Category: Process

Feedstock

A petroleum refining study starts with describing its feedstock, the crude oil and the range of products that are produced by the various processes. Crude oil comes from different parts of the world and has different physical and chemical characteristics. On the other hand, the products that are produced have to meet market requirements and as such, should comply with certain specifications.

Figure 1 shows the ratio of C/H in some of chemical compounds:

Crude oil is a non-uniform material. The composition depends on its location. The composition of crude oil, on an elemental basis, falls within certain ranges regardless of its origin.

Table 1 shows that carbon and hydrogen contents vary within narrow ranges. For this reason, crude oil is not classified on the basis of carbon content. Despite their low concentrations, impurities such as:

• Sulphur
• Nitrogen
• Oxygen
• Metals

The above item are undesirable because they cause concerns in the processability of crude feedstock and because they affect the quality of the produced products.

Elemental composition of crude oils

Products

Figure 2 shows typical refinery products with their carbon atom contents and boiling ranges. There are specifications for over 2000 individual refinery products. Intermediate feed stocks can be routed to various units to produce different blend products depending on market demand.

Fig 2 The typical refinery products with their carbon atom contents and boiling ranges

Liquefied Petroleum Gas (LPG)

Liquified petroleum gas is a group of hydrocarbon-based gases derived from crude oil refining or natural gas fractionation. They include ethane, ethylene, propane, propylene, normal butane, butylene, isobutane and isobutylene.

Gasoline

Gasoline is classified by octane ratings (conventional, oxygenated and reformulated) into three grades:

• Regular
• Midgrade
• Premium

Kerosene

Kerosene is a light petroleum distillate that is used in space heaters, cook stoves and water heaters and which is suitable for use as a light source.

Jet Fuel

This category comprises both gasoline and kerosene and meets specifications for use in aviation turbine power units.

Diesel Fuel

The quality of diesel fuels can be expressed as cetane number or cetane index. The cetane number (CN) is expressed in terms of the volume percent of cetane (C16H34) which has high ignition (CN = 100) in a mixture with alpha-methyl-naphthalene (C11H10) which has low ignition quality (CN = 0).

Fuel Oil

The fuel oils are mainly used in space heating and thus the market is quite high specially in cold climates.

• No. 1 fuel oil is similar to kerosene
• No. 2 fuel oil is very similar to No. 2 diesel fuel
• Heavier grades of No. 3 and 4 are also available

Residual Fuel Oil

It is mainly composed of vacuum residue. Critical specifications are viscosity and sulphur content. Low sulphur residues are in more demand in the market.

Lube Oil

Lubricants are based on the viscosity index. Paraffinic and naphthenic lubricants have a finished viscosity index of more than 75.

Asphalt

Asphalt is an important product in the construction industry and comprises upto 20% of products. It can be produced only from crude containing asphaltenic material.

Petroleum Coke

Carbon compounds formed from thermal conversion of petroleum containing resins and asphaltenes are called petroleum cokes. Fuel grade coke contains about 85% carbon and 4% hydrogen. The balance is made up of sulphur, nitrogen, oxygen, vanadium and nickel.

Storage feed and product

In most processes, there are many different types of equipment used for the storage of liquids and gases.

Importance of using the correct storage:

• Minimizing evaporation losses
• Reducing the cost of unnecessary capital investment in building additional tanks, lines, pumps, and loading or unloading facilities
• Improve safety (decrease flammability)

The following Considerations are necessary in choosing a suitable storage system:

• Type of storage unit
• Amount of storage
• Production of these units

The general types of storage units can be divided into two groups

• Low Pressure Storage
• High Pressure Storage

Low-pressure liquid storage tanks commonly contain fuels such as gasoline and diesel, but they often contain other hazardous materials such as other flammable solvents, oxidizers, corrosives, and toxic liquids. Typically working pressures on these containers range from 2.5-15 psi.  The low pressure tanks are most common in refineries and the most common types of low pressure tanks are:

• Open Top Tanks
• Fixed Roof Tanks
• Floating Roof Tanks

Pressure vessels play an important role in various industries. These tanks are used as a tool to store gases, liquids and solids under pressure and are used in oil and gas, petrochemical, pharmaceutical and many other industries. In this article, we will introduce the types of pressure vessels and their applications. The most common types of high pressure tanks are

