Impact of nozzle operation on mass transfer in jet aerated loop reactors. Characterization and comparison to an aerated stirred tank reactor

The impact of mass transfer on productivity can become a crucial aspect in the fermentative production of bulk chemicals. For highly aerobic bioprocesses the oxygen transfer rate (OTR) and productivity are coupled. The achievable space time yields can often be correlated to the mass transfer performance of the respective bioreactor. The oxygen mass transfer capability of a jet aerated loop reactor is discussed in terms of the volumetric oxygen mass transfer coefficient k_La [h^?1] and the energetic oxygen transfer efficiency E [kgO₂ kW^?1 h^?1]. The jet aerated loop reactor (JLR) is compared to the frequently deployed aerated stirred tank reactor. In jet aerated reactors high local power densities in the mixing zone allow higher mass transfer rates, compared to aerated stirred tank reactors. When both reactors are operated at identical volumetric power input and aeration rates, local k_La values up to 1.5 times higher are possible with the JLR. High dispersion efficiencies in the JLR can be maintained even if the nozzle is supplied with pressurized gas. For increased oxygen demands (above 120 mmol L^?1 h^?1) improved energetic oxygen transfer efficiencies of up to 100 % were found for a JLR compared to an aerated stirred tank reactor operating with Rushton turbines.     

Prediction of gas-liquid mass transfer coefficient in sparged stirred tank bioreactors

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 kLa 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 kLa values, due to the consumption of oxygen for both cell growth and metabolite production. In this work, theoretical equations for kLa 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 kLa 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.     

低高径比喷射环流生化反应器流体力学和发酵性能的研究

对高径比s≤2．5喷射环流生化反应器的流体力学和传质特性进行了系统的研究,选出反应器的最佳结构,关联出氧的体积传递系数(k_La)表达式。在此基础上,进行了谷氨酸发酵试验,摸索出用该设备进行各氨酸发酵的最佳工艺条件,使5批一次性投糖发酵的糖酸转化率达到50％以上。     

Mass transfer limitation as a tool to enhance the enantiomeric excess in the enzymatic synthesis of chiral cyanohydrins

The enantioselective synthesis of cyanohydrins catalyzed by R-hydroxynitrile lyase in an aqueous-organic liquid two-phase system using, mass transfer limitation to enhance enantiomeric excess at 5°C and pH 5.5 is described. Benzaldehyde, a good substrate, and cinnamaldehyde, a notoriously difficult substrate, were used as model substrates and compared in order to establish the mass transfer limitation concept in a two-liquid phase system, where the non-enzymatic-racemic reaction competes. Enzyme concentration and phase volume ratio between organic and buffer phase were geared to one another to enhance the enantiomeric excess for each substrate. In both cases, after optimization, excellent chemical conversion (>99% on a 60 mmol scale), high throughput and high enantiomeric excess (benzaldehyde >99% and cinnamaldehyde >96%) were achieved.     

Measurement of heat transfer coefficients in stirred single‐use bioreactors by the decay of hydrogen peroxide

Single‐use bioreactors are barely described by means of their heat transfer characteristics, although some of their properties might affect this process. Steady‐state methods that use external heat sources enable precise investigations. One option, commonly present in stirred, stainless steel tanks, is to use adjustable electrical heaters. An alternative are exothermic chemical reactions that offer a higher flexibility and scalability. Here, the catalytic decay of hydrogen peroxide was considered a possible reaction, because of the high reaction enthalpy of –98.2 kJ/mole and its uncritical reaction products. To establish the reaction, a proper catalyst needed to be determined upfront. Three candidates were screened: catalase, iron(III)‐nitrate and manganese(IV)‐oxide. Whilst catalase showed strong inactivation kinetic and general instability and iron(III)‐nitrate solution has a pH of 2, it was decided to use manganese(IV)‐oxide for the bioreactor studies. First, a comparison between electrical and chemical power input in a benchtop glass bioreactor of 3.5 L showed good agreement. Afterwards the method was transferred to a 50 L stirred single‐use bioreactor. The deviation in the final results was acceptable. The heat transfer coefficient for the electrical method was 242 W/m²/K, while the value achieved with the chemical differed by less than 5%. Finally, experiments were carried out in a 200 L single‐use bioreactor proving the applicability of the chemical power input at technical relevant scales.     

