Improvements to needle-coke quality by pressure reductions from a tube reactor

Needle coke of excellent uniaxial orientation and thermal expansion of 0.6 × 10^-6/°C was produced at 460°C in a tube bomb from a low sulfur petroleum vacuum residue by reducing the carbonization pressure from 15 to 7 kg/cm²G at the solidification stage. Both timing and rate of pressure reduction are critical to the uniaxial arrangement of mesophase molecules that governs the coke quality. Reduction of pressure before the formation of bulk mesophase or after the solidification did not induce an improvement. Excess gas evolution by the larger rate of pressure reduction creates excessive porosity, although the uniaxial arrangement may be improved. A too-small rate is not effective in modifying the lamellar arrangements at the solidification stage.