In today’s refining climate, the economics of a visbreaking unit are mainly related to the conversion levels achieved. Conversion is limited by the impact of cracking reactions on the progressive destabilisation of the asphaltenes present in the unit feed. This destabilisation causes fouling phenomena, due to precipitation of the asphaltenes and conversion of some of the asphaltenes to generate coke particles at cracking temperatures above 400°C (752°F), and these phenomena limit unit run lengths. On the other hand, destabilised asphaltenes can easily aggregate over time and, since produced visbreaker resid is often a base stock for the production of heavy fuel oils, this can lead to problems with hot filtration test (HFT) results.
Visbreaker severity is monitored to maximise conversion, taking into account excessive coke generation and the tendency of asphaltenes to precipitate, either inside process equipment or as sediments in the heavy fuel oil produced. As fouling increases exponentially at higher conversions, process control becomes more and more important to prevent drastic negative outcomes.
Severity is a function of operating temperatures and velocity steam. This article presents a case of optimisation of these two variables at the Repsol Tarragona visbreaker, resulting in major improvements to visbreaker conversion.
Courtesy of PTQ Q1 2011, www.eptq.com.
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