O2 Mass Transfer Research Articles

Performance evaluation and effect of biogas circulation rate of a bubble column for biological desulfurization

Biological desulfurization using a bubble column reactor was investigated in a continuous biogas treatment. Rapid biogas circulation between the digester and the bubble column for biological desulfurization was used to stimulate the gas-liquid mass transfer of H(2)S. A positive correlation between the biogas circulation rate and H(2)S removal rate was observed. Moreover, the increase in the circulation rate stimulated the O(2) mass transfer, eventually translating into an increase in sulfate production from the oxidation of H(2)S. Throughout the continuous experiment, the reactor retained sufficient levels of sulfide-oxidizing bacteria. A comparison of the results of the continuous biogas treatment and batch tests suggests that the gas-liquid mass transfer rate of H(2)S was the rate-limiting step in the biological desulfurization in the reactor, indicating that the mass transfer efficiency of H(2)S needs to be improved to enhance the desulfurization performance.

Oxygen Transfer in a Fluctuating Capillary Fringe

Dynamic fluctuations in water table elevation cause the entrapment of air, which affects the hydraulic properties of the porous medium in the capillary fringe as well as the biogeochemical status of the underlying, potentially O2–depleted groundwater. In this study, we conducted quasi two-dimensional flow-through experiments at the laboratory bench scale to investigate in detail the mass transfer of O2 in a fluctuating capillary fringe. We evaluated the effects of different boundary conditions such as single drainage and imbibition events as well as periodic fast and slow water table fluctuations. High-resolution vertical profiles of O2 concentration, measured at two distances in the horizontal groundwater flow direction, and mass fluxes, determined in the effluent of the flow-through chamber, were used to quantify O2 transfer under the different boundary conditions applied. The results show that the partitioning between the aqueous and the gaseous phases plays a significant role in the supply of O2 to groundwater at medium time scales. In the case of fast water table fluctuations, the specific yield has to be considered. The experiments with a periodically changing boundary condition demonstrate that highly dynamic fluctuations of the water table enhance the mass transfer of O2 from the atmosphere into the groundwater when compared with steady-state conditions. Moreover, the characteristics of the water table fluctuations determine a specific dynamic response of the system: we observed an approximately double amount of O2 transferred to the groundwater when applying slow fluctuations compared with the case of cyclic, abrupt changes in the water table elevation.

Kinetics Behavior and Sulfur Transformations of Iron Sulfide during Supercritical Water Oxiation

Oxidation of iron sulfide in supercritical water was investigated in the batch reactor. Iron sulfide was converted in two parallel processes: gasification by water and oxidation by oxygen. Assuming that the reaction order of H2O was 0, the activation energy and pre-exponential factor of the gasification process were determined to be 43kJ mol-1 and 22.4 min-1, correspondingly. It is found that above 773K the oxidation process was limited by the mass transfer of O2 to particles surface. Below 773K, with an assumption of zero order in H2O concentration and first-order reaction in oxygen concentration, the activation energy and pre-exponential factor for the rate of oxidation were estimated as154kJ mol-1 and 1.7×106m3 mol-1 min-1, respectively. With supercritical water oxidation under the experimental conditions, the sulfur-containing components in the product were sulfide, sulfite and sulfate, in which sulfide and sulfate were predominant. It is likely to completely convert the sulfur to the sulfate by supercritical water oxidation using high temperature and long reaction time. The reaction pathway of iron sulfide could be expressed as: iron sulfide → sulfide → sulfite → sulfate.

Experimental and Modeling Analysis of the Thermal Behavior of an Autothermal C3H8Catalytic Partial Oxidation Reformer

In this work, a spatially resolved sampling technique is applied to characterize the performance of a C3H8 CPO reformer and to compare it with that of a CH4 reformer. The case of Rh-coated honeycomb catalysts is examined. The axial profiles show that higher temperatures are reached in C3H8 CPO, especially at the reactor inlet. Surface hot-spot temperatures around 950 °C lead the catalyst to rapid loss of activity. A detailed model analysis is also applied to better understand the reasons for the observed differences of the thermal behavior. On one hand, the heat release via oxidation reactions is controlled by O2 mass transfer rate and thus proportional to O2 inlet concentration, which is ∼20% higher in the C3H8/air mixture at equal C/O ratio. On the other hand, while CH4 steam reforming is partly chemically controlled, C3H8 steam reforming is mainly limited by gas–solid diffusion. Thus, a less efficient balance between exo- and endothermic reactions occurs in the case of C3H8 CPO, and this results in muc...

