Single-event kinetic modeling of the hydrocracking of hydrogenated vacuum gas oil

The primary objective of the research project was to further develop a computer program modeling the hydrocracking of partially hydrogenated vacuum gas oil (HVGO), and to use the model to compare the theoretical product distribution to experimental data describing the product distribution of an industrial pilot reactor. The hydrocracking of HVGO on acid zeolites is effectively modeled utilizing a single-event kinetic approach developed by Froment and coworkers. The hydrocracking of HVGO can be described in terms of the fundamental reaction steps involving carbenium ions. Some 45 single-event rate parameters are used to dictate the rate of each single-event in the reaction network. The composition of the partially hydrogenated feed stock is detailed up to C33. Each component and lump is considered in terms of the elementary steps to generate a network of continuity equations and single-event rate parameters. A reactor model comprising this kinetic model can be used to simulate the isothermal and nonisothermal hydrocracking of a HVGO feed stock. The results are represented in terms of the yields of 241 lumps and components in the gas phase and 241 components and lumps in the liquid phase. The predicted yields of various commercial oil fractions and particular components are then compared to experimental data from an industrial pilot reactor to verify the accuracy of the model and the single-event rate parameters

Main-chain polybenzoxazine nanofibers via electrospinning

Cataloged from PDF version of article.Here we report the successful production of nanofibers from main-chain polybenzoxazines (MCPBz) via electrospinning without using any other carrier polymer matrix. Two different types of MCPBz (PBA-ad6 and PBA-ad12) were synthesized by using two types of difunctional amine (1,6-diaminohexane and 1,12-diaminododecane), bisphenol-A, and paraformaldehyde as starting materials through a Mannich reaction. 1H NMR and FTIR spectroscopy studies have confirmed the chemical structures of the two MCPBz. We were able to obtain highly concentrated homogeneous solutions of the two MCPBz in chloroform/N,N-dimethylformamide (DMF) (4:1, v/v) solvent system. The electrospinning conditions were optimized in order to produce bead-free, uniform and continuous nanofibers from these two MCPBz by varying the concentrations of PBA-ad6 (30–45%, w/v) and PBA-ad12 (15–20%, w/v) in chloroform/DMF (4:1, v/v). The bead-free fiber morphology was evidenced under SEM imaging when PBA-ad6 and PBA-ad12 were electrospun at solution concentration of 40% and 18% (w/v), respectively. The nanofibrous mats of MCPBz were obtained as free-standing material, yet, PBA-ad12 mat was more flexible than and PBA-ad6 mat. Furthermore, the curing studies of these MCPBz nanofibrous mats were performed to obtain cross-linked materials

Domain Wall Depinning in Random Media by AC Fields

The viscous motion of an interface driven by an ac external field of frequency omega_0 in a random medium is considered here for the first time. The velocity exhibits a smeared depinning transition showing a double hysteresis which is absent in the adiabatic case omega_0 --> 0. Using scaling arguments and an approximate renormalization group calculation we explain the main characteristics of the hysteresis loop. In the low frequency limit these can be expressed in terms of the depinning threshold and the critical exponents of the adiabatic case.Comment: 4 pages, 3 figure

Sensitive surface states and their passivation mechanism in CdS quantum dots

Cataloged from PDF version of article.We report on phase sensitive surface states of CdS quantum dots (QDs), where it is noticed that a simple phase change from dispersion to solid has shown significant influence on the emission spectrum. As the solvent evaporates from the dispersion, apparently yellow dispersion transforms into a white light emitter because of the conformal changes in the polymer that surrounds the QDs. In turn, these changes catalyze the emission from three specific wavelengths in the blue region of the spectrum, shifting the surface defects closer to the conduction band of CdS. In the phase change from dispersion to solid, flexible and dangling polymer chains are transformed into rigid moieties that can be treated as a modified chemical environment. Furthermore, to ascertain the origin of the new emission lines, we have studied a dipole interaction-based passivation mechanism between QDs and the polymer. The proposed mechanism may be valuable for designing future QD-based fluorophores and explains the sensitivity of the surface states in the case of CdS

Low temperature vortex phase diagram of Bi2Sr2CaCu2O8 : a magnetic penetration depth study

We report measurements of the magnetic penetration depth \lambda_m(T) in the presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals. Warming, after magnetic field is applied to a zero-field cooled sample, results in a non-monotonic \lambda_m(T), which does not coincide with a curve obtained upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the possible relation of our results to the vortex decoupling, unbinding, and dimensional crossover.Comment: M2S-HTSC-V

Fabrication of cellulose acetate/polybenzoxazine cross-linked electrospun nanofibrous membrane for water treatment

Herein, polybenzoxazine based cross-linked cellulose acetate nanofibrous membrane exhibiting enhanced thermal/mechanical properties and improved adsorption efficiency was successfully produced via electrospinning and thermal curing. Initially, suitable solution composition was determined by varying the amount of the benzoxazine (BA-a) resin, cellulose acetate (CA) and citric acid (CTR) to obtain uniform nanofibrous membrane via electrospinning. Subsequently, thermal curing was performed by step-wise at 150, 175, 200 and 225 °C to obtain cross-linked composite nanofibrous membranes. SEM images and solubility experiments demonstrated that most favorable result was obtained from the 10% (w/v) CA, 5% (w/v) BA-a and 1% (w/v) CTR composition and cross-linked nanofibrous membrane (CA10/PolyBA-a5/CTR1) was obtained after the thermal curing. Chemical structural changes (ring opening) occurred by thermal curing revealed successful cross-linking of BA-a in the composite nanofibrous membrane. Thermal, mechanical and adsorption performance of pristine CA and CA10/PolyBA-a5/CTR1 nanofibrous membranes were studied. Char yield of the pristine CA nanofibrous membrane has increased notably from 12 to 24.7% for composite CA10/PolyBA-a5/CTR1 membrane. When compared to pristine CA membrane, CA10/PolyBA-a5/CTR1 nanofibrous membrane has shown superior mechanical properties having tensile strength and Young's modulus of 8.64 ± 0.63 MPa and 213.87 ± 30.79 MPa, respectively. Finally, adsorption performance of pristine CA and CA10/PolyBA-a5/CTR1 nanofibrous membranes was examined by a model polycyclic aromatic hydrocarbon (PAH) compound (i.e. phenanthrene) in aqueous solution, in which CA10/PolyBA-a5/CTR1 nanofibrous membrane has shown better removal efficiency (98.5%) and adsorption capacity (592 μg/g). © 2017 Elsevier Lt

