Distributed parameter model of an airlift fermentor

A distributed parameter model for an airlift fermentor is presented. A riser represents the airlift fermentor, with plug flow in both gas and liquid phases, a well-mixed section that acts as gas separator, and a downcomer with plug flow. The set of equations proposed makes possible both the understanding and design of the system. Macroscopic balances shows a behavior that is very close to conventional continuous stirred tank fermentor from the viewpoint of biomass production. In addition, the model predicts concentration profiles of biomass, substrate and oxygen in the liquid, and oxygen in the gas phase. This allows estimation of optimal gas flow rate for sufficient oxygen transfer with minimum energy input.     

Design and performance characteristics of a continuous multistage tower fermentor

The design of a continuous multistage tower fermentor is described. The fermentor consists of five stages separated by perforated plates. Each stage includes mechanical mixing provided by two disc turbine impellers and has its own impeller shaft with bearing assembly and flexible coupling that enables the operation of an arbitrary number of stages. The normal operation of this system enables the co-current flow of gas and liquid, but the system can function countercurrently as well. The purpose of this study was to examine the hydrodynamic performance, i.e., the pressure gradient along the tower, the mixing time, gas holdup, the residence lime distribution of the continuous phase, the value of the backflow coefficient, and the oxygen transfer rate under conditions usually used during fermentations. From the interrelations between parameters influencing the proper performance of this system, an optimal design of plate geometry for processes requiring high oxygen transfer rate was formulated.     

Design and performance of a new microcalorimetric system for aerobic cultivation of microorganisms

A twin-type heat-conduction calorimeter for an aerobic fermentation process was designed and constructed fermentor both in batch and continuous runs. The time constant of the calorimeter was 3.3 min. The volumetric oxygen transfer coefficient (k_La) of the vessel was measured by a sulfite oxidation method in a continuous flow system under various rotation speeds and gas flow rates. Sufficient thermal stability of the calorimeter was obtained both in batch and continuous runs within the operation time period.     

Design and oxygen-transfer potential of a pulsed continuous tubular fermentor

A new type of continuous fermentor has been developed which features an agitation-aeration system based on pulsed flow across perforated plates the use of plug flow to achieve a concentration gradient. The influence of the agitation-aeration parameters (in this case the pulsed speed) has been measured, and mathematical models have been produced for the gas hold-up, the power dissipated in agitation and aeration, the oxygen-transfer rate, and efficiency. The oxygen transfer is high, up to 600 mmol/liter hr with a transfer efficiency much higher than that published for any other technique.     

Inhibitory effect of carbon dioxide on the fed-batch culture of Ralstonia eutropha: evaluation by CO2 pulse injection and autogenous CO2 methods

In order to see the effect of CO(2) inhibition resulting from the use of pure oxygen, we carried out a comparative fed-batch culture study of polyhydroxybutyric acid (PHB) production by Ralstonia eutropha using air and pure oxygen in 5-L, 30-L, and 300-L fermentors. The final PHB concentrations obtained with pure O(2) were 138.7 g/L in the 5-L fermentor and 131.3 g/L in the 30-L fermentor, which increased 2.9 and 6.2 times, respectively, as compared to those obtained with air. In the 300-L fermentor, the fed-batch culture with air yielded only 8.4 g/L PHB. However, the maximal CO(2) concentrations in the 5-L fermentor increased significantly from 4.1% (air) to 15.0% (pure O(2)), while it was only 1.6% in the 30-L fermentor with air, but reached 14.2% in the case of pure O(2). We used two different experimental methods for evaluating CO(2) inhibition: CO(2) pulse injection and autogenous CO(2) methods. A 10 or 22% (v/v) CO(2) pulse with a duration of 3 or 6 h was introduced in a pure-oxygen culture of R. eutropha to investigate how CO(2) affects the synthesis of biomass and PHB. CO(2) inhibited the cell growth and PHB synthesis significantly. The inhibitory effect became stronger with the increase of the CO(2) concentration and pulse duration. The new proposed autogenous CO(2) method makes it possible to place microbial cells under different CO(2) level environments by varying the gas flow rate. Introduction of O(2) gas at a low flow rate of 0.42 vvm resulted in an increase of CO(2) concentration to 30.2% in the exit gas. The final PHB of 97.2 g/L was obtained, which corresponded to 70% of the PHB production at 1.0 vvm O(2) flow rate. This new method measures the inhibitory effect of CO(2) produced autogenously by cells through the entire fermentation process and can avoid the overestimation of CO(2) inhibition without introducing artificial CO(2) into the fermentor.     

