Five-Lump Mild Thermal Cracking Reaction Model of Crude Oils and Bitumen with VLE Calculations

Abstract

Thermal cracking involves the breakdown of large hydrocarbons to smaller hydrocarbons via the scission of carbon−carbon, carbon−nitrogen, and carbon−sulfur bonds. Mild thermal cracking reactions occur between 350 and 450 °C and primarily in the liquid phase. A reaction model implementing vapor−liquid equilibrium (VLE) calculations was developed in this work to better represent the reacting system−the liquid phase. The resulting model can be used to determine reaction kinetics in experimental systems that have a VLE component, which would normally make it inadequate for reaction kinetics determination. A mild thermal cracking experimental program was conducted with three crude assays: Hardisty (MBL), Albian heavy synthetic (AHS), and Christina Lake diluted bitumen (CDB). The experiments were conducted in a pilot-scale semibatch vertical reactor with a sweeping nitrogen gas blanket in the head space. The three crude oils were reacted at mild conditions: 350, 400, 425, and 450 °C at 1.24 MPa(a). The reaction model with VLE calculations was used to determine reaction rates representative of the experimental data collected. The coefficient of determination between the reaction model's predicted system composition and the experimental data for MBL, AHS, and CDB were determined to be 0.99, 0.99, and 0.98, respectively. Moreover, 80, 85, and 89% of the model's predicted system composition had less than 10% error for MBL, AHS, and CDB, respectively.

Effect of Pressure on Visbreaking Product Composition and Properties

“…At the same time, there are assumptions associated with the ERT . Two of those assumptions are the description in terms of first-order kinetics with a single average activation energy, which is common practice in modeling, ,,, but it is also an oversimplification. Reported activation energy values for the thermal conversion of heavy oil are in the range 102–268 kJ/mol, , and the activation energies for the thermal conversion of different species can be quite different.…”

“…9 In more sophisticated models, VLE calculations may be explicitly incorporated as stepwise corrections to the volumetric flow rate as the reaction progresses. 11,12 Of importance is that the vapor and liquid in this well-mixed fluid progress through the coil at the same velocity as one would expect from near-ideal PFR behavior.…”

“…Even in simplified design calculations, the pressure-dependent volume of the mixed vapor-and-liquid passing through the coil must be calculated to determine the average space time . In more sophisticated models, VLE calculations may be explicitly incorporated as stepwise corrections to the volumetric flow rate as the reaction progresses. , Of importance is that the vapor and liquid in this well-mixed fluid progress through the coil at the same velocity as one would expect from near-ideal PFR behavior.…”

Exploring the Conversion Limits of Bitumen Visbreaking through a Molecular Reaction Model

“…Modeling efforts to date devoted to thermal cracking have mostly sought to predict product yields, ,− without much emphasis on understanding changes in product stability and coking behavior. The majority of the available models use the conventional, but limited, approach where reactions are represented using broad hydrocarbon fractions or “lumps”. ,,, Very few studies have approached these processes at the molecular level − but focused on severe thermal cracking as seen in the delayed coking process. As far as the author is aware, there have been no such published studies addressing the conversion boundaries of visbreaking from a molecular-level perspective.…”

Enhanced Representation of Thermal Cracking Chemistry in the Context of Bitumen Partial Upgrading

“…A model that is able to capture relevant behaviors of thermal cracking, including olefin formation and changes in the solubility of aromatic species, would require more depth in terms of the description of hydrocarbon components and their reactions than the classic modeling approach based on the lumping of hydrocarbon components by boiling point. The range of application of lump-based models is limited to tracking how the yields of different boiling fractions evolve within a certain range of conditions, as illustrated by recent studies. − By contrast, modeling approaches that treat the reaction chemistry at the molecular structure level can offer a much higher resolution for describing a thermal cracking process. There has been increasing interest in applying such detailed models to complex refinery processes, such as hydrotreating, − hydrocracking, − and fluid catalytic cracking .…”