C. G. Dussap

A simplified monodimensional approach for modeling coupling between radiant light transfer and growth kinetics in photobioreactors

Abstract Local information is essential to photobioreactor modeling because of medium anisotropy in radiant light energy. Local available energy can be calculated using complex equations, applying the physical laws of radiative transfer and independently accounting for light absorption and scattering in the reactor. In this paper, these equations are simplified postulating monodimensional approximation for the radiation field. This simplification is established for rectangular, cylindrical and spherical coordinates, leading to simple analytical solutions for available radiant energy profiles inside the reactor. This approach provides a method of determining working illuminated volume defined as the photobioreactor volume with sufficient radiant light energy for microorganism growth. This enables the coupling between radiant light transfer and growth kinetics to be easily studied. Physical light transfer models are used to simulate volumetric biomass growth rates in a cylindrical photobioreactor with kinetic parameters obtained from batch cultures of the cyanobacterium Spirulina platensis in rectangular photoreactors. These calculations are compared with experimental data obtained on continuous cultures in a wide range of incident radiant energy fluxes. The model is found to have a good predictability and robustness.

Modeling of the water-sucrose state diagram below 0 °C

The state diagram of sucrose-water solutions was constructed by the use of model calculations; the activity coefficient from the UNIQUAC model allowed determination of the solution-ice equilibrium curve and the glass transition temperature as a function of the composition was evaluated from the Gordon-Taylor equation. The accuracy of the glass transition temperature (T′g) of the maximally freeze-concentrated fraction, determined from the intersection of both curves, was analyzed and discussed. While the UNIQUAC model predicted reliable ice melting temperatures in the high sucrose concentration range, a significant variation of the fitted glass transition curve was obtained due to the selected temperature values for the glass transition temperature. The calculated glass temperatures (T′g) were compared with direct DSC experiments. n nWhile the UNIQUAC model predicted reliable ice melting temperatures in the high sucrose concentration range, a significant variation of the fitted glass transition curve was obtained due to the selected temperature values as the glass transition temperature. The calculated glass temperatures (Tg′ were compared with direct DSC experiments.

Metabolic flux distribution in Corynebacterium melassecola ATCC 17965 for various carbon sources

The distribution of carbon in the metabolic network of a bacterial cell was estimated by a mass‐balance‐based intracellular flux computation method. It was applied to the growth phase of Corynebacterium melassecola, a glutamic acid producing bacterium, using experimental production yields of biomass, lactate and acetate measured during batch cultures on glucose, fructose, and various mixtures of both sugars. This flux computation method identifies the direction of the 86 reactions that ensure proper metabolic function during the growth phase of C. melassecola. Flux ratios allow comparison of calculated and relevant experimental yields. The results highlight the key influence of the biomass production yield Y u2009X−Ou20092 on the overall distribution of carbon; the proportion of carbon drained in the pentose‐P pathway fell from a value in the range of 54% to 47% on media containing glucose (Y u2009X−Ou20092 = 1.75 to 1.56 g X/g O2) to 37% on fructose medium (Y u2009X−Ou20092 = 1.36 g X/g O2). The highest maintenance requirement was calculated on fructose medium (Jm = 290 mol ATP/100 mol fructose) which must be connected to a lower efficiency of cell multiplication observed on this substrate. Another important result was that the significant decreases in experimental values of production yields and rates observed on fructose medium which were related to the operation of the FBPase. In particular, it was estimated that, as long as the proportion of glucose in the carbon source remains above 22% (78% fructose), the operation of the FBPase is not necessary and the bacteria exhibit behavior similar to that observed on glucose alone; this result is consistent with experimental observations.

Xanthan batch fermentations: compared performances of a bubble column and a stirred tank fermentor

Fermentations of Xanthomonas campestris, NRRL B-1459, were carried out in a bubble column fermentor (BCF) and in a stirred tank fermentor (STF) to allow comparison of representative variables measured during the microbial growth and the gum production.The microbial growth phase was described by a logistic rate equation where maximum cell concentration was provided by nitrogenous compounds balance. The average value of the maximum specific growth rate was higher in the bubble column (μM=0.5 h−1) than in the stirred reactor (μM=0.4 h−1).The upper values of xanthan yield (Yg-x=0.65 kg xanthan/kg glucose; YO2−x xanthan/kg oxygen) and specific production rate (qx=0.26 kg xanthan/kg biomass · h) were measured when the oxygen transfer coefficient was kept up above 80 h−1 in the STF fermentor. In the bubble column the fermentation achieved in the same culture medium lasts two times longer than in the stirred aerated tank; this was attributed to the low value of the oxygen transfer coefficient (KLa =20 h−1) at the beginning of the gum synthesis phase. The results obtained in the stirred tank were the basis to estimate the optimal biomass concentration which enables to achieve a culture in non-limiting oxygen transfer conditions.Nevertheless, the transfer characteristics were more homogeneous in the bubble column than in the stirred tank where dead stagnant zones were observed. This is of primary importance when establishing fermentation kinetics models.

Estimation of O2 and CO2 Solubility in Microbial Culture Media

A simple model for predicting gas solubilities of O2 and CO2 at low pressure and near ambient temperature in solutions of salts, sugars, and organic solvents (alcohols, ketones, ethers, aldehydes, etc.) is proposed. It is derived from the Van Laar assumptions and takes account of size differences between molecules in solution by their volumetric fraction. It is a group contribution model where anions and cations are considered as groups and other molecules are treated as in the UNIFAC (UNIQUAC Functional group Activity Coefficients) procedure. Pseudo‐Henrys constants for groups were determined using solubility data in aqueous solutions containing only one salt, one sugar, or one organic compound at 25 °C. The predictive performance of the model was evaluated by comparison with experimental data using multicomponent aqueous salt−sugar mixtures. The model was used to estimate solubilities of oxygen and carbon dioxide in fermentation media.

