Oxygen transfer in slurry bioreactors

Applied Microbiology and Biotechnology, 1989

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 holdup are examined. The liquid velocity at the reactor axis and the gas holdup 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 holdup 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 reexamined data for actual fermentation broths which are Chaetomium cellulolyticum and Neurospora sitophila cultured in a 1000-1 pilot-plant scale airlift fermentor.

Chemical Product and Process Modeling, 2000

Bubble column bioreactors used to perform aerobic fermentations consist of a liquid medium, containing microorganisms that uptake oxygen for metabolic reactions, and gas bubbles that supply this oxygen. Mass transfer rate from gas to liquid phase is a crucial factor for the performance of bioreactors, because microorganisms' life and metabolic reactions depend, directly, upon it. The maximum transfer rate of oxygen from gas bubbles to the liquid medium is function of two important parameters, the specific interfacial area (a), and the mass transfer coefficient (k L ); these two parameters are lumped into the volumetric specific transfer coefficient k L a. Since size of gas bubbles is not constant along the bioreactor, gas-liquid mass transfer rate changes, continuously. In order to optimize mass transfer rates, it is essential to know the bubble size distribution and the interfacial phenomena in each particular system at different operating conditions. Due to the complexity of hydrodynamics and bubble interphase characteristics, the current state of the problem does not consider a universal model to evaluate mass transfer rates in gas-liquid systems; moreover, information about bubble size and its distribution is often neglected. This work presents a model to evaluate axial distribution of values of volumetric mass transfer coefficients (k L a), considering changes in bubble size and its influence on bubble area along the reactor. Simultaneously, this model evaluates changes of volumetric mass transfer coefficient and its effect on the fermentation kinetics, which influences performance of the bioreactor.

Revista De Chimie Bucharest Original Edition, 2014

Various strategies for improving the mass transfer rate of oxygen have been used in order to obtain a good efficiency of aerobic fermentation processes. The paper focuses on studies concerning the influence of solid carrier addition in the liquid phase on oxygen volumetric mass transfer coefficient in a stirred and aerated tank bioreactor. The selected oxygen carriers (vectors) can be easily separated and they have no harmful effect on the microbial population implied in bioprocesses. Fine particle of activated carbon, silicon oil impregnated activated carbon, bacterial cellulose, magnetite, and bacterial cellulose-magnetite composite were used as oxygen-vectors. An enhancement of oxygen transfer was indicated by an increase in volumetric mass transfer coefficient, k l a, which was determined by a dynamic method. A significant improvement of oxygen mass transfer in the presence of magnetite and bacterial cellulose-magnetite composite was highlighted.

Biotechnology advances, 2009

In aerobic bioprocesses, oxygen is a key substrate; due to its low solubility in broths (aqueous solutions), a continuous supply is needed. The oxygen transfer rate (OTR) must be known, and if possible predicted to achieve an optimum design operation and scale-up of bioreactors. Many studies have been conducted to enhance the efficiency of oxygen transfer. The dissolved oxygen concentration in a suspension of aerobic microorganisms depends on the rate of oxygen transfer from the gas phase to the liquid, on the rate at which oxygen is transported into the cells (where it is consumed), and on the oxygen uptake rate (OUR) by the microorganism for growth, maintenance and production. The gas-liquid mass transfer in a bioprocess is strongly influenced by the hydrodynamic conditions in the bioreactors. These conditions are known to be a function of energy dissipation that depends on the operational conditions, the physicochemical properties of the culture, the geometrical parameters of the bioreactor and also on the presence of oxygen consuming cells. Stirred tank and bubble column (of various types) bioreactors are widely used in a large variety of bioprocesses (such as aerobic fermentation and biological wastewater treatments, among others). Stirred tanks bioreactors provide high values of mass and heat transfer rates and excellent mixing. In these systems, a high number of variables affect the mass transfer and mixing, but the most important among them are stirrer speed, type and number of stirrers and gas flow rate used. In bubble columns and airlifts, the lowshear environment compared to the stirred tanks has enabled successful cultivation of shear sensitive and filamentous cells. Oxygen transfer is often the rate-limiting step in the aerobic bioprocess due to the low solubility of oxygen in the medium. The correct measurement and/or prediction of the volumetric mass transfer coefficient, (k L a), is a crucial step in the design, operation and scale-up of bioreactors. The present work is aimed at the reviewing of the oxygen transfer rate (OTR) in bioprocesses to provide a better knowledge about the selection, design, scale-up and development of bioreactors. First, the most used measuring methods are revised; then the main empirical equations, including those using dimensionless numbers, are considered. The possible increasing on OTR due to the oxygen consumption by the cells is taken into account through the use of the biological enhancement factor. Theoretical predictions of both the volumetric mass transfer coefficient and the enhancement factor that have been recently proposed are described; finally, different criteria for bioreactor scale-up are considered in the light of the influence of OTR and OUR affecting the dissolved oxygen concentration in real bioprocess.

