There are two types of visbreaking processes: coil or soaker visbreaking. Figure 6.5a shows a schematic diagram of the coil visbreaking process. For visbreaking, the feedstock is introduced into the coil heated in the furnace where the thermal cracking reactions take place. At the furnace outlet, the reaction products are immediately quenched using a portion of the gas oil product from the fractionator to stop the thermal cracking reactions. The quenched products are sent to the fractionator for separation into gas, gasoline, light gas oil, and visbroken residue streams. A steam stripper can be used with the fractionator for better separation of the visbreaking products. In the soaker visbreaking process, a soak drum is placed after the furnace, Figure 6.5b. Most of the thermal cracking reactions, in this case, take place in the soaker drum.
Depending on the process objectives and feedstock characteristics, reaction temperatures range from 450°C to 485°C and pressures ranging from 3 to 10 bar. Higher temperatures and lower residence times are used in the coil visbreaking process.
Residence times can vary from 1 min (associated with high temperatures in coil visbreaking) to 10 min (for lower temperatures used in soaker visbreaking).
Similar to deasphalting and distillation, the environmental impact of visbreaking is associated with burning fuel in the furnace to provide energy for thermal cracking, and, to a lesser extent, burning off the coke deposited in the coil or soaker drum leading to emissions of CO2, oxides of nitrogen (NOx), and oxides of sulfur (SOx) in the flue gases.