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%.     

KLa measurement in two‐phase partitioning bioreactors: new insights on potential errors at low power input

BACKGROUND: Two‐phase partitioning bioreactors (TPPBs) are based on the addition of a non‐aqueous phase (NAP) to a biological process in order to overcome a limited delivery of gaseous substrates to the microorganisms in the case of compounds with low affinity for water. However, the high power input (P_g/V) required to disperse the NAP is often the major limitation for TPPB applications at full scale. Therefore, the accurate determination of the overall mass transfer coefficient (K_La) at low P_g/V values is a critical issue as these operational conditions are more attractive from a scale‐up point of view. RESULTS: NAP addition altered the typical shape of the dissolved oxygen curves used for K_La determination at the lowest P_g/V values tested (70–80 W m^?3). Below a threshold P_g/V value of 600 W m^?3, the presence of the NAP increased the error in K_La measurements up to 115% relative to controls deprived of NAP. CONCLUSIONS: The error in K_La measurements at low P_g/V values might be related to failures in the fundamental assumption regarding liquid phase homogeneity in the mass transfer model used. Copyright © 2010 Society of Chemical Industry     

Mass transfer and deodorization efficiency in a countercurrent spray tower for low superficial gas velocities

The aim of this study was to characterize mass transfer and deodorization efficiency in a countercurrent spray tower for low superficial gas velocities. The influence of operating parameters (U_G = 0.005 to 0.025 m s^?1, U_L = 6.1 × 10^?5 to 2.4 × 10^?4 m s^?1) on the liquid retention (ε_L), the drop diameter (d_g), the interfacial area (a) and the overall liquid and gas phase mass transfer coefficients (K_La, K_Ga) were estimated. The spray efficiency of some malodorous compounds was also estimated. A negative influence of the superficial gas velocity was demonstrated, during the spraying of water or chemical neutralizing scrubbing solutions. There was also an increase with the liquid flow rate. Abatements obtained were very good with respect to ammonia (>90%), and acceptable for the other compounds.     

Mass transfer in external-loop airlift bioreactors using static mixers

The influence of static mixers on the overall gas-liquid volumetric mass transfer coefficient (K_La_L) was examined in an external-loop type airlift bioreactor (approximately 15 L volume, 1.8 m static liquid height, A_r/A_d = 0.444). The study was conducted with aqueous salt solution (0.15 kmol ? m^?3 NaCl) and with pseudoplastic solutions of carboxymethyl cellulose (0.2 ? 0.6 wt./vol. % (g/100 mL) CMC). Over a broad range of power law parameters K (10^?3 ? 10 Pa ? sⁿ) and n (0.5 ? 1.0), the presence of static mixers in the riser was found to enhance the K_La_L relative to mixer-free mode of operation. The extent of increase in K_La_L depended on the fluid “thickness”, K: the higher the K, the greater the K_La_L intensification due to static mixers. For otherwise identical conditions, the presence of static mixers improved K_La_L by 30-500%, depending on the fluid. The boost in K_La_L was associated with increased gas holdup and gas-liquid interfacial area, which arose due to bubble breakup accomplished by the static mixing elements. Potential advantages of static mixers in upgrading the performance of oxygen-limited fermentations were pointed out.     

Mass transfer in liquid-liquid (horizontal) pipeline contactors

The mass transfer characteristics of liquid-liquid (horizontal) pipeline contactors were studied for the bubble and plug flow regimes. The contactors were operated co-currently. The theory of mass transfer with chemical reaction was used to obtain values of physical mass transfer coefficient (k_La) and effective interfacial area (a) for a number of systems. In all the experiments the resistance to mass transfer was confined to the continuous phase. The effect of the flow rates and viscosities of the two phases, interfacial tension, contactor length, pipe diameter, the presence of solids, etc. on both, k_La and a was studied. The values of k_La and a were found to vary from 1 to 50 × 10^?3 sec^?1, and 0.4 to 25 cm^?1, respectively.     

