Bitumen does occur naturally, but for all intents and purposes it is petroleum on which the world relies for its supplies of bitumen today. The bitumen content of crude can vary between 15 percent and 80 percent but the more normal range is 25 percent to 40 percent. In fact the three broad classifications for crude oils are: bitumen based, paraffin based bitumen and paraffin based
Bitumen is found in nature in several forms, from the hard, easily crumbled bitumen in rock asphalt to the softer, more viscous material found in tar sands and asphalt 'lakes'. It is commonly mixed with varying proportions of mineral or vegetable impurities that need to be extracted before it can be used effectively as an engineering material. However it may be found as an asphaltite, a natural bitumen without impurities that varies in the extent to which it is soluble in carbon disulphide.
Natural bitumen occurs, as does petroleum, as the result of the special decomposition of marine debris. It will have been moved over many thousands of years through porous rocks such as limestone or sandstone, often by volcanic action. In some areas notable for their petroleum resources, for example the Middle East, semi-fluid bitumen can be found oozing out of fissures near hot springs or seeping out of the ground.
Rock asphalt, with its variable and relatively low content of bitumen, tends to be found away from the places where bitumen is needed. It is costly to move around and to process.It is important to draw the distinction between bitumen and coal tar. The latter is obtained from the carbonization of coal and, although it is black and viscous in appearance, it has very different chemical properties.
Engineering projects in every part of the world, from the construction of transcontinental highways to the waterproofing of flat roof surfaces, rely on the particular properties of bitumen. Crude oil processed by the petroleum industry provides all but a small percentage of this vital material.
There are five major classifications of petroleum bitumen produced by the refining and manufacturing process:
Paving grade bitumen: (or asphalt cement) is refined and blended to meet road engineering and industrial specifications that take into account different climatic conditions. As a product it is the most widely used bitumen. It may also be considered as the parent bitumen from which the other types, listed below, are produced.
Cutback bitumen: consist basically of bitumen that has been diluted in order to make it more fluid for application, mainly in road making. Their fluidity depends on the degree of hardness of the bitumen base and the proportion of diluent (or flux) to bitumen.
Bitumen emulsions: the basic bitumen has also been diluted in order to facilitate application. Hot bitumen, water and emulsifier are processed in a high-speed colloid mill that disperses the bitumen in the water in the form of globules that are normally in the 5-10 micrometre size range but may be even smaller.
Industrial bitumen: (or oxidized bitumens) are made by blowing air through hot paving grade bitumen. The result is a product that softens at a higher temperature than that at which paving grade bitumen softens. It also has more rubberlike properties and its viscosity is much less affected by changes in temperature than is the case with paving grade bitumen.
Modified bitumen: are formulated with additives to improve their service performance by changing such properties as their durability, resistance to ageing, elasticity and/or plasticity.
In an oil refinery, the crude oil being processed is separated into different 'cuts' on the basis of their boiling range. In atmospheric distillation, the first stage of this continuous process, the crude oil is heated to 300°C-350°C and the more volatile components, e.g. gasoline and kerosene, are distilled off. This leaves a residue which is further distilled under vacuum. The actual processing temperature of the residue is 350°C-390°C, but because of the applied vacuum, the further effective distillation cut point is 500°C-560°C. This yields further volatile products known as vacuum distillates and leaves a non-volatile residue of high viscosity. Depending on the crude origin, the final residue may be used as bitumen without further processing. For some crude oils though, or to meet particular specifications, additional processing may be required such as air blowing of the residue. The air blowing process demands careful selection and control of process temperature, air rate and residence time t ensure consistent product. An important aspect of bitumen manufacture which requires expertise and experience is the selection of crudes which will yield a satisfactory product not all crude can be used to make bitumen.
Bitumen is stored at refineries and large depots in large permanent tanks made of mild-steel plate, with capacities of between 100 and 5 000 tons. Steam or hot oil is pumped through heating coils in the heavily insulated tanks so that the bitumen remains fluid. Smaller depots and big users store bitumen in small permanent or semi-mobile tanks of up to 60 tones capacity, heated by electricity or flame tubes.
As bitumen cools, it gradually becomes unworkable. Therefore the delivery of paving grade bitumen not only calls for the supply of bitumen to be provided at the right quality and quantity at the right time, but the product must also be kept between 150-200°C throughout the supply chain. When it is loaded into a road tanker, the temperature can drop by about 7-10°C and continue to fall by a further one or two degrees per hour while in transit.
Road tankers with capacities of between 10 and 40 tons take most of the bitumen from the refineries. They have insulated single-compartment tanks, usually with some kind of heating such as heating tubes. To transport bitumen to intermediary depots with reheating facilities, insulated rail cars are used. Their capacity ranges from 10 tons to 50 tons and they are also fitted with electric heaters or direct oil-fired flame tube heaters. When bitumen is moved by sea, it can be shipped in a variety of vessels from 300 ton barges to ocean-going tankers of up to 30,000 tons. The temperature of the hot-loaded bitumen is maintained by high pressure steam or hot oil passing through coils in the bitumen tanks
© 2000 Mena Report (www.menareport.com)
