Cell Operation Mode Research Articles

Physiological heterogeneities in microbial populations and implications for physical stress tolerance

BackgroundTraditionally average values of the whole population are considered when analysing microbial cell cultivations. However, a typical microbial population in a bioreactor is heterogeneous in most phenotypes measurable at a single-cell level. There are indications that such heterogeneity may be unfavourable on the one hand (reduces yields and productivities), but also beneficial on the other hand (facilitates quick adaptation to new conditions - i.e. increases the robustness of the fermentation process). Understanding and control of microbial population heterogeneity is thus of major importance for improving microbial cell factory processes.ResultsIn this work, a dual reporter system was developed and applied to map growth and cell fitness heterogeneities within budding yeast populations during aerobic cultivation in well-mixed bioreactors. The reporter strain, which was based on the expression of green fluorescent protein (GFP) under the control of the ribosomal protein RPL22a promoter, made it possible to distinguish cell growth phases by the level of fluorescence intensity. Furthermore, by exploiting the strong correlation of intracellular GFP level and cell membrane integrity it was possible to distinguish subpopulations with high and low cell membrane robustness and hence ability to withstand freeze-thaw stress. A strong inverse correlation between growth and cell membrane robustness was observed, which further supports the hypothesis that cellular resources are limited and need to be distributed as a trade-off between two functions: growth and robustness. In addition, the trade-off was shown to vary within the population, and the occurrence of two distinct subpopulations shifting between these two antagonistic modes of cell operation could be distinguished.ConclusionsThe reporter strain enabled mapping of population heterogeneities in growth and cell membrane robustness towards freeze-thaw stress at different phases of cell cultivation. The described reporter system is a valuable tool for understanding the effect of environmental conditions on population heterogeneity of microbial cells and thereby to understand cell responses during industrial process-like conditions. It may be applied to identify more robust subpopulations, and for developing novel strategies for strain improvement and process design for more effective bioprocessing.

Production of C2 hydrocarbons and H2 from CH4 in a proton conducting cell

The production of C2 hydrocarbons and H2 from methane was studied in a proton conducting cell-reactor, at 973–1153K and atmospheric pressure. Under open-circuit, the La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) catalyst (anode) was found to be active towards both, H2 and C2 formation. Under closed-circuit, when protons were “pumped” away from the catalyst, an increase in C2's and H2 production was observed. The fraction of produced H2 that was transported to the cathode reached values as high as 95%. In the fuel cell operation mode and at the maximum power output, the C2 formation rate exceeded the open circuit rate by up to 20%.

An improved tank in series model for PEMFC

This study presents an improved Tank in Series Reactor (TSR) Proton Exchange Membrane (PEM) fuel cell model with mass and energy balance equations accounting for evaporation and condensation in cathode channels. The TSR model includes the modified charge balance equation suitable for potentiostatic fuel cell operation mode. Polarization curves calculated with the improved model agree with experimental data from the literature. The developed TSR model is able to predict the limiting two-dimensional profiles in PEMFC. Simulation results illustrate the influence of co-current and counter current mode on PEM fuel cell performance.

An Improved Tank in Series Model for the Direct Methanol Fuel Cell

This study presents an improved TSR model for Direct Methanol Fuel Cell (DMFC) with mass and energy balance equations taking into account gas evolution in the anode channels. The TSR model includes a modified charge balance equation suitable for potentiostatic fuel cell operation mode. Polarization curves calculated with the improved TSR model agree with experimental data from the literature. The developed TSR model is able to approximate two-dimensional concentration, temperature, and current density profiles in DMFC with parallel flow field.