BioMed Research International, 2013

Volumetric mass transfer coefficient (kLa) is an important parameter in bioreactors handling viscous fermentations such as xanthan gum production, as it affects the reactor performance and productivity. Published literatures showed that adding an organic phase such as hydrocarbons or vegetable oil could increase thekLa. The present study opted for palm oil as the organic phase as it is plentiful in Malaysia. Experiments were carried out to study the effect of viscosity, gas holdup, andkLaon the xanthan solution with different palm oil fractions by varying the agitation rate and aeration rate in a 5 L bench-top bioreactor fitted with twin Rushton turbines. Results showed that 10% (v/v) of palm oil raised thekLaof xanthan solution by 1.5 to 3 folds with the highestkLavalue of 84.44 h−1. It was also found that palm oil increased the gas holdup and viscosity of the xanthan solution. ThekLavalues obtained as a function of power input, superficial gas velocity, and palm oil fraction were ...

Biotechnology and Bioengineering, 1977

Chemical Engineering Science, 2019

Oxygen transfer is a key element in aerobic fermentations, especially if the culture broth's rheology is non-Newtonian, as in the case of cultures of filamentous fungi. It is well known that viscosity negatively affects the volumetric mass transfer coefficient (k L a), but mechanisms involved in terms of change of interfacial area (a) and liquid-side mass transfer coefficient (k L) have still not been clearly identified. This lack of knowledge is in part due to the difficulty in measuring bubble size in aerated media, especially in viscous fluids. In the present study, a recently developed dual-probe method was validated and then used to measure bubble Sauter mean diameter (d 32) in water and in xanthan gum solutions, which exhibit rheological behaviors similar to filamentous fungi's broths. Ethanol was used to modify the coalescence of the dispersed phase and the results were compared to data obtained in a filtered fermentation culture of Trichoderma reesei. Additional experiments were carried out to obtain the volumetric mass transfer coefficient and the global gas holdup (α G). It was then possible to obtain the liquid-side

Biotechnology and Bioengineering, 1988

For viscous mycelial fermentations it was demonstrated at the pilot-plant scale that the replacement of standard radial flow Rushton turbines with larger-diameter axialflow Prochem hydrofoil impellers significantly improved oxygen transfer efficiency. It was also determined that the Streptomyces broth under evaluation is highly shear thinning. Separate experiments using a Norcardia broth with similar rheological properties demonstrated that the oxygen transfer coefficient, KLa, can be greatly increased by use of water additions to reduce broth viscosity. These observations are consistent with the hypothesis that the improvement in oxygen transfer by changing agitator types is primarily due to an improvement in bulk mixing. A model is presented, based on the concepts of Bajpai and Reuss, which explains this improvement i n performance in terms of enlargement of the well mixed micrornixer region for viscous mycelial broths. sity, Medford, MA.

In many fermentation processes, oxygen transfer is the rate limiting step. Correct measurement and subsequent estimation of the volumetric mass transfer coefficient is a crucial step in the design procedure of bioreactors. This article discusses some of the methods that are commonly used for the measurement of the mass transfer coefficient and their applicability for measurement in large scale bioreactors. It has been found that among the methods discussed here, the dynamic pressure method appears most useful for industrial scale bioreactors, with a small degree of approximations for gas-liquid mixing in the reactor and is suitable for large scale bioreactors with errors less than 10%, over the entire range of the operating conditions encountered in the fermentor operation.