• Spherical Storage Vessels
• Cylindrical  Storage Vessels
• Gas Cylinders

Petroleum refining processes stabilize and upgrade petroleum products by separating them from less desirable products and by removing objectionable elements. Undesirable elements such as sulfur, nitrogen and oxygen are removed by hydrodesulfurization, water treatment, chemical sweetening and removal of acid gases. Refining processes, mainly used to separate petroleum products, include processes such as deasphalting. Desalination is used to remove salt, minerals, sand and water from raw crude oil before refining. Asphalt blowing is used to polymerize and stabilize asphalt in order to improve its weathering characteristics. The most important oil refining processes are:

• Hydro-desulphurization
• Hydro-treating
• Chemical sweetening
• De-asphalting

Hydro-desulphurization

Although reactions related to the catalytic hydrogenation of organic materials were known before 1897, the property of finely divided nickel to catalyze the fixation of hydrogen on hydrocarbon double bonds. Hydrodesulfurization (HDS) is a catalytic chemical process widely used to remove sulfur compounds from refined petroleum products such as:

• Gasoline or Petrol
• Jet fuel
• Diesel fuel
• Fuel oils

One of the goals of sulfur removal is to reduce sulfur dioxide emissions from the use of those fuels in automobiles, airplanes, railroad locomotives, ships or oil-burning power plants, residential and industrial furnaces, and other forms of fuel combustion.

Another important reason for removing sulfur from intermediate product naphtha streams in an oil refinery is that sulfur, even at very low concentrations, poisons the platinum and rhenium noble metal catalysts in the catalytic reformer units that are subsequently used to upgrade the naphtha stream. will be Hydrogenation of sulfur compounds leads to the formation of hydrogen sulfide, an undesirable and toxic gas. Industrial hydrogen desulfurization processes include facilities for absorbing and removing hydrogen sulfide gas. In oil refineries, hydrogen sulfide gas is subsequently converted to byproduct elemental sulfur. In fact, the vast majority of the 68,000,000 metric tons of sulfur produced worldwide in 2010 was by-product sulfur from oil refining and natural gas processing plants.

Desulfurization or desulfurization with the help of hydrogen is a catalytic process that is carried out in refineries to remove sulfur compounds from fossil fuels. Most metals catalyze the hydrodesulfurization process, but intermediate metals are more active. Ruthenium disulfide is the most active of the catalysts, but a relatively high binary combination of cobalt and molybdenum is usually used as the main catalyst in the process.

The reaction takes place in a trickle bed reactor, where the direction of movement of the liquid phase is always from top to bottom, but the direction of movement of the gas phase can be from top to bottom or from bottom to top. This reactor is usually used in hydrogenation reactions. The process temperature and operating pressure are 330°C and 4000 kPa respectively.

In this reactor, sulfur compounds are converted into hydrogen sulfide in the presence of hydrogen. All reactions are exothermic and the reaction rate decreases as the reactor length increases. Some of the most important reactions that occur in this process are listed below:

Reactions A and B for mercaptans and reactions C and D are related to dimethyl disulfide and dimethyl disulfide, which are in the flow of disulfide oils with hydrogen, respectively. Also, reaction E is for benzothiophene.

1. CH₃SH+H₂ CH₄+H₂S
2. C₂H₆S₂+H₂ 2CH₄+2H₂S
3. C₃H₉S₂+7/2H₂ 3CH₄+2H₂S
4. C₄H₁₀S₂+3H₂ 2C₂H₆+2H₂S
5. C₃H₆S+3H₂ C₈H₁₀+H₂S

Hydro-treating

The term hydrogen refining is used to describe the process of removing sulfur, nitrogen and metal impurities in the raw material by hydrogen in the presence of a catalyst. Hydrocracking is the process of catalytic cracking of raw materials into lower boiling point products by reacting them with hydrogen. Hydrogenation is used when aromatics are saturated by hydrogen to the corresponding naphthenes. The use of hydraulic conversion technique depends on the type of raw material and desired products, which are shown in the table below.

The Hydrotreating process achieves the following objectives:

• Removing impurities, such as sulphur, nitrogen and oxygen for the control of a final product specification or for the preparation of feed for further processing (naphtha reformer feed and FCC feed);
• Removal of metals, usually in a separate guard catalytic reactor when the organo-metallic compounds are hydrogenated and decomposed, resulting in metal deposition on the catalyst pores (e.g. atmospheric residue desulphurization (ARDS) guard reactor);
• Saturation of olefins and their unstable compounds.

