Refining Crude Oil and Products
Refining Crude Oil – Lesson Overview
The Crude Oil and Products Lesson consists of the following topics
- What is Crude Oil?
- Crude Oil Characteristics
- Types of Crude Oil
- Why do we Refine?
- Refining – a Boiling Process
- Distillation – The First Step
- Crude Oil Assays
- What is Oil Refining?
- What are Petroleum Products?
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What is Crude Oil?
Before learning about refining, it is important to understand a few basic properties of crude oil, the key raw material processed in a refinery.
Crude oils were formed millions of years ago as the remains of countless microscopic marine organisms, such as plankton, were deposited on the seabed and gradually buried deeper by the sediments accumulating on top of them. Over millions of years these remains were transformed by bacteria, heat and pressure into crude oil. As the result of variations in this process, crude oils vary in color, composition, and consistency. They are generally classified as” light” or “ heavy”, depending on proportion of heavy vs. light molecules present.
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Unlike water, crude oil is not a chemical compound. Rather, it is a complex mixture of molecules, consisting of compounds formed from hydrogen and carbon atoms, called “hydrocarbons”. Just as water boils at 212 degrees Fahrenheit, each of the compounds in crude oil has its own boiling temperature. In general, the more carbon atoms in the hydrocarbon molecule, the higher the temperature at which it boils and vaporizes. It is this phenomenon that permits crude oil to be refined into so many different products.
Crude Oil Characteristics
Scientists usually measure the density of a liquid using specific gravity, which is the weight of the liquid divided by the weight of same volume of water. However, the popular approach in the oil industry, defined by the American Petroleum Institute (API), for measuring crude oil density is “API Gravity”, which is expressed in degrees API.
As opposed to specific gravity, API gravity decreases as the density of the compound being measured increases, i.e. the higher the API gravity the lighter the compound. For example, the API gravity of naphtha is about 50 degrees API, but the API gravity of asphalt is around 11 degrees API.
Another important variable in determining the quality of crude is the sulfur content. Crudes with low sulfur content, less than 0.5%, are referred to as “sweet” crudes, and those with sulfur contents greater than 2.5% are known as “sour” crudes. Crudes with sulfur content in between these values are considered “intermediate”. Click here to learn about the origin of the “Sweet” and “Sour” terminology.
In the US crude oil volumes are measured in barrels, with each barrel equaling 42 US gallons. Note: this converts to 35 Imperial gallons in Europe. This custom stems from the early days of the Pennsylvania oilfields when oil was shipped in 50 gallon wine barrels. To allow for spillage during transportation, payment at the destination was only for 42 gallons. Shippers soon learned to seal the barrels and include only 42 gallons.
Sweet vs Sour Crude Oil
The origin of the “sweet” and “sour” terminology to describe the sulfur content in crude oil has more to do with taste than you may think. In the early days of Pennsylvania crude oil production petroleum was being used as a substitute for whale oil for indoor lighting. If a batch of kerosene had too much sulfur, it would emit an unpleasant odor when burned. The method used to determine if kerosene was suitable for the New York and Philadelphia markets was to taste it. If the taster thought it sweet, it passed; if it tasted sour, it was rejected as having too much sulfur.
Types of Crude Oil
Different oil producing areas yield dramatically different varieties of crude oil. This is why it’s common for crude oils to be identified by their origin, such as “West Texas Intermediate”, “Indonesian Duri”, or “Mexican Maya”.
This chart shows the sulfur content and API gravity for several crudes from different parts of the world . The economic value of a crude increases as it’s API gravity increases (i.e. as it gets “lighter”) and as it’s sulfur content decreases.
Thus a ”sweet” light crude will command a higher price than a ”sour” heavy crude. This is because light crudes contain more valuable components such as gasoline than do heavy crudes, and it is costly to remove the high content of sulfur in a sour crude. As a result, a refiner would clearly pay more per barrel for West Texas Intermediate than for Venezuelan Pilon.
Crude oil benchmarks are those crudes used on an international basis as pricing bases for the crude oil markets. A benchmark crude requires that a large number of buyers and sellers are interested enough in trading the crude so that there is good price reporting and transparency. The key global benchmark crudes are circled on the chart. They are often termed “marker crudes.”
Even though the qualities of crude oil from an area are generally known, the exact specifications for each lot or cargo can vary widely. Therefore, certain minimum and maximum specifications are included in contracts and a laboratory analysis of the crude is performed at the time of sale.
Why Do We Refine Crude Oil?
Other than burning for fuel, which is wasteful, crude oil cannot be used as it occurs in nature. It must be refined to manufacture finished products from its components such as gasoline and heating oil.