Design and physical characteristics of a multistage,continuous tower fermentor

A multistage tower laboratory fermentor has been constructed consisting of eight compartments separated by sieve plates. Flow of substrate and air is concurrent from the bottom to the top of the column. It, was hoped that this system could be used to reproduce, simultaneously on a continuous basis, eight distinct phases of a batch growth curve. It was believed that the extent of batch curve simulation would depend upon the character of hydraulic mean residence time of broth in the column and in the individual compartments. The expected relationship did not occur. Rather it was found that growth in the column involved residence time characteristics not only for the fluid but also for the microorganisms, and for the growth limiting substrate. Depending upon the column operation, these could be distinct and different. The purpose of this investigation was to study the residence time distribution (RTD) of the continous (fluid) and dispersed (microorganisms) phases for model systems as well as for a yeast fermentation. Various degrees of flow nonideality, i.e., fluid blackflow and dispersed phase sedimentation, were noticed. The former seems to be due to interaction of the concurrent gas and liquid flow; it is particularly dependent upon void area of the sieve plate holes. Sedimentation is probably a function of plate design as well as cell size and density. It wa concluded that for a particular plate design the gas hold-up wass controlled by superficial air velocity and was the main parameter governing the differences between dispersed and continous phase(Rt1). This conclusion was supported by a computeraided styudy utilizing a mathematical model of fluid flow to fit the growth kinetics and cell distribution observed experimentally throughout the fermentor. Some advantages of foam control in the tower fermentor by surface active compounds are mentioned. Also, suggestions are made for carrying out fermentations that have two liquid phases, such as a hydrocarbon fermentation. The possibility of closely approximating plug-flow conditions in the multistage tower fermentor, a necessary condition for batch growth simulation, is discussed from a practical point of view.     

Experimental simulation of oxygen profiles and their influence on baker's yeast production: II. Two-fermentor system

A reactor configuration consisting of two reactors with an exchange flow was used for the experimental simulation of large-scale conditions. The influence of fluctuations in oxygen concentration on the growth and metabolite production of baker's yeast was investigated by sparging one fermentor with air and one with nitrogen gas. It was found that the biomass yield decreased and the metabolite formation increased with rising circulation time (longer oxygen-unlimited and oxygen-limited periods). Not only was the performance of the oxygen-limited fermentor characterised by (partly) reductive metabolism, but that of the oxygen-unlimited fermentor as well. The results of the experiments in this reactor system were compared with those from the experiments carried out in a one-fermentor system with periodically changing oxygen concentrations. The formation of acetic acid, which is characteristic for transient states, showed a distinct difference between the two reactor systems.     

Some comments on respiratory quotient (RQ) determination from the analysis of exit gas from a fermentor

Recently, the respiratory quotient (RQ) of microbes measured in situ in a fermentor by exit-gas analysis has been used successfully, for instance, in a fed-batch culture of baker's yeast as a criterion to control the feeding rate.(1-3) It is significant here to keep RQ values close to unity throughout; any deviations of RQ from unity give rise to deterioration of the cell growth yield.However easy it might be to keep RQ values around unity by controlling the feeding rate, the question of whether or not RQ values determined by gas analysis at the fermentor exit could generally represent those in vivo deserves attention. Indeed, for a fermentation carried out at an alkaline side, gas analysis would give RQ values that differ remarkably from true values because of the medium's "storage" of CO(2) released from microbes. The purpose of this communication is to make clear those factors that would affect true RQ values in the analysis of exit gas from a fermentor.     

Control of packed column fouling in the continuous fermentation and stripping of ethanol

By recycling the contents of a 14 L fermentor through a stripping column to continuously remove ethanol and reduce product inhibition, continuous complete conversion of nutrient feed containing 600 g/L glucose was achieved in a small pilot plant. Ethanol was recovered from the carbon dioxide stripping gas in a refrigerated condenser, and the gas was reheated with steam and recycled by a blower. Productivity of ethanol in the fermentor as high as 15.8 g/L/h and condensate production of up to 10 L/day of almost 50% by volume ethanol were maintained for up to 60 days of continuous operation. Weekly washing of the column packing in situ was required to prevent loss of performance caused by attached growth of yeast cells, which restricts the gas flow rate through the stripping column. (c) 1996 John Wiley & Sons, Inc.     

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.     

