Sustainability of algae derived biodiesel: a mass balance approach

A rigorous chemical engineering mass balance/unit operations approach is applied here to bio-diesel from algae mass culture. An equivalent of 50,000,000 gallons per year (0.006002 m3/s) of petroleum-based Number 2 fuel oil (US, diesel for compression-ignition engines, about 0.1% of annual US consumption) from oleaginous algae is the target. Methyl algaeate and ethyl algaeate diesel can according to this analysis conceptually be produced largely in a technologically sustainable way albeit at a lower available diesel yield. About 11 square miles of algae ponds would be needed with optimistic assumptions of 50 g biomass yield per day and m2 pond area. CO2 to foster algae growth should be supplied from a sustainable source such as a biomass-based ethanol production. Reliance on fossil-based CO2 from power plants or fertilizer production renders algae diesel non-sustainable in the long term.     

Photobioreactor design: Mixing, carbon utilization, and oxygen accumulation

Photobioreactor design and operation are discussed in terms of mixing, carbon utilization, and the accumulation of photosynthetically produced oxygen. The open raceway pond is the primary type of reactor considered; however small diameter (1-5 cm) horizontal glass tubular reactors are compared to ponds in several respects. These are representative of the diversity in photobioreactor design: low capital cost, open systems and high capital cost, closed systems. Two 100-m(2) raceways were operated to provide input data and to validate analytical results. With a planktonic Chlorella sp., no significant difference in productivity was noted between one pond mixed at 30 cm/s and another mixed from 1 to 30 cm/s. Thus, power consumption or CO(2) outgassing limits maximal mixing velocities. Mixing power inputs measured in 100-m(2) ponds agreed fairly well with those calculated by the use of Manning's equation. A typically configured tubular reactor flowing full (1 cm diameter, 30 cm/s) consumes 10 times as much energy as a typical pond (20 cm deep flowing at 20 cm/s). Tubular reactors that flow only partially full would be limited by large hydraulic head losses to very short sections (as little as 2 m length at 30 cm/s flow) or very low flow velocities. Open ponds have greater CO(2) storage capacity than tubular reactors because of their greater culture volume per square meter (100-300 L/m(2) vs. 8-40 L/m(2) for 1-5-cm tubes). However, after recarbonation, open ponds tend to desorb CO(2) to the atmosphere. Thus ponds must be operated at higher pH and lower alkalinity than would be possible with tubular reactors if cost of carbon is a constraint. The mass transfer coefficient, K(L), for CO(2) release through the surface of a 100-m(2) pond was determined to be 0.10 m/h. Oxygen buildup would be a serious problem with any enclosed reactor, especially small-diameter tubes. At maximal rates of photosynthesis, a 1-cm tubular reactor would accumulate 8-10 mg O(2)/L/min. This may result in concentrations of oxygen reaching 100 mg/L, even with very frequent gas exchange. In an open pond, dissolved oxygen rises much more slowly as a consequence of the much greater volume per unit surface area and the outgassing of oxygen to the atmosphere. The maximum concentration of dissolved oxygen is typically 25-40 mg/L. The major advantage of enclosed reactors lies in the potential for aseptic operation, a product value which justifies the expense. For most products of algal mass cultivation, open ponds are the only feasible photobioreactor design capable of meeting the economic and operating requirements of such systems, provided desirable species can be maintained.     

Cultivation of microplantlets derived from the marine red alga Agardhiella subulata in a stirred tank photobioreactor

