Crude Oil

Crude oil consists of four different types of hydrocarbon groups:

• Paraffin (alkane) molecules are straight chains of five or more carbon atoms that have only saturated bonds. If the chain of carbons consists of more than 18 carbon atoms then it is a wax and forms a waxy crude oil. General formula: CnH2n+2.

Figure 12: Paraffin Molecule (Hyne 2001)

• Naphthene (cycloparaffin) molecules are chains of closed circle with saturated bonds with carbon atoms. Also, these molecules consist of five carbon atoms or more in length. Since crude oil with high naphthene content usually gives high asphalt

content (heavier oil components) the value of such oils are lower. General formula:

CnH2n-6.

Figure 13: Naphthene Molecule (Hyne 2001)

• Aromatic (benzene) molecules are also chains of closed ring but there are one or more unsaturated bonds between the carbon atoms. The aromatic molecules consist of six or more carbon atoms in length. For a refinery, aromatic rich crude oil gives the highest octane gasoline and thereby being a valuable input. The refiner must pay a higher price for such crude oil.

Figure 14: Aromatic Molecule (Hyne 2001)

• Asphaltic molecules are large with more than 40 carbon atoms and therefore have a high boiling point since it is so heavy. Under surface condition, the aspaltic molecules appear in a solid or a semi-solid phase.

Figure 15 The content of different Hydrocarbons in Crude Oil, Hyne 2001

The refining process (discussed in details later in this thesis) uses chemicals, catalysts, heat, and pressure to separate and combine the basic types of hydrocarbon molecules naturally found in crude oil into groups of similar molecules. The refining process also rearranges their structures and bonding patterns into different hydrocarbon molecules and compounds.

Therefore it is the type of hydrocarbon (paraffinic, naphthenic, or aromatic) rather than its specific chemical compounds that is significant in the refining process

There are mainly two types of crude oil that a refinery uses: the asphalt based crude oil and the paraffin based crude oil. The asphalt based crude oil has little paraffin wax and its colour is black. The paraffin based crude oil has little asphalt and is a bit greenish. Also, a combination of these two crude oils is used.

The asphalt based crude oil will after a refinery process yield much high-grade gasoline and asphalt. While the paraffin based crude oil will yield much paraffin wax, lubricating oil and kerosene. The figure below shows the yield of the average crude oil but it is strongly dependent on type crude oil and refinery system.

Figure 16 The average yield of Crude Oil in percent, Source: American Petroleum Institute

The different crude oils have different properties regarding to density, sulphur content and viscosity. We will shortly discuss these different properties but first we present a table of the properties for the different crude streams. A crude stream consists of oil from one or several production sites that are blended as one product.

Figure 17 Properties of different types of Crude Oil, Source: Hyne 2001

Other Hydrocarbons Alkenes

Alkenes are mono-olefins with the general formula CnH2n and contain only one carbon-carbon double bond in the chain. The simplest alkene is ethylene, with two carbon-carbon atoms joined by a double bond and four hydrogen atoms. Olefins are usually formed by thermal and catalytic cracking and rarely occur naturally in unprocessed crude oil. Example of simplest Alkene: Ethylene (C2H4), Typical Alkenes with the same chemical formula (C4H8) but different molecular structures: 1-Butene and Isobutene

Dienes and Alkynes

Dienes, also known as diolefins, have two carbon-carbon double bonds. The alkynes, another class of unsaturated hydrocarbons, have a carbon-carbon triple bond within the molecule. Both these series of hydrocarbons have the general formula CnH2n-2. Diolefins such as 1, 2-butadiene and 1, 3-butadiene, and alkynes such as acetylene, occur in C5 and lighter fractions from cracking. The olefins, diolefins, and alkynes are said to be unsaturated because they contain less than the amount of hydrogen necessary to saturate all the valences of the carbon atoms. These compounds are more reactive than paraffins or naphthenes and readily combine with other elements such as hydrogen, chlorine, and bromine. Example of simplest Alkynes: Acetylene (C2H2), Typical Diolefins with the same chemical formula (C4H6) but different molecular structures: 1, 2-Butadiene and 1, 3-Butadiene.

Density of Crude Oil

Crude oil is commonly reported in terms of degrees API (American Petroleum Institute) to standardise equipment and procedures in the petroleum industry, obtained from the relative density using the following formula:

Formula I: Calculation of API - the density of Crude Oil, Source: Favennec 2001

The density of water at 22ºC is approximately 1 kg/m3, while the density of oil varies around 0.8 kg/m3. This gives clean water at 22ºC and ^◦API of 10 while crude oils mainly vary from 25 to 35. However, the ^◦API of crude oil can vary as much as from 5 to 55. Crudes can be classified by gravity into:

• light crude oils, with gravities higher than 33^ºAPI

• medium crude oils, with gravities between 22 and 33^ºAPI

• heavy crude oils, with gravities less than 22^ºAPI

A light crude oil contains a higher proportion of the smaller molecules that make up the light and middle distillates than heavy crude. A heavy crude oil contains a high proportion of the very large molecules, present in fuel oil.

