Refinery Processes
Refinery Processes – Lesson Overview
The Refinery Processes Lesson consists of the following topics:
- Generalized Refinery Plant Layout
- Generalized Refinery Processes
- Separation
- Conversion
- Treatment
- Blending
- Refinery Yield Flexibility – Feedstocks
- Refinery Yield Flexibility – Operations
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General Refinery Plant Layout
To the layperson, a refinery appears to be a strange maze of towers, pipes, tanks, and buildings. In reality, a refinery is an organized and coordinated arrangement of manufacturing processes designed to produce physical and chemical changes in crude oil. 
A refinery includes control rooms, onsite and offsite process units, miles of pipeline, and vast storage tanks for crude oil and products. Every unit has a particular task, but all the processes are integrated—with technology playing a key part in the smooth operation of the refinery.
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When you visit a refinery, you learn that it is laid out like the flow of crude and product through the plant. You usually enter through an Administration Building which houses management, the technical services, and support staff.
The largest set of tanks observed are crude tanks – many with floating roofs. The roof floats on the oil like a raft with a sliding seal against the tank walls, preventing the oil and associated gases from escaping. To reduce the risk of fire or explosion, there is no substantial empty space beneath the floating roof.
The processing units – that bewildering array of pipes, exchangers, pumps and towers – are termed onsite units. This is where the crude is converted to products through a variety of processes.
The offsite units have the support services and components needed by the onsite processing units and include: boilers to generate steam, furnace fuel oil and plant gas systems, fresh water and salt water cooling systems, compression to provide plant and instrument air, substations for electric power, the flare system, effluent collection/treatment/disposal and the safety systems.
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Highly automated control rooms allow operators to regulate refinery operations. Using computer-driven process control systems with console screens displaying real-time graphics and data on the status of the plant, operators can respond immediately to changes and fine-tune processes.
“Tank Farm” is the term used for an installation or group of tanks used by refineries, gathering and trunk pipeline companies, crude oil producers, and terminal operators to store crude oil and products. Product can move from the refinery to inland destinations via road and rail.
General Refinery Processes
In theory, the refiner can meet the demands of the market place by careful selection of a mix of crude oils (a crude slate) to produce the desired products (the “demand barrel” or “yield pattern”). In practice, there is insufficient flexibility in the supplies of available crude oil to allow the world’s refineries to meet the demand barrel through crude oil selection alone.
Therefore, refiners turn to a variety of chemical processes to enable them to produce the required products of the right quality.
Some of these refinery processes act on the entire barrel of oil, while others process only a specific portion of the barrel or refined product. 
The material presented now discusses the key processes:
- Separation
- Conversion
- Treatment
- Blending
Separation
The first phase in the refining process involves separating the crude oil or unfinished oils into different hydrocarbon groups, or “fractions”. Fractional distillation using heat is the most common approach used. 
Distillation takes advantage of the fact that each of the different hydrocarbon compounds that comprise crude oil has a characteristic boiling point. The boiling point of a liquid is the temperature at which it vaporizes when heated. The boiling point is also the temperature at which a vapor condenses when cooled.
By heating crude oil until all or most of it vaporizes and then cooling the hydrocarbon vapor to specific temperatures, refiners can separate out or “cut” the crude oil into groups of hydrocarbons with particular boiling-point ranges.
The separating of crude oil into specific hydrocarbon groups, or “fractions,” is the central feature of the distillation process, and the products produced are called “straight-run products”.
Two major types of distillation – atmospheric and vacuum
Atmospheric Distillation
Because the various hydrocarbons in crude oil have different boiling temperatures, the first step is to separate them using a process called atmospheric distillation. The distillation process, which takes place in a column, follows this sequence:
- Crude oil is cleaned and then heated in high-pressure furnaces to temperatures of about 850-1000 degrees Fahrenheit/600 degrees Celsius.
- The mixture boils, forming hydrocarbon vapors (gases) and liquids that enter the bottom of the distillation column.
- Inside the distillation column, the vapors and liquids separate into components or fractions according to weight and boiling point. The lightest fractions rise to the top of the tower where they condense back into liquids, medium weight liquids stay in the middle of the tower, and the heaviest fractions settle at the bottom.
