Petroleum Fractions and their Components
The petroleum fractions obtained from the distillation towers can be generally classified as gases and naphtha, middle distillates and heavy fraction. This section focuses on the components of these fractions.
Gases and Naphtha
The main hydrocarbon component of petroleum gases is methane. There are however other higher hydrocarbon compound in lesser quantities. Ethane, propane, butane, isobutane are present depending on the nature of the crude oil.
There may be presence of other non-desirable gases, such as hydrogen, carbon dioxide, hydrogen sulphide, and carbonyl sulphide which must be removed in the processing.
Naphtha fraction covers the boiling range of gasoline and is made up of mainly saturated constituents with small amounts of aromatics (mono- and di). Unfortunately, most of the raw petroleum naphtha molecules have low octane number and not valuable for direct application in gasoline.
The raw naphtha is processed further by reforming, isomerization and combined with other process naphtha and additives to formulate commercial gasoline. The saturated constituents in petroleum gases and naphtha are paraffin from methane (CH4) to n-decane (n- C10H22).
The naphtha fraction is the low boiling paraffin which may be the most abundant compound in a crude oil depending on the source of the crude oil. The paraffin in the naphtha begin at C4 with isobutane as the only isomer.
Additionally, the saturated constituents of naphtha fraction include cycloalkanes (naphthenes) with five- or six-carbon rings being the most prominent. Naphthenes such as methyl derivatives of cyclopentane and cyclohexane are more common than the parent unsubstituted structures.
Fused ring dicycloalkanes such as cis- decahydronaphthalene (cis-decalin) and trans-decahydronaphthalene (trans-decalin) are also found in naphtha fraction. Finally, petroleum naphtha contains numerous aromatic constituents beginning with benzene and its C1 to C3 alkylated derivatives, benzene derivatives having fused cycloparaffin rings such as tetralin, and naphthalene are all included in this fraction.
However, the 1- and 2-methyl naphthalene and higher homologs of fused two-ring aromatics appear in the middle distillate fraction.
Middle Distillates
The middle distillate is the raw material (the feedstock) for kerosene, aviation jet fuel, and diesel fuel production.
The mid-distillate fraction of petroleum is rich in saturated species but in contains some quantities of naphthenes and aromatics (either simple single ring compounds: BTX or fused rings with up to three aromatic rings, or fused with heterocyclic compounds).
Within the saturated constituents of the middle distillates, the concentration of n-paraffin is regularly from C11 to C20. Mono- and di-cycloparaffins with five or six carbons per ring constitute the bulk of the naphthenes in the middle distillate boiling range.
The most prominent aromatics in the mid-distillate boiling fractions are the di- and tri-methyl naphthalenes. Other one- and two-ring aromatics are also present but in small quantities.
Kerosene (stove oil) and light gas oil fractions are the most prominent products of the middle distillates of the crude oil physical process and usually represent the last fractions to be separated by distillation at atmospheric pressure.
These middle distillates leave the fractions from the heavy gas oil and higher boiling material. In this middle distillate is also the vacuum gas oil which has less saturate constituents compared to the aromatic constituents.
The vacuum gas oil is used often as heating oil and as feedstock for diesel fuel. Its most commonly subjected to catalytic cracking to produce naphtha (gasoline feedstock) or extracted to yield lubricating oil.
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In this vacuum gas oil, the various components’ distribution such as saturates (paraffin or iso-paraffin) and naphthenes are dependent on the crude oil source and type.
Nevertheless, naphthene constituent is about 60-70% of the saturate constituents unlike the lower boiling fraction that is more of paraffin. The paraffin from C20– C44arestill present in sufficient quantity in the vacuum gas oil.
Heavy Fractions
The heavy fractions often referred to as the low boiling fraction or the vacuum residua for the vacuum distillation are the most complex of petroleum. It contains the majority of the heteroatoms components of the crude oil.
The molecular weight of the constituents range up to several thousands. This vacuum residua fraction is so complex that it cannot be characterized individually.
Separation of vacuum residua by group type is made difficult by the multi-substitution of aromatic and naphthenic species and the presence of multiple functionalities in single molecules present.
This fraction is often the feedstock for thermal cracking or catalytic hydrocracking. It is not often used for direct catalytic reforms and cracking without hydro-treatment because of the presence of heteroatoms that can poison the catalyst and metals like Ni, V that can promote side reactions.
Chemical Refining Processes
The chemical conversion/refining processes in the petroleum industry are generally used for the following purposes:
To upgrade lower-value materials such as heavy residues to more valuable high demand products such as naphtha and LPG. Naphtha is mainly used to supplement the gasoline pool, while LPG is used as a fuel or as a petrochemical feedstock.
Thus, these two products are in higher demand than the others and are produced from the others by chemical conversion processes.
To improve the characteristics of a fuel. For example, a lower octane naphtha fraction is reformed to a higher octane reformate product.
The reformate is mainly blended with naphtha for gasoline formulation or extracted for obtaining aromatics needed for petrochemicals production.
Reduce harmful impurities in petroleum fractions and residues to control pollution and to avoid poisoning certain processing catalysts.
This section will discuss the thermal and catalytic cracking as examples of chemical refining processes.
Chemistry of Thermal Cracking
Thermal cracking is often used to increase gasoline production. The first step in cracking is the initiation process which occurs by thermal decomposition of hydrocarbon molecules to two free radical fragments.
