Petrochemicals

Energy Worldwide

Human energy requirements begin with human existence itself. From the moment humans discovered fire, they have burned whatever fuel they found. Initially using readily accessible fuels such as wood and dung; later coal, petroleum and gas became important sources. Wood, the oldest known fuel, has lost its commercial importance in developed countries but still holds commercial value in less developed nations. Over 90% of the world's energy requirements are met through the burning of fossil fuels. These fuels, formed over millions of years through various geological processes, are unfortunately being consumed rapidly in a very short timeframe. Since replacement cannot occur in the near term, people have naturally turned to new sources beyond conventional energy resources. Nuclear energy is one of the most important, considering the sensitivity in both its acquisition and use stages. While energy sources such as solar, geothermal, wind and biomass increase their share in meeting energy requirements with each passing day, their contribution remains quite low. Oil continues to maintain its importance as the largest fuel and energy source of our time.

Formation of Crude Oil

It is known that petroleum is formed from organic matter in the seas through various organic processes. However, two important theories have been proposed regarding crude oil formation. The most valid theory, known as Engler Theory or the organic theory, examines the formation process in four main stages.

First Stage:

The death and burial of living organisms in the seas such as fish, algae, plankton and others. This stage is defined as the biological formation stage.

Second Stage:

The transformation of solid organic matter into proto-petroleum and micro-petroleum, the first liquids, through catalytic pathways. During the decomposition stage, bacteria provide the most important catalytic effect. This stage is defined as the catalytic formation stage.

Third Stage:

The movement of the formed liquid from its initial bed or reservoir to its final reservoir. This stage is defined as the migration stage.

Fourth Stage:

The composition of proto-petroleum changes in this final reservoir and becomes petroleum. It is also known that crude oil contains natural structures such as porphyrins and glutamine. This is because these structures contain nitrogen (N). Nitrogen is present in the peptide bonds of amino acids, which are the building blocks of proteins. Porphyrins are derived from chlorophyll. The structure of crude oil consists of hydrocarbons. However, nitrogen (N), sulphur (S) and oxygen (O) atoms are heteroatoms present in these organic compounds. The other theory was proposed by a Russian scientist. It appears as an inorganic theory. Petroleum, as is known, is a mixture of hydrocarbons composed of organic compounds containing hydrogen (H) and carbon (C) atoms. It proposes that simple organic molecules can be composed of certain inorganic-origin substances. It proposes that under the catalysis of molten iron in magma, reactions of water gas and similar molecules result in the formation of carbon monoxide (CO) and hydrogen gas (H2), which through certain syntheses under the catalysis of minerals containing Ni and Co found in the earth's crust, produce petroleum. However, the absence of free alkali metals in nature weakens this theory.

Classification, Chemical and Physical Properties of Crude Oil

The chemical composition and structure of crude oil vary depending on its location. Even oil extracted from the same location can sometimes contain different components. Crude oil also contains various compounds with oxygen, nitrogen and sulphur. Some olefins (alkenes) with general formulas (CnH2n) as hydrocarbons have been found to exist in sections of oil boiling at high temperatures. Olefins boiling at low temperatures are not very abundant. Depending on the chemical structure of the hydrocarbons it contains, petroleum is divided into four main categories.

1. Paraffinic Base:

This category contains saturated hydrocarbons (Alkanes CnH2n+2). They are thermodynamically very stable compounds.

2. Naphthenic Base (Cycloparaffins):

This category contains saturated cyclic alkanes (Cycloalkanes CnH2n). As they are saturated compounds, their reactivity is lower than olefins and aromatics. However, due to ring strain, they are more reactive than straight-chain paraffins.

3. Aromatic Base:

This category contains cyclic aromatic compounds with high unsaturation. Compounds with properties similar to benzene (CnH2n-6), the first aromatic compound studied, are found. Small single-ring aromatics such as benzene and toluene are not commonly found in crude oil. Polyaromatics with two or more rings are always present in the high-boiling fractions of petroleum.