Macrophytic marine red algae are a diverse source of bioactive natural compounds. "Microplantlet" suspension cultures established from red algae are potential platforms for biosynthesis of these compounds, provided suitable bioreactor configurations for mass culture can be identified. The stirred tank bioreactor offers high rates of gas-liquid mass transfer, which may facilitate the delivery of the CO(2) in the aeration gas to the phototrophic microplantlet suspension culture. Therefore, the effects of impeller speed and CO(2) delivery on the long-term production of microplantlet biomass of the model red alga Agardhiella subulata was studied within a stirred tank photobioreactor equipped with a paddle blade impeller (D(i)/D(T) = 0.5). Nutrient medium replacement was required for sustained biomass production, and the biomass yield coefficient based on nitrate consumption was 1.08 +/- 0.09 g dry biomass per mmol N consumed. Biomass production went through two exponential phases of growth, followed by a CO(2) delivery limited growth phase. The CO(2)-limited growth phase was observed only if the specific growth rate in the second exponential phase of growth was at least 0.03 day(-)(1), the CO(2) delivery rate was less than 0.258 mmol CO(2) L(-)(1) culture h(-)(1), and the plantlet density was at least 10 g fresh mass L(-)(1). Increasing the aeration gas CO(2) partial pressure from 0.00035 to 0.0072 atm decreased the cultivation pH from 8.8 to 7.8, prolonged the second exponential phase of growth by increasing the CO(2) delivery rate, and also increased the photosynthetic oxygen evolution rate. Impeller speeds ranging from 60 to 250 rpm, which generated average shear rates of 2-10 s(-)(1), did not have a significant effect on biomass production rate. However, microplantlets cultivated in a stirred tank bioreactor ultimately assumed compact spherical shape, presumably to minimize exposure to hydrodynamic stress.     

The techno-economic potential of renewable energy through the anaerobic digestion of microalgae

The potential of microalgae as feedstock for methane production is evaluated from a process technical and economic point of view. Production of mixed culture algae in raceway ponds on non-agricultural sites, such as landfills, was identified as a preferred approach. The potential of straightforward bio-methanation, which includes pre-concentration of microalgae and utilization of a high rate anaerobic reactor was examined based on the premises of achievable up-concentration from 0.2-0.6 kg m(-3) to 20-60 kg dry matter (DM) m(-3) and an effective bio-methanation of the concentrate at a loading rate of 20 kg DM m(-3) d(-1). The costs of biomass available for bio-methanation under such conditions were calculated to be in the range of €86-€124 ton(-1) DM. The levelized cost of energy by means of the process line "algae biomass--biogas--total energy module" would be in the order of €0.170-0.087 kWh(-1), taking into account a carbon credit of about €30 ton(-1) CO2(eq).     

Rapid, automated analysis of 13C and 18O of CO2 in gas samples by continuous-flow, isotope ratio mass spectrometry.

A rapid automated method for isotopic analysis of 13C and 18O in CO2 has been developed. A variety of gas samples containing CO2 can be swept from serological tubes into a helium carrier flow; impurities are separated on a GC column so that a pure pulse of CO2 in He flows into the mass spectrometer. Isotopic ratio determinations are carried out as the pulse passes through the mass spectrometer, allowing a sample to be measured approximately every 4 min. A double, concentric needle-probe is used to flush the sample from the tube so that 100% sample recovery is achieved, maximizing sensitivity and preventing the possibility of fractionation. The precision of the technique, sigma(m-1), is better than 0.2% (0.0002 atom per cent excess) for 13C and 0.4% (0.83 p.p.m.) for 18O for 10 micromol of CO2 at natural abundance. Samples containing only atmospheric concentrations of CO2 can also be analyzed.     

A versatile method for preparation of hydrated microbial–latex biocatalytic coatings for gas absorption and gas evolution