Carbon Dioxide Absorption into Stirred Emulsions of n‐Alkanes

Volumetric mass transfer coefficients k_La for CO₂ absorption into n‐alkane/water emulsions were determined at oil volume fractions of 0–100 % in a stirred tank at a stirring speed of 1000 min^?1. The oil was n‐heptane, n‐hexadecane, or n‐dodecane. The decrease of k_La with increasing volume fraction of dispersed oil can be uniformly correlated to the emulsion viscosity with the power of ?0.72. Only the addition of n‐heptane caused a strong increase of the mass transfer coefficient. Upon addition of the surfactant sodium dodecyl sulfate to n‐heptane emulsions, k_La decreased as for the other oils. The increase can therefore be attributed to the spreading of n‐heptane on the bubble surface enabling gas‐oil contact, whereas spreading is inhibited by the ionic surfactant.     

Performance evaluation of a water/silicone oil two‐phase partitioning bioreactor using Rhodococcus erythropolis T902.1 to remove volatile organic compounds from gaseous effluents

BACKGROUND: In the framework of biological processes used for waste gas treatment, the impact of the inoculum size on the start‐up performance needs to be better evaluated. Moreover, only a few studies have investigated the behaviour of elimination capacity and biomass viability in a two‐phase partitioning bioreactor (TPPB) used for waste gas treatment. Lastly, the impact of ethanol as a co‐substrate remains misunderstood. RESULTS: Firstly, no benefit of inoculation with a high cellular density (>1.5 g L^?1) was observed in terms of start‐up performance. Secondly, the TPPB was monitored for 38 days to characterise its behaviour under several operational conditions. The removal efficiency remained above 63% for an inlet concentration of 7 g isopropylbenzene (IPB) m^?3 and at some time points reached 92% during an intermittent loading phase (10 h day^?1), corresponding to a mean elimination capacity of 4 × 10^?3 g L^?1 min^?1 (240 g m^?3 h^?1) for a mean IPB inlet load of 6.19 × 10^?3 g L^?1 min^?1 (390 g m^?3 h^?1). Under continuous IPB loading, the performance of the TPPB declined, but the period of biomass acclimatisation to this operational condition was shorter than 5 days. The biomass grew to approximately 10 g L^?1 but the cellular viability changed greatly during the experiment, suggesting an endorespiration phenomenon in the bioreactor. It was also shown that simultaneous degradation of IPB and ethanol occurred, suggesting that ethanol improves the biodegradation process without causing oxygen depletion. CONCLUSION: A water/silicone oil TPPB with ethanol as co‐substrate allowed the removal of a high inlet load of IPB during an experiment lasting 38 days. Copyright © 2008 Society of Chemical Industry     

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.     

Mass Transfer of Ozone Using a Microporous Diffuser Reactor System

An ozone reactor was constructed using a tubular gas diffuser made of microporous stainless steel to significantly reduce gas bubble size and increase overall mass transfer area. Overall mass transfer coefficient, K_La [s ^?1], was correlated with gas (G) and liquid (L) flow rates using K_La = AL^αG^β , with A = 3.96 × 10 ⁸ [s^?1], α = 1.53, and β = 0.40, with L and G in [m ³s^?1]. The reactor is essentially plug flow at high G or L. This system achieves one of the highest ozone mass transfer rates observed in the literature.     

The effect of a surfactant on mixer—settler operation

A single stage mixer—settler was used to investigate the effect of surfactant on the mass transfer rate in the system water—HNO₃—30 vol.% TBP/dodecane. The interfacial tension of this system first falls then rises with increasing sodium lauryl sulphate (SLS) concentration. The addition of SLS makes the stage efficiency, which is closely related to k_ha, the product of the individual mass transfer coefficient of HNO₃ in the aqueous phase, and average interfacial area per unit volume of mixing chamber, to increase significantly due to an increase in the value of a. A maximum k_ha value of 0.53 litres^?1, a minimum value of interfacial tension, and phase inversion which converted the aqueous phase from continuous to dispersed were observed at around the critical micellar concentration (100 parts 10^?6) of SLS in the system of an aqueous to organic phase ratio of 0.2.     

