Page 85 - Synthetic Fuels Handbook

72 CHAPTER THREE

Gas +
gasoline

Fractionator
Quench Gas oil

Internals
for reducing
Soaker
backmixing

Cracked or
Furnace visbroken residue

Feed
FIGURE 3.5 A soaker visbreaker.

TABLE 3.2 Comparison of Visbreaking with Delayed Coking and Fluid Coking

Visbreaking
Purpose: to reduce viscosity of fuel oil to acceptable levels
Conversion is not a prime purpose
Mild [470–495°C (878–923°F)] heating at pressures of 50–200 psi
Reactions quenched before going to completion
Low conversion (10%) to products boiling less than 220°C (428°F)
Heated coil or drum (soaker)
Delayed Coking
Purpose: to produce maximum yields of distillate products
Moderate [480–515°C (896–959°F)] heating at pressures of 90 psi
Reactions allowed to proceed to completion
Complete conversion of the feedstock
Soak drums (845–900°F) used in pairs (one on stream and one off stream being decoked)
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 (428°F): ca 30% (but feedstock dependent)
Fluid Coking
Purpose: to produce maximum yields of distillate products
Severe [480–565°C (896–1049°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 (<C ) than delayed coking
5
Less coke made than delayed coking (for one particular feedstock)