We describe a latex wet coalescence method for gas-phase immobilization of microorganisms on paper which does not require drying for adhesion. This method reduces drying stresses to the microbes. It is applicable for microorganisms that do not tolerate desiccation stress during latex drying even in the presence of carbohydrates. Small surface area, 10-65?μm thick coatings were generated on chromatography paper strips and placed in the head-space of vertical sealed tubes containing liquid to hydrate the paper. These gas-phase microbial coatings hydrated by liquid in the paper pore space demonstrated absorption or evolution of H(2), CO, CO(2) or O(2). The microbial products produced, ethanol and acetate, diffuse into the hydrated paper pores and accumulate in the liquid at the bottom of the tube. The paper provides hydration to the back side of the coating and also separates the biocatalyst from the products. Coating reactivity was demonstrated for Chlamydomonas reinhardtii CC124, which consumed CO(2) and produced 10.2?±?0.2?mmol?O(2)?m(-2)?h(-1), Rhodopseudomonas palustris CGA009, which consumed acetate and produced 0.47?±?0.04?mmol?H(2)?m(-2)?h(-1), Clostridium ljungdahlii OTA1, which consumed 6?mmol CO?m(-2)?h(-1), and Synechococcus sp. PCC7002, which consumed CO(2) and produced 5.00?±?0.25?mmol O(2)?m(-2)?h(-1). Coating thickness and microstructure were related to microbe size as determined by digital micrometry, profilometry, and confocal microscopy. The immobilization of different microorganisms in thin adhesive films in the gas phase demonstrates the utility of this method for evaluating genetically optimized microorganisms for gas absorption and gas evolution.     

Outdoor helical tubular photobioreactors for microalgal production: modeling of fluid-dynamics and mass transfer and assessment of biomass productivity

The production of the microalga Phaeodactylum tricornutum in an outdoor helical reactor was analyzed. First, fluid dynamics, mass-transfer capability, and mixing of the reactor was evaluated at different superficial gas velocities. Performance of the reactor was controlled by power input per culture volume. A maximum liquid velocity of 0.32 m s(-1) and mass transfer coefficient of 0.006 s(-1) were measured at 3200 W m(-3). A model of the influence of superficial gas velocity on the following reactor parameters was proposed: gas hold-up, induced liquid velocity, and mass transfer coefficient, with the accuracy of the model being demonstrated. Second, the influence of superficial gas velocity on the yield of the culture was evaluated in discontinuous and continuous cultures. Mean daily values of culture parameters, including dissolved oxygen, biomass concentration, chlorophyll fluorescence (F(v)/F(m) ratio), growth rate, biomass productivity, and photosynthetic efficiency, were determined. Different growth curves were measured when the superficial gas velocity was modified-the higher the superficial gas velocity, the higher the yield of the system. In continuous mode, biomass productivity increased by 35%, from 1.02 to 1.38 g L(-1) d(-1), when the superficial gas velocity increased from 0.27 to 0.41 m s(-1). Maximal growth rates of 0.068 h(-1), biomass productivities up to 1.4 g L(-1) d(-1), and photosynthetic efficiency of up to 15% were obtained at the higher superficial gas velocity of 0.41 m s(-1). The fluorescence parameter, F(v)/F(m), which reflects the maximal efficiency of PSII photochemistry, showed that the cultures were stressed at average irradiances within the culture higher than 280 microE m(-2) s(-1) at every superficial gas velocity. For nonstressed cultures, the yield of the system was a function of average irradiance inside the culture, with the superficial gas velocity determining this relationship. When superficial gas velocity was increased, higher growth rates, biomass productivities, and photosynthetic efficiencies were obtained for similar average irradiance values. The higher the superficial gas velocity, the higher the liquid velocity, with this increase enhancing the movement of the cells inside the culture. In this way the efficiency of the cells increased and higher biomass concentrations and productivities were reached for the same solar irradiance.     

Mesophyll conductance to CO2 in Arabidopsis thaliana

The close rosette growth form, short petioles and small leaves of Arabidopsis thaliana make measurements with commercial gas exchange cuvettes difficult. This difficulty can be overcome by growing A. thaliana plants in 'ice-cream cone-like' soil pots. This design permitted simultaneous gas exchange and chlorophyll fluorescence measurements from which the first estimates of mesophyll conductance to CO(2) (g(m)) in Arabidopsis were obtained and used to determine photosynthetic limitations during plant ageing from c. 30-45 d. Estimations of g(m) showed maximum values of 0.2 mol CO(2) m(-2) s(-1) bar(-1), lower than expected for a thin-leaved annual species. The parameterization of the response of net photosynthesis (A(N)) to chloroplast CO(2) concentrations (C(c)) yielded estimations of the maximum velocity of carboxylation (V(c,max_Cc)) which were also lower than those reported for other annual species. As A. thaliana plants aged from 30 to 45 d, there was a 40% decline of A(N) that was entirely the result of increased diffusional limitations to CO(2) transfer, with g(m) being the largest. The results suggest that in A. thaliana A(N) is limited by low g(m) and low capacity for carboxylation. Decreased g(m) is the main factor involved in early age-induced photosynthetic decline.     

