• Ateration. Alkylation
• Barrel
• BTU (British Thermal Unit)
• Cracking
• Fractional distillation
• Gasoline (petrol)
• Refining process. Refining
• Trap (oil or gas trap)
• Unification. (Catalytic) Reformig

ALTERATION - ALKYLATION

The “alteration” or “alkylation” process is used in order to rearrange molecules in hydrocarbons which have been divided through cracking (see the related term in the glossary). The structure of the molecules in one fraction obtained through the fractional distillation or cracking are rearranged to produce another type of hydrocarbon. This process is usually called “alkylation”. It is a refining process which enables the chemical combination of isobutene with olefin hydrocarbons (propylene or butylenes). The process is controlled through a certain pressure and temperature in presence of an acid catalyst (sulfuric acid or hydrofluoric acid) [1].

Low molecular weight compounds (propylene or butylenes) are combined together in presence of a catalyst. The product of the process are high octane hydrocarbons which are then used in gasoline blends. The alteration process is therefore a crucial part of the refining process of crude oil when this is aimed at producing gasoline or other fuels which need a high level of calorific power. High octane hydrocarbons are used in motor or aviation gasoline blends to reduce the anti-knock value [2]. These types of fuels are also the most profitable product of the oil refining process.

BARREL

The word “barrel” (bbl) is generally used to indicate a measure of volume for petroleum products. One barrel is the equivalent of 42 U.S. gallons or 0.15899 cubic meters (9,702 cubic inches) [3]. 7.3 barrels are equal to one ton. 6.29 barrels are equal to one cubic meter and one barrel is equal to 159 liters approx.

7.3 bbls = One ton: 6.29 bbls = One cubic meter: One bbl = 159 liters approx [4].

A total production of one barrel per day is equal to 50 tonnes per year.

The “barrel” has been used for the first time in the United States and then adopted on world energy markets to measure quantities of crude oil traded worldwide. However, the “barrel” is also a unit of measurement frequently used to refer to quantities of oil derivatives such as fuel or other liquids. The price of crude oil is calculated on the barrel on world energy markets.

Oil production at a specific well or field is generally measured in barrels of oil per day (bbl/d). Besides crude, the “barrel” is also used to measure the level of water daily produced from a field or a well (barrels of water per day) and to measure the total production of both oil and water from a field or a well (barrels of liquid per day) [5]. The barrel of oil equivalent (bboe) is a unit of energy based on the approximate energy released by burning one barrel of crude oil [6]. The bboe is used by oil and gas companies in their financial statements as a way of combining oil and natural gas reserves and production into a single measure. A bboe is equivalent to 6 mcf (thousand cubic feet) of natural gas. This is derived by valuing an mcf as containing about 1/6 of the energy of a barrel of oil [7].

TOTAL PRODUCTION FROM ONE BARREL [8]:

BTU – BRITISH THERMAL UNIT

The word BTU (which stand for British Thermal Unit) indicates a unit of measurement of heating value of a fuel [9]. This unit of measurement is used to identify the quantity of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit [10]. The BTU is generally used to measure the energy that can be obtained from gas hydrocarbons.

In North America, the term "BTU" is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills, and air conditioners. When used as a unit of power, BTU per hour (BTU/h) is understood, though this is often confusingly abbreviated to just "BTU". In the UK and other parts of the world it is written BTU.

BTU can be converted into other more popular units of measurements. In fact, the BTU is a unit of measurement generally used in the power and stream generation sector and energy-related industries while it has been replaced in scientific use by the International system of measures.

A MMBTU (one million BTU) is approximately equal to 1.054615 GJ. Conversely, 1 gigajoule is equivalent to 26.8 m³ of natural gas at defined temperature and pressure. One MMBtu is equal to 28.263682 m³ of natural gas at defined temperature and pressure. One standard cubic foot of natural gas is approximately equal to 1000 BTU (to within a few percent).

A BTU per hour (BTU/h) is the unit most commonly associated with BTU. 1000 BTU/h are approximately 293 W and it is equal to 25 cubic meters of gas [11]. Consequently, one thousand cubic meters of gas = 4MBTU = 11625 Kwh = 1 ton of oil.

CRACKING

The term “cracking” refers to the process through which large hydrocarbon molecules are split into smaller ones in order to obtain lighter hydrocarbons. This process requires very high temperatures and sometimes the use of a “catalyst”. In fact, there are different types of cracking [12]. There are two types of cracking which have additional variations in the way they are implemented.