Insights into large‐scale cell‐culture reactors: II. Gas‐phase mixing and CO2 stripping

Most discussions about stirred tank bioreactors for cell cultures focus on liquid-phase motions and neglect the importance of the gas phase for mixing, power input and especially CO(2) stripping. Particularly in large production reactors, CO(2) removal from the culture is known to be a major problem. Here, we show that stripping is mainly affected by the change of the gas composition during the movement of the gas phase through the bioreactor from the sparger system towards the headspace. A mathematical model for CO(2)-stripping and O(2)-mass transfer is presented taking gas-residence times into account. The gas phase is not moving through the reactor in form of a plug flow as often assumed. The model is validated by measurement data. Further measurement results are presented that show how the gas is partly recirculated by the impellers, thus increasing the gas-residence time. The gas-residence times can be measured easily with stimulus-response techniques. The results offer further insights on the gas-residence time distributions in stirred tank reactors.

A shrinking core model for slow combustion of solid 5-O-caffeoylquinic acid

The oxidation of 5-O-caffeoylquinic acid (5-CQA) was investigated in the temperature range from 161 to 188 °C. The kinetic experiments were carried out in small batch reactors made of glass, where a very thin (1.1 μm) fixed bed of powder 5-CQA was in contact with air at approximately 91.4 kPa. Aqueous 5-CQA solutions were prepared with the solid found in the reactors at different reaction times and used to measure the decrease of 5-CQA concentrations with a spectrophotometer operated at 323 nm. A shrinking core model was applied to describe the non-catalytic heterogeneous reaction between 5-CQA and oxygen. It basically assumes mass transfer of O2 from bulk air to the outer surface of nonporous particles of solid 5-CQA, where an irreversible reaction with no formation of solid products takes place. The proposed model correctly reproduced the obtained kinetic results in all the investigated operating conditions when an exponential reduction of external area of solid with reaction time was taken into account. Arrhenius-type expressions successfully described the dependence of the calculated global reaction and shrinking rate constants on temperature. An activation energy close to 77046 J mol−1 was found for the slow combustion reaction of 5-CQA.

Diagenetic Fe-carbonates in Paleoarchean felsic sedimentary rocks (Hooggenoeg Formation, Barberton greenstone belt, South Africa): Implications for CO2 sequestration and the chemical budget of seawater

In order to evaluate the potential of felsic sediments as a CO2-sink in the Archean, we studied felsic volcaniclastic/epiclastic sedimentary rocks of the 3.45Ga Hooggenoeg Formation, Barberton greenstone belt, which were affected by metasomatic processes during seafloor alteration and diagenesis. Water–rock interactions leading to K-, Si- and CO2-metasomatism were quantified. The precursor rock, a K-metasomatized dacite, was leached of Ca, Mg, Fe, Na, Sr, and Ba and enriched in K, Rb and Si prior to erosion and deposition. The formation of K-mica and quartz (and minor K-feldspar) in the dacites suggests low pH metasomatic conditions, likely produced by hydrothermally induced circulation of hot, acidic and reduced Archean seawater in equilibrium with a high PCO2 atmosphere. Erosion and transport of K-metasomatized dacitic detritus away from the felsic volcanic centers resulted in the deposition of conglomerate, sandstone and shale by mass flow processes. Early diagenetic silicification affected mainly the fine-grained sediments with higher silica sorption capacity, forming impermeable layers, while sand-rich sediments were partly silicified and remained permeable. Trapped fluids precipitated two generations of Fe-rich dolomites and finally calcite. Up to 30vol.% of siliciclastic coarse-grained sediment was replaced by carbonates in a shallow-burial, high heat-flow diagenetic regime (depth: ∼750m, temperature: 80–160°C), and likely throughout deposition of overlying volcano–sedimentary units. The carbon isotopic composition of Fe-rich dolomites (δ13CPDB=+1.9 to +2.4‰) and the strong Fe–Ca–Mg leaching of the Paleoarchean volcanic formations support the influence of seawater-derived fluids throughout CO2-metasomatism. For each gram of eroded dacite, the overall chemical exchange involved by K–Si–CO2-metasomatism was characterized by a mass transfer of Fe (1.2mmol/g), Na (2.1mmol/g) and O2− (2.0mmol/g) to seawater. In contrast, seawater was depleted in Si (10mmol/g), Ca (0.51mmol/g), Mg (0.43mmol/g), K (1.5mmol/g) and H (0.93mmol/g) during incorporation of these elements in the volcanic and sedimentary rocks. The average CO2 uptake by the sedimentary rocks studied here is estimated to be 1.8mmol/g, in the same order of magnitude as previous estimates for the Paleoarchean basaltic crust. Although mafic-ultramafic rocks are the most abundant rocks in Paleoarchean greenstones belts, and represent the most important atmospheric CO2-sink upon seafloor alteration in the Paleoarchean, coarse felsic sedimentary rocks provide a non-negligible contribution to the build-up of the continental CO2 reservoir.