Chemical sweetening

Sweetening of distillates is accomplished by the conversion of mercaptans to alkyl disulfides in the presence of a catalyst. Conversion may be followed by an extraction step for removal of the alkyl disulfides. In the conversion process, sulfur is added to the sour distillate with a small amount of caustic and air. The mixture is then passed upward through a fixed-bed catalyst, counter to a flow of caustic entering at the top of the vessel. In the conversion and extraction process, the sour distillate is washed with caustic and then is contacted in the extractor with a solution of catalyst and caustic. The extracted distillate is then contacted with air to convert mercaptans to disulfides. After oxidation, the distillate is settled, inhibitors are added, and the distillate is sent to storage. Regeneration is accomplished by mixing caustic from the bottom of the extractor with air and then separating the disulfides and excess air.

Amine gas treating, also known as gas sweetening and acid gas (AG) removal, refers to a group of processes that use aqueous solutions of various amines to remove H2S and CO from gases. Sweetening processes involve the removal of H2S and mercaptans from refinery streams.

Amines have a functional group that contains nitrogen. Primary amines arise when one of the three hydrogen atoms in ammonia is replaced by an organic substituent. Secondary amines have two organic substituents bound to N together with one H. The most commonly used amines in gas treating are:

• Primary monoethanolamine (MEA)
• Secondary diethanolamine (DEA)
• Tertiary methyldiethanolamine (MDEA)

Propane Deasphalting

Solvent Deasphalting (SDA) is a unique separation process in which the residue is separated by molecular weight (density) instead of boiling point, producing a low pollutant deasphalting oil (DAO) that is rich in paraffin. . This has the advantage of being a relatively low-cost process that has the flexibility to accommodate a wide range of DAO qualities. As with vacuum distillation, there are limitations to how far the SDA unit can upgrade the residue or how much DAO it can produce. These restrictions usually include:

• The DAO quality specifications required by downstream conversion units
• The final high-sulphur residual fuel oil stability and quality

The well-proven SDA process normally separates vacuum residue feedstock into relatively low metal/carbon DAO and a heavy pitch stream containing most of the contaminants. A solvent (typically C3–C7) is used and recovered from both product streams by supercritical recovery methods, thereby minimizing utilities consumption.

One of the well-known solvent deasphalting process is the ROSE process. The ROSE process is an energy efficient and cost-effective solvent deasphalting technology. The following Figure is a simplified process flow diagram of the Rose process.

In the Rose process the feedstock is mixed with a portion of the solvent and fed to an asphaltene separator where additional solvent is contacted with the feed in a countercurrent mode at an elevated temperature and pressure. The heavy asphaltene fraction drops out of the solution and is withdrawn from the bottom. The solvent dissolved in the asphaltenes is separated, recovered and recycled.

Oil refineries are complex industrial facilities that convert crude oil into a variety of valuable products, including gasoline, diesel, jet fuel, and numerous petrochemicals. One of the fundamental aspects of refining is the separation of crude oil into its components based on their distinct physical and chemical properties. This separation is done through various processes, each of which is specifically designed for certain fractions of crude oil.

The first phase in petroleum refining operations is the separation of crude oil into its major constituents using 3 petroleum separation processes:

• Atmospheric distillation
• Vacuum distillation
• Light ends recovery (gas processing)

Crude oil consists of a mixture of hydrocarbon compounds including paraffinic, naphthenic, and aromatic hydrocarbons with small amounts of impurities including sulfur, nitrogen, oxygen, and metals. Refinery separation processes separate these crude oil constituents into common boiling-point fractions. Below is an overview of the primary separation processes used in oil refineries:

1. Distillation

A. Atmospheric distillation

This initial step in the refining process involves heating the crude oil and introducing it into the distillation column. Here the oil is separated into different parts according to their boiling point. Lighter fractions, such as gases and naphtha, rise to the top of the column, while heavier fractions, including gas oils and residues, settle to the bottom.

Crude distillation unit (CDU) is at the front-end of the refinery, also known as topping unit, or atmospheric distillation unit. It receives high flow rates hence its size and operating cost are the largest in the refinery. Many crude distillation units are designed to handle a variety of crude oil types. The design of the unit is based on a light crude scenario and a heavy crude scenario. The unit should run satisfactorily at about 60% of the design feed rate. Seasonal temperature variation should be incorporated in the design because changes in the cut point of gasoline can vary by 20 C (36 F) between summer and winter.

Typical products from the unit are:

• Gases
• Light straight run naphtha (also called light gasoline or light naphtha)
• Heavy gasoline (also called military jet fuel)
• Kerosene (also called light distillate or jet fuel)
• Middle distillates called diesel or light gas oil (LGO)
• Heavy distillates called atmospheric gas oil (AGO) or heavy gas oil(HGO)
• Crude column bottoms called atmospheric residue or topped crude.

