Characterization of visbreaker feeds

Abstract

A visbreaker should be operated at high severity for better process economics, but the severity is limited by heater fouling and product stability. Both of these limitations are due to coke formation. Visbreaking feed can be characterized by its coking propensity, which is derived from the kinetics of coke formation. Coke formation can be calculated from the coking propensity and the process severity, so that the process may be controlled at the maximum severity permissible at all times. Coking propensity can be determined with a simple laboratory visbreaker. The coking propensities are found to be 2.1 × 10 −3, 4.2 × 10 −3 and 7.3 × 10 −3 wt% s −1 for mixed residue, Boscan crude and Arab heavy residue, respectively, and are in good agreement with commercial visbreaking experiences. Coking propensity is directly proportional to the asphaltene content of the feed. It is also indirectly related to the viscosity, but not to the Conradson carbon residue (CCR) content or the density (API gravity) of the feed.