Rapid sampling for analysis of in vivo kinetics using the BioScope: a system for continuous-pulse experiments

In this article we present a novel device, the BioScope, which allows elucidation of in vivo kinetics of microbial metabolism via perturbation experiments. The perturbations are carried out according to the continuous-flow method. The BioScope consists of oxygen permeable silicon tubing, connected to the fermentor, through which the broth flows at constant velocity. The tubing has a special geometry (serpentine channel) to ensure plug flow. After leaving the fermentor, the broth is mixed with a small flow of perturbing agent. This represents the start of the perturbation. The broth is sampled at different locations along the tubing, corresponding to different incubation times. The maximal incubation time is 69 s; the minimally possible time interval between the samples is 3-4 s. Compared to conventional approaches, in which the perturbation is carried out in the fermentor, the BioScope offers a number of advantages. (1) A large number of different perturbation experiments can be carried out on the same day, because the physiological state of the fermentor is not perturbed. (2) In vivo kinetics during fed-batch experiments and in large-scale reactors can be investigated. (3) All metabolites of interest can be measured using samples obtained in a single experiment, because the volume of the samples is unlimited. (4) The amount of perturbing agent spent is minimal, because only a small volume of broth is perturbed. (5) The system is completely automated. Several system properties, including plug-flow characteristics, mixing, oxygen and carbon dioxide transfer rates, the quenching time, and the reproducibility have been explored, with satisfactory results. Responses of several glycolytic intermediates in Saccharomyces cerevisiae to a glucose pulse, measured using a conventional approach are compared to results obtained with the BioScope. The agreement between the results demonstrates that the BioScope is indeed a promising device for studying in vivo kinetics.     

Development of a new commercial-scale airlift fermentor for rapid growth of yeast

To grow yeast rapidly, it is necessary to supply sufficient oxygen to the yeast and to effectively remove the heat of the fermentation. We succeeded in developing a commercial-scale fermentor for growing a food yeast (Candida utilis) to produce RNA rapidly. This fermentor is an internal-loop airlift type with vertical heat transfer tubes between inner and the outer columns. The volume of the fermentor is 145 m³ (working volume 75 m³). The oxygen transfer rate (OTR) was 9.9 kg-O₂/m³/h using a superficial gas velocity of 30 cm/s based on the outer column. Much of the heat of fermentation and the energy resulting from aeration could be removed effectively by the heat transfer tubes. This unique airlift fermentor was driven at a dilution rate of 0.43 h⁻¹ for about 70 d, with the yeast concentration being maintained at 22.8 kg-dry cell/m³. The yeast production rate was 9.79 kg-dry cell/m³/h. Compared with a traditional stirred-type fermentor, two Vogelbush-type fermentors and another airlift fermentor, our fermentor was far superior with respect to OTR and yeast productivity.     

In vivo NMR system for evaluating oxygen-dependent metabolic status in microbial culture

To optimize an appropriate microbial culture in a fermentor, precise control of the medium's dissolved oxygen tension (DOT) is crucial. In particular, to study the effect of DOT on cellular metabolic status by using in vivo nuclear magnetic resonance (NMR) measurements, the set-up of the experiment must be optimized to maintain DOT in the culture. In the conventional method, DOT is monitored by a sensor inside a fermentor and is controlled by changing the agitation rate. Here, we report a novel and accurate system that minimizes time lag by an automated aeration flow control device, allowing an NMR spectrometer to monitor representative metabolites in real-time. To fulfill these two objects, the fermentor was composed of a fermentation vessel and two outer tubes, through which the medium was circulated by rotary pumps. One tube monitored DOT in via a sensor, and at the same time the other tube monitored metabolites via an NMR spectrometer.In this study, we used this system to analyze the responses of Escherichia coli cells under various oxygen conditions. The results validated the use of this system in the study of microbial metabolism.     

A continuous,multistage tower fermentor. I. Design and performance tests

The design of a continuous column fermentor with a multiple staging effect is described. The column is divided into four compartments by horizontal perforated plates and is provided with a central agitator shaft driving an impeller in each compartment. A tube at the center of each plate forms a liquid seal around the shaft and also acts as a “downcomer.” The fermentor is normally operated with counter-current flow of gas and medium. Fresh medium is added to the top stage and product is withdrawn from the bottom. The effect of plate and agitator design on fermentor performance was studied in terms of factor such as oxygen transfer rate, gas holdup, and interstage mixing. By proper choice of the design parameters, the fermentor was made to approximate a perfect four-stage cascade in terms of reactor performance. Preliminary experiments were performed with air-water systems, but a more realistic picture of fermentor performance was obtained in experience involving propagation of Escherichia coli. Data for business and substrate concentrations in each stage confirmed the staging effect of the apparatus. The fermentor operated in a stable manner for periods of more than two weeks.     