Photooxidative Death in Blue-Green Algae

When incubated in the light under 100% oxygen, wild-type blue-green algae (Anacystis nidulans, Synechococcus cedrorum) die out rapidly at temperatures of 4 to 15 C, and at 35 C (or at 26 C in the case of S. cedrorum) in the absence of CO(2). Photosynthesis is impaired in these cells long before they die. Blocking of photosystem II at high temperatures in the presence of CO(2) sensitizes the algae to photooxidative death. Photooxidative death and bleaching of photosynthetic pigments are separable phenomena. Photooxidative conditions were demonstrated in Israeli fish ponds using A. nidulans as the test organism during dense summer blooms, when dissolved CO(2) is low, and in winter, when water temperatures generally drop below 15 C. This finding suggests that photooxidative death may be responsible for the sudden decomposition of blue-green blooms in summer, and may be a factor in the absence of blue-green blooms in winter.     

亚热带河口养殖塘沉积物有机碳矿化及其影响因子

亚热带河口的养殖塘是温室气体二氧化碳(CO2)的重要排放源,但其沉积物有机碳矿化动力学特征目前尚未厘清.以我国东南沿海亚热带闽江、木兰溪以及九龙江河口区的6个凡纳滨对虾(Penaeus vannamei)养殖塘为研究对象,对沉积物取样,并进行为期60 d的室内厌氧培养,对养殖塘沉积物CO2累积产量曲线利用一阶动力学模型...     

Hydrodynamics and mass transfer coefficient in activated sludge aerated stirred column reactor: experimental analysis and modeling

The aerated stirred reactor (ASR) has been widely used in biochemical and wastewater treatment processes. The information describing how the activated sludge properties and operation conditions affect the hydrodynamics and mass transfer coefficient is missing in the literature. The aim of this study was to investigate the influence of flow regime, superficial gas velocity (U(G)), power consumption unit (P/V(L)), sludge loading, and apparent viscosity (mu(ap)) of activated sludge fluid on the mixing time (t(m)), gas hold-up (epsilon), and volumetric mass transfer coefficient (k(L)a) in an activated sludge aerated stirred column reactor (ASCR). The activated sludge fluid performed a non-Newtonian rheological behavior. The sludge loading significantly affected the fluid hydrodynamics and mass transfer. With an increase in the U(G) and P/V(L), the epsilon and k(L)a increased, and the t(m), decreased. The epsilon, k(L)a, and t(m), were influenced dramatically as the flow regime changed from homogeneous to heterogeneous patterns. The proposed mathematical models predicted the experimental results well under experimental conditions, indicating that the U(G), P/V(L), and mu(ap) had significant impact on the t(m), epsilon, and k(L)a. These models were able to give the t(m), epsilon, and k(L)a values with an error around +/-8%, and always less than +/-10%.     

Does the Prostrate-leaved Geophyte Brunsvigia orientalis Utilize Soil-derived CO2 for Photosynthesis?

