Hi-Tech Process Control
Using the latest electronic technology to monitor and control the plants, operators run the process units around the clock, 7 days a week. From control rooms located in each Operations area, operators use a computer-driven process control system with console screens that display color interactive graphics of the plants and real-time (current) data on the status of the plants. The process control system allows operators to "fine tune" the processes and respond immediately to process changes. With redundancy designed into the control system, safe operations are assured in the event of plant upset.
Refining's Basic Steps
Most refineries, regardless of complexity, perform a few basic steps in the refining process: DISTILLATION, CRACKING, TREATING andREFORMING. These processes occur in our main operating areas – Crude/Aromatics, Cracking I, RDS/Coker, Cracking II, and at the Sulfur Recovery Unit.
 Pascagoula Refinery skyline |
1. Distillation
Modern distillation involves pumping oil through pipes in hot furnaces and separating light hydrocarbon molecules from heavy ones in downstream distillation towers – the tall, narrow columns that give refineries their distinctive skylines.
The Pascagoula Refinery's refining process begins when crude oil is distilled in two large Crude Units that have three distillation columns, one that operates at near atmospheric pressure, and two others that operate at less than atmospheric pressure, i.e., a vacuum.
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During this process, the lightest materials, like propane and butane, vaporize and rise to the top of the first atmospheric column. Medium weight materials, including gasoline, jet and diesel fuels, condense in the middle. Heavy materials, called gas oils, condense in the lower portion of the atmospheric column. The heaviest tar-like material, called residuum, is referred to as the "bottom of the barrel" because it never really rises.
This distillation process is repeated in many other plants as the oil is further refined to make various products.
In some cases, distillation columns are operated at less than atmospheric pressure (vacuum) to lower the temperature at which a hydrocarbon mixture boils. This "vacuum distillation" (VDU) reduces the chance of thermal decomposition (cracking) due to over heating the mixture.
As part of the 2003 Clean Fuels Project, the Pascagoula Refinery added a new low-pressure vacuum column to the Crude I Unit and converted the RDS/Coker's VDU into a second vacuum column for the Crude II Unit. These and other distillation upgrades improved gas oil recovery and decreased residuum volume.
Using the most up-to-date computer control systems, refinery operators precisely control the temperatures in the distillation columns which are designed with pipes to withdraw the various types of products where they condense. Products from the top, middle and bottom of the column travel through these pipes to different plants for further refining.
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2. Cracking
Since the marketplace establishes product value, our competitive edge depends on how efficiently we can convert middle distillate, gas oil and residuum into the highest value products.
At the Pascagoula Refinery, we convert middle distillate, gas oil and residuum into primarily gasoline, jet and diesel fuels by using a series of processing plants that literally "crack" large, heavy molecules into smaller, lighter ones.
Heat and catalysts are used to convert the heavier oils to lighter products using three "cracking" methods: fluid catalytic cracking (FCC), hydrocracking (Isomax), and coking (or thermal-cracking).
The Fluid Catalytic Cracker (FCC) uses high temperature and catalyst to crack 63,000 barrels (2.6 million gallons) each day of heavy gas oil mostly into gasoline. Hydrocracking uses catalysts to react gas oil and hydrogen under high pressure and high temperature to make both jet fuel and gasoline.
Also, about 58,000 barrels (2.4 million gallons) of lighter gas oil is converted daily in two Isomax Units, using this hydrocracking process.
We blend most of the products from the FCC and the Isomaxes directly into transportation fuels, i.e., gasoline, diesel and jet fuel. We burn the lightest molecules as fuel for the refinery's furnaces, thus conserving natural gas and minimizing waste.
In the Delayed Coking Unit (Coker), 105,000 barrels a day of low-value residuum is converted (using the coking, or thermal-cracking process) to high-value light products, producing petroleum coke as a by-product. The large residuum molecules are cracked into smaller molecules when the residuum is held in a coke drum at a high temperature for a period of time. Only solid coke remains and must be drilled from the coke drums.
Modifications to the refinery during its 2003 Clean Fuels Project increased residuum volume going to the Coker Unit. The project increased coke handling capacity and replaced the 150 metric-ton coke drums with new 300 metric-ton drums to handle the increased residuum volume.
The Coker typically produces 6,200 tons a day of petroleum coke, which is sold for use as fuel or in cement manufacturing.
Combining
While the cracking processes break most of the gas oil into gasoline and jet fuel, they also break off some pieces that are lighter than gasoline. Since Pascagoula Refinery's primary focus is on making transportation fuels, we recombine 14,800 barrels (622,000 gallons) each day of lighter components in two Alkylation Units. This process takes the small molecules and recombines them in the presence of sulfuric acid catalyst to convert them into high octane gasoline.
3. Treating (Removing Impurities)
The products from the Crude Units and the feeds to other units contain some natural impurities, such as sulfur and nitrogen. Using a process called hydrotreating (a milder version of hydrocracking), these impurities are removed to reduce air pollution when our fuels are used.
Because about 80 percent of the crude oil processed by the Pascagoula Refinery is heavier oils that are high in sulfur and nitrogen, various treating units throughout the refinery work to remove these impurities.
In the RDS Unit's six 1,000-ton reactors, sulfur and nitrogen are removed from FCC feed stream. The sulfur is converted to hydrogen sulfide and sent to the Sulfur Unit where it is converted into elemental sulfur. Nitrogen is transformed into ammonia which is removed from the process by water-washing. Later, the water is treated to recover the ammonia as a pure product for use in the production of fertilizer.
The RDS's Unit main product, low sulfur vacuum gas oil, is fed to the FCC (fluid catalytic cracker) Unit which then cracks it into high value products such as gasoline and diesel.
4. Reforming
Octane rating is a key measurement of how well a gasoline performs in an automobile engine. Much of the gasoline that comes from the Crude Units or from the Cracking Units does not have enough octane to burn well in cars.
The gasoline process streams in the refinery that have a fairly low octane rating are sent to a Reforming Unit where their octane levels are boosted. These reforming units employ precious-metal catalysts ‑ platinum and rhenium – and thereby get the name "rheniformers." In the reforming process, hydrocarbon molecules are "reformed" into high octane gasoline components. For example, methyl cyclohexane is reformed into toluene.
The reforming process actually removes hydrogen from low-octane gasoline. The hydrogen is used throughout the refinery in various cracking (hydrocracking) and treating (hydrotreating) units.
Our refinery operates three catalytic reformers, where we rearrange and change 71,000 barrels (about 3 million gallons) of gasoline per day to give it the high octane cars need.
Blending
A final and critical step is the blending of our products. Gasoline, for example, is blended from treated components made in several processing units. Blending and Shipping Area operators precisely combine these to ensure that the blend has the right octane level, vapor pressure rating and other important specifications. All products are blended in a similar fashion.
Quality Control
In the refinery’s modernly-equipped Laboratory, chemists and technicians conduct continuous quality assurance tests on all finished products, including checking gasoline for proper octane rating. Techron®, Chevron’s patented performance booster, is added to gasoline at the company’s marketing terminals, one of which is located at the Pascagoula Refinery.
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