A reassessment of mixing cost in fermentation processes

The gas–liquid oxygen transfer rate is a key step in the production of antibiotics in submerged fermentation. If the gas–liquid oxygen mass transfer rate is not equal to the required liquid–solid oxygen mass transfer rate at a particular cell concentration, then productivity of the particular fermentation operation will not be the maximum possible value. One way to increase the productivity of a given fermentation tank installation is to increase the cell concentration and to increase the oxygen transfer by changing the mixer and air supply to match the new requirements. In order to evaluate the cost of making this change to the larger mixing equipment, a typical cost example is given which can easily be modified for other combinations of production cost and mixer cost. As an example, it is seen that a considerable savings can result from a given installation by primarily changing the oxygen transfer ability of the equipment to utilize a given fermentor more efficiently. Production cost savings of 8 to 25% are shown in the example cited.     

Studies on Oxygen Transfer in Submerged Fermentations

This paper refers to the application of gas analyzers for the determination of oxygen transfer rate, showing examples in the studies and the performances of submerged fermentations. Oxygen and carbon dioxide analyzers were set to monitor the gas streams to and from the fermentor. Continuous data on the concentrations of oxygen and carbon dioxide in the air streams were thus provided throughout the fermentation. Distinctive characters of this method were applicability to fermentors in practice and ability of obtaining data directly relating to the fermentations.

The modification of sulfite oxidation method for the determination of oxygen transfer rate from air into liquid or of a measure of aeration effectiveness was made. The proposed method was the application of gas analyzers in the studies on submerged fermentation. Some comparative discussions were made between this and the conventional titrimetric method. This modified method could be applied to biological systems with no alteration, therefore, it was made possible to compare the sulfite solution with the biological systems in relation to the problems on oxygen transfer.     

Russian Article pp. 335-340

Industrial processes of cultivation of aerobic microorganisms are characterized by too small transportation rates of oxygen from a gas into a cell. The search for new ways of productivity increase by means of intensification of oxygen mass transfer in the fermentation process is of great importance. The possibilities for use of chelate transition metal complexes (CTMC) capable of binding oxygen in biosynthetic processes have been studied. The formation of labial complex of the studied substances with oxygen is shown by spectrophotometry. The increase of the oxygen delution rate in water in the presence of CTMC is shown by physical absorbtion method. The rate increase of oxygen mass transfer from gas into liquid is calculated on the base of the sulphite method. The use of CTMC in the role of oxygen carriers in the yeast cultivation leads to the biomass increase up to 20%.     

New methods of oxygen monitoring in the gas phase of cultivation processes

Summary The principles of two new techniques for monitoring oxygen in the gas phase of fermentation processes are presented. In addition, the application of both methods on real fermentation processes is given to show their potential in comparsion with well established methods.     

Coupling of microbial kinetics and oxygen transfer for analysis and optimization of gluconic acid production with Aspergillus niger

Gluconic acid fermentation has been widely used for the analysis of various aspects of kinetics and gas liquid transfer of oxygen. Most of these studies are, however, restricted to processes with bacteria. Mathematical models for industrially important productions with fungi have not been elaborated. In the experimental part of this work computer coupled fermentations of gluconic acid production with Aspergillus niger NRRL 3 have been performed. Knowledge of the stoichiometric relationship in the key reaction (glucose oxidase) provides an excellent opportunity for on-line estimation of glucose, biomass and product gluconate from oxygen uptake and carbon dioxide evolution rates. Starting then from experimental observations on the pH-depending oxygen kinetics of gluconic acid formation and influences of product concentrations on the growth of Aspergillus niger a mathematical framework is developed in which the kinetics of growth and production are coupled with gas liquid oxygen transfer. The model can be successfully applied to simulations of the experimental results of gluconic acid fermentations with cyclic addition of glucose. An important aspect in the coupling of transport and microbial reaction in this model is the incorporation of the influence of sugar and gluconate on the solubility of oxygen and k _La via changes of viscosities and molecular diffusivities. With the development of such a comprehensive model, it appears feasible to investigate the influence of various process conditions (sugar feeding, pressure, optimal pH profiles) and to study their possible impacts on the productivity of the overall process.     

