Asphalt (petroleum)

Asphalt production from petroleum crude oil

(PD) Image: Milton Beychok
Schematic 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, heating 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 psia (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.