Visbreaking is the process of cracking heavy vacuum tar to reduce the need for fuel-oil cutter-stock
Visbreaker reactors comprise two sections: a heater, heating up the feed to cracking temperatures, and a soaker drum where much of the cracking occurs.
Visbroken tar is a suspension of asphaltenes in heavy oil, and visbreaking severity is limited by asphaltene solubility constraint.
However it is difficult to determine the correct severity because feed properties, which affect fuel stability are not fully known
Further, severity is a function of residence time, whereas visbreaker feed availability varies daily or sometimes hourly
Secondary parameters: pressure and steam injection into the transfer line, affect the severity in a way that cannot be readily estimated
Laboratory fuel stability tests are typically carried out only once per day, not frequent enough for APC to approach the severity constraint.
How VISBS works
Residence time model
The first VISBS step is to calculate coil by coil density, volume flow and residence time.The visbreaking reaction rate doubles every 17ºC and VISBS uses that relation to calculate equivalent residence time at 468.33ºC (875ºF).This model responds to throughput, coil steam or pressure changes.
Conversion model
Conversion is a function of residence time and the APC controller can correct heater COT, avoiding over or undercracking
Actual conversion is a function of feed quality, it is calculated from fractionator data.
Conversion definition on the TBP curve
How VISBS works (cont.)
Maximum conversion model
The maximum conversion model is a function of feedcutpoint and Watson K factor.We install a GCC inferential model on the vacuum unit plus a vacuum residue density meter
Visbroken product properties calculated by a GCC package on the fractionator:
Naphtha ASTM 90% point
LGO flashpoint, ASTM 90% point
Internal reflux on key trays and flooding constraints.