Heat and Mass Transfer, 2000

Mass transfer in bioreactors has been examined. In the present work, dynamic methods are used for the determination of K L a values for water, model media and a fermentation broth (Candida utilis) in an airlift reactor. The conventional dynamic method is applied at the end of the microbial process in order to avoid an alteration in the metabolism of the microorganisms. New dynamic methods are used to determine K L a in an airlift reactor during the microbial growth of Candida utilis on glucose. One of the methods is based on the continuous measurement of carbon dioxide production while the other method is based on the relationship between the oxygen transfer and biomass growth rates. These methods of determining K L a does not interfere with the microorganisms action. A theoretical mass transfer model has been used for K L a estimation for the systems described above. Some differences between calculated and measured values are found for fermentation processes due to the model is developed for two-phase air-water systems. Nevertheless, the average deviation between the predicted values and those obtained from the relationship between oxygen transfer and biomass production rates are lower than 25% in any case.

Chemical Engineering Science, 2017

In the industrial fermentation processes, most liquids are non-coalescent and often exhibit increased viscosity. However, due to the limitations of most measurement methods, there is a lack of reliable data for predicting volumetric mass transfer coefficients (k L a) for viscous batches, especially under high dissipated energies, as the accurate determination of oxygen concentration profile in viscous liquids is not as easy as in low viscosity ones. Our goal is to develop reliable technique for k L a determination in viscous liquids and to establish suitable correlation shapes to describe k L a data. We used the dynamic pressure method (DPM), the experimental setup which has been modified for the measurement in viscous liquids. Dissolved oxygen (DO) probes were placed in bypass measuring cells. This set up brings well defined transient characteristics of DO probes, which is crucial for correct k L a evaluation. Measurements were conducted in two phase multiple-impeller fermenters with a non-coalescent viscous Newtonian batch under a wide range of experimental conditions and in the apparatuses of two scales. Using pure oxygen as gas phase, it was confirmed that DPM yields k L a's independent of the driving force of absorption even in viscous batch. The improved set up of DPM enabled to use also optical DO probes as well as polarographic ones. It was confirmed that optical DO probe can be used for k L a values up to 0.4 s-1. Based on the experimental data, correlations were developed to predict k L a in industrial fermenters. Standard correlation k L a = 2.99•10-3 •(P g /V L) 0.891 v s 0.556 with SD 30%, based on gassed power input P g and superficial gas velocity v s , has low standard deviation but it is scale specific. On the other hand, when the term of impeller tip speed (ND) is used instead of P g , predicted data exhibit neither over-nor underestimation of 2 k L a for particular apparatus scale; so the effect of the vessel scale is properly described using this term. In addition, the impeller power number P o was found to be a reliable predictor of k L a in a common correlation for various impeller types, when used together with the impeller tip speed term. The correlation k L a = 0.295•(ND) 2.083 v s 0.461 P o 0.737 with SD 28% suggested in this work can be used for fairly accurate design of industrial fermenters. Both the experimental technique and the correlation shape are ready to be used to obtain the design tool for other batches with various viscosities.

2014

The study of oxygen mass transfer was conducted in a laboratory scale 5 liter stirred bioreactor equipped with one Rushton turbine impeller. The effects of superficial gas velocity, impeller speed, power input and liquid viscosity on the oxygen mass transfer were considered. Air/ water and air/CMC systems were used as a liquid media for this study. The concentration of CMC was ranging from 0.5 to 3 w/v. The experimental results show that volumetric oxygen mass transfer coefficient increases with the increase in the superficial gas velocity and impeller speed and decreases with increasing liquid viscosity. The experimental results of k l a were correlated with a mathematical correlation describing the influences of the considered factors (the overall power input and the superficial gas velocity) over the studied rages. The predicted k l a values give acceptable results compared with the experimental values. The following correlations were obtained: Air/water system () 4. 30 43. 1 088...

