Gas-liquid Mass Transfer Limitations Research Articles

Perfluorocarbon nanoemulsions as hydrogen carriers to promote the biological conversion of hydrogen and carbon dioxide to methane

The formation of methane (CH4) via a biomethanation process using hydrogen (H2) and carbon dioxide (CO2) as feedstock is impeded by H2 gas-liquid mass transfer limitations. To address these limitations and improve the CH4 production rate in H2/CO2 biomethanation, perfluorocarbon (PFC) nanoemulsions exhibiting a high solubility for gases were proposed as H2 transfer carriers to promote the H2 utilization rate and methanogenesis kinetics. The presence of PFC nanoemulsions in a volume fraction of 1.5% increased the peak rate of CH4 production by 30.6% and decreased the lag time by 84.5%. These nanoemulsions, composed of PFC and surfactants, efficiently transported H2 molecules to alleviate gas-liquid transport limitations in biological solutions. Furthermore, lipophilic PFC, which is nonspecifically bound to cell membranes of Methanosarcina barkeri (M. barkeri) cells, also provided the transport of H2 molecules from PFC nanoemulsions to the cells. The PFC nanoemulsions separated from these cell membranes primarily through physical oscillations. However, physical separation was difficult when using 2 vol% PFC nanoemulsions due to their strong combination with M. barkeri cells, which hindered H2 transmission and utilization.

Biotransformation of Methane and Carbon Dioxide Into High-Value Products by Methanotrophs: Current State of Art and Future Prospects.

Conventional chemical methods to transform methane and carbon dioxide into useful chemicals are plagued by the requirement for extreme operating conditions and expensive catalysts. Exploitation of microorganisms as biocatalysts is an attractive alternative to sequester these C1 compounds and convert them into value-added chemicals through their inherent metabolic pathways. Microbial biocatalysts are advantageous over chemical processes as they require mild-operating conditions and do not release any toxic by-products. Methanotrophs are potential cell-factories for synthesizing a wide range of high-value products via utilizing methane as the sole source of carbon and energy, and hence, serve as excellent candidate for methane sequestration. Besides, methanotrophs are capable of capturing carbon dioxide and enzymatically hydrogenating it into methanol, and hence qualify to be suitable candidates for carbon dioxide sequestration. However, large-scale production of value-added products from methanotrophs still presents an overwhelming challenge, due to gas-liquid mass transfer limitations, low solubility of gases in liquid medium and low titer of products. This requires design and engineering of efficient reactors for scale-up of the process. The present review offers an overview of the metabolic architecture of methanotrophs and the range of product portfolio they can offer. Special emphasis is given on methanol biosynthesis as a potential biofuel molecule, through utilization of methane and alternate pathway of carbon dioxide sequestration. In view of the gas-liquid mass transfer and low solubility of gases, the key rate-limiting step in gas fermentation, emphasis is given toward reactor design consideration essential to achieve better process performance.

Simulating hydrodynamics in a Rushton turbine at different stirring velocities applied to non-Newtonian fluids

In this work, gas-liquid hydrodynamics of a Rushton turbine was studied using Computational Fluid Dynamics. Different stirring conditions commonly used in fungal culture applications are simulated. Several scenarios are predicted related to gas-liquid mass transfer limitation. The above, reflected by low air dispersion reached and bubble size determinations caused by the non-Newtonian rheology, leading the process to obtain kLa values only in the order of 30 h−1 at high, stirring speeds. However, the high-power consumption in fungal culture in agitated tank bioreactors can be disadvantages in large-scale prototypes applied in non-Newtonian fluids. These Findings shown in this research should be considered as a primary criterion for optimizing mass transfer problems in large scale fungal culture applications.