Heavy oils are cheaper since they yield much less of the valuable refined products like gasoline. In addition, heavy oils are more viscous, making them more difficult to transport in pipes. Often the heavy oils have to be heated up somewhat to make the transportation smoother. Sometimes the heavy oil is mixed with lighter oil like the mix of heavy oil from Grane production site with the much lighter oil from Oseberg. In this way, the seller of the oil can achieve a higher sales price than otherwise.

Sweetness of Crude Oil

The sulphur content in a certain crude oil decides if the oil is sweet or sour. Sweet crude oils have less than one per cent sulphur by weight and sour crude oils have more than one per cent sulphur. A refinery is willing to pay 1-3 USD per barrel (Hyne 2001) in premium for sweet crude oil. Thus sulphur is an unwanted element due to its ability of being an environmental hazard and harming refinery equipment.

Sulphur may be present in crude oil as hydrogen sulphide (H2S), as sulphur compounds such as mercaptans, sulphides, disulfides, thiophenes, etc. or as elemental sulphur. Each crude oil has different amounts and types of sulphur compounds but as a rule the proportion, stability, and complexity of the compounds are greater in heavier crude-oil fractions. Hydrogen sulphide is a primary contributor to corrosion in refinery processing units. Other corrosive substances are elemental sulphur and mercaptans. Moreover, the corrosive sulphur compounds have an obnoxious odour. Pyrophoric iron sulphide results from the corrosive

action of sulphur compounds on the iron and steel used in refinery process equipment, piping, and tanks. The combustion of petroleum products containing sulphur compounds produces undesirables such as sulphuric acid and sulphur dioxide. Catalytic hydro treating processes such as hydrodesulphurisation remove sulphur compounds from refinery product streams. Sweetening processes either remove the obnoxious sulphur compounds or convert them to odourless disulfides, as in the case of mercaptans.

Pour Point of Crude Oil

As mentioned earlier, if the paraffin molecules are 18 carbon atoms or more in chain they are waxes. A crude oil with significant amount of wax is called a waxy crude oil. A waxy crude oil is liquid in the reservoir where it is very hot but when arriving to the surface it cools down and becomes solid. This can stop the flow in pipes and force maintenance work. The amount of wax is indicated by the pour point of oil. The pour point gives the lowest temperature of the crude oil where the crude oil goes from liquid phase to solid phase, and can vary between minus 60ºC to plus 52ºC.

On the NCS, heat exchangers in the transporting pipeline system are used to heat up cold crude oil with the heat from a warm crude oil. By this way, one can secure the flow of crude oil in the pipelines.

Other Elements of Crude Oil

Oxygen compounds such as phenols, ketones, and carboxylic acids occur in crude oils in varying amounts.

Nitrogen is found in lighter fractions of crude oil as basic compounds, and more often in heavier fractions of crude oil as non-basic compounds that may also include trace metals such as copper, vanadium, and/or nickel. Nitrogen oxides can form in process furnaces. The decomposition of nitrogen compounds in catalytic cracking and hydro cracking processes forms ammonia and cyanides that can cause corrosion.

Metals, including nickel, iron and vanadium are often found in crude oils in small quantities and are removed during the refining process. Burning heavy fuel oils in refinery furnaces and boilers can leave deposits of vanadium oxide and nickel oxide in furnace boxes, ducts,

and tubes. It is also desirable to remove trace amounts of arsenic, vanadium, and nickel prior to processing as they can poison certain catalysts.

Crude oils often contain inorganic salts such as sodium chloride, magnesium chloride, and calcium chloride in suspension or dissolved in entrained water (brine). These salts must be removed or neutralized before processing to prevent catalyst poisoning, equipment corrosion, and fouling. Salt corrosion is caused by the hydrolysis of some metal chlorides to hydrogen chloride (HCl) and the subsequent formation of hydrochloric acid when crude is heated. Hydrogen chloride may also combine with ammonia to form ammonium chloride (NH4Cl), which causes fouling and corrosion.

Carbon dioxide may result from the decomposition of bicarbonates present in or added to crude, or from steam used in the distillation process.

Some crude oils contain naphthenic (organic) acids, which may become corrosive at temperatures above 232° C when the acid value of the crude is above a certain level.