Petroleum refining

April 30, 2013, 5:36 pm
Source: Eia::OSHA
Topics:
Content Cover Image

HOVENSA petroleum refinery, St. Croix, U.S. Virgin Islands (By Cumulus Clouds, via Wikimedia Commons)

 

Introduction

A petroleum refinery is an installation that manufactures finished petroleum products from crude oil, unfinished oils, natural gas liquids, other hydrocarbons, and alcohol. Refined petroleum products include but are not limited to gasolines, kerosene, distillate fuel oils (including No. 2 fuel oil), liquefied petroleum gas, asphalt, lubricating oils, diesel fuels, and residual fuels.

Simple Distillation

 
caption By Crude_Oil_Distillation-fr.svg: Image originale:Psarianos, Theresa knott ; image vectorielle:Rogilbert derivative work: Utain (Crude_Oil_Distillation-fr.svg) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
By Crude_Oil_Distillation-fr.svg: Image originale:Psarianos, Theresa knott ; image vectorielle:Rogilbert derivative work: Utain (Crude_Oil_Distillation-fr.svg) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons

The core refining process is simple distillation (Figure 1). Because crude oil is made up of a mixture of hydrocarbons, this first and basic refining process is aimed at separating the crude oil into its "fractions," the broad categories of its component hydrocarbons. Crude oil is heated and put into a still—a distillation column—and different products boil off and can be recovered at different temperatures. The lighter products—liquid petroleum gases (LPG), naphtha, and so-called "straight run" gasoline—are recovered at the lowest temperatures. Middle distillates—jet fuel, kerosene, distillates (such as home heating oil and diesel fuel)—come next. Finally, the heaviest products (residuum or residual fuel oil) are recovered, sometimes at temperatures over 1000 degrees F. The simplest refineries stop at this point. Other refineries reprocess the heavier fractions into lighter products to maximize the output of the most desirable products, as shown schematically in Figure 1, and as discussed below.

Downstream Processing

caption Oil refinery in Kentucky. Photo:Ross Grady (Flickr)

Additional processing follows crude distillation, "downstream" (or closer to the refinery gate and the consumer) of the distillation process. Downstream processing is grouped together in this discussion, but encompasses a variety of highly-complex units designed for very different upgrading processes. Some change the molecular structure of the input with chemical reactions, some in the presence of a catalyst, and some with thermal reactions.

In general, these processes are designed to take heavy, low-valued feedstock—often itself the output from an earlier process—and change it into lighter, higher-valued output. A catalytic cracker, for instance, uses the gasoil (heavy distillate) output from crude distillation as its feedstock and produces additional finished distillates (heating oil and diesel) and gasoline. Sulfur removal is accomplished in a hydrotreater. A reforming unit produces higher-octane components for gasoline from lower-octane feedstock that was recovered in the distillation process. A coker uses the heaviest output of distillation—the residue or residuum—to produce a lighter feedstock for further processing, as well as petroleum coke.

Crude Oil Quality

The physical characteristics of crude oils differ. Crude oil with a similar mix of physical and chemical characteristics, usually produced from a given reservoir, field, or sometimes even a region, constitutes a crude oil "stream." Most simply, crude oils are classified by their density and sulfur content. A common unit of measurement is API gravity—the American Petroleum Institute's measure of specific gravity of crude oil or condensate in degrees. It is an arbitrary scale expressing the gravity or density of liquid petroleum products.

Less dense (or "lighter") crudes generally have a higher share of light hydrocarbons—higher value products—that can be recovered with simple distillation. The denser ("heavier") crude oils produce a greater share of lower-valued products with simple distillation and require additional processing to produce the desired range of products. Some crude oils also have a higher sulfur content, an undesirable characteristic with respect to both processing and product quality. For pricing purposes, crude oils of similar quality are often compared to a single representative crude oil, a "benchmark," of the quality class.

The quality of the crude oil dictates the level of processing and re-processing necessary to achieve the optimal mix of product output. Hence, price and price differentials between crude oils also reflect the relative ease of refining. A premium crude oil like West Texas Intermediate (WTI), the U.S. benchmark, has a relatively high natural yield of desirable naphtha and straight-run gasoline. Another premium crude oil, Nigeria's Bonny Light, has a high natural yield of middle distillates. By contrast, almost half of the simple distillation yield from Saudi Arabia's Arabian Light, the historical benchmark crude, is a heavy residue ("residuum") that must be reprocessed or sold at a discount to crude oil. Even West Texas Intermediate and Bonny Light have a yield of about one-third residuum after the simple distillation process.