BACKGROUND AND AIMS: A test was made of the hypothesis that the prostrate growth habit of the leaves of the geophyte Brunsvigia orientalis enables utilization of soil-derived CO(2) and is related to the presence of lysigenous air-filled channels characteristic of B. orientalis leaves. METHODS: Brunsvigia orientalis was sampled at a field site. Leaf anatomy, stomatal density, leaf/soil gas exchange characteristics and soil atmosphere and leaf delta(13)C isotope abundances were examined. KEY RESULTS: The leaves of B. orientalis have large lysigenous air-filled channels separating the upper and lower surfaces of the leaves. The upper surface comprised approx. 70 % of the leaf mass and 75 % of the leaf N (mmol g(-1)). Between 20 % and 30 % of the stomatal conductance and CO(2) assimilation was through the lower surface of the leaf. CO(2) efflux rates from the soil surface were up to 5.4 micromol m(-2) s(-1) while photosynthetic fluxes through the lower surface of the leaves were approx. 7 micromol m(-2) s(-1). However, the utilization of soil-derived CO(2) only altered the leaf delta(13)C isotope abundance of the prostrate leaves by a small amount. Using delta(13)C values it was estimated that 7 % of the leaf tissue C was derived from soil-derived CO(2). CONCLUSIONS: A small proportion of photosynthetically fixed CO(2) was derived from the soil, with minimal associated transpirational H(2)O loss into the space between the leaf and soil. The soil-derived CO(2), taken up through the lower surface was probably assimilated by the palisade tissue in the upper surface of the leaf which was exposed to sunlight and where most of the leaf N was located. The occurrence of lysigenous air channels in the leaves may provide longitudinal strength without impaired transfer of CO(2) taken up through the lower surface to the upper surface.     

Characterization of an experimental miniature bioreactor for cellular perturbation studies

A mini bioreactor (3.0 mL volume) has been developed and shown to be a versatile tool for rapidly screening and quantifying the response of organisms on environmental perturbations. The mini bioreactor is essentially a plug flow device transformed into a well-mixed reactor by a recycle flow of the broth. The gas and liquid phases are separated by a silicone membrane. Dynamic mass transfer experiments were performed to determine the mass transfer capacities for oxygen and carbon dioxide. The mass transfer coefficients for oxygen and carbon dioxide were found to be 1.55 +/- 0.17 x 10(-5) m/s and 4.52 +/- 0.60 x 10(-6) m/s, respectively. Cultivation experiments with the 3.0 mL bioreactor show that (i) it can maintain biomass in the same physiological state as the 4.0 L lab scale bioreactor, (ii) reproducible perturbation experiments such as changing substrate uptake rate can be readily performed and the physiological response monitored quantitatively in terms of the O2 and CO2 uptake and production rates.     

Transfer of carbon dioxide within cultures of microalgae: plain bubbling versus hollow-fiber modules

In attempts to improve the metabolic efficiency in closed photosynthetic reactors, availability of light and CO(2) are often considered as limiting factors, as they are difficult to control in a culture. The carbon source is usually provided via bubbling of CO(2)-enriched air into the culture medium; however, this procedure is not particularly effective in terms of mass transfer. Besides, it leads to considerable waste of that gas to the open atmosphere, which adds to operation costs. Increase in the interfacial area of contact available for gas exchange via use of membranes might be a useful alternative; microporous membranes, in hollow-fiber form, were tested accordingly. Two hollow-fiber modules, different in both hydrophilicity and outer surface area, were tested and duly compared, in terms of mass transfer, versus traditional plain bubbling. Overall volumetric coefficients (K(L)a) for CO(2) transfer were 1.48 x 10(-2) min(-1) for the hydrophobic membrane, 1.33 x 10(-2) min(-1) for the hydrophilic membrane, and 7.0 x 10(-3) min(-1) for plain bubbling. A model microalga, viz. Nannochloropsis sp., was cultivated using the two aforementioned membrane systems and plain bubbling. The produced data showed slight (but hardly significant) increases in biomass productivity when the hollow-fiber devices were used. However, hollow-fiber modules allow recirculation of unused CO(2), thus reducing feedstock costs. Furthermore, such indirect way of supplying CO(2) offers the additional possibility for use of lower gas pressures, as no need to counterbalance hydrostatic heads exists.     