Immobilized metal complexes in porous hosts: catalytic oxidation of substituted phenols in CO2 media

Development of processes that utilize heterogeneous catalysis in environmentally beneficial media is of fundamental and practical importance. The oxidation of 2,6-di-tert-butylphenol (DTBP) to 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) and 3,5,3′,5′-tetra-tert-butyl-4,4′-diphenoquinone (TTBDQ) has been investigated to evaluate the factors necessary to achieve high product conversion and selectivity in various media. A series of porous materials with immobilized Co(II) complexes served as catalysts and their reactivities using O2 as the terminal oxidant were screened in neat acetonitrile, supercritical carbon dioxide (scCO2), and CO2-expanded acetonitrile. The highest conversions were found with the catalysts that had high affinity for dioxygen. Moreover, the greatest conversions (∼60%) were obtained when reactions were done in scCO2, which is attributed to improved mass transfer of O2 and substrates through the porous catalysts. Furthermore, the heterogeneous catalysts can be recycled with some loss of activity (∼30%) after three cycles; nonetheless these results suggest that the polymer hosts efficiently protect the immobilized catalytic sites from destructive bimolecular routes.

On-line oxygen uptake rate and culture viability measurement of animal cell culture using microplates with integrated oxygen sensors.

O2 uptake rates of animal cells (Chinese hamster ovary-CHO) were measured in 96-well microtiter plates by integrating with fluorescent sensors thereby measuring fluorescence intensity ratios of an O2-sensitive and an insensitive fluorophor. O2 consumption rate was estimated from measured dissolved O2 and from O2 mass transfer coefficient determined in advance. Specific uptake decreased with time from 3.2 x 10(-13) mol O2 cell(-1) h(-1) at 15 h cultivation to 1.8 x 10(-13) mol O2 cell(-1) h(-1) at 48 h. Specific O2 uptake was also determined by sampling from a spinner-flask culture giving identical values. A cell viability assay for cultures based on O2 measurements is described in which cells are incubated outside the fluorescence reader and then the dissolved O2 is measured only once at a fixed time after the start of incubation. This protocol can be directly applied for high-throughput measurements.

Oxidation of Carbon Particles in Supercritical Water: Rate and Mechanism

Oxidation of a carbon particle in supercritical water was studied in a flow-type reaction cell by directly observing the change in the size and shape of the particle through a sapphire window attached to the cell. Three types of carbon were investigated: activated carbon, synthetic graphite, and highly oriented pyrolytic graphite (HOPG). The reaction was observed at a pressure of 23−30 MPa and a temperature of 673−873 K. The spherical particles (3−4 mm in diameter) of both the activated carbon and the synthetic graphite became smaller while maintaining their shape, indicating that the reaction occurred nearly at the surface of the sphere. The reaction was first order with respect to O2 concentration. The rate of the decrease in the radius was in the order of μm s-1 for the activated carbon, and the rate was about an order of magnitude slower for the synthetic graphite. The rate of O2 mass transfer was estimated by computational fluid dynamics (CFD) calculations, showing good agreement with the reaction r...

Supercritical water oxidation of a carbon particle by Schlieren photography

The flow pattern around a carbon particle (∼ 3.5 mm in diameter) during the supercritical water oxidation (SCWO) process (25 MPa, 723 K, 3.6 wt % O2 concentration) was visualized for the first time by the use of Schlieren optics and a SCWO reactor equipped with transparent sapphire windows. To link Schlieren images with flow fields, computational fluid dynamics (CFD) simulations were executed, assuming temperature-dependent density of supercritical water and heat of a surface reaction, and then the obtained density fields around the particle were numerically converted to Schlieren images. Numerically obtained Schlieren images agreed well with observed images, confirming that the CFD simulation was able to predict the flow field in SCWO processes. Such a combined approach of CFD and flow visualization revealed that, in SCWO of solid substances with a fast reaction rate, the heat of surface reaction increased the temperature of the fluid and induced an upward flow from the particle with higher fluid temperature than that of the surroundings. Such flow can increase reaction rates by enhancing the mass transfer of O2 to solids. Therefore, a flow field in SCWO is an important factor controlling the progress of reaction and it can be reasonably predicted by CFD simulations. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2082–2089, 2004