Other Refinery Inputs

In addition to crude oil that runs through a simple distillation, a variety of other specialized inputs, usually to downstream units, enhance the refiner's capability to make the desired mix of products. Among these products might be unfinished (partly refined) oil, or imported residual fuel oil used as input to a vacuum distillation unit. The supply pattern for "reformulated gasoline" or RFG, the mandated low-pollution product first required in 1995, includes an important share of blending components that are classified as refinery inputs. These blending components include oxygenates but consist mainly of products that could be classified as finished gasoline in other jurisdictions or products that require little additional blending to be classified as finished gasoline. While they are counted as "refinery inputs," they are brought to saleable specifications in terminals and blending facilities, not in conventional refineries.

Major Refinery Outputs

The most important refinery product is motor gasoline, a blend of hydrocarbons with boiling ranges from ambient temperatures to about 400° F. The important qualities for gasoline are octane number (antiknock), volatility (starting and vapor lock), and vapor pressure (environmental control). Additives are often used to enhance performance and provide protection against oxidation and rust formation.

Kerosene is a refined middle-distillate petroleum product that finds considerable use as a jet fuel and around the world in cooking and space heating. When used as a jet fuel, some of the critical qualities are freeze point, flash point, and smoke point. Commercial jet fuel has a boiling range of about 375°-525° F, and military jet fuel 130°-550° F. Kerosene, with less-critical specifications, is used for lighting, heating, solvents, and blending into diesel fuel.

Liquified petroleum gas (LPG) consists principally of propane and butane and is produced for use as fuel and is an intermediate material in the manufacture of petrochemicals. The important specifications for proper performance include vapor pressure and control of contaminants.

Distillate fuels such as diesel fuels and domestic heating oils have boiling ranges of about 400°-700° F. The desirable qualities required for distillate fuels include controlled flash and pour points, clean burning, no deposit formation in storage tanks, and a proper diesel fuel cetane rating for good starting and combustion.

Residual fuels are heavier oils, known as No. 5 and No. 6 fuel oils, that remain after the distillate fuel oils and lighter hydrocarbons are distilled away in refinery operations. Many marine vessels, power plants, commercial buildings and industrial facilities use residual fuels or combinations of residual and distillate fuels for heating and processing. The two most critical specifications of residual fuels are viscosity and low sulfur content for environmental control.

A variety of solvents, whose boiling points and hydrocarbon composition are closely controlled, are produced in refineries. These include benzene, toluene, and xylene.

Petrochemicals are products derived from crude oil refining, such as ethylene, propylene, butylene, and isobutylene, that are primarily intended for use as chemical feedstocks in the production of plastics, synthetic fibers, synthetic rubbers, and other products.

Lubricants are produced in special refining processes. Additives such as demulsifiers, antioxidants, and viscosity improvers are blended into the base stocks to provide the characteristics required for motor oils, industrial greases, lubricants, and cutting oils. The most critical quality for lubricating-oil base stock is a high viscosity index, which provides for greater consistency under varying temperatures.

Coke is a residue high in carbon content and low in hydrogen that is the final product of thermal decomposition in the condensation process in cracking. It is almost pure carbon with a variety of uses from electrodes to charcoal briquets.

Asphalt is dark brown-to-black cement-like material obtained by petroleum processing and containing bitumens as the predominant component. It used primarily for road construction and roofing materials, and thus must be inert to most chemicals and weather conditions.

U.S. Refining Capacity

U.S. refining capacity, as measured by daily processing capacity of crude oil distillation units alone, has appeared relatively stable in recent years, at about 16 million barrels per day of operable capacity. While the level is a reduction from the capacity of twenty years ago, the first refineries that were shut down as demand fell in the early 1980's were those that had little downstream processing capability. Limited to simple distillation, these small facilities were only economically viable while receiving subsidies under the Federal price control system that ended in 1981. Some additional refineries were shut down in the late 1980's and during the 1990's, always, of course, those at the least profitable end of a company's asset portfolio. At the same time, refiners improved the efficiency of the crude oil distillation units that remained in service by "debottlenecking" to improve the flow and to match capacity among different units and by turning more and more to computer control of the processing. Furthermore, following government mandates for environmentally more benign products as well as commercial economics, refiners enhanced their upgrading (downstream processing) capacity. As a result, the capacity of the downstream units ceased to be the constraining factor on the amount of crude oil processed (or "run") through the crude oil distillation system. Thus crude oil inputs to refineries ("runs") have continued to rise, and along with them—given the stability of crude oil distillation capacity—capacity "utilization" rose throughout much of the 1990's. Utilization—the share of capacity filled with crude oil—reached truly record levels in the last half of the decade, nominally exceeding 100 percent for brief periods.