The first type of cracking is called “Thermal cracking”. It basically consists in heating the hydrocarbons until they reach high temperatures using also high pressures in some cases. This allows the hydrocarbons to break apart forming simpler hydrocarbons. The simple word “cracking” is often used to refer to this type of cracking as this is the oldest and most common type of cracking. However, thermal cracking can be achieved in different ways. There are three methods to implement thermal cracking:

• Steam - high temperature steam (1500 degrees Fahrenheit / 816 degrees Celsius) is used to break ethane, butane and naptha into ethylene and benzene, which are used to manufacture chemicals.

• Visbreaking - residual from the distillation tower is heated (900 degrees Fahrenheit / 482 degrees Celsius), cooled with gas oil and rapidly burned (flashed) in a distillation tower. This process reduces the viscosity of heavy weight oils and produces tar.

• Coking - residual from the distillation tower is heated to temperatures above 900 degrees Fahrenheit / 482 degrees Celsius until it cracks into heavy oil, gasoline and naphtha. When the process is done, a heavy, almost pure carbon residue is left (coke); the coke is cleaned from the cokers and sold [13].

The second type of cracking is called “Catalytic cracking” and it uses a catalyst to separate different hydrocarbons. This method of cracking generally uses zeolites as catalysts [14]. Catalytic cracking can be also done through other catalyst such as aluminum hydrosilicate, bauxite and silica-alumina. As in the case of thermal cracking there are different methods to implement catalytic cracking:

• Fluid catalytic cracking - a hot, fluid catalyst (1000 degrees Fahrenheit / 538 degrees Celsius) cracks heavy gas oil into diesel oils and gasoline [15].

• Hydrocracking - similar to fluid catalytic cracking, but uses a different catalyst, lower temperatures, higher pressure, and hydrogen gas. It takes heavy oil and cracks it into gasoline and kerosene (jet fuel). Hydrocracking is basically a refining process that uses hydrogen and catalysts with relatively low temperatures and high pressures for converting middle boiling or residual material to high-octane gasoline, reformer charge stock, jet fuel, and/or high grade fuel oil. The process uses one or more catalysts, depending upon product output [16].

Once hydrocarbons have been cracked into smaller ones they pass through another fractional distillation column to be further distilled and to separate different components inside them [17].

FRACTIONAL DISTILLATION

Fractional distillation constitutes the oldest and most common way to separate hydrocarbons in different “fractions”. This technique is generally used in the refining process of crude oil (also unprocessed oil) to separate the various hydrocarbons that constitute crude oil in different fractions in order to blend these hydrocarbons at a later stage and obtain various products (gasoline, etc…). It is the most important phase in the refining process.

Fractional distillation basically consists in heating up crude oil in order to let it vaporize. Different hydrocarbons inside crude oil have different boiling temperatures. By heating up the crude it is then possible to separate the hydrocarbons inside oil and then use them separately one from the other in order to take advantage of their specific characteristics to obtain different fuels for various applications (see “Refining process”).

In order to understand how the fractional distillation process actually works, it is important to understand first the “boiling point principle” and how it makes it possible to separate the various components inside crude oil:

• “The boiling points of organic compounds can give important clues to other physical properties. A liquid boils when its vapor pressure is equal to the atmospheric pressure. Vapor pressure is determined by the kinetic energy of molecules while at the same time kinetic energy is related to temperature and the mass and velocity of the molecules. When the temperature reaches the boiling point, the average kinetic energy of the liquid particles is sufficient to overcome the forces of attraction that hold molecules in the liquid state. As a consequence, these molecules break away from the liquid forming the gas state”.

• “Vapor pressure is caused by an equilibrium between molecules in the gaseous state and molecules in the liquid state. When molecules in the liquid state have sufficient kinetic energy, they may escape from the surface and turn into a gas. Molecules with the most independence in individual motions achieve sufficient kinetic energy (velocities) to escape at lower temperatures. The vapor pressure will be higher and therefore the compound will boil at a lower temperature”.

• “Molecules which strongly interact or bond with each other through a variety of intermolecular forces can not move easily or rapidly and therefore, do not achieve the kinetic energy necessary to escape the liquid state. Therefore, molecules with strong intermolecular forces will have higher boiling points. This is a consequence of the increased kinetic energy needed to break the intermolecular bonds so that individual molecules may escape the liquid as gases”.

A series of alkanes demonstrates the general principle that boiling points increase as molecular weight or chain length increases.

BOILING POINTS OF ALKANES:

“The principle which is used is that the longer the carbon chain, the higher the temperature at which the compounds will boil. The crude petroleum is heated and changed into a gas. The gases are passed through a distillation column which becomes cooler as the height increases. When a compound in the gaseous state cools below its boiling point, it condenses into a liquid. The liquids may be drawn off the distilling column at various heights”  [18].

