Asphaltene

Asphaltenes are defined generally as the insoluble part of bitumen in n-heptane and soluble in methylbenzene (toluene). Other definitions exist, like the insolubility in n-pentane or n-hexane, which leads to different weight percentages – the lower the carbon number, the higher the content obtained from bitumen. By the definition of insoluble in n-heptane and soluble in toluene also long-chained hydrocarbons (with more than about 40 carbons) will count as asphaltenes, due to their solubility. Also asphaltene extraction methods, especially the used filter, and extraction temperature have an impact on the amount of asphaltenes and their properties.

asphaltenes after bitumen filtration

At room temperature asphaltenes form a black powder and constitute 5 to 31wt% of bitumen. The density of asphaltenes is about 1,15 g/cm³. Isolated dry asphaltenes do not melt in oxygen atmosphere, but in inert atmosphere they form a liquid-like product with no clear melting point and start to decompose at about 350 °C, leaving carbonaceous residue (coke).

Asphaltenes have a H/C ratio between 0,98 and 1,56 and have a higher content of hetero elements (nitrogen and oxygen). An average molecule consists of 4 to 10 fused aromatic rings and aliphatic chains. Some aliphatic chains are assumed to link multiple groups of rings (archipelago type). Asphaltenes contain more condensed rings and polar groups compared to the fractions from maltenes (n-heptane soluble part of bitumen. The condensed rings form almost planar sections of the asphaltene molecule, which can associate through pi-pi bonding to stacks. These stacks (nanoaggregates) are assumed to be the reason for structural formation and for short-range order. Additional almost all metals from bitumen are present in the asphaltenes. Usually the total amount of metals is below 0,1wt%, which leads to a content of metalorganic molecules (like porphyrines) of about 1 to 2wt% of asphaltenes. Three classes of compounds can be identified in asphaltenes: polyaromatics with relatively few saturated substituents, porphyrines (insoluble in n-heptane) and other metal organic compounds and n-alkanes with more than about 40 carbons.

porphyrine

possible molecular asphaltene structure

Asphaltenes have a large contribute on all physical properties, for example with increasing asphaltene content viscosity and density increases.

crude oil - origin of bitumen part 2

The first step in petroleum refinery is rectification. Since the boiling points of the compounds of petroleum lie close to each another, differential distillation is employed for the primary separation. 

The refinery involves one or more distillation towers, depending on the desired products. The classification into atmospheric towers and vacuum towers is according to the predominant pressure inside the operating tower. Atmospheric towers operate under "normal" pressure and vacuum towers operate at lower pressure (1333 to 13332 Pa), induced by vacuum pumps or steam ejectors and require different tower geometries. Due to lower pressure and temperature under 425 °C thermal cracking of components is avoided.

During these treatments of separation no chemical changes occur, it is based on the physical properties of petroleum. Further processing of bitumen involves chemical changes and are thermal cracking, catalytic cracking, visbreaking, coking and hydroprocessing.

The vacuum residue has not always the desired properties and so it is post processed. To receive “harder” bitumen (decreasing needle penetration value, change in temperature-viscosity property) the bitumen can be oxidized with air. There are two grades of oxidised bitumen: semi-blown and blown bitumen. The blowing column is about 70% filled with 230 to 260 °C hot vacuum residue. The air is introduced at the bottom and occasionally for more intense mixing baffles or agitation systems are installed. To control the temperature of the generally exothermic reaction water can be sprayed from the top or injected laterally.

Further the vacuum residue can be deasphalted. With solvent (alkane) the asphaltene fraction is removed from bitumen.

For the final product bitumen is blended with itself or other compounds resulting in modified bitumen. The compounds for modified bitumen are for example emulsifier (for production of emulsion), petroleum distillates, polymers, sulphur or waxes. The final product is stored in heated tanks at a temperature of about 185 °C. To avoid oxidation (oxidative hardening) the tanks are pressurise with nitrogen. The delivery too is done by tank lorries at temperature 185 °C ±5 °C.

crude oil - origin of bitumen part 1

The properties of bitumen depend on the properties of the crude oil from which it is manufactured and the treatment during manufacturing. There are similarities in relation to elemental composition between crude oil and bitumen. Nitrogen and sulphur contents are increasing, due to higher concentration of these elements in heavier compounds and hydrogen is decreasing, due to extraction of lighter hydrocarbon chains during refinery. The metal content varies in crude oil from 0,01 to 0,04wt%. About thirty different metals are found in different crude oils. The most common are vanadium, nickel, iron, zinc, mercury, boron, sodium, potassium, calcium and magnesium.

The origin of crude oil, base on the organic petroleum hypothesis, is the biomass (special microorganisms like phytoplankton) that inhabited the seas and oceans a long time ago. The concentration of organic material increases with depth, so the highest concentration is found at the bottom. Due to mineral accumulation these organic material can be trapped and encapsulated from decomposition. With the build up of additional layers of minerals and the movements of the lithosphere plates, trapped organic material can pass in zones of rift or subduction where the temperature increases by 100 to 400 °C (minimum depth of 2 to 3 km). In that temperature range the dissociation of various compounds takes place and provides the necessary energy for the formation of crude oil.

Nowadays we extract crude oil from those entrapped depositions. The extraction happens under the natural layer pressure, or by pumping. With both operation methods the petroleum layer and added water layer are concurrently extracted. Petroleum and water then form an emulsion. The continuous phase is most frequently petroleum and the discontinuous phase is accordingly water. The emulsion intensifies depending on the extraction method and additionally during transportation through pipes. Further the presence of emulsifiers influences the formation and properties of an emulsion. The used method to dry petroleum depends on the amount and condition of water. Non-stabilized emulsion can be separated by settling methods or by settling with moderate heating. A stabilized emulsion is more difficult to separate. The methods include intense heating, chemical processing, electrical processing or a combination of those methods. Often during drying a desalting takes place, by removing the water with the dissolved minerals (chemical or electrostatic separation at 110 to 160 °C).

After this the crude oil is ready for refinery.