The Shell Soaker Visbreaking process is ideally suited for the reduction of heavy fuel oil product via residue viscosity reduction and maximum production of distillates. Typical applications include processing of atmospheric residue, vacuum residue or solvent Deasphalter pitch.
Over 70% of the total visbreaking capacity built during the last 15 years is based on this Shell technology. When compared to coil visbreaking technology, it offers demonstrated advantages that include significantly lower investment costs, lower fuel requirements, increased heater run length, and higher conversion operation with better viscosity reduction.
The technology provides refiners with the means to conserve valuable cutter stock while still producing high quality, stable fuel oil. This conservation of valuable cutter stock, combined with fuel savings derived from the technology, offers an overall cost advantage that leads to project payouts of one to two years.
With more than 35% of the vacuum residue feed converted to distillate and lighter products, Shell Soaker Visbreaking is one of the lowest cost conversion process options.
The Shell Soaker Visbreaking process can be tailored to meet the reifner's specific needs. The most basic configuration of the Visbreaking process includes the Heater, Soaker and Fractionator. The cutpoint of the light gasoil fraction is approximately 350 ºC, and heavier material is included in the visbroken residue. Light gasoil is typically sent to a hydrotreater, or alternatively used internally as cutterstock.
Because of the drive to reduce fuel production, and to maximize distillate yield, more refiners now decide to add a Vacuum Flasher to recover more distillate products. The cutpoint of the heavy gasoil stream taken from the Vacuum Flasher is approximately 520 ºC. This heavy gasoil stream is ideally routed to a Thermal Distillate Cracker, or a Catalytic Cracker, to convert it to lighter distillate products.
The heavy gasoil fraction from the Vacuum Flasher can be routed to a cracker. This could be either a cracker already available in the refinery, or a new cracker. Ideally, the heavy gasoil fraction is routed to a Shell Thermal Distillate Cracker. The Thermal Distillate Cracker combined with the Shell Soaker Visbreaker is called the Shell Thermal Gasoil process.
The descriptions that follow are based on traditional Shell Soaker Visbreaker technology, with a Vacuum Flasher to recover additional heavy gasoil.
Visbreaker feed is pumped through preheat exchangers before entering the visbreaker heater, where the residue is heated to the required cracking temperature. In the convection section of the visbreaker heater, superheated steam is generated. Heater effluent is sent to the soaker drum where most of the thermal cracking and viscosity reduction takes place under controlled conditions. The pressure in the Soaker drum can be adjusted, which results in a change in residence time, and the amount of heavies that reside in the liquid phase. This flexibility in pressure provides the possibility to reach optimum selectivity. Soaker drum effluent is flashed and then quenched in the fractionator. Heat integration is maximized in order to keep fuel consumption to a minimum. The flashed vapors are fractionated into gas, naphtha, gasoil and visbreaker residue.
Liquid visbreaker residue is steam-stripped in the bottom of the fractionator and pumped through the cooling circuit to battery limits. Visbreaker gasoil, which is drawn off as a side stream, is steam-stripped, cooled and sent to battery limits. Alternately, the gasoil fraction can be included with the visbreaker residue as cutterstock. Other cutter stocks, such as light cycle oil or heavy atmospheric gasoil, are typically added to the visbreaker residue/gas oil mixture to meet the desired fuel oil specification.
Products yields are dependent on feed type and product specifications. Typical product yields for Middle East crude are given below.
Based on vacuum residue feed:
|
Product |
Yield (wt%) |
|
|
Gas |
2 % |
|
|
Naphtha |
4 % |
Endpoint 165 °C |
|
Light Gasoil |
12 % |
Endpoint 350 °C |
|
Heavy Gasoil |
18 % |
Endpoint 520 °C |
|
Vacuum Flashed Residue |
64 % |
|
The investment is in the order of 1000 - 1200 US$/BPSD installed capacity excluding treating facilities and depending on capacity.
|
Utilities, typical per bbl @ 180°C: |
|
|
16 |
|
|
Electricity, kWh |
0.5 |
|
Net steam production, kg |
18 |
|
Cooling water, m3 |
0.10 |