Intermediate accumulation and efficiency of anaerobic digestion treatment of surfactant (alcohol sulfate)‐rich wastewater at increasing surfactant/biomass ratios

Textile (eg cotton) finishing industry wastewater is characterised by high concentrations of surfactants (up to 2 g dm^?3) and of readily biodegradable biopolymers (COD 5–15 g dm^?3). The anionic surfactant decyl sulfate (DS) was chosen as model surfactant and soluble starch (size) as model compound for the readily biodegradable fraction of the wastewater. Twenty‐two batch experiments with increasing DS/biomass ratio (and starch/biomass ratio) were started simultaneously. Biomass concentrations ranged from 50 to 15 000 mg dm^?3. Minor inhibition effects were found for the surfactant degradation itself at all DS/biomass ratios (maximum biodegradation rate 7.7 mg_DS g_biomass^?1 h^?1). The starch hydrolysis started without a lag‐phase at DS/biomass ratios of up to 0.15 g_DS g_biomass^?1. The lag‐phase was prolonged to about 100 h at a very high DS/biomass ratio (3 g_DS g_biomass^?1). The relative importance of the accumulated intermediates was dependent on the DS/biomass ratio. Above 0.3 g_DS g_biomass^?1 10% of the substrate organic carbon accumulated as ethanol, but no ethanol accumulation was observed at low DS/biomass ratios. Moderate DS/biomass ratios caused a considerable delay of the methanogenesis; high DS/biomass ratios prevented the methanogenesis almost completely. © 2002 Society of Chemical Industry     

The cocurrent downflow contactor–a novel reactor for gas-liquid-solid catalysed reactions

The Cocurrent Downflow Contactor (CDC) has been developed as a mass transfer and reactor device, with and without addition of tangential (swirl) flow, giving gas hold-up (E_g) values of 0.5–0.75, interfacial areas in the range 1000–6000 m²m^?3 liquid and k_La values in the range of 0.15–1.55 s^?1 for absorption using the O₂/H₂O system. It has been studied as a catalytic slurry reactor for the hydrogenation of (i) itaconic acid and (ii) triglycerides catalysed by Pd and Ni catalysts. The reactions were observed to be largely surface-reaction rate controlled, due to the very efficient mass transfer (k_La up to 11.75 s^?1 under reaction conditions) and application of swirl flow-enhanced reaction rates. The CDC has recently been found to be capable of operating as a fixed bed reactor, thus eliminating a downstream catalyst separation problem (therefore more cost effective), and is superior in its mass transfer characteristics to other known devices. Scale-up can be undertaken without loss of performance efficiency.     

Non‐dispersive absorption of CO2 in parallel and cross‐flow membrane modules using EMISE

BACKGROUND: This paper reports an analysis of the mass transfer behaviour of CO₂ absorption in hollow fibre membrane modules in parallel and cross‐flow dispositions. The ionic liquid EMISE, 1‐ethyl‐3‐methylimidazolium ethylsulfate, is used to achieve a zero solvent emission process and the experimental results are compared with CO₂ permeation through the membrane, without solvent in the lumenside. RESULTS: Overall mass transfer coefficients K_{overall, CF} = (0.74 ± 0.02) × 10^?6 m s^?1 and K_{overall, PF} = (0.37 ± 0.018) × 10^?6 m s^?1 were obtained for cross‐flow and parallel flow, respectively. These values are one order of magnitude lower than the coefficient obtained in permeability experiments, K_{overall, PERM} = (6.16 ± 0.1) × 10^?6 m s^?1, indicating the influence of the absorption in the process. Including the specific surface and gas volume of each contactor in the analysis, a similar value of a first‐order kinetic rate constant, K_R = 2.7 × 10^?3 s^?1 is obtained, showing that the interfacial chemical reaction CO₂‐ionic liquid is the slow step in the absorption process. CONCLUSION: An interfacial chemical reaction rate constant K_R = 2.7 × 10^?3 s^?1, describes the behaviour of the CO₂ absorption in the ionic liquid EMISE using membrane contactors in parallel and cross‐flow dispositions. Copyright © 2012 Society of Chemical Industry     

Gas‐Liquid Mass Transfer in Fibrous Bed Reactor with Counter‐Current Liquid Recycle

In this work, the gas‐liquid mass transfer in a lab‐scale fibrous bed reactor with liquid recycle was studied. The volumetric gas‐liquid mass transfer coefficient, k_La, is determined over a range of the superficial liquid velocity (0.0042–0.0126 m.s^–1), gas velocity (0.006–0.021 m.s^–1), surface tension (35–72 mN/m), and viscosity (1–6 mPa.s). Increasing fluid velocities and viscosity, and decreasing interfacial tension, the volumetric oxygen transfer coefficient increased. In contrast to the case of co‐current flow, the effect of gas superficial velocity was found to be more significant than the liquid superficial velocity. This behavior is explained by variation of the coalescing gas fraction and the reduction in bubble size. A correlation for k_La is proposed. The predicted values deviate within ± 15 % from the experimental values, thus, implying that the equation can be used to predict gas‐liquid mass transfer rates in fibrous bed recycle bioreactors.     