Effect of organic solvents on oxygen mass transfer in multiphase systems: Application to bioreactors in environmental protection

The absorption of oxygen in aqueous–organic solvent emulsions was studied in a laboratory-scale bubble reactor at a constant gas flow rate. The organic and the gas phases were dispersed in the continuous aqueous phase. Volumetric mass transfer coefficients (k_La) of oxygen between air and water were measured experimentally using a dynamic method. It was assumed that the gas phase contacts preferentially the water phase. It was found that addition of silicone oils hinders oxygen mass transfer compared to air–water systems whereas the addition of decane, hexadecane and perfluorocarbon PFC40 has no significant influence. By and large, the results show that, for experimental conditions (organic liquid hold-up ≤10% and solubility ratio ≤10), the k_La values of oxygen determined in binary air–water systems can be used for multiphase (gas–liquid–liquid) reactor design with applications in environmental protection (water and air treatment processes).     

Application of equilibrium and mass transfer models to dynamic removal of Cr(VI) ions by Chitin in packed column reactor

The dynamic removal of hexavalent chromium by chitin flakes was studied in a packed column reactor. The values of column parameters were predicted as a function of flow rate, bed depth, particle size and inlet metal ion concentration. On evaluating the breakthrough curves, sorption isotherms were obtained and modelled according to the Langmuir, the Redlich–Peterson and the Freundlich models. Kinetic and mass transfer aspects of the dynamic removal of Cr(VI) ions by chitin were investigated using several mathematical models. Column studies showed a good correlation between the experimental data and the calculated breakthrough curves obtained by the Adams–Bohart or the Wolborska models and the Clark model. The simulation of the whole breakthrough curve was effective with the Clark model, but the breakthrough was best predicted by Adams–Bohart, or related derived models.     

Syngas fermentation of Clostridium carboxidivoran P7 in a hollow fiber membrane biofilm reactor: Evaluating the mass transfer coefficient and ethanol production performance

Gasification followed by syngas fermentation is a unique hybrid process for converting lignocellulosic biomass into fuels and chemicals. Current syngas fermentation faces several challenges with low gas–liquid mass transfer being one of the major bottlenecks. The aim of this work is to evaluate the performance of hollow fiber membrane biofilm reactor (HFM-BR) as a reactor configuration for syngas fermentation. The volumetric mass transfer coefficient (K_La) of the HFM-BR was determined at abiotic conditions within a wide range of gas velocity/flowrate passing through the hollow fiber lumen and liquid velocity/flowrate passing through the membrane module shell. The K_La values of the HFM-BR were higher than most reactor configurations such as stir tank reactors and bubble columns. A continuous syngas fermentation of Clostridium carboxidivorans P7 was implemented in the HFM-BR system at different operational conditions, including the syngas flow rate, liquid recirculation between the module and reservoir, and the dilution rate. It was found that the syngas fermentation performance such as syngas utilization efficiency, ethanol concentration and productivity, and ratio of ethanol to acetic acid depended not only on the mass transfer efficiency but also the characteristics of biofilm attached on the membrane module (biofouling or abrading of the biofilm). The HFM-BR results in a highest ethanol concentration of 23.93 g/L with an ethanol to acetic acid ratio of 4.79. Collectively, the research shows the HFM-BR is an efficient reactor system for syngas fermentation with high mass transfer.     

