Z. İlsen Önsan

ONBOARD FUEL CONVERSION FOR HYDROGEN-FUEL-CELL-DRIVEN VEHICLES

Increasingly stringent legislation controls emissions from internal combustion engines to the point where alternative power sources for vehicles are necessary. The hydrogen fuel cell is one promising option, but the nature of the gas is such that the conversion of other fuels to hydrogen on board the vehicle is necessary. The conversion of methanol, methane, propane, and octane to hydrogen is reviewed. A combination of oxidation and steam reforming (indirect partial oxidation) or direct partial oxidation are the most promising processes. Indirect partial oxidation involves combustion of part of the fuel to produce sufficient heat to drive the endothermic steam reforming reaction. Direct partial oxidation is favored only at high temperatures and short residence times but is highly selective. However, indirect partial oxidation is shown to be the preferred process for all fuels. The product gases can be taken through a water–gas shift reactor, but still retain ∼2% carbon monoxide, which poisons fuel-cell catalysts. Selective oxidation is the preferred route to removal of residual carbon monoxide. Low-temperature oxidation in the absence and presence of an excess of hydrogen is reviewed. Au-based catalysts show much promise, but precious metal catalysts such as Pt/zeolite have some advantages.

Mathematical description of ethanol fermentation by immobilised Saccharomyces cerevisiae

Abstract Fermentation characteristics of Saccharomyces cerevisiae ATCC 9763 immobilised in Ca-alginate gel beads have been investigated in a stirred batch system at 2, 4, 8, 10% (w/v) initial glucose concentrations. The experimental results were tested using eleven different kinetic models relating biomass and ethanol production and glucose utilisation in different forms. The models proposed by Monod and Hinshelwood were found to be more appropriate for describing the batch growth and ethanol production of immobilised S. cerevisiae at low (2–4%) and high (8–10%) initial glucose concentrations, respectively. The validation of the models chosen was done using data obtained from experiments in an inclined reactor with nutrient recirculation.

Heterogeneous reactor modeling for simulation of catalytic oxidation and steam reforming of methane

Abstract An autothermal, dual catalyst, fixed-bed reaction system proposed for hydrogen production from methane is mathematically investigated using different catalyst bed configurations and feed ratios. Consecutive placement or physical mixture of the oxidation and reforming catalysts, Pt/ δ –Al 2 O 3 and Ni/MgO–Al 2 O 3 , respectively, are the two configurations of interest. Reactor operation at different feed ratios is analyzed for both catalyst bed configurations on laboratory scale and industrial scale via a series of simulations by using one-dimensional heterogeneous fixed-bed reactor model. The type of heterogeneous components implemented into the model is decided by checking related criteria. Hydrogen production is predicted to be higher when the catalysts are in a physically mixed state as well as at low methane-to-oxygen and high steam-to-methane ratios, which are in agreement with the experimental results reported for a bench scale integral reactor. The optimum operating conditions for obtaining maximum hydrogen production are also investigated.

Ethanol production and fermentation characteristics of recombinant Saccharomyces cerevisiae strains grown on starch

The production of ethanol from starch has been investigated in three genetically modified Saccharomyces cerevisiae strains (YPG/AB, YPG/MM, and YPB-G). Two of the three strains produce the Aspergillus awamori glucoamylase together with either the Bacillus subtilis (YPG/AB) or the mouse (YPG/MM) α-amylase as separately secreted polypeptides. YPB-G, on the other hand, secretes a bifunctional fusion protein that contains both the B. subtilis α-amylase and the A. awamori glucoamylase activities. Substrate utilization, biomass growth, and ethanol production were all studied in both starch- and glucose-containing media. Much higher growth rates were found when any of the three strains were grown on glucose. YPG/AB showed the most efficient utilization of starch for ethanol production with the lowest levels of reducing sugars accumulating in the medium. The superior performance of YPG/AB as compared to YPB-G was found to correlate with its higher level of α-amylase activity. The ethanol production levels of YPG/AB in starch- and glucose-containing media were found to be comparable. YPB-G, which secretes the bifunctional fusion protein, could produce ethanol in media with starch concentrations above 100 g l−1 while YPG/MM did not produce ethanol from starch because of its negligible secretion of glucoamylase.

On-board fuel conversion for hydrogen fuel cells: comparison of different fuels by computer simulations

Abstract The conversions of methane, propane, octane and methanol to hydrogen under conditions pertinent to fuel cell operation have been described quantitatively and examined by a series of computer simulations. Catalytic conversion may be achieved by direct partial oxidation or by a combination of total oxidation and steam reforming. Both systems have been simulated, using conversion data and kinetic equations reported in the literature for various catalyst configurations and hydrocarbons. The results show that, in terms of hydrogen produced per weight of fuel and water carried, direct partial oxidation of propane or oxidation/steam reforming of octane are the best alternatives. The latter possess the ease of operation but coke formation may be more of a problem. Methanol, often suggested as a fuel, is much less efficient. Operation of a vehicle using the catalytic conversion system would require fueling by both hydrocarbon and water.

Structure/activity relationships in coprecipitated nickel-alumina catalysts using CO2 adsorption and methanation

Abstract A series of coprecipitated Ni/Al 2 O 3 catalysts containing 0–25 wt.−% Ni were examined for total surface area, total pore volume, metal surface area, CO 2 adsorption and CO 2 methanation activity in order to study the relation between metal content, structure and catalyst activity. Coprecipitated Ni/Al 2 O 3 catalysts are found to be efficient promoters for methanation. Methanation activity is dependent on the nickel content and the degree of CO 2 adsorption at the reaction considered. Although Al 2 O 3 does not exhibit methanation activity, it is found to be active for CO 2 adsorption. Reverse spillover increases methane production per unit nickel surface particularly for catalysts with low Ni loadings.