Effects of surfactant contamination on oxygen mass transfer in fine bubble aeration process

The effects of anionic, cationic, and non-ionic surfactants (SDS, SDBS, CTAB and Tween20) on oxygen mass transfer (OMT) in fine bubble aeration systems were investigated. The overall gas-liquid volumetric mass transfer coefficient (K _L a), specific interfacial area (a), and liquid-side mass transfer coefficient (K _L ) parameters were used to assess the influence of the surfactants. At the same concentration, the different surfactants were observed to influence the K _L a value as follows: K _L a (SDBS)>K _L a (SDS)>K _L a (tween20)>K _L a (CTAB). For all used surfactants, the overall trends showed a significant decrease in the K _L a value at low concentrations (0–5mg/L), while the K _L a value recovered somewhat at high concentrations (10–20mg/L). The decrease to the KL value was found to be much larger than increase in the a value in the presence of surfactants. Furthermore, a simple model was established that provides an OMT prediction for different surfactants.     

Production of poly‐3‐hydroxybutyrate by Cupriavidus necator from corn syrup: statistical modeling and optimization of biomass yield and volumetric productivity

BACKGROUND: The paper reports an investigation into the possibility of producing poly‐3‐hydroxybutyrate (P(3HB)) polyester using corn syrup, a relatively low cost by‐product from the starch industries. The concentrations of medium components, corn syrup, dipotassium hydrogen phosphate (K₂HPO₄), sodium dihydrogen phosphate (NaH₂PO₄) and ammonium sulfate [(NH₄)₂SO₄] were optimized using design of experiments (DOE). RESULTS: Response surface methodology (RSM) under central composite face design (CCFD) was used to obtain the optimum values of medium components and responses in terms of biomass yield and volumetric P(3HB) productivity. The highest P(3HB) productivity and biomass yield obtained were 0.224 g L^?1 h^?1 and 0.57 g g^?1, respectively. A limited‐nitrogen concentration had a higher volumetric P(3HB) productivity (0.170 g L^?1 h^?1) than that of the excess nitrogen batch experiment (0.0675 g L^?1 h^?1). The optimum corn syrup:N:P ratio of 50:0.078:1 was based on numerical optimization of the desirability function between biomass yield and volumetric P(3HB) productivity by Cupriavidus necator DSMZ 545. CONCLUSION: The results obtained in this study demonstrated that P(3HB) could be efficiently produced to a high concentration with high productivity by applying nitrogen limitation in a defined medium, indicating this agricultural by‐product to be a suitable nutrient source in further studies to develop biomaterials through biotechnology. Copyright © 2010 Society of Chemical Industry     

Combined Effect of Agitation/Aeration and Fed‐Batch Strategy on Ubiquin‐ one‐10 Production by Pseudomonas diminuta

The effects of aeration rate and agitation speed on ubiquinone‐10 (CoQ10) submerged fermentation in a stirred‐tank reactor using Pseudomonas diminuta NCIM 2865 were investigated. CoQ10 production, biomass formation, glycerol utilization, and volumetric mass transfer coefficient (k_La) were affected by both aeration and agitation. An agitation speed of 400 rpm and aeration rate of 0.5 vvm supported the maximum production (38.56 mg L^–1) of CoQ10 during batch fermentation. The fermentation run supporting maximum production had an k_La of 27.07 h^–1 with the highest specific productivity and CoQ10 yield of 0.064 mg g^–1h^–1 and 0.96 mg g^–1 glycerol, respectively. Fermentation kinetics performed under optimum aeration and agitation showed the growth‐associated constant (a = 5.067 mg g^–1) to be higher than the nongrowth‐associated constant (β = 0.0242 mg g^–1h^–1). These results were successfully utilized for the development of fed‐batch fermentation, which increased the CoQ10 production from 38.56 mg L^–1 to 42.85 mg L^–1.     