PERFORATION-MEDIATED MODIFIED ATMOSPHERE PACKAGING: INFLUENCE OF PACKAGE GEOMETRY AND PERFORATION LOCATION ON OXYGEN AND CARBON DIOXIDE TRANSFER

ISHS VIII International Controlled Atmosphere Research Conference PERFORATION-MEDIATED MODIFIED ATMOSPHERE PACKAGING: INFLUENCE OF PACKAGE GEOMETRY AND PERFORATION LOCATION ON OXYGEN AND CARBON DIOXIDE TRANSFER

Shadowgraph Observation of Supercritical Water Oxidation Progress of a Carbon Particle

Supercritical water oxidation (SCWO) progress for a spherical activated carbon particle was pursued using a flow-type reaction cell equipped with sapphire windows, where the change in the size and shape of the particle inside the cell was observed in real time by means of a shadowgraph. While maintaining its shape, the particle radius decreased almost linearly with time at a rate on the order of micrometers per second. Phenomenologically, the reaction occurred at the surface of the sphere. The decreasing rate of the radius was nearly first order with respect to the O2 concentration and was almost independent of the temperature and pressure in the cell. On the other hand, the rate increased with the flow rate of the supercritical water. These trends suggested that the reaction was almost limited by the mass transfer of O2 from the bulk to the surface of the particle. The observed rate roughly agreed with the rate of O2 mass transfer which was estimated using empirical relations.

Hydrodynamics and mass transfer in a tubular airlift photobioreactor

In photobioreactors, which are usually operated under light limitation,sufficient dissolved inorganic carbon must be provided to avoid carbonlimitation. Efficient mass transfer of CO2 into the culture mediumisdesirable since undissolved CO2 is lost by outgassing. Mass transferof O2 out of the system is also an important consideration, due tothe need to remove photosynthetically-derived O2 before it reachesinhibitory concentrations. Hydrodynamics (mixing characteristics) are afunctionof reactor geometry and operating conditions (e.g. gas and liquid flow rates),and are a principal determinant of the light regime experienced by the culture.This in turn affects photosynthetic efficiency, productivity, and cellcomposition. This paper describes the mass transfer and hydrodynamics within anear-horizontal tubular photobioreactor. The volume, shape and velocity ofbubbles, gas hold-up, liquid velocity, slip velocity, axial dispersion,Reynoldsnumber, mixing time, and mass transfer coefficients were determined intapwater,seawater, and algal culture medium. Gas hold-up values resembled those ofvertical bubble columns, and the hydraulic regime could be characterized asplug-flow with medium dispersion. The maximum oxygen mass transfer coefficientis approximately 7 h−1. A regime analysisindicated that there are mass transfer limitations in this type ofphotobioreactor. A methodology is described to determine the mass transfercoefficients for O2 stripping and CO2 dissolution whichwould be required to achieve a desired biomass productivity. This procedure canassist in determining design modifications to achieve the desired mass transfercoefficient.

Ferrous oxidation chemistry in passive abiotic systems for the treatment of mine drainage

The rates of chemical oxidation and gas transfer were determined for two passive abiotic systems for the treatment of mine drainage. One system (HB) is a series of ponds whose discharge was net acidic. The second treatment system (CK) has shallow channels and net alkaline discharge. The O 2 mass transfer coefficient was 2 cm h −1 for HB Pond #2 and ranged from 4 to 40 cm h −1 for the CK channels, depending on channel velocity and depth. The CO 2 mass transfer coefficient was 1.3 cm h −1 for HB Pond #2 and ranged from 1.4 to 15 cm h −1 for the CK channels. Oxygen transfer appeared to be the rate-limiting step for oxidation of Fe(II) at HB Pond #2. The oxidation rate for Fe(II) adsorbed to already-formed ferric oxides (heterogeneous oxidation) appeared to control the rate of oxidation of Fe(II) at the CK channels. Based on field data, the heterogeneous rate constant for oxidation of Fe(II) was 6.0×10 −9 mg l −1 s −1 at 17°C or k 2 of 5.5×10 −8 mg l −1 s −1 at 25°C. This value was consistent with previously reported values for k 2 based on laboratory experiments. The annual-averaged iron removal rates were 18 g d −1 m −2 for HB and 42 g d −1 m −2 for CK, compared to empirical design expectations of 10 to 20 g d −1 m −2 . Heterogeneous oxidation accounted for >80 of the oxidation of Fe(II) at HB and >90% of the oxidation of Fe(II) at CK.