Using modeling to select catalyst dilution methods for mass transfer intensification in lab gas–liquid fixed-bed reactors

Heterogeneous catalyst testing methodology at the lab scale must provide intrinsic kinetics data for reactor design purposes as well as the intrinsic activity ranking during catalyst screening in the field of refining and petrochemistry. The significant downscaling of the past century coupled with the increasingly active catalytic formulations may introduce considerable momentum, mass and heat effects in experiments at small scale. Catalyst dilution has emerged as one versatile and robust way to reduce the impact of momentum and heat effects on heterogeneous catalyst testing. This paper presents a methodology based on global phenomena and catalyst dilution modeling to assess and optimize reactor loading techniques for specific problems. More particularly, the aim is to provide catalysts developers and kinetics experts with concrete guidelines for intensifying gas-liquid mass transfer in lab gas–liquid fixed-bed reactors through catalyst dilution. The methodology is applied to the kinetics determination of the oil residue Hydrodemetallation (HDM) and to the screening of catalysts for benzene hydrogenation. Layered dilution, consisting on the split of the catalyst in two beds separated by an intermediate bed containing an inert material of the same size as the catalyst, poorly improves gas–liquid mass transfer. Uniform dilution, based on the direct mixture of catalyst and inert material of the same size, significantly enhances gas–liquid mass transfer as the reactant local consumption per reactor unit volume is strongly reduced. Combinations of both abovementioned dilution techniques can be used with fast and/or high stoichiometric factor chemical systems operated at conversions higher than 70%. A new criterion is proposed to calculate the minimum dilution factor to guarantee negligible gas–liquid mass transfer limitations as a function of conversion, external mass transfer and global pseudo second-order kinetics.

Reversible retrofitting of a stirred-tank bioreactor for gas-lift operation to perform synthesis gas fermentation studies

A retrofit approach enables the non-invasive and reversible modification of a commercially available stirred-tank bioreactor into a gas-lift bioreactor within a few minutes. The gas-lift bioreactor configuration was characterised using a non-coalescent medium at working volume-specific volumetric gas flow rates of up to 20 L L−1 h−1, showing mean gas bubble diameters of up to 0.57 mm, volumetric power inputs of up to 23 W m-3, gas-liquid mass transfer coefficients of up to 153 h−1 (CO) and up to 122 h−1 (H2), mixing times (θ90) of down to 0.5 min and gas hold-ups of up to 1.2%. A comparison of autotrophic batch processes with Clostridium aceticum and Clostridium carboxidivorans in the stirred-tank (2.0 L) and the retrofitted gas-lift bioreactor (2.5 L) at identical operating conditions showed differences in the cell dry weight and product concentrations (acetate, ethanol, butyrate, 1-butanol, hexanoate, 1-hexanol) due to gas-liquid mass transfer limitations caused by the reduced power input in the gas-lift bioreactor. CO limitation was demonstrated by a myoglobin-protein assay for CO measurement in the liquid phase. The simple retrofitting of a stirred-tank bioreactor for gas-lift operation thus provided comparative data on process performances of acetogenic bacteria that convert synthesis gas in gas-lift and stirred-tank bioreactors on a lab-scale.

Visible light driven CO2 reduction to methanol by Cu-porphyrin impregnated mesoporous Ti-MCM-48

Mesoporous Ti-MCM-48 photocatalyst, with Si/Ti (wt/wt) ratio of 100, 50 and 25 was effectively applied for CO2 reduction into methanol under UV–visible light irradiation bearing mid gap energy states and Ti3+ sites. The bare Ti-MCM-48 with Si/Ti (wt/wt) ratio of 25 displayed highest photocatalytic methanol yield (85.88 μmol g−1 L−1) with a BET surface area of 1528 m2 g−1 and mid gap energy states as determined from the XPS analysis, compared to the other composite ratios. The Ti-MCM-48(25) impregnated with Cu-porphyrin (CuTPP) resulted in methanol yield of 297 μmol·g−1 under 33 mW·cm−2 simulated light intensity, using 0.1 M Na2SO3 in 0.1 M NaOH as supporting electrolytes, which is 3.45 times higher than the bare Ti-MCM-48(25) due to the visible light excitation of the porphyrin macrocycle and charge transition from the lowest occupied molecular orbital (LUMO) of CuTPP to the Ti3+ metal centers. Also, methanol yield was studied with CO2 gas-liquid mass transfer and mass transfer limitations prevailing in the reactor. Mass transfer limitation experiments revealed that metal loading, catalyst concentration, stirring speed and light intensity influence the methanol yield.