In aerobic bioprocesses, oxygen is a key substrate; due to its low solubility in broths (aqueous solutions), a continuous supply is needed. The oxygen transfer rate (OTR) must be known, and if possible predicted to achieve an optimum design operation and scale-up of bioreactors. Many studies have been conducted to enhance the efficiency of oxygen transfer. The dissolved oxygen concentration in a suspension of aerobic microorganisms depends on the rate of oxygen transfer from the gas phase to the liquid, on the rate at which oxygen is transported into the cells (where it is consumed), and on the oxygen uptake rate (OUR) by the microorganism for growth, maintenance and production. The gas-liquid mass transfer in a bioprocess is strongly influenced by the hydrodynamic conditions in the bioreactors. These conditions are known to be a function of energy dissipation that depends on the operational conditions, the physicochemical properties of the culture, the geometrical parameters of the bioreactor and also on the presence of oxygen consuming cells. Stirred tank and bubble column (of various types) bioreactors are widely used in a large variety of bioprocesses (such as aerobic fermentation and biological wastewater treatments, among others). Stirred tanks bioreactors provide high values of mass and heat transfer rates and excellent mixing. In these systems, a high number of variables affect the mass transfer and mixing, but the most important among them are stirrer speed, type and number of stirrers and gas flow rate used. In bubble columns and airlifts, the lowshear environment compared to the stirred tanks has enabled successful cultivation of shear sensitive and filamentous cells. Oxygen transfer is often the rate-limiting step in the aerobic bioprocess due to the low solubility of oxygen in the medium. The correct measurement and/or prediction of the volumetric mass transfer coefficient, (k L a), is a crucial step in the design, operation and scale-up of bioreactors. The present work is aimed at the reviewing of the oxygen transfer rate (OTR) in bioprocesses to provide a better knowledge about the selection, design, scale-up and development of bioreactors. First, the most used measuring methods are revised; then the main empirical equations, including those using dimensionless numbers, are considered. The possible increasing on OTR due to the oxygen consumption by the cells is taken into account through the use of the biological enhancement factor. Theoretical predictions of both the volumetric mass transfer coefficient and the enhancement factor that have been recently proposed are described; finally, different criteria for bioreactor scale-up are considered in the light of the influence of OTR and OUR affecting the dissolved oxygen concentration in real bioprocess.

The Canadian Journal of Chemical Engineering, 1984

The volumetric mass transfer coefficients, kLa, measured in a pilot plant (0.1 m3) and an industrial (67.5 m3) fermentor during an actual fermentation process are presented. Problems related to the estimation of the phsyical properties as well as to the correlation of experimental data and to scale up procedures are discussed. Although the scale up factor was rather high, both sets of data could be represented by single correlation. Comparison of the experimental data with several available correlations demonstrated the need for pilot plant experiments and scale up procedures, since it is almost impossible to take into account all relevant system properties.

Biotechnology and Bioengineering, 2005

Oxygen mass transfer in sparged stirred tank bioreactors has been studied. The rate of oxygen mass transfer into a culture in a bioreactor is affected by operational conditions and geometrical parameters as well as the physicochemical properties of the medium (nutrients, substances excreted by the micro-organism, and surface active agents that are often added to the medium) and the presence of the micro-organism. Thus, oxygen mass transfer coefficient values in fermentation broths often differ substantially from values estimated for simple aqueous solutions. The influence of liquid phase physicochemical properties on k L a must be divided into the influence on k L and a, because they are affected in different ways. The presence of micro-organisms (cells, bacteria, or yeasts) can affect the mass transfer rate, and thus k L a values, due to the consumption of oxygen for both cell growth and metabolite production. In this work, theoretical equations for k L a prediction, developed for sparged and stirred tanks, taking into account the possible oxygen mass transfer enhancement due to the consumption by biochemical reactions, are proposed. The estimation of k L a is carried out taking into account a strong increase of viscosity broth, changes in surface tension and different oxygen uptake rates (OURs), and the biological enhancement factor, E, is also estimated. These different operational conditions and changes in several variables are performed using different systems and cultures (xanthan aqueous solutions, xanthan production cultures by Xanthomonas campestris, sophorolipids production by Candida bombicola, etc.). Experimental and theoretical results are presented and compared, with very good results. ß 2005 Wiley Periodicals, Inc.