A novel membrane bioreactor for microbial growth

A novel membrane bioreactor, previously assessed for its gas transfer characteristics, was used in various size and membrane configurations for the growth of the strictly aerobic bacterium Pseudomonas aeruginosa. The bioreactor was found to readily support growth, and the initial growth rates showed the previously demonstrated enhanced effect in gas O2 mass transfer of the dimpled membrane bioreactor over flat membrane bioreactors. The production of a secondary metabolite by a Pseudomonas sp. following growth was demonstrated, as was the biotransformation of a nitrile by Nocardia rhodochrous with the removal of the biotransformation products across a membrane. The potential of the bioreactor, in terms of other applications in the field of biotechnology, is discussed.

Studies on the production of lipase from recombinant Staphylococcus carnosus.

Production of lipase from recombinant Staphylococcus carnosus pLipPS1 was studied in standard stirred tank bioreactors. Only low lipase activity was obtained under conventional operating conditions, i.e., moderate to high stirring speeds and aeration rates for keeping the dissolved oxygen concentration at high levels. Additional targetted experiments indicated that the reason for the observed low lipase activity is lipase inactivation due to surface forces and shear stress at the gas/liquid interface. Therefore, a cultivation strategy is proposed that minimizes gas/liquid interfacial area and maximizes the driving concentration for O2 mass transfer by controlling the dissolved oxygen to low values by gentle stirring and low aeration rates. Thus, high lipase activities can be obtained even in larger scale standard stirred tank bioreactors.

The Dickinson W. Richards lecture. New concepts in assessing cardiovascular function.

The primary role of the heart is to provide energy for the circulatory transport of oxygen (O2) to cells at rates commensurate with their metabolic activity. At rest, even a "sick" heart may be capable of transporting O2 adequately. But during exercise, the increase in O2 required by muscle cells demands that their blood flow be increased. The supply of O2 needed to meet the O2 requirement for muscle mitochondrial high-energy phosphate generation during exercise is a critical function of the circulation. Thus, the adequacy of cardiovascular function can be estimated, noninvasively, from the pattern of O2 uptake in response to an exercise stimulus. While arterial O2 tension (PaO2) is dependent on pulmonary function (except for intracardiac right-to-left shunt), the mass transfer of O2 (VO2) between the cells and lungs depends on pulmonary blood flow (i.e., cardiac output) and O2 concentration difference between the pulmonary arterial and pulmonary venous blood, C(a-v)O2 (Fick principle). Thus, VO2 in the first 15 seconds of exercise can be used to describe the initial increase in pulmonary blood flow and stroke volume, while the subsequent rise in VO2 results from the further increase in VO2 in response to work rate increase are used to detect circulatory disturbances. Also, the rate of CO2 output (VCO2) has been valuable in the assessment of cardiovascular function when related to VO2. Inadequate O2 availability results in anaerobic metabolism, causing increased muscle lactic acid production. At the pH of cell water, most of the hydrogen ions produced with lactate are buffered by bicarbonate. The CO2 generated by the buffering reaction (22 ml for each milliequivalent) causes a net increase in VCO2 relative to VO2 at the work rate at which buffering begins. This provides a useful estimate of the anaerobic threshold. Thus, study of the dynamic coupling of external to cellular respiration during a work rate stimulus provides valuable, direct, and noninvasive information about cardiovascular mechanisms in health and disease.

Kinetics of the Oxidation of CaS

The kinetics of the oxidation of dense pellets of CaS by the reaction: $$2CaS(s) + 3O_2 (g) \to 2CaO(s) + 2SO_2 (g)$$ was examined by continuous thermogravimetric analysis. The experiments covered a temperature range of 1673 to 1853 K, Ar-O2-SO2 mixtures varying from 1 to 40 pct O2 and 0 to 20 pct SO2, and gas velocities ranging from 20 to 57 cm/s at the reaction temperature. Analysis of the data shows that the initial rate of the reaction is controlled by the mass transfer of O2 through the gaseous boundary layer of the pellet and is subsequently controlled by the diffusion of O2 through the porous layer of the reaction products. To verify these results, the permeation rates of SO2 through the reaction products were measured at room temperature and compared to values calculated from the thermogravimetric data.