Dual-Function Electrocatalytic and Macroporous Hollow-Fiber Cathode for Converting Waste Streams to Valuable Resources Using Microbial Electrochemical Systems.

Dual-function electrocatalytic and macroporous hollow-fiber cathodes are recently proposed as promising advanced material for maximizing the conversion of waste streams such as wastewater and waste CO2 to valuable resources (e.g., clean freshwater, energy, value-added chemicals) in microbial electrochemical systems. The first part of this progress report reviews recent developments in this type of cathode architecture for the simultaneous recovery of clean freshwater and energy from wastewater. Critical insights are provided on suitable materials for fabricating these cathodes, as well as addressing some challenges in the fabrication process with proposed strategies to overcome them. The second and complementary part of the progress report highlights how the unique features of this cathode architecture can solve one of the intrinsic bottlenecks (gas-liquid mass transfer limitation) in the application of microbial electrochemical systems for CO2 reduction to value-added products. Strategies to further improve the availability of CO2 to microbial catalysts on the cathode are proposed. The importance of understanding microbe-cathode interactions, as well as electron transfer mechanisms at the cathode-cell and cell-cell interface to better design dual-function macroporous hollow-fiber cathodes, is critically discussed with insights on how the choice of material is important in facilitating direct electron transfer versus mediated electron transfer.

Umgehung des Gas‐flüssig‐Stofftransports von Sauerstoff durch Kopplung der photosynthetischen Wasseroxidation an eine biokatalytische Oxyfunktionalisierung

AbstractDer Gas‐flüssig‐Stofftransport gasförmiger Reaktanten limitiert die Raum‐Zeit‐Ausbeute O2‐abhängiger (bio)katalytischer Reaktionen in wässrigen Lösungen. In dieser Studie wurde die oxygene Photosynthese für den homogenen Sauerstoffeintrag durch In‐situ‐Erzeugung von O2 in der Flüssigphase genutzt, um diese Limitierung zu umgehen. Hierfür produziert das phototrophe Cyanobakterium Synechocystis sp. PCC6803 als Ganzzellbiokatalysator heterolog die Alkanmonooxygenase AlkBGT aus Pseudomonas putida GPo1. Die chemo‐ und regioselektive Hydroxylierung von Pelargonsäuremethylester zu ω‐Hydroxypelargonsäuremethylester kann so bei Belichtung, jedoch ohne externe Zugabe von molekularem Sauerstoff katalysiert werden. Der Sauerstoff stammt aus der photosynthetischen Wasseroxidation. Die Photosynthese liefert ebenfalls die nötigen Reduktionsäquivalente zur Regeneration von Fe2+ in AlkB für den Sauerstofftransfer zur terminalen Methylgruppe. Die In‐situ‐Kopplung der oxygenen Photosynthese an O2‐übertragende Enzyme ermöglicht nun das Design schneller Oxyfunktionalisierungsreaktionen.

Modeling Mass Transfer in a Three-Phase Minireactor Filled with Trilobe Catalytic Extrudates in Series

The internal and external gas-liquid mass transfer limitations in a three phase spiral string bed mini-reactor are estimated, using as a model reaction the benzene hydrogenation over a commercial tri-lobe Ni/Al2O3 catalyst at elevated pressures and temperatures. A detailed two phase flow model incorporating liquid volatility, intrinsic catalytic reaction rates, gas-liquid mass transfer effects, liquid-solid mass transfer effects, internal mass transfer effects and wetting of the catalytic particles, was used for the simulation of the spiral string bed reactor performance. The benzene intrinsic reaction kinetics was estimated conducting benzene hydrogenation experiments over crushed catalyst particles. The gas-liquid mass transfer coefficient was estimated from the data of this study and the liquid-solid mass transfer effects were estimated from data of a previous communication. The proposed correlation for the prediction of gas-liquid mass transfer coefficients can describe the gas-liquid mass transfer li...