The effects of non-Newtonian fermentation broth viscosity and small bubble segregation on oxygen mass transfer in gas-lift bioreactors: a critical review

This review paper deals with the effects of non-Newtonian fermentation broth viscosities on gas–liquid mixing and oxygen mass transfer characteristics to provide knowledge for the design and development of gas-lift bioreactors, which can operate satisfactorily with high viscosity fermentation broths. The effect of small bubble segregation is also examined.     

Studies on Oxygen Transfer in Submerged Fermentations

The unsteady-state gas analysis method for the determination of oxygen transfer rate in shaken cultures was applied to the glutamic acid fermentation. This method was essentially based on the measurement of oxygen tension of gas phase in flasks. Major intentions of this application were to clarify the problems of oxygen transfer, to establish the measure of aeration effectiveness and to design the best aeration condition in the glutamic acid fermentation. Correlations of the fermentation to the level of dissolved oxygen, the rate of oxygen transfer, overall oxygen transfer coefficient and oxygen demand of cells were examined. As a measure of aeration effectiveness, the rate of oxygen transfer actually occurring in the fermentation seemed to be preferable, and was found to be above 7 × 10^?7 mol/ml-min for attaining the maximum productivity. Extremely high or low oxygen supply resulted in less productivity.     

影响灵芝三萜类物质液态发酵合成的关键因素研究进展

灵芝是我国名贵的食药两用型菌类,具有广泛的药用价值,其三萜类物质为灵芝中最主要的药理活性物质之一。灵芝液态发酵因具有生长周期短、环境条件可控、目标产物质量稳定及易实现规模化制备等特点而成为获得灵芝三萜类物质最有前景的方式。灵芝三萜代谢途径、发酵工艺及参数、溶解氧控制等是影响灵芝三萜类物质液态发酵合成的关键因素。本文总结了灵芝三萜生物合成的代谢途径和相关的酶(基因)、液态发酵方式和发酵参数调节的溶解氧控制这3个层面对灵芝三萜类物质生物合成的影响,并对今后的研究方向进行了展望,为液态培养灵芝三萜类物质调控及高产提供参考,也为下一步研究提供借鉴。     

A comparison of some methods of estimating volumetric mass-transfer coefficients in an external-loop airlift fermenter

Volumetric mass-transfer coefficients were measured in an 11-L external-loop airlift fermenter with deionized water, a fermentation medium, and during a fermentation. Both a Mackareth oxygen electrode and a novel rapid-response probe were used. When the conventional step-change dynamic method was used for water, the long, nonlinear response time of the Mackareth electrode made correction of its readings difficult; this problem did not occur when the rapid-response probe was used. A comparison was made with a method of mass-transfer coefficient determination which does not involve any assumptions about the gas residence time distribution. However, this method requires that the liquid phase is well-mixed and this requirement was not met in the airlift fermenter. Comparison of the present results with other K(L) a determinations for airlift fermenters showed that K(L) a in water depends on the active gas holdup, the value of K(L) a/epsilon at 20 degrees C being ca. 0.37 s(-1). Although higher gas holdups were obtained with the fermentation medium than for water, the values of K(L) a/epsilon were lower, ca. 0.22 s(-1) at 20 degrees C.     

