Asit Kumar Das

Effect of surface nonuniformity on the kinetics of simultaneous adsorption of SO2-NOx over Na-γ-Al2O3 sorbent: a coverage-dependent stoichiometry

Abstract The effect of nonuniformity of the surface of Na–γ-Al2O3 on the kinetics of surface reactions is investigated for the simultaneous adsorption of NO, O2 and SO2. A nonuniform kinetic model is developed by considering the variations in the rate coefficients of important surface reactions with species coverages. Experimental data from a transient fixed bed microreactor and a steady-state riser reactor are used for the estimation of model parameters. The model explains the large variation in the NO removal in the above two reactors by the difference in the coverage of the sulfite species SO 2 ∗ and accounting for the nonuniformity in the adsorption and surface reactions. The SO 2 ∗ species is produced from the primary adsorption of gas phase SO2 on a free site. The adsorption of NO occurs first on a sulfate species SO 2 ∗∗ , resulting from the interaction of SO 2 ∗ with a free site. Subsequently, a complex formation occurs in multiple series-parallel steps involving many SO 2 ∗ and O2 per mole of NO. In the nonuniform model, the rate of the series-parallel steps and hence the average stoichiometry of the complexes depend strongly on the degree of SO 2 ∗ coverage. The calculated average number of SO 2 ∗ entities in the complex is much smaller at the riser (2–3 moles/mole of NO) than at the fixed bed conditions (9 moles/mole of NO). A lower consumption of SO 2 ∗ in the complex allows a higher rate of formation of SO 2 ∗∗ , leading to a higher NO removal as observed in the riser. There is an optimum inlet SO2/NO ratio of 2.5 at which maximum NO removal in the riser is achieved.

Transient 3D Simulation of the Turbulent Gas Solid Flow in Large Scale Risers Using a Density Based Solution Algorithm

Transient 3D simulation of the gas-solid flow in large diameter (> 0.3 m) risers is performed using a new density based solution algorithm. Unlike the conventional pressure based algorithm used so far for riser simulation, the density based solution method uses the pre-conditioning of time derivatives, does not have the internal pressure and velocity correction loop and hence provides a much faster convergence speed. The two phase flow is highly oscillatory with many small scale high frequency (10Hz) fluctuations and a few dominating low frequency (0.02 Hz) oscillations. The transient simulation of a case belonging to a fast fluidized regime with Geldart B particles, demonstrated the density inversion phenomena, experimentally observed before. Cluster formation is simulated on a real time scale for a dilute phase riser having solid fraction < 0.0007, although the time averaged flow fields still resemble a core-annular flow structure.Copyright

Effect of reactor configuration on performance of vacuum gas oil (VGO) hydrotreater: Modelling studies

Abstract While kinetics is independent of either scale or configuration of hydrotreating reactor, hydrodynamics of reactor depend on both. The hydrodynamics of reactor which comprise of phase mass-transfer, catalyst wetting and pressure drop affect its performance significantly and should be addressed adequately while deciding on configuration or scale up issues. This study evaluates and compares the performance of different configurations of commercial VGO hydrotreater by employing mathematical model encompassing kinetics and hydrodynamics. Two configurations have been studied: • Conventional trickle-bed reactor, subdivided into (a) beds in parallel and (b) beds in series. • Pre-saturated one liquid flow (POLF) reactor, subdivided into (a) reactor with single pre-saturator (POLF-SP) and (b) multiple reactors in series with intermittent pre-saturators (POLF-MP). The performance of conventional reactor in series/parallel is found to be superior to POLF configurations. The inferior performance of POLF configurations is attributed to mixed flow behaviour due to recycle as against plug flow behaviour in conventional trickle-bed reactors.