Thermodynamics and economic feasibility of acetone production from syngas using the thermophilic production host Moorella thermoacetica

BackgroundSyngas fermentation is a promising option for the production of biocommodities due to its abundance and compatibility with anaerobic fermentation. Using thermophilic production strains in a syngas fermentation process allows recovery of products with low boiling point from the off-gas via condensation.ResultsIn this study we analyzed the production of acetone from syngas with the hypothetical production host derived from Moorella thermoacetica in a bubble column reactor at 60 °C with respect to thermodynamic and economic feasibility. We determined the cost of syngas production from basic oxygen furnace (BOF) process gas, from natural gas, and from corn stover and identified BOF gas as an economically interesting source for syngas. Taking gas–liquid mass transfer limitations into account, we applied a thermodynamics approach to derive the CO to acetone conversion rate under the process conditions. We estimated variable costs of production of 389 $/t acetone for a representative production scenario from BOF gas with costs for syngas as the main contributor. In comparison, the variable costs of production from natural gas- and corn stover-derived syngas were determined to be higher due to the higher feedstock costs (1724 and 2878 $/t acetone, respectively).ConclusionWe applied an approach of combining thermodynamic and economic assessment to analyze a hypothetical bioprocess in which the volatile product acetone is produced from syngas with a thermophilic microorganism. Our model allowed us to identify process metrics and quantify the variable production costs for different scenarios. Economical production of bulk chemicals is challenging, making rigorous thermodynamic/economic modeling critical before undertaking an experimental program and as an ongoing guide during the program. We intend this study to give an incentive to apply the demonstrated approach to other bioproduction processes.

Kinetics of homogeneous 5‐hydroxymethylfurfural oxidation to 2,5‐furandicarboxylic acid with Co/Mn/Br catalyst

2,5‐furandicarboxylic acid (FDCA) is a potential non‐phthalate based bio‐renewable substitute for terephthalic acid‐based plastics. Herein, we present an investigation of the oxidation rate of 5‐hydroxymethylfurfural (HMF) to FDCA in acetic acid medium using Co/Mn/Br catalyst. Transient concentration profiles of the reactant (HMF), intermediates [2,5‐diformylfuran (DFF), 5‐formyl‐2‐furancarboxylic acid (FFCA)], and the desired product (FDCA) were obtained for this relatively fast reaction in a stirred semi‐batch reactor using rapid in‐line sampling. Comparison of the effective rate constants for the series oxidation steps with predicted gas–liquid mass transfer coefficients reveals that except for the FFCA → FDCA step, the first two oxidation steps are subject to gas–liquid mass transfer limitations even at high stirrer speeds. Novel reactor configurations, such as a reactor in which the reaction mixture is dispersed as fine droplets into a gas phase containing oxygen, are required to overcome oxygen starvation in the liquid phase and further intensify FDCA production. © 2016 American Institute of Chemical Engineers AIChE J, 63: 162–171, 2017

Pilot-scale semi-continuous cultivation of microalgae Chlorella vulgaris in bubble column photobioreactor (BC-PBR): Hydrodynamics and gas–liquid mass transfer study

A 56L compressed-air aerated bubble column photobioreactor (BC-PBR) was used to cultivate Chlorella vulgaris under indoor conditions via semi-continuous cultivation method. The effects of hydrodynamics and gas–liquid mass transfer limitations towards the biomass production of C. vulgaris at different compressed-air aeration rates were studied. The optimum biomass production of microalgae was achieved at compressed-air aeration rate of 0.16VVM, with superficial velocity, UG of 0.185ms−1. The experimental volumetric gas–liquid mass transfer coefficient, kLaL(CO2), was 4.5×10−3s−1 under the optimum growth condition. Besides that, the physical properties such as density, viscosity and surface tension remained constant throughout the semi-continuous cultivation mode. Low biomass production was observed at the highest aeration rate (0.19VVM), which indicated that shear sensitivity has significant effect on microalgae growth in BC-PBR. Apart from that, the effect of shear stress on the growth of C. vulgaris was also reported, especially on the reproduction of microalgae cells. The semi-continuous cultivation system had shown the possibility to partially harvest microalgae biomass during the exponential growth phase and thus, shorten the overall cultivation time in the subsequent cycles (elimination of lag phase) which resulted in higher biomass production.