A novel split-cylinder airlift reactor for fed-batch cultures

Conventional airlift reactors are not adequate to carry out variable volume processes since it is not possible to achieve a proper liquid circulation in these reactors until the liquid height is higher than that of the downcomer. To carry out processes of variable volume, the use of a split-cylinder airlift reactor is proposed, in the interior of which two multi-perforated vertical baffles are installed in order to provide several points of communication between the reactor riser and downcomer. This improves the liquid circulation and mixing at any liquid volume. In fed-batch cultures, it is important to know how liquid height affects the hydrodynamic characteristics and the volumetric oxygen transfer coefficient since this impacts on the kinetic behavior of any fermentation. Thus, in the present work, the effect of the liquid height on the mixing time, the overall gas hold-up, and the volumetric oxygen transfer coefficient of the proposed airlift reactor were determined. The mixing time was increased and the volumetric oxygen transfer coefficient decreased due to the increase of the liquid height in the reactor in all the superficial gas velocities tested, which corresponded to a pseudohomogeneous flow regime. The experimental values of the mixing time and the mass-transfer coefficient were properly described through correlations in which the U_GR/H_L ratio was used as the independent variable. Thus, this variable might be used to describe the hydrodynamic behavior and the oxygen transfer coefficient of airlift reactors when such reactors are used in processes where the liquid volume changes with time. However, these correlations are useful for the particular device and for the particular operating conditions at which they were obtained. These empirical correlations are useful to understand some factors that influence the mixing time and volumetric oxygen transfer coefficient, but such correlations do not have a sufficient predictive potential for a satisfactory reactor design. The overall gas hold-up values were not significantly affected when the liquid height was lower than the downcomer height. However, the values decreased abruptly when the reactor was operated with liquid heights over the downcomer height, especially at high superficial gas velocities.     

Studies on the Ventilation in Submerged Fermentations

Quantitative studies on the dissolution and dissociation of carbon dioxide in a cultured system were made. The inosine fermentation and the glutamic acid fermentation were employed for this study. According to the results obtained in this experiment, the quantity of dissociated carbonic acid in cultured liquid was given by Henderson-Hasselbalch’s equation with experimental pK′. The method for the direct determination of bicarbonate ion concentration was also investigated. The Warburg direct method gave a satisfactory result for this purpose.

By using the modified Severinghaus CO₂ electrode, the relationship between partial pressure of carbon dioxide in effluent gas and that in culturing system was investigated. Partial pressure of carbon dioxide in gas phase was almost equivalent to the average value of dissolved carbon dioxide tension in liquid phase for a given short time of the fermentation. The term of r_e was introduced in order to study the dynamic characteristics of carbon dioxide evolution in submerged fermentors. The dynamic characteristics of respiration in submerged fermentation was also studied by using biological r_ab and r_e.     

Effects of controlled gas environments in solid-substrate fermentations of rice

The gas environment is solid-substrate fermentations of rice significantly affected levels of biomass and enzyme formation by a fungal species screened for high amylase production. Constant oxygen and carbon dioxide partial pressures were maintained at various levels in fermentations by Aspergillus oryzae. Control of the gas phase was maintained by a “static” aeration system admitting oxygen on demand and stripping excess carbon dioxide during fermentation. Constant water vapor pressures were also maintained by means of saturated salt solutions. High Oxygen pressures stimulated amylase productivity significantly. On the other hand, amylase production was severely inhibited at high carbon dioxide pressures. While relatively insensitive to oxygen pressure, maximum biomass productivities were obtained at an intermediate carbon dioxide pressure. High oxygen transfer rates were obtained at elevated oxygen pressures, suggesting, in view of the stimulatory effect of oxygen on amylase production, a stringent oxygen requirement for enzyme synthesis. Solid-substrate fermentations were highly advantageous as compared with submerged cultures in similar gas environments. Not only were amylase productivities significantly higher, but the enzyme was highly concentration in the aqueous phase of the semisolid substrate particles and could be extracted in a small volume of liquid. Results of this work suggest that biomass and product formation in microbial processes may be amenable to control by the gas environment. This is believed to offer an interesting potential for optimizing selected industrial fermentation processes with respect to productivity and energy consumption.     

裂殖壶菌发酵产DHA过程呼吸参数的在线检测分析

利用尾气分析仪对发酵过程的尾气中的O2、CO2含量进行实时检测,建立了裂殖弧菌发酵生产DHA过程中的呼吸参数在线检测方法,实现了裂殖壶菌补料分批发酵过程及双阶段供氧控制发酵过程中的呼吸参数在线检测分析。通过呼吸参数在线检测分析,从氧消耗机制方面解释了双阶段氧传递控制工艺能获得较高生物量、油脂和DHA含量的原因,从而为该工艺过程提供了理论指导。根据发酵过程中菌体生长不同时期的呼吸参数的变化情况,建立了基于呼吸商变化的在线补料控制方法,设计了一种基于RQ-Stat的补料工艺。RQ-Stat补料方式最终获得的油脂含量、DHA产量和产率比间歇式补料工艺分别提高了11.58%、12.19%和11.40%。     

