Shell
Thermal Distillate Cracking technology provides a cost-effective solution to convert heavy distillate product like heavy gasoil, or vacuum gasoil, to light
gasoil. Given the low investment costs, the ease of operation and the great
return, this technology has been selected as the cracking technology of choice
in 30% of Shell's refineries. Thermal cracking of heavy gasoil takes place in the liquid phase in a
furnace, at elevated pressure and temperature.
A small part of the heavy gasoil is converted to residue and light products. The majority of the heavy gasoil is converted to light gasoil.
The feed and product requirements of the Thermal Distillate Cracking process are flexible, and the process has the capability to optimize conversion through adjustment of the HGO recycle rate. Given that the endpoint requirements for light gasoil are likely to change in the near future, this process provides an opportunity for refiners to meet those revised requirements at a very moderate investment.
Distillate feed (for example: heavy gasoil from the Atmospheric Distillation Unit, or vacuum gasoil from the Vacuum Distillation Unit) is routed to a Surge Drum. Liquid from this drum is pumped to the Distillate Heater, which typically operates at a pressure of approximately 20 bar and 490 °C. At these conditions, the cracking reactions take place in the liquid phase.
Fluid from the Distillate Heater is then routed to the Combi Tower. In this column, separation is achieved between residue, gasoil, and lighter products. In addition, a heavy gasoil fraction is taken from the Combi Tower that is returned to the Surge Drum, and then recycled through the Distillate Heater
The bottom product of the Combi Tower is typically routed to a Vacuum Flasher. In the Vacuum Flasher, heavy gasoil is recovered from the residue stream, and routed back to the Distillate Heater. The vacuum flashed residue product from the Vacuum Flasher can be routed to fuel oil blending, or can be used internally as refinery fuel.
Because heavy gasoil from both the Vacuum Flasher and the Combi Tower is recycled to the Distillate Heater, all heavy gasoil is converted. Light gasoil from the Combi Tower is first stripped, and is then routed to a Hydrotreater. Alternatively, light gasoil can be used as cutterstock.
Typical product yields from the Shell Thermal Distillate Cracking process are:
|
Product |
Yield (wt%) |
|
|
Gas |
9 % |
|
|
Naphtha |
17 % |
Endpoint 165 °C |
|
Gasoil |
54 % |
Endpoint 350 °C |
|
Residue |
20 % |
|