Bioprocessing analysis of Pyrococcus furiosus strains engineered for CO₂-based 3-hydroxypropionate production.

Metabolically engineered strains of the hyperthermophile Pyrococcus furiosus (T(opt) 95-100°C), designed to produce 3-hydroxypropionate (3HP) from maltose and CO2 using enzymes from the Metallosphaera sedula (T(opt) 73°C) carbon fixation cycle, were examined with respect to the impact of heterologous gene expression on metabolic activity, fitness at optimal and sub-optimal temperatures, gas-liquid mass transfer in gas-intensive bioreactors, and potential bottlenecks arising from product formation. Transcriptomic comparisons of wild-type P. furiosus, a genetically-tractable, naturally-competent mutant (COM1), and COM1-based strains engineered for 3HP production revealed numerous differences after being shifted from 95°C to 72°C, where product formation catalyzed by the heterologously-produced M. sedula enzymes occurred. At 72°C, significantly higher levels of metabolic activity and a stress response were evident in 3HP-forming strains compared to the non-producing parent strain (COM1). Gas-liquid mass transfer limitations were apparent, given that 3HP titers and volumetric productivity in stirred bioreactors could be increased over 10-fold by increased agitation and higher CO2 sparging rates, from 18 mg/L to 276 mg/L and from 0.7 mg/L/h to 11 mg/L/h, respectively. 3HP formation triggered transcription of genes for protein stabilization and turnover, RNA degradation, and reactive oxygen species detoxification. The results here support the prospects of using thermally diverse sources of pathways and enzymes in metabolically engineered strains designed for product formation at sub-optimal growth temperatures.

A Critical Assessment of Microbiological Biogas to Biomethane Upgrading Systems.

Microbiological biogas upgrading could become a promising technology for production of methane (CH(4)). This is, storage of irregular generated electricity results in a need to store electricity generated at peak times for use at non-peak times, which could be achieved in an intermediate step by electrolysis of water to molecular hydrogen (H(2)). Microbiological biogas upgrading can be performed by contacting carbon dioxide (CO(2)), H(2) and hydrogenotrophic methanogenic Archaea either in situ in an anaerobic digester, or ex situ in a separate bioreactor. In situ microbiological biogas upgrading is indicated to require thorough bioprocess development, because only low volumetric CH(4) production rates and low CH(4) fermentation offgas content have been achieved. Higher volumetric production rates are shown for the ex situ microbiological biogas upgrading compared to in situ microbiological biogas upgrading. However, the ex situ microbiological biogas upgrading currently suffers from H(2) gas liquid mass transfer limitation, which results in low volumetric CH(4) productivity compared to pure H(2)/CO(2) conversion to CH(4). If waste gas utilization from biological and industrial sources can be shown without reduction in volumetric CH(4) productivity, as well as if the aim of a single stage conversion to a CH(4) fermentation offgas content exceeding 95 vol% can be demonstrated, ex situ microbiological biogas upgrading with pure or enrichment cultures of methanogens could become a promising future technology for almost CO(2)-neutral biomethane production.

High-rate hydrogenotrophic methanogenesis for biogas upgrading: the role of anaerobic granules

Hydrogenotrophic methanogenesis has been proved to be a feasible biological method for biogas upgrading. To improve its performance, the feasibility of typical anaerobic granules as the inoculum was investigated in both batch and continuous experiments. The results from batch experiments showed that glucose-acclimated granules seemed to perform better than granules acclimated to acidified products (AP, i.e. acetate, propionate and ethanol) in in situ biogas upgrading systems and a slightly higher H2 consumption rate (1.5 mmol H2 g VSS−1 h−1) was obtained for glucose-acclimated granules. For AP-acclimated granules, the inhibition on anaerobic digestion and pH increase (up to 9.55±0.16) took place, and the upgrading performance was adversely affected. In contrast, better performance for AP-acclimated granules was observed in ex situ systems, possibly due to their higher hydrogenotrophic methanogenic activities (HMA). Moreover, when gas–liquid mass transfer limitations were alleviated, the upgrading performance was significantly improved (three-fold) for both glucose-acclimated and AP-acclimated granules. The HMA of anaerobic granules could be further enhanced to improve biogas upgrading performance via continuous cultivation with H2/CO2 as the sole substrate. During the three months' cultivation, secondary granulation and microbial population shift were observed, but anaerobic granules still remained intact and their HMA increased from 0.2 to 0.6 g COD g VSS−1 d−1. It indicated that the formation of hydrogenotrophic methanogenic granules, a new type of anaerobic granules specialized for high-rate hydrogenotrophic methanogenesis and biogas upgrading, might be possible. Conclusively, anaerobic granules showed great potential for biogas upgrading.

