Oil and Gas Handbook

6 Refining

Up to the early 1970s, crude oil prices were kept reasonably stable by major international oil companies and industrialized nations. Less value was created in the upstream production operations and relatively more profits were generated in refining and distribution operations. With the 1973 oil crisis and rising crude oil prices, more of the value was created upstream.

Now, the success of a modern refinery depends more on economies of scale and the ability to process a wide range of crudes into the maximum quantity of high value fuels and feedstock. A refinery that is able to handle multiple types from heavy to light crude is said to have to have a large swing. Trade specifications such as "West Texas Intermediate" (WTI) API 38.3°, "Brent Blend" API 38.3°, "Heavy Arab Crude" API 27.7° or "Grane" API 18.7° are examples of such crudes.

Medium light crudes can be used directly in early engines and burners. Modern consumers, such as gas and diesel engines, aviation turbojet engines and ship bunkers need fuels manufactured to precise specifications. This includes removing contaminants and pollutants, such as sulfur.

6.1 Fractional Distillation

The basic refinery uses fractional distillation. Incoming crude is heated to its boiling point. It then enters the distillation column, which separates the different fractions. The column is of the reflux type, where colder condensed fluids running down are reheated by rising vapors that in turn condense. This produces clear thermal zones where the different products can be drained.

NOTE:The schematic in figure 19 is simplified. Both continuous and vacuum distillation is used in separate columns to avoid heating the raw crude to more than 370 °C. Overheating would cause thermal cracking and excessive coke that may also plug pipes and vessels. Also a sidecut stripper is used, in addition to the main column, to further improve separation. Sidecut is another name for the fractions emerging from the side (rather than top and bottom) of the main column, i.e., naphtha, gasoline, kerosene and diesel.

The fractions are a mix of alkanes and aromatics and other hydrocarbons, so there is not a linear and uniformly rising relationship between carbon number and boiling point and density, although there is a rough fit. Even so, this means that each fraction contains a distribution of carbon numbers and hydrocarbons.

6.2 Basic Products

The basic products from fractional distillation are:

Figure 19. Fractionating continuous distillation, overview
Figure 19. Fractionating continuous distillation, overview

Liquid petroleum gas (LPG) has carbon numbers of 1–5 and a boiling point up to 20 °C. Most of the LPGs are propane and butane, with carbon number 3 and 4 and boiling points -42 °C and -1 °C, respectively. Typical usage is domestic and camping gas, LPG vehicles and petrochemical feedstock.

Naphtha, or full range naphtha, is the fraction with boiling points between 30 °C and 200 °C and molecules generally having carbon numbers 5 to 12. The fraction is typically 15–30% of crude oil by weight. It is used mainly as a feedstock for other processes:

  • In the refinery for producing additives for high octane gasoline
  • A diluent for transporting very heavy crude
  • Feedstock to the petrochemical olefins chain
  • Feedstock for many other chemicals
  • As a solvent in cleaning

Gasoline has carbon numbers mainly between 4 and 12 and boiling points up to 120 °C. Its main use is as fuel for internal combustion engines. Early on, this fraction could be sold directly as gasoline for cars, but today’s engines require more precisely formulated fuel, so less than 20% of gasoline at the pump is the raw gasoline fraction. Additional sources are needed to meet the demand, and additives are required to control such parameters as octane rating and volatility. Also, other sources such as bioethanol may be added, up to about 5%.

Kerosene has main carbon numbers 10 to 16 (range 6 to 16) boiling between 150 °C and 275 °C. Its main use is as aviation fuel, where the best known blend is Jet A-1. Kerosene is also used for lighting (paraffin lamps) and heating.

Diesel oil, or petrodiesel, is used for diesel engines in cars, trucks, ships, trains and utility machinery. It has a carbon number range of 8 to 21 (mainly 16–20) and is the fraction that boils between 200 °C and 350 °C.

White and black oils: The above products are often called white oils, and the fractions are generally available from the atmospheric distillation column. The remaining fraction below are the black oils, which must be further separated by vacuum distillation due to the temperature restriction of heating raw crude to no more than 370–380 °C. This allows the lighter fractions to boil off at a lower temperature than with atmospheric distillation, avoiding overheating.

