K _L a). Maximum K _L a value was determined at a mixing rate of 600 rpm and air flow rate of 3.4 Lmin⁻¹. For control purpose, manipulated variable was selected as air flow rate due to its effectiveness on the K _L a. To examine the dynamic behavior of the bioreactor, various input signals were utilized as a forcing function and three different model orders were tested. A second0order controlled auto regressive moving average (CARMA) model was used as the process model in the control algorithm and in the system identification step. It is concluded that the ternary input is more suitable than the other input types used in this work for system identification. Recursive least squares method (RLS) was used to determine the model parameters. GMV control results were compared with the traditional PID control results by using performance criteria of IAE and ITAE for different types of DO set point trajectories. DO concentration in the batch bioreactor was controlled more successfully with an adaptive controller structure of GMV than the PID controller with fixed parameters.     

A coupled population balance model and CFD approach for the simulation of mixing issues in lab‐scale and industrial bioreactors

Lab‐scale (70 L) and industrial scale (70 m³) aerated fermenters are simulated using a commercial computational fluid dynamics code. The model combines an Euler‐Euler approach for the two‐phase flow, a population balance model for biological adaptation to concentration gradients, and a kinetic model for biological reactions. Scale‐up at constant volumetric mass transfer coefficient is performed, leading to concentration gradients at the large scale. The results show that for a given concentration field and a given circulation time t_c, the population (physiological) state depends on the characteristic time of biological adaptation T_a. The population specific growth rate (T_a?t_c) is found independent of the spatial location and closely related to the volume average concentration. Oppositely, the population specific uptake rate (T_a～t_c) is spatially heterogeneous. The resulting local disequilibria between the uptake rate and the growth rate provide an explanation for the decreased performances of poorly macromixed industrial bioreactors. © 2013 American Institute of Chemical Engineers AIChE J, 60: 27–40, 2014     

Gas-liquid transfer of atmospheric CO2 in microalgal cultures

Transfer of CO₂ in an aerated microalgal culture system has been studied. The CO₂ absorption rate was not visibly affected by the kinetics of the reactions occurring in the culture medium. Biomass concentration in the bioreactor, within the interval studied, did not significantly affect the overall volumetric coefficient of CO₂ transfer (K_1a(CO₂)). An expression has been proposed for the estimation of K_1a(CO₂) in the culture as a function of the effective power input to dispersion. Several empirical equations, found in the literature for estimation of the mass transfer parameters, were studied and those which best predicted experimental results are presented. The study concluded that empirical equations for a bubble diameter analogous to that found in the experiments should be used.     

Modelling Oxygen Transfer and Aerobic Growth in Shake Flasks and Well‐Mixed Bioreactors

Oxygen transfer is an important aspect of aerobic metabolism. In this work, microbial growth on glucose (fast metabolism) and phenol (slow metabolism) have been studied using Pseudomonas putida in shake flasks and a mixed bioreactor considering both substrate and oxygen depletion. Under typical operating conditions, the highest mass transfer coefficient (K_La) for the aerated well‐mixed bioreactor was found to be 50.8 h^?1, while the maximum non‐aerated shake flask K_La was 21.1 h^?1. The presence of media and/or dead cells did not have significant effect on measured values of K_La. A new equation for prediction of K_La in shake flasks with an absolute average deviation of 11.1% is introduced, and a combined model for oxygen mass transfer and microbial growth is shown to fit experimental data during growth on glucose and phenol in both shake flasks and the mixed bioreactor with an absolute average deviation of 19.3%.     

Gas—liquid mass transfer in pulp suspension mixing operations

The rate of mass transfer from the gas to water phases was measured in a commercial, high-shear, laboratory mixer under conditions typical of medium-consistency bleaching. The gas—liquid volumetric mass transfer coefficient, k_La, was measured using the cobalt-catalyzed sulfite oxidation technique. Suspensions of fully-bleached kraft pulp and synthetic nylon fibres were used, with mass transfer rates measured over a range of suspension compositions and mixer operating conditions. In the presence of pulp fibre, mass transfer rates were significantly reduced over the comparable water cases. The same dramatic decrease in mass transfer was not observed for the nylon suspensions, although k_La did decrease with increasing suspension concentration. Comparison of this data with that obtained from ozone bleaching experiments confirmed that at medium-consistency gas—liquid mass transfer controls ozone bleaching.     

Comparison of Overall Gas‐Phase Mass Transfer Coefficient for CO2 Absorption between Tertiary Amines in a Randomly Packed Column

The mass transfer performance of CO₂ absorption into an innovative tertiary amine solvent, 1‐dimethylamino‐2‐propanol (1DMA2P), was investigated and compared with that of methyldiethanolamine (MDEA) in a packed column with random Dixon‐ring packing. All experiments were conducted under atmospheric pressure. The effects of inert gas flow rate, amine concentration, liquid flow rate, CO₂ loading, and liquid temperature on mass transfer performance were analyzed and the results presented in terms of the volumetric overall mass transfer coefficient (K_Ga_v). The experimental findings clearly indicate that 1DMA2P provided better mass transfer performance than MDEA. For both 1DMA2P and MDEA solutions, the K_Ga_v increased with rising amine concentration and liquid flow rate, but decreased with higher CO₂ loading. The inert gas flow rate only slightly affected the K_Ga_v. A satisfactory correlation of K_Ga_v was developed for the 1DMA2P‐CO₂ system.