Novel use of a perfluorocarbon for supplying oxygen to aerobic submerged cultures

Summary A novel method for supplying oxygen by using a perfluorocarbon as an oxygen carrier in submerged cultures was demonstrated. Oxygen was transferred to the aqueous phase by liquid-liquid contacting in a spray column fermentor. Advantages of using such a system for supplying oxygen are discussed.Nomenclature F₁ recirculation rate of PP9 through nozzle - P_B back pressure at nozzle - SA_PP9 oxygen concentration in PP9 in aerator as a percent of saturation when air equilibrated at the end of the exponential phase of growth - T_F fermentation time to reach final cell concentration - V_A air flow rate through sparger - X_e cell concentration at the end of the exponential phase of growth - X_F final cell concentration - Ø_A gas (air) hold-up - Ø_L liquid (PP9) hold-up in aqueous system     

Gas holdup, power consumption, and oxygen absorption coefficient in a stirred-tank fermentor under foam control

For a laboratory stirred-tank fermentor (STF) with foaming system of 0.5M sulfite solution containing an anionic soft detergent, the performing of a foam-breaking apparatus with a rotating disk (FARD)fitted to the STF was evaluated. The gas holdup in a mechanical foam-control system (MFS), i.e., the stirred-tank fermentor with the rotating disk foambreaker, was confirmed to be larger than that in a nonfoaming system (NS), i.e., the STF with an antifoam agent added. The agitation power in the mechanical foam control system was found to be smaller compared with the agitation power in the nonfoaming system, due to the increased gas holdup. Comparison of the oxygen absorption coefficient between the mechanical foam control system and the nonfoaming system in terms of the specific power input also demonstrated the superiority of the mechanical foam control system, not only in oxygen transfer performance but also in power input economy.     

Mechanistic analysis of xanthan gum production in a stirred tank

The overall effect of agitation on xanthan gum production by Xanthomonas campestris ATCC13951 in a stirred vessel was mechanistically analyzed considering local variation of the specific production rate due to variation of shear stress in the vessel. The whole liquid volume in a fermentor was roughly divided into three regions; the micromixing region around the impeller with high shear stress, the macromixing region dominated by a circulating flow and the stagnant region. The value of the shear rate was first ascertained by experiments in order to obtain a picture of shear rate variation in a radial direction from the impeller, and the equivalence between the volumes of the high shear stress region and micromixing region was confirmed. The shear stress obtained using a correlation between the shear rate at the impeller tip and Reynolds number of Wichterle et al. was used as a representative of the shear stress in the micromixing region, and the shear stress estimated by use of an empirical correlation between the average shear rate in a fermentor and agitation speed derived by Metzner et al. was adopted as a representative of the shear stress in the macromixing region. The information about the circulation time distribution was also used to take into account oxygen deficiency during circulation of liquid elements in the macromixing region, considering that oxygen from the gas phase was supplied mainly in the high shear region. The calculated values of xanthan gum concentrations which were obtained by the proposed simulation method agreed well with the experimental data in the time course of xanthan gum production at various agitation speeds. Experimental results of the relationship between the overall specific production rate and ND (N, agitation speed, and D, impeller diameter) was also verified by the proposed method.     

Effect of ventilation on the production of acetaldehyde by Zymomonas mobilis

The production of acetaldehyde, a flavoring agent in food, by Zymomonas mobilis was carried out in batch culture. The volatilization rate constant (k_v) of acetaldehyde and the initial volumetric oxygen transfer coefficient (k_La⁰) in an Erlenmeyer flask with a cotton-plug (cotton-flask) and an aerated-flask with a forced-air system (aerated-flask) were measured. The culture environment in the aerated-flask was found to be very different from that in the cotton-flask. Cell growth in a cotton-flask was strongly inhibited, making practical acetaldehyde production in cotton-flask very difficult. On the other hand, acetaldehyde production in the aerated-flask increased while the fermentation time decreased with increases in the air flow rate. The k_v value of acetaldehyde in a jar fermentor was affected mainly by air flow rate. By considering both the oxygen transfer rate and the ventilation effect on the culture, it was possible to scale-up from the aerated-flask to a jar fermentor. In the jar fermentor, production of acetaldehyde and growth inhibition by acetaldehyde were affected mainly by the k_La⁰ and k_v, values, respectively. The overall production of acetaldehyde in the jar fermentor under the optimum k_La⁰ and k_v conditions was 6.64 g/l (Y_p/s: 0.27), which was about 1.5 times higher than the maximum concentration obtained in the aerated-flask.     

An experimental study of acid production rate controlled operations of a continuous fermentor

The efficacy of acid production rate (APR) controlled operations of a continuous fermentor supporting the growth of a methylotroph, L3, was experimentally examined. Direct digital control of pH at a constant value allowed for on-line estimation of APR during the fermentation. Two types of APR controlled operations were studied. In the first type of operation, the APR was controlled at a constant value according to a predetermined program by manipulating the feed flow rate to the fermentor. Such an operation effectively stabilized the cell mass productivity of a continuous fermentor subjected to disturbances in the feed nutrient concentration. It resulted in a near complete conversion of methanol to yield a cell mass product with very low amounts of unutilized methanol at both steady state and transient fermentation situations. In the second type of operation, the feed flow rate was manipulated to optimize the steady state value of APR during the fermentation. This method shows promise for on-line steady state optimization of cell mass productivity in a continuous fermentor.