Abhash Nigam

Molecular representation of complex hydrocarbon feedstocks through efficient characterization and stochastic algorithms

Abstract A stochastic method was devised to transform efficient sets of analytical characterizations into molecular representations of complex petroleum feedstocks. Important structural attributes of petroleum molecules (e.g. number of aromatic rings, number of naphthenic rings, number and length of aliphatic side chains) were assembled into molecules according to quantitative probability density functions for each attribute. The outcome was the atomic detail of a large ensemble of representative molecular structures from which both molecular and global product properties were deduced. Critical steps in the stochastic method were the generation of a chemical logic diagram, the compilation of cumulative probability functions for the structural attributes, stochastic sampling of each distribution, and the molecular construction. A general Monte Carlo algorithm provided an unbiased sampling of the probability density functions. The method was applied to three different complex petroleum feedstock fractions: an offshore California asphaltene, a Kern River heavy oil, and sour import heavy gas oil. The asphaltene example predicted the defining solubility protocol to within 1%. The heavy oil and gas oil simulations reproduced boiling point fractionation curves to within standard deviations of 20.8 and 25°C.

Monte Carlo simulation of complex reaction systems : molecular structure and reactivity in modelling heavy oils

Abstract The problem of asphaltene precipitation during their thermolysis in a hypothetical inert oil motivated development of a stochastic model of asphaltene structure, reactions, and product identification. This amounted to stochastic construction of 10,000 asphaltene molecules as a representation of those found in a real resid. The reactions of each molecule, deduced from related model compound reaction pathways and kinetics, provided the reaction of the asphaltene representation after Monte Carlo simulation. A regular solution theory based thermodynamic model assembled the 10,000+ product molecules into global solubility-based product fractions. The simulation data were in good agreement with laboratory data for an off-shore California crude-derived asphaltene.

Asphaltene and resid pyrolysis: Effect of reaction environment

ABSTRACT Hondo and Maya vacuum resids and their isolated asphaltenes were pyrolyzed at 400, 425, and 450°C (752, 797, 842°F) for batch holding limes ranging from 20 to 180 minutes. Maltene, asphaltene, and coke product fractions were isolated by a solvent extraction sequence; gas yield was determined gravimetrically. Results were summarized in terms of a lumped reaction network. The variation of product yields, kinetics, and apparent activation energies with feedstock and asphaltene environment provided insight into asphaltene structure and thermal reaction pathways.

Mechanism based lumping of pyrolysis reactions: Lumping by reactive intermediates

Abstract A mechanism-oriented kinetic model has been developed with a small CPU requirement which allows the model to be incorporated into a reactor simulation with fluid dynamics and heat and mass transfer. The model achieves a small CPU requirement by lumping rdicals with similar reactivity together. A 42-lump subset of the 10 5 or more radicals is used to describe all the elementary reactions with a high degree of accuracy. Structure/reactivity relationships are utilized to provide rate constants for these elementary steps. The model predictions are compared to the results of full mechanistic simulations and the reaction of pure and synthetic mixtures of model compounds.

Semiempirical rate laws for rice-Herzfeld pyrolysis of mixtures : capturing chemistry with reasonable computational burden

Mechanism-derived rate laws for kinetically coupled Rice-Herzfeld pyrolysis were used to deduce the form of semiempirical rate laws (SERLs) that nevertheless represent the mechanistic chemistry. These SERLs strike a balance between the CPU demands of mechanistic models and the lack of chemical significance of purely empirical models. The mechanism-derived pyrolysis rate laws were phrased in terms of a pure component, initiation, propagation, and termination groups, akin to the kinetic term, the driving force, the adsorption group, and exponent of Langmuir-Hinshelwood-Hougen-Watson models