How do we use all those towers and pipes?
Crude oil contains hundreds of different hydrogen and carbon – or hydrocarbon – molecules as well as other naturally occurring materials such as nitrogen, salt and sulfur. At Shell’s Puget Sound Refinery, these materials are removed, and the crude oil is separated into different types of hydrocarbons (or “fractions”) based on their boiling points. Then these fractions are processed into an array of useful products, such as gasoline, diesel fuel and heating oil. Keep reading to learn more about the different units in the refinery where these processes take place.
In this unit, water, salt and sediments are removed from the crude oil. Then the oil is routed into the Atmospheric Distillation Tower, where it is heated under pressure. The “lightest” fractions – those, such as propane, naphtha, kerosene and diesel, which have the lowest boiling points – vaporize.
They rise to the top of the tower, where they cool and condense and are sent to other units for processing. The remaining crude oil is sent to the Vacuum Pipestill (VPS). Here, the crude is heated in a vacuum, which lowers the boiling point of the fractions. Finally, the remaining oil, called heavy residuum, is sent to the Gas Oil Distillation Tower, where gas oils (or diesel distillates) are removed.
Delayed Coking Unit (DCU)
In the DCU, the heavy residuum from the Crude Unit is poured into a large drum, where it is heated to break down, or “crack,” it into fractions that are sent to other units for processing. Then a high-pressure “blade” of water is used to cut the product remaining in the drum – petroleum coke – into chunks for removal.
Fluid Catalytic Cracking Unit (FCCU)
The gas oils removed from the crude oil in the Gas Oil Distillation Tower are sent to the FCCU. In the unit’s reactor, a reusable silica-alumina catalyst helps crack large oil molecules into more valuable products. A "fractionator" separates out the diesel fuel; the remaining crude oil is sent through three more distillation towers, which divide it into gasoline, fuel gas, propane and butanes. The propane and butanes serve as feedstocks for the Alkylation and Polymerization units.
In the Polymerization Unit, propylene – a byproduct of the cracking in the DCU and FCCU – is exposed to phosphoric acid-impregnated catalyst pellets. This process re-forms it into polymer gasoline, which is used to help blend gasoline as well as nonene – a feedstock for making petrochemicals.
In the Alkylation Units, propylene and another byproduct of the FCCU called butylene are mixed with isobutane and a sulfuric acid catalyst. Then the sulfuric acid is removed and the remaining product is pumped to distillation towers. There, it’s separated into liquefied petroleum gas (LPG), mixed butanes and alkylate, which is a high-octane blending component used in lead-free premium gasolines.
Kerosene or low-octane naphtha from the Crude Unit, and naphtha and diesel from the FCCU and DCU are pumped to the Hydrotreating Units. They are combined with a catalyst in a high-pressure, hydrogen-rich atmosphere, which removes sulfur and nitrogen contaminants, producing not only desulfurized hydrocarbons, but also hydrogen sulfide and ammonia. The desulfurized hydrocarbons are distilled further into low-octane naphtha and jet fuel.
Catalytic Reforming Units
In the Catalytic Reforming Units, the low-octane, desulfurized naphtha is heated and exposed to a platinum catalyst to produce reformate, a high-octane blending component for gasoline. Chemical reactions in these units also produce hydrogen, which is used in the Hydrotreating Units.
Sulfur Recovery Units
Some crude oil, called “sour” crude, contains higher levels of sulfur. In the Sulfur Recovery Unit, controlled combustion and then a catalyst are used to liquefy and remove the sulfur, which helps reduce emissions and allows the refinery to process this type of crude oil . The liquid sulfur is sold as a fertilizer ingredient.
The Puget Sound Refinery generates electricity as a byproduct of the refining process. It uses about 350,000 pounds of steam per hour to produce 140 megawatts of electricity— enough power for 70,000 homes. In addition, the Boiler House, which is part of the cogeneration facility, provides steam, instrument and plant air, boiler feedwater and fire and service water for the refinery.
Wastewater Treatment Plant
All sewage and wastewater from the plant is treated and then tested before being discharged into Fidalgo Bay. This helps ensure that the treated water meets standards required by the refinery’s National Pollution Discharge Elimination System permit. The plant also handles ballast water from ships and recovers oil for recycling.