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What is Bitumen and How is it Made?

Posted by Shell Zhang on

bitumen asphalt


Bitumen is defined in the Oxford English Dictionary as ‘a tarlike mixture of hydrocarbons derived from petroleum naturally or by distillation, and used for road surfacing and roofing’ (Oxford University Press, 1996).

It is widely believed that the term bitumen originated in the ancient and sacred language of Hindus in India, Sanskrit; in which jatu means ‘pitch’ and jatu-krit means ‘pitch creating’. These terms referred to the pitch produced by some resinous trees. The Latin equivalent is claimed by some to be originally gwitu-men (‘pertaining to pitch’) and by others to be pixtu-men (‘bubbling pitch’), which was subsequently shortened to bitumen before passing via French into English.

There are several references to bitumen in the Bible, although the terminology used can be confusing. In Genesis, Noah’s ark is ‘pitched within and without with pitch’, and Moses’ juvenile adventure is in ‘an ark of bulrushes, daubed with slime and with pitch’. Even more perplexing are the descriptions of the building of the Tower of Babel. The Authorised Version of the Bible says ‘they had brick for stone, and slime had they for mortar’; the New International Version states that ‘they used bricks instead of stone and tar instead of mortar’; Moffat’s 1935 translation says ‘they had bricks for stone and asphalt for mortar’; but the New English Bible states that ‘they used bricks for stone and bitumen for mortar’.

The ancient uses of natural bitumens or pitch continued in those inhabited parts of the world where deposits were readily available. However, there seems to have been little development of usage elsewhere. In many countries, none of the present major uses of bitumen were introduced until the end of the nineteenth century. However, there would appear to have been some wider knowledge of large sources of natural bitumen such as lake asphalt in Trinidad. In the middle of the nineteenth century, attempts were made to utilise rock asphalt from European deposits for road surfacing and, from this, there was a slow development of the use of natural products for this purpose, followed by the advent of coal tar and, later, of refined bitumen manufactured from crude oil.

Definition of bitumen

The term ‘bitumen’ is used to describe refined bitumen, a hydrocarbon product produced by removing the lighter fractions (such as liquid petroleum gas, petrol and diesel) from crude oil during the refining process. In North America, bitumen is commonly known as asphalt binder or asphalt. For the purpose of this post, the term ‘bitumen’ is used.

A comprehensive definition of refined bitumen is used in the industry document The Bitumen Industry – A Global Perspective (Eurobitume and the Asphalt Institute, 2011) and is reproduced here verbatim:

Bitumen is an engineering material and is produced to meet a variety of specifications based upon physical properties. Bitumen is the residual product from the distillation of crude oil in petroleum refining. The basic product is sometimes referred to as ‘straight run’ bitumen and is characterised by CAS# 8052-42-4 or 64741-56-6 which also includes residues obtained by further separation in a deasphalting process. Bitumen can be further processed by blowing air through it at elevated temperatures to alter its physical properties for commercial applications. The general characteristics of oxidized bitumen are described by CAS# 64742-93-4. The vast majority of petroleum bitumens produced conform to the characteristics of these two materials as described in their corresponding CAS definitions.

Bitumen is produced to grade specification either directly by refining or by blending. Bitumen should not be confused with coal derived products such as coal tar or coal tar pitches. These are manufactured by the high temperature pyrolysis (.8008C) of bituminous coals and differ from bitumen substantially in comparison and physical characteristics. The differences between bitumen and coal-tar products are well defined in the literature.

Similarly, bitumen should not be confused with petroleum pitches (CAS# 68187-58-6), which are often aromatic residues, produced by thermal cracking, coking or oxidation from selected petroleum fractions. The composition of petroleum pitches differs significantly from bitumen.

Bitumen also should not be confused with natural or lake asphalt such as Trinidad Lake Asphalt, Gilsonite, rock asphalt and Selenice. These products are unrefined and not produced by refining of crude oil. They often contain a high proportion of mineral matter (up to 37% by weight) and light components, leading to a higher loss of mass when heated.

Bitumen is manufactured during the distillation of crude oil. It is generally agreed that crude oil originates from the remains of marine organisms and vegetable matter deposited with mud and fragments of rock on the ocean bed. Over millions of years, organic material and mud accumulated into layers some hundreds of metres thick, the substantial weight of the upper layers compressing the lower layers into sedimentary rock. Conversion of the organisms and vegetable matter into the hydrocarbons of crude oil is thought to be the result of the application of heat from within the Earth’s crust and pressure applied by the upper layers of sediments, possibly aided by the effects of bacterial action and radioactive bombardment. As further layers were deposited on the sedimentary rock where the oil had formed, the additional pressure squeezed the oil sideways and upwards through porous rock. Where the porous rock extended to the Earth’s surface, oil seeped through to the surface. Fortunately, the majority of the oil and gas was trapped in porous rock, which was overlaid by impermeable rock, thus forming gas and oil reservoirs. The oil remains here until its presence is detected by seismic surveys and recovered by drilling through the impermeable rock.