4. Mixed Structures:

This category contains mixed components in certain proportions from the other three main groups mentioned above. Crude oil is generally dark brown in color. Rarely it can be light brown, yellow or reddish. Crude oil rich in asphaltenes is dark brown-black or brown; those without asphaltenes are red and yellow in color. All oils exhibit yellow-green to blue-green fluorescence properties. Oils can be fluid or viscous depending on the light, medium and heavy oils they contain and according to paraffins and asphaltenes. The odor of petroleum is produced from compounds with low boiling points in its composition. Oils containing sulphur compounds such as hydrogen sulphide (H2S) and mercaptans (RSH) are odorous. Their specific gravities are generally determined as (0.72-1) g/cm3. Their specific gravities depend on the amount and type of compounds with low boiling points they contain. The calorific value of petroleum also depends on its composition and has a heat value of (9,600-11,500) kcal/kg. The flash point varies depending on the amounts of hydrocarbons boiling at low temperatures and is (0-200)°C.

Basic Refining Processes

Through basic refining operations applied to crude oil in petroleum refineries, primary products are obtained. Crude oil as extracted from underground is not usable in its raw form. These operations consist of various physical and chemical processes. The following refining processes are applied to desalted crude oil.

1 - Atmospheric Distillation:

Crude oil contains hydrocarbons boiling up to 500°C in mixed form. It is the process of separating hydrocarbons boiling up to approximately 350°C by taking advantage of the differences in their boiling points. Continuous distillation takes place in a distillation tower (distillation column) consisting of trays, each with a different temperature. This distillation is carried out under atmospheric pressure. As a result of the process, those with boiling points close to each other are collected in fractions to form primary products. From the top of the column, the lightest products are liquefied petroleum gas (LPG); and from the section immediately below between (38-120)°C, light gasoline (naphtha) is obtained. As we move downward, the fraction between (120-180)°C is taken as heavy gasoline, the fraction between (170-270)°C is kerosene, and the fraction between (200-360)°C is taken as heavy fuel oil products. From the bottom of the tower, the heaviest products are separated and sent to the vacuum distillation unit. In the vacuum distillation unit, hydrocarbons in the bottom product (the heaviest ones) from the atmospheric distillation unit are distilled. The distillation column used here is shorter in height. This is because the number of products is small. High distillation temperature (350-500°C) requires high preheating temperature. This accelerates formations that would cause defects such as coking in the furnace. To separate substances boiling at very high temperatures without breaking down their structures, vacuum distillation is applied. As the pressure in the tower decreases, the boiling point also decreases. Vacuum distillation is used to produce mineral oil, asphalt and heavy fuel oil to be fed to the catalytic cracking unit.

2 - Cracking:

The process of breaking down hydrocarbons with high boiling points (large molecular weight) through thermal or catalytic methods and converting them into products with lower boiling points but higher commercial value.

3 - Reforming:

Producing high-value gasoline by changing the structures of hydrocarbons in the heavy gasoline fractions through catalytic reactions without changing their carbon numbers. The production of aromatic compounds such as benzene and toluene is achieved.

4 - Polymerization:

The process of combining two, three or four C3 and C4 olefins (alkenes CnH2n) resulting from cracking to produce higher-value products.

5 - Alkylation:

The production of a new hydrocarbon by combining a small olefin with an isoparaffin to create a branched structure with high octane number. Paraffins with branched structures burn more efficiently. For this reason, to increase the octane number of gasoline, the increase in the number of branched molecules is specifically targeted. In this way, the numbers of compounds that are both stable in storage and efficient in combustion reactions are increased.

6 - Isomerization:

Obtaining the hydrocarbon with high commercial value or the one required for a specific process from hydrocarbons with the same number of carbons.

7 - Sulphur Removal:

Since most of the products sold are used as fuels, to prevent the formation of gases such as sulphur dioxide (SO2) after combustion, sulphur in the products is chemically separated. The most important sulphur removal method is the "Hydrodesulphurization" method. Thus, sulphur in the product is converted to hydrogen sulphide (H2S) compound and removed. Hydrogen sulphide is then separated into its components through another reaction and hydrogen gas and elemental sulphur are made available for use.

Main and Intermediate Products Obtained from Crude Oil

A large portion (more than 90%) of petroleum products produced as a result of various physical and chemical operations in oil refineries are used as transportation fuels. The remaining portion forms important feedstocks for various chemical industries, primarily petrochemicals.