This initiation step can occur by a homolytic carbon-carbon bond scission at any position along the hydrocarbon chain.
Further β bond scission of the new free radical R˙can continue to produce ethylene until the radical is terminated. Free radicals may also react with a hydrocarbon molecule from the feed by abstracting a hydrogen atom.
In this case the attacking radical is terminated, and a new free radical is formed. Abstraction of a hydrogen atom can occur at any position along the chain. However, the rate of hydrogen abstraction is faster from a tertiary position than from a secondary, which is faster than from a primary position.
The secondary free radical can crack on either side of the carbon carrying the unpaired electron according to the beta scission rule, and a terminal olefin is produced.
Hydrogen transfer (chain transfer) may occur when a free radical reacts with other hydrocarbons. There are two major commercial thermal cracking processes, delayed coking and fluid coking.
Coking
Coking is a severe thermal cracking process designed to handle heavy residues with high asphaltene and metal contents. These residues cannot be fed to catalytic cracking units because their impurities deactivate and poison the catalysts.
Products from coking processes vary considerably with feed type and process conditions. These products are hydrocarbon gases, cracked naphtha, middle distillates, and coke.
The gas and liquid products are characterized by a high percentage of unsaturation. Hydrotreatment is usually required to saturate olefinic compounds and to desulphurize products from coking units.
Coking can be carried out in a delay coking unit (Figure 3.5). In the delayed coking unit, the reactor system consists of a short contact-time heater coupled to a large drum in which the preheated feed “soaks” on a batch basis. Coke gradually forms in the drum. A delayed coking unit has at least a pair of drums.
A delayed coking unit has at least a pair of drums. When the coke reaches a predetermined level in one drum, flow is diverted to the other so that the process is continuous. Vapours from the top of the drum are directed to the fractionator where they are separated into gases, naphtha, kerosene, and gas oil.
Figure – Delayed Coking Unit (Speight 2006)
Chemistry of Catalytic Cracking
Catalytic cracking is a process used to crack heavy fraction, lower values stocks and produce higher valued, light and middle distillates. The process produces valuable gasoline with higher octane rating than thermal cracking.
It also produces light hydrocarbon gases, which are valuable raw materials (i.e. feed-stocks) for petrochemicals’ production. Catalytic cracking process’s mechanism promotes isomerization reactions which results in branched chain products which is responsible for the better octane number.
The cracking catalysts can be acid-treated clays; synthetic amorphous silica-alumina; zeolites (crystalline alumina-silica).
Acid treated clays were initially used in catalytic cracking processes before they have been replaced by synthetic amorphous silica-alumina. Amorphous silica- alumina is more active and stable than the acid treated clays.
More recently, zeolite which is crystalline alumina-silica catalyst are incorporated into the amorphous silica-alumina to improve selectivity towards aromatics. Cracking catalysts possess both two sites which are Lewis and Bronsted acid sites.
These sites promote the formation of carbonium ion which then promotes the cracking and the formation of branched isomers. The structure of zeolites involves holes in the crystal lattice formed by the silica-alumina tetrahedral shape.
The catalytic cracking reaction is different from thermal cracking in that reactions’ mechanism is through formation of carbocation intermediate as against the free radical intermediate in thermal cracking.
The carbocations undergo shift in the methyl group to give secondary or tertiary carbocation which is more stable. This makes carbocations undergo process more stable and selective than free radicals. The following reactions illustrate the catalytic cracking process with zeolite:
Abstraction of hydride ion from the hydrocarbon feed stock by the Lewis acid site of the zeolite.
Finally, the cracking reaction involves the carbon-carbon beta bond scission which is the reaction that leads to lower chain product. A bond at a position beta to the positively-charged carbon breaks heterolytically, yielding an olefin and another carbocation which continues the reaction.
A typical catalytic cracking is carried out in a Fluid Catalytic Cracking (FCC) reactor. The reaction system contains two compartments which are the fluidized bed reactor and the regenerator.
The catalyst is feed and fluidized in the fluidized bed reactor after which the hydrocarbon feed is loaded. When the catalyst is deactivated by carbon (coke) deposit, it is taken into the regenerator where it is regenerated with hot air which converts the coke to carbon dioxide.
Figure: Typical Fluid Catalytic Cracking reactor with regenerator (Mata and Hatch, 2000)
In summary,valuable products are obtainable from crude oil after the refining processes. Crude oil after exploration is subjected to desalting and dewatering because associate water and salts in crude oil are detrimental to its refining.
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The physical refining processes include atmospheric distillation to obtain fractions like gasses, light naphtha (gasoline), heavy naphtha, kerosene, and gas oil.
The residuum from the atmospheric distillation can be further distilled in a vacuum distillation column to obtain vacuum gas oil and lubricating oil.
Generally, the petroleum fractions obtained from the distillation towers can be classified as gases and naphtha, middle distillates and heavy fraction.
Though crude oil in itself is considered valuable but its values are manifested in the different products that are derivable from it.
The value addition processes to crude oil are the refining processes which include physical processes such as desalting and dewatering; atmospheric and vacuum distillation and chemical processes such as thermal and catalytic cracking among others.
The chemistry of thermal cracking is by free radical mechanism while that of catalytic cracking is by carbocation formation leading to branching of the hydrocarbon chain in catalytic cracking. Catalytic cracking is therefore preferred in the production of higher octane rating gasoline compared to thermal cracking.