Improvement of ethanol production from starch by recombinant yeast through manipulation of environmental factors

The production of ethanol from starch has been investigated in a genetically modified Saccharomyces cerevisiae strain, YPB-G, which secretes a bifunctional fusion protein that contains both the Bacillus subtilis α-amylase and the Aspergillus awamori glucoamylase activities. The effects of a number of environmental factors on starch degradation, ethanol production, and plasmid stability have been assessed in batch culture. These include initial glucose supply, colony selection methodology prior to inoculation, and medium formulation. Cultures containing 40 g/l starch were observed to degrade starch effectively and produce higher amounts of ethanol in shorter periods. The provision of glucose in the growth medium during the early phases of fermentation resulted in faster growth and higher ethanol productivities. YE-Salts medium was found to support plasmid-containing cells throughout the whole fermentation; only 15% of the recombinant cells had lost the plasmid content by the end of the fermentation of 120 h. Fed-batch cultures produced high yields of ethanol on starch (0.46 g ethanol/g substrate) through the longer production period.

Effect of carbon source perturbations on transcriptional regulation of metabolic fluxes in Saccharomyces cerevisiae

BackgroundControl effective flux (CEF) of a reaction is the weighted sum of all fluxes through that reaction, derived from elementary flux modes (EFM) of a metabolic network. Change in CEFs under different environmental conditions has earlier been proven to be correlated with the corresponding changes in the transcriptome. Here we use this to investigate the degree of transcriptional regulation of fluxes in the metabolism of Saccharomyces cerevisiae. We do this by quantifying correlations between changes in CEFs and changes in transcript levels for shifts in carbon source, i.e. between the fermentative carbon source glucose and nonfermentative carbon sources like ethanol, acetate, and lactate. The CEF analysis is based on a simple stoichiometric model that includes reactions of the central carbon metabolism and the amino acid metabolism.ResultsThe effect of the carbon shift on the metabolic fluxes was investigated for both batch and chemostat cultures. For growth on glucose in batch (respiro-fermentative) cultures, EFMs with no by-product formation were removed from the analysis of the CEFs, whereas those including any by-products (ethanol, glycerol, acetate, succinate) were omitted in the analysis of growth on glucose in chemostat (respiratory) cultures. This resulted in improved correlations between CEF changes and transcript levels. A regression correlation coefficient of 0.60 was obtained between CEF changes and gene expression changes in the central carbon metabolism for the analysis of 5 different perturbations. Out of 45 data points there were no more than 6 data points deviating from the correlation. Additionally, up- or down-regulation of at least 75% of the genes were in qualitative agreement with the CEF changes for all perturbations studied.ConclusionThe analysis indicates that changes in carbon source are associated with a high degree of hierarchical regulation of metabolic fluxes in the central carbon metabolism as the change in fluxes are correlating directly with the change in transcript levels of genes encoding their corresponding enzymes. For amino acid biosynthesis there was, however, not found to exist a similar correlation, and this may point to either post-transcriptional and/or metabolic regulation, or be due to the absence of a direct perturbation on the amino acid pathways in these experiments.

Integrative investigation of metabolic and transcriptomic data

BackgroundNew analysis methods are being developed to integrate data from transcriptome, proteome, interactome, metabolome, and other investigative approaches. At the same time, existing methods are being modified to serve the objectives of systems biology and permit the interpretation of the huge datasets currently being generated by high-throughput methods.ResultsTranscriptomic and metabolic data from chemostat fermentors were collected with the aim of investigating the relationship between these two data sets. The variation in transcriptome data in response to three physiological or genetic perturbations (medium composition, growth rate, and specific gene deletions) was investigated using linear modelling, and open reading-frames (ORFs) whose expression changed significantly in response to these perturbations were identified. Assuming that the metabolic profile is a function of the transcriptome profile, expression levels of the different ORFs were used to model the metabolic variables via Partial Least Squares (Projection to Latent Structures – PLS) using PLS toolbox in Matlab.ConclusionThe experimental design allowed the analyses to discriminate between the effects which the growth medium, dilution rate, and the deletion of specific genes had on the transcriptome and metabolite profiles. Metabolite data were modelled as a function of the transcriptome to determine their congruence. The genes that are involved in central carbon metabolism of yeast cells were found to be the ORFs with the most significant contribution to the model.

Ignition Characteristics of Pt, Ni and Pt-Ni Catalysts Used for Autothermal Fuel Processing

Oxidation of propane and n-butane over supported Pt, Ni and Pt-Ni catalysts was studied under fuel-rich conditions. Light-off temperatures followed the order of propane > n-butane and of Ni > Pt-Ni > Pt and were found to have minimum values at optimal fuel:oxygen ratios over Pt-Ni. The bimetallic Pt-Ni catalyst is likely to involve (i) synergistic interactions between the two metals and (ii) pronounced effect of Pt metal during surface ignition. Differences in oxidation activities of Pt and Pt-Ni catalysts seem to be related to the degree of dispersion of Pt metal. For both hydrocarbons, coke formation was not observed under the conditions employed.