A Network of Chemical Reactions for Modeling Hydrocracking Reactors
Advancing the chemical engineering fundamentals
Chemical Reaction Engineering (T2-2P)
Keywords: hydrocracking, compositional model, molecule-based kinetics
The current petroleum market exhibits a trend of gradual increase in the participation of low quality crudes characterized by high contents of sulfur and/or nitrogen, besides high carbon/hydrogen ratios. In this scenario, the technology of Hydrocracking Processes (HCC) can guarantee stringent specified urban fuels by providing huge increases of hydrogen/carbon ratio of heavy fractions as well destroying contaminant heteroatoms and enhancing yields of naphtha and middle fractions by aromatic hydrogenation, paraffin cracking and ring opening of naphthenic molecules. This technology is, on the other hand, costly because it demands high usage of hydrogen and extremely severe reactor conditions (temperature, pressure and spatial time). Thus it is not surprising that HCC plants are rare in the refinery context around the world. Inherently connected to this, modeling studies of HCC are also scarce. Plausible reasons derive from a large set of theoretical obstacles that characterize HCC like: (i) complex (true) chemical description of HCC feeds; (ii) complex chain of chemical transformations involved and associated chemical reaction network; (iii) complex behavior of reaction rates; and (iv) complex hydrodynamic, kinetic and thermal effects through the reactor.
In this work we study some of the phases involved in the development of a HCC reactor model within a molecular-structure-based approach.
Phase 1 considers a chemical description of HCC feeds. We use a discrete compositional model for a pre-hydrotreated heavy vacuum gasoil which constitutes a typical feed of a hydrocracking bed in the second stage of a HCC process. A set of hydrocarbon families is formulated to cover relevant functional molecular sub-structures quantifiable by analytical procedures of feedstocks and products. Each family has parameters defining its concentration and molecular weight distribution, and is complemented by a framework of rules for generation of molecular structures belonging to it. Feed parameters were estimated by reconciliating property predictions with available characterizing data.
Phase 2 is concerned with the HCC reactions network and the corresponding kinetic mechanisms. Empirical kinetic rules from the Literature were applied for proposing a HCC reaction network adopting molecule-based kinetics. Reactions rates were modeled according to several mechanisms involving gas-liquid equilibrium and adsorption equilibrium along an experimental isothermal reactor. In order to keep the model within tractable limits, kinetic and adsorption parameters were grouped into a primary and a secondary sets. The secondary set is calculated from the primary set via empirical proportionality factors. With this dependence, the primary set can be estimated via non-linear regression of predicted properties over characterizing data of HCC products.
Statistical analysis was used to measure the quality of all results of Phases 1 and 2.
See the full pdf manuscript of the abstract.
Presented Tuesday 18, 13:30 to 15:00, in session Chemical Reaction Engineering (T2-2P).