LPG (Liquefied Petroleum Gas):

After light hydrocarbons released during refinery operations are separated—propane (CH3CH2CH3) and butane (CH3CH2CH2CH3) from other gases (H2, CH4, C2H6, C2H4)—a mixture is prepared in 50:50 or 40:60 ratios, liquefied under pressure and sold in steel cylinders. Butane and propane are gases with high calorific value (Butane: 291,000; Propane: 22,450 kcal/m3). In addition to their use in homes for cooking and heating, they are used in many areas as fuel in car engines or as raw materials in the chemical industry.

Gasoline:

The most fundamental product of the petroleum industry worldwide. All operations performed to increase, modify and develop gasoline yield in petroleum refining have been parallel with developments in the automotive industry. Gasoline is a liquid containing hydrocarbons boiling between 38-204°C in its composition. This mixture contains C5-C12 hydrocarbons. Operations performed to improve the properties and efficiency of gasoline are synonymous with increasing the "octane number". The octane number is based on n-heptane, a straight-chain paraffin rated as "0" and 2,2,4-trimethylpentane (isooctane), a branched compound rated as "100".

Kerosene:

The product obtained after gasoline in the refinery. These fuels are hydrocarbon mixtures boiling between 180-360°C; they contain C10-C25 hydrocarbons. While previously used only for lighting and heating purposes, today diesel and jet fuels are produced from within this wide fraction. Each fuel used for military aircraft has a wider boiling range than that used for civil jet aircraft.

Fuel Oils:

Fuel oils are fractions heavier than kerosene with high calorific value (10,000 kcal/kg). They are generally divided into two types: light and heavy. Fuel oils have replaced coal in industry. The most important properties sought in these are as follows: a) Low pour point to ensure easy flow in cold weather, b) Low sulphur content to prevent corrosion and odor, c) Sufficient amount of light hydrocarbons to allow quick ignition of fuel at the burner, d) Homogeneous mixing when preparing medium-weight fuels to ensure product stability.

Lubricating Oils (Mineral Oils):

Lubricating oils used to reduce friction between moving motor, machine and similar parts are collected in two main categories: liquids and solids. Liquids are called mineral oil and solids are called grease. Mineral oils are petroleum fractions boiling at high temperatures with high viscosity. The most important property of mineral oils is the viscosity index. Generally, as temperature increases, oils become thinner and their viscosity decreases. The viscosity index is a relative measure showing resistance to this thinning. The desired properties of these oils are that they be homogeneous, have a viscosity range that does not change much at the temperatures at which they are used, and be chemically stable substances. Mineral oils can be examined in three main categories: 1-Motor oils; 2-Industrial oils; classified as light, medium and heavy duty oils. 3-Oils used in metal processing; used in significant quantities as cooling agents.

Grease (Solid Oil):

Semi-solid materials made by adding 3-30% solid thickener into petroleum oils. Alkali salts of fatty acids and Al-salts (various soaps) are used as thickeners. They are used instead of liquid oils in systems that are not sealed, where leakage cannot be prevented, and where foreign substances such as dust enter from outside. There are various greases ranging from very soft to very hard. Their melting points vary between 70-180°C. In addition to soap, additives are added to grease that delay oxidation, increase resistance to rust and water. They are classified according to the type of thickener. These additives are metal salt soaps, organic substances, special clays and advanced silicas.

Petroleum Waxes (Waxy Structures):

They are of two types: paraffin and microcrystalline. Paraffin waxes are white and somewhat hard. They soften and melt between 40-80°C. They predominantly contain n-paraffins with 23-29 carbons. Microcrystalline ones are hard substances with colors varying from white to brown. They predominantly contain paraffins with naphthenes containing 34-70 carbons. Applications include; mainly the paper and cardboard industry, and wax, match, insulation material industry, anti-rust material and dental filling material.