Improvement of heat removal in solid‐state fermentation tray bioreactors by forced air convection

BACKGROUND: Heat removal is one of the major constraints in large‐scale solid‐state fermentation (SSF) processes. The effect of internal air circulation by forced convection on heat and water transfer has not been studied in SSF tray bioreactors. Formulation of a mathematical model for SSF requires a good estimation of the mass and heat transfer coefficients. RESULTS: A stainless steel tray bioreactor (80.6 L capacity) was used. Aspergillus niger C28B25 was cultivated under SSF conditions on an inert support. Temperature, moisture content, biomass and substrate concentrations were measured. Water and energy integral balances were used to estimate the heat and mass transfer coefficients involved in the process. The Reynolds number (N_Re) in the headspace of the tray bioreactor ranged from 2.5 to 2839, which increased the global heat transfer coefficient from 4.2 to 6.9 (W m^?2 K^?1) and the mass transfer coefficient from 1.0 to 2.1 (g m^?2 s^?1). Mathematical model predictions of the temperature and moisture content of the fermentation bed showed a high goodness‐of‐fit with the experimental results. CONCLUSIONS: This is the first report describing the effect of N_Re of air in the headspace of a SSF tray bioreactor on the heat and mass transfer coefficients and temperature regulation in SSF. Copyright © 2011 Society of Chemical Industry     

Improved gas'liquid contacting in co-current flow

A new method of increasing interfacial area for gas-liquid contacting in co-current flow using screen packings has been evaluated in a 51/2-in. I. D. column using CO₂ chemisortpion in sodium hydroxide solution and CO² physical desorption from water. The study investigated the effect of gas velocity (0.3-2.7-ft/sec.), liquid velocity (0.018-0.1-ft/sec), and of column height (0.67-7.8-ft.) on the interfacial area, a, and the physical desorption liquid phase mass transfer coefficient, K_L. The two types of screen packing tested produced interfacial areas of 2 to 4 times that generated in an unpacked column. K_L showed no effect of gas velocity but increased with liquid velocity, a and K_L decreased with increasing column height. Generally, photographic evaluation gave unrealistically high values of a.     

A Zwitterionic Surfactant Bearing Unsaturated Tail for Enhanced Oil Recovery in High‐Temperature High‐Salinity Reservoirs

High‐temperature/high‐salinity (HTHS) reservoirs contain a significant fraction of the world's remaining oil in place and are potential candidates for enhanced oil recovery (EOR). Selection of suitable surfactants for such reservoirs is a challenging task. In this work, two synthesized zwitterionic surfactants bearing a saturated and an unsaturated tail, namely 3‐(N‐stearamidopropyl‐N,N‐dimethyl ammonium) propanesulfonate and 3‐(N‐oleamidopropyl‐N,N‐dimethyl ammonium) propanesulfonate, respectively, were evaluated. The surfactant with the unsaturated tail showed excellent solubility in synthetic seawater (57,643 ppm) and in formation brine (213,734 ppm). However, the unsaturated surfactant with a saturated tail showed poor solubility, and therefore it was not evaluated further. The thermal stability of the synthesized unsaturated surfactant solution in seawater was evaluated by heating the solution at 90 °C in a sealed aging tube for 2 weeks. The thermal stability of the unsaturated surfactant was confirmed by FTIR and NMR analysis of the aged samples at such harsh conditions. The critical micelle concentration (CMC) of the synthesized unsaturated surfactant in seawater was 1.02 × 10^?4 mol L^?1, while the surface tension at CMC was 30 mN m^?1. The synthesized unsaturated surfactant was able to reduce the oil–water interfacial tension to ~10^?1 mN m^?1 at different conditions. A commercial copolymer of acrylamide and 2‐acrylamido‐2‐methylpropane sulfonic acid (AM‐AMPS) was tested for EOR applications in HTHS conditions. The addition of the synthesized unsaturated surfactant to the AM‐AMPS copolymer increased the viscosity of the system. The increase in oil recovery by injecting the unsaturated surfactant solution and the surfactant–polymer mixture in solution was 8 and 21%, respectively. The excellent properties of the synthesized unsaturated surfactant show that surfactants with an unsaturated tail can be an excellent choice for HTHS reservoirs.