B. Vacuum distillation

For heavier fractions that cannot be effectively separated at atmospheric pressure, vacuum distillation is used. By reducing the pressure in the distillation column, the boiling points of these heavier components are lowered, allowing them to evaporate at lower temperatures and minimizing the risk of thermal cracking.

Topped crude withdrawn from the bottom of the atmospheric distillation column is composed of high boiling-point hydrocarbons. When distilled at atmospheric pressures, the crude oil decomposes and polymerizes and will foul equipment. To separate topped crude into components, it must be distilled in a vacuum column at a very low pressure and in a steam atmosphere.

To extract more distillates from the atmospheric residue, the bottom from the atmospheric CDU is sent to the vacuum distillation unit. The vacuum unit distillates are classified as light vacuum gas oil (LVGO),  medium vacuum gas oil (MVGO), and heavy vacuum gas oil (HVGO).

In addition a vacuum residue is produced. If the distillates are feed to down stream conversion process, their the sulphur, metal and asphaltene content should be reduced by hydrotreating or hydroprocessing. In some refineries the whole atmospheric residue is hydroprocessed before vacuum distillation. The vacuum unit can also be used to produce lubrication oil grade feed stocks. This depends on the quality of the crude oil feed to the refinery as only special types of crude can produce lube grade feed stocks.

The atmospheric residue can be sent directly to the vacuum unit after heat extraction in the crude preheat exchangers train. If it is sent to storage, the temperature should not be below 150 C (300 F) to control the viscosity necessary for proper flow. It is then heated in several exchangers by the hot products and pumparounds of the vacuum unit. Final heating to 380–415 C (716–779 F) is done in a fired heater. To minimize thermal cracking and coking, steam is injected in the heater tube passes. The feed enters the vacuum tower at the lower part of the column. As in the case of atmospheric distillation, a 3–5 vol% overflash is maintained (i.e., 3–5 vol% vapours are produced more than the total products withdrawn above the flash zone).

Vacuum distillation units have a system to create the vacuum that uses either ejectors or a combination of ejectors and liquid ring pumps. Ejectors recompress the gases through a nozzle where vapours from the column are sucked into the venturi section of the nozzle by a stream of medium or low pressure steam. The vapour phase at the ejector exit is partially condensed in an exchanger with cooling water. The liquid phase is then sent to the overhead drum. The vapour phase goes from the condenser to another ejector-condenser stage. Liquid ring pumps are similar to rotor gas compressors. One pump can replace two or three stages of ejectors in dry or wet type vacuum distillation. They do not use steam and can significantly reduce hydrocarbon-rich aqueous condensates in a system using ejectors. Systems with ejectors are much more flexible and rapid to put into operation. The higher investments required by liquid ring pumps are offset by reduced steam consumption and lower installation costs.

3. Light ends recovery (gas processing)

Light end recovery is a critical process in oil refining that focuses on the separation and recovery of light hydrocarbon fractions from crude oil or refinery intermediate streams. Commonly referred to as the “light end,” these light hydrocarbons typically include methane, ethane, propane, butane, and light naphtha. Light end recovery is essential to maximize the value of the refining process, optimize product yield and ensure efficient refinery operation.

The liquid sidestream withdrawn from the tower will contain low-boiling components which lower the flashpoint. These ‘‘light ends’’ are stripped from each sidestream in a separate small stripping tower containing four to ten trays with steam introduced under the bottom tray. The steam and stripped light ends are vented back into the vapor zone of the atmospheric fractionator above the corresponding side draw tray.

The light end recovery unit uses absorption and distillation steps to remove propane and heavier components from the refinery gas stream before it is used as fuel gas. The recovered C3+ components are then separated and used in different product streams.

In the absorption stage, naphtha and kerosene are used to absorb heavier hydrocarbons producing fuel gas and heavier liquid stream. The naphtha-rich liquid product that exits the bottom of the absorber enters a series of columns that step by step remove the lighter products.

These sub-columns are named in a self-explanatory way:

• Deethanizer: ethane recovery
• Depropanizer: propane recovery
• Debutanizer: Recovery of butanes
• Deisobutanizer: separates overhead isobutane and regular butane as a side draw

The remaining C5/C6 fraction is typically utilized for gasoline blending, whereas the heavy naphtha fraction is directed to catalytic reforming processes to generate high-octane gasoline.  It is important to note that one or more of these light end columns may present challenges when expanding, reconfiguring, or altering refinery feedstocks. However, these challenges can be effectively addressed through the implementation of modern upgraded internals and/or the re-rating of rotating equipment.”

