Asphalt (petroleum)

This article is about Asphalt (petroleum). For other uses of the term Asphalt, please see Asphalt (disambiguation).

Petroleum asphalt is a sticky, black and highly viscous liquid or semi-solid that is present in most petroleum crude oils and in some natural deposits. Petroleum crude oil is a complex mixture of a great many different hydrocarbons. Petroleum asphalt is defined as that part of crude oil which is separated from the higher-boiling hydrocarbons in crude oil by precipitation upon the addition of lower-boiling hydrocarbon solvents such as propane, pentane, hexane or heptane. The precipitated material consists of asphaltenes which have an average molecular weight of about (800 - 2500 g/mole) and exist in the form of flat sheets of polyaromatic condensed rings with short aliphatic chains.

Over the years, petroleum asphalt has been referred to as bitumen, asphaltum or pitch. The terminology varies from country to country and from individual to individual. Asphalt is often confused with coal tar (or coal pitch) derived from the pyrolysis of coal and which has a different chemical structure than asphalt.

When petroleum asphalt is combined with construction aggregate (sand, gravel, crushed stone, etc.) for use in road construction or paving, it has often been referred to as asphaltic concrete, asphaltic cement, bituminous concrete, blacktop or road tar.

The natural deposits of asphalt (often referred to as tar) include asphaltic lakes such as Bermudez Lake in Venezuela, and Pitch Lake in Trinidad. Other natural deposits include oil sands (often called tar sands) and the two largest deposits of oil sands are such as in Alberta, Canada and the Orinoco Oil Belt area of Venezuela.

History of asphalt

The documented use of naturally occurring asphalt dates back almost to 4000 B.C.:

• 3800 B.C.: Asphalt used for caulking boats made of reeds.
• 3500 B.C.: Asphalt used as cement for jewelry.
• 3000 B.C.: Asphalt used as construction cement by the Sumerians of ancient Mesopotamia (now known as Iraq). Also used to seal a bathing pool or water tank in the city of Mohenjo-Daro in the Indus Valley Civilization located in what is now Pakistan.
• 2500 B.C.: Asphalt and other petroleum oils used in ancient Egypt for embalming mummies. (The Persian word for asphalt is mumiyah which may be related to the English word for mummy).
• 1000 B.C.: Asphalt used for waterproofing by lake dwellers in what is now Switzerland.
• 625 – 650 B.C.: The first recorded use of asphalt as a road-building material was in Babylon during the reigns of King Nabopolassar and his son, King Nebuchadnezzar.
• 500 B.C.: Asphalt mixed with sulfur was used as an incendiary device in the Greek wars. (The word asphalt comes from the Greek word asphaltos, meaning secure in English.)
• 300 B.C. – A.D. 250: Reported occurrences of asphalt and oil seepages in Mesopotamia and the use of liquid asphalt as an illuminant in lamps.
• A.D. 750: First reported use in Italy of asphalt as a coloring material in paintings.

Europeans exploring the Americas discovered natural deposits of asphalt. Writing in 1595, Sir Walter Raleigh described a lake of asphalt on the island of Trinidad, near Venezuela. He used it to recaulk his ships.

In the late 1700s and early 1800s, first Pierre-Marie-Jérôme Trésaguet of France, then Thomas Telford and subsequently John Loudon McAdam (both of Scotland) perfected the leveling, draining and construction of roads using layers of broken stones and gravel. In the period of 1860 – 1880, to reduce road dust and road maintenance, builders began using hot coal tar to bond the stones together. Such roads became named after McAdam and known as tarmacadam roads, later shortened to tarmac.

In 1870, Belgian chemist Edmond J. DeSmedt laid the first true asphalt pavement in the United States of America in Newark, New Jersey. He also paved Pennsylvania Avenue in Washington, D.C. in 1876 using 54,000 square yards (45,140 square metres) of sheet asphalt from Pitch Lake in Trinidad.

During the early 1900s, coal gasification was being widely used to produce town gas and the by-product tar produced during coal gasification was a readily available product. That tar was extensively used in the construction of tarmacadam (or, more simply, tarmac) roads.

By 1907, asphalt from petroleum refineries had outstripped the use of natural asphalt from Trinidad or elsewhere.. Later in the 1900s, when natural gas replaced town gas, asphalt from petroleum refineries dominated the asphalt paving market from that point on. By the early 1990s, asphalt paving mixture producers in the United States used more than 50 × 10⁶ barrels (7.95 × 10⁶ cubic metres) of petroleum asphalt per year. Of the 2.27 × 10⁶ miles (3.65 × 10⁶ kilometers) of paved road in the United States, 94 percent of them are surfaced with asphalt, including 65 percent of the interstate system.

Asphalt production from petroleum crude oil

(PD) Image: Milton Beychok
Schematic block flow diagram of how petroleum asphalt is produced in petroleum refineries.