Lubricating oils, or mineral base lubricating oil (as opposed to synthetic lubricants), form the basis for lubricating waxes and polishes. These typically contain 90% raw material with carbon numbers from 20 to 50 and a fraction boiling at 300–600 °C. 10% additives are used to control lubricant properties, such as viscosity

Fuel oils is a common term encompassing a wide range of fuels that also includes forms of kerosene and diesel, as well as the heavy fuel oil and bunker that is produced at the low end of the column before bitumen and coke residues. Fuel oil is graded on a scale of 1 to 6 where grade 1 and 2 is similar to kerosene and diesel, 3 is rarely used anymore. 4–6 are the heavy fuels, also called Bunker A, B and C, where B and C are very high viscosity at normal ambient temperatures and requires preheating to about 100 °C and 120 °C respectively, before it flows enough to be used in an engine or burner. Fuel oil grade 4 does not require preheating and is sometimes mixed with off spec products, such as tank residue and interface liquid from multiphase pipelines or with grade 2 fuel oil to achieve low-enough viscosity at ambient temperatures. Fuel oil 6 is the lowest grade, its specification also allows 2% water and 0.5% mineral soil and is consumed almost exclusively by large ships in international waters, where pollutants such as sulfur is less regulated.

Bitumen and other residues like coke and tar has carbon numbers above 70 and boiling points above 525 °C. Low sulfur coke can be used for anodes in the metals industry (aluminum and steel) after processing (calcining). The remainder is a problem fuel, because of high sulfur content and even higher CO2 emissions than coal (typically 15% higher). Bitumen in the form of asphalt boiling above 525 °C is used for roofing and road paving. Asphalt concrete pavement material is commonly composed of 5% asphalt/bitumen and 95% stone, sand, and gravel (aggregates).

6.3 Upgrading and Advanced Processes

The Refinery make up differs from an upstream plant, in that the overall site is divided up in to process types or 'units'. The refining plant type processes are generally licensed, and a license is required to build and operate one of these. Each license will be the same but scaled to meet the processing capacity in tons per day. A full explanation of these processes is beyond the scope of this book, but a non-exhaustive description is given below.

The following figure gives a more detailed process flow diagram of an actual modern refinery. It shows the extent of treatment that takes place after initial fractional distillation, to improve fuel yield and functional properties, and an explanation of why modern gasoline at the pump contains less than 20% raw gasoline straight from the column. Additional processes may also be included, e.g., for crude pre-treatment to be able to source lower quality crudes with less processing at the production site.

Refinery process
Figure 20. Refinery process flow diagram. Source: Wikipedia

Most of these reactions take place at elevated temperature and pressure over a catalyst such as platinum or rhenium or sometimes iron, and need precise control for optimal throughput. A few process flow diagrams have been included to give an indication of the complexity of these processes compared to the relative simplicity of many upstream processes.

Atmospheric distillation is the fractional distillation unit already described. In actual designs, it is combined with vacuum distillation. Raw crude cannot be heated to more than 370–380 °C. It is often called the Crude Oil Distillation Unit (CDU)

Vacuum distillation unit (VDU) further distills the black oils into fuel oils and residual bitumen and coke to avoid overheating the crude and to extract additional valuable product that could be upgraded.

Naphtha hydrotreater: Various sulfur compounds are present in the hydrocarbon mixture and, if burnt with the other carbons, will cause sulfuric emissions. The hydrotreater uses hydrogen to remove some of these compounds. As an example, the hydrodesulfurization (HDS) reaction for ethanethiol can be expressed as:

ethanethiol + hydrogen → ethane + hydrogen sulfide
C2H5SH + H2 → C2H6 + H2S

A catalytic reformer unit is used to convert the naphtha molecules (C5-C12) into higher octane reformate (reformer product). These are mixed with raw gasoline to achieve a higher octane product. The process creates more aromatics (ring formed hydrocarbons) by dehydrocyclization or more complex hydrocarbons with double bonds or side groups by dehydrogenation. These processes release hydrogen which is recovered and can be reused in hydrotreaters or hydrocrackers.

Catalytic reformer
Figure 21. Catalytic reformer. Source: Wikipedia

Distillate hydrotreater units desulfurize distillates (such as diesel) after fractional distillation, in the same way as the naphtha hydrotreater.

Fluid catalytic crackers (FCC) units upgrade heavier fractions into lighter, more valuable products. Long chain molecules (high carbon numbers) are split into shorter molecules to achieve more of the high value fuel components. A typical design uses a reactor and a regenerator. A fine powdered porous catalyst with zeolite (silicate and alumina) is fluidized in the hydrocarbon vapor, where a reaction takes place at 535 °C and 0.172 MPa. The catalytic reaction takes place within a few seconds, after which the reformate and catalyst is separated in a cyclone. The spent catalyst then goes back to a regenerator that heats it to 715 °C at 0.241 MPa and releases flue gas. The catalyst powder can then be reused. The reformates go to a distillation column for separation into fractions.

A hydrocracker unit performs essentially the same function as the FCC when more saturated hydrocarbons are desirable in the product. This means alkane carbon chains with single bonds, not double bonds or cyclic rings like aromatics, or more complex molecules. For this, additional hydrogen is needed. The reaction takes place with hydrogen under pressure over a catalyst. The relative market need for diesel, kerosene and gasoline will influence the choice of FCC versus hydrocracker. In the US, with a higher relative volume of gasoline, more FCC capacity is needed, while in Europe and Asia, with higher diesel consumption, more hydrocracking is used.