Kinetic Investigations of p-Xylene Oxidation to Terephthalic Acid with a Co/Mn/Br Catalyst in a Homogeneous Liquid Phase

Kinetic investigations of the liquid phase oxidation of p-xylene (pX) to terephthalic acid (TPA) with Co/Mn/Br catalyst were performed in a stirred 50 mL Parr reactor at 200 °C and 15 bar pressure under conditions wherein product precipitation is avoided. The oxidant (O2) was introduced by sparging into the liquid phase at constant gas-phase O2 partial pressure. Apparent kinetic rate constants, estimated by regressing experimental conversion data to a pseudo-first order lumped kinetic model, are at least an order of magnitude greater than those reported in the literature for similar catalytic reactions. We attribute this difference to the presence of gas–solid and liquid–solid mass transfer resistances in the previous studies wherein the TPA product precipitates as it forms, trapping intermediate products and slowing down their oxidation rates. Our results also indicate that it is not possible to completely eliminate the gas–liquid mass transfer limitations associated with the fast intermediate oxidation steps, even when operating without solids formation and at high stirrer speeds. Other types of reactor configurations are therefore needed to better overcome gas–liquid mass transfer limitations. Systematic studies of bromide concentration effects show that the observed reaction rates become zero order in bromide concentration at sufficiently high bromide levels where the elimination of intermediate 4-(bromomethyl)benzoic acid by oxidation is favored. Further, the rate constants do not show any statistically significant dependence on pX concentration as suggested in other reports involving the traditional three-phase gas–liquid–solid reaction system. This again confirms that the formation of a solid phase hinders the overall oxidation rate, resulting in much smaller apparent rate constants.

Fundamental study on gas–liquid mass transfer in a biotrickling filter packed with polyurethane foam

AbstractBACKGROUNDBiotrickling filtration is one of the most promising biotechnologies for off‐gas treatment. However, gas–liquid mass transfer limitations constitute the most important drawback of biotrickling filters (BTFs) when treating hydrophobic pollutants. Oxygen limitations might also arise when treating soluble pollutants at high loading rates. In the present study, an easy‐to‐implement method for mass transfer characterization in BTFs based on dissolved oxygen measurements was investigated.RESULTSkLa values for oxygen in the range 30–300 h−1 were obtained by fitting the experimental data to a mathematical model with high correlation coefficients (average r2 = 0.995). In addition, the kLa values for other gas pollutants of industrial and environmental interest (NH3, H2S, toluene and hexane) were estimated by means of an empirical correlation, which allowed calculation of their mass transfer rates in order to identify potential mass transfer limiting scenarios.CONCLUSIONSThe methodology proposed allowed for a rapid, easy and accurate determination of the kLa for oxygen under varied operational conditions, the mass transfer data obtained being in agreement with previous literature. The experimental kLa data obtained for oxygen and the methodology applied for the estimation of kLa for the target gaseous pollutants selected constitutes a valuable tool for the optimization, design and operation of BTFs. © 2013 Society of Chemical Industry

A spray reactor concept for catalytic oxidation of p-xylene to produce high-purity terephthalic acid

