Thermal Conversion Modeling of Visbreaking at Temperatures below 400 °C

Abstract

This paper is in honor of Michael (Mike) T. Klein, and his contributions to the modeling of thermal conversion are highlighted within the context of this study. The question that was posed is whether thermal conversion models developed for conventional visbreaking (430–490 °C) are adequate for the description of visbreaking at lower temperatures? This topic is relevant to partial upgrading of bitumen by visbreaking at temperatures of <400 °C. Insights from thermal conversion performed at 100–430 °C were employed to revisit the description of the free radical chemistry and how temperature affected the relative importance of thermal reactions. With a decreasing temperature, the increasing contribution of reactions, such as molecule-induced homolysis and the presence of “persistent” free radical species in the feed, results in a higher free radical concentration than is predicted by initiation through thermally induced homolytic bond dissociation. With a decreasing temperature, transfer reactions are also increasing in relative importance. This affects propagation and termination reactions. One of the important consequences of the increased contribution of transfer reactions at lower temperatures is that the apparent activation energy of cracking is reduced. The threshold temperature below which conventional visbreaking models no longer provide an adequate description of conversion is 380–400 °C. Drawing on the differences that must be captured to model low-temperature thermal conversion, it was shown that the development work by Mike Klein provided a solid basis for such modeling. Of particular importance is the ability to incorporate reactive intermediates that include radical isomers and to model transfer reactions. Quantitative structure–reactivity relationships captured the essence of the chemistry that must be reflected to model low-temperature thermal conversion.

Is Solubility Classification a Meaningful Measure in Thermal Conversion?

“…The contribution of the heat-up, reaction, and cool-down periods to the actual equivalent residence time (ERT) at 380 °C was calculated by the method of Yan . This method works well for most visbreaking studies, but it was recognized that its use in this study was at the limit of its validity . In the absence of a better strategy to account for heat-up and cool-down periods, it was nevertheless decided to use the equivalent residence time method of Yan …”

“…28 This method works well for most visbreaking studies, but it was recognized that its use in this study was at the limit of its validity. 29 In the absence of a better strategy to account for heat-up and cool-down periods, it was nevertheless decided to use the equivalent residence time method of Yan. 28 Once the reactor was cold, the reactor was depressurized and opened to collect the thermally converted product.…”

“…In terms of reaction fundamentals, the reaction rate is proportional to the free radical concentration, and at temperatures below 380 °C, the calculation increasingly underpredicts the free radical concentration. 29 The experiment with an equivalent residence time of 38 min had an uncommonly long heat-up period. It spent 50 min in the temperature range of 360−380 °C due to an issue with the sand-bath heater.…”

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

“…There has been increasing interest in applying such detailed models to complex refinery processes, such as hydrotreating, − hydrocracking, − and fluid catalytic cracking . However, fewer efforts , have focused on developing process models of this nature for thermal cracking with commercially relevant feedstocks despite the considerable foundational work on hydrocarbon pyrolysis theory. − …”

Special Issue of Energy & Fuels Honoring Michael T. Klein

“…Ramos-Pallares et al apply a regular solution theory to describe the partitioning of petroleum distillate into solvent and asphaltene-rich phases. de Klerk treats us to an engineering model suitable for representing the kinetics observed in the lower temperature required for visbreaking of bitumen, which he traces to the methodology devised by Mike Klein for modeling the kinetics of high-temperature thermal cracking. Vedachalam et al describe an unconventional approach to an upgrading step in petroleum refining, namely, oxidative desulfurization.…”