Cross-linked main-chain polybenzoxazine nanofibers by photo and thermal curing; Stable at high temperatures and harsh acidic conditions

In this study, for the first time cross-linking of linear aliphatic diamine-based main-chain polybenzoxazine (MCPBz) electrospun nanofibers were accomplished by two-step approach consisting of photo and thermal curing. Initially, two novel MCPBz resins which comprise of a benzophenone unit in the polymer main-chain were synthesized and uniform MCPBz nanofibers were produced by electrospinning. At first step, photo curing was performed by free radical polymerization initiated by UV-light and thermal stability of nanofibers was enhanced. At second step, thermal curing was carried out at different temperatures (150-225 °C) and ring opening and cross-linking of benzoxazine groups in the fiber structure were achieved. After two-step curing, cross-linked MCPBz nanofibers were obtained as free-standing material with good mechanical properties. Moreover, it was shown that these two cross-linked MCPBz nanofibers were structurally stable and maintained their fibrous morphology at high temperatures (400 °C), in good solvents (chloroform, DMF, 1,4-dioxane, DMAc, THF) and highly concentrated strong acids (HCl, HNO3, H2SO4). © 2015 Elsevier Ltd. All rights reserved

Melting of Flux Lines in an Alternating Parallel Current

We use a Langevin equation to examine the dynamics and fluctuations of a flux line (FL) in the presence of an {\it alternating longitudinal current} $J_{\parallel}(\omega)$. The magnus and dissipative forces are equated to those resulting from line tension, confinement in a harmonic cage by neighboring FLs, parallel current, and noise. The resulting mean-square FL fluctuations are calculated {\it exactly}, and a Lindemann criterion is then used to obtain a nonequilibrium `phase diagram' as a function of the magnitude and frequency of $J_{\parallel}(\omega)$. For zero frequency, the melting temperature of the mixed phase (a lattice, or the putative "Bose" or "Bragg Glass") vanishes at a limiting current. However, for any finite frequency, there is a non-zero melting temperature.Comment: 5 pages, 1 figur

Polybenzoxazine-Based Nanofibers by Electrospinning

In this chapter recent progress in the production of polybenzoxazine-based nanofibrous mats by electrospinning is highlighted. The benzoxazine monomers could easily form thermosetting polybenzoxazines by in situ thermally initiated ring-opening polymerization, hence, they are promising materials for both the surface modification of polymeric nanofibrous mats and the production of polybenzoxazine-based composite nanofibers. After curing, polybenzoxazines provide hydrophobic characteristic for the modified polymeric nanofiber surfaces by in situ polymerization of the benzoxazines because of their highly cross-linked structure. Also, they allow for the further functionalization of the surfaces as superhydrophobic and superleophilic by the incorporation of SiO2 nanoparticles into the benzoxazine solution. In addtion, benzoxazine monomers could be directly added into electrospinning solutions and the thermal curing of the obtained nanofibrous mat could yield hydrophobic composite nanofibers. Moreover, because of outstanding properties, such as near-zero volumetric change upon curing, low water absorption, high glass transition temperature, high char yield, and no by-products without any catalysts during curing, polybenzoxazines are good a candidate as a precursor for the production of carbon nanofibers. In addition, the molecular structure of polybenzoxazines facilitates immense design flexibility, which enables the tailoring of the properties of the cured material. Therefore, suitable polybenzoxazines can be synthesized, and cross-linked polybenzoxazine nanofibers, with enhanced thermal and mechanical properties, can be obtained by electrospinning without the blending of other polymers. By combining the unique properties of nanofibers and the facinating properties of polybenzoxazines, highly cross-linked polybenzoxazine-based nanofibrous mats can be obtained and these materials are quite useful, especially in filtration applications. © 2017 Elsevier Inc. All rights reserved

Enhanced thermal stability of eugenol by cyclodextrin inclusion complex encapsulated in electrospun polymeric nanofibers

Polyvinyl alcohol (PVA) nanofibers encapsulating eugenol (EG)/cyclodextrin (CD) inclusion complexes (IC) (EG/CD-IC) were produced via electrospinning technique in order to achieve high thermal stability and slow release of EG. In order to find out the most favorable CD type for the stabilization of EG, three types of native cyclodextrins (α-CD, β-CD, and γ-CD) were used for the formation of EG/CD-IC. In the case of PVA/EG/α-CD nanofibers, uncomplexed EG was detected indicating that α-CD is not a proper host for EG/CD-IC formation. However, for PVA/EG/β-CD-IC and PVA/EG/γ-CD-IC nanofibers, enhanced durability and high thermal stability for EG were achieved due to the inclusion complexation. The electrospun nanofibers encapsulating CD-IC of active compounds such as eugenol may be quite useful in the food industry due to the extremely large surface area of nanofibers along with specific functionality, enhanced thermal stability, and slow release of the active compounds by CD inclusion complexation. © 2013 American Chemical Society