The therapeutic potential of immune cell-derived exosomes as an alternative to adoptive cell transfer

Adoptive cell transfer (ACT), a form of cell-based immunotherapy that eliminates cancer by restoring and strengthening the body’s immune system, has revolutionized cancer treatment. ACT entails intravenous transfer of either tumor-resident or peripheral blood-modified immune cells into cancer patients to mediate anti-tumor response. Although these immune cells control and eradicate cancer via enhanced cytotoxicity against specific tumor antigens, several side effects have been frequently reported in clinical trials. Recently, exosomes, potential cell-free therapeutics, have emerged as an alternative to cell-based immunotherapies, due to their higher stability under same storage condition, lower risk of GvHD and CRS, and higher resistance to immunosuppressive tumor microenvironment. Exosomes, which are nano-sized lipid vesicles, are secreted by living cells, including immune cells. Exosomes contain proteins, lipids, and nucleic acids, and the functional role of each exosome is determined by the specific cargo derived from parental cells. Exosomes derived from cytotoxic effectors including T cells and NK cells exert anti-tumor effects via proteins such as granzyme B and FasL. In this mini-review, we describe the current understanding of the ACT and immune cell-derived exosomes and discuss the limitations of ACT and the opportunities for immune cell-derived exosomes as immune therapies.     

Influence of sparger on energy dissipation, shear rate, and mass transfer to sea water in a concentric-tube airlift bioreactor

Data on volumetric mass-transfer coefficient, K_La_L, in a 12 × 10⁻³ m³ airlift bioreactor are reported. Measurements were made in sea water. The superficial gas velocity ranged up to 0.21 m/s. Four cylindrical spargers (60–1000 μm pore size) were tested. In bubbly flow, the sparger pore size strongly influenced the K_La_L; the highest K_La_L values were obtained with the smallest pore size. In contrast, in the transition and heterogeneous flow regimes, the pore size had little influence on K_La_L. The best correlation of the mass transfer data was obtained when both gas holdup and liquid superficial velocity were taken as independent variables. Shear rates were estimated in the different zones of the reactor. The highest values were found in the bottom zone of the reactor and in the gas-liquid separator. The penetration and isotropic turbulence models were used to develop a semi-theoretical equation relating the volumetric mass-transfer coefficient to shear rate; hence providing a better understanding of how the operational variables may be manipulated to attain a moderate shear rate and an appropriate level of mass transfer, two extremely important parameters for the growth of sensible microorganisms as those used in marine biotechnology.     

A comparative evaluation of oxygen mass transfer and broth viscosity using Cephalosporin-C production as a case strategy

The production of Cephalosporin-C (CPC) a secondary metabolite, using a mold Acremonium chrysogenum was studied in a lab scale Internal loop air lift reactor. Cephalosporin-C production process is a highly aerobic fermentation process. Volumetric gas–liquid mass transfer coefficient (k_La) and viscosity (η) were evaluated, during the growth and production phases of the microbial physiology. An attempt has been made to correlate the broth viscosity, η and volumetric oxygen transfer coefficient, k_La during the Cephalosporin-C production in an air lift reactor. The impact of biomass concentration and mycelial morphology on broth viscosity has been also evaluated. The broth exhibits a typical non-Newtonian fermentation broth. Rheology parameters like consistency index and fluidity index are also studied.     

Biotransformation of glucose to gluconic acid by Aspergillus niger—study of mass transfer in an airlift bioreactor

A glucose–gluconic acid biotransformation system was suggested for the experimental study of oxygen transfer in bioreactors. This biosystem was used for the investigation of the effect of the flow rate and biomass concentration on the volumetric oxygen transfer coefficient k_La in a 10 dm³ internal-loop airlift bioreactor. For this purpose, the fermentation broth of the mycelial strain Aspergillus niger was employed, representing a three-phase system, where bubbles come into contact with dense rigid pellets. The results showed that the presented biotransformation system can be successfully utilised for the determination of the oxygen transfer rate in airlift bioreactors. The experiments showed a strong positive influence of the air flow rate on the rate (r_Glu), specific rate of gluconic acid production (k_Glu/X) as well as on the volumetric oxygen transfer coefficient (k_La). This confirmed an expected limitation of production rate by the oxygen transport from the gas to the liquid phase in the whole range of air flow rates applied. Moreover, consistent curves of the production rate r_Glu and k_La values vs. biomass concentration c_X (amount of enzymes) were observed. These exhibited a local maximum for c_X equal to 6.68 g dm⁻³. On the other hand, the specific production rate monotonously decreased with increasing biomass concentration. A decline of k_La values at higher c_X values was attributed to a bubble coalescence promoting effect of mycelial pellets.     

