Semih Eser

Effects of activation method on the pore structure of activated carbons from apricot stones

Abstract Two series of activated carbons were prepared from apricot stones by using carbonization followed by steam activation and one-step pyrolysis/activation in steam. The pore structure of the activated carbons was characterized by CO 2 adsorption at 273 K and by N 2 adsorption at 77 K. The macro- and mesoporosity were determined by mercury porosimetry. Optical microscopy, scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM) were used to examine the microstructure of activated carbons. A considerable difference observed between the measured CO 2 and N 2 surface areas indicates a narrow microporosity in all the carbons. The two-step method produced better developed meso- and especially macroporosity. The SEM and optical micrographs show that one-step pyrolysis/steam activation preserves the original cell structure of the raw apricot stones. In general, the apparent surface areas of the two-step carbons are slightly higher than those of the one-step carbons. However, the one-step method would provide lower production costs because it would eliminate the separate carbonization stage.

Carbonization of petroleum feedstocks I: Relationships between chemical constitution of the feedstocks and mesophase development

Abstract The relationships between the chemical constitution of the petroleum feedstocks and the mesophase development were investigated. A range of coker feedstocks that were characterized by solvent fractionation, elemental analysis, and FT-IR were carbonized in conventional tubing bomb reactors. The solid carbonization products were examined by polarized-light microscopy. In general, a direct relationship was observed between the degree of aromaticity of the feedstocks and the extent of mesophase development. The separate carbonization of the asphaltene and maltene fractions of the feedstocks indicated that the asphaltenes dominated in determining the optical textures of the semicokes produced from the petroleum feedstocks.

Carbonization of coker feedstocks and their fractions

Abstract Carbonization studies of petroleum vacuum distillation residua have been carried out on a laboratory scale with the objective of developing a better understanding of the causes and mechanisms of the incidence of shot coke formation in delayed coking. The rates of carbonization of the residua are found to inversely relate to the extent of mesophase development in the resulting semicoke. The asphaltenic fractions of the petroleum stocks carbonized most rapidly and produced cokes containing small anisotropic regions (fine mosaics), similar to that of commercially produced shot coke and consistent with arrested mesophase development. The addition of aromatic petroleum materials enhanced mesophase development commensurate with their reducing the rate of carbonization. While the microstructural features of shot coke are reproducibly produced in the laboratory, the characteristic spherical morphology is not and appears to be determined by the conditions obtaining in large-scale operation. It is suggested that this morphology may arise from the segregation of regions of rapidly carbonizing constituents from a less reactive matrix.

Kinetics, in situ X-ray diffraction and environmental scanning electron microscopy of activated charcoal gasification catalyzed by vanadium oxide, molybdenum oxide and their eutectic alloy

Mechanisms of carbon oxidation catalyzed by vanadium pentaoxide (V2O5) and molybdenum trioxide (MoO3) have been a subject of controversy. Two complementary in situ techniques, X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM), were used in this work to study the gasification of an activated charcoal catalyzed by the two metal oxides, their eutectic alloy and the binary mixture with the eutectic composition. Gasification experiments were carried out at relatively low temperatures (300–650 °C) in an XRD cell (1 atm) and ESEM (2.2 Torr) to monitor phase transformations and morphological changes of oxide catalysts, respectively. The experimental results showed that MoO3 and V2O5 particles in contact with active carbon surfaces are reduced to oxides with lower oxidation states, e.g. MoO2 and V6O13, respectively. The reduction of MoO3 to MoO2 on the carbon surface prevents the sublimation of MoO3 which takes place readily on a quartz surface under the same conditions. The formation of V6O13, on the other hand, causes more extensive spreading of the catalyst on carbon surfaces, since V6O13 has a lower melting point than V2O5. Based on the XRD and ESEM observations, it is clear that phase transformations of metal oxides during gasification depend on their interactions with carbon surfaces. The phase transformations of the metal oxides, play, in turn, a significant role in carbon gasification, as evident from the kinetic data obtained for the catalytic gasification of the activated charcoal sample. The synergy observed between the components of the eutectic mixture is discussed, comparing the XRD and ESEM observations.