caption http://www.window.state.tx.us/specialrpt/energy/nonrenewable/images/exhibit4-16.png
http://www.window.state.tx.us/specialrpt/energy/nonrenewable/images/exhibit4-16.png

As with most aspects of the U.S. oil industry, the Gulf Coast is by far the leader in refinery capacity, with more than twice the crude oil distillation capacity as any other United States region. (The difference is even greater for downstream processing capacity, because the Gulf Coast has the highest concentration of sophisticated facilities in the world.) As discussed in the section on Trade, the Gulf Coast is the nation's leading supplier in refined products as in crude oil. It ships refined product to both the East Coast (supplying more than half of that region's needs for light products like gasoline, heating oil, diesel, and jet fuel) and to the Midwest (supplying more than 20 percent of the region's light product consumption.)

There are seasonal patterns in refinery input. In the United States, refinery runs mirror the overall demand for products—lower in the colder months and higher in the warmer months. In addition, as they move out of the gasoline season in the early autumn and then into the next gasoline season in the late winter, refiners routinely perform maintenance. The duration and depth of the cutback in refining activity during each maintenance season is affected by a variety of factors, including the relative strength of the market for refined products. Therefore, when stocks are high and demand slack, the refinery maintenance season is likely to be longer and deeper. Refinery activity will also respond to the market's need (and hence relative prices) for product, with changes in the level of crude oil throughput as well as emphasis on one product over another.

World Refining Capacity

Broadly speaking, refining developed in consuming areas, because it was cheaper to move crude oil than to move product. Furthermore, the proximity to consuming markets made it easier to respond to weather-induced spikes in demand or to gauge seasonal shifts. Thus, while the Mideast is the largest producing region, the bulk of refining takes place in the United States, Europe or Asia.

caption http://www.intertanko.com/Members-Information/Research-and-Projects/Research-and-Projects/World-refinery-capacity-is-tight/
http://www.intertanko.com/Members-Information/Research-and-Projects/Research-and-Projects/World-refinery-capacity-is-tight/
 

There have historically been a few exceptions—concentrations of refining capacity that were not proximate to consuming markets. A refining center in the Caribbean, for instance, supplied heavy fuel oil to the U.S. East Coast where it was used for power, heat, and electric generation. As the demand for this heavy fuel oil, or residual fuel oil, waned, so did those dedicated refineries. While the Caribbean refineries, as well as refineries in the Middle East and in Singapore, were built for product export, they are the exception. As such, most refineries meet their "local" demand first, with exports providing a temporary flow for balancing supply and demand.

The largest concentration of refining capacity is in North America (in fact, the United States), accounting for about one-quarter of the crude oil distillation capacity worldwide, as shown in the graph in Figure 5, and as discussed more fully below. Asia and Europe follow as refining centers. As also shown in the graph, North America (again, the United States) has by far the largest concentration of downstream capacity—the processing units necessary to maximize output of gasoline. The gasoline emphasis of course mirrors the demand barrel and hence refinery output in the different regions, since no other global region uses as much of its oil in the form of gasoline as North America does.

In addition to gravity and sulfur content, the type of hydrocarbon molecules and other natural characteristics may affect the cost of processing or restrict a crude oil's suitability for specific uses. The presence of heavy metals, contaminants for the processing and for the finished product, is one example. The molecular structure of a crude oil also dictates whether a crude stream can be used for the manufacture of specialty products, such as lubricating oils or of petrochemical feedstocks.

Refiners therefore strive to run the optimal mix (or "slate") of crudes through their refineries, depending on the refinery's equipment, the desired output mix, and the relative price of available crudes. In recent years, refiners have confronted two opposite forces—consumers' and government mandates that increasingly required light products of higher quality (the most difficult to produce) and crude oil supply that was increasingly heavier, with higher sulfur content (the most difficult to refine).

Further Reading

  • James H. Gary, Glenn Handwerk, Petroleum Refining: Technology and Economics (Hardcover), (CRC Press, 2001), ISBN: 0824704827

 

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Energy Information Administration and Occupational Safety & Health Administration. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Energy Information Administration and Occupational Safety & Health Administration should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

Glossary

Citation

(2013). Petroleum refining. Retrieved from http://www.eoearth.org/view/article/155206

0 Comments

To add a comment, please Log In.