Visbreaking units upgrade heavy residual oils by thermally cracking them into lower viscosity product that can be blended into lighter, more valuable products. Visbreaking is characterized by its thermal severity, ranging from mild cracking at 425 °C to severe cracking at 500 °C. Depending on the residual oil, as much as 15–25% lighter fractions like diesel, kerosene and gasoline could be obtained. The residue is tar and coke.

The Merox unit treats LPG, kerosene or jet fuel by oxidizing thiols (mercaptans) to organic disulfides. The purpose is to reduce strong odors caused by thiol presence.

Coking units (delayed coking, fluid coker, and flexicoker), like the visbreaker, uses thermal cracking of very heavy residual oils into gasoline and diesel fuel. The residue is green coke, and is further processed to fuel coke or, if too low in sulfur and contaminants, to anode coke for the metallurgic industries.

An alkylation unit produces high-octane components for gasoline blending. The main use is to convert isobutane (C4H10, but arranged differently than nbutane) to alkylates, mainly isooctane or isoheptane by adding an alkyl group such as propene or butene over a strong acid catalyst, such as sulfuric or hydrofluoric acid.

Dimerization is similar to alkylation, but uses a dimer group instead of an alkyl group. For example, butenes can be dimerized into isooctene, which may be hydrogenated to form isooctane.

Isomerization units convert linear molecules to higher-octane branched molecules by rearranging the same atoms arranged in a different way. For example, C4H10 n-butane has the carbon atoms in a chain, while isobutane has a central carbon with one hydrogen and three CH3 groups attached. The isobutane can then be fed to the alkylation unit.

Steam reforming produces hydrogen for the hydrotreaters or hydrocracker. Typical is the steam methane reformer (SMR), where steam reacts with Methane at 425 °C with a nickel catalyst to produce syngas, which is a source for many different reactions:

CH4 + H2O ↔ CO + 3H2

If more hydrogen is needed, followed by a gas shift reaction with CO:

CO + H2O ↔ CO2 + H2

Amine gas treater, Claus unit, and tail gas treatment converts hydrogen sulfide from hydrodesulfurization into elemental sulfur, which is a valuable traded product.

Claus process
Figure 22. Claus process. Source: Wikipedia

The Claus process is the most common with the overall reaction:

2 H2S + O2 → S2 + 2 H2O

The reactor runs at 1,000 °C and 0.15 MPa, with three steps: one thermal and two catalytic to improve yield.

Using these processes, a modern refinery can raise the basic gasoline yield depending on crude quality from 10–40% to around to 70%.

6.4 Blending and Distribution

After the refining processes, the various fractions are stored in intermediate tanks, then blended into marketable products for loading onto railcars, trucks or ships, and distribution to gas stations or industries.

Each product is blended to a specification of up to 25 parameters such as octane rating, energy content, volatility and sulfur content. The task is to achieve the specification (and not exceed, where applicable) with the minimum amount of over-spec "giveaway". The blending quality is managed with infrared or chromatograph type process analyzers. These can determine the precise fractions of a sample by molecule type.

Terminal operations
Terminal operations

The standard specification gasoline is therefore standard from company to company in the individual markets, ensuring compatibility with vehicle manufacturer requirements. Also, the terminal operator may be an independent third party or run as co-distribution, so that a terminal in one region distributes for several companies based on the same products in the same tanks.

Each company then seeks to differentiate its product by adding small quantities of unique additives that are marketed to increase engine performance, lifetime, clean combustion and so forth. These additives are added as the product is dispensed to trucks for delivery to that brand gasoline station

Gasoline dispensing
Gasoline dispensing

A main task is to ensure the balance of incoming and outgoing products and consolidate with stored volumes. These have to be compensated for temperature, water content (water may be absorbed from air humidity and released at low temperatures as bottom slop in tanks) and vapor loss. Vapor or volatile organic compounds (VOCs), form above the product in fixed roof tanks and when filling up compartments in cars or vessels. VOC loss can be significant for high volatility products like gasoline, and must be recovered and/or handled to reduce emissions and explosive hazard.

The terminal management system tracks batches of product received or dispensed, as well as those eventually received by gasoline stations, airports or other consumers and consolidates with stored volumes. Each operation should be validated against orders, bills of lading and positive identification of trucks, vessels and their operators. In countries where this process is not well managed, losses of product due to theft and other factors can be as high as 15% or more in the distribution operations.

Eventually the main goal is to ensure that orders are met, and stakeholders pay or get paid in the form of VAT, taxes, product, delivery charges, etc.

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