The Shell Thermal Gasoil process is a combination of Shell Soaker Visbreaking technology, and Shell Thermal Distillate Cracking technology. Heavy gasoil from the Visbreaking process is converted to lower boiling fractions, to maximize yield to light gasoil.
Feed to the Shell Thermal Gasoil process is typically atmospheric residue, or alternatively vacuum residue. Heavy gasoil, that is normally separated from atmospheric residue in a Vacuum Distillation Unit, can be converted to light gasoil in the Thermal Gasoil process. The advantage is that the investment for a Vacuum Distillation Unit is not required, or that an existing Vacuum Distillation unit is debottlenecked.
Vacuum residue, or atmospheric residue, is pumped through feed preheat exchangers, where the feed is heated against cracked residue, and then routed to the visbreaker heater. In the heater, the feed is heated to the required cracking temperature and routed to the soaker where the majority of the thermal cracking occurs under controlled conditions. The soaker effluent is routed to a cyclone and the cyclone overheads are charged to the flash zone of the atmospheric fractionator.
In the top section of the Fractionator, the soaker effluent is split into four fractions: heavy gasoil, gasoil, naphtha and offgas. The gasoil is taken from the Fractionator as a draw off, steam-stripped in a side stripper to improve the flash point, and sent to the battery limit. The overhead vapors are condensed in a two-stage condensing system: in the first stage, only the reflux is condensed; in the second stage, the naphtha product is condensed. From the overhead system, the offgas and naphtha are sent to the battery limit.
Inside the Fractionator, the liquid is quenched to prevent further cracking and then steam-stripped. The hot Fractionator bottoms, together with the cyclone bottoms, are routed to the vacuum flasher where the vacuum gasoil (VGO) is recovered. The VGO is sent, together with the heavy gasoil from the atmospheric Fractionator, to a distillate thermal cracking heater where it is mostly converted into lower boiling fractions. The heater effluent is routed to the flash zone of the atmospheric Fractionator. The unconverted heavy gasoil is recovered in the Fractionator and Vacuum Flasher and is recycled back to the distillate thermal cracking heater to maximize the gasoil yield.
The vacuum-flashed residue is sent to fuel oil blending where it is blended with gasoil product and/or other cutter-stocks to meet the specified fuel oil viscosity.
Products yields are dependent on feed type and product specifications. Typical product yields for Middle East crude are given below.
Based on atmospheric residue feed:
|
Product |
Yield (wt%) |
|
|
Gas |
6 % |
|
|
Naphtha |
13 % |
Endpoint 165 °C |
|
Light Gasoil |
39 % |
Endpoint 350 °C |
|
Vacuum Flashed Residue |
42 % |
|
The investment amounts to 1400 - 1600 US$/BPSD vacuum residue (or approximately 800 - 1000 US$/BPSD atmospheric residue) installed excluding treating facilities and depending on capacity and configuration.
|
Utilities, typical per bbl VR @ 180°C: |
|
|
Electricity, kWh |
0.8 |
|
Net steam production, kg |
29 |
|
Cooling water, m3 |
0.17 |