Optimized aeration by carbon dioxide gas for microalgal production and mass transfer characterization in a vertical flat-plate photobioreactor

To optimize the aeration conditions for microalgal biomass production in a vertical flat-plate photobioreactor (VFPP), the effect of the aeration rate on biomass productivity was investigated under given conditions. Air enriched with 5% or 10% (v/v) CO(2) was supplied for the investigation at rates of 0.025-1 vvm. The CO(2) utilization efficiency, change of pH in the medium, and the optimum aeration rate were determined by evaluating biomass productivity. To investigate the VFPP mass transfer characteristics, the overall volumetric mass transfer coefficient, k(L)a, was evaluated for several different flat-plate sizes. Increasing the height of the VFPP could improve both the mass transfer of CO(2) and the illumination conditions, so this appeared to be a good method for scaling up. Based on a comparison of the k(L)a value at the optimum aeration rate with previously reported results, it was confirmed that the range of CO(2) concentration used in the experiments was cost-effective for mass culture.     

Ethene removal from a synthetic waste gas using a dry biobed

A packed granular activated carbon (GAC) biobed, inoculated with the ethane-degrading strain Mycobacterium E3, was used to study ethene removal from a synthetic waste gas. Ethene, for which the dimensionless partition coefficient for an air-water system at 20 degrees C is about 7.6, was used as a model compound for poorly water soluble gaseous pollutants. In a first mode or operation, the GAC biobed was sprinkled intermittently and the waste gas influent was continuously pre-humidified, establishing relatively moist conditions (water content >40% to 45%). A volumetric ethene removal rate of 0.382 kg COD . m(-3) . d(-1) (0.112 kg ethene . m(-3) . d(-1)) was obtained for an influent concentration of 125 ppm, a superficial waste gas velocity of 3.6E-3 m . s(-1) and a pseudo residence time of 45 s. However, in the second mode of operation, omitting the pre-humidification of the waste gas influent and establishing a "dry" biobed (water content <40% to 45%), and thus obtaining better mass transfer to the biofilm, the ethene removal could be doubled for otherwise comparable operating parameters. Furthermore, under decreased wetting and for the given experimental conditions (influent concentration 125 to 816 ppm, waste gas superficial velocity 3.0E-3 m .s(-1), pseudo waste gas residence time 43 s), the ethene removal was not limited by mass transfer of ethene through the water layer covering the biofilm. (c) 1994 John Wiley & Sons, Inc.     

In vivo kinetics with rapid perturbation experiments in Saccharomyces cerevisiae using a second-generation BioScope

We present a robust second-generation BioScope: a system for continuous perturbation experiments. Firstly, the BioScope design parameters (i.e., pressure drop, overall oxygen (O2) and carbon dioxide (CO2) mass transfer, mean residence time distribution and plug flow characteristics) were evaluated. The average overall mass transfer coefficients were estimated to be 1.8E-5 m s(-1) for O2 and 0.34E-5 m s(-1) for CO2. It was determined that the O2/CO2 permeable membrane accounted for 75% and 95% of the overall resistance for O2 and CO2, respectively. The Peclet number (Pe) of the system was found to be >500 for liquid flow rates between 1 and 4 ml min(-1), ensuring plug flow characteristics. Secondly, steady-state intracellular metabolite concentrations obtained using direct rapid sampling from the fermentor were compared with those obtained by rapid sampling via the pre-perturbation sample port of the BioScope. With both methods the same metabolite levels were obtained. Thirdly, glucose perturbation experiments were carried out directly in the fermentor as well as in the BioScope, whereby steady-state Saccharomyces cerevisiae cells from a glucose/ethanol limited chemostat were perturbed by increasing the extracellular glucose concentration from 0.11 to 2.8 mM. Intracellular and extracellular metabolite levels were measured within a time window of 180 s. It was observed that the dynamic metabolite concentration profiles obtained from both perturbations were nearly the same, with the exception of the C4 metabolites of the TCA cycle, which might be due to differences in culture age.     