A spray reactor in which the liquid phase containing dissolved p-xylene (pX) and the catalyst is dispersed as fine droplets by a nozzle into a continuous vapor phase containing the oxidant (O2) is shown to produce high-purity terephthalic acid (TPA) with less than 25ppm 4-carboxybenzaldehyde (4-CBA) in the solid TPA product. In sharp contrast, the solid TPA product obtained from a conventional stirred reactor similar to the configuration used in the conventional Mid-Century (MC) process contains nearly 1000ppm 4-CBA even though the reactor is operated at similar pressure and temperature (15bar and 200°C) but with the gas phase dispersed into the liquid phase. The dramatic improvement in TPA product quality during spray reactor operation is attributed to two main factors: the alleviation of interphase gas–liquid mass transfer limitations that facilitates more complete oxidation of the pX and the intermediate oxidation products to TPA, and reduced backmixing that enhances the oxidation rates. Theoretical calculations show that the time constants for O2 diffusion in typical spray droplets (50μm diameter) is 1–2 orders of magnitude lower than the kinetic rate constant confirming complete O2 penetration and saturation of the droplets. Further, the usage of CO2 as an inert gas and the dominance of acetic acid (>50mol.%) in the vapor phase under reaction conditions create an environment that falls outside of the flammability envelope. Mathematical modeling of the stirred reactor using MC process conditions accurately predicts the steady state temperatures observed in industrial reactors. The model also clearly divulges that the cooling provided by partial evaporation of the acetic acid solvent is vital to maintain stable steady state operation. By eliminating the need for a hydrogenation step, the spray reactor has the potential to not only reduce capital and operating costs but also provide environmental benefits.

A critical evaluation of CO2 supplementation to algal systems by direct injection

Microalgae are currently cultivated on a relatively small scale for nutritional products, supplements and aquaculture feed. In recent years there has been renewed interest in algal bioenergy, which would require cultivation at far greater scales. A key component of large-scale production systems is the delivery of CO2 to the algal cells, which is often a limiting factor in ponds and air sparged systems. Although many methods of CO2 supplementation to algal reactors have been investigated, the most commonly suggested method is still the direct injection of CO2 enriched gas into the growth medium. A sound understanding of CO2 gas–liquid mass transfer is critical to efficient operation of cultivation systems as mixing and gas compression may represent significant operational expenses. For carbon capture or sequestration through algal culture, CO2 recovery is equally important, particularly where carbon trading is involved. Chlorella vulgaris was grown in internal loop airlift reactors under varied CO2 partial pressures in the inlet gas. In these reactors, with an overall mass transfer coefficient of 0.0094s−1, an inlet CO2 partial pressure of 0.0012atm (1200ppm CO2 by volume) was sufficient to overcome any mass transfer limitations. Under these operating conditions, a CO2 recovery of 26% resulted. Increasing the partial pressure of CO2 in the inlet gas above 0.0012atm did not increase the algal productivity and caused significant decreases in CO2 recovery to 9.7% and 2.1% at inlet CO2 partial pressures of 0.00325atm and 0.0145atm respectively. Much research into algal growth is done without analysis of CO2 gas–liquid mass transfer, using influent CO2 partial pressures well in excess of the minimum value required to overcome CO2 gas–liquid mass transfer limitations, even in poorly mixed systems. This could result in algal growth being optimised under conditions that are not industrially practical or desirable.

Determination of oxygen gas–liquid mass transfer rates in heap bioleach reactors

A detailed experimental study is described which was conducted to determine the rate of oxygen gas–liquid mass transfer within the packed bed of heap bioleach reactors at different temperatures (22–68 °C), using the Na 2SO 3 method. The raw data was analysed using a simplified film mass transfer model, making corrections for oxygen solubility in concentrated solution and for increased water vapour partial pressure at elevated temperatures. The results compared favourably against two independent experimental leach studies, indicating k La values between 33 and 46 h −1. The value varied with the particle size distribution of the packing, with k La assuming larger values for those packings that had a higher fines contents. While k La increases with temperature, the solubility of oxygen decreases simultaneously, resulting in net mass transfer rates being relatively unaffected by temperature in the range studied. This indicates that thermophile heap bioleach reactors are likely to operate under gas–liquid mass transfer limitations, especially at high altitude.