Mesophase and pyrolytic carbon formation in aircraft fuel lines

Exposure of jet fuel to high temperatures in aircraft fuel lines triggers pyrolysis reactions which eventually lead to deposition of carbonaceous solids on metal surfaces. This is a particularly important problem for advanced future aircraft which may expose fuel to very high temperatures. Different optical textures were observed in samples of deposits formed in different sections of aircraft fuel systems. Deposits from a burner fuel line consist only of pyrolytic carbon, indicating that gas phase reactions were responsible for solid formation. Afterburner line deposits, on the other hand, contain both pyrolytic carbon and carbonaceous mesophase structures, implicating also liquid phase carbonization reactions. The FTIR data shows that solids containing mesophase consist of polynuclear aromatic hydrocarbons with a low degree of condensation and alkyl substitution. In contrast, solids with pyrolytic carbon structure are composed of highly condensed, large polyaromatic species. It is clear that jet fuels go through extensive cracking, aromatization, and aromatic polymerization reactions before solid deposition takes place in the fuel lines.

Carbonaceous Mesophase Formation and Molecular Composition of Petroleum Feedstocks

Carbonaceous mesophase forms during carbonization of some organic compounds, thermoplastic polymers, petroleum feedstocks, and certain coals and coal-derived materials. It is an intermediate, anisotropic phase with properties similar to those of nematic liquid crystals. The formation of this plastic phase at relatively low temperatures (400–500°C) is critically important in determining the properties (e.g. strength, reactivity, and conductivity) of carbon materials (e.g. graphite electrodes) derived from the resulting semi-cokes or cokes. A study has been undertaken to relate the molecular composition of petroleum feedstocks and the degree of mesophase development during carbonization. Gas chromatography/mass spectrometry and high performance liquid chromatography with a photodiode array detector were used to analyze the molecular composition of fluid catalytic cracking (FCC) decant oils and thermal tars from gas oils. Automated manual point counting techniques were used in conjunction with polarized-light microscopy to measure the degree of mesophase development, expressed as the optical texture index of the resulting cokes. Correlations were observed between the optical texture indices of cokes and the molecular composition of the feedstocks. For example, high concentrations of methylated pyrenes in the feedstocks appear to lead to a high degree of mesophase development which produces highly anisotropic cokes, i.e. high quality needle cokes.

High temperature swelling of coal/tetralin mixtures in a high pressure microdilatometer

High pressure microdilatometer experiments were performed on a subbituminous (Wyodak) and a bituminous (Illinois no. 6) coal in helium and hydrogen atmospheres with and without added tetralin. Wyodak coal samples showed no swelling but contractions ranging between 24 and 40 vol% upon heating at 20 and 100 °C min− 1 under helium or hydrogen pressures between 150 and 1000 psig (~1.0–6.9 MPa). Under the same conditions, Illinois no. 6 coals displayed contractions (25–60 vol%) prior to swelling up to 117 vol%. Upon tetralin addition (at 35–190 wt% of the coal), Wyodak coal samples did not swell but showed an increasing contraction with increasing helium or hydrogen pressure due to a slight softening and fusion of the coal particles. In contrast, addition of tetralin at much lower concentrations (5–35 wt%) had a marked effect on the contraction and swelling behaviour of Illinois no. 6. A maximum swelling of 200 vol% was obtained at a tetralin addition of 30 wt%. The increased swelling results from more extensive softening and fusion of coal particles in the presence of tetralin. Both coals showed a decreasing char yield with increasing tetralin concentration. The substantially lower extent of interaction observed between Wyodak coal samples and tetralin compared to Illinois no. 6 coal can be attributed to the differences in pore structure and/or chemical constitution of the two coal samples. Examination of the resultant solids by optical microscopy revealed the microstructural changes produced by thermal treatment in dilatometer experiments.