As mentioned earlier above, petroleum crude oil is essentially a complex mixture of a great many hydrocarbons. There are a great many different crude oil sources and each of the crude oils from those sources has its own unique mixture of hydrocarbons. Upon being fed into a petroleum refinery, the crude oil is initially distilled (i.e., boiled) to remove and recover various products such as naphtha which is subsequently further refined to produce gasoline (petrol), jet fuel, diesel oil, fuel oil and so-called vacuum oils which may also be further refined to produce more gasoline.

The initial distillation of the petroleum crude oil is done in two steps as shown in the adjacent diagram:

• The first step is atmospheric distillation, at an absolute pressure slightly above atmospheric pressure, after heating the crude oil in a process furnace to a temperature of about 395 °C. That temperature cannot be exceeded because, above that temperature, the hydrocarbon molecules will crack (break) into smaller molecules and coke that is essentially solid carbon which would plug up the furnace tubes.

• The second step is vacuum distillation at an absolute pressure of 10 – 40 mmHg. The temperature must still be limited to being no higher than about 395 °C. However, under the vacuum of 10 – 40 mmHg, that is equivalent to a boiling point of about 565 °C at atmospheric pressure.

The residual bottoms product from the atmospheric distillation is referred to as atmospheric resid or atmospheric residuum. Similarly, the residual bottoms product from the vacuum distillation is referred to as vacuum resid or vacuum residuum. In some refineries that process very heavy crude oils (i.e., crude oil with a higher than average crude oil density), the atmospheric resid may be suitable for further processing to obtain a petroleum asphalt end-product. However, again as shown in the adjacent diagram, it is more generally the vacuum resid that is further processed to derive an end-product of petroleum asphalt.

The vacuum resid may be used as: a heavy fuel oil blending stock; for further processing in a delayed coker to produce hydrocarbon gases, coker naphtha, coker gas oil and petroleum coke; as a feed stock for processing into lubricating oil; or routed through a de-asphalting process to produce petroleum asphalt. The adjacent diagram depicts a portion of the vacuum resid being routed to use as a heavy fuel oil blending stock and a portion being routed through a de-asphalting process.

There are a number of de-asphalting processes. Perhaps the most common one is known as propane de-asphalting which uses supercritical propane (meaning the propane is at temperature and pressure conditions above its critical point) as a solvent to extract and separate the lower-boiling, lower-density hydrocarbon oil molecules from the asphaltene molecules. The extraction occurs in a vertical tower operating at an absolute pressure of about 500 psi (34 atmospheres), a bottom temperature of 105 °F (40 °C) and a top temperature of 140 °F (60 °C). The propane enters the extraction tower at the bottom and travels upward counter-current to the asphalt that enters the tower at the top and flows downward.

The propane solvent in the separated deasphalted oil from the top of the extraction tower is stripped out and recycled for re-use in the extraction tower. The solvent-free, deasphalted oil may then be used as a feedstock component in other petroleum refinery processes such as a fluid catalytic cracker or a hydrocracker to produce a gasoline blending component.

Solvent in the asphalt stream from the bottom of the extraction tower is also stripped from the asphalt stream and recycled for re-use. The solvent-free asphalt may then be marketed as end-product petroleum asphalt. Alternatively, all or some of the asphalt product from the de-asphalter may be processed in an air-blowing process to produce what is known as air-blown or oxidized asphalt.

As shown in the adjacent diagram, an air-blowing process consists of using an air compressor to blow air through the liquid asphalt at a temperature ranging from 235 to 290 °C and being careful to avoid any combustion of the asphalt by remaining about 25 °C below the flash point of the feedstock asphalt. In brief, the asphalt fed into an air-blowing process is oxidized by the oxygen in the air. The three most important operating variables in the asphalt air-blowing process are the rate of air injection, the system temperature and the amount of time that the asphalt is kept in contact with the air.

The air-blown product asphalt has a higher temperature softening point than asphalt which has not been air-blown and that is a desirable property for certain uses of petroleum asphalt.

The end-product petroleum asphalt is typically maintained at a temperature of about 150 °C during storage at the petroleum refinery as well as during transportation to the asphalt end-users.

Some important physical properties of petroleum asphalt

There are a number of physical properties that are important to the end-users of petroleum asphalt, including:

• Softening Point: The range of temperatures at which the asphalt softens (as determined by ASTM test D-36). The softening point is sometimes thought of as the melting point.

• Penetration: The distance that a weighted needle or cone will sink into the asphalt during a set period of time at a prescribed temperature (as determined by ASTM test D-5). Penetration results are presented in units of 0.1 mm (i.e., the units are given as dmm). Thus, a penetration of 40 means the needle has penetrated 4 mm.

• Viscosity: An indication of how viscous the liquid asphalt is at various temperatures (as determined by ASTM tests D-88 and D-2170).

• Flash Point: The temperature at which that the heated asphalt will ignite briefly (as determined by ASTM test D-92).

The softening point and penetration are the most commonly used measurements for classifying an asphalt's properties. Generally, as the softening point increases, the viscosity also increases, the penetration drops and the flash point rises.