Influence of properties of gas diffusion layers on the performance of polymer electrolyte-based unitized reversible fuel cells

Polymer electrolyte-based unitized reversible fuel cells (URFCs) combine the functionality of a fuel cell and an electrolyzer in a single device. In a URFC, titanium (Ti)-felt is used as a gas diffusion layer (GDL) of the oxygen electrode, whereas typical carbon paper is used as a GDL of the hydrogen electrode. Different samples of Ti-felt with different structural properties (porosity and fiber diameter) and PTFE content were prepared for use as GDLs of the oxygen electrode, and the relation between the properties of the GDL and the fuel cell performance was examined for both fuel cell and electrolysis operation modes. Experimental results showed that the cell with a Ti-felt GDL of 80 μm fiber diameter had the highest round-trip efficiency due to excellent fuel cell operation under relatively high-humidity conditions despite degradation in performance in the electrolysis mode.

The Effects of Operating Conditions on the Performance of a Solid Oxide Steam Electrolyser: A Model‐Based Study

AbstractTo support the development of hydrogen production by high temperature electrolysis using solid oxide electrolysis cells (SOECs), the effects of operating conditions on the performance of the SOECs were investigated using a one‐dimensional model of a cathode‐supported planar SOEC stack. Among all the operating parameters, temperature is the most influential factor on the performance of an SOEC, in terms of both cell voltage and operation mode (i.e. endothermic, thermoneutral and exothermic). Current density is another influential factor, in terms of both cell voltage and operation mode. For the conditions used in this study it is recommended that the SOEC be operated at 1,073 K and with an average current density of 10,000 A m–2, as this results in the stack operating at almost constant temperature along the cell length. Both the steam molar fraction at the inlet and the steam utilisation factor have little influence on the cell voltage of the SOEC but their influence on the temperature distribution cannot be neglected. Changes in the operating parameters of the SOEC can result in a transition between endothermic and exothermic operation modes, calling for careful temperature control. The introduction of air into the anode stream appears to be a promising approach to ensure small temperature variations along the cell.

Performance of La2–xSrxCo0.5Ni0.5O4±δ as an Oxygen Electrode for Solid Oxide Reversible Cells

AbstractLa2 – xSrxCo0.5Ni0.5O4 ± δ (LSCN) is presented as a novel electrode for both solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). LSCN/10Sc1CeSZ (scandia and ceria stabilised zirconia)/LSCN symmetrical cells were fabricated and characterised by AC impedance spectroscopy under different oxygen partial pressures at temperatures of up to 900 °C. At 850 °C polarisation resistances of 5.15 and 5.74 Ω cm2 were found applying air and oxygen, respectively. Electrolysis and fuel cell j–V experiments were also performed at temperatures between 700 and 850 °C using a LSCN/10Sc1CeSZ/Ni‐YSZ. Electrochemical results showed that the cell performs equally in both SOFC and SOEC modes. Detailed results in terms of performance are presented and discussed. LSCN is presented as a good candidate for both SOFC and SOEC. The similar performance obtained for j–V (current density–voltage) curves in electrolysis and fuel cell operation mode is thought to be related to the flexible oxygen non‐stoichiometry of the LSCN (K2NiF4‐type structure).

Power generation/energy storage by a fuel cell/battery system: Regeneration of the MnO 2 positive electrode with gaseous oxygen

Fuel cell/battery (FCB) systems are promising power generation/energy storage systems because of their bi-functionality as fuel cells and as secondary batteries. We investigated the required charging after the discharged manganese dioxide (MnOOH) by oxygen gas under the rest condition and during the fuel cell operation mode using manganese dioxide as a positive electrode for the FCB system. Electrochemical characterization was performed using cyclic voltammetry and galvanostatic measurements. Additionally, changes in the crystal structure and the chemical functional groups during the electrode reactions were monitored by X-ray diffractometry and Fourier transform infrared spectroscopy. The results indicated that MnOOH formed via the electrochemical discharge of manganese dioxide (MnO 2) and that the oxyhydroxide can be chemically transformed back to MnO 2 with gaseous oxygen (O 2). The recharged MnO 2 can be used as the cathode in a fuel cell with an O 2 supply and it can also be electrochemically discharged without an O 2 supply. In addition, we confirmed that MnO 2 does not convert to Mn 3O 4 during the charge/discharge cycles if the redox reaction is maintained within a restricted range where a homogeneous process exists between MnO 2 and MnOOH. The results in this study suggest that the FCB system can be constructed using MnO 2 as the positive electrode and a metal hydride (MH) as the negative electrode, which can be rapidly charged to more than 70% of the theoretical capacity within 10 min using pressurized H 2 and electrochemically discharged, in an alkaline electrolyte. This system possesses a high-power generation efficiency, a high-energy density and a high load-following capability.