The fractured distillation process consequently consists of different steps. The first one is to heat a mixture of substances in liquid form with different boiling points to a high temperature. Subsequently, once the liquids boil, the majority of substances goes in a vapor form. Thirdly, the vapor enter the so-called “fractional distillation column” filled with trays which may have holes to let the vapor go through the column while increasing the contact time between the vapor and the liquid formed at various heights in the column. The trays also help to collect the liquids formed at various heights in the column while the vapor cools down while passing from the bottom to the top of the column (hot at the bottom and cold at the top). When a substance in the vapor reaches a height where the temperature of the column is equal to that substance’s boiling point, it will condense to form a liquid (the substance with the lowest boiling point will condense at the highest point in the column; substances with higher boiling points will condense lower in the column.). In the end, the trays collect the various liquid fractions which may pass through condensers to cool them further and then go to storage tanks or go through further chemical processing [19].

THE REFINERY SYSTEM [20]:

“Although all fractions of petroleum find uses, the greatest demand is for gasoline. One barrel of crude petroleum contains only 30-40% gasoline. Transportation demands require that over 50% of the crude oil be converted into gasoline. To meet this demand some petroleum fractions must be converted to gasoline. This may be done by "cracking" - breaking down large molecules of heavy heating oil; "reforming" - changing molecular structures of low quality gasoline molecules; or "polymerization" - forming longer molecules from smaller ones” [21].

GASOLINE (ALSO PETROL)

Gasoline (or petrol or fuel – sometimes in North America called “gas” for abbreviation) is a byproduct of the oil refining process. It is a liquid mix of aliphatic hydrocarbons and enhanced with aromatic hydrocarbons toluene, benzene, or iso-octane to increase octane ratings, primarily used as fuel in internal combustion engines. Gasoline is produced in refineries. Gasoline created through the first distillation process would not meet today engine requirements and consequently it must be refined or distilled again to be sold on the market as engine fuel. There are therefore several types of processes beside fractional distillation that can be implemented to produce high quality gasoline. Unification (catalytic reforming), cracking, and alteration are among these processes (see relative terms in the glossary) necessary to refine crude oil. Once one of these processes has been completed it is possible to blend the various hydrocarbon components obtained in order to produce gasoline.

Gasoline is usually a mixture of paraffins (alkanes), naphthenes (cycloalkanes), aromatics and olefins (alkenes). The exact ratios can depend on the process used at the oil refinery to produce gasoline, the type of crude oil used at the refinery and the grade of gasoline (octane grade in particular). However, gasoline can contain some organic compounds and in particular sulfur compounds.

As with crude oil there are in fact different types of gasoline with different qualities. As the blends of hydrocarbons used to produce gasoline differ one from the other some type of gasoline may have higher octane ratings and therefore a higher calorific power. The majority of gasoline is compose by hydrocarbons with between 5 and 12 carbon atoms per molecule. An important characteristic in gasoline is the octane rating level, which is a measure of how resistant gasoline is to the abnormal combustion phenomenon known as detonation (also known as knocking, pinging, spark knock, and other names). Deflagration is the normal type of combustion. Octane rating is measured relative to a mixture of 2,2,4-trimethylpentane (an isomer of octane) and n-heptane [22].

REFINING PROCESS - REFINING

Hydrocarbons, such as oil and gas, which are extracted from the soil usually contain a mixture of different chemical components all mixed together. In order to able to use these hydrocarbons to produce energy it is necessary to separate the different components inside them. The refining process has the aim to separate these components and make it possible to obtain fuels and gasoline from crude oil (also unprocessed oil). Crude oil goes through the refining process in order to be commercialized as gasoline or other type of fuels.

The refining process can be carried out in different ways in order to separate hydrocarbons that constitute crude oil in useful substances. However, crude oil is firstly refined through sulfuric acid or other substances to remove heavier impurities. Different components inside hydrocarbons are then separated in different “fractions”. These will be refined (or distilled) again later in order to remove the lightest impurities and they will eventually be combined together again in different form to create fuel or new hydrocarbons. There are at least two different processes used to separate chemical components in extracted hydrocarbons in different fractions or to combine smaller hydrocarbons into larger ones:

• Fractional distillation (traditional method);
• “Cracking” - Catalytic distillation or cracking (chemical distillation or processing) – also used to change one fraction into another. Other methods include:
• “Unification” (or catalytic reforming - reforming);
• “Alteration” (Alkylation).