Uses of petroleum asphalt

(CC) Photo: Asphalt Education Partnership
Close-up photograph of asphalt paving.

Road construction

The largest use of petroleum asphalt is for making asphaltic concrete for road construction and accounts for approximately 80% of the petroleum asphalt consumed in the United States. The asphalt is used as the binder or glue that holds together the aggregate of sand, gravel, crushed stone, slag or other material.

There are various mixtures of asphalt with other materials that are used in road construction and other paving applications:

• Rolled asphaltic concrete that contains about 95% aggregate and 5% petroleum asphalt binder.
• Mastic asphalt that contains about 90–93% aggregate and 7–10% petroleum asphalt binder.
• Asphalt emulsions that contain about 70% petroleum asphalt and 30% water plus a small amount of chemical additives.
• Cutback asphalt that contains petroleum solvents (referred to as cutbacks).

Only rarely, if ever, are air-blown asphalts used in asphalt-aggregate mixtures for paving purposes.

Roofing shingles

Roofing shingles account for most of the remaining 20% of asphalt consumption in the United States. Most of the petroleum asphalt used in manufacturing shingles is air-blown asphalt. There are three major grades (types) of asphalt used for roofing:

• Type 1: Penetration at 77 °F (25 °C) = 25 − 50 dmm and softening point of 140 − 150 °F (60 − 66 °C).
• Type 2: Penetration at 77 °F (25 °C) = 20 − 30 dmm and softening point of 166 − 175 °F (74 − 79 °C).
• Type 3: Penetration at 77 °F (25 °C) = 15 − 25 dmm and softening point of 190 − 205 °F (88 − 96 °C).

Asphalt roofing shingles are the dominant roofing material in the United States, representing more than three-fifths of the total installed shingle. They are a very economical roofing choice, especially so for house with sloping roofs.

There are two basic types of asphalt shingles, those with an organic base and those with a fiberglass base:

• Fiberglass shingles have a glass fiber reinforced mat coated with asphalt that contains mineral fillers. The glass fiber mat is not waterproof by itself, but the asphalt coating makes it waterproof.

• Organic base shingles are generally paper saturated with asphalt for waterproofing. Then a top coating of asphalt is applied and ceramic granules are embedded in that top coating. Organic shingles contain about 40% more asphalt than fiberglass shingles which makes them heavier and more resistant to being blown off in windy conditions.

In hot climate areas, the amount of sunlight reflectance provided by the shingles is an important property in evaluating the performance of a specific brand of shingle. Reflectance is provide by the embedded granules and by colored shingle coatings.

According to the U.S. Census Bureau, asphalt shingles accounted for more than half of the residential roofing market during the early 2000s.

Other uses

• Asphaltic concrete is widely used for paving vehicle parking lots and aircraft landing and take-off runways in airports around the world
• Canal and reservoir linings as well as dam facings
• Floor tiles
• Battery casings
• Waterproofing of fabrics and various other materials
• Treatment of fence posts and other wooden objects
• Cattle sprays

Asphalt production from oil sands

Athabasca Oil Sands
Operating facilities (February 2009)

Type of project	Number of projects	bbl/day of asphalt
In situ extraction	12	595,000
Surface mining	4	1,018,000
Upgrading	3	1,002,000
Note: 1 bbl/day = 158.987 L/day = 0.159 m3/day

There are three large natural deposits of asphalt (or bitumen) in Alberta, Canada that are known as the Athabasca Oil Sands and their total surface area is about 54,000 square miles (141,000 square kilometres). The oil sands consist of about 83% sand, 3% clay, 4% water and 10% asphalt.

The proven reserves of asphalt in those deposits are about 1.7 × 10¹² barrels (270 Gm³ ). About 10% of that is recoverable by current (2009) technology and it is estimated that, with new technologies, the recoverable amount could be about 18-19% which would be about 315 × 10⁹ barrels (50 Gm³ ). For comparison, the estimated reserves of petroleum crude oil in Saudi Arabia (as of early 2008) are about 265 × 10⁹ barrels (42 Gm³ ).

Asphalt is currently being extracted from the Athabasca oil sands and being converted into synthetic petroleum crude oil (referred to as syncrude). About 20% of the Athabasca oil sands can be and is being extracted by surface mining techniques. In addition, in situ techniques are being used to extract asphalt from various depths beneath the surface of the oil sands deposits. After extraction, the asphalt is converted into synthetic petroleum crude oil in refining facilities that are referred to as upgraders. Currently, about 1,000,000 barrels (159,000 m³ ) of the asphalt are being converted into synthetic crude oil (see adjacent table).

On average, upgrading of about 1.16 barrels of asphalt is required to produce 1 barrel of syncrude. The synthetic crude oil is subsequently transported to conventional petroleum refineries for processing.

Very little, if any, of the extracted asphalt is used for road construction because the Athabasca oil sands are far from the major markets for road construction asphalt and the transportation costs would be too high.