Asphalt:

Asphalt is a colloid of asphaltenes and polymer substances in petroleum within oils. Brown-black asphaltenes are hydrogen-poor hydrocarbons. They contain considerable sulphur, oxygen and nitrogen. Polymer substances called resins are brown, sticky semi-solid materials. Asphalts made from the heaviest fraction of petroleum are also found naturally in various forms. Their main uses are road construction and waterproofing. A typical road asphalt contains approximately 30% asphaltenes and 40% resins.

Other Products:

Heavy liquids and solids remaining from various refinery operations are consumed individually or mixed with other fuels. Depending on the type of crude oil processed, various organic solvents and chemical substances are obtained. Light and heavy naphthas are used as solvents in various industrial branches. These chemicals produced from the gasoline fraction are:

1 - Petroleum Ether:

The lightest gasoline fraction. It is the fraction distilled between 25-89°C. It is used in extraction operations and pharmaceuticals.

2 - Test Gasoline:

The gasoline fraction distilled between 60-140°C. It is used in the extraction of vegetable oils and the cleaning of textile materials. In refineries processing high-sulphur petroleum, sulphur production also exists separately. Petroleum is simultaneously, in order to meet the requirements of daily civilization in various fields; the natural production source of monomers, which are raw materials for numerous polymers of industrial importance in terms of global production and consumption. We can list the most important of these as follows:

1 - Ethylene:

The smallest two-carbon alkene with molecular formula C2H4. It is obtained from cracking reactions. Primarily for the production of polyethylene (HDPE and LDPE) polymers; vinyl chloride obtained by its chlorination is also the monomer used in the production of polyvinyl chloride (PVC) polymer. PVC is one of the polymers with the most industrial use, particularly in the manufacture of doors and windows.

2 - Propylene:

A three-carbon alkene with molecular formula C3H6. It is obtained from cracking reactions. The polymer obtained from the polymerization of this monomer has applications as synthetic fiber.

3 - Butylene and Isobutylene:

Isomers of four-carbon alkene with molecular formula C4H8. They are obtained from catalytic cracking reactions of heavy gasoline fractions or from the steam cracking unit of naphtha. Isobutylene is particularly an important monomer used in butyl rubber production.

4 - 1,3-Butadiene:

A monomer with molecular formula C4H6 obtained from catalytic dehydrogenation reactions of n-butane. It is used in synthetic rubber production.

5 - Styrene:

A compound with molecular formula C6H5-CH=CH2, this monomer known by the name phenyl ethylene; is obtained by alkylating benzene through Friedel-Crafts alkylation reaction to obtain ethyl benzene and then dehydrogenating it over aluminum chloride, solid phosphoric acid or silica-alumina catalysts. This monomer is used in the production of light packaging materials popularly called "foam" (polystyrene foam), styrene rubber and polystyrene plastics.

6 - Chloroprene:

The monomer known by molecular formula C4H5Cl (2-chloro-1,3-butadiene). It is obtained from acetylene and hydrogen chloride. Acetylene is first dimerized to monovinyl acetylene. The resulting molecule then reacts with hydrogen chloride and chloroprene is formed. The polymer obtained from this monomer is used in synthetic rubber production.

7 - Benzene and Its Derivatives:

Benzene with molecular formula C6H6 is obtained in the catalytic reforming unit of refining operations from cyclohexane. As a result of alkylation reactions of benzene, toluene (methylbenzene) and xylene's (dimethyl benzene) ortho, meta and para isomers are obtained. Phenol (hydroxybenzene) is also obtained as a benzene derivative through the reaction of chlorobenzene with sodium hydroxide. All of these produced compounds are organic solvents of very high industrial importance.

Instructor Ayşın Küçükparmaksız / Kocaeli University Kocaeli Vocational School / Chemical and Chemical Processing Technologies Refinery and Petrochemistry Technology / Program Coordinator

 

References

1. Introduction to Petrochemical Technology-I 2. Introduction to Petrochemical Technology-II course notes. 3. Modern Petroleum Technology, PART 2, Fifth Edition, Edited by G. D. HOBSON, JOHN WILEY & SONS, Copyright 1984 by The Institute of Petroleum, London. 4. Petroleum Refining Technology, Assoc.Prof. Ömer KULELİ, CAĞ-LAYAN KİTABEVİ, 1st Edition (1981).