Once this process has been completed the different distilled or chemically processed fractions are treated to remove impurities, such as organic compounds containing sulfur, nitrogen, oxygen, water, dissolved metals and inorganic salts. Treating is usually done by passing the fractions through the following:

• a column of sulfuric acid - removes unsaturated hydrocarbons (those with carbon-carbon double-bonds), nitrogen compounds, oxygen compounds and residual solids (tars, asphalt);
• an absorption column filled with drying agents to remove water;
• sulfur treatment and hydrogen-sulfide scrubbers to remove sulfur and sulfur compounds.

Once all the fractions have been treated, they are cooled and then blended together to make various products, such as:

• gasoline of various grades, with or without additives;
• lubricating oils of various weights and grades (e.g. 10W-40, 5W-30);
• kerosene of various grades;
• jet fuel;
• diesel fuel;
• heating oil;
• chemicals of various grades for making plastics and other polymers [23].

Each fraction contains a different hydrocarbon which can be used to produce a specific kind of fuel or chemical product with various applications.

TRAP (OIL OR GAS TRAP)

An oil or gas trap is usually a configuration or conglomerate of rocks which is suitable for containing hydrocarbons. This configuration however is sealed by impermeable rocks which make it impossible for hydrocarbons to migrate to the surface [24]. Even if the extraction of oil and gas becomes more complicated when hydrocarbons are trapped, the absence of traps will stop hydrocarbons from accumulating in large quantities underground. Oil for example will not accumulate in large quantities unless something traps it underground. Oil traps can be divided in three geological types [25]:

Various traps

• Structural traps: These traps usually include a geologic structure which has changed over time and it is capable of retaining hydrocarbons due to change in rocks’ composition or type (movement of rocks which created an impediment for hydrocarbons to move) [26]. These traps can be found in three different forms in petroleum geology:

• Anticline trap: A type of structural hydrocarbon trap whose closure is controlled by the presence of an anticline. An anticline is “an arch-shaped fold in rock in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. Anticlines form many excellent hydrocarbon traps, particularly in folds with reservoir-quality rocks in their core and impermeable seals in the outer layers of the fold. A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core” [27].

• Fault trap: A type of trap which is caused by the movement of rocks along a “fault line”. The rock containing the reservoir has generally moved near a layer of impermeable rock. The latter blocks hydrocarbons from moving from the reservoir to the surface. It is also possible that the fault line constitutes a trap itself. Clays within the fault zone are smeared as the layers of rock slip past one another. This is known as fault gouge [28].

• Salt dome trap: A type of trap made by a salt dome. This is a “mushroom-shaped or plug-shaped diapir made of salt, commonly having an overlying cap rock. salt domes form as a consequence of the relative buoyancy of salt when buried beneath other types of sediment. The salt flows upward to form salt domes, sheets, pillars and other structures. Hydrocarbons are commonly found around salt domes because of the abundance and variety of traps created by salt movement and the association with evaporite minerals that can provide excellent sealing capabilities” [29].

• Stratigraphic traps: These traps can block hydrocarbons and accumulate large quantities underground because of a change in the character and characteristics of the rocks involved and not because of movement of the rocks involved. “Stratigraphy” literally indicates “the study of the rocks and their variations”. The trap is composed by impermeable rocks or sedimentary features such as reefs or unconformities or changes in rock type [30].

• Combination traps: This trap combines two or more of the characteristics shown by the above type of traps. In reality many traps are a combination of the situations described above.

UNIFICATION – (CATALYTIC) REFORMING

Unification or catalytic reforming (or just reforming) can be used sometimes during the refining process in order to create bigger hydrocarbons once they have been separated through cracking. This is usually done in order to obtain particular types of hydrocarbons generally used to produce specific fuels or gasoline (high octane fuels). The major unification process is called “catalytic reforming” or simply “reforming”. As in the cracking process a catalyst is used in order to combine hydrocarbons together. Common catalysts are platinum, platinum-rhenium mix. These are used to combine low weight naphtha into aromatics, which are used in making chemicals and in blending gasoline. A significant by-product of this reaction is hydrogen gas, which is then either used for hydrocracking or sold [31].

Catalytic reforming is therefore part of the refining process. Controlled heat and pressure together with the catalyst are used in order to rearrange hydrocarbons together. This is generally done in order to convert paraffinic and naphthenic type of hydrocarbons (low-octane gasoline) into higher octane stocks in order to blend them together and obtain finished gasoline. There are two types of catalytic reforming:

• Low pressure: the processing unit operates at less than 225 pounds per square inch measured at the outlet separator.

• High pressure: the processing unit operates at a pressure which is either equal or higher than 225 pounds per square inch measured at the outlet separator [32].