A new method for studying microaerobic fermentations. I. A theoretical analysis of oxygen programmed fermentation

An experimental method for studying microaerobic fermentation, called oxygen programmed fermentation, is introduced. The method if based on a chemostat. The mathematical equations governing the dynamics of the system are derived and simulations are made for two principally different cases: a purely respirative organism, and an organism capable of fermentation during oxygen limitation. It is shown that at a suitably chosen ramp rate, the dissolved oxygen concentration in the broth can be made to decrease almost linearly. It is suggested that the greatest use of oxygen programmed fermentation will be in initial experiments. Compared with chemostat studies, a scan of different oxygenation rates will provide a time-saving method of finding the interacting regions for metabolic transitions. Furthermore it is shown that the methods makes it possible to study cell physiology at condition which would normally lead to washout. (c) 1994 John Wiley & Sons, Inc.     

Gasometric measurement of sulfite oxidation

In the commonly used sulfite method the consumption of sulfite is determined by iodometry. Since however, the addition of organic substances may interfere with iodometry (e.g. due to chemical reactions with iodine) the gasometric measurement of sulfite oxidation has been developed for analysis of how different culture media may influence the oxygen transfer rate. The striking decrease of sulfite oxidation rate due to addition of culture media to the sulfite solution suggests that adsorption of orgnic components in the gas liquid interface may account for an additional diffusion barrier and thus for a decrease of the oxygen transfer coefficient which in addition gives an explanation for differences between values found by the sulfite method and by aerobic cultivations. Consequently identical values of oxygen transfer rate have been obtained for both systems whenever the sulfite system has been properly adjusted to the aerobic cultivation conditions. In so far, the gasometric sulfite method proved to be a unique tool for rapid determination of factors influencing oxygen transfer rate in fermentation processes which may give rise to a reappraisal as to the relevance of the sulfite method for oxygen transfer optimization.     

Oxygen mass transfer into aerated CMC solutions in a bubble column

Differing findings on the volumetric mass transfer coefficients k(L)a in CMC solutions in bubble column bioreactors have been reported in the literature. Therefore, oxygen mass transfer was studied again in CMC solutions in a 14-cm-i.d. x 270-cm-height bubble column using different spargers. The k(L)a values were determined along with the dispersion coefficients by fitting the prediction of the axial dispersed plug model with the experimental oxygen concentration profiles in the liquid phase. Surprisingly, the obtained liquid phase dispersion coefficients for CMC solution are higher than one would expect from correlations. The k(L)a data depend largely on the flow regime. In general, they are lower than those reported in the literature. The data for developing slug and established slug flow are dependent on the gas velocity and the effective viscosity of the solution and can br correlated by a simple correlation. This correlation describes k(L)a values measured on fermentation broth of Penicillium chrysogenum with striking agreement.     

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.     

Gas hold-up in external-loop airlift bioreactors

A new model of gas hold-up is proposed for external-loop airlift bioreactors. It is based on the similarity between the liquid circulation due to the local variation of gas hold-up in airlift bioreactors and the natural convection due to temperature difference. The model is developed to include the case of non-Newtonian fermentation media which are involved in many industrially bioprocesses. The capability of the model is examined using a wide range of experimental results including the present data. Reasonable agreement is obtained between the proposed model and the experimental data both for Newtonian and non-Newtonian media.     

Optimal design of airlift fermenters

In this article a modeling of a draft-tube airlift fermentor (ALF) based on perfect backmixing of liquid and plugflow for gas bubbles has been carried out to optimize the design and operation of fermentation units at different working capacities. With reference to a whey fermentation by yeasts the economic optimization has led to a slim ALF with an aspect ration of about 15. As far as power expended per unit of oxygen transfer is concerned, the responses of the model are highly influenced by k_La However, a safer use of the model has been suggested in order to assess the feasibility of the fermentation process under study.