However, crude oil can be split into its various parts, or fractions, by carefully applying heat and capturing products within certain boiling ranges. The quality of the fractions produced is not sufficient to be sold directly as petroleum products without further treatment.
Moreover, the yield of products from straight distillation of crude oil is not the same as the “demand barrel”. The crude oil must, therefore, be further refined to improve qualities and to produce the yield demanded by the market place. A large part of refinery processing is concerned with converting unwanted fuel oil into wanted gasoline, using various processing methods.
Refining – A Boiling Process
The distillation of crude oil is the start of the refining process, and is primarily a boiling operation.
The crude oil is first washed to remove salt, heated in a furnace, and introduced to the Crude Distillation Unit (CDU). In the tower, the crude oil is separated according to the boiling range into a number of fractions.
A fraction is the term used for a specific hydrocarbon which is produced and captured according to its molecular weight and boiling point. Some fractions from the distillation process have all the qualities needed and become refined products – ready for sale. Others require further processing before they are sold to customers.
The lightest fractions, including liquid petroleum gas (LPG), are boiled off the crude at the lowest temperature (150º F in this diagram.)
The second fraction boiled off is kerosene or distillates which occurs at 450º F.
Next is gas oil which is an intermediate fraction that requires further processing in the plant (750º F).
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The heaviest fractions (fuel oil), with the highest boiling points, are the last fractions produced in the distillation process. Heavy fractions require more severe processing to be useful.
Distillation – The Final Step
Before crude oil is processed in a refinery, a sample of the crude is taken to the laboratory.
Using standardized testing methods a distillation curve analysis is performed by measuring the temperature of the distillate fraction at specific cumulative volume percentages. These boiling point measurements are then related to the physical properties of the crude using correlations developed by the industry.
The correlations estimate the properties important to the refinery processes such as API gravity, molecular weight, viscosity and critical temperatures and pressures based on the crude oil assay described in the next slide.
A typical distillation curve for a crude oil is shown graphically in the picture above, and each crude oil batch or cargo has a unique distillation curve.
These temperature ranges are indicative only, because they will vary among refineries. Another term for a distillation temperature range is “cut point.”
Crude Oil Assays
A crude oil assay is the term used for the complete chemical evaluation of a particular crude oil type.
Assay data help refiners determine if a crude oil is compatible for a particular refinery to meet the product yield requirements of the marketplace. The assay is also used for studies of refinery expansions because the crude characteristics and components define the detailed refinery process design.
Crude oil assays vary field-by-field, and the assay changes over time. Therefore assays are updated and published by the producer as the crude quality changes.
Many oil companies have links to crude oil assays on their web pages. Assays are extensive and some crude assay libraries have 800-1,000 different crudes. A typical assay format comparing a set of international crudes is shown above. Note the variation in assay caused by the differences in API gravity.
What is Oil Refining?
The composition of crude oil is such that it has to be processed to produce salable products. The refiner has a variety of methods by which to produce these components:
- Separation – by distillation
- Conversion – changing the shape or the size of the molecules to improve their qualities
- Treatment – to remove impurities
- Blending – to mix blending stocks and additives to obtain finished products
The tools the refiner can use to further process the components to achieve these qualities are:
- Heat – for distillation
- Pressure – to promote chemical reactions
- Catalysts – to speed chemical reactions
- Additives – to improve quality directly
All of these processes could be carried out on a small scale in a laboratory, but many refineries are capable of processing hundreds of thousands of barrels of crude oil per day.
Petroleum Products
Just as it is important to know the basics of crude oil in order to understand petroleum refining, it is also worth reviewing the main types of petroleum products manufactured and their key uses.
In general, refinery products can be categorized by the temperature range over which they boil, which generally corresponds to crude oil quality.
- Those products with low boiling points are called “light products” like LPG, Gasoline and Kerosene. LPG is made up of gasses, and is so “light” that is is kept in containers, under high pressure, in a liquid state to be useful.
- Those with high boiling points are called “heavy products” and include residual fuel oil, lubricants, and asphalts.
Petroleum products have a wide variety of uses.
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Liquefied petroleum gas (LPG) and naphtha are primarily used as chemical feed stocks. LPG is also used for heating and “bottled” gas.
Gasoline and diesel are used for automotive fuels and aviation turbine kerosene for aircraft fuel.
Gas oil is used for burning to produce heat for room heating and water heating.
Finally, heavy fuel oil is used for burning in industrial furnaces and to produce electricity; it is also used as marine fuels.
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Although I’m not an engineer, the course gave me many useful tools to understand the technicalities of the industry and now I can easily communicate with my contacts in the sector”.