- Stacks of trays at various intervals collect the liquid hydrocarbons. Bubble caps on the trays slow down the vapors as they rise through the column.
- The collected liquid fractions may pass to other areas via pipes for further processing or to condensers, which cool them before moving them to the tanks.
Vacuum Distillation
Atmospheric process residue is usually sent to a second distillation column where the process is repeated under vacuum to recover additional heavy distillates. This process is called vacuum distillation. 
This allows heavy hydrocarbons with boiling points of 850° F and higher to be separated without their being cracked into unwanted products such as coke and gas at the higher temperatures.
The heavy distillates recovered by vacuum distillation can be converted into asphalt and lubricating oils by a variety of processes.
Conversion
Relatively small amounts of motor gasoline and other light fuels are produced through distillation alone (10 to 25 percent, depending on the crude oil quality). The growing demand for these fuels spurred refiners to develop new technologies designed to increase the yield of motor gasoline and its quality.
The processes they developed convert hydrocarbons not found in the gasoline range into more valuable, gasoline-range hydrocarbons. 
These Downstream refining processes fall into three general categories: combining hydrocarbon molecules, rearranging molecules, and cracking hydrocarbon molecules.
Combining Hydrocarbon Molecules
Combining operations link two or more hydrocarbon molecules to form a larger molecule. In this way, gases can be converted into a liquid and used as an additive to motor gasoline and other motor fuels.
Two common procedures for combining hydrocarbon molecules are alkylation and polymerization.
Rearranging Hydrocarbon Molecules
Rearranging operations alter the original structure of a molecule, producing a new molecule with different characteristics from the original but with the same number of carbon atoms.
Catalytic reforming and isomerization are two of the most commonly used techniques for rearranging hydrocarbons.
Cracking of Hydrocarbon Molecules
One method refiners use to increase the yield of gasoline is cracking. Cracking is a process by which large, heavy hydrocarbon molecules are broken down, or “cracked”, into smaller, gasoline range hydrocarbon molecules.
Refiners discovered that cracking raises the yield of motor gasoline and improves its quality. Cracking can be achieved either through the application of heat or the use of a catalyst.
The main types of cracking operations are catalytic cracking, catalytic hydrocracking using hydrogen and thermal cracking.
Conversion Definitions
Alkylation
Alkylation refers to the chemical bonding of light molecules with isobutane to form isoparaffins that make high octane gasoline.
Polymerization
Polymerization is the technique which employs a catalyst and links together molecules of olefin gases produced during thermal and catalytic cracking operations. High octane motor gasoline blend stock is produced from these reactions.
Catalytic Reforming
Reforming is a process which uses heat, pressure, and a catalyst (usually containing platinum) to bring about chemical reactions that upgrade naphthas into high-octane gasoline and petrochemical feedstock.
Isomerization
Isomerization is a process in which straight-chained hydrocarbon molecules are converted to branch-chained molecules with the same chemical composition. This reaction is advanced through the use of a specially prepared platinum catalyst.
The products of isomerization are blended into motor gasoline and aviation fuel to raise the octane level.
Cracking Definitions
The most prevalent cracking processes break down heavier hydrocarbon molecules (high boiling point oils) into lighter products such as gasoline and diesel. These processes include:
Catalytic cracking
- This process is used to convert heavy hydrocarbon fractions obtained by vacuum distillation into a mixture of more useful products such as gasoline and light fuel oil.
Hydrocracking:
- Hydrocracking is catalytic cracking in the presence of hydrogen.
- It can increase the yield of gasoline components, as well as being used to produce light distillates. It produces no residues, only light oils.
Thermal cracking or Coking:
- The oldest technology – uses heat to break down the residue from vacuum distillation.
- The lighter elements produced from this process can be made into distillate fuels and gasoline.
The heavy residue is converted into residual oil or coke, which is used in the manufacture of electrodes, graphite, and carbides.
Treatment
With the increased emphasis on producing higher yields of high-octane gasolines and low-sulfur fuel oil, it is necessary to upgrade the components used in gasoline, aviation fuel, and fuel oil blending. Blend stocks produced directly from atmospheric distillation and thermal cracking often contain unacceptable amounts of sulfur, nitrogen, and other impurities. 
