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The Chemistry and Technology of Petroleum
fuel oils of acceptable viscosity. By reducing the viscosity of the nonvolatile fraction, visbreaking
reduces the amount of the more valuable light heating oil that is required for blending to meet the
fuel oil specifications. The process is also used to reduce the pour point of a waxy residue.
The visbreaking process (Speight and Ozum, 2002) uses the approach of mild thermal cracking
as a relatively low-cost and low-severity approach to improving the viscosity characteristics of the
residue without attempting significant conversion to distillates. Low residence times are required
to avoid coke formation, although additives can help to suppress coke deposits on the tubes of the
furnace. The process also offers potential for future use as a preliminary treatment for heavy feedstocks prior to full conversion (Radovanović and Speight, 2011; Speight, 2012).
Two visbreaking processes are commercially available: the soaker visbreaker and the coil
visbreaker.
The soaker visbreaking process (Figure 18.1) achieves some conversion within the heater but the
majority of the conversion occurs in a reaction vessel or soaker that holds the two-phase effluent
at an elevated temperature for a predetermined length of time. Soaker visbreaking is described as
TABLE 18.1
Comparison of Visbreaking with Delayed Coking and Fluid Coking
Thermal cracking
Purpose: to produce volatile products of low-volatile or nonvolatile feedstocks
Conversion was the prime purpose
Cracking with simultaneous removal of distillate (semi-continuous)
Batch cracking (noncontinuous)
High conversion
Process configuration: various
Visbreaking
Purpose: to reduce viscosity of fuel oil to acceptable levels
Conversion is not a prime purpose
Mild (470°C–495°C; 880°F–920°F) heating at pressures of 50–200 psi
Reactions quenched before going to completion
Low conversion (10%) to products boiling less than 220°C (430°F)
Heated coil or drum (soaker)
Delayed coking
Purpose: to produce maximum yields of distillate products
Moderate (480°C–515°C; 900°F–960°F) heating at pressures of 90 psi
Reactions allowed to proceed to completion
Complete conversion of the feedstock
Soak drums (845°F–900°F) used in pairs (one on-stream and one off-stream being de-coked)
Coked until drum solid
Coke removed hydraulically from off-stream drum
Coke yield: 20%–40% by weight (dependent upon feedstock)
Yield of distillate boiling below 220°C (430°F): ∼30% (but feedstock dependent)
Fluid coking
Purpose: to produce maximum yields of distillate products
Severe (480°C–565°C; 900°F–1050°F) heating at pressures of 10 psi
Reactions allowed to proceed to completion
Complete conversion of the feedstock
Oil contacts refractory coke
Bed fluidized with steam; heat dissipated throughout the fluid bed
Higher yields of light ends (
The Chemistry and Technology of Petroleum
fuel oils of acceptable viscosity. By reducing the viscosity of the nonvolatile fraction, visbreaking
reduces the amount of the more valuable light heating oil that is required for blending to meet the
fuel oil specifications. The process is also used to reduce the pour point of a waxy residue.
The visbreaking process (Speight and Ozum, 2002) uses the approach of mild thermal cracking
as a relatively low-cost and low-severity approach to improving the viscosity characteristics of the
residue without attempting significant conversion to distillates. Low residence times are required
to avoid coke formation, although additives can help to suppress coke deposits on the tubes of the
furnace. The process also offers potential for future use as a preliminary treatment for heavy feedstocks prior to full conversion (Radovanović and Speight, 2011; Speight, 2012).
Two visbreaking processes are commercially available: the soaker visbreaker and the coil
visbreaker.
The soaker visbreaking process (Figure 18.1) achieves some conversion within the heater but the
majority of the conversion occurs in a reaction vessel or soaker that holds the two-phase effluent
at an elevated temperature for a predetermined length of time. Soaker visbreaking is described as
TABLE 18.1
Comparison of Visbreaking with Delayed Coking and Fluid Coking
Thermal cracking
Purpose: to produce volatile products of low-volatile or nonvolatile feedstocks
Conversion was the prime purpose
Cracking with simultaneous removal of distillate (semi-continuous)
Batch cracking (noncontinuous)
High conversion
Process configuration: various
Visbreaking
Purpose: to reduce viscosity of fuel oil to acceptable levels
Conversion is not a prime purpose
Mild (470°C–495°C; 880°F–920°F) heating at pressures of 50–200 psi
Reactions quenched before going to completion
Low conversion (10%) to products boiling less than 220°C (430°F)
Heated coil or drum (soaker)
Delayed coking
Purpose: to produce maximum yields of distillate products
Moderate (480°C–515°C; 900°F–960°F) heating at pressures of 90 psi
Reactions allowed to proceed to completion
Complete conversion of the feedstock
Soak drums (845°F–900°F) used in pairs (one on-stream and one off-stream being de-coked)
Coked until drum solid
Coke removed hydraulically from off-stream drum
Coke yield: 20%–40% by weight (dependent upon feedstock)
Yield of distillate boiling below 220°C (430°F): ∼30% (but feedstock dependent)
Fluid coking
Purpose: to produce maximum yields of distillate products
Severe (480°C–565°C; 900°F–1050°F) heating at pressures of 10 psi
Reactions allowed to proceed to completion
Complete conversion of the feedstock
Oil contacts refractory coke
Bed fluidized with steam; heat dissipated throughout the fluid bed
Higher yields of light ends (