Growth monitoring of a photosynthetic micro-organism (Spirulina platensis) by pressure measurement

An on-line measurement technique for estimating biomass production rate in a photosynthetic micro-organism culture was developed and tested experimentally. The technique is based on monitoring O2 production from the increase in pressure inside a closed photobioreactor. The data obtained by this method correlated with the direct measurement of the biomass concentration. A material balance on the components in the system allows the validity domain of the method to be defined. The method was applied to batch cultures of the cyanobacterium, Spirulina platensis, in a cylindrical photobioreactor validating existing physiological and light energy models.

Modelling continuous culture of Rhodospirillum rubrum in photobioreactor under light limited conditions

AbstractRhodospirillum rubrum was grown continuously and photoheterotrophically under light limitation using a cylindrical photobioreactor in which the steady state biomass concentration was varied between 0.4 to 4xa0kgxa0m−3 at a constant radiant incident flux of 100xa0Wxa0m−2. Kinetic and stoichiometric models for the growth are proposed. The biomass productivities, acetate consumption rate and the CO2 production rate can be quantitatively predicted to a high level of accuracy by the proposed model calculations.n Nomenclature:CX, biomass concentration (kgxa0m−3) D, dilution rate (h−1) Ea, mean mass absorption coefficient (m2 kg−1) IΣ, total available radiant light energy (Wxa0m−2) K, half saturation constant for light (Wxa0m−2) RW, boundary radius defining the working illuminated volume (m) rX, local biomass volumetric rate (kgxa0m−3xa0h−1) , mean volumetric growth rate (kgxa0m−3xa0h−1) VW, illuminated working volume in the PBR (m−3).n Greek letters:n γ, working illuminated fraction (−) ρM, maximum quantum yield (−) barη, mean energetic yield (kg J−1).

Dynamic model of a nitrifying fixed bed column: Simulation of the biomass distribution of Nitrosomonas and Nitrobacter and of transient behaviour of the column

Abstract A dynamic model for a fixed bed nitrifying column with recirculation of the liquid and gas phases was developed. Liquid RTD experiments demonstrated that the liquid phase was perfectly mixed inside the column. Hete- rogeneity of biomass distribution on the solid phase (beads) was represented by an N-tanks in series model, and a back-mixing term was set to account for the well-mixed liquid phase throughout the column. In autotrophic conditions, competition for oxygen is the cause of the spatial segregation of the two species. Nitrosomonas is concentrated on beads at the bottom of the bed whereas Nitrobacter is more widely distributed. This is consistent with biomass distribution results reported by Cox et al. [17] in a nitrifying fixed bed column. Nitrification takes place at the bottom of the column, always in oxygen gas-liquid mass transfer limiting conditions. Nevertheless, considering the whole process, nitrification is complete (>98% of NH3 oxidised) and there is no oxygen limitation (the outlet dissolved oxygen concentration is not limiting). The dynamic behaviour of the column, in conditions set up to avoid biofilm diffusion limitation, was simulated for different NH3-load variations and oxygen shutdowns. The simulated behaviour of the column can be compared to results reported by Bazin et al. [16]. This confirms that the output transient nitrite peaks are higher when changes in the process conditions produce a rearrangement of biomass distribution in the fixed bed.

Metabolic Flux Modelling as a Tool to Analyse the Behavior of a Genetically Modified Strain of Saccharomyces Cerevisiae

Flux distribution for a wild and a mutant strain of Saccharomyces cerevisiae are compared and investigated in terms of metabolic flux calculation and thermodynamic analysis of central metabolism under anaerobic conditions. Starting from a redundant set of measured rates obtained from batch cultures on glucose or fructose as carbon source, an original data reconciliation technique associated with the calculation of metabolic flux is used. Comparative analysis of carbon split in the metabolic network for the mutant yeast strain lacking the glucose6P-dehydrogenase (CD101-1A) and for the reference wild strain (ATCC 7754) allows to conclude that the pentose phosphate is in priority devoted to its anabolic function rather than to the production of NADPH cofactors. This last function seems to be as well assumed by the specific NADP acetaldehyde dehydrogenase enzyme; this explains the significantly higher production of acetate by the mutant strain.

Intra- and extracellular concentrations of glutamate, lactate and acetate during growth of Corynebacterium glutamicum on different media

Corynebacterium glutamicum ATCC 17965 was cultivated in a 4-L batch aerated fermentor with glucose, fructose and mixtures of these two sugars in various proportions as carbon sources and with different concentrations of minerals and vitamins. A multilayer centrifugation technique was devised to obtain cell extracts in order to assess intracellular production of glutamate and partitioning between intracellular and extracellular spaces for lactate and acetate, the main by-products produced during the growth phase. Glutamate production increased with the proportion of glucose in the carbon source. The average value for the intracellular concentration of glutamate obtained with basic glucose medium was increased three-fold when initial concentrations of vitamins and minerals were increased four-fold. In this case, overall production of glutamate (16.3u2009mM) reached the highest value obtained. Production of acetate was weak on all media types (<u20091.6u2009mm). it was the same for lactate synthesis in media where glucose remained the major carbon source (<u20092.3u2009mm). production of lactate was significantly higher on media where fructose was the main carbon source (>u200910u2009mM to 60u2009mM). The increase in lactate production and the decrease in glutamate production were correlated to a modification of carbon flux distribution between the metabolic pathways as the fructose proportion was increased. An increase in the concentration of minerals favoured production of glutamate during growth. This was correlated with an increase in the NADPH,H+ production rate.