Crude oil fractions contain impurities such as sulfur, nitrogen, oxygen, water, dissolved metals, and inorganic salts.
Treatment is the removal of these impurities through several processes to enhance the performance of end products, reduce pollution, and prevent damage to refining equipment. In particular, sulfur must be removed because, when burnedin fuels, sulfur combines with oxygen to form sulfur dioxide.
This gas is soluble in water and makes it acidic. Two commonly used methods of removing impurities from petroleum products are hydrotreating and chemical treating to remove sulfur.
Hydrotreating
Hydrotreating is one way of removing many of the contaminants from many of the intermediate or final products. In the hydrotreating process, the entering feedstock is mixed with hydrogen and heated to 550°–700°F. The crude oil combined with the hydrogen and then enters a reactor loaded with a catalyst that promotes several reactions:
- Hydrogen combines with sulfur to form hydrogen sulphide (H2S).
- Nitrogen compounds are converted to ammonia.
- Any metals contained in the crude oil are deposited on the catalyst.
Note that hydrogen sulphide (H2S) – created from hydrotreating- is a toxic gas that needs further treatment. The customary processing involves two steps:
- Removal of the hydrogen sulphide gas from the hydrocarbon stream
- Conversion of hydrogen sulphide to elemental sulfur, a non-toxic chemical used in the production of fertilizer
A number of other chemical and physical methods are used for treating, including:
- Dehydrating and desalting fractions, which removes water and salt from the fractions by heating them and allowing the water-salt solution to settle out
- Washing fractions with caustic or acid solutions
- Exposing fractions to absorbent clays
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Blending
The last phase of the refining process is blending. Blending operations involve the mixing of blending stocks and additives to obtain a finished product. Most refinery products are ultimately produced by combining various blend stocks.
For any given product, the exact ratio of each of the components depends on the particular specifications and characteristics which the fuel must meet. 
The exact composition of motor gasoline, for example, can vary depending on such factors as octane rating, lead content, volatility (vapor pressure), and alcohol content.
Flash point, viscosity, sulfur content, and cloud point are other factors considered in blending other petroleum products. In the past, batch or tank blending procedures were employed, but today, in-line blending is practiced at most refineries. Both accurate and reliable, in-line blending also is much faster than the older methods.
Using this method, blended product can be moved directly to a pipeline or loaded unto a tanker.
The blending process is relatively straightforward. Once the specifications and quantity of a fuel have been selected, the appropriate volume of the various component parts is determined. To begin the blending operation, a series of valves connecting the various component tanks with the blending line are opened.
Metering devices attached to the valves monitor the flow of the components to ensure that the proper mix is achieved.
Analyzers along the blending line automatically sample and examine the product for quality. Where necessary, adjustments are made to correct any variations to the product mix.
Refinery Yield Flexibility – Feedstock Selection
In an existing facility, refiners can use a number of methods to change the yield pattern of products, including:
- Feedstock changes: A feedstock is the crude oil or a fraction thereof to be sent to or “charged” to any process equipment. Refiners can vary the amount and type of light and heavy crude oils purchased for processing. One example of a feedstock or intermediate is naphtha purchased for processing to supplement the gasoline produced from the crude. This will typically result in a significant change in the preparation of heavy product that is produced. However, as discussed previously, refineries cannot meet their objectives with crude oil blending alone.
- Purchase blendstocks, such as Butane, Naphtha and Ethanol for gas blending.
- Purchase intermediate products such as Naphtha, long residue, cat cracker feed, condensates, etc.
Operating Mode
Within an existing design, there are three major operating methods or modes that refiners have to vary product yield pattern:
- To change the distillation cut points on the atmospheric distillation unit – one method is to vary the naphtha end point which will increase or decrease the middle distillate yields.
- To change the mode of operation of the product recovery (distillation) section – for example, most Northern Hemisphere countries change from maximum gasoline mode in the summer time to maximum distillate mode in the winter to produce heating oil for home heat.
- To alter the temperature and pressure in the operation of downstream units (reforming and conversion) – typically by changing the “severity” of the operation.
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Related Resources:
What is the difference between Upstream and Downstream?
Drilling Wells for Oil and Gas and Offshore Drilling
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.