Syngas fermentation to biofuel: Evaluation of carbon monoxide mass transfer coefficient (kLa) in different reactor configurations

Lignocellulosic biomass such as agri‐residues, agri‐processing by‐products, and energy crops do not compete with food and feed, and is considered to be the ideal renewable feedstocks for biofuel production. Gasification of biomass produces synthesis gas (syngas), a mixture primarily consisting of CO and H₂. The produced syngas can be converted to ethanol by anaerobic microbial catalysts especially acetogenic bacteria such as various clostridia species.One of the major drawbacks associated with syngas fermentation is the mass transfer limitation of these sparingly soluble gases in the aqueous phase. One way of addressing this issue is the improvement in reactor design to achieve a higher volumetric mass transfer coefficient (k_La). In this study, different reactor configurations such as a column diffuser, a 20‐μm bulb diffuser, gas sparger, gas sparger with mechanical mixing, air‐lift reactor combined with a 20‐μm bulb diffuser, air‐lift reactor combined with a single gas entry point, and a submerged composite hollow fiber membrane (CHFM) module were employed to examine the k_La values. The k_La values reported in this study ranged from 0.4 to 91.08 h^?1. The highest k_La of 91.08 h^?1 was obtained in the air‐lift reactor combined with a 20‐μm bulb diffuser, whereas the reactor with the CHFM showed the lowest k_La of 0.4 h^?1. By considering both the k_La value and the statistical significance of each configuration, the air‐lift reactor combined with a 20‐μm bulb diffuser was found to be the ideal reactor configuration for carbon monoxide mass transfer in an aqueous phase. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2011     

A simple method to estimate the contribution of biological floc and reactor-solution to mass transfer of oxygen in activated sludge processes

In this study, the mass transfer coefficient of biological floc (K(L)a(bf)) was estimated from the mass transfer coefficient of the mixed-liquor (K(L)a(f)) and the reactor-solution (K(L)a(e)). The biological floc resistance (BFR) and reactor-solution resistance (SR) were defined as the reciprocal of K(L)a(bf) and K(L)a(e), respectively, by applying the concept of serial-resistance originally presented in two-film theory (Lewis and Whitman (1924) Ind Eng Chem 16:1215-1220). The specific biological floc resistance (SBFR) was defined as biological floc resistance per unit biomass concentration. The data indicated that an activated sludge process yielding low BFR/MLR and BFR/SR tended to produce higher oxygen transfer efficiency. Surprisingly, the reactor-solution posed the same level of resistance as clean water in all experiments, except in a 5-day SRT, non-nitrifying, completely mixed activated sludge (CMAS) process run. Furthermore, SBFR successfully represented biological floc and showed a positive correlation to sludge volume index (SVI). In addition, SBFR/SR and oxygen transfer efficiency (OTE(f)) followed an exponential relationship for the complete data set. The method of separating the mixed-liquor into biological floc and reactor-solution improved the understanding of oxygen transfer under process conditions, without resorting to intrusive techniques or direct handling of fragile biological floc.     

Mathematical analysis of solid-liquid mass transfer in a batch reactor for the enzymatic transformation of testosterone to 4AD

A previous mathematical analysis of mass transfer in a two-phase (solid-liquid) batch reactor for enzymatic transformation of testosterone to 4AD (Pereira et al., 1987) is extended to incorporate the effect of convective mixing. The results of the analysis showed that for a given enzyme loading, the mass transfer resistance in the solid (a function of the bead size) and the intensity of convective mixing (as embodied in the mass transfer coefficient) are two parameters that can be varied such that the overall mass transfer rate from the solid to the liquid phase ensures optimal reactor performance.