Maximum Carbon Yield Research Articles

Optimization and scale-up of CNF production based on intrinsic kinetic data obtained from TEOM

Optimizing the operating parameters for growing carbon nanofibers (CNFs) is an important step toward the large-scale production of CNFs with a high degree of structure control. The present work demonstrates that a tapered-element oscillating microbalance (TEOM) is an excellent tool for the kinetic study of the highly dynamic process of CNF growth. Initial CNF growth rates, deactivation rates, and yields can be studied simultaneously as a function of temperature, pressure, hydrogen partial pressure, or residence time. The kinetic data from TEOM studies are directly implemented to scale up the production of CNF. For a hydrotalcite (HT)-derived Ni catalyst (77 wt% Ni), the highest CNF growth rate was obtained at a temperature of around 580 °C and a hydrogen partial pressure of around 0.1 bar. High temperatures and low partial pressures of hydrogen resulted in a high initial growth rate and deactivation rate, whereas low temperatures and high partial pressures of hydrogen resulted in low initial growth rate and deactivation rate. A carbon capacity (maximum carbon yield) of 50 g/g cat could be achieved at optimized conditions. Growth rates and CNF yields were found to increase with increasing total pressure (0.3–3.8 bar). The mechanisms of CNF growth and deactivation were explored based on kinetic data and DFT calculation studies from the literature. The results obtained from the TEOM studies were verified in a PFR fixed-bed reactor and a horizontally placed fixed-bed reactor similar to a CSTR reactor. The residence time of methane was identified as the most important parameter for scaling up the process. The data on the effect of hydrogen from the TEOM were reproduced, and a successful scale-up of 10,000 times was achieved.

Preparation of activated carbon from asphalt and heavy oil fly ash and coal fly ash by pyrolysis

Activated carbons were prepared by the pyrolysis of a mixture of fly ash and road paving asphalt dissolved in kerosene at 650 °C in N 2 atmosphere. Asphalt provides a porous carbonaceous structure and also acts as a binder to hold the fly ash particles together. The fly ash acts as a support for this carbon structure. The preparation variables studied were the pretreatment of asphalt–fly ash composite in air or N 2; and the asphalt ratio (amount of asphalt in grams added to 100 g of fly ash) of the raw mixture. The pretreatment of asphalt–fly ash composite at 250 °C in air resulted in a higher effective asphalt carbon yield than no pretreatment or pretreatment in N 2. The effective asphalt carbon yield increased with asphalt ratio up to 20 and then decreased at higher ratios. For heavy oil fly ash, the optimum asphalt ratio was 50–60 for maximum carbon yield. The specific surface areas of the asphalt and fly ash carbons were in the range of 500–800 m 2/(g-asphalt derived carbon), depending on the asphalt derived carbon content of the sample. The asphalt ratio in the raw materials also affected the macropore structure of the product carbon: the macropore diameter of the sample increased with asphalt ratio.

Kinetic Studies and Biochemical Pathway Analysis of Anaerobic Poly-( R )-3-Hydroxybutyric Acid Synthesis in Escherichia coli

Poly-(R)-3-hydroxybutyric acid (PHB) was synthesized anaerobically in recombinant Escherichia coli. The host anaerobically accumulated PHB to more than 50% of its cell dry weight during cultivation in either growth or nongrowth medium. The maximum specific PHB production rate during growth-associated synthesis was approximately 2.3 +/- 0.2 mmol of PHB/g of residual cell dry weight/h. The by-product secretion profiles differed significantly between the PHB-synthesizing strain and the control strain. PHB production decreased acetate accumulation for both growth and nongrowth-associated PHB synthesis. For instance under nongrowth cultivation, the PHB-synthesizing culture produced approximately 66% less acetate on a glucose yield basis as compared to a control culture. A theoretical biochemical network model was used to provide a rational basis to interpret the experimental results like the fermentation product secretion profiles and to study E. coli network capabilities under anaerobic conditions. For example, the maximum theoretical carbon yield for anaerobic PHB synthesis in E. coli is 0.8. The presented study is expected to be generally useful for analyzing, interpreting, and engineering cellular metabolisms.

Metabolic pathway analysis of a recombinant yeast for rational strain development.

Elementary mode analysis has been used to study a metabolic pathway model of a recombinant Saccharomyces cerevisiae system that was genetically engineered to produce the bacterial storage compound poly-beta-hydroxybutyrate (PHB). The model includes biochemical reactions from the intermediary metabolism and takes into account cellular compartmentalization as well as the reversibility/irreversibility of the reactions. The reaction network connects the production and/or consumption of eight external metabolites including glucose, acetate, glycerol, ethanol, PHB, CO(2), succinate, and adenosine triphosphate (ATP). Elementary mode analysis of the wild-type S. cerevisiae system reveals 241 unique reaction combinations that balance the eight external metabolites. When the recombinant PHB pathway is included, and when the reaction model is altered to simulate the experimental conditions when PHB accumulates, the analysis reveals 20 unique elementary modes. Of these 20 modes, 7 produce PHB with the optimal mode having a theoretical PHB carbon yield of 0.67. Elementary mode analysis was also used to analyze the possible effects of biochemical network modifications and altered culturing conditions. When the natively absent ATP citrate-lyase activity is added to the recombinant reaction network, the number of unique modes increases from 20 to 496, with 314 of these modes producing PHB. With this topological modification, the maximum theoretical PHB carbon yield increases from 0.67 to 0.83. Adding a transhydrogenase reaction to the model also improves the theoretical conversion of substrate into PHB. The recombinant system with the transhydrogenase reaction but without the ATP citrate-lyase reaction has an increase in PHB carbon yield from 0.67 to 0.71. When the model includes both the ATP citrate-lyase reaction and the transhydrogenase reaction, the maximum theoretical carbon yield increases to 0.84. The reaction model was also used to explore the possibility of producing PHB under anaerobic conditions. In the absence of oxygen, the recombinant reaction network possesses two elementary modes capable of producing PHB. Interestingly, both modes also produce ethanol. Elementary mode analysis provides a means of deconstructing complex metabolic networks into their basic functional units. This information can be used for analyzing existing pathways and for the rational design of further modifications that could improve the system's conversion of substrate into product.