TABLE 1.1 Major crude oil streams and their properties
1.3.2 Oil Products
Crude oil is too volatile to be used on its own, and hence distillation of crude oil into various fractions of different volatility is needed. The main types of oil products in descending order of volatility are:
• gases and LPGs
• gasolines/naphthas
• kerosenes
• gasoils/diesels
• fuel oils
• lubricating oils, paraffin wax, asphalt, tar, and other residuals.
Methane and ethane are gases found with petroleum. Methane, which is also referred to as “natural gas,” is used for energy generation while ethane is used as a feedstock for petrochemical production, where it is converted into plastics. LPGs refer to propane, butane, or and a mixture of the two. They are used for cooking and industrial purposes. Gasoline is used mainly for motor transportation. Gasolines or naphthas are also used as feedstock for the petrochemical industry and refineries.
Kerosenes are mainly used as aviation turbine fuel (ATF). They are also still used for lighting and cooking in some parts of the world. Gasoils are used principally for home heating or as diesel engine fuel. They are also used as petrochemical feedstock. Fuel oils are used in marine transportation (also known as bunker oil) or as a source of fuel at refineries or power stations.
The refining process involves the separation of hydrocarbons by state and size, processing and treating individual products for the purpose of removing impurities and converting, or cracking, heavier hydrocarbons into lighter, more desirable compounds (Figure 1.3). The first stage of refining involves fractional distillation, whereby the crude oil is heated to a high temperature, usually around 350 °C, and pumped into a distillation column where a temperature gradient is maintained between the top and the bottom. Lighter components of the crude oil, which boil at lower temperatures, condense at higher levels of the column while heavier compounds settle at lower levels of the column. Off-take pipes at different heights of the column withdraw fractions of different compounds, with gases and LPG at the top of the tower and fuel oils and residuals at the bottom. This residue from atmospheric distillation can further be subjected to vacuum distillation to remove more volatile components of the residue, leaving behind asphaltenes and other heavy residues.
FIGURE 1.3 Simplified refining process diagram
Following distillation, the oil products are subjected to hydro-treating or Merox treating, whereby the sulfur present in the products is removed. Hydro-treating involves mixing hydrogen gas with the oil product (usually naphtha or gasoline) and passing the mixture over a catalyst at high temperature and pressure, resulting in the sulfur being removed as hydrogen sulfide gas.
The next major step in the refining process is the conversion of fractions into lighter, more desirable compounds. Naphthas are subjected to a process of catalytic reforming or platforming, whereby the “octane number,” a measure of performance of motor fuels, is increased using a catalyst like platinum. Heavy residues are subjected to thermal cracking (heating to temperatures in excess of 400 °C) or catalytic cracking, where a finely divided catalyst is mixed with the feedstock and heated, to produce catalytic-cracked gasoline and other light products. Hydro-cracking, another catalytic cracking process that uses hydrogen, can also be used for this purpose.
The final step in the process is blending, where different products produced at the refinery are mixed in certain proportions to form the finished products, which conform to certain standards. For example, oxygenates are blended with motor gasoline to reduce the lead content and increase the octane number of the fuel.
Prior to refining, a crude oil assay is conducted to get a good idea of the product yield (i.e., the fraction of each product that can be obtained from the particular grade of crude oil). With crude oils of a similar origin, the crude grade with a higher API gravity value is likely to yield higher-end products; however, an assay is the best means of getting a reliable estimate of product yield. Sample product yields from primary distillation of Brent Crude Oil and Dubai Crude Oil are shown in Tables 1.2 and 1.3.
TABLE 1.2 Brent Crude Oil distillation yields by percentage of weight
TABLE 1.3 Dubai Crude Oil distillation yields by percentage of weight
A test of the types of hydrocarbons present in the feedstock for the refinery can also be conducted to identify the appropriate feedstock to be used. This is called a PONA (paraffins, olefins, naphthenes, and aromatics) analysis. Feedstock that is rich in paraffins is better used as a petrochemical feedstock as it cracks easily. Olefins do not occur naturally in crude oils but are produced by refining processes and are present in other feedstock like naphthas and gasolines. Naphthenes and aromatics have higher octane numbers and are more suitable for refineries.
The product yields are used to calculate the gross refining margin. This is calculated by multiplying the product yields with the prevailing product prices and subtracting the cost of crude oil used. Some of the popular local product benchmarks are listed in Table 1.4. Calculating refining margins is essential to maintain the profitability of the refining operation, as refineries have flexibility in terms of choosing the optimum crude oil grade to use, changing the operation of the refinery to produce different fractions of products, blending, and the storage of products.
TABLE 1.4 Selected local product benchmarks
Natural Gas
Natural gas is another fossil fuel, which is naturally found along with crude oil or coal and is formed in a similar manner (i.e., the exertion of high pressure and temperature over millions of years, by geological processes, on the remains of plants and animals). The main constituent of natural gas is methane (CH4). Natural gas, when produced along with crude oil, is called associated gas. When crude oil is found in small quantities along with primarily natural gas, it is called condensate. Natural gas can also be extracted from coal reservoirs (known as coalbed methane), and landfill gas and biogas also contain high quantities of methane. Natural gas usually occurs with impurities such as water vapor, carbon dioxide, mercury, nitrogen, and hydrogen sulfide, as well as other gases such as ethane, propane, butane, and heavier hydrocarbons, which when liquefied are called natural gas liquids (NGLs). These impurities need to be removed before natural gas can be transported.
Natural gas is transported through pipelines or is liquefied to transport using liquefied natural gas (LNG) carriers. In this case, regasification facilities are required at the terminal where LNG is transported to. Since the heating use of natural gas is seasonal, gas needs to be stored for the winter season. Natural gas is “injected” into underground facilities like depleted gas reservoirs, salt caverns, and aquifers or stored within pipelines or as LNG.
Natural gas is the cleanest-burning hydrocarbon and is increasingly being used for electricity generation. It is used for heating and cooking and as feedstock for chemical manufacturing. It is also used as fuel for vehicles, which run on either compressed or liquid natural gas, and it can further be converted to other fuels using gas-to-liquid processes. Ethane is used for manufacturing plastics, while propane and butane are