No abstract

ABSTRACT:The adsorption of simple Lennard-Jones fluids in a carbon slit pore of finite length was studied with Canonical Ensemble (NVT) and Gibbs Ensemble Monte Carlo Simulations (GEMC). The Canonical Ensemble was a collection of cubic simulation boxes in which a finite pore resides, while the Gibbs Ensemble was that of the pore space of the finite pore. Argon was used as a model for Lennard-Jones fluids, while the adsorbent was modelled as a finite carbon slit pore whose two walls were composed of three graphene layers with carbon atoms arranged in a hexagonal pattern. The Lennard-Jones (LJ) 12-6 potential model was used to compute the interaction energy between two fluid particles, and also between a fluid particle and a carbon atom.Argon adsorption isotherms were obtained at 87.3 K for pore widths of 1.0, 1.5 and 2.0 nm using both Canonical and Gibbs Ensembles. These results were compared with isotherms obtained with corresponding infinite pores using Grand Canonical Ensembles. The effects of the number of cycles necessary to reach equilibrium, the initial allocation of particles, the displacement step and the simulation box size were particularly investigated in the Monte Carlo simulation with Canonical Ensembles. Of these parameters, the displacement step had the most significant effect on the performance of the Monte Carlo simulation. The simulation box size was also important, especially at low pressures at which the size must be sufficiently large to have a statistically acceptable number of particles in the bulk phase. Finally, it was found that the Canonical Ensemble and the Gibbs Ensemble both yielded the same isotherm (within statistical error); however, the computation time for GEMC was shorter than that for canonical ensemble simulation. However, the latter method described the proper interface between the reservoir and the adsorbed phase (and hence the meniscus).

Solid chars were prepared from coconut shell at different carbonization temperatures in the range from 250-750°C and gasified in a thermogravimetric analyzer under the atmosphere of carbon dioxide at 850 o C. The kinetic analysis showed an accelerating increasing of char conversion with reaction time, indicating an increase in the instantaneous gasification rate as the reaction proceeded. Four kinetic models for gas-solid reactions including, the volume-reaction model (VRM), the shrinking-core model (SCM), the random-pore model (RPM) and the modified volume-reaction model (MVRM) were tested against the measured kinetic data and the MVRM was found to predict the gasification kinetics most accurately. The char reactivity index was computed from the apparent rate constant of the MVRM and used to assess the reactivity of char towards carbon dioxide gasification. It was found that the char reactivity index decreased with increasing carbonization temperature, with the char produced at the lowest temperature of 250°C giving the highest reactivity. Surface area of activated carbon, produced from the gasification of various chars at 850°C for 60 and 120 min, correlated well with the char reactivity index, showing a proportional increasing of surface area with increasing reactivity index and passing through a maximum near the reactivity index of 0.02 min-1 .

In this study, a Grand Canonical Monte Carlo simulation (GCMC) method is used to study the adsorption of different probe molecules on activated carbon, while the experimental tests are performed by using a Gravimetric Analyzer. In addition the simulation results together with the measured isotherm data are used for the determination of micropore size distribution. Nitrogen at 77 K and carbon dioxide at 273 and 300 K are proposed as molecular probes. The simulation results obtained for various pore sizes represent the structure of molecular probe packing in the individual pores at different pressures. The reconstructed adsorption isotherm obtained by using these results and a postulated pore size distribution (PSD) function is used to determine the PSD of activated carbon which provides the best match between the simulation isotherm and the experimental isotherm. The PSD obtained using the GCMC agrees very well with the Density Functional Theory (DFT) method. The PSD for carbon dioxide differs from that for nitrogen due to the molecular structure and size. The advantage of GCMC is that it can provide not only adsorption isotherm but also the snapshot that presents the mechanism inside the pore

No abstract