Crude oil is a complex hydrocarbon mixture that mainly includes the following hydrocarbons:

• Alkanes (paraffins)
• Cycloalkanes (naphthenes)
• Aromatic

The specific composition of crude oil varies depending on the geological source and affects its physical and chemical properties. Linear alkanes, while abundant, generally have lower octane ratings, making them less desirable for gasoline formulation.

The process of oil conversion in oil refineries is necessary to convert crude oil into a variety of valuable products such as gasoline, diesel, jet fuel, and petrochemicals.

Oil conversion processes include the following processes:

• Distillation (Atmospheric and Vacuum)
• Cracking (thermal and catalytic)
• Reforming
• Polymerization and Alkylation
• Isomerization
• Coking
• Visbreaking

Distillation (atmosphere and vacuum)

Crude oil is heated in a furnace to a temperature of about 350-400 degrees Celsius and then fed into a distillation column. The key deductions obtained are:

• Gases: Propane and butane.
• Naphtha: Used for gasoline production
• Kerosene: Jet fuel and solvents
• Diesel: Fuel for vehicles
• Residuum: Heavy oils for lubricants or asphalt

The heavier fractions remaining after atmospheric distillation are subjected to vacuum distillation. This process produces vacuum gas oil (VGO) and other heavier products.

Cracking (thermal and catalytic)

Cracking is a critical step that breaks down larger hydrocarbon molecules into smaller, more valuable molecules. For higher molecular weight fractions such as atmospheric residues (AR) and vacuum gas oils (VGOs), cracking in the presence of hydrogen is required to get light products. In this case a dual function catalyst is used. It is composed of a zeolite catalyst for the cracking function and rare earth metals supported on alumina for the hydrogenation function. The main products are:

• Kerosene
• Jet fuel
• Diesel
• Fuel oil

Reforming

Reforming process converts naphtha into high-octane gasoline components. It rearranges molecular structures through heat and catalysts, enhancing the performance of gasoline.

Polymerization and alkylation

Polymerization and alkylation processes are used to combine small petroleum molecules into larger molecules.

Isomerization process

Isomerization of light naphtha is the process in which low octane number hydrocarbons (C₄, C₅, C₆) are transformed to a branched product with the same carbon number.

This process produces high octane number products. One main advantage of this process is to separate hexane (C₆) before it enters the reformer, thus preventing the formation of benzene which produces carcinogenic products on combustion with gasoline. The main catalyst in this case is a Pt-zeolite base.

The purpose of this process: to convert low-octane n-paraffins into high-octane isoparaffins. Primary Process Technique: Isomerization is carried out in a fixed bed reactor with chloride where n-paraffins are converted to isoparaffins. The catalyst is sensitive to incoming pollutants (sulfur and water).

Process steps:

• Desulfurized feed and hydrogen are dried in fixed beds
• The mixed feed is heated and passed through a hydrogenation reactor to saturate the olefins to paraffin and benzene.
• The hydrogenation effluent is cooled and passes through an isomerization reactor
• The final effluent in the form of hydrogen and LPG, which usually goes to gas fuel, and the isomerite product is cooled and separated for gasoline blending.

Feedstocks for isomerization typically consist of straight light naphtha containing C5 to C6 hydrocarbons. Before isomerization, this raw material is pre-purified to remove impurities such as sulfur and nitrogen compounds.

Catalysts, usually platinum or palladium, are used in the isomerization process on acidic substrates such as alumina or zeolites. This catalytic system facilitates molecular rearrangement at high temperatures (150-250 °C) and pressure (up to 20 atmospheres).

Separation and purification

After the reaction, the product mixture contains unbranched linear hydrocarbons. Distillation is used to effectively separate these components and ensure that the final product adheres to gasoline composition specifications.

Coking

Coking is a thermal cracking process used to convert low value residual fuel oil into higher value gas oil and petroleum coke. Vacuum residues and thermal filaments are cracked in the coking process at high temperature and low pressure. Products are petroleum coke, gas oil and lighter oil reserves. Delayed coking is the most widely used process today, but fluid coking is expected to become an important process in the future.

Visbreaking

Topped crude or vacuum residuals are heated and thermally cracked in the visbreaker furnace to reduce the viscosity, or pour point, of the charge. The cracked products are quenched with gas oil and flashed into a fractionator. The vapor overhead from the fractionator is separated into light distillate products. A heavy distillate recovered from the fractionator liquid can be used as either a fuel oil blending component or catalytic cracking feed.