Current Density Distribution Mapping in PEM Fuel Cells as An Instrument for Operational Measurements

A newly developed measurement system for current density distribution mapping has enabled a new approach for operational measurements in proton exchange membrane fuel cells (PEMFC). Taking into account previously constructed measurement systems, a method based on a multi layer printed circuit board was chosen for the development of the new system. This type of system consists of a sensor, a special electronic device and the control and visualization PC. For the acquisition of the current density distribution values, a sensor device was designed and installed within a multilayer printed circuit board with integrated shunt resistors. Varying shunt values can be taken into consideration with a newly developed and evaluated calibration method. The sensor device was integrated in a PEM fuel cell stack to prove the functionality of the whole measurement system. A software application was implemented to visualize and save the measurement values. Its functionality was verified by operational measurements within a PEMFC system. Measurement accuracy and possible negative reactions of the sensor device during PEMFC operation are discussed in detail in this paper. The developed system enables operational measurements for different operating phases of PEM fuel cells. Additionally, this can be seen as a basis for new opportunities of optimization for fuel cell design and operation modes.

Determination of PASHs by various analytical techniques based on gas chromatography–mass spectrometry: Application to a biodesulfurization process

Polycyclic aromatic sulphur heterocyclic (PASH) compounds, such as dibenzothiophene (DBT) and alkylated derivatives are used as model compounds in biodesulfurization processes. The development of these processes is focused on the reduction of the concentration of sulphur in gasoline and gas–oil [D.J. Monticello, Curr. Opin. Biotechnol. 11 (2000) 540], in order to meet European Union and United States directives. The evaluation of biodesulfurization processes requires the development of adequate analytical techniques, allowing the identification of any transformation products generated. The identification of intermediates and final products permits the evaluation of the degradation process. In this work, seven sulfurated compounds and one non-sulfurated compound have been selected to develop an extraction method and to compare the sensitivity and identification capabilities of three different gas chromatography ionization modes. The selected compounds are: dibenzothiophene (DBT), 4-methyl-dibenzothiophene (4-m-DBT), 4,6-dimethyl-dibenzothiophene (4,6-dm-DBT) and 4,6 diethyl-dibenzothiophene (4,6 de-DBT), all of which can be used as model compounds in biodesulfurization processes; as well as dibenzothiophene sulfoxide (DBTO 2), dibenzothiophene sulfone (DBTO) and 2-(2-hydroxybiphenyl)-benzenesulfinate (HBPS), which are intermediate products in biodesulfurization processes of DBT [ A. Alcon, V.E. Santos, A.B. Martín, P. Yustos, F. García-Ochoa, Biochem. Eng. J. 26 (2005) 168]. Furthermore, a non-sulfurated compound, 2-hydroxybiphenyl (2-HBP), has also been selected as it is the final product in the biodesulfurization process of DBT [A. Alcon, V.E. Santos, A.B. Martín, P. Yustos, F. García-Ochoa. Biochem. Eng. J. 26 (2005) 168]. Since, typically, biodesulfurization reactions take place in a biphasic medium, two extraction methods have been developed: a liquid–liquid extraction method for the watery phase and a solid phase extraction method for the organic phase. Recoveries of the selected compound in both media were studied. They were in the range of 80–100% for the watery and in the range of 40–60% for the organic phase, respectively. Gas chromatography coupled to mass spectrometry (GC–MS) has been employed for the identification of these selected compounds. Three different ionization modes were applied: conventional electron impact (EI); positive chemical ionization (PCI), using methane as the reagent gas; and a recently developed ionization mode known as hybrid chemical ionization (HCI), using perfluorotri- n-butylamine as the reagent gas. Limits of detection and identification capabilities have been compared between the three analytical techniques. The sensitivity of the three analytical techniques was studied and LOD between 0.05 and 1, between 0.09 and 2 and between 0.001 and 0.043 were achieved for PCI, EI and HCI, respectively. The developed method was applied in samples from a biodesulfurization process. The biodesulfurization reactions were conducted in resting cell operation mode, using Erlenmeyer flasks or an agitated tank bioreactor. The microorganism employed was Pseudomonas putida CECT 5279. The reaction was performed under controlled air flow, stirring and temperature conditions.