Characterization of oxygen transfer in miniature and lab-scale bubble column bioreactors and comparison of microbial growth performance based on constant k(L)a

This work describes the engineering characterization of miniature (2 mL) and laboratory-scale (100 mL) bubble column bioreactors useful for the cultivation of microbial cells. These bioreactors were constructed of glass and used a range of sintered glass gas diffusers with differently sized pores to disperse humidified air within the liquid biomedium. The effect of the pressure of this supplied air on the breakthrough point for gas diffusers with different pore sizes was examined and could be predicted using the Laplace-Young equation. The influence of the superficial gas velocity (u(g)) on the volumetric mass transfer coefficient (k(L)a) was determined, and values of up to 0.09 s(-1) were observed in this work. Two modeling approaches were considered in order to predict and provide comparison criteria. The first related the volumetric power consumption (P/V) to the k(L)a and a good correlation was obtained for differently sized reactors with a given pore size, but this correlation was not satisfactory for bubble columns with different gas diffusers. Values for P/V ranged from about 10 to 400 W.m(-3). Second, a model was developed predicting bubble size (d(b)), bubble rising velocity (u(b)), gas hold-up (phi), liquid side mass transfer coefficient (k(L)), and thus the k(L)a using established theory and empirical correlations. Good agreement was found with our experimental data at different scales and pore sizes. Values for d(b) varied from 0.1 to 0.6 mm, and k(L) values between 1.7 and 9.8 x 10(-4) m.s(-1) were determined. Several E. coli cultivations were performed in the miniature bubble column at low and high k(L)a values, and the results were compared to those from a conventional stirred tank operated under identical k(L)a values. Results from the two systems were similar in terms of biomass growth rate and carbon source utilization.     

Incorporating hydrodynamics into spatiotemporal metabolic models of bubble column gas fermentation

Gas fermentation has emerged as a technologically and economically attractive option for producing renewable fuels and chemicals from carbon monoxide (CO) rich waste streams. LanzaTech has developed a proprietary strain of the gas fermentating acetogen Clostridium autoethanogenum as a microbial platform for synthesizing ethanol, 2,3-butanediol, and other chemicals. Bubble column reactor technology is being developed for the large-scale production, motivating the investigation of multiphase reactor hydrodynamics. In this study, we combined hydrodynamics with a genome-scale reconstruction of C. autoethanogenum metabolism and multiphase convection–dispersion equations to compare the performance of bubble column reactors with and without liquid recycle. For both reactor configurations, hydrodynamics was predicted to diminish bubble column performance with respect to CO conversion, biomass production, and ethanol production when compared with bubble column models in which the gas phase was modeled as ideal plug flow plus axial dispersion. Liquid recycle was predicted to be advantageous by increasing CO conversion, biomass production, and ethanol and 2,3-butanediol production compared with the non-recycle reactor configuration. Parametric studies performed for the liquid recycle configuration with two-phase hydrodynamics showed that increased CO feed flow rates (more gas supply), smaller CO gas bubbles (more gas–liquid mass transfer), and shorter column heights (more gas per volume of liquid per time) favored ethanol production over acetate production. Our computational results demonstrate the power of combining cellular metabolic models and two-phase hydrodynamics for simulating and optimizing gas fermentation reactors.     

Toluene removal from waste air using a flat composite membrane bioreactor

In this report, gaseous toluene biodegradation results in a flat composite membrane reactor inoculated with Pseudomonas putida TVA8 are presented. Preliminary abiotic experiments showed that transport of toluene through the membrane was linearly and negatively correlated with the gas residence time (tau). During a 339-day biofiltration experiment, the influence of gas residence time (2-24 sec) and mass loading rate (B(v); 10-483 g x m(-3) h(-1)) on the toluene elimination capacity was investigated. A maximum elimination capacity (EC(max)) of 397 g x m(-3) h(-1) was achieved at tau = 24 sec and B(v) = 473 g x m(-3) h(-1). Expressed per unit membrane area, the EC(m,max) was 0.793 g x m(-2) h(-1), which is five times higher than results obtained with other membrane bioreactor experiments in the same range of loading rates. At low gas residence times, reactor performance was limited by mass transfer. Toluene concentration profiles along the membrane were measured for several biotic and abiotic conditions. For inlet concentrations (C(in)) up to 1 g x m(-3), more than 90% was eliminated at 15 cm from the reactor inlet. For C(in) > 1.65 g x m(-3), longer membranes are necessary to obtain these high removal efficiencies.