Impact of cell design and operating conditions on the performances of SOFC fuelled with methane

An in-house-model has been developed to study the thermal and electrochemical behaviour of a planar SOFC fed directly with methane and incorporated in a boiler. The usual Ni-YSZ cermet has been considered for the anode material. It has been found that methane reforming into hydrogen occurs only at the cell inlet in a limited depth within the anode. A sensitivity analysis has allowed establishing that anode thicknesses higher than ∼400–500 μm are required to achieve both the optimal methane conversion and electrochemical performances. The direct internal reforming (DIR) mechanisms and the impact of operating conditions on temperature gradients and SOFC electrical efficiencies have been investigated considering the anode supported cell configuration. It has been shown that the temperature gradient is minimised in the autothermal mode of cell operation. Thermal equilibrium in the stack has been found to be strongly dependent on radiative heat losses with the stack envelope. Electrochemical performance and cell temperature maps have been established as a function of methane flow rates and cell voltages.

Mass transfer coefficients in liquid–liquid extraction

The paper concerns with mass transfer in liquid–liquid extraction, considering that the experimental determination of the individual mass transfer coefficients during liquid–liquid extraction is still considered to be a difficult problem. In this work, the liquid–liquid extraction using a ternary system, water–acetone–carbon tetrachloride, was studied. The investigations were performed in an improved Lewis cell, in continuous and batch operation modes. The immiscible components (water–carbon tetrachloride) were mixed separately, while the acetone transfer was carried out in the two directions through an interface with a constant area of the surface, created between the two phases. The specific fluxes, the driving forces and the individual and overall mass transfer coefficients were determined by measuring the inflow and outflow acetone concentrations in the two phases, together with the equilibrium data. The values of the individual mass transfer coefficients in each phase were correlated and expressed in the form of criterial equation, as Sh= bRe p Sc m.

Separation of particulate environmental samples by SPLITT fractionation using different operating modes

Procedures for gentle preseparation of particle suspensions prior to sedimentation field flow fractionation (SdFFF) analysis based on the use of Split-flow thin cell (SPLITT) fractionation (SF), were examined for isolating size fractions from both polydisperse standard silica samples and natural river particle samples. Two modes of SPLITT cell operation (conventional SF, full feed depletion SF (FFDSF)) are discussed and their performances compared. Recycling procedures for enhancing the resolution were tested and the degree of the separation attained was checked by optical microscopy and, in the case of the natural river particle samples, by coupling SdFFF with scanning electron microscopy (SEM) of the fractionated samples. The FFDSF has the advantage that no external dilution is introduced during the binary separation, so that the total sample concentration is unaffected and the separation obtained appears to be equally good for the two methods.

An improved mode of Kerr cell operation

An improved mode of Kerr cell operation

Sensitivity of Resonant Cavity and Travelling-wave E.S.R. Spectrometers†

ABSTRACT The paper discusses the three methods of using a resonant cavity for microwave spectroscopic measurements, in terms of sensitivity and the ability to discriminate between absorption and dispersion. The experimental results obtained confirm this analysis. The theory for the equivalent modes of waveguide cell operation is also derived and is extended to the travelling